Nowadays, fiber reinforced plastic (FRP) has been widely used in many areas such as auto mobile, airplane and marine vessel due to its high specific strength, good corrosion resistance, relatively low cost and so on. However, it still remains unknown that what kind of damage will happen in the internal structure when an automobile, which is made from FRP, has a slight impact with something such as a wall. Then the following road safety of the automobile cannot be guaranteed because certain parts may be exposed to damage in what seems even like a slight impact. In addition, it is well known that initial fracture can bring damage and great effect to the mechanical properties of the FRP material. The novelty of this paper is that the object of this research is micro crack such as transverse crack. While, almost previous report is aimed at delamination. Actually, before the delamination happens, micro crack has already occurred. The mechanical property of FRP is beginning to decrease by delamination. However, when the delamination occurred in the FRP is examined, it is already too late because the delamination can bring great influence to the safety of the FRP products. Therefore, it is important to investigate and detect the presence of micro crack with ultrasonic wave. In this way, some accidents might be avoided. While, because of the variety of the constraints in the fracture mechanism, the damage behavior is very difficult to evaluate and there are rarely researches and data on it. Although damage assessment by visual observation and the durable service life on FRP has become a general tendency in these recent years, the appropriate way of non-destructive examination has not been confirmed yet.

The purpose of this study is to investigate the possibility of non-destructive examination with ultrasonic wave testing for mechanical damage of glass fiber reinforced plastics (GFRP). The possibility of dividing of Lamb wave modes by reducing the thickness of samples was confirmed and the variance of distribution of frequency of S0 mode wave by micro fracture in GFRP.

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