WCNDT '96 - New Delhi
Table of Contents
|AET - Applications|
Fatigue failure in advanced composites is very complex owing to a large number of parameters involved. Unlike metallic materials where fatigue failure is governed by a dominant crack propagation and the stress intensity factor associated with the crack tip, in fiber reinforced composite materials fatigue is progressive in nature and is governed by accumulation and interaction of different failure mechanisms viz., matrix cracking, interface failure, delamination growth, fiber breakage.
Fatigue life prediction of composite materials has been the subject of many investigations in the last two decades. Though it appears that these materials have superior fatigue resistance compared to metallic materials, the understanding is incomplete. And, the existing failure theories are inadequate for design. Several theories have been proposed based on stiffness reduction, residual strength reduction, cumulative damage theory etc. However, many of these if not contradicting are inconsistent and show extensive scatter. Though the onset of damage in composite materials is known to take place early in the fatigue life and further, the damage progression is known to occur in stages before the final failure, degradation in terms of measurable mechanical properties such as reduction in stiffness or residual strength has been observed to be very gradual and insignificant almost till the ultimate failure making it very difficult when it comes to life prediction.
Thus, study of fatigue phenomenon in composites requires a dynamic tool which can detect different failure mechanisms and further monitor as these cause cumulative damage progression in different stages along the fatigue life. Acoustic emission technique appears to be the most suitable tool since each source generating these emissions is expected to have some unique features reflected in the AE signals collected. Also the technique can be used on-line for monitoring the cumulative damage progression.
In the current experimental investigations unidirectional carbon fiber reinforced epoxy composite specimens prepared as per ASTM D-3479 specifications were subjected to tension-tension fatigue under constant amplitude load controlled mode at 10 cycles per second and with a stress ratio of 0.2. A closed loop servo hydraulic system was utilized for the tests. Acoustic emission monitoring was carried out on-line and the data acquired using a 25 MHz sampling rate A/D card which has both on-line and off-line signal analysis capabilities. Acoustic isolators were used to suppress the hydraulic noise generated by the dynamic loading system.
Amplitude as well as frequency distribution of AE signals have been studied to detect and characterize different failure mechanisms. For a quantitative measure of degradation of the material with fatigue load cycles, reduction in stiffness of the specimen was measured intermittently. Further, ultrasonic imaging was carried out on the specimens subjected to fatigue load upto different stages of fatigue life. Correlation between NDE parameters and mechanical property degradation has been attempted with an eventual objective of obtaining a quantitative measure of cumulative damage at a given point of time and for prediction of final failure.
|AET - Applications|