Improvement of End Boundary Conditions for Off-Axis Tension Specimen Use.
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH SCHOOL OF ENGINEERING
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Inaccuracies in using the off-axis tension test are caused by the nonuniform stresses produced by the end constraints. The purpose of this thesis is to show that these nonuniformities can be virtually eliminated by adjusting the amount of tab clamping and selectively locating the point about which the clamp may rotate. A second phase of the study is to conduct a limited verification of a strain energy failure criteria that can account for the material nonlinearity found in composite materials. Determination of the ideal amount of clamping and point of rotation was carried out using a linear finite element method. The nonlinear behavior of the specimen was simulated using a nonlinear finite element method. The failure criteria was used in conjunction with the nonlinear finite element algorithm in determining failure. Experimentation was also carried out to verify the finite element solutions. The results indicate that a nearly uniform state of stress can be produced with ideal tab clamping and rotation. The results indicate that the nonlinear behavior of the specimen was accurately predicted by the nonlinear finite element method but that the failure state predicted by the failure criteria was in error. Suggestions are made for improving the accuracy of the failure criteria. Author