This paper is a review of low-velocity impact responses of composite materials. First the term ‘low-velocity impact’ is defined and major impact-induced damage modes are described from onset of damage through to final failure. Then, the effects of the composite's constituents on impact properties are discussed and post-impact performance is assessed in terms of residual strength.

Review of low-velocity impact properties of composite materials

The delamination process in thin composite plates subjected to low-velocity impact is simulated using a specially developed 2D cohesive/volumetric finite element scheme. Cohesive elements are introduced along the boundaries of the inner layers and inside the transverse plies to simulate the spontaneous initiation and propagation of transverse matrix cracks and delamination fronts. The analysis is performed within the framework of the finite deformation theory of elasticity to account for the nonlinear stiffening of the thin composite plate and the large rotations which accompany the fracture process. The simulation is dynamic and uses an explicit time stepping scheme. Comparison with existing experiments performed on graphite/epoxy laminates indicates that the cohesive/volumetric finite element scheme is able to capture the complex mechanisms leading to the delamination, including the initial micro-cracking of the matrix, the appearance of critical transverse matrix cracks and the rapid propagation of delamination cracks initiated at the intersections between the critical matrix cracks and the adjacent plies.

Impact-induced delamination of composites: A 2D simulation

An investigation of the low velocity impact load level at which a composite laminate will delaminate is presented. The delamination threshold load is described as the load level, obtained from the load–time history or load–displacement plot, at which a sudden load drop occurs due to specimen stiffness loss as a result of laminate level damage. Approximately 500 low velocity impact load–time histories from the Air Force Research Laboratory (AFRL) Low Velocity Impact Database are used to investigate the delamination threshold load. The database contains laminate impact test data for graphite/epoxy (AS4/3501-6), graphite/PEEK (AS4/APC-2), and graphite/BMI (IM7/5260) material systems. The delamination threshold load observations are compared to C-scan damage measurements of impacted specimens to determine if the sudden load drop corresponds to delamination development.

Delamination threshold loads for low velocity impact on composite laminates

The problem of impact damage in laminated composite structures, and the consequent reduction in residual strength, has been a topic of continual research for over two decades. The number of journal papers on the subject now runs into four figures and most have been conscientiously reviewed by Abrate(1991, 1994, 1998). This review is not intended to be in the academic tradition, with emphasis on acknowledging the authorship of all the various research initiatives. Instead we present our opinions so that the reader can appreciate our current understanding of the problem, our capability of predicting by analysis, and the scope of the design tools for avoiding structural damage, or at least designing damage tolerant aerospace structures.

Impact on composite structures

Impact loading of plates and shells by free-flying projectiles: A review

Impact damage prediction in carbon composite structures

Impactor mass and specimen geometry effects in low velocity impact of laminated composites

Thick glass/polyester laminates of four different dimensions subject to low-velocity impact have been investigated using a guided drop-weight test rig with a flat-ended impactor in ascending energy order up to 3100 J. The characteristics of impact response and energy absorption have been determined by impact force and absorbed energy histories, and impact damage incurred was examined by cross-sectioning and ultrasonic C-scanning. Residual compressive strengths were measured, and the damage tolerance of the laminates was assessed by the retaining ability of these strengths. It is found that the salient features in force-time history curves can be related to fracture processes occurring in the laminates, and that the established relationships between impact force and incident kinetic energy (IKE) can be used to identify damage initiation without examining impacted specimens, which is later confirmed by the damage force maps. The constructed damage force and energy maps have shown not only damage initiation in an unstable fashion but also increase of damage size with IKE and force until reaching their load-bearing capabilities. Residual compressive strengths are reduced very rapidly with the increase of impact damage due to extensive delamination.

G.A.O. Davies

Papers

Impact damage prediction in carbon composite structures

Impactor mass and specimen geometry effects in low velocity impact of laminated composites

Impact damage and residual strengths of woven fabric glass/polyester laminates

Buckling and postbuckling of composite structures

Impact damage with compressive preload and post-impact compression of carbon composite plates