Nondestructive Evaluation of Fiberglass Reinforced Plastic Subjected to Localized Heat Damage, Using Acoustic Emission

Abstract

Fiberglass reinforced composites are extensively used in electronic and aerospace applications due to their high specific strength. In these applications, they are often subjected to localized heat damage due to various sources such as electronic over heating, electric arching, and laser beams. In order to ensure their reliability, it is important to predict their residual properties using nondestructive evaluation techniques. Unidirectional fiberglass composite specimens were manufactured using three layers of a fiber glass prepreg. Some of the specimens were subjected to a localized heat damage using a heated copper tip with a diameter of 12.5 mm at 360° C and other specimens were subjected to a laser beam operated at 0.64 Watts/mm2 for various exposure time. In addition, the number of laser damaged spots varied among similar specimens. The specimens were then subjected to tension tests while acoustic emission activities of specimens were collected. The AE activity of all specimens showed three distinct regions. An early activity, followed by a relatively dormant activity period and a high exponential activity before final failure. The period of the dormant activity was independent of the contact heat duration of less than 15 minutes. However, the dormant period for the laser damaged specimens was a function of the number of laser damaged spots. The majority of the early activities for all specimens were related to mechanisms other than fiber fracture. The activity in the dormant period for contact heat damage was mainly controlled by the fiber fracture, while for the undamaged and laser damaged specimens was by the interfacial failure. This could be justified since laser damaged specimens contained numerous damaged fibers leading to a significant interfacial shear stress. The failure modes of specimens further supported this conclusion. The state of the damage in the composite was predicted using the AE-stress delay concept.