The impact resistance of composite materials — a review

A constitutive model for anisotropic damage in fiber-composites

A review of experimental data and elementary theoretical formulas for compressive failure of polymer matrix fibre composites indicates that the dominant failure mode is by plastic kinking. Initial local fibre misalignment plays a central role in the plastic kinking process. Theoretical analyses and numerical results for compressive kinking are presented, encompassing effects of strain-hardening, kink inclination, and applied shear stress. The assumption of rigid fibres is assessed critically, and the legitimacy of its use for polymer matrix composites is established.

http://www-mech.eng.cam.ac.uk/profiles/fleck/papers/38.pdf

Compressive failure of fibre composites

One dimensional modelling of failure in laminated plates by delamination buckling

A review of the recent developments in the analysis of laminated beams and plates with an emphasis on shear effects and buckling is presented. A discussion of various shear-deformation theories for plates and beams is given. The available theories are derived assuming a variation of either the in-plane displacement components or the stress components or both in the thickness coordinate. A review of the recently developed finite elements to analyze thin and thick laminated beams and plates is given next. These elements have been derived using the displacement methods, or the mixed methods or the hybrid methods. Recent studies on the buckling and postbuckling behavior of perfect and geometrically imperfect plates are described next. These behaviors have been studied using analytical, numerical, and experimental techniques. Finally, a review of the various studies on the delamination buckling and growth in beams and plates is given. Once again, the studies have been conducted using analytical, numerical, and experimental techniques. The energy release rates have been determined using closed-form solutions or using numerical differentiation. Mention also is made of studies on multiple delaminations and on dynamic response of composite laminates under impact loads.

Recent advances in analysis of laminated beams and plates. Part I - Sheareffects and buckling.

Compression failure mechanisms in unidirectional composites were examined. Possible failure modes of constituent materials are summarized and analytical models for fiber microbuckling are reviewed from a unified viewpoint. Due to deficiencies in available models, a failure model based on nonlinear properties and initial fiber curvature is proposed. The effect of constituent properties on composite compression behavior was experimentally investigated using two different graphite fibers and four different epoxy resins. The predominant microscopic scale failure mode was found to be shear crippling. In a soft resin, shear crippling was in the form of buckling of fibers on a microscopic scale. However, stiff resins failure was characterized by the formation of a kink band. For unidirectional laminates, compressive strength, and compressive modulus to a less extent, were found to increase with increasing magnitude of resin modulus. The change in compressive strength with resin modulus was predicted using the proposed nonlinear model.

/pdf/compression-failure-mechanisms-in-unidirectional-composites-2208njapq0.pdf

Compression Failure Mechanisms in Unidirectional Composites

An experimental investigation has been conducted to determine the effect of low-velocity impact damage and unloaded circular holes on the compressive strength of a 48-ply orthotropic graphite/epoxy flat laminate. Specimens were impacted by a 1.27-cm-diameter aluminum sphere with speeds from 52 to 101 m/x to simulate momenta typical of low-velocity impact hazards that can occur in commercial aircraft service. It is shown that low-velocity impact damage can significantly degrade the static compressive strength of the laminate. Specimens that fail at axial strains above 0.008 in the undamaged condition can fail at strains as low as 0.0031 when impacted at 100 m/s. Circular holes also reduce the static compressive strength of the laminate. The failure strain decreases as the hole diameter increases.

Effect of Impact Damage and Holes on the Compressive Strength of a Graphite/Epoxy Laminate

Structural technology of laminated filamentary-composite stiffened-panel structures under combined in-plane and lateral loadings is discussed. Attention is focused on (1) methods for analyzing the behavior of these structures under load and for determining appropriate structural proportions for weight efficient configurations, and (2) effects of impact damage and geometric imperfections on structural performance. Recent improvements in buckling analysis involving combined in-plane compression and shear loadings and transverse shear deformations are presented. A computer code is described for proportioning or sizing laminate layers and cross-sectional dimensions, and the code is used to develop structural efficiency data for a variety of configurations, loading conditions, and constraint conditions. Experimental data on buckling of panels under in-plane compression is presented to validate the analysis and sizing methods and to illustrate structural performance and efficiency obtained from representative structures. Experimental results show that strength of panels under in-plane compression can be degraded by low-velocity impact damage. Mechanisms of impact-damage initiation and propagation are described. Finally, data are presented that indicates the matrix is a significant factor influencing tolerance to impact damage.

/pdf/recent-developments-in-the-design-testing-and-impact-damage-t2nuq69wwk.pdf

Recent Developments in the Design, Testing and Impact-Damage Tolerance of Stiffened Composite Panels

The results of an extensive imperfection survey on a 10-ft-diameter integrally stiffened cylindrical shell are presented. The shape of the measured initial imperfections is clearly influenced by details of the shell construction. The modal components of the measured imperfection surface as a function of the circumferential and of the axial wave numbers are calculated. The discrete axial power spectral density functions and the corresponding root-mean-square values of the imperfections are also determined for given circumferential wave numbers. Using the Fourier coefficients of the measured initial imperfections, buckling loads are calculated by solving the nonlinear Donnell-type imperfect shell equations iteratively. The calculated lowest buckling load compares favorably with the values usually recommended for similar shell structures.

Imperfection Surveys on a 10-ft-Diameter Shell Structure

An experimental investigation was conducted to identify the failure mechanisms and to understand damage propagation in compression-loaded composite structures. The tests were conducted on several laminates of different ply orientation with thickness that ranged from 0.56 to 0.75 cm. The panels were damaged by 1.27-cm-diameter aluminum spheres propelled normal to the specimen surface at velocities ranging from 30 m/s to 140 m/s. Results indicate that there is significant internal laminate damage due to low-velocity impact with no surface damage. The internal damage consists of delamination and intraply cracking. Three damage propagation modes were identified as causing specimen failure; which are delamination, axial load-lateral deformation coupling, and local shear failure.

Jerry G. Williams

Papers

Compression Failure Mechanisms in Unidirectional Composites

Effect of Impact Damage and Holes on the Compressive Strength of a Graphite/Epoxy Laminate

Recent Developments in the Design, Testing and Impact-Damage Tolerance of Stiffened Composite Panels

Imperfection Surveys on a 10-ft-Diameter Shell Structure

Low-velocity impact damage in graphite-fiber reinforced epoxy laminates