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.

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