Mixed Experimental‐Structural Model for Interlaminar Shear Fracture Toughness

For brittle composite materials, a mixed experimental and structural model is developed to determine a consistent interlaminar shear fracture toughness $\left(G_{\mathrm{II}c}\right)$ in an end notched flexure (ENF) test. The ENF procedure presented here differs from the customary ENF test in that one specimen is used for multiple measurements of compliances and the corresponding critical loads and crack growths; this procedure reduces the number of specimens needed to determine $G_{\mathrm{II}c}\text{.}$ Using this test procedure, $G_{\mathrm{II}c}$ is difficult to compute from only the experimental results because the critical load is sensitive to the inhomogeneity of the material and the specimen variation; thus a model that integrates the experimental results with structural beam theory is developed. Using the experimental results, a functional relationship between the critical load and the important geometric and material properties is obtained. A structural model is developed from Castigliano's second theorem to compute the flexural and transverse shear deflection of the delaminated beam. Experimental results are presented for three 32‐ply multidirectional graphite‐epoxy laminates: $0{°}_{32};$ $0{°}_4\text{,}$ $90{°}_8\text{,}$ $0{°}_4;$ and $0{°}_4\text{,}$ $\pm 45{°}_2\text{,}$ $\mp 45{°}_2\text{,}$ ldquo;$0{°}_4\text{.}$ The structural model is compared with the experimental compliance and deflection results, and $G_{\mathrm{II}c}$ is determined using the experimental and structural model. $G_{\mathrm{II}c}$ obtained by this experimental procedure is compared to the $G_{\mathrm{II}c}$ determined using the customary ENF test procedure.