The transverse tensile strength of a unidirectional laminate is generally accepted as a material property. The transverse strength of a single ply in angle-ply laminates is influenced by the neighboring layers (effect of constraint) and is for this reason not a material property. In the present investigation of four different carbon fiber-reinforced plastics (CFRP) this influence of constraint and the effect of fiber, matrix, and interface on transverse cracking are investigated. An intermediate ductile resin system proved to increase the transverse strength significantly (if the fiber matrix bond is good) in comparison with a brittle resin system. A highly ductile matrix (polyetheretherketone—PEEK), however, did not further or additionally improve the transverse strength significantly. The intermediate ductile resin system proved to be sensitive to the formation of voids under the cure conditions applied. The inhomogeneous distribution of voids disturbed the expected constraining effect.

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