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Paperboard strength under biaxial normal plus shear stresses is predicted using Hill-type criteria (including TsaiHill, Norris, Fischer, and Ashkenazi). Results are compared throughout ail four quadrants with experimental data obtained from on-and off-axis uniaxial coupons, cylinders, and cruciform specimens. The Norris and Fischer theories predict observation quite well except at higher values of shear stress where they become too conservative. The Tsai-Hill theory is conservative throughout. Simplicity of both analysis and implementation, plus need for only minimal experimental input data, are significant advantages of Hill-type criteria. Moreover, their reliability for paper is comparable to that of other published results. Other strength theories will be addressed in subsequent papers.

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Ability to determine the individual stresses photoelastically in orthotropic composites from isochromatic data, but without employing isoclinics or isopachics, is developed and demonstrated. This new capability is achieved by combining isochromatic information, complex stress functions and numerical nonlinear least-squares concepts into a new and effective hybrid method for stress analyzing composite materials. Unlike previous methods of photo-orthotropic-elasticity, the present technique simultaneously smoothes the recorded isochromatic data, provides reliable boundary information, and separates the isochromatics into individual stress components throughout the area of interest. The method is illustrated by evaluating the three individual stresses throughout a region adjacent to a hole in a uniaxially loaded orthotropic composite plate from the measured isochromatics.