Compressive behaviour of thin-skin stiffened composite panels with a stress raiser

The in-plane compressive behaviour of thin-skin stiffened composite panels with a stress concentrator in the form of an open hole or low velocity impact damage is examined analytically. Drop weight impact in laminated polymer composites causes matrix cracking, delaminations and fibre breakage, which together can seriously degrade the laminate compressive strength. Experimental studies, using ultrasonic C-scan images and X-ray shadow radiography, indicated that the overall damage resembles a hole. Under uniaxial compression loading, 0° fibre microbuckling surrounded by delamination grows laterally (like a crack) from the impact site as the applied load is increased. These local buckled regions continued to propagate, first in discrete increments and then rapidly at failure load. The damage pattern is very similar to that observed in laminated plates with open holes loaded in compression. Because of this resemblance, a fracture mechanics model, developed initially to predict notched compressive strength, was applied to estimate the compression-after-impact (CAI) strength of a stiffened panel; in the analysis the impact damage is replaced with an equivalent open hole. Also, the maximum stress failure criterion is employed to estimate the residual compressive strength of the panel. The unnotched compressive strength of the composite laminate required in the analysis is obtained from a three-dimensional stability theory of deformable bodies. The influence of the stiffener on the compressive strength of the thin-skin panel is examined and included in the analysis. A good agreement between experimental measurements and predicted values for the critical failure load is obtained.     

Translaminar fracture toughness: The critical notch tip radius of 0° plies in CFRP

The relationship between translaminar fracture toughness measured at initiation and specimen initial notch root radius is investigated for the translaminar failure mode of cross-ply IM7/8552 carbon/epoxy laminates. Compact tension specimens with four sizes of notch root radii were tested; the true initiation toughness of the laminate was measured from specimens with notch tip radii of ρ ? 250 μm. Testing of specimens with larger notch root radii, ρ = 750 μm, yielded an apparent toughness that was found to be 30% higher than the true toughness of the laminate. The propagation toughness corresponding to the R-curve plateau was found not to be affected by the initial notch tip radius. Investigation of the fracture surfaces of failed specimens revealed that there is no interaction between the 0° and 90° ply failure mechanisms, and that the critical notch radius is a property intrinsic to the 0° plies of the laminate.     

Measurement of the static compressive strength of carbon-fibre/epoxy laminates

This paper describes an experimental study of the compressive failure of T800/924C carbon-fibre/efoxy composite laminates. Undirectional laminates loaded parallel to the fibres have compressive strengths that are 70% of the tensile strength and fail by fibre-microbuckling. During microbuckling the fibre debonds from the matrix, and the fibres break in bending. Multidirectional [(±45/0₂)₃]_sm laminates were also tested in compression, and the critical failure mechanism observed was microbuckling of the 0° plies. The failure strain was almost the same as for the undirectional laminate, The failure strain was almost the same as for the unidirectional laminate, which indicated that the ±45° plies have no significant influence on the failure strength of the 0° plies.     

An experimental and numerical investigation into the damage mechanisms in notched composites

Investigations of the effect of size on the tensile strength of composite laminates containing circular holes show that there is a large difference both in failure stress and mechanism due to changes in test configuration. This is particularly true of the ply and laminate thickness, and hole diameter. Interrupted tests have been performed on open hole tensile specimens at different load levels to determine the progressive damage development, evaluated through non-destructive testing (X-ray and C-scanning). The tests were also analysed using a novel Finite Element Modelling technique. This was able to accurately predict the wide range of ultimate strengths measured with variation in test parameters, principally through incorporation of the sub-critical damage in the analysis. A significant damage mechanism was seen to be delamination at the hole edge which generally occurred at a lower stress for a smaller hole diameter to ply block thickness ratio. Delaminations allowed damage to join up through the thickness of the laminate and propagate. In ply-level scaled specimens, the delamination propagation was the ultimate failure mode of most of the specimens. In sub-laminate level scaled specimens, localised damage relieved stress in the 0° fibres at the hole edge, delaying the onset of fibre failure. Less damage was seen for larger holes, thus leading to a decreasing failure stress with increasing hole diameter.     

Notch and strain rate sensitivity of non-crimp fabric composites

The notch and strain rate sensitivity of non-crimp glass fibre/vinyl-ester laminates subjected to uniaxial tensile loads has been investigated experimentally. Two sets of notch configurations were tested; one where circular holes were drilled and another where fragment simulating projectiles were fired through the plate creating a notch. Experiments were conducted for strain rates ranging from 10⁻⁴ s⁻¹ to 10² s⁻¹ using servo hydraulic machines. A significant increase in strength with increasing strain rate was observed for both notched and un-notched specimens. High speed photography revealed changes in failure mode, for certain laminate configurations, as the strain rate increased. The tested laminate configurations showed fairly small notch sensitivity for the whole range of strain rates.     

Effect of geometry on compressive failure of notched composites

The compressive failure of carbon fibre-epoxy laminates is investigated theoretically and experimentally. Panels with a single edge notch, a central notch or a central hole are considered. The failure mechanism is by microbuckling in the 0° plies and is accompanied by delamination and plastic deformation in the off-axis plies [1]. To predict the critical length of the microbuckle and the failure load, the microbuckle is modelled as a cohesive zone. The magnitude of the normal compressive traction across the microbuckle is assumed to decrease linearly with increasing overlap of material on either side of the microbuckle. The relative effect of the specimen size and a bridging length scale is investigated to illustrate the transition between small-scale and large-scale bridging. If the bridging length scale is small compared with the specimen dimensions, the specimen fails when the stress intensity at the notch tip equals a critical compressive stress intensity factorK _IC . When the bridging length scale is not small compared with either the initial defect size or the unnotched ligament length then it is necessary to include the details of the traction across the microbuckle to predict the failure load accurately.     

A method for predicting the fracture toughness of CFRP laminates failing by fibre microbuckling

Cohesive zone models have had notable success in the prediction of damage from notches in engineering materials loaded in tension. They have also been used to determine the growth of fibre microbuckling from a hole in a composite laminate under compression, (Soutis C, Fleck NA, Smith PA. Failure prediction technique for compression loaded carbon fibre/epoxy laminate with an open hole. J Compos Mater, 1991;25(11):1476–1498). The usual strategy is to replace the inelastic deformation associated with plasticity or microbuckling with a line-crack and to assume some form of stress-displacement bridging law across the crack faces. For instance, in the Dugdale analysis of plastic deformation in metals from the root of a notch (Dugdale DS. Yielding of steel sheets containing slits, J Mech Phys Solids, 1960;8:100–104), it is assumed that the bridging normal traction across the crack faces equals the tensile yield strength of the solid. The material response elsewhere in the cracked structure is assumed to be linear elastic. In this paper, a new method is presented for the calculation of the fracture energy G_c associated with fibre microbuckling, a parameter required in the Soutis et al. linear softening cohesive zone model for the prediction of the open hole compression strength. Theoretical results are found to be in good agreement with experimental data for several carbon fibre–epoxy laminates.     

Is there a thickness effect on compressive strength of unnotched composite laminates?

The effect of laminate thickness was investigated on the compressive behavior of unidirectional and crossply composites. A recently developed compression test method for thick composites was used to test specimens from 16 to 200-plies thick. In all cases the stress-strain behavior to failure is nonlinear and failure strength is matrix dominated. Longitudinal compressive failure is triggered by matrix failure accompanied by fiber microbuckling and the compressive strength is greatly degraded by initial fiber misalignment. The longitudinal compressive strength shows a mild trend of decreasing values with increasing thickness. It can be explained that, even if such a trend is significant, increasing size would have a diminishing effect on compressive strength for initial fiber misalignments greater than 1.5 to 2°. This revised version was published online in July 2006 with corrections to the Cover Date.     

Using thin-plies to improve the damage resistance and tolerance of aeronautical CFRP composites

Thin-ply composites are currently receiving specific attention from researchers due to their capabilities to delay matrix cracking. In this paper, the aim is to design a hybrid laminate that contains both thin- and normal plies. The objective is to improve the tolerance of normal plies by adding thin-plies to the composite in different configurations. Two alternatives were designed, tested, and compared to the specimens made of traditional plies. Impact and compression after impact tests were conducted on each configuration at different impact energies. After being impacted, the specimens were c-scanned to define the delamination pattern. Results showed that surrounding each normal ply with two thin-plies improved the delamination threshold by 15% as compared to the specimens made all of normal plies. Under compression, 15% improvements in the compression after impact strength were obtained. By using thin-plies, the size of each individual delamination was reduced, resulting in small threads instead of peanut delaminations.     

Tension-compression fatigue behaviour of fibre-reinforced ceramic matrix composite with circular hole

The tension-compression fatigue behaviour of a silicon carbide fibre-reinforced glass ceramic matrix composite, SiC/1723, with a circular hole was investigated at room temperature. Two laminate lay-ups were studied: cross-ply, [0/90]_2s, and unidirectional, [0]₈. At first, the fatigue limit based on one million cycles was established for the tension-tension fatigue condition. Then, the fatigue response under fully reversed (tension-compression) cycling loading with a maximum stress equal to the tension-tension fatigue limit was investigated. This tension-compression loading resulted in an increased amount of damage and ultimately led to the specimen failure well before one million cycles. In the cross-ply laminate, the damage mechanisms in the 90° plies involved transverse cracks only during tension-tension cycling, and transverse and longitudinal cracks during tension-compression cycling. In the unidirectional laminate, the longitudinal cracks which initiated at the hole periphery grew longer in tension-compression fatigue than in tension-tension fatigue. On the other hand, no damage and consequently no effect on fatigue life was observed during the compression-compression fatigue condition only.     

The effect of environment on the compression strength of notched CFRP — a fractographic investigation

Compression strength tests were carried out on notched 0° ± 45° CFRP coupons under various environmental conditions. In addition, the fracture of a number of specimens was arrested to minimize post-failure damage and thus facilitate fractographic analysis. The investigation revealed three basic environment-related failure modes. Significant compressive notch sensitivity occurred only under hot-wet conditions and, apart from this condition, laminates in which axial plies were distributed singly were weaker than those in which the 0° plies were in groups of two or three.     

Perforation of flexible laminates by projectiles of different geometry

This paper investigates the response of flexible laminates to ballistic impacts by projectiles of various geometries, namely, flat-ended, hemispherical, ogival (CRH 2.5) and conical (30° half-angle) projectiles. The laminate of interest is Spectra Shield^® comprising [0°/90°] extended chain polyethylene filaments embedded in a thermoplastic resin. Ballistic tests show that flat-ended projectiles cut the laminate through a shearing action, effectively punching a circular hole in the laminate whereas hemispherical projectiles perforate the laminates by stretching the Spectra filaments to failure resulting in a rectangular hole in the laminates. While the manner in which they are perforated are different, many similarities are observed in specimens perforated by flat ended and hemispherical projectiles such as the formation of a generator strip, the extent of delamination, the creasing of the laminate, tearing of the laminate at the edges, etc. Ogival and conical projectiles, on the other hand, perforate the laminates with minimal delamination and tearing of the specimens. Interestingly, the region of the specimens affected by the projectiles appears to increase in size instead of becoming more localised at higher impact velocities as often reported for most ballistic impacts events, including the ballistic perforation of woven fabric. This suggests flexible laminates are more effective in dissipating energy than woven fabric in the application of flexible armour.     

Notched strength of carbon fibre/epoxy composite laminates with a circular hole

This paper improves the two stress fracture criteria proposed by Whitney and Nuismer (known as the point stress criterion and the average stress criterion) to predict the strength of composite laminates with a circular hole. In the point stress criterion, it is assumed that the failure occurs when the stress over some distance (d ₀) away from the notch is equal to or greater than the unnotched laminate strength. In the average stress criterion it is assumed that failure occurs when the average stress over some distance (a ₀) ahead of the notch equals the unnotched laminate strength. Both stress fracture criteria are two parameter models based on the unnotched strength (σ₀) and a characteristic dimension (d ₀ or a ₀). A simple relation is used for the characteristic length to improve the accuracy while evaluating the notched strength of carbon/epoxy composite laminates. The analytical results are compared well with the existing test results of AS4-carbon/948 A1 epoxy [0/90]_{4 s} and [0/±45/90]_{2 S} composite laminates with various hole diameters and specimen widths.     

Notched strength of carbon fibre/epoxy composite laminates with a circular hole

This paper improves the two stress fracture criteria proposed by Whitney and Nuismer (known as the point stress criterion and the average stress criterion) to predict the strength of composite laminates with a circular hole. In the point stress criterion, it is assumed that the failure occurs when the stress over some distance (d ₀) away from the notch is equal to or greater than the un-notched laminate strength. In the average stress criterion it is assumed that failure occurs when the average stress over some distance (a ₀) ahead of the notch equals the unnotched laminate strength. Both stress fracture criteria are two parameter models based on the unnotched strength (σ ₀) and a characteristic dimension (d ₀ ora ₀). A simple relation is used for the characteristic length to improve the accuracy while evaluating the notched strength of carbon/epoxy composite laminates. The analytical results are compared well with the existing test results of AS4-carbon/948 Al epoxy [0/90]_4s and [0/ ± 45/90]_2S composite laminates with various hole diameters and specimen widths.     

Unification of strength scaling between unidirectional,quasi-isotropic,and notched carbon/epoxy laminates

An investigation into size effects and notch sensitivity in quasi-isotropic carbon/epoxy laminates was carried out. The purpose is to draw a complete picture of the strength scaling in unidirectional, quasi-isotropic, and notched carbon/epoxy laminates. A link was established between the strength scaling of the unidirectional and quasi-isotropic laminates. Efforts were made to understand the relationship between unnotched and open-hole strengths. For very small holes, the notched strengths approach the unnotched strength limit. A scaling law based on Weibull statistics was used to predict the unnotched laminate strengths. For very large holes, the same scaling law in conjunction with a detailed 3D ply-by-ply FE analysis with matrix cracks in the 90° plies and delamination cohesive interface elements was used to predict the large notched strengths. A good agreement between the modelling and experimental results was achieved. The effects of 90° matrix cracks on unnotched and notched strengths were also studied.     

Fatigue life prediction of carbon fibre reinforced laminates by using cycle-dependent classical laminate theory

In this work a study about the adaption of the classical laminate theory for fatigue loads is presented. Cycle dependent stiffnesses of single UD 0°, UD 45° and UD 90° plies are implemented in order to calculate the fatigue-induced stiffness decrease of a multidirectional lay-up with the stacking sequence [0°/+45°/−45°/90°/90°/−45°/+45°/0°]. As second input alternative, UD 0°, UD 90° and ±45° plies are used. The calculated cycle-dependent stiffness parameters are compared to experimentally measured fatigue data of the multidirectional lay-up. The experimental test procedure used for the measurement of cycle-dependent stiffness parameters has been published previously. Results show that the experimentally measured stiffness decreases of the multidirectional lay-up can be estimated accurately based on the cyclic unidirectional input parameters.     

Determination of damage micromechanisms and fracture resistance of glass fiber/epoxy cross-ply laminate by means of X-ray computed microtomography

The onset and evolution of the damage in three dimensions was studied by X-ray computed micro-tomography (XCT) in a notched glass fiber/epoxy cross-ply laminate subjected to three-point bending. It was found that damage began by formation of intraply cracks in the 90° plies followed by intraply cracking the 0° plies. Fiber fracture in front of the notch tip occurred at 65% of the maximum load and finally fiber kinking and interply delamination took place under the loading point. Finite element (FE) simulations were carried out to understand crack initiation and the redistribution of stresses upon crack propagation. The crack area corresponding to each damage mechanism was quantified from the XCT images, and this information was used to determine the effective fracture resistance curve of the cross-ply laminate.     

Unit cell modelling of textile laminates with arbitrary inter-ply shifts

Deformation and failure mechanisms of textile laminates are strongly affected by mutual shift of the plies. To model arbitrarily stacked laminate within a traditional framework of multi-scale modelling, one must construct a representative volume element (RVE), which includes all the plies. This is a time consuming and computationally expensive work. As an alternative, the paper suggests a technique that allows setting problems on one unit cell of a single ply, i.e. a volume smaller than RVE of the laminate. The technique approximates the stress field in a ply by combination of stress fields obtained in two additional problems. Boundary conditions (BC) in these problems imitate the interaction of the unit cell with surrounding media. Once these problems are solved, the solution for arbitrary number of plies is composed analytically. The proposed technique respects inter-ply configurations, accounts for the number of plies, distinguishes the ply position, and reproduces the meso stress state with a good accuracy. The technique is validated against reference solutions obtained for the entire laminate.     

Fracture of quasi-isotropic composite sheets with sharp notches

Continuous fibre composites are materials that exhibit rather linear elastic deformation behaviour: suggesting brittleness and notch sensitivity. However, notched composites may sustain significant mechanical load. The notch resistance of composites is investigated on quasi-isotropic composite sheets with sharp crack like notches. This allows the use of analytic solutions of the stress field around a crack in a similar way as is used for linear elastic fracture mechanics (LEFM) in homogeneous isotropic solids. Similar to the small scale yielding boundary condition in fracture mechanics, applied on homogeneous isotropic solids, a small-scale non-linear damage condition should be fulfilled for valid LEFM application on quasi-isotropic composites. Indeed, it appeared to be possible to define critical stress intensity factors (K_1c) for the quasi-isotropic composite. Moreover, K_1c values can quantitatively be related to laminate parameters and to the related damage and deformation processes occurring in a small near crack tip zone with intense non-linearity and strain gradients in the thickness direction. Before the final explosive fracture occurred, stable crack growth was observed. This could be described with R-curves, as done for homogeneous metal sheet specimens. Indeed, also in this case, the R-curves were identical, independent of the length of the initial crack-like notch. The R-curves can be estimated adopting a crack-bridging model. Crack growth occurs at the notch tip in the 0° plies. The other plies bridge the fractured 0° plies. The fracture mechanisms, determining the K_1c-values and the shape of the R-curve, are quite different for composites and metals. Yet, the method of fracture mechanics, well established for metals, can obviously also be applied to quasi-isotropic composites.