Influence of thermal cycles on the low velocity impact response of CFRP laminates

Carbon Fiber Reinforced Polymers (CFRP) are increasingly used in several applications, mainly in the aeronautical field, due to their better mechanical properties in terms of specific strength and stiffness. These materials, however, at the same time are characterized by high sensitivity to impacts of foreign objects as well as variation of environmental conditions. Both of these issues have been studied for several decades, this research resulting in a number of papers regarding impact damage characterization and prediction and influence of thermal aging. The present study focuses on the combined effects of thermal cycles and low velocity impact damage on unidirectional carbon fiber laminates. Indeed no specific contribution can be found dealing with the influence of thermal aging on the impact response of unidirectional CFRP. Previous studies by other investigators either regard different composite materials (such as glass fiber or PEEK/carbon fiber laminates), or the effect of thermal aging on other mechanical properties, or examine how preexisting impact damage is affected by subsequent thermal cycles. Low velocity impact tests, according to the relevant ASTM code, were carried out by means of a drop-weight tower on unidirectional CFRP laminates previously aged with different numbers of thermal cycles, ranging from 1 to 80. The extreme temperatures of each cycle were -16 °C and +80 °C. The kinetic energy of the impactor was kept constant at 6 J in all tests, in order to induce matrix cracking and delaminations but not extensive fiber fracture in the specimens. After impact, the laminates were inspected both visually and by means of ultrasonic scanning. The analysis of data recorded during impact tests revealed that the energy absorbed by the laminates was lower after the longest aging period. In addition, the load drops that are visible in the contact force history of impacted laminates, and are usually attributed to the propagation of delamination, tend to disappear as the number of thermal cycles increases. These results suggest that thermal aging caused a change in the mechanical response of CFRP laminates to low velocity impact. Possible explanation of such change are discussed.