“Medium Pressure (<70 MPA) Flexible Boundary

Cubical Triaxial Tester”

Inventors: V.M. Puri, et al.

PSU Invention Disclosure No. 97-1789

Mechanical properties of powders are of critical importance in storage, flow, handling, compaction and processing. In addition, powder properties are of central importance in the implementation of constitutive models for evaluating and predicting stress-strain behavior of powders. The state-of-the-art computational methods such as finite element method are dependent upon fundamental properties to model the powder’s response. Therefore, there is a definite need for devices and methods for measuring mechanical properties of powders.

The subject invention disclosure is a medium pressure (<70 MPa) flexible boundary cubical triaxial tester, which has been developed by the Penn State researchers to measure the stress-strain and mechanical properties of powders over a wide range of loading conditions and stress paths. In this tester, gas or air pressure up to 35 MPa (5000 psi) can be applied to all the six surfaces of a 50.8x50.8x50.8 mm (2x2x2 inch) cube-shaped powder specimen. The pressure in vertical direction (top-bottom faces of the powder specimen) and the pressure in horizontal direction (left-right faces and front-back faces) can be controlled independently. The tester can handle displacement as large as 50.8 mm (2 inch) in each of the three principal directions.

This invention can be used to conduct tests over many stress paths such as: hydrostatic triaxial compression (HTC), conventional triaxial compression (CTC), and mean effective stress (MES) stress paths. Tests have been conducted on many test materials. For example, HTC test results for a microcrystalline cellulose powder, an aluminum oxide powder and three loading-unloading cycles, i.e., 0-3.5-0.7-7-0.7-10.5-0 MPa, are shown in Figures 12, 13, 14, 15, 16 and 17. Two powders showed very different load-deformation behavior and compressibility. Microcrystalline cellulose powder was much more compressible than the aluminum oxide powder. The microcrystalline cellulose powder had a maximum displacement of 6.4 mm (strain of 11.2%) and the aluminum oxide powder had only 2.3 mm (strain of 4.0%). Both powders showed some elastic bulk modulus, K, the loading index, l, and unloading index, k, were determined for the two test powders. The values of K, l, k increased at higher pressure. The cubical triaxial tester worked well during these tests in terms of pressure control, loading rate control, loading boundary conditions and deformation measurement. This test apparatus can be used to measure the true load-response of powders.