The effect of localized heat damage on the first natural frequency of fiber glass reinforced plastic is studied. The study is performed to determine the possibility of using natural frequency to evaluate the damage state of a composite. The natural frequency and its variation with the damage (size, location, and severity) are obtained using finite element analyses, continuum and lumped models. The results show that the natural frequency of composite specimen decreases with the growth of the damage size and increase of damage severity. However, its variation is much more sensitive to the damage size and location than to its severity. The natural frequency apparently does not change significantly with the number of damage spots as long as the total damage area is constant. A correlation between the natural frequency and the residual tensile strength of the composite is obtained, indicating its potential in predicting the residual strength of damaged composites.

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.