A comparative study of the fatigue and post-fatigue behavior of carbon–glass/epoxy hybrid RTM and hand lay-up composites

The paper presents a study of the fatigue and post-fatigue behavior of a hybrid carbon–glass biaxial fabric reinforced epoxy composite manufactured by the resin transfer molding (RTM) and the hand lay-up (HL) processes, with the main objective of assessing whether a material characterization run at the prototype level of a handicraft technology could be significant for a mass production technology and whether a comparison on static properties (a viable task at an industrial level) could ensure the same level of agreement for the fatigue life and residual properties. Tensile and flexural static tests as well as displacement-controlled bending fatigue tests (R ratio of 0.10) were conducted on two sets of standard specimens, having fiber orientation parallel to the loading direction (on-axis specimens) and at 45° to the loading direction (off-axis specimens). Specimens were subjected to different fatigue loading, with the maximum load level up to 60% of the average ultimate flexural strength, and damage in the laminate was continuously monitored through the loss of bending moment during cycling. After 106 cycles, the fatigue test was stopped and residual properties were measured. Micrographs of sample sections revealed some voidage for HL specimens while resin rich areas were observed for RTM specimens. Results of the static tensile and flexural tests pointed out lower mechanical properties for the RTM specimens when tested on-axis and slightly higher properties when tested off-axis. Regardless of specimen fiber orientation, the fatigue and post-fatigue performance of RTM samples was inferior to that of HL specimens with the gap increasing for increasing fatigue load levels. The result was ascribed to the presence in RTM samples of resin-rich areas, which are reported to have limited influence on the laminate static properties but which may act as initiation sites for fatigue cracks.