Showing papers by "Daniel B. Miracle published in 1998"

Experiments and analysis of fiber fragmentation in single and multiple-fiber SiC/Ti-6Al-4V metal matrix composites

Abstract: Single-fiber and multiple-fiber single-ply fragmentation experiments were performed at room temperature on SiC/ Ti-6A1-4V specimens, to understand interface shear failure under fragmentation conditions and to assess load-sharing behavior in longitudinally loaded composites. Tensile specimens were instrumented with two acoustic emission sensors and an extensometer to monitor the strain at which fiber breaks occurred. Following testing, the break locations were determined using a novel ultrasonic shear-wave back reflection (SBR) technique. Data analysis was performed using Curtin's exact fiber fragmentation model, wherein the in situ Weibull strength and Weibull modulus of the fiber, and the average shear stress under fragmentation conditions, were determined based on best fit with two essentially independent sets of data from the experiments, i.e. the breaking stress of the fibers, and the fragment length distribution. Results for the SCS-6/Ti-6A1-4V samples are presented in this paper, and they are compared with results from other SiC fibers in the same Ti-alloy matrix. The average shear stress from the fragmentation test was significantly higher than that obtained by push-out tests, and is explained on the basis of high radial clamping stress on the fiber in the immediate vicinity of a fiber break. Experiments were also performed on multi-fiber single-ply specimens. Comparison with the single-fiber results showed evidence of correlated fracture even for the relatively weak interface of the SCS-6 fiber. SBR image and macroscopic slip bands indicate that localized plasticity plays a dominant role in promoting correlated fiber fractures at room temperature, and the mechanism is outlined.

Effects of thickness and precracking on the fracture toughness of particle-reinforced al-alloy composites

Abstract: The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for a valid J Ic test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation, with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for the thick and thin specimens were in good agreement with the experimental data.

Modification of transverse creep behavior of an orthorhombic titanium aluminide based Ti–22Al–23Nb/SiCf composite using heat treatment

Abstract: The feasibility of improving the transverse creep properties of an orthorhombic titanium aluminide matrix composite reinforced with unidirectional SCS-6 SiC fiber via modification of matrix microstructure was investigated using post-consolidation heat treatments. A Ti–22Al–23Nb/SCS-6 4-ply composite was subjected to heat treatments which consisted of solutionizing at either above or below the β -transus temperature of the matrix alloy, followed by controlled cooling and aging within the orthorhombic (O) phase field. Mechanical testing showed that the transverse (90°) creep behavior of the titanium alloy composite in the temperature range 650–760°C is significantly improved by a supra-transus solution heat treatment followed by the aging, without any adverse effect on the longitudinal (0°) tensile strength of the composite. This improvement in the creep response was attributed to an increase in the volume fraction of the orthorhombic phase and the lath morphology of the (O+ β ) two-phase regions in the matrix microstructure. The effect of sub-transus heat treatment was less pronounced and the transverse creep resistance of the composite decreased with increasing volume fraction of the α 2 phase in the matrix. Examination of creep ruptured 90° composite specimens showed that transverse creep damage in the form of fiber/matrix interfacial debonding occurs predominantly at very closely spaced or touching fibers.