Failure analysis of impact-damaged/hygrothermally aged fiber-reinforced polymer matrix composite joints subjected to bend loading

Resistance-welded thermoplastic and adhesive-bonded thermosetting composite lap joints were compared in terms of mechanical properties and fracture aspects in pristine and after impact at 10 J and/or hygrothermally ageing conditions. Ultimate strength under quasi-static four-point bend loading (4PB) of virgin joints was higher than simply impacted ones, followed by simply aged test coupons, and finally by impacted and subsequently aged test pieces. Environmental ageing showed the same detrimental side effect, in relative extent terms, in both types of joint. Deleterious effects on 4PB stiffness of epoxy thermosseting composite joint was clearly visible and of the same intensity in both simply impacted and simply aged conditions, although impacting and subsequent ageing of test pieces curiously caused a much lower deterioration of this property. Thermoplastic PPS joints also displayed a somewhat unusual performance in this regard, as their stiffness in response to simply impacted and simply aged conditions was substantially higher than the pristine joint, although, as expected, the impacted and subsequently aged test coupons exhibited the lowest stiffness among all the tested conditions. A detailed failure analysis was carried out to determine the main operating failure mechanisms and developed fracture types in the macro and microscopic levels.