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Scaling effects in the threaded pieces made of the spatially reinforced carbon-carbon composite materials
Abstract
The paper presents results of studying the impact of the strength and stiffness scaling effects in modern spatially reinforced carbon-carbon composite materials on bearing capacity of the products made from them. These materials are widely used in equipment operating under extreme conditions of simultaneous loading with force and temperature factors. When designing such products, it becomes necessary to understand the influence degree of the frame damaged during machining on observed properties of the material used, including dimensions, at which it is required to consider reduction in strength and stiffness, as well as the value, which could reduce the considered properties.
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An experimental analysis of the scale effects of the strength and rigidity of CCCM with a 4-DL reinforcement scheme under compression has been carried out. The influence of the size and shape of the sample on the observed tensile strength and modulus of elasticity of the material is described. The features of nonlinear deformation of the material, the effect of multiple loading are considered. The X-ray computed tomography method analyzed the processes occurring in the structural components of the material.
This paper defines the structural strength criterion for 4DL-reinforced carbon-carbon materials. For this scheme, fiber reinforcement consists of four groups of reinforcing elements, three of them are located in parallel planes with the angles of 120° between them and the fourth one is normal to them. The paper addresses the first failure of the material corresponding to its yield stress, in this point, one of the material components deviates from linear elastic behavior. A composite material is considered to be non-uniform structurally and consists of a matrix and reinforcing elements, rods. Those rods, in their turn, represent a unidirectional composite. To analyze the stress-strain state of individual components of the material, a three-level elastic model is built that uses the analytic approach at the micro level, while at higher levels it uses the finite element method. For numerical calculations, a structural cell of the material is taken. The boundary conditions provide small to negligible influence of the edge effects, thus simulating the behavior of the infinite volume of the material. For the material components, local strength criteria are introduced, where the fields of the criterion quantities are averaged over the volume of the structural cell. The strength surface of the material that corresponds to its first failure is obtained, and the conclusion is made that the suggested criterion provides a reasonable agreement with the available data on the typical carbon-carbon composite characteristics. Based on the calculated dependencies of the material’s yield stress on the load direction, a procedure is suggested to identify the model parameters based on the material failure behavior analysis using standard tensile and compressive tests. Estimated discrepancies between the results calculated using the suggested criterion and those obtained using the limiting stress criterion for biaxial stress states are given. It is shown that the discrepancy may reach tens of percent and in some cases the material strength increases in comparison with that in the uniaxial stress state. The results are subject to verification tests in order to verify the model for advanced spatially reinforced carbon-carbon composite materials.
Based on previous research, in this paper, the compressive shear failure behavior and mechanical properties of bonded–bolted hybrid single-lap joints of C/C composites at high temperature were studied. The compression shear test was performed on the joints at 800 °C to obtain the load–displacement curve and failure morphology. The failure modes of joints were observed by digital microscopy and scanning electron microscopy. A numerical analysis model was implemented in finite element code Abaqus/Explicit embedded with the user material subroutine (VUMAT). The numerical results were compared with the test results to verify the correctness of the model. The interrelationship of the compression shear loading mechanism and the variations in stress distribution between bonded joints and bonded–bolted hybrid joints at high temperature were explored. The progressive damage of hybrid joints and the variations in the ratio of the bolt load to the total load with displacement were obtained.
In the present work, an experimental investigation on the failure of 3D woven carbon/carbon all-composites mechanically fastened bolted joints with protruding-head and countersunk-head bolts has been implemented under out-of-plane loading in air at room temperature and up to 1100 °C for the first time. The load-displacement curves and main damage mechanisms for the required temperature points were determined and evaluated, respectively. It follows that, for room temperature, the global mechanical response was characterized by pseudoplastic, and the failure was dominated by the damage mechanisms including matrix cracking, interface debonding and threads fracture, especially, edge bending to breakage occurring for countersunk fastener. For 1100 °C, silicon carbide ceramic anti-oxidation coating was applied to prevent from oxidizing. Due to the relief of the thermal stresses, joints bearing capacity was enhanced by fiber/matrix interface performance improvement accompanying with a brittleness tendency of mechanical response, and the dominating failure mechanisms were carbon/carbon substrate oxidation caused by air entering through the cracks in SiC coating.
In this paper, by use of test combined with numerical simulation, the out-of-plane loading-bearing failure process of CFRP and C/C joints were studied, while Hashin failure criterion was selected to predict the damage and the failure morphologies were observed. Results show that the failure process of CFRP joint is characterized by a three-peak mechanical response and bending failure accompanied the failure mechanism of interface debonding and cracking of fiber and matrix, and that of C/C joint is characterized by pseudoplasticity and shearing failure accompanied a large number of matrix cracks, local interface debonding and fiber kink and buckling.
Fasteners made of carbon/carbon composites (C/Cs) have been developed for joining C/Cs components in high-temperature environment. A numerical damage model based on post-failure behavior was developed for simulating shear strength of 3D weave C/Cs fastener under several load angles. 3D pixel method was employed to simulate the heterogeneous damage process of constituents considering weave geometry in the macro model. Experiments were performed and the main damage as well as failure modes are also provided. It shows that the damage model can provide reasonable predictions compared to experiment data. And the damage mechanism observed in experiment can be reproduced in the numerical simulation. Furthermore, the effect of load angle on shear strength and failure modes was investigated which shows the strength of specimen decreases with increase of load angle. Numerical results showed different damage progression under several load angles which also reflected different load curve trends and post-failure behavior.
A method was proposed to test the pull-through mechanical behavior of fastener threads, which were fabricated from weave carbon/carbon (C/C) composites. The damage morphologies of the C/C fastener threads were observed through an optical microscope and high-resolution micro-CT systems. The acoustic emission (AE) technique was utilized to track the damage progression of threads during loading up to fracture in terms of AE event rate which has an exponential type profile. Finally, A 3D finite element damage evolution model of composite threads was established based on continuum damage mechanics to calculate the thread load distribution and damage progression. The relations between the pitch and the load distribution, as well as between different fabrication directions and ultimate loads, were investigated by using this model. The stress in the first thread was analyzed based on the tapered cantilever assumption. The results show that, the first thread is brittle fracture at the root where is the higher stress level region of the threads and it is the initial damage. The load distribution in C/C threads is not uniform and not improved as the value of pitch decreases. Load capacity of C/C threads is different result from the fabrication direction. Numerical results agree well with experimental results.
Carbon-Carbon (C-C) threaded fasteners have been developed for joining C-C composites experiencing temperature above 1500 °C. The fasteners were fabricated from spun yarn graphitized carbon fabric through resin route and from needle punched carbon felt through pitch route techniques. The preforms were processed with multiple cycles of impregnation, carbonisation, and graphitisation up to densities of 1.78 gcm -3 and 1.90 gcm -3. Densification process cycles were repeated to obtain composites with reduced porosity and improved thermo-mechanical properties. Material evaluation of C-C fasteners is discussed and compared with existing graphite fasteners used in industrial and aerospace sectors. Scanning electron microscopy was also carried out to study microstructure of fractured specimens.
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