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Size effect of carbon fiber-reinforced silicon carbide composites (C/C-SiC): Part 2 - tensile testing with alignment device
Abstract
The appropriate assessment of mechanical properties is essential to design ceramic matrix composites. The size effect of strength plays a key role for the material understanding and the transfer from lab-scale samples to components. In order to investigate the size effect for carbon fiber-reinforced silicon carbon (C/C-SiC) under tensile load, a tensile testing with a minimum of deviation from the pure tensile loading is necessary. Hence, a hybrid edge/face-loading test device for self-alignment and centering of C/C-SiC tensile samples was developed, evaluated and proved to ensure pure tensile load. The mechanical analysis of more than 190 samples with two different cross-sections fabricated from the same material population revealed no significant difference in tensile strength. Although the volume under load was increased from 129 to 154 mm³, the tensile strengths of 162 ± 7 and 164 ± 6 MPa did not change. These results are discussed regarding the weakest link and energetic size effect approaches.
... During the shaping process, the polymeric thermoset undergoes irreversible covalent crosslinking, rendering it non-meltable and conferring a stable shape. However, during the last years, the use of thermoplastic-based matrices for CMC applications increased [26][27][28][29][30][31][32]. ...
... This process requires the use of polymeric matrix precursors with a carbon yield after pyrolysis (1000 � C, nitrogen) of at least 30 wt.%, which drastically limits the number of potential thermoplastic candidate materials. Previous investigations identified polyetheretherketone (PEEK) as well as carbon fiber reinforced PEEK (CF-PEEK) as suitable [26][27][28][29][30], due to the high carbon yield of PEEK after pyrolysis in nitrogen of about 40-50 wt.% [34][35][36][37]. Furthermore, PEEK is employed in numerous medical applications, including tissue engineering, and is a well-established material for additive manufacturing processes [38][39][40][41][42][43][44]. ...
... Tensile tests were performed on specimens with four different layups: "T0" ([0] 20 layup), "T45" ([45/ − 45] 10s layup), "T30" ([30/ − 30] 10s layup), and "TQI" or "quasi-isotropic" ([0/45/90/ − 45] 5s layup). The standard ASTM C1275-15 [35] was taken as a guideline to design the specimens and perform the tests, which for this class of materials may require specific and tight precautions [36]. All the specimens were dog-bone shaped, with a total length of 200mm, a gauge length of 45mm, and an average thickness of 4.21 ± 0.08mm. ...
... Such morphology further supports the hypothesis of relative slip between fiber and matrix and is a clear indication that the final failure is triggered by the progressive accumulation of cracks in the material surrounding the fibers. This was also discussed in [30,31,36], where it was indicated that the fractures that can be observed in bending initially develop in the matrix, and that fibers are eventually broken because of stress concentrations induced by matrix damaging. ...
This work presents a Finite Element approach to model the in-plane mechanical behavior of C/C-SiC fabrics at
the lamina level, including the non-linear response of diverse lay-ups and the bending-to-tensile strength ratio at
failure. The material model employs a decomposition into two idealized phases, which can be exploited to
capture matrix- and fiber-dominated responses at a high level of abstraction. The constitutive law of the matrix
phase adopts a Continuum Damage approach driven by two Tsai-Wu surfaces, while a quasi-brittle behavior is
attributed to the fibers phase. This decomposition effectively represents the influence of matrix degradation on
the response and failure of the laminates. Moreover, simulations reveal that a statistical distribution of the
strength is required to represent some of the experimental outcomes. The correlation with experimental data that
was achieved points out that the technique is a promising tool for supporting the early design phase of CMC
structures.
... To overcome the intrinsic brittleness of UHTCs, recent investigations have proven that the introduction of fibers, either continuous or discontinuous, tremendously improve the failure tolerance over a broad temperature range without hampering the hot corrosion resistance [6][7][8]. It has been also highlighted that the fiber distribution and architecture impact the final mechanical and thermo-mechanical properties of the ceramic matrix composite (CMC) [9][10][11][12][13][14], no matter if the matrix is a polymer, a metal or a ceramic [15,16]. ...
... This dichotomy between the promise of 3D printing and its current limitations underscores the critical importance of comprehending the size effect. This becomes especially relevant when considering that numerous materials, ranging from silicon carbide (SiC) ceramics [11] to sea ice [12], composites [13], steel [14], vinyl foam [15], concrete [16], rock [17], lime [18], and an array of polymers [19], exhibit characteristics that are contingent on size. This phenomenon poses a formidable challenge to the scientific community, emphasizing the pressing need for systematic investigations into size effects in additively manufactured polymers. ...
Most of the experiments on additively manufactured polymers are on a small scale, and it remains uncertain whether findings at a small scale can be extrapolated to their larger-scale counterparts. This uncertainty mainly arises due to the limited studies on the effect of size on three-dimensional (3D)-printed polymers, among many others. Given this background, this preliminary study aims to investigate the effect of geometric dimensions (i.e., the size effect) on the mechanical performance of four representative types of 3D-printable polymers, namely, (1) polycarbonate acrylonitrile butadiene styrene (PC/ABS), (2) acrylonitrile-styrene-acrylate (ASA), (3) polylactic acid (PLA) as a bio biodegradable and sustainable material, and (4) polyamide (PA, nylon), based on compression, modulus of elasticity, tension, and flexural tests. Eight different sizes were investigated for compression, modulus of elasticity, and tension tests, while seven different sizes were tested under flexure as per relevant test standards. A material extrusion technique was used to 3D-print the polymers in a flat build orientation and at an infill orientation angle of 45°. The results have shown that the mechanical properties of the 3D-printed polymers were size-dependent, regardless of the material type, with the most significant being flexure, followed by tension, compression, and modulus of elasticity; however, no clear general trend could be identified in this regard. All the materials except for nylon showed a brittle failure pattern, characterized by interfacial failure rather than filament failure. PLA outperformed the other three polymer specimens in terms of strength, irrespective of the type of loading.
... 1,16,17,[19][20][21][22] Ceramic matrix composites (CMCs), such as SiC f /SiC (where the subscript f represents fibers), F I G U R E 1 Applications of thermal barrier coatings (TBCs) in gas turbine engines. 1 C f /C, SiC f /C, and C f /SiC have been considered to replace alloys owing to their high melting points (>2700 • C), low density, and high strength. [23][24][25][26][27][28] However, C and SiC tend to oxidize to form CO 2 and SiO 2 , respectively, in environments containing high-temperature air, preventing the direct application of CMCs in aviation and gas turbine engines. To protect CMCs from damage caused by hightemperature air or steam, environmental barrier coatings (EBCs) have been proposed, which delay their oxidization and thus lower the probability of their failure. ...
This perspective outlines the research directions of ferroelastic rare‐earth (RE) tantalates (RETaO4), which have been studied as multifunctional thermal/environmental barrier coatings (T/EBCs) with working temperatures above 1600°C. Ferroelastic RETaO4 ceramics exhibit several distinct features, including the reversible second‐order ferroelastic phase transition, high toughness, low thermal conductivity, low oxygen ion conductivity, and adjustable thermal expansion coefficients. This perspective provides a concise summary of the research progress on tantalate coatings synthesized via air plasma spraying and electron beam physical vapor deposition. The service performance of tantalate coatings is typically evaluated through thermal fatigue and shock measurements. Uncovering the failure mechanisms and understanding influencing factors are key aspects for future studies. In this regard, establishing the synthesis‐structure‐property relationship of RETaO4 coatings is essential. This brief perspective can serve as a guide for future work on RETaO4 coatings and further advancements in their applications across various fields, including aviation engines and gas turbines.
... The influence of the geometry of the specimen on the properties of the composite is also not negligible (Kueh, 2014;Flauder et al., 2021). Li et al., (2021) found through experimental studies that different preparation angles have significant effects on the tensile properties and damage mechanisms of 3D braided composites. ...
Free-edge effect is one of the factors affecting the mechanical properties of three-dimensional woven composites under tensile load. However, current research is relatively poorly understood regarding the effect of free-edge on the stiffness and strength of the material. This paper aims at examining the influence of free-edge effect on the mechanical properties of 3D woven composites under tension through experimental and simulation methods. The three-dimensional digital image correlation (DIC) technique is used to collect the full-field strains on the specimen surface during the test, and the stress-strain differences in different regions in the width direction are analyzed, and the overlap of the curves in each region is found to be high, indicating that the boundary effect has a small influence on the tensile properties of 3D woven composites. Experimental studies are conducted on specimens of different widths (within the range of 15–20 mm), and the results indicate that the differences in mechanical properties of 3D woven composites under tension loading in this width range are not significant. A progressive damage finite element model is developed for calculation and compared with experimental results. It is found that the tensile properties of the material decreased when the width of the specimen is less than twice the size of the single cell. This study can provide certain data support for the study of the mechanical properties of 3D woven composites and enable the subsequent more in-depth study to provide a certain foundation.
... The influence of the geometry of the specimen on the properties of the composite is also not negligible [9] [10]. Li et al [11]found through experimental studies that different preparation angles have significant effects on the tensile properties and damage mechanisms of 3D braided composites. ...
Free-edge effect is one of the factors affecting the mechanical properties of three-dimensional woven composites under tensile load. However, current research is relatively poorly understood regarding the effect of free-edge on the stiffness and strength of the material. This paper aims at examining the influence of free-edge effect on the mechanical properties of 3D woven composites under tension through experimental and simulation mathods. The three-dimensional DIC technique is used to collect the full-field strains on the specimen surface during the test, and the stress-strain differences in different regions in the width direction are analyzed, and the overlap of the curves in each region is found to be high. Experimental studies are conducted on specimens of different widths (within the range of 15-20 mm), and the results indicate that the differences in mechanical properties of 3D woven composites under tension loading in this width range are not significant. A progressive damage finite element model is developed for calculation and compared with experimental results.
Because of the excellent fracture toughness and oxidation resistance, carbon fiber reinforced silicon carbide (C/C-SiC or C/SiC) exhibits a sound potential in various application areas such as aerospace technology and high-performance braking systems. For the composite’s reliable design, production, examination, quality assurance and verification, however, the statistical distribution of mechanical properties is of crucial interest and has not been investigated in detail yet. In this work, the strength values of C/C-SiC composite, which was developed via Liquid Silicon Infiltration at the Institute of Structures and Design of German Aerospace Center (DLR), were measured under tensile, bending and compression load. The results were analyzed by normal and Weibull distribution statistics and verified by Kolmogorov–Smirnov-test (KS-test) and Anderson–Darling-test (AD-test). Based on the statistical analysis, the 4PB-strength of C/C-SiC composite can be better described by Weibull distribution. In comparison, normal distribution is more suitable for the compression strength. The influence of different numbers of coupons on the mechanical properties has been identified. A scanning electron microscope (SEM) was employed to analyze the fracture surface, which confirmed that the different statistical distribution of strength values was caused by various failure mechanisms.
Two novel approaches are proposed for elimination of stress concentrations in tensile and compressive testing of unidirectional carbon/epoxy composites. An interlayer hybrid specimen type is proposed for tensile testing. The presented finite element study indicated that the outer continuous glass/epoxy plies suppress the stress concentrations at the grips and protect the central carbon/epoxy plies from premature failure, eliminating the need for end-tabs. The test results confirmed the benefits of the hybrid specimens by generating consistent gauge-section failures in tension. The developed hybrid four point bending specimen type and strain evaluation method were verified and applied successfully to determine the compressive failure strain of three different grade carbon/epoxy composite prepregs. Stable failure and fragmentation of the high and ultra-high modulus unidirectional carbon/epoxy plies were reported. The high strength carbon/epoxy plies exhibited catastrophic failure at a significantly higher compressive strain than normally observed.
Woven SiC-based ceramic matrix composites (CMCs) are promising materials for several systems running at high temperatures. They provide excellent examples of microstructure–behavior relationships in long fiber-reinforced brittle matrices. Minicomposite test specimens represent an appropriate length scale for investigation of multidirectionally reinforced CMCs like the 2D woven SiC SiC and C SiC. The chapter discusses microstructure–behavior relationships for 2D woven composites. It focuses on matrix damage by multiple cracking, ultimate fracture, delayed fracture at high temperatures, nonlinear stress–strain behavior, influence of constituent properties, and stochastic features induced by flaw populations. Damage evolution and ultimate failure are modeled on the basis of micromechanics and fracture probabilities.
For the manufacture of carbon fibre reinforced SiC materials, three different processes are used in principle: Chemical Vapour Infiltration (CVI), Liquid Polymer Infiltration (LPI), also called Polymer Infiltration and Pyrolysis (PIP) as well as Melt Infiltration or Liquid Silicon Infiltration (MI / LSI). Depending on the differ-ent methods for building up the SiC-matrix and embedding the carbon fibres in the brittle ceramic matrix, the resulting C/SiC and C/C-SiC materials vary significantly in their properties as well as in their manufacturing costs. The advantages of the MI processes are their short manufacturing times and the use of low cost raw materials, leading to the most cost efficient Ceramic Matrix Composites (CMC), compared to materials de-rived from CVI and PIP. In combination with their unique thermal and mechanical properties, MI based CMC`s opened up a wide field of new applications beyond aerospace.
This presentation gives an overview of the manufacturing processes and resulting properties of C-fibre rein-forced SiC materials. Typical applications of C/C-SiC or C/SiC materials based on MI processes, like friction materials, hot structures for solid propellant rocket motors and temperature stable structures for optical tele-communication systems are presented.
The mechanical characterisation of Ceramic Matrix Composites (CMCs) poses challenges because of the high testing temperature and their particular material structures. Choosing the correct specimen geometry is critical to obtain valid test results. This is particularly true for composite materials which have high strength in the fibre direction, but low strength in the transverse direction and in shear mode. In this paper, a generic tensile specimen shape is investigated by varying four of its geometric parameters. A finite element model is built to determine the stress field, which is compared with the material strengths to ascertain failure mode and location. Recommendations are made for the parameters to ensure tensile failure within gauge section. They are checked using both CFRP and CMC specimens. Some of the modelling issues are also discussed.
Formulas for the effective volumes and effective surfaces for rectangular bars loaded in flexure are reviewed. The ratio of strengths from any two configurations, such as three-point flexural to four-point, is independent of whether the flaws are volume- or surface-distributed, if the cross-sectional sizes are the same. Discrimination between volume- or surface-distributed flaw types, based on the strength ratios, is not possible in such cases. On the other hand, conversion of strengths from one configuration to another is very easy if testing is performed in accordance with one of the current world standard test methods.
The material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 ◦) and off-axis (±45 ◦) direction are summarized. The tensile properties are determined at room temperature and 1300 ◦C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 ◦C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 ◦C.
Volume 21 provides a working knowledge of the capabilities and applications of commercially significant composites, including metal-matrix composites, ceramic-matrix composites, and polymer and organic-matrix types. It covers constituent materials, design considerations, and manufacturing processes. It includes detailed information on post-processing and assembly, quality control, testing and certification, and properties and performance. In addition, the volume addresses product reliability and repair, failure analysis, and recycling and disposal. It provides application examples and engineering data as well. For information on the print version of Volume 21, ISBN 978-0-87170-703-1, follow this link.
Carbon fiber reinforced silicon carbide (C/SiC) composites are enabling materials for components working in ultra-high-temperature extreme environments. However, their mechanical properties reported in the literature are mainly limited to room and moderate temperatures. In this work, an ultra-high-temperature testing method for the mechanical properties of materials in inert atmosphere is presented based on the induction heating technology. The flexural properties of a 2D plain-weave C/SiC are studied up to 2600 °C in inert atmosphere for the first time. The deformation characteristics and failure mechanisms at elevated temperatures are gained. Theoretical models for the high-temperature Young’s modulus and tensile strength of 2D ceramic matrix composites are then developed based on the mechanical mechanisms revealed in the experiments. The factors contributing to the mechanical behaviors of C/SiC at elevated temperatures are thus characterized quantitatively. The results provide significant understanding of the mechanical behaviors of C/SiC under ultra-high-temperature extreme environment conditions.
The synthesis of C/C-SiC via liquid silicon infiltration (LSI) with thermoplastic carbon precursors can lead to the formation of macropores in the C/C state. The macro-pore pattern formation can be controlled and used as a new level in microstructure for ceramic matrix composites. The pore formation occurs in the CFRP state with the remelting of the thermoplastic matrix and can be fixed to the C/C state after liquid phase pyrolysis. The macro-pores are primarily induced by a non-linear elastic recovery of the fiber preform and not via gas formation and release during pyrolysis. The formation of the macroporosity is described and explained. These pores can be tailored by process control in size and shape. The pore shapes can vary from isolated spherical pores to an interconnected tube like macro-pore network. The relationship between starting setting variables of the fiber preform, the process conditions, and the resulting structure are discussed.
Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC CMCs) are promising candidates for components in the hot gas section of jet engines, as they exhibit high temperature resistance and low density compared to their metal alloy counterparts. Three common manufacturing routes are chemical vapor infiltration, reactive melt infiltration (RMI) and polymer infiltration and pyrolysis (PIP). This work describes a new approach of a combined PIP and RMI process. A combination of the processes seems attractive: the remaining porosity after PIP process can be closed by subsequent siliconization, resulting in a dense material. SiC/SiC CMCs were manufactured by PIP process using Hi-Nicalon Type S fibers. Generally, the processing of SiC/SiC, produced solely by PIP route, is rather time-consuming and the composites show a certain residual porosity. In order to obtain a dense matrix and to reduce the processing time, an additional RMI with silicon alloy is carried out after a reduced number of PIP cycles. To protect the fibers during the siliconization, a CVD fiber coating was applied. Microstructure was examined via microCT, SEM and EDS. Bending strength was determined to 433 MPa; strain to failure was 0.60%. The overall processing time was reduced by 55% compared to standard PIP route. The hybrid material contained 70% less unreacted carbon than material produced by LSI process alone.
In order to study the effects of temperature on the material behavior of Liquid Silicon Infiltration (LSI) based continuous carbon fiber reinforced silicon carbide (C/C-SiC), the mechanical properties at room temperature (RT) in in-plane and out-of-plane directions are summarized and the tensile properties of C/C-SiC were then determined at high temperature (HT) 1200 °C and 1400 °C under quasi static and compliance loading. The stress-strain response of both HT tests is similar and almost no permanent strain can be observed compared to the RT, which can be explained through the relaxation of residual thermal stresses and the crack distribution under various states. The different fracture mechanisms are confirmed by the analysis of fracture surface. Furthermore, based on the analysis of hysteresis measurements at RT, a modeling approach for the prediction of material behavior at HT has been developed and a good agreement between test and modeling results can be observed.
Being damage tolerant, the CMCs exhibit nonlinear deformations as a result of cracks that form in the matrix, in the interfaces, and in the fibers. The sequence of cracking modes displays several features that depend on the arrangement of fibers, the microstructure, and the respective properties of constituents. Being ceramic materials, the constituents are highly sensitive to inherent microstructural flaws generated during processing. The flaw populations govern matrix cracking and fiber failures, so that strengths of constituents exhibit statistical distributions. A bottom-up multiscale approach based on micromechanics must account for the contribution of inherent fracture-inducing flaws, variability of constituent strengths, and associated size effects. The chapter deals with modeling of the stochastic processes of multiple fracture of the matrix and the fibers that govern damage and failure on fiber-reinforced ceramic matrix composites. The models are based on probabilistic approaches to brittle fracture, including the Weibull phenomenological model and the physics-based elemental strength model that considers the flaws as physical entities. The probabilistic models that are discussed permit determination of stresses at crack initiation from microstructural flaws and resulting crack pattern. Applications to the prediction of tensile behavior of unidirectional or woven composites are then discussed.
Studies of fiber-reinforced ceramic matrix composites (CMCs) have shown non symmetric monotonic stress-strain response for tensile and compressive uniaxial loading. Because most CMC components experience multi-axial loading, it is imperative that the composite behavior and resulting cumulative damage be predictable using the data from uniaxially-loaded laboratory tests. A finite element analysis (FEA) model of a prismatic, rectangular beam composed of separately meshed fiber and matrix elements was subjected to four-point flexural loading. Cumulative damage was incorporated into the model using an element “kill” command to change the stiffnesses of elements whose probabilistically-distributed strengths were exceeded by the resulting stresses. Unsupported fibers were not allowed to support compression. Although the model does not directly incorporate the behavior of the interphase material, good agreement between the FEA model and experimental results were found for a 3-D, braided SiC fiber/CVI SiC matrix CMC. The probabilistically-based cumulative damage FEA model gave a variety of force-displacement curves which formed an envelope of curves encompassing the experimental results.
This communication addresses some common misconceptions about weakest link theory and Weibull statistics as they pertain to strength distributions of brittle fibers. After describing the nature of the problem, the flaws in ensuing proposed models of strength distributions are highlighted and discussed. A way forward that obviates the problems is suggested.
For the manufacture of carbon fibre reinforced SiC materials, three different processes are used in principle: Chemical Vapour Infiltration (CVI), Liquid Polymer Infiltration (LPI), also called Polymer Infiltration and Pyrolysis (PIP) as well as Melt Infiltration or Liquid Silicon Infiltration (MI / LSI). Depending on the differ-ent methods for building up the SiC-matrix and embedding the carbon fibres in the brittle ceramic matrix, the resulting C/SiC and C/C-SiC materials vary significantly in their properties as well as in their manufacturing costs. The advantages of the MI processes are their short manufacturing times and the use of low cost raw materials, leading to the most cost efficient Ceramic Matrix Composites (CMC), compared to materials de-rived from CVI and PIP. In combination with their unique thermal and mechanical properties, MI based CMC`s opened up a wide field of new applications beyond aerospace. This presentation gives an overview of the manufacturing processes and resulting properties of C-fibre rein-forced SiC materials. Typical applications of C/C-SiC or C/SiC materials based on MI processes, like friction materials, hot structures for solid propellant rocket motors and temperature stable structures for optical tele-communication systems are presented.
Übersicht zu kohlenstofffaserverstärkten Siliciumcarbidkeramiken. Herstellungsmethoden, Werkstoffeigenschaften und Anwendungsbereiche.
Carbon/Carbon (C/C) composites derived from the thermoplastic polymer polyetherimide (PEI) were pyrolized up to 1000 °C, subsequently carbonized in inert atmosphere up to 2200 °C and afterwards infiltrated with liquid silicon. The investigation of fibers and matrix with Raman microspectroscopy revealed, that an increased carbonization temperature leads to an increased carbon order as well as an incipient stress-induced graphitization of the carbon matrix close to the fiber surfaces at 2200 °C. The derived C/C-SiC samples show a maximum flexural strength of 180 MPa with C/C composites treated at 2000 °C and monotonically increasing Young’s moduli ranging from 49 GPa with C/C preforms treated at 1600 °C up to 59 GPa after carbonization at 2200 °C. The carbon fiber strength was evaluated with a single fiber tensile test, which showed a monotonically increased Young’s modulus and a decrease of the strength after carbonization at 2200 °C.
An experimental investigation was performed to study the influence of fatigue damage introduced by different loading cycles on the residual tensile strength (RTS) of plain-weave reinforced Cf/C-SiC composites (2D C/C-SiC). The specimens were subjected to the fatigue stress of 57 MPa for the preselected numbers of cycles as follows: 102, 104 and 105, respectively, before the static tensile test. The microstructures and fractured surfaces after the tensile test were examined by optical and scanning electron microscopy, respectively. The results showed that the RTS of the specimens after the preselected fatigue cycles numbers of 102, 104 and 105 increase to 89.8, 94.1 and 82.4 MPa, respectively, which are somewhat higher compared to the virgin samples (79.7 MPa). Additionally, we found that the linear part of the tensile stress-strain curve is independent on the fatigue cycles. Finally, the increased fatigue damage in C/C-SiC composites could determine a reduction of elastic modulus in all cases of fatigue tests.
Ceramic matrix composites (CMCs) are non-brittle, tough and damage tolerant materials as compared with monolithic ceramics. The non-brittle dissipative behaviour of CMCs results from the achievement, during their processing, of a fibre-matrix bond sufficiently weak to allow, during loading, multi-cracking of the matrix to develop without breaking the fibre. This unique feature, together with the good high temperature environmental behaviour of these materials resulting from the nature of the matrix, either silicon carbide or oxide, accounts for their rapid development during the last two decades. Therefore, the applications of these materials, restricted in a first step to military use, are now entering civil programmes such as hypersonic vehicles or turbojet engines.
Depending on the manufacturing method, the carbon (C) matrix is either replaced completely by silicon carbide (SiC) matrix, leading to C/SiC materials, or tailored C/C preforms are densified with an additional SiC or SiSiC matrix, leading to C/C-SiC materials. For the manufacture of C/SiC materials, the processes based on chemical vapor deposition (CVD), pyrolysis of preceramic polymers, and reaction bonding have been adapted for the buildup of SiC matrices in fibrous preforms, leading to three different manufacturing methods such as chemical vapor infiltration (CVI), polymer infiltration and pyrolysis (PIP) and melt infiltration (MI). This chapter describes the manufacture of C/C-SiC materials and components based on in situ fiber embedding and liquid silicon infiltration (LSI). The industrial use of C/SiC materials is still focused on niche markets. However, existing application areas have been expanded and novel application areas, such as rocket propulsion and aircraft brakes, could be opened in recent years.
Two thermoplastic matrices, polyetheretherketone (PEEK) and polyetherimide (PEI), are examined in this work in terms of their suitability as carbon precursors for C/C-SiC composites. The carbon fibers are pretreated between 400 and 1600 °C in nitrogen atmosphere to modify the surface functionality, which was characterized with XPS. An analysis of the C/C-SiC composition shows an increasing SiC content as a function of the increased fiber pretreatment temperature and the polymer precursor. Bending tests of C/C-SiC composites exhibit enhanced fiber-pull-out effects at a fiber pretreatment temperatures from 400 to 800 °C.
Ceramic materials in general have a very attractive package of properties: high strength and high stiffness at very high temperatures, chemical inertness, low density, and so on. This attractive package is marred by one deadly flaw, namely, an utter lack of toughness. They are prone to catastrophic failures in the presence of flaws (surface or internal). They are extremely susceptible to thermal shock and are easily damaged during fabrication and/or service. It is therefore understandable that an overriding consideration in ceramic matrix composites (CMCs) is to toughen the ceramics by incorporating fibers in them and thus exploit the attractive high-temperature strength and environmental resistance of ceramic materials without risking a catastrophic failure. It is worth pointing out at the very outset that there are certain basic differences between CMCs and other composites. The general philosophy in nonceramic matrix composites is to have the fiber bear a greater proportion of the applied load. This load partitioning depends on the ratio of fiber and matrix elastic moduli, E
f/E
m. In nonceramic matrix composites, this ratio can be very high, while in CMCs, it is rather low and can be as low as unity; think of alumina fiber reinforced alumina matrix composite. Another distinctive point regarding CMCs is that because of limited matrix ductility and generally high fabrication temperature, thermal mismatch between components has a very important bearing on CMC performance. The problem of chemical compatibility between components in CMCs has ramifications similar to those in, say, MMCs. We first describe some of the processing techniques for CMCs, followed by a description of some salient characteristics of CMCs regarding interface and mechanical properties and, in particular, the various possible toughness mechanisms, and finally a description of some applications of CMCs.
The stress–strain curves of fiber − reinforced ceramic − matrix composites (CMCs) exhibit obvious non-linear behaviour under tensile loading. The occurrence of multiple damage mechanisms, i.e., matrix multicracking, fiber/matrix interface debonding and fibers fracture, is the mainly reason for the non-linear characteristic. The micromechanics approach has been developed to predict the tensile stress–strain curves of unidirectional, cross-ply and woven CMCs. The shear-lag model was used to describe the micro stress field of the damaged composite. The damage models were used to determine the evolution of micro damage parameters, i.e., matrix crack spacing, interface debonded length and broken fibers fraction. By combining the shear-lag model with damage models and considering the effect of transverse multicracking in the 90° plies or transverse yarns in cross-ply or woven CMCs, the tensile stress–strain curves of unidirectional, cross-ply, 2D and 2.5D woven CMCs have been predicted. The results agreed with experimental data.
Ceramic matrix composites (CMC) are considered the next generation of advanced materials used in space and aviation due to their high-temperature strength, creep resistance, chemical resistance, low porosity, and low density. However, these materials are difficult to process owing to the large cutting force and high cost on tool consumption. electrical discharge machining (EDM), featured by the negligible machining force and acceptable tooling cost, is a potential nontraditional machining technique for CMC. In this paper, EDM was used to process a new class of advanced CMC, that is, those with continuous ceramic fiber reinforcements. The challenge is its low material removal rate (MRR) due to the low workpiece conductivity and debris evacuation efficiency. Electrode vibration and dielectric deep flushing were used to promote debris evacuation, and an increase of MRR and surface quality without sacrificing tool wear ratio was observed. Gap voltage, peak current, pulse duration, and duty ratio were studied using design of experiments for deeper understanding of the process. The effect of these parameters was investigated, and an analysis of variance was conducted. The optimal condition was also predicted and experimentally validated. It was found that a high gap voltage or low duty ratio leads to a high machining rate due to improved debris evacuation efficiency. The material removal mechanism was found to be cracking due to thermal expansion of the matrix and breakage of the nonconductive fibers.
This paper surveys the available results on the size effect on the nominal strength of structures — a fundamental problem of considerable importance to concrete structures, geotechnical structures, geomechanics, arctic ice engineering, composite materials, etc., with applications ranging from structural engineering to the design of ships and aircraft. The history of the ideas on the size effect is briefly outlined and recent research directions are emphasized. First, the classical statistical theory of size effect due to randomness of strength, completed by Weibull, is reviewed and its limitations pointed out. Subsequently, the energetic size effect, caused by stress redistributions due to large fractures, is discussed. Attention is then focused on the bridging between the theory of plasticity, which implies no size effect and is applicable for quasibrittle materials only on a sufficiently small scale, and the theory of linear elastic fracture mechanics, which exhibits the strongest possible deterministic size effect and is applicable for these materials on sufficiently large scales. The main ideas of the recently developed theory for the size effect in the bridging range are sketched. Only selected references to the vast amount of work that has recently been appearing in the literature are given.
The mechanical properties of various 2D ceramic matrix fiber composites were characterized by tension testing, using the gripping and alignment techniques developed in this work. The woven fabric composites used for the test had the basic combinations of Al{sub 2}O{sub 3} fabric/Al{sub 2}O{sub 3}, SiC fabric/SiC, and SiC monofilament uniweave fabric/SiC. Tension testing was performed with strain gauge and acoustic emission instrumentation to identify the first-matrix cracking stress and assure a valid alignment. The peak tensile stresses of these laminate composites were about one-third of the flexural strengths. The SiC monofilament uniweave fabric (14 vol%)/SiC composites showed a relatively high peak stress of 370 MPa in tension testing.
This chapter describes the stochastic damage evolution and failure in fiber-reinforced composites. The accomplishments in the area of modeling of the mechanical properties of fiber-reinforced composites are reviewed with an emphasis on accurately predicting ultimate tensile strength, stress–strain behavior, and reliability. The model composite studies consists of a volume fraction of continuous cylindrical fibers embedded in a matrix material in a unidirectional arrangement. The reinforcing fibers are generally brittle materials and so are linearly elastic up to the point of failure. The point of failure in any individual length of fiber is determined by the largest flaw or crack in that particular fiber. The interface between the fibers and matrix occupies a vanishing fraction of the total composite volume but plays a critical role in determining many composite properties related to damage and strength. In polymer and metal matrices, the interface becomes important when fibers break. In ceramic matrices, the interface is critical first when the matrix cracks and then again when the fibers break. It is found that increasing stress causes flaws to fail, slip/exclusion zones to form or increase in length, and exclusion zones to increasingly overlap until the entire fiber is subsumed within the exclusion zones and the test saturates. The in situ fiber strength and fracture mirrors are also elaborated.
Ceramic matrix composites (CMCs) and, in particular, continuous fibre ceramic composites (CFCCs) are targeted for industrial, aerospace and other high-technology applications that require the high-temperature properties and the wear/corrosion resistance of advanced ceramics while providing inherent damage tolerance (i.e. increased 'toughness') without the volume/ surface area-dependent strengths of monolithic ceramics. To utilize CFCCs designers need reliable and comprehensive data bases (and the design codes that contain them). Generating reproducible information for these data bases requires standards. Presently, there are relatively few (compared to metals) national (e.g. ASTM, CEN, JIS, etc.) or international standards (e.g. ISO) for testing CFCCs. In this paper, the various standards for CFCCs are reviewed and additional areas requiring normalization are discussed (e.g. mechanical, thermal, electrical, electro-magnetic, optical, and biological testing). 'Design codes' such as the ASME Boiler and Pressure Vessel Code discussed here, are widely accepted, general rules for the construction of components or systems (for performance, efficiency, usability, or manufacturabilty) with emphasis on safety. Wide-ranging codes incorporate figurative links between materials, general design, fabrication techniques, inspection, testing, certification, and finally quality control to insure that the code has been followed. Implicit in design codes are many of the standards for materials testing, characterization, and quality control. Logical outcomes of design codes are data bases of material properties and performance 'qualified' for inclusion in the code. As discussed in this paper, data bases (such as those contained in the Mil-Hdbk-17 CMC effort) may be in print, electronic or worldwide web-based formats and may include primary summary data (e.g. mean, standard deviation, and number of tests) along with secondary data (e.g. graphical information such as stress-strain curves).
The effect of a Weibull distribution of flaws on the strength on non-linear composite materials is considered. The tensile strength applying in bending is compared with that in direct tension. This leads to an estimate of the ratio modulus of rupture to ultimate tensile strength for composite materials.
The Weibull distribution is widely used to describe the scatter of the strength in brittle (but also quasi-brittle) materials, often assuming that the Weibull modulus is a “material constant”. One possible motivation of this perhaps comes from the classical Freudenthal’s interpretation of Weibull modulus depending on the crack size distribution, which however assumes the cracks to be at large distance one from the other. It is here found with simple numerical experiments with collinear cracks that Weibull distributions tend to be obtained also with interaction taken into account, but the Weibull modulus depends on both the crack size distribution and the distribution of ligaments. Hence, Weibull modulus should not be considered a “material constant” or to correspond to an “intrinsic” microstructure of the material, as assumed in many industrial applications and commercial postprocessors of FEM softwares, even in the case of a varying stress fields. In the limit case of a crack or sharp notch this leads to paradoxically a zero scale parameter (and the usual Weibull modulus). Hence, in the case of a blunt notch, we suggest the Weibull modulus would vary depending on the distribution of cracks, their distances, and the interaction with the geometry and stress field. Only numerical simulations where the distribution of cracks is directly included in the geometry under consideration can provide the correct scale factor and Weibull modulus.
The chain-of-bundles model for fibrous composites is reviewed, and an
approximation to the probability of failure is derived. This leads to
formulae for predicting the strength of such a composite. These formulae
are developed in the context of an asymptotic theory, and the Monte
Carlo method is used to study a specific case in more detail. We also
discuss the size effect. The probabilistic analysis relies heavily on
extreme value theory, and a brief survey of the relevant parts of that
theory is included.
Uniaxial tensile testing is a method used throughout the world to measure the strength and ductility of materials. An important aspect of uniaxial tensile testing which often goes unrecognized is test system alignment. Poor alignment can significantly influence test results at small strains, especially the fracture strengths of materials in a brittle state. The prupose of this review paper is to enable a reader to identify sources of misalignment, recognize the effects of misalignment on tests results, evaluate the extreme surface bending strains and stresses, and become acquainted with some techniques for reducing misalignments to within tolerable limits. Numerous references are made to the literature which describes how misalignment may be influenced by couplings in the loading train and by specimen design. A quantitative assessment of the devices and techniques discussed in this literature is made in those cases where sufficient data have been provided. The literature surveyed indicates that misalignment in carefully designed and precisely machined testing systems ranges between 3 and 15% bending. The need for reporting the misalignment at which a given test result is obtained is pointed out.
This paper (part II) proposes a macroscopic probabilistic model of ultimate failure for CMCs. The model is based on the features displayed by the strength of 2D woven SiC/SiC composites that were reported in part I. These features were attributed to variability in the local stress state. An analogy with the ergodic theory is used to illustrate the heuristic fluctuations of the local stress-state and to construct the model. The probabilistic model that is proposed is then used to predict the ultimate failure of 2D woven SiC/SiC composites under 4-point bending conditions, from the strength distributions obtained using tensile and three-point flexure tests. Predictions compare satisfactorily to experimental results.
Commercialization of advanced ceramic composites requires standardization of test methods and data bases both for bench marking processing improvements and for use as input for the design of components. National and international standardized test methods for CFCCs have been increasingly introduced over the past decade. Widely-available data bases are just now being developed. Three ASTM test methods (C1341 for flexure, C1275 for tension and C1292 for shear) were used to evaluate of a commercial CFCC (ceramic grade Nicalon™ fabric-reinforced silicon nitrocarbide matrix). Nuances of the test methods were investigated through the use of rosette strain gages, dual data acquisition systems, multiple parameter measurements and other experimental techniques. Quantities such as off-axis strains, non-uniform stresses, differences in displacement measurement techniques, and variability in dimensional determination were used to interpret differences between inter and intralaboratory results.
The bending strength of fiber-reinforced glasses and ceramics is often observed to be higher than their tensile strength; the difference varies, however, from one material to another. To gain an understanding of the relationship between these two measure of strength, we have carried out an analysis of bending which accounts for the deviations from linearity that occur on the tensile side of the beam. The results of this analysis indicate that the strength ratio (bending strength/tensile strength) depends most sensitively on the rate at which the stress drops after the ultimate tensile strength. In particular, composites failing gracefully (with a gradual decay in stress) tend to have comparatively higher strengths in bending. A method of inferring the: tensile strength from simply the load-deflection curve in bending is proposed. In addition, by accounting for the weakness in interlaminar shear, we can predict the variation in bend strength with beam aspect ratio. The various theories are compared with experimental data.
Although such national test standards for CFCCs as ASTM C1275-94 “Standard Practice for Monotonic Tensile Strength Testing of Continuous Fibre-Reinforced Advanced Ceramics with Solid Rectangular Cross-Sections at Ambient Temperatures” have helped clarify certain issues in testing CFCCs, the complete quantification of the effects of certain test parameters is still ongoing. In this study, the effects of test mode (load vs. displacement), test rate (fast vs. slow), specimen geometry (straight-sided vs. reduced-gage section), specimen volume (long/thin vs. short/fat), and bending in tension were evaluated for a 12-ply, 2-D, plain weave SiC fibre reinforced/CVI SiC matrix CFCC. Results show that ‘graceful failure’ is often accentuated by displacement control and strengths (and fracture strains) are not affected by test rates. For certain fibre architectures (e.g. 2-D plain weave) no strong tendency to more frequent non gage section failures was noted for straight-sided specimens. Type of specimen volume had some effect since long and thin specimens exposed larger volumes of individual fibres to maximum stresses than did short and thick specimens. The limited number of tests prevented definite conclusions on the effect of bending (e.g. >5%) although proportional limit stress showed a more pronounced sensitivity to bending that did ultimate tensile strength.
A theory is presented to predict the pullout work and ultimate tensile strength of ceramic-matrix composite (CMC) materials tested under uniaxial tension as functions of the underlying material properties. By assuming that the fibers fracture independently and that global load redistribution occurs upon fiber fracture, the successive fragmentation of each fiber in the multifiber composite becomes identical to that of a single fiber embedded in a homogeneous large-failure-strain matrix, which has recently been solved exactly by the present author. From single-fiber fragmentation, the multifiber composite distribution of pullout lengths, work of pullout, and ultimate tensile strength are easily obtained. The trends in these composite properties as a function of the statistical fiber strength, the fiber radius and fill fraction, and the sliding resistance τ between the fibers and the matrix easily emerge from this approach. All these properties are proportional to a characteristic gauge length δc and/or the associated characteristic stress σ, with proportionality constants depending only very weakly on the fiber Weibull modulus: the pullout lengths scale with δc, the work of pullout scales with σcδc, and the ultimate strength scales with σc. The key length δc is the generalization of the “critical length,” defined by Kelly for single-strength fibers, to fibers with a statistical distribution of strengths. The theory also provides an interpretation of fracture-mirror measurements of pulled-out fiber strengths so that the in situ key strength σc and Weibull modulus of the fibers can be determined directly. Comparisons of the theoretical predictions of the ultimate tensile strength to literature data on Nicalon/lithium aluminum silicate (LAS) composites generally show good agreement.
The strength and reliability of fiber-reinforced ceramic-matrix composites (CMCs) are dependent on whether conditions of local or global load sharing prevail. Global load sharing is promoted by a low interfacial sliding stress and is manifested in a zero-tangent modulus at the point of tensile failure along with random fiber failures and extensive fiber pullout. In this paper, it is demonstrated that conditions of global load sharing are not present in two commonly studied CMCs, despite the fibrous appearance of their fracture surfaces. This behavior is manifested in a volume-dependent strength, as evidenced by strength differences measured in tension and flexure (accounting for the nonlinear stress distribution in flexure). Methods of weakest-link statistics are used to relate the strengths measured in the two test configurations. Estimates for the Weibull moduli of the two systems are obtained from the experiments and compared with values obtained through Monte Carlo simulations based on a three-dimensional-lattice Greens function method. The implications of these results on the strength of large components and of small regions of high stress concentration are discussed briefly.
Few engineering materials are limited by their strength; rather they are limited by their resistance to fracture or fracture toughness. It is not by accident that most critical structures, such as bridges, ships, nuclear pressure vessels and so forth, are manufactured from materials that are comparatively low in strength but high in toughness. Indeed, in many classes of materials strength , and toughness are almost mutually exclusive. From a fracture-mechanics perspective, the ability of a microstructure to develop toughening mechanisms ' acting either ahead or behind the crack tip can result in resistance-curve (R-curve) behavior where the fracture resistance actually increases with crack extension; the implication here is that toughness is often developed primarily during crack growth and not for crack initiation. Biological materials are perfect examples of this; moreover, they offer microstructural design strategies for the development of new materials for structural applications demanding combinations of both strength and toughness.
The use of end tabs is often necessary when performing quasi-static uniaxial tests on fibre-reinforced composites. However, finding a suitable combination of material and geometry for these end tabs to have acceptable and reproducible results may be a problem. In this article four different geometries and four different materials of the tabs are numerically examined for the tensile testing of a carbon fabric reinforced polyphenylene sulphide. First, it is assessed if a simplified finite element model of a tensile grip is acceptable. Then, this simplified model is used to examine the proposed set-ups. It may be concluded that, for the given material, short straight end tabs with a [(0°,90°)]4s layup should be used and the specimen should be mounted in such a way that the end tabs are completely between the grips. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers
Mechanisms of failure in a unidirectional Sic-fibe/glass-ceramic composite are investigated using in situ observations during tensile and flexural loading. These experiments show that failure in tension occurs in several stages (similar to certain other brittle fiber composites): multiple matrix cracking, followed by fiber fracture and pullout. In flexural loading the failure process is more complex. Consequently, the flexural test cannot be used for measurement of tensile strength (although it can be used for measurement of the stress for matrix cracking). The application of conventional fracture mechanics to describe tensile failure is discussed. The in situ observations provide direct indication of the importance of frictional bonding between the matrix and fibers. Some novel methods for measuring the frictional forces and residual stresses are investigated, and the influence of surface damage on strength is assessed.
Ceramic matrix composites (CMC), based on reinforcements of carbon fibres and matrices of silicon carbide (called C/SiC or
C/C-SiC composites) represent a relatively new class of structural materials. In the last few years new manufacturing processes
and materials have been developed. Short fibre reinforcements, cheap polymer precursors and liquid phase processes reduced
the costs by almost one order of magnitude in comparison to first generation C/SiC composites which were originally developed
for space and military applications. Besides high mass specific properties and high thermal stability, functional properties
like low thermal expansion and good tribological behaviour play an increasing importance for new commercial applications like
brake disks and pads, clutches, calibration plates or furnace charging devices.
It is well documented that nominally identical specimens of brittle materials, e.g. ceramics, show a large variation of tensile fracture stresses and in order to use brittle materials as engineering materials the strength has to be characterized. The most widely used expression for characterization is the cumulative distribution function proposed by Weibull [ 1 ]. The Weibull function is also known to statisticians as Fisher-Tipper Type Ill distribution of smallest values or as the third asymptotic distribution of smallest extreme value [2]. The Weibull statistics is based on the the "weakest link-hypothesis" which means that the most serious flaw in the specimen will control the strength. The most serious flaw is not necessarily the largest one because its severity also depends on where it is situated. In other words, the flaw which is subjected to the highest stress intensity factor will be strength controlling. The flaws initiating fracture can conveniently be classified as intrinsic or extrinsic [3]. The intrinsic flaws are introduced during fabrication and are predominantly inclusions and voids. The extrinsic flaws are stress-induced cracks, such as surface cracks introduced during machining and microcracks resulting from large residual stresses, e.g. due to thermal contraction