Ye T. Chou

Fractal characterization of the fracture surface of a high-strength low-alloy steel

The fractal dimension of fracture surfaces of a high-strength low-alloy steel (ASTM A710) has been measured using different ruler lengths. The fractal dimension increases with increasing impact toughness, E, and can be expressed by the quantitative relationship, E=E0e25D′, where E0=1.9 J representing the impact toughness in Euclidean space, and D′ is the fractal dimensional increment, defined by D′=DS−2=DL−1 and DS and DL are respectively, the surface and linear fractal dimensions.

EFFECT OF SILVER ADDITION ON THE MICROSTRUCTURE OF YBA2CU3O7-X

The variation of grain morphology in silver-doped YBa[sub 2]Cu[sub 3]O[sub 7[minus][ital x]] was investigated as a function of sintering temperature, atmosphere, and amount of Ag addition. In the presence of the liquid phase formed at 925 [degree]C for undoped samples, and at 910 [degree]C for silver-doped samples, the grain shape and size changed drastically from small and nearly equiaxed to large and elongated. The anisotropy in the grain shape was sensitive to both the silver content and the atmosphere. On the other hand, while the grain size was generally insensitive to the atmosphere, it decreased as the silver content increased. The silver phase, if sufficiently large, would block the grain growth in the matrix. The amount of silver for effective blocking was predicted from a microstructural model, and the prediction was in agreement with experimental results.

Superplastic flow in fine-grained YBa2Cu3O7−x high Tc superconductors☆

One of the important compounds in the recently discovered high temperature oxide superconductor series is YBa[sub 2]Cu[sub 3]O[sub 7[minus]x], which has a critical temperature ([Tc]) of 93K. Known as the 123 compound, its physical and chemical properties, including its mechanical behavior, have stimulated much extensive research. This polycrystalline compound is, however, very brittle and does not yield plastically at ambient temperature. Even at high temperatures, the range of deformation remains small. A study of high temperature deformation of the 123 compound, especially in the range of superplastic flow, has been conducted by the present authors for the past three years. This paper summarizes part of the findings concerning the superplastic nature of the material as affected by the deformation parameters.

Laser-induced controlled cracking in ceramic crystals

Abstract Interior cracking was generated in KCl, LiF and MgO single crystals using a focused, short pulse laser. The laser-induced damage region in LiF and MgO crystals was composed of a damage core and three orthogonal penny-like cracks along {100} planes. In KCl crystals an extended cleavage crack was generated on just one side of the core. The detailed features were examined by optical and scanning electron microscopy.

Deformation mechanism map for creep in YBa2Cu3O7−x

A deformation mechanism map with grain size and stress as variables was constructed for creep in YBa2Cu3O7−xat 850 and 950 °C. Theoretical models of Nabarro-Herring, Coble, and power-law creep were used for the construction. The values of various physical constants for creep of YBa2Cu3O7−x were taken from the literature, or estimated with appropriate assumptions. The constructed map showed that the Nabarro-Herring creep would dominate at high temperatures in the practical range of grain size and stress, and that the power-law creep would occur at large stress (> 1 GPa) and grain size. A review of previous creep studies showed that the map is in close agreement with the experimental results. Discrepancies in the values of stress exponent and activation energy for creep of YBa2Cu3O7−x given in the literature are explained by the use of the constructed map.

The anomalous flow behavior in single-phase Ti44Al56 single crystals. The effect of deformation history

Abstract In this paper we present our studies of effect of deformation temperature on the anomalous flow behavior of [001] and [011] oriented Ti 44 Al 56 single crystals grown in an ASGAL optical imaging floating zone crystal growth furnace. The specimens were compression tested at room temperature (RT), 973 and 1173 K and they displayed anomalous flow behavior. However, if the sample is deformed at high temperature, at about 973 K (below the peak temperature), and then retested at room temperature, the high temperature strength is retained, i.e. the flow stress is irreversible. But if the initial deformation at high temperature is performed at about 1173 K (above the peak temperature), and then deformed at room temperature the room temperature flow strength is observed, i.e. the flow stress is reversible. These results suggest that the reversibility of flow stress anomaly is determined by the deformation mechanism occurring at the elevated temperature. If the deformation mechanism at the two temperatures is the same, then the flow stress is irreversible, but if they are different, then it is reversible.

Controlled generation of internal cracks in polymethyl methacrylate

Abstract A simple technique is presented for generating internal cracks in polymethyl methacrylate (PMMA) using laser irradiation. With precise control of the laser beam, a well-defined single crack can be produced in the matrix. The shape of the crack is flat and regular, close to an ellipse with nearly equal major and minor axes. The size and orientation of the crack can be controlled through the laser energy and the adjustment of the optical system. The laser-induced crack in PMMA consists of three regions: the central damage region caused by direct energy absorption, the intermediate cracking region, and the outer circumferential region associated with crazing. These controlled internal cracks can be used to study crack propagation, interaction, and related problems.

Effect of silver on superplastic deformation in YBa2Cu3O7-x/Ag composites

Superplastic deformation was studied in fine-grained (0.7-1. 1 μm) YBa 2 Cu 3 O 7 - x /Ag composites containing 2.5-25 vol% Ag. The compression tests were conducted in the temperature range of 750-875 °C and at strain rates of 10 - 5 to 10 - 3 /s. For the YBa 2 Cu 3 O 7 - x /25%Ag composites with grain size of 0.7-1.1 μm, deformed at 800-850 °C and 10 - 5 to 10 - 3 /s, the stress exponent, grain size exponent, and the activation energy of deformation were 2.0 ′ 0.1, 2.5 ′ 0.7, and 760 ′ 100 kJ/mol, respectively. These values were the same as those of the pure YBa 2 Cu 3 O 7 - x , indicating that the deformation of the composite was controlled by that of the rigid YBa 2 Cu 3 O 7 - x phase. However, the strain rate was increased by the addition of silver as explained by the soft inclusion model of Chen. The dependence of the flow stress on the silver content was in close agreement with the prediction of the model.

An experimental study of the interaction of internal cracks in PMMA

Internal cracks in polymethyl methacrylate (PMMA) specimens were generated by pulsed laser light. The interactions of coplanar cracks, parallel cracks, and cracks in T and H configurations were investigated. The tensile strength of specimens with a single internal crack decreased with increasing crack size, and the strength correlated well with the initial crack and unstable crack size in the Griffith relationship. For specimens with coplanar and parallel cracks, the strength increased and decreased with the crack distance, respectively. For the T and H crack configurations, the presence of delamination cracks decreased the strength, and the reduction in strength became more significant when the crack distance was small. All fracture surfaces showed similar fracture morphology in the sequence of laser-generated crack, smooth fracture mirror, mist with hyperbolic markings, and rough hackle region with rib markings. Examination of the fracture surfaces revealed crack arrest by the delamination cracks in both T and H configurations, and crack bowing between delamination cracks in the H configuration. The propagating crack was eventually able to circumvent the delamination cracks. The experimental results are compared with the available theoretical analyses, and the relevance of the present study to the toughening of brittle materials is discussed.

Observation of fracture features in fused quartz with laser-induced internal flaws

Abstract Pulsed-laser light was used to generate internal flaws in fused quartz. The size of the flaw was proportional to the laser pulse energy above a threshold value of 2.5 mJ. Specimens which fractured under four-point bending, either from surface origins or from the internal flaws with a size less than 0.25 mm, exhibited the commonly observed fracture features of sequential mirror, mist, hackle and branching, and the transition from the mirror to other features was gradual. However, in specimens with internal flaws of a size greater than 0.25 mm, a new “demarcation” line (a smoothly curved boundary line between the mirror and other fracture features) was observed. Its characteristic varied with the flaw size and the relative position along the line. It is suggested that the formation of the demarcation line is caused by the combination of the dynamic process of crack propagation and environmental effects.