Fixture for Accurate Load Path in Axial Compression

A design is presented for a fixture to test uniaxially and eccentrically loaded column specimens of high performance ceramics. This new design is needed to study the phenomenon of spalling as well as crack propagation on tensile surfaces. The testing apparatus consists of three parts: loading blocks, an alignment jig, and a loading frame. The hardened 4340 steel components of the loading blocks, starting from each end of the specimen, consist of a truncated pyramid, cylindrical roller, V-grooved block, a second cylindrical roller transverse to the first, and a second V-grooved block. All steel components fit into an alignment jig. The eccentricity is machined into the jig. This eccentricity induces a bending moment on the specimen, producing a tensile stress on one surface while maintaining a much higher compressive stress on the opposite surface. Loading is achieved by using a modified 445-kN (100 000-lbf) beam balance machine capable of maintaining the load to within 0.2% indefinitely. Tests over periods of several days showed no tendency for the alignment or eccentricity to change with time. Accuracies achieved by this method are well within 5% for both uniaxial and eccentric loading. This newly designed system has advantages over several existing methods: (1) it is simple and inexpensive compared with the spherical air bearing, (2) it is accurate and reproducible compared with the spherical joint, and (3) it has a high load capacity and ease of alignment compared with the spherical bearing on a flat plate. Formulas are given to scale the apparatus for different load ranges.