Redesign of the Mixed-Mode Bending Delamination Test to Reduce Nonlinear Effects

TLDR

The mixed‑mode bending (MMB) test applies simultaneous Mode I and Mode II loading to a split‑beam specimen using a lever. The authors redesigned the MMB apparatus to minimize geometric nonlinearity, avoiding the need for nonlinear analysis. An iterative nonlinear analysis guided the redesign, producing an apparatus that applies loads just above the specimen midplane via a roller, thereby reducing nonlinearity. The redesigned apparatus achieved nonlinearity errors below ±3 % and produced load‑displacement and G data comparable to those from the original apparatus and the nonlinear analysis, confirming its effectiveness.

Abstract

The mixed-mode bending (MMB) test uses a lever to apply simultaneously Mode I and Mode II loading to a split-beam specimen. An iterative analysis that accounts for the geometric non-linearity of the MMB test was developed. The analysis accurately predicted the measured load-displacement response and the strain energy release rate, G, of an MMB test specimen made of APC22 (AS4/PEEK). The errors in G when calculated using linear theory were found to be as large as 30% in some cases. Because it would be inconvenient to use a nonlinear analysis to analyze MMB data, the MMB apparatus was redesigned to minimize the nonlinearity. The nonlinear analysis was used as a guide in redesigning the MMB apparatus. With the improved apparatus, loads are applied just above the midplane of the test specimen through a roller attached to the lever. This apparatus was demonstrated by measuring the mixed-mode delamination fracture toughness of APC2. The nonlinearity errors associated with testing this tough composite material were less than ±3%. The data from the improved MMB apparatus analyzed with a linear analysis were similar to those found with the original apparatus and the nonlinear analysis.