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Abstract

Short gauge length coupons of plain woven Carbon/Epoxy with layup [#(0/90)] 24 and [#(45/−45)] 24 are subjected to Compression-Compression and Tension-Compression fatigue loading at several stress levels. Inspection of the gauge length with Digital Image Correlation (DIC) allows measurement of the strain field. The hysteresis loops, axial and shear stiffness as a function of the number of cycles are obtained from the strain field and measured load. For most of the tests a general degradation of the specimen stiffness is observed. For the coupons subjected to uniaxial Compression-Compression with layup [#(45/−45)] 24 , however, a 17% stiffness increase is seen during the first 10 ⁴ cycles. Post-mortem inspection revealed that the aluminium tabs of the coupons, to facilitate load introduction from the clamps, had partially debonded during testing. A 3D finite element analysis of the coupons including tabs is carried out to investigate the phenomena. The degradation of the tab-coupon bond is modelled using contact with debond capability. This confirmed that tab debonding is responsible for significantly altering the stress state in the (short) gauge section which results in an apparent stiffening of the sample during fatigue loading. Hereby, the authors offer a new explanation for stiffness increase of composite coupons in fatigue tests. For future tests it is shown that quantification of tab debonding is necessary to provide a decisive answer about the experimental boundary conditions in the gauge section of the coupon.

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... • Compression and compression after impact tests combined with DIC are useful in elucidating buckling behaviour and damage development, including delamination. Compression is often tested as a tabbed rectangular specimen with a short gauge length [76,77], thick multi-layer block [78], using a frame [79,80], or a tubular section [81,82]. The possibility of buckling and out-ofplane movement is inherent in compressive tests compared to their tensile counterparts, making the use of stereo-DIC highly desirable and typical in the aforementioned studies. ...
... Similarly, compressive testing also links architectural variations and damage development for 2D woven [77] and 3D woven composites [78,80]. Moreover, off-axis compressive behaviour [142], compressive fatigue testing [76] and the influence of delamination on compressive strain response [143] have been investigated. DIC can be especially powerful for determining the location and progression of damage, as shown in Fig. 7i, by broken z-binder tows in a 3D woven composite [79]. ...
... Environmental conditions such as temperature or water immersion influence matrix properties, the resulting strain field, and can similarly be observed with DIC [108]. As the strain behaviour varies significantly [76], (b) vertical displacement of a carbon fibre PLA cross-chiral metamaterial structure [152], (c) out-of-plane deformation for impact loading at 20 000 fps [100], (d) braided carbon, glass and aramid fibre 39 mm diameter tubes deflection under torsional loading [75], (e) longitudinal strain of fibre-reinforced polymer lap joint [153], (f) 2D-DIC showing strain of 3.5 mm thick carbon woven composite undergoing 3 point bending [87], (g) shear angle of woven carbon/PEEK with softened matrix at 325 • C [111], (h) longitudinal strain with multiple holes for woven carbon/PPS laminate (red = maximum ∼1% strain) [154], (i) in-situ 3D printing DIC analysis during manufacture using natural speckle pattern [155] and (j) fibre strains during stamp forming of a woven composite [50]. depending on the constituents and structure of the composite, experimental DIC results from actual components and coupons are extremely valuable for part validation and design optimisation. ...
Article
Full-text available
Digital image and volume correlation (DIC and DVC) are increasingly being used to measure the deformation and damage behaviour of composite materials. The heterogeneous nature of the composite microstructure results in complex local behaviour, which cannot be captured using traditional experimental characterisation techniques. DIC and DVC offer significant advantages in monitoring this behaviour for fibre-reinforced composite materials. However, as acceptance of these techniques is growing, careful consideration of testing setup and approach is nevertheless crucial for achieving accurate results. This paper reviews digital image and volume correlation methods for the characterisation of composite materials and reports current research applications. The review discusses essential considerations and provides recommendations to aid researchers in achieving quality deformation measurement and damage identification in composites. Finally, an outlook on possible future applications is presented as a motivator for further studies.
... The elastic and damage laws have been tested and validated for balanced woven carbon/epoxy plies in static [76,78] and extended for fatigue loads [75,76]. The proposed simplified model of using two equivalent UD plies to model the behaviour of a woven ply has been used recently by other authors [79][80][81]. ...
... It should be noted that for each type of fibre, resin or weaving, it is necessary to repeat these tests to identify the damage model coefficients of the equivalent UD ply. Table 2. Damage properties of the cured textile composite [80]. The part of the model presented here relates mainly to the nonlinear evolution of the matrix damage for static loadings. ...
... Damage properties of the cured textile composite[80]. ...
Article
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The internal structure of composite materials is modified during manufacturing. The formation of woven prepregs or dry preforms changes the angle between the warp and weft yarns. The damage behaviour of the consolidated composite is modified by these changes of angle. It is important when designing a composite part to consider this modification when calculating the damage in order to achieve a correct dimensioning. In this paper, a damage calculation approach of the consolidated textile composite that takes into account the change in orientation of the yarns due to forming is proposed. The angles after forming are determined by a simulation of the draping based on a hypoelastic behaviour of the woven fabric reinforcement. Two orthogonal frames based on the warp and weft directions of the textile reinforcement are used for the objective integration of stresses. Damage analysis of the cured woven composite with non-perpendicular warp and weft directions is achieved by replacing it with two equivalent Unidirectional (UD) plies representing the yarn directions. For each ply, a model based on Continuum Damage Mechanics (CDM) describes the progressive damage. Two examples are presented, a bias extension specimen and the hemispherical forming coupon. In both cases, the angles between the warp and weft yarns are changed. It is shown that the damage calculated by taking into account these angle changes is greatly modified.
... However, especially when testing laminates with high Poisson's ratio, the support restrictions on transverse deformation may influence the test outcome. Sevenois et al. 4 investigated the tab bonding influence on specimen testing and showed a large influence of the tabs on results. With compression testing, low clamping forces as suggested for D6641 may lead to undesired stick-slip in transverse direction and undefined stress and strain states in the gauge length, invalidating test results relying on single-axis strain measurement. ...
... Furthermore, because of the restrained transversal contraction, the modulus is calculated over a different shear strain range. An additional potential reason is the onset of end tab constraint relaxation reported by Sevenois et al. 4 . Shear modulus estimation on short specimen is therefore not recommended with the current formula, and further research is necessary. ...
Article
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... For example, tab debonding can influence significantly the stress distribution in specimens with short gauge length. [45] This is especially true for layups with large Poisson's ratio. The comparison of tensile specimens with different layups showed that for [AE45] laminates, tabbing leads to undesirable differences in the strain distribution, whereas for cross-ply and unidirectional layups, the influence is less significant. ...
Article
Full-text available
The positive combination of lightweight design and high fatigue resistance of fiber reinforced materials has led to their broad application in many different structural applications. During the design phase, it is often only considered that these materials are subjected to tensile loading conditions to make use of their excellent strength and fatigue resistance properties. However, in the current challenge to reduce weight of transportation vehicles, a broadening range of loading conditions for composites may arise, whereby it is not always possible to restrict loading to pure tensile conditions. In contrast to metals, compressive loading is a challenging load case for composites. Much research is undertaken to understand the compressive behavior of composites and to develop valid methods for their characterization. Especially for compressive fatigue testing (load ratio R < 0 and R > 1), the generally accepted methods are rare, and not much is reported in the literature on how characterization should be done. This review provides an overview of existing methods, i.e., setups for testing fiber reinforced polymer composites under compression and discusses their applicability to fatigue testing.
... Measurements using strain gauges to determine the stress conditions that are inserted into mathematical models have been used, for example, to test wind turbine blades [8], testing and modelling in composites [9], crack initiation in alloys [10] or predicting the life of welds [11]. Strain gauges are performed on agricultural machines to determine tractor tensile strength measurements [12], operational dynamic tests on a polygon for an agricultural machine [13], fatigue life assessment of the machine [14]. ...
Article
Full-text available
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Article
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Article
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A mode-I type of crack propagation and branching on account of fracture energy failure mechanisms methodology is investigated in unidirectional (UD) fiber reinforced epoxy sheet molding compounds (SMC). In order to eliminate the influence of structural boundary conditions with various crack lengths on tip K (stress intensity factor) value to a greatest extent, a tapered double cantilever beam (TDCB) tensile sample is selected as the far-field boundary, the SMC slices with pre-cracks are embedded into TDCB samples, which are referred to as the embedded TDCB (E-TDCB) geometries. By applying the transvers tensile load to E-TDCB, embedded SMC is provided with far-field driving force for crack propagation, so as to obtained a great agreement between stable crack propagation and minor tip K oscillation. Both E-TDCB and dummy TDCB (D-TDCB) samples were selected for experimental test with comparation. The dynamic crack propagation parameters (work components of far-field driving force as well as crack length and Crack tip open displacement et al.) were measured and analyzed with progressive damage criterion. Although in disparate principle, the numerical prediction with proposed constitutive law correlates well with experimental results.
Article
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Thesis
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This paper develops a mathematical model for fatigue damage evolution in composite materials. The characteristics of damage growth in composite materials are studied and compared with those of damage growth in homogeneous materials. Continuum damage mechanics concepts are used to evaluate the degradation of composite materials under cyclic loading. A new damage accumulation model is proposed to capture the unique characteristics of composite materials. The proposed model is found to be more accurate than existing models, both in modelling the rapid damage growth early in life and near the end of fatigue life. The parameters for the proposed model are obtained with experimental data. A numerical example is implemented to illustrate that the proposed model is able to accurately fit several different sets of experimental data.
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Initiation of cracking and delamination growth in a unidirectional glass/epoxy composite were evaluated under mode I, mode II, and mixed mode I + II static loading. They have been expressed in terms of the total critical strain energy release rate, GTC, and the total fracture resistance, GTR. For the mixed mode I + II, a mixed-mode bending apparatus was used. The loading in this test is a simple combination of the double cantilever beam mode I and the end notch flexure mode II tests. In addition to characterisation of delamination initiation, whatever the value of the modal ratio, this apparatus allows the plotting of an R curve, from which we obtain the total fracture resistance, GTR.Experimental results were correlated with computations of a semi-empirical criterion through the plotting of the total critical strain energy release rate, GTC, versus the modal ratio, and of the total fracture resistance, GTR, versus the modal ratio.
Article
A new approach for modelling the post-fatigue strength and stiffness of notched fibre composite laminates has been developed. It is based on the observation of notch tip damage which can be quantified by the extent of the individual failure processes, splitting in the 0° plies and delamination between the 0° ply and off-axis plies. The notch tip damage zone grows stably under tensile cyclic loading in a self-similar manner and the size and shape of this damage zone is dependent on laminate geometry and constituent properties of the fibre, matrix and interface. The post-fatigue strength and stiffness of the laminate can be related uniquely to the split length, which defines the extent of damage growth. In this first paper in a series of four, observation is made and measurements taken of the damage growth mechanisms that make up the damage zone in carbon fibre/epoxy laminates. Radiographs are used to characterise the notch tip damage zone and to establish a qualitative relationship between post-fatigue strength (or stiffness), cyclic stress, damage size and numbers of cycles.
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