This paper presents a review of the effect of crack tip cyclic plasticity on retarded crystallographic (stage I) crack growth in metal single crystals, bicrystals, engineering polycrystals and particulate-reinforced metal matrix composites (PMMC). The cyclic plastic deformation at the tip of a crystallographic crack, is shown to be affected by tortuous crack wake, adjacent grain boundary, or by the surrounding particles in a PMMC. Evidence gathered shows that change in the crack growth direction results from the redistribution of crack tip plastic strain. Similarly, the crack growth rate deceleration is related to the reduction of the cyclic plastic strain range due to the crack tip shielding. The role of the cyclic plasticity as a driving force in the shielded crystallographic crack growth, is demonstrated. The threshold for both the short and long crack growth corresponds to a critical loading condition which produces a persistent slip band of the characteristic microstructural size.
Blends of Poly (methyl methacrylate) (PMMA) and poly (α-methylstyrene-co-acrylonitrile) (αMSAN) exhibit an LCST behavior and are miscible at lower temperatures than the cloud points. The temperature dispersion of dynamic viscoelasticity of the blend system was measured in the single-phase and phase-separated states. It was found that some thermal histories during sample preparation bring the structural equilibrium to PMMA and αMSAN. Such a thermal condition was used for all the measurements to attain the experimental reproducibility. Temperature dispersion curves of dynamic viscoelastic functions, E', E" and tan δ, for the blends in single-phase and phase-separated cases located between those of PMMA and αMSAN in order of the component ratio. The shapes of the curves of phase-separated blends, however, become quite different from those of single-phase ones at temperatures above 80°C. The curves of E' and tanδ of blend samples in phase-separation show the broad relaxation and locate close to those of the rich component. Correspondingly, the peaks of the E" curves of phase-separated samples become broader than those for single-phase. Only the PMMA/αMSAN(40/60) blend in a phase-separated state shows two peaks on the E" curve when it was annealed at 200°C. These two peaks become more apparent with the annealing time suggesting the phase separation proceeds.
Alloying reaction in production of galvannealed steel was studied in unsteady-state diffusion conditions analogous to the industrial galvannealing process along with varying aluminum concentrations in the molten zinc bath. High bath aluminum concentration and rapid heating rate suppress the growth of not only the ζ phase, but also the Γ1 phase. The alloying temperature should be chosen in the vicinity of 773K in order to provide a coating predominantly composed of the δ1 phase. These results are discussed in accordance with the concept that the galvannealing process proceeds as a nonequilibrium zinc-iron reaction in an unsteady-state diffusion condition.
Directors and slip vectors are introduced to the theory of plasticity in order to take account of microstructures in crystalline materials in the range of the finite deformation. The notion of additive decomposition of the strain and curvature rate into elastic and plastic components is introduced in terms of directors and slip vectors, which represent the elastic and plastic microstructural change. The relation between the proposed theory and the couple stress theory of elasticity is discussed. The constitutive relation considering couple stress for plastic materials is also developed and an example of the rate-type constitutive equation is derived, where the yield condition depends on not only the usual Cauchy stress but also on the couple stress.
We have been investigating in the past the compaction behaviour of powder by particulate modeling in two-dimensions and three-dimensions. This model incorporates plastic deformation of particles at contact besides inter-particle friction, viscous force and gravity. An attempt is made in this study to perform simulation of three-dimensional compaction of powder, where powder is compressed with arbitrary strain rate ratios, and to examine its constitutive behavior, i.e. yield surface and the normality of strain rate vector to the surface. The simulated results show that the shape of the yield surface in principal stress space is a part of an ellipsoid whose axis coincides with hydrostatic stress axis; this qualitatively agree with experimental observations. The normality rule, on the other hand, does not necessarily hold, in particular in a stress state near uni-axial strain compaction.
In the present study, fatigue tests under axial, torsional and combined axial-torsional loading were conducted using the cylindrical specimen with circumferential blunt notch of Ti-6Al-4V titanium alloys. Two kinds of alloys with different microstructure, the (α+β) and the β alloys, were investigated in the fatigue tests. The fatigue life was correlated with the range of equivalent plastic strain. In both alloys, the fatigue life for the same equivalent plastic strain became longer as increasing shear component in the stress state at the notch root. The dominant surface cracks propagated in Mode I under the axial and combined loading in the two alloys. When the crack growth rate under the axial and combined loading was correlated with the range of J-integral, the result almost coincided with the growth relation for large through-thickness cracks in each material. Although the growth of Mode II type was predominant under the torsional loading, the growth direction of the main crack was the axial one in the (α+β) alloy, but the circumferential one in the β alloy. The cracking morphology depending on the microstructure, under the torsional loading, was simulated successfully by using a proposed model for crack initiation.
Reversed plane bending fatigue tests were carried out on three kinds of particle-reinforced aluminum matrix composites, Si/A2024, SiC/A2024 and SiC/AC4CH. SiC/A2024 and two kinds of Si/A2024 which have different size and distribution of Si particles were consolidated by Powder Metallurgy(PM) process, and SiC/AC4CH was made by casting process. The initiation and growth behavior of small surface cracks was continuously monitored by the replication technique and investigated in detail. It was found that both fatigue crack initiation strength and crack growth resistance became higher in the order, SiC/AC4CH, , Si/A2024 with coarse Si particle, Si/A2024 with fine Si particle, SiC/A2024. When stress amplitude was normalized by tensile strength, the difference in the fatigue strength of P/M composites was scarcely observed, but the casting composite exhibited lower strength than P/M one.
This paper describes the low cycle fatigue and creep-fatigue of HIPed Ti-48at%Al and Ti-50at%Al intermetallics at elevated temperature. Powders of the two kinds of intermetallics made by Plasma Rotating Electrode method were HIPed to form specimens. The two types of intermetallics were simply tensioned and fatigued with fast-fast, slow-fast and fast-slow strain waves. Both the intermetallics had almost the same tensile strength which did not decrease with increasing testing temperature up to 1073K. The Ti-48at%Al intermetallic had a superior fatigue strength in comparison with the Ti-50at%Al intermetafic under the three types of strain waves. Both of the intermetallics exhibited a superior fatigue strength to a cast TiAl intermetallic.
The influence of friction welding conditions on the mechanical-joint tensile and bend ductility -properties of as-welded dissimilar friction joints between pure titanium (Ti) and aluminum-magnesium alloy (A5083) base materials was investigated. The tensile strength increased when high friction pressure (≥200MPa) and high upsetting pressure (≥300MPa) were used during friction welding. Joint tensile strength also increased when friction time decreased, and the highest strength was obtained using the friction time of 0.5s. The higher joint strength was observed for the contacting surfaces polished using a buff than that prepared using No.240 emery paper.The use of high friction and upsetting pressure values and short friction times resulted in dissimilar joints which fractured in A5083 substrate during joint tensile testing. The joint bend ductility also increased when high friction pressures (≥200MPa) and a short friction time (0.5s) were employed during joining. It was concluded that the selection of joining parameters that promotes the formation of thin transition and intermetallic layers produces dissimilar joints with satisfactory mechanical properties.
The metallurgical properties of the interface regions of as-welded dissimilar friction joints between pure titanium (Ti) and aluminum-magnesium alloy (A5083) base materials were investigated. Hardness values in the region about Imm from the joint interface were higher than those in the bulk Ti and A5083 substrates. No transition layer was observed at the joint interface using optical microscopy, X-ray diffraction analysis and scanning electron microscope (SEM) including EDS-analysis. However, transmission electron microscope (TEM) observation of the joint interface region confirmed that intermetallic phases with thickness of 200nm or less were produced during joining. The intermetallic phases were estimated to be Mg2Al3, Al3Ti and τ-Al (Ti2Mg3Al18) by TEM-EDS analysis. It was concluded that the selection of joining parameters that promotes the formation of thin transition and intermetallic layers produces dissimilar joints with satisfactory mechanical properties.