Volume 24, Issue 3 (September 2001)
Influence of the Loading Apparatus on the Stresses within Biaxial Specimens
The design and construction of a biaxial loading machine is presented in this paper. Two major concerns are addressed in designing the machine: alignment between the loading axes and the specimen, and shear stresses (friction) that may develop between the loading platens and the specimen. The alignment requirement is solved by constructing a multi-frame machine in which the horizontal frame hangs from the top of a 890 kN (200 Kip) Baldwin machine by cables and springs, thus allowing displacements and rotations of the horizontal loading axis with respect to the vertical axis. Shear stresses between the platens and the specimen are reduced by using brush platens. These platens are made of independent plates flexible enough to bend and thus reduce the transmitted friction. Numerical modeling of the interaction between the machine, platens, and the specimen shows the advantage of the multi-frame machine with brush platens over other combinations. With a multi-frame machine with brush platens, the stress field is uniform at the center of the specimen; differences between the applied and the obtained stresses are mostly within a 10% error, and are produced at the top boundary of the specimen. A single-frame machine with brush platens produces a nonsymmetric stress field and increases the confinement in the specimen as much as 5%. Rigid platens produce unloading within the specimen, which will cause an overprediction in the strength of the material.