Plastic Microbuckling Research Articles

Quantification of damage evolution in cross-ply polymer composites under longitudinal compression by fast computed tomography and semi-automated segmentation

Compressive failure in a notched cross-ply IM7/8552 carbon fiber reinforced polymer (CFRP) laminate is studied experimentally by time-lapse in-situ synchrotron radiation computed tomography (CT). The focus is on tracking and quantifying the fiber damage evolution in the 0° plies under compression for the first time via a specimen design that exhibits quasi-stable damage accumulation. A novel segmentation algorithm has been developed and applied to extract morphological features of interest including the kink band width, inclination angle, and the extent of propagation from the notch. In the four innermost 0∘ plies, shear-driven fiber fracture initiates from the notch tip and propagates as a single fracture plane inclined about 45° to the compression axis. This shear-driven fiber fracture transitions to a kink band at a distance of about 300μm from the notch, which propagates at a shallower angle. During the later stages of the test, the kink bands propagate in the reverse direction to areas that previously initiated by shear-driven fiber fracture, which may explain why post-mortem observations seldom reveal the fiber shear fracture mechanism. The fiber damage initiation and evolution appears distinctly different from the classical parallel plane kink bands formed by plastic microbuckling under uniaxial compression in that the dominant mechanism appears to be shear-driven fiber fracture.

Effect of high-low temperature on the compressive and shear performances of composite sandwich panels with pyramidal lattice truss cores

Composite sandwich panels with pyramidal lattice truss cores (CSPPLTCs) were manufactured employing waterjet-cutting technology and interlocking assembly. The effect of high-low temperature environments on the out-of-plane compressive and shear performance of the panels was studied using theoretical and experimental methods. In particular, the out-of-plane compressive stiffness, strength of sandwich panels with pyramidal lattice truss cores, their shear stiffness, and strength in high-low temperature environments were predicted using theoretical equations. The study found that in low-temperature environments, the movement of molecular chain segments in the epoxy is hindered, futhermore, the mismatched thermal expansivity of carbon fiber and epoxy resin leads to the improvement of interface bonding performance, thus improving the out-of-plane compressive and shear performance of CSPPLTCs; in high temperature environments, the movement of molecular chain segments in the epoxy gets more active and tends to move freely, so the resin turns into a flexible state with high elasticity from the stiff glass state, which finally causes a deterioration in out-of-plane compressive and shear performance of CSPPLTCs. According to observations, the failure mode depends on temperature environments. Brittle fracture and crushing failure happen in the composite struts at low temperature; plastic microbuckling may happen in the composite struts at high temperature.

Effect of Fiber Misalignment on Mechanical and Failure Response of Kenaf Composite under Compressive Loading

The use of kenaf fibres has grown unexpectedly in the world as they help to establish green materials in automobile, sports and food packaging industries. Over the past few decades, unidirectional fiber-reinforced composites have been extensively used in industry due to their high specific strength characteristics. During manufacturing process, several defects especially fiber misalignment might exist in the unidirectional composite structure. This kind of deviation from its optimal parallel packing in a unidirectional fibre reinforced composite would influence its overall load-bearing efficiency. Performance data of kenaf composite due to this imperfection, however, is very limited in the literature. In this regard, the effects of fibre misalignments on the unidirectional kenaf composite compressive reaction have been studied. For this reason, pultruded kenaf composite specimens with different fibre alignment from 00 to 20 at 2.1 and 8.4 mm/s strain rates were subjected to a range of compression measures. The findings revealed that, the failure strain seems to be almost constant at value of 0.05 and 0.063 while the failure stress decreases from 140Mpa until 120MP when the fibre alignment increases when loaded within a range of 2.1~8.4s-1. Additionally, under increased fiber misalignment and strain rate, fibre plastic microbuckling, fibre breakage, fibre splitting and fibre matrix debonding was progressively formed on the specimen.

Kink band evolution in polymer matrix composites under bending: A digital image correlation study

Polymer matrix composites are attractive structural materials in automotive, defense, and aerospace industries due to their high strength to low weight ratios. However, due to their low shear strength, compression dominated failure mechanisms such as plastic microbuckling lead to the development of kink bands, which are a key strength-limiting factor in modern polymer matrix composites. This phenomenon has been studied extensively, particularly for uniaxial compression; however, experimental measurements of the strain fields leading to and developing inside these bands under bending are not well explored. In this study, digital image correlation is used to measure strains inside kink bands developing during three-point bending of cross-plied [0/90] laminated composite Dyneema™ HB80. Measurements indicated large normal and shear strains developed inside the band in a way that suggested systematic increases in ply rotation angle as the band evolved with increased bending deflection. Results also suggested intermittent buckling events involving fiber bundles that correlate with oscillations observed in the load–displacement curve. Optical microscopy of failed samples showed failure resulted from a combination of plastic microbuckling and axial splitting.

Experimental study on the compression-after-impact behavior of foam-core sandwich panels

This paper presents the experimental results pertaining to the deformation, failure mode, failure mechanism and residual strength for the compression-after-impact tests of woven polymer-based foam-core sandwich panels. Sandwich panels with impact-type damage, inflicted via low-velocity impact, were tested to failure in compression. The optical strain measurement system ARAMIS was used to obtain the complete non-uniform strain and displacement fields of the damaged face-sheet. Consequently, damage in the face-sheets was characterized by fractography. It has been found that the dominant damage mechanism is kink band in the impacted face-sheets due to fiber plastic micro-buckling. Besides, the effects of impactor diameter, impact energy, face-sheet thickness and foam core thickness on the residual strength under in-plane compressive loading were studied. This study will be useful for characterizing and predicting the residual compressive strength and understanding the failure mechanism of woven polymer-based foam-core sandwich panel.

Influence of matrix toughness and ductility on the compression-after-impact behavior of woven-ply thermoplastic- and thermosetting-composites: A comparative study

This study was aimed at comparing the residual compressive strength and behavior of TS-based (epoxy) and TP-based (PPS or PEEK) laminates initially subjected to low velocity impacts. Provided that the impact energy is not too low, the permanent indentation is instrumental in initiating laminates local buckling under compressive loadings. CAI tests revealed that matrix toughness is not the primary parameter ruling the damage tolerance of the studied materials. However, matrix ductility seems to slow down the propagation of transverse cracks during compression thanks to plastic micro-buckling which preferentially takes place at the crimps in woven-ply laminates. It could therefore justify why the matrix toughness of TP-based laminates does not result in significantly higher CAI residual strengths. Finally, the compressive failure mechanisms of impacted laminates are discussed depending on matrix nature, with a particular attention paid to the damage scenario (buckling and propagation of 0° fibers failure).

Compressive response of glass fiber composite sandwich structures

Sandwich panels with crushable foam cores have attracted significant interest for impulsive load mitigation. We describe a method for making a lightweight, energy absorbing, glass fiber composite sandwich structure and explore it is through thickness (out-of-plane) compressive response. The sandwich structure utilized corrugated composite cores constructed from delamination resistant 3D woven E-glass fiber textiles folded over triangular cross section prismatic closed cell, PVC foam inserts. The corrugated structure was stitched to 3D woven S2-glass fiber face sheets and infiltrated with a rubber toughened, impact resistant epoxy. The quasi-static compressive stress–strain response of the panels was experimentally investigated as a function of the strut width to length ratio and compared to micromechanical predictions. Slender struts failed by elastic (Euler) buckling which transitioned to plastic microbuckling as the strut aspect ratio increased. Good agreement was observed between experimental results and micromechanical predictions over the wide range of core densities investigated in the study.

Deformation behaviour and failure mechanisms of Al–TiB2 in situ composites

The deformation behaviour and failure mechanisms of Al–TiB2 in situ metal matrix composites were examined through uniaxial tension and compression tests. The influence of particle size and holding time was found to have stronger influence. Elastic modulus of the composite during tension and compression was higher than that of the unreinforced matrix. Fractographic morphology studies were done using optical and scanning electron microscopy. Results reveal that the primary failure was due to matrix yielding initiated by the slip lines. A dimpled structure of the matrix is observed indicating that the failure occurred by microvoid coalescence. The specimens also exhibit plastic microbuckling phenomena at the circumference, followed by particle fracture adding up to the failure mechanisms during compression. Compressive strength and tensile elastic modulus were very sensitive to change in the percentage reinforcement. During compression, majority of the samples failed by splitting or splaying mode. In tension, the particles influence the failure mechanism, whereas in compression they act as barriers to the matrix flow.

ChemInform Abstract: Hierarchical Structure and Mechanical Properties of Nacre: A Review

AbstractReview: 123 refs.

Hierarchical structure and mechanical properties of nacre: a review

Nacre (known as mother of pearl) is the iridescent inner shell layer of some mollusks. Nacre is composed of 95 wt% aragonite (a crystallographic form of CaCO3) and 5 wt% organic materials (proteins and polysaccharides). It is well known that it exhibits high fracture toughness, much greater than that of monolithic aragonite, because of its ingenious structure. It also exhibits energy absorption properties. It has a complex hierarchical microarchitecture that spans multiple length scales from the nanoscale to the macroscale. It includes columnar architectures and sheet tiles, mineral bridges, polygonal nanograins, nanoasperities, plastic microbuckling, crack deflection, and interlocking bricks, which exhibit a remarkable combination of stiffness, low weight and strength. Nacre's special self-assembly characteristics have attracted interest from materials scientists for the development of laminated composite materials, molecular scale self-assembly and biomineralization. This paper reviews the characteristics of hierarchical structure and the mechanical properties of nacre that provide the desired properties, and the latest developments and biomimetic applications.

The Through-Thickness Compressive Strength of a Composite Sandwich Panel With a Hierarchical Square Honeycomb Sandwich Core

Sandwich panels with aluminum alloy face sheets and a hierarchical composite square honeycomb core have been manufactured and tested in out-of-plane compression. The prismatic direction of the square honeycomb is aligned with the normal of the overall sandwich panel. The cell walls of the honeycomb comprise sandwich plates made from glass fiber/epoxy composite faces and a polymethacrylimide foam core. Analytical models are presented for the compressive strength based on three possible collapse mechanisms: elastic buckling of the sandwich walls of the honeycomb, elastic wrinkling, and plastic microbuckling of the faces of the honeycomb. Finite element calculations confirm the validity of the analytical expressions for the perfect structure, but in order for the finite element simulations to achieve close agreement with the measured strengths it is necessary to include geometric imperfections in the simulations. Comparison of the compressive strength of the hierarchical honeycombs with that of monolithic composite cores shows a substantial increase in performance by using the hierarchical topology.

Quasistatic deformation and failure modes of composite square honeycombs

Carbon fibre epoxy matrix composite honeycombs have been fabricated by slotting, assembling and adhesively bonding composite laminate sheets with various fibre architectures. Their out-of-plane compressive and in-plane shear responses were measured as a function of relative density, ratio of the cell height to width and the number of cells in the specimen. The measurements indicate that the response is relatively insensitive to the ratio of the cell height to cell width and number of cells in the specimen but is strongly dependent on the laminate type and fibre orientation. For example, the compressive strength of the honeycombs made from 0 90 laminates with fibres aligned with the compression direction was greater than that of honeycombs made from a woven material with fibres at ±45 . However, the shear strengths exhibited the opposite trend. These differences were attributed to a change in failure mode. In compression the honeycombs failed by either elastic buckling or plastic microbuckling while in shear the two main failure modes were elastic buckling or shear failure of the composite sheet material. Analytical models are derived for these collapse modes and used to predict the strength of the honeycomb structure. The predicted strengths are substantially higher than the measurements due to the presence of manufacturing imperfections in the honeycombs that are not accounted for in the analytical models. A limited finite element (FE) investigation is also reported to quantify the effects of imperfections on the compressive strength of the composite honeycombs. The measurements and analytical predictions reported here indicate that composite cellular materials such as honeycombs reside in a gap in the strength versus density material property space, providing new opportunities for lightweight, high strength structural design.

Statistical Energy and Failure Analysis of CFRP Compression Behavior Using a Uniaxial Microbuckling Model

Compression failure followed by fragmentation is a pre-eminent energy absorption mechanism in fiber-reinforced composites and, therefore, it is often used in crash absorption devices. Fragmentation is the last stage of degradation leading to total failure of a carbon fiber reinforced plastic (CFRP) laminate in compression. Micromechanical studies have shown that this phenomenon is initiated mainly by a microbuckling mechanism which depends on microstructural imperfections (fiber misalignment and matrix plasticity). As far as energy absorption and dedicated modeling strategies are concerned, the question is whether the failure scenario is dependent on these imperfections in terms of dissipated energy. In this context, we used a one-dimensional microscale approach to model plastic microbuckling and evaluate the corresponding dissipated energy for a realistic range of microstructural parameters (with statistical treatment). Contrary to our expectations and to the well-known sensitivity of the peak loads, these energies seem unaffected by microscopic parameters. In conclusion, we propose some ideas on how to include the main aspects of fragmentation, which can occur in composite materials under in-plane compression loading, on the mesoscale.

Compressive failure analysis of non-crimp fabric composites with large out-of-plane misalignment of fiber bundles

Failure initiation under compressive loading in non-crimpfabric composites containing bundles with out-of-plane orientation imperfections was analyzed using FEM in plane stress and linear elastic formulation. The bundle orientation imperfection in a composite unit was described by a sine function. Failure initiation strain was determined comparing failure functions corresponding to two alternative failure mechanisms: (a) plastic microbuckling in bundle due to mixed compressive and shear load; (b) plastic matrix yielding according to von Mises criterion. Parameters for compressive failure initiation analysis were bundle misalignment angle, fiber volume fraction inside the bundle and bundle volume fraction inside the composite unit. The support effect of the neighbouring material was analyzed varying boundary conditions and solving cases with particular configuration of surrounding material. Model prepreg tape GF/EP composite with different introduced levels of out-of-plane waviness of layers was used to validate the conclusions from parametric analysis.

Compressive strength of fibre composites with random fibre waviness

The compressive strength of unidirectional long fibre composites is predicted for plastic microbuckling from a random two-dimensional distribution of fibre waviness. The effect of the physical size of waviness is addressed by using couple stress theory, with the fibre bending resistance scaling with the fibre diameter d . The predicted statistical distribution of compressive strength is found using a Monte Carlo method. An ensemble of fibre waviness profiles is generated from an assumed spectral density of waviness and the compressive strength for each such realisation is calculated directly by the finite element method. The average predicted strength agrees reasonably with practical values, confirming the hypothesis that microbuckles can be initiated by fibre misalignment. It is found that the probability distribution of strength is well matched by a Weibull fit, and the dependence of the Weibull parameters upon the spectral density of waviness is determined. For the practical range of fibre distributions considered, it is concluded that the strength depends mainly upon the root mean square amplitude of fibre misalignment, with the shape of the power spectral density function playing only a minor role. An engineering model for predicting the compressive strength is proposed, akin to weakest link theory for materials containing flaws. A specimen containing randomly distributed waviness is examined to locate regions of high-fibre misalignment. The strength of each of these weak regions is estimated from a look-up table derived from calculations with idealised circular or elliptical patches of waviness. The strength of the composite is given by the failure stress associated with the weakest such patch. For random distributions of waviness, the predictions using this engineering approach are in good agreement with the direct calculations of strength using the finite element method.

Compressive strength of fibre composites with random fibre waviness

The compressive strength of unidirectional long fibre composites is predicted for plastic microbuckling from a random two-dimensional distribution of fibre waviness. The effect of the physical size of waviness is addressed by using couple stress theory, with the fibre bending resistance scaling with the fibre diameter d. The predicted statistical distribution of compressive strength is found using a Monte Carlo method. An ensemble of fibre waviness profiles is generated from an assumed spectral density of waviness and the compressive strength for each such realisation is calculated directly by the finite element method. The average predicted strength agrees reasonably with practical values, confirming the hypothesis that microbuckles can be initiated by fibre misalignment. It is found that the probability distribution of strength is well matched by a Weibull fit, and the dependence of the Weibull parameters upon the spectral density of waviness is determined. For the practical range of fibre distributions considered, it is concluded that the strength depends mainly upon the root mean square amplitude of fibre misalignment, with the shape of the power spectral density function playing only a minor role. An engineering model for predicting the compressive strength is proposed, akin to weakest link theory for materials containing flaws. A specimen containing randomly distributed waviness is examined to locate regions of high-fibre misalignment. The strength of each of these weak regions is estimated from a look-up table derived from calculations with idealised circular or elliptical patches of waviness. The strength of the composite is given by the failure stress associated with the weakest such patch. For random distributions of waviness, the predictions using this engineering approach are in good agreement with the direct calculations of strength using the finite element method.

Experimental study of microbuckle initiation in a model composite system

Model composites comprised of alternate layers of steel plates and epoxy adhesive have been tested in compression. The early stages of microbuckle growth were captured on video, confirming the mechanisms of initiation and growth assumed by theoretical models of plastic microbuckling for compressive failure of long-fibre engineering composites.

A structural approach of plastic microbuckling in long fibre composites: comparison with theoretical and experimental results

The aim of this paper is to compare some predictions obtained from a structural plastic microbuckling model presented in detail by Drapier et al. (1999), with theoretical and experimental results from the literature. After a short presentation of this model, it is established that with our approach it is possible to find the elastic modes determined by Drapier et al. (1996) on the composite microstructure. The plastic instability mechanism is then investigated and its understanding is refined. Some simulations are carried out varying the fibre initial imperfection, and the results are detailed and compared with predictions from a kink-band model (Budiansky and Fleck, 1993). Compared to the present knowledge, the understanding of the influence of the imperfection shape and of its distribution across the ply thickness is improved and new results are exposed. Validation of the present approach is completed by comparing the influence of both matrix and fibre behaviours as predicted by Budiansky and Fleck (1993) with the ones obtained from our numerical tool. Results demonstrate the influence of the change in the matrix tangent stiffness. Secondly, we have quantified the effects of the applied loading, thickness and stacking sequence on the compressive strength of laminates. Numerical predictions provide new results that yield a proper justification of the very high compressive strength measured with bending tests. These predictions also fit well experimental measurements from the literature showing the effects of the thickness (Wisnom, 1992) and of the stacking sequence (Grandsire-Vinçon, 1993) on the compressive strength. For the first time, the effect of the gradient of loading across the laminate thickness is predicted. Results are shown to correlate well with experimental results from Wisnom et al. (1997).

Quasi-static and dynamic mechanical response of Haliotis rufescens (abalone) shells

Quasi-static and dynamic compression and three-point bending tests have been carried out on Haliotis rufescens (abalone) shells. The mechanical response of the abalone shell is correlated with its microstructure and damage mechanisms. The mechanical response is found to vary significantly from specimen to specimen and requires the application of Weibull statistics in order to be quantitatively evaluated. The abalone shell exhibited orientation dependence of strength, as well as significant strain-rate sensitivity; the failure strength at loading rates between 10×10 3 and 25×10 3 GPa/s was approx. 50% higher than the quasi-static strength. The compressive strength when loaded perpendicular to the shell surface was approx. 50% higher than parallel to the shell surface. The compressive strength of abalone is 1.5–3 times the tensile strength (as determined from flexural tests), in contrast with monolithic ceramics, for which the compressive strength is typically an order-of-magnitude greater than the tensile strength. Quasi-static compressive failure occurred gradually, in a mode sometimes described as “graceful failure”. The shear strength of the organic/ceramic interfaces was determined to be approx. 30 MPa by means of a shear test. Considerable inelastic deformation of the organic layers (up to a shear strain of 0.4) preceded failure. Crack deflection, delocalization of damage, plastic microbuckling (kinking), and viscoplastic deformation of the organic layers are the most important mechanisms contributing to the unique mechanical properties of this shell. The plastic microbuckling is analysed in terms of the equations proposed by Argon ( Treatise of Materials Science and Technology. Academic Press, New York, 1972, p. 79) and Budiansky ( Comput. Struct. 1983, 16, 3).

Towards a numerical model of the compressive strength for long fibre composites

The compressive failure of long fibre composites is tackled as a structural instability that is initiated by plastic microbuckling. A homogenised model is proposed to take into account the fibre initial waviness, the non-linear matrix behaviour and some structural effects. The problem is formulated at the ply scale by using a two-scale displacement field and a specific finite element is developed to reduce the extent of computations. This numerical tool permits us to rapidly determine the failure of various unidirectional ply configurations by use of a maximum load criterion. The results establish the influence of some structural parameters on the compressive failure strains, as has been observed in related experiments. They also explain the high compressive strength achieved under flexural loading.