Residual strains and stresses in tungsten/Kanthal composites

Abstract

Neutron diffraction was used to measure thermal residual strains in tungsten/Kanthal metal matrix composites containing 10, 20, 30 and 70 vol.% fibers at room temperature. In addition, thermal residual strains were measured in the 20 and 70 vol.% composites as a function of temperature. The study was conducted over the temperature range 20–800°C. The results were compared with those estimated using finite element modeling. As the matrix has a higher coefficient of thermal expansion than the fiber, the measured bulk axial strain at room temperature in fibers is compressive and varies from about −0.0020 to −0.0004, whereas for the matrix, the strain is tensile and increase from 0.0003 to 0.0027 as the amount of tungsten fibers in the composite increases from 10 to 70 vol.%. For a given volume percent of fibers, the sum, of the absolute values of axial strains measured in the fibers and the matrix in the composite, averaged over 2.5×25×25 mm samples sectioned from 2.5×25×200 mm as-fabricated composites bars, is about 0.0030 or less. Based on the temperature drop from the hot, press temperature to room temperature, the total axial strain should be about 0.0050. This indicates that the thermal residual stresses relax at high temperatures and that the stress-free temperature is lower than the hot, press temperture (1065°C). This agrees with the measured stress-free temperature of 600–650°C. Based on the finite element model, it is estimated that the residual axial strains in tungsten fibers are compressive and are about −0.0020 and −0.0005 in 20 and 70 vol.% composite samples, respectively. The corresponding strains in the Kanthal matrix are about 0.0010 and 0.0022 (tensile). The calculated residual axial stresses in tungsten fibers are about −1015 and −258 MPa and in the Kanthal matrix are about 251 and 602 MPa in 20 and 70 vol.% composite samples, respectively. For the 70 vol.% composite, the equivalent stress in the matrix is about 400 MPa, which is less than the 530 MPa yield stress of Kanthal.