D. A. Koss

Interfacial stress state present in a “thin-slice” fibre push-out test

An analysis of the stress distributions along the fibre-matrix interface in a “thin-slice” fibre push-out test is presented for selected test geometries. For the small specimen thicknesses often required to displace large-diameter fibres with high interfacial shear strengths, finite element analysis indicates that large bending stresses may be present. The magnitude of these stresses and their spatial distribution can be very sensitive to the test configuration. For certain test geometries, the specimen configuration itself may alter the interfacial failure process from one which initiates due to a maximum in shear stress near the top surface adjacent to the indentor, to one which involves mixed mode crack growth up from the bottom surface and/or yielding within the matrix near the interface.

Interfacial shear and matrix plasticity during fiber push-out in a metal-matrix composite

Abstract Interfacial shear during thin-slice fiber push-out of sapphre fibers bonded to a niobium matrix has been examined both experimentally and computationally. Observations indicate a failure process involving the combination of interfacial crack growth and diffuse matrix deformation at opposite ends of the fibers being displaced. The result is a stage during which ‘stable’ fiber displacement occurs under increasing axial loads applied to the fiber. A straightforward analysis of the extent of this stage suggests that it obeys a load instability criterion, depending on the competition between geometric softening due to interfacial crack growth and strain-hardening within the matrix near the interface.

The interfacial failure sequence during fiber pushout in metal matrix composites

Abstract Laser profilometry measurements of fiber displacements during thin-slice fiber pushout tests of three MMC systems indicate that inter-facial failure initiates at the specimen backface, opposite the indenter location. Backface failure initiation occurs at loads as low as 50% of the maximum “debond” load, even under test conditions designed to minimize specimen flexure under load. Finally, at least in the two sapphire-TiAl systems examined, there is evidence that the entire fiber displaces as a unit under increasing loads prior to the maximum debond load. The above failure sequence is consistent with recent analyses (9,15) which predict backface failure initiation in thin-slice MMC specimens in which large thermally induced residual stresses exist. The present data also suggest a difficulty in associating the maximum debond load, PMA~, solely with crack growth.

On the strain-induced growth of neighboring voids

The purpose of this communication is to report preliminary experimental results which describe the strain-induced growth of a pair of spherical cavities, spaced about one cavity diameter apart, and embedded in material (titanium) subjected to far-field uniaxial tension. Ultrasonic imaging is used to monitor cavity dimensions during the incremental straining of the specimens, and data are presented for both longitudinal and transverse cavity growth for extensional strains up to [approximately] 0.5. As will be evident, the data indicate a significant degree of void interaction even in this uniaxial tension case.

Interfacial Shear Behavior of Sapphire-Reinforced NiAi Composites

The interfacial shear behavior in near-equiatomic NiAl reinforced by sapphire filaments has been examined at room temperature using a fiber pushout test technique. The load-displacement data indicate a large variability in the initial interface failure stress, although reverse push behavior indicates a comparatively constant interfacial sliding friction stress. The observed behavior suggests that the presence of asperities on the fiber surfaces and nonuniformities in fiber diameter require constrained plastic flow within the NiAl matrix in order for interfacial shear to occur. The location, shape, severity, and distribution of fiber asperities as well as the uniformity of fiber diameter are critical to the interfacial shear process.

Mechanics of Interfacial Failure during Thin-Slice Fiber Pushout Tests

Interfacial failure along the fiber-matrix interface during fiber push-out tests is examined for conditions where (1) a “thin-slice” specimen geometry is used and (2) matrix plasticity is necessary for large scale fiber displacement. The influence of both test geometry and the combination of thermally and mechanically induced stresses on the potential failure process is discussed. Experimental results are presented which illustrate a range of interfacial failure processes as a consequence of different combinations of specimen geometries, thermally induced residual stresses, and mechanically applied stresses. It is concluded that mode I crack opening and growth induced by specimen bending can be a major contributor to the interfacial failure process. Accurate quantitative measurements of interfacial shear properties must therefore rely on test configurations in which specimen bending is eliminated.

On Improving the Fracture Toughness of a Niai-Based Alloy by Mechanical Alloying

Mechanical alloying (MA) has been used to process the NiAl-based alloy Ni-35Al-20Fe, such that a fine-grain (about 2 microns) microstructure is obtained through the addition of 2 vol pct Y2O3 particles. When compared to a conventionally processed, coarse-grained (about 28 microns) Ni-35-20 alloy without the Y2O3 particles, the MA alloy exhibits two to three times higher fracture toughness values, despite a 50-percent increase in yield strength. Room-temperature K(O) values as high as 34 MPa sq rt m are observed, accompanied by a yield strength in excess of 1100 MPa. Fractography confirms a change in fracture characteristics of the fine-grained MA alloy.

ON INTERFACIAL SHEAR IN SAPPHIRE FIBER-REINFORCED NIOBIUM

The interfacial shear response of sapphire fibers embedded in a niobium matrix has been studied using thin-slice fiber pushout tests. The combination of finite element analyses of the stress states along the interface and experimental observations suggest the following interfacial shear process: (a) initially a combination of crack initiation and propagation as well as matrix deformation causes stable fiber displacement under increasing applied stresses, (b) a shear instability within the niobium, but near the fiber-matrix interface, results in abrupt fiber displacement under decreasing stress, and (c) frictional sliding and galling govern the final stage as the fiber is pushed out of the matrix, leaving a layer of niobium adhered to the fiber. The radial stress component normal to the fiber-matrix interface, the equivalent stress within the matrix, as well as changes in the interfacial shear stress all influence the fiber-matrix displacement process.

On the Macroscopic Flow Behavior of the Phases Present in Ti-24Al-llNb

The deformation behavior of two alloys whose compositions (Ti-25Al-9Nb and Ti-11Al-23Nb in at.%) correspond to alpha-two and beta phases in the Ti-24Al-11Nb alloy has been investigated. Results from both compression and tensile tests over a range of temperatures from 27° to 650°C indicate that the yield stress as well as the strain and strain-rate hardening characteristics of the Ti-24-11 alloy are controlled by the Ti 3 Al-base alpha-two phase. In contrast to the Alpha-two alloy, the Beta alloy has a flow stress which is very sensitive to temperature and strain rate at 650°C, suggesting the onset of high temperature creep processes.