STUDIES ON FATIGUE DAMAGE PROGRESSION IN POST-IMPACTED CFRP COMPOSITE THROUGH PASSIVE THERMOGRAPHY AND STIFFNESS MEASUREMENT

Reinforced polymer composite materials are widely used in several areas of aerospace, other civilian structures, in view of tailor-suiting to the design requirements. During service, barely visible damages are induced due to accidental tool drops, hail storms and bird strikes, and they can propagate due to fatigue cycles applied during a mission. The damage progression can result in loss of load carrying capacity and ultimate failure. Damage progression due to fatigue in composites has been an important aspect of study as it can result in loss of load carrying capacity and ultimate failure. In this study, the stiffness degradation in a quasi-isotropic carbon fiber polymer composite specimen subjected to FALSTAFF (Fighter Aircraft Loading Standard for Fatigue) spectrum was assessed, after it has been subjected to a drop-impact. Fatigue test was carried out post-impact till specimen failure, which meant testing over several days. The unloading stiffness of the specimen was estimated from the load versus displacement data that was recorded after every block of FALSTAFF loading. It is observed that the stiffness of the specimen degrades with the progression of damage. An Infrared thermal imaging camera (TIM 160 from MicroEpsilon, Germany) was used in passive mode to monitor the temperature changes in the specimen during fatigue cycling. In view of the long duration of fatigue test spanning several days and IR camera cooling requirements, the test was periodically interrupted after certain blocks of FALSTAFF loading. Temperature data during fatigue cycling was compared with stiffness degradation to understand the fatigue damage progression in specimens. The first derivative of temperature response data was found to have a reasonable correlation with the first derivative of stiffness.