Mechanical Behavior of Metals Under Triaxial Stress: Apparatus and Experiments
by
Mark Anthony Balzer
Abstract
A novel materials testing system has been designed and constructed to conduct monotonic and cyclic triaxial stress-controlled tests on solid, cylindrical specimens. The system can simultaneously generate any combination of tensile/compressive axial load and lateral pressure up to 750 MPa. Details of the test chamber design, high pressure fluid production and sealing, load and strain measurement under high pressure, computer control and data acquisition are described.
Under combined axial-stress/external-pressure loadings, a solid specimen experiences a truly three-dimensional, homogenous stress state comprised of a uniform axial stress in conjunction with radial and circumferential stresses equal to the negative of the pressure, -P. Through independent computer control of the pressure and axial stress, material behavior under a variety of multiaxial stress states is investigated. Experimental results obtained from conventional uniaxial and from triaxial monotonic and cyclic tests conducted using this unique apparatus are presented for carburized 4320 steel, hardened 4340 steel, nickel-titanium shape memory alloy and normalized 1070 steel. For the first three materials, stress state is shown to have a dramatic effect on the yield strength, the shape of the stress-strain curves and the phase transformation behavior. Pressure raised the fatigue strength of the 1070 steel, but did not affect the flow stress.
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