John R. Hellmann

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.

Test methodology for the thermal shock characterization of ceramics

An experimental methodology is proposed to evaluate the thermal shock resistance of ceramics. A technique based on infrared heating has been developed to perform systematic and well controlled thermal shock experiments. This novel technique was used to evaluate the resistance of yttria-stabilized zirconia–alumina foams to thermal loads. Foams of varying cell sizes were subjected to thermal shock and the damage was evaluated using retained strength and non-destructive elastic modulus measurements. The transient thermal gradients and the resulting thermoelastic stresses in the foams were predicted using finite element analysis and the extent of damage was correlated to the maximum thermal strains generated in foams.

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.

Fiber pushout and interfacial shear in metal-matrix composites

The mechanical response of a fiber/matrix interface is very important in determining the strength and the fracture behavior of metal-matrix composites. As a means of examining interfacial shear behavior, the use of the “thin-slice” fiber pushout test is becoming increasingly common. However, recent thin-slice pushout tests suggest interfacial failure processes depend not only on intrinsic factors (e.g., interfacial bond strength and toughness and matrix plasticity), but also on extrinsic factors (e.g., specimen configuration, thermally induced residual stresses, and the mechanics associated with the test). In light of these factors, this article briefly describes the contrasts in the mechanics of fiber pullout and fiber pushout. In addition, selected aspects of thin-slice fiber pushout behavior are examined to illustrate the physical nature of the interfacial shear response and its dependence on both intrinsic and extrinsic factors.

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.

Thermal fatigue resistance of open cell ceramic foams

Abstract A variety of open cell ceramic foams were subjected to rapid thermal cycles by infrared heating and forced air cooling to study the thermal fatigue behaviour of these materials. After cycling, the extent of damage in the samples was determined by measuring the elastic modulus using dynamic resonance (non-destructive test) and the retained strength in three-point bending (destructive test). It was found that the retained elastic modulus and strength gradually decreased with an increase in the number of cycles, followed by a saturation behaviour indicating a damage accumulation mechanism. The extent of damage was found to depend on the cellular structure parameters (i.e. cell size and density), composition, as well as thermal cycling variables, such as maximum temperature, cooling rate, etc.

High temperature, tube burst test apparatus

A testing apparatus is described that enables both single and double-ended tubular members to be tested under pressure and at elevated temperatures. For double-ended tubular members, the apparatus comprises first and second pressure seals at either end of the tubular member under test, both seals including annular compliant members that bear upon the internal surface of the tubular member. A heater is positioned within the tubular member and one of the pressure seals has an orifice through which the heater is connected to a power source. Pressurization occurs through an orifice in the other pressure seal and cooling apparatus surrounds the first and second ends of the tubular member to cool the pressure seals, thereby enabling the annular compliant members to retain their compliancy when the tubular member is heated to test temperature. For single-ended tubular members, a single pressure seal is used having pathways for both electrical and pressurization connections to the interior of the tubular member.

Thin-slice fiber pushout and specimen bending in metallic matrix composite tests

Abstract For researchers interested in fiber pushout testing of MMCs (including intermetallic matrix composites), we have shown that • • Composites containing optically transparent fibers can be metal backplated using photolithographic techniques to provide a support with a hole size matching the fiber diameter. • • Computational results indicate that backplated specimens exhibit negligible specimen flexure during thin-slice fiber pushout testing. • • Results from thin-slice fiber pushout tests of three different sapphire-reinforced MMCs indicate that the interfacial shear strength values are the same regardless of whether the specimen was supported by the Ni backplate or by a hole 1.6 times the fiber diameter or by a groove 2 times the fiber diameter. Thus at least for fiber aspect ratios of ≥2 and groove width/specimen thickness ratios ≤2, specimen bending does not significantly affect the measured value of the average interfacial shear strength at maximum fiber push-out load in the MMC systems studied.

Oriented microchannel membranes via oxidation of carbon-fiber-reinforced glass composites

Abstract Oriented microchannel membranes were fabricated by the selective oxidation of carbon-fiberreinforced Pyrex glass composites. The oxidation behavior of two PAN-based fibers (T-300R and IM-7) was studied in the as-received condition, and after incorporation into the glass matrix, in order to elucidate differences in oxidation behavior controlled by differences in fiber structure and catalysis by matrix constituents. As-received T-300R fibers oxidized preferentially in an axial fashion from the fiber ends, whereas IM-7 fibers oxidized uniformly on all fiber surfaces. The oxidation rate for the as-received T-300R fiber decreased steadily throughout the oxidation process as a result of a loss of fiber mass, without an increase in the concentration of exposed fiber surface area. The oxidation behavior of the as-received IM-7 fiber was dramatically different than the T-300R fiber. The oxidation rate of the IM-7 fiber increased through 35% burn-off and then steadily decreased, corresponding to the decrease in fiber mass. Incorporation of both fibers in a glass matrix resulted in composites which exhibited similar oxidation behavior. However, the initial oxidation rates for the IM-7 and T-300R fibers in the composite were greater than the initial oxidation rates for the as-received fibers. Interrupted oxidation tests revealed possible catalytic effects of minor constituents in the glass (most notably, sodium) on the oxidation behavior of these fibers embedded in a Pyrex matrix.

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.