Yang Leng

Shape instabilities of plate-like structures—I. Experimental observations in heavily cold worked in situ composites

Abstract The capillary shape instabilities associated with ribbon or finite plate dispersoids are described and documented with experimental evidence from three composite systems: (1) drawn castings of Cuue5f8Fe containing ribboned Fe fibers (14.3 vol.% Fe), (2) rolled liquid-phase sintered Niue5f8W alloys containing elliptic W plates (48 vol.% W), (3) rolled “hypoeutectic” Niue5f8W alloys containing W blades which occupy 17% of the eutectic microconstituents volume. The primary instabilities observed include cylinderization via edge recession, boundary splitting via the thermal grooving of internal boundaries, and edge spheroidization via the ovulation of ridges formed by edge recession. For a given plate-shaped dispersoid, the dominant instability is determined by cross sectional aspect ratio (width/thickness) and, when internal boundaries are present, the ratio of the internal boundarys energy to that of the phase interface.

Multiple shear band formation in metallic glasses in composites

Multiple shear banding is observed in metallic glasses during tensile deformation of laminated composites containing such glasses. The phenomenon is related to (1) the local stress concentration that develops as a result of the formation of the first shear band, (2) the distribution in stress required to initiate shear banding in tensile loading, and (3) the properties of the surrounding matrix. The tendency for localized and uniform multiple shear banding has been determined. This was done by utilizing a finite element method (FEM) to simulate the local stress state in the vicinity of the shear band first formed, and by determining the distribution in shear band initiation strengths. The experimental data were combined with the FEM analysis to “predict” locations of secondary shear band initiation. Localized secondary banding is predicted for large initial slip displacements, whereas uniform banding is expected when the initial slip displacement is small.

Fracture Behavior of Laminated Metal Metallic Glass Composites

The crack growth behavior of metallic glass in laminated metal-metallic glass composites was investigated and compared to the crack growth characteristics of monolithic metallic glass. The composite arrangement significantly increases the crack growth resistance of the glass. Growth in the monolithic glass is catastrophic, whereas in the composite, it is stable. The behavior is described in terms of crack growth resistance(R) curves and discussed in terms of extrinsic and intrinsic contributions to toughness. It is found that an extrinsic factor,i.e., matrix bridging, makes the major contribution to increased crack growth resistance and that a limiting crack opening displacement model interprets the experimental data quite well. Enhanced glass deformation in the crack tip region, manifested by multiple shear band formation, is responsible for the intrinsic toughening observed. Physical models are developed to estimate the level of intrinsic toughening due to this effect.

Some tensile properties of metal-metallic glass laminates

The tensile properties of brass (Cu-30% Zn)-nickel base metallic glass (MBF-35 Metglas) laminates have been investigated. Laminates were prepared by soldering these constituents together with a Pb−Sn alloy. The metallic glass exhibited an enhanced tensile ductility in the metal matrix environments, and such enhanced ductility depended on the metal matrix strength and elongation. Multiple shear bands have been observed in the metallic glass ribbon with enhanced tensile ductility. The failure modes of the laminates have been analysed based on stress-strain concentration concepts, following failure of the glass. The results were consistent with the experimental observations.

Tensile deformation of 2618 and AlFeSiV aluminum alloys at elevated temperatures

The present study experimentally characterizes the effects of elevated temperature on the uniaxial tensile behavior of ingot metallurgy 2618 Al alloy and the rapidly solidified FVS 0812 P/M alloy by means of two constitutive formulations: the Ramberg/Osgood equation and the Bodner-Partom (1975) incremental formulation for uniaxial tensile loading. The elastoplastic strain-hardening behavior of the ingot metallurgy alloy is equally well represented by either formulation. Both alloys deform similarly under decreasing load after only 1-5 percent uniform tensile strain, a response which is not described by either constitutive relation.

Elevated temperature cracking of RSP aluminum alloy 8009: Characterization of the environmental influence

Degradation of the material properties of the Al alloy is examined to determine the effects of moist air and predissolved hydrogen on elevated-temperature fatigue and fracture resistance. Experiments are conducted at 175 C in both moist air and high vacuum with as-processed specimens and specimens that are vacuum-heat-treated. Fracture mechanics characterizations are made for initiation and propagation fracture toughnesses during rising load, fatigue-crack propagation kinetics, and sustained-load crack-growth rates. Time-dependent embrittlement at intermediate temperatures is identified in both plate and extrusion samples of the Al-Fe-Si-V alloy 8009. Intermediate temperature cracking is found to be the same for each case in both vacuum and moist air, and the vacuum heat treatment does not significantly affect the results.

Study of creep crack growth in 2618 and 8009 aluminum alloys

Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower thanKic; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower thanKic. Alloys 8009 and 2618 exhibit creep brittle behavior,i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with the stress intensity factor,K, and theJ integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also, no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between the creep crack growth rate and stress intensity factor,K.

Ineffective lengths in metal matrix composites

Abstract A finite element model (FEM) analysis has been conducted to evaluate ineffective lengths in metal matrix composites. The results are compared with those of a modified analytical treatment based on a previous analysis of Rosen. Ineffective lengths obtained by both treatments are in good agreement, but differ considerably from those predicted by Rosens original formulation. The influence of different boundary loading conditions at the fiber end on the efficiency of stress transfer to the fiber has also been investigated by the FEM. In addition, an elastic-plastic FEM analysis has been used to study the influence of matrix plastic deformation on ineffective lengths. As is also discussed, the FEM approach may be useful for defining criteria for delamination at the fiber-matrix interface.

Analytical estimation of asperity-induced crack closure

The concept of crack closure has been used for over twenty years to explain the characteristics of fatigue crack growth. It is generally accepted that crack closure reduces the applied stress intensity amplitude, and is particularly significant at the near threshold regimes. The asperity-induced crack closure has been considered as one of the main closure mechanisms, since roughness-induced and oxide-induced closures can be treated as special cases of it. Vasudevan, Sadananda and Louat have reviewed crack closure effects and concluded that the closure effect is much less than that usually believed. They argued that the asperity-induced closure was only about one quarter of that computed from the slope change point on load-displacement curves. The argument was based on their theoretical evaluation using a dislocation crack model in which a sharp asperity was located near the crack tip. Note that Louat et al. have only evaluated an extreme case in which the asperity width can be ignored. In this work, the authors evaluate the crack closure when the asperity thickness varies from a point to finite values, in order to obtain more realistic estimation of asperity-induced closure during fatigue crack propagation.