Polymer composites have a wide share among engineering materials. It is important that the material properties are known before being used in industrial applications. Damage behavior needs to be determined in order to safely forming of laminated composites. Propagation characteristics of existing cracks for determining damage are among the current research topics of the researchers. In this study, the fracture toughness of the composite structure was investigated by performing compact tensile

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... d compact compression tests for hybrid fiber reinforced polypropylene composite laminates which have three types of composition having various thicknesses and fiber contents, woven and/or chopped glass fiber reinforcement. The critical energy release rates of fiber and matrix in both tensile and compressive fracture cases were determined in pre-cracked specimens under plane-strain loading conditions. The damage mechanisms of the composite materials used in the present study were described as fiber breakage/buckling of longitudinal and matrix crack/crushing of transverse. As a result of the longitudinal tension, the damage progressed gradually as translaminar fiber breaking in materials containing continuous fibers. In the transverse tension process, fiber-matrix separation caused 2 intralaminar deformation in the materials. The highest fracture critical energy release rate was found in the material with the maximal fiber layer. Nomenclature CC Compact compression CT Compact tension f E Elasticity modulus (Pa) F Deformation load (N) f Geometrical calibration factor G Fracture energy release rate (J/m 2 ) K Critical stress intensity (N/m 3/2 ) h Specimen thickness (m) s Crack displacement (m) B W Energy at break (J) w Specimen width (m)  Energy calibration factor