Repeated Impact Failure of Continuous Fiber Reinforced Thermoplastic and Thermoset Composites

Abstract

An effort was made to study the damage mechanisms in both thermoplastic and thermoset composites in response to repeated low-velocity impacts. An instrumented falling-dart impact tester was used that provided the load-deflection and energy-deflection curves plus other numerical data on material properties. Both poly(phenylene sulfide) (PPS) and epoxy composites prepared from different preform styles and stacking sequences were tested. For any composite with given dimensions a threshold incident impact energy (Ec,) could be identified above which delamination would occur at the first impact. As a consequence, both the stiffness and the strength of this composite were reduced and these properties could be measured using the same impact tester as a function of impact cycles. If instead a subcritical incident energy (Ein < Ec) was imposed on the material, no appreciable property loss would be observed until a critical number of impact cycles (Nc) was reached. In this subcritical stage, matrix cracking preceded delamination as impacts were repeated. Both the number and size of delamination cracks were then found to increase as the number of repeated impacts was increased. An elastic strain energy approach was developed to predict the threshold incident energy values of various composites and the prediction was found to be in good agreement with the experimental data. Distinct failure mechanisms were found to exist between epoxy and PPS composites.

Fatigue Mechanisms Under Low Energy Repeated Impact of Composite Laminates

“…[7]. Numerous papers were published on the subject of impact fatigue and have encompassed a variety of materials such as concrete [8,9], metals [10,11], polymers [12][13][14][15] and fiber reinforced plastics [5,[16][17][18][19][20]. Each of these works was performed in totally different testing conditions (test configuration, stress state, etc.)…”

“…Some authors consider repeated impact as a convenient method for assessing the damage tolerance of composites [20]. Repeated impact tests performed by Jang et al [20] show that for a given composite laminate, there exists a threshold incident energy above which a significant damage in the form of a delamination crack will occur in the composite in response to just one impact.…”

“…Repeated impact tests performed by Jang et al [20] show that for a given composite laminate, there exists a threshold incident energy above which a significant damage in the form of a delamination crack will occur in the composite in response to just one impact. Below this critical incident energy the number of cycles for the damage state in composite to reach a critical level can be used as an index for damage tolerance of this composite.…”

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“…Relatively few studies have been directed towards the problem of repeated impact or impact fatigue of laminated composites and the corresponding damage mechanism. Again also here most studies reported in literature are related to CFRP [29][30][31][32][33]. Rotem [30] showed that when a low energy impact was exerted on a laminate, invisible damage might occur as a result of contact stresses between impactor and the laminate.…”

“…When impacts were repeated, the damage zone grew and caused a reduction in strength and modulus. Jang et al [31,32] studied the damage mechanisms in response to repeated low-velocity impacts in both thermoplastic (Polyphenylene sulphide, PPS) and thermoset (epoxy) composites reinforced with glass, aramid and carbon fibres. For any composite an incident impact energy threshold could be identified above which delamination would occur at the first impact.…”