One of the most elementary concepts in elasticity theory is that of a uniform state of stress. Producing such a state of stress in the laboratory, however, is not a trivial task. A common experiment in composite mechanics—the tension test of off-angle composites-is discussed in this paper and the influence of end constraint on the uniform stress field is investigated. Analytical and experimental evidence is presented to show the serious effects caused by con ventional clamping devices.

Pagano and Halpin - Influence of End Constraint in the Testing of Anisotropic Bodies

Effect of rock anisotropy on wellbore stresses and hydraulic fracture propagation

The purpose of this study is to gain a better understanding of the parameters affecting crack growth direction in unidirectional composite materials. To achieve this, the effect of anisotropy and biaxial, far field, loading on the direction of crack growth in unidirectional off-axis composite materials is investigated. Specific emphasis is placed on defining the crack-tip-stress field and finding a consistent criterion for predicting the direction of crack growth. An anisotropic crack-tip-stress analysis was implemented using three criteria (the normal stress ratio theory, the tensor polynomial failure criterion, and the strain energy density theory) to predict the direction of crack extension in unidirectional off-axis graphite-epoxy. The theoretically predicted crack extension directions were then compared with experimental results. It was determined that only the normal stress-ratio criterion correctly predicts the direction of crack extension.

Predicting crack growth direction in unidirectional composites

Analysis of crack extension in anisotropic materials based on local normal stress

Experimental biaxial data are essential to validate failure criteria for composite laminates. In this study an optimized design of cruciform specimen used for in-plane biaxial loading was proposed. The uniformity of strain field in the gauge region of the smooth specimen under biaxial loadings was confirmed via finite element analysis. Both smooth and open-hole specimens fabricated by π/4 quasi-isotropic carbon epoxy laminates were tested under three different biaxial stress schemes, where stresses {σx, σy} are expressed in normalized forms as {1, 0}, {1, 1}, and {1, -1}, respectively. The relations between far-field stresses and global biaxial loadings were established based on test data, and the strengths of both smooth and open-hole specimens under different biaxial loading schemes were acquired. Digital image correlation (DIC) technique was employed to measure strain field in the neighborhood of the central hole of the open-hole specimens during test, and the measurements were compared with strain dis...

Effects of an Open Hole on the Biaxial Strengths of Composite Laminates

Center-notched coupons of 16-ply unidirectional AS4/3501-6 graphite-epoxy were loaded in tension to failure in order to determine the direction of initial crack growth, the load required to cause initial crack growth, and the load required to cause failure. Fifteen-degree off-axis tensile coupons provided a far-field biaxial state of stress away from the notch. Specimens of three different length-to-width ratios were tested in order to vary the biaxial stress states. Two notch configurations were used, one perpendicular to the loading direction, the other perpendicular to the material fiber direction. In all of the cases studied experimentally, crack growth was parallel to the fiber direction. An anisotropicelasticity crack-tip stress analysis was subsequently implemented in order to compare experimental crack-growth directions to those predicted by three crack-extension-direction criteria. Of the three criteria studied, (Strain-energy density, the tensor polynomial, and the normal stress ratio), only one, the normal-stress-ratio criterion, provided crack-growth-direction predictions that agreed with the experimental results.

Crack growth in unidirectional graphite-epoxy under biaxial loading

An anisotropic elasticity crack tip stress analysis is implemented using three crack extension direction criteria (the normal stress ratio, the tensor polynominal and the strain energy density) to predict the direction of crack extension in unidirectional off axis graphite-epoxy. The theoretical predictions of crack extension direction are then compared with experimental results for 15 deg off axis tensile coupons with center cracks. Specimens of various aspect ratios and crack orientations are analyzed. It is shown that only the normal stress ratio criterion predicts the correct direction of crack growth.

M. A. Gregory

Papers

Crack growth in unidirectional graphite-epoxy under biaxial loading

Crack growth direction in unidirectional off-axis graphite-epoxy