A study of residual stresses in titanium metal matrix composites

Residual stress analysis of as-processed SCS-6/Ti-24Al-11Nb (at.%) (0) {dollar}sb8{dollar} metal matrix composite (MMC) was performed using the x-ray diffraction (Sin{dollar}sp2sb{lcub}Psi{rcub}{dollar}) technique. Residual stresses were measured in the {dollar}alphasb2{dollar} phase of the matrix in orientations parallel, transverse, and 45{dollar}spcirc{dollar} to the fiber direction. Stress profiles were determined by successive removal of the matrix from the composite surface by electropolishing. Within 10 to 30 microns of the first fiber plane, the stress levels were found to be lower than those far from the fiber plane. Residual stress measurements performed on the matrix material between the first two fiber rows (second-ply) revealed lower stress levels than those in the outermost matrix layers. The presence of stress gradients normal to the surface of the composite was also revealed in the second-ply region. Residual stresses were also measured after thermal cycling from 150{dollar}spcirc{dollar}C to 815{dollar}spcirc{dollar}C in an inert atmosphere.; Residual stress profiles were also determined in SCS-6/Beta-21S composites in unidirectional (0) {dollar}sb4{dollar}, cross-ply (0/90) {dollar}sb{lcub}rm s{rcub}{dollar}, and quasi-isotropic (0/{dollar}pm{dollar}45/90) {dollar}sb{lcub}rm s{rcub}{dollar} lay-ups in directions parallel and perpendicular to the fiber direction on the surface ply. For the unidirectional MMC the longitudinal stresses were almost twice the transverse stresses. The stress profile showed a slight increasing gradient towards the fiber-matrix interface in both the longitudinal and transverse directions. On the surface the stresses measured 235 MPa and 130 MPa in the longitudinal and transverse directions respectively, and at a distance of 25 microns from the fiber-matrix interface, the stresses measured 250 MPa and 150 MPa in the same directions. In the cross-ply MMC the longitudinal and transverse stresses were similar in magnitude and measured 200 MPa at the surface and 220 MPa at distance of 25 microns from the fiber-matrix interface. For the quasi-isotropic MMC, the stresses in the longitudinal and transverse directions were intermediate between the unidirectional and crossply MMCs.; FEM models based on the conventional unit cell (quarter-fiber), one-fiber and two-fibers have been developed and analyzed to predict residual stresses on the surface of the composite. In addition, using the element birth/death options available in the FEM code, the surface layer removal process that is conventionally used in the residual stress measuring technique (x-ray diffraction) has been simulated in the model.; Generally, the longitudinal stresses predicted on the surface layers in the unidirectional (0) {dollar}sb4{dollar} and (0) {dollar}sb8{dollar} composite were almost twice the transverse stresses, while for the cross-ply (0/90) {dollar}sb{lcub}rm s{rcub}{dollar} composite, the longitudinal and the transverse stresses were similar in magnitude. The stress profile for the quasi-isotropic (0/{dollar}pm{dollar}45/90) {dollar}sb{lcub}rm s{rcub}{dollar} lay-up were intermediate between the unidirectional and cross-ply composites. These results show the strong influence of the underlying ply on the residual stresses in the outer ply. A 90{dollar}spcirc{dollar} ply has greater influence than a 45{dollar}spcirc{dollar} ply on the outer 0{dollar}spcirc{dollar} ply of the composite. The stresses predicted in the second-ply region of the composite were slightly lower than the surface stresses. (Abstract shortened by UMI.)...