Interfacial Strength Evaluation in a Glass Fiber Reinforced Composite Using Cruciform Specimen Method

In fiber reinforced composite materials, the interface between the fiber and the matrix plays akey role in mechanical properties of composite materials. Therefore, a more accurateevaluation method of the interface is necessary to develop better fiber reinforced compositematerials. Many techniques are used for evaluating the interfacialproperties. Recently, anexperimental method of evaluating interfacial tensile strength that uses a cruciform shapespecimen is proposed from such aviewpoint. In the cruciform specimen, a single fiberwhose direction is perpendicular to the loading direction is embedded in the specimen centralregion where the specimen width is enlarged. This method can avoid the influence ofinterfacial stress singularity at the specimen edge on the debonding initiation. Although thereare some studies on this method and it may be very useful in evaluating the interfacial tensilestrength, it is not well established as an evaluation method and is not widely used yet. Thepurpose of the present study is to verify the validity of the cruciform specimen experimentallyand analytically. A GF/Epoxy model composite is used. The initiation and propagation ofinterfacial debonding in both cruciform specimen and straight specimens are experimentallyclarified. Moreover, stress analysis using finite element method (FEM) is conducted.Following results are experimentally obtained. In the straight specimens, debonding initiatedfrom the free edge at a lower stress compared to the cruciform specimens. It was alsoobserved that the debonding propagates gradually as the load increased (Fig. 1). In thecruciform specimens, debonding initiated at a higher stress compared to the straight specimen.In cruciform specimens, no debonding initiation at the free edge was observed. It was alsoobserved that the debonding propagation after initiation was much faster than in the straightspecimens (Fig. 2). From FEM, following results are found. In the straight specimens, it wasseen that the influence of the stress singularity at free edge exists, and the interfacial normalstress was very high in the vicinity of the free edge. In the cruciform specimens, it was seenthat the interfacial normal stress is vanishing in the vicinity of the free edge. This correspondsto the debonding initiation behavior obtained from the experiment. The interfacial strengthwas evaluated by the result of the experiment and FEM. REFERENCES1. J.K.Kim,et al., “Engineered Interfaces in Fiber Reinforced Composites”, Elsevier,1998.2. D.B.Gundel, et al., “Scripta Metallurgica Materialia”, 33, 12, pp 2057-2065, 1995.3. S.G.Warrier et al., “Metallurgical and Materials Transactions”, 27A, pp 2035-2043,1996. 4. B.S.Majumdar et al., “Evaluation of the Tensile Interface Strength in Brittle-MatrixComposite Systems”, J.Am.Ceram.Soc., 81, 6, pp 1600-161