Effect of specimen dimensions on interlaminar fracture toughness of composites

Oleksandr Kravchenko, Purdue University

Abstract

The influence of specimen geometry on Mode I interlaminar fracture toughness of composite materials was examined. Groups of double cantilever beams with different thicknesses were tested. A number of tests was conducted for each sample and C-scan was used to confirm a uniform crack length over the width of the sample. If the crack front was curved due to anticlastic bending, a razor blade was used to further propagate a crack to obtain a more uniform crack front. By performing a finite element simulation of a double cantilever beam, it was shown that a non-singular stress term in addition to a singular stress term is present in the vicinity of the crack tip. It was shown that different apparent fracture toughness values are obtained for different sample thicknesses. This undermines conventional understanding of the critical stress intensity factor as the only parameter that defines fracture toughness of a material. Thus, to make a complete prediction of fracture toughness, a second non-singular opening stress term resulting from William's expansion was used. By knowing the magnitude of the non-singular stress term it is possible to relate it to apparent fracture toughness. It was noticed that a smaller magnitude of the nonsingular stress results in bigger values of fracture toughness. This is especially important because the present ASTM standard suggests testing double cantilever beam specimens with a thickness between 3 and 5 mm that appear to have higher apparent fracture toughness values than of thicker samples.

Degree

M.S.A.A.

Advisors

Sun, Purdue University.

Subject Area

Aerospace engineering

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