RESIDUAL STRAINS AND STRESSES IN TUNGSTEN KANTHAL COMPOSITES/

Neutron diffraction was used to measure thermal residual strains in tu ngsten/Kanthal metal matrix composites containing 10, 20, 30 and 70 vo l.% fibers at room temperature. In addition, thermal residual strains were measured in the 20 and 70 vol.% composites as a function of tempe rature. The study was conducted over the temperature range 20-800 degr ees C. The results were compared with those estimated using finite ele ment modeling. As the matrix has a higher coefficient of thermal expan sion than the fiber, the measured bulk axial strain at room temperatur e in fibers is compressive and varies from about -0.0020 to -0.0004, w hereas 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 ma trix in the composite, averaged over 2.5 x 25 x 25 mm samples sectione d from 2.5 x 25 x 200 mm as-fabricated composites bars, is about 0.003 0 or less. Based on the temperature drop from the hot press temperatur e to room temperature, the total axial strain should be about 0.0050. This indicates that the thermal residual stresses relax at high temper atures and that the stress-free temperature is lower than the hot pres s temperature (1065 degrees C). This agrees with the measured stress-f ree temperature of 600-650 degrees C. Based on the finite element mode l, 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.% c omposite samples, respectively. The corresponding strains in the Kanth al matrix are about 0.0010 and 0.0022 (tensile). The calculated residu al 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.% com posite samples, respectively. For the 70 vol.% composite, the equivale nt stress in the matrix is about 400 MPa, which is less than the 530 M Pa yield stress of Kanthal. (C) 1997 Elsevier Science S.A.