The fracture characteristics of a superplastic single phase copper alloy

A superplastic single phase copper alloy exhibits a sigmoidal relationship between strain rate and stress at 823 K, dividing the behaviour into three regions. Maximum elongation to fracture (380%) occurs at intermediate strain rates at the lower end of region II, and there is a decrease in total elongation at both low (region I) and high (region III) strain rates. No necking is observed in regions I and II, and there is only very slight necking in region III. Internal cavities are formed at all strain rates, but the appearance of the cavities depends critically on the imposed strain rate. At high strain rates, the cavities are small and lie in strings parallel to the tensile axis; but as the strain rate is reduced the cavities become larger, more rounded, and essentially randomly distributed. The mode of failure is ductile rupture in region III, but void growth and interlinkage become increasingly important with decreasing strain rate.     

A microstructural examination of the flow behaviour of a superplastic copper alloy

There are important differences in the microstructures of specimens of a superplastic copper alloy deformed in the three regions of flow associated with superplasticity. There is very extensive dislocation activity at high strain rates in Region III, whereas at intermediate and low strain rates in Regions II and I the dislocation density is low and many of the grains appear to be dislocation-free. Measurements show that grain-boundary sliding is important in Region II but decreases in magnitude in the less superplastic Regions I and III.     

Localization of Strain Rate at Mesoscale and Its Effect on the Measured Strain Rate Sensitivity During Superplastic Deformation

Microstructural observations on specimens deformed in regions I, II and III of superplasticity suggest that the dislocation activity is restricted to limited grains in region I but spreads to almost all grains in the region III. Analysis has been carried out to show that the transition from the localized plastic flow in region I to relatively homogeneous plastic flow in region III significantly contributes to the high strain rate sensitivity observed in region II.     

Dependence of sub-critical crack growth on effective stress intensities

Sub-critical crack growth in high strength steels exhibit three regions of growth rates. In region I and III the growth is dependent on the stress intensity (computed for a sharp crack tip), while in region II the growth rate appears insensitive to the stress intensity. An investigation of the behavior in region II indicates that the insensitivity may be due to blunting of the crack tip resulting in a lower effective stress intensity. Based on experimental results a ratio for approximation of the effective stress intensity at the crack tip is proposed, and the cause of the anomalous behavior in region II is suggested.     

High strain rate superplasticity in powder metallurgy aluminium alloy 6061 + 20 vol.-%SiCpcomposite with relatively large particle size

Abstract

The possibility of high strain rate superplasticity (HSRS) was examined over a wide range of temperatures in a powder metallurgy aluminium alloy 6061/SiC_p composite with a relatively large SiC particle size of ~8 μm. A maximum tensile elongation of 350% was obtained at 600°C and 10^-2 s^-1. Tensile elongations over 200% were obtained in a narrow temperature range between 590 and 610°C at high strain rates of 10^-2 and 10^-1 s^-1. The current testing temperature range could be divided into two regions depending on the rate-controlling deformation mechanism. Region I is in the lower temperature range from 430 to 490°C, where lattice diffusion controlled dislocation climb creep (n = 5) is the rate-controlling deformation process, and region II is in the higher temperature range from 520 to 610°C, where lattice diffusion controlled grain boundary sliding controls the plastic flow. An abnormally large increase in activation energy was noted at temperatures above 590°C, where large tensile elonga tions over 200% were obtained at high strain rates. This increase in activation energy and high tensile ductility may be explained in terms of presence of a liquid phase created by partial melting, but such evidence could not be provided by the current differential scanning calorimetry (DSC) test. This may be because the DSC is not sensitive enough to detect the small amount of liquid phase.     

Superplasticity and associated activation energy in Ti-3Al-8V-6Cr-4Mo-4Zr Alloy

The high temperature deformation characteristics of a commercial β -titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr have been studied in the temperature range 830–925∘C. The alloy exhibited superplasticity in a narrow temperature and strain rate range i.e. 850–865∘C and 5× 10^{− 5}–3× 10^{− 3} s^{− 1} respectively, with a maximum elongation of 634% at 855∘C. The superplastic behaviour in the alloy is considered to arise as a result of subgrain formation at the higher strain rates (region III) which enhances diffusional creep at lower strain rates (region II). The activation energy values for regions II and III were found to be close to the lower of the two activation energy values (129.2 KJ/mole) proposed to describe self diffusion in β -phase suggesting that the rate controlling mechanism during high temperature deformation of the alloy was that for lattice diffusion.     

密度和应变率对聚苯乙烯泡沫(EPS)准静态压缩力学行为的影响

分别研究了聚苯乙烯泡沫(Expanded polystyrene,简称EPS)在三种不同密度和三种不同加载速率下的无侧限单轴准静态压缩力学行为.结果表明:EPS的压缩与一般多孔材料的压缩特征相似,其应力-应变曲线也分为三阶段(弹性段、塑性屈服平台段及致密段).并验证了聚苯乙烯泡沫(EPS)在线弹性阶段的弹性模量与其密度近似符合二次函数关系;通过对实验结果的拟合得出了EPS的密度与其屈服强度呈线性关系并给出了关系表达式.同时表明:同一密度的EPS在不同加载速率下其线弹性模量基本不变而屈服强度随加载速率的增加而显著增加,其应变率敏感度m值较大且变化显著,EPS表现出明显的应变率效应.     

Load controlled low cycle fatigue of [0]8 Sigma fibre reinforced titanium matrix composite

Fibre reinforced titanium matrix composites (TMCs) are being considered for use in future aeronautical gas-turbine compressor discs. Low cycle fatigue is thought to be one of the mechanisms most damaging to such a component. Here, the low cycle fatigue behaviour of Ti-6-4, reinforced with SM1140+ fibre, is investigated over the temperature range 22°C to 600°C. SN curves have a characteristic S shape and can be split into three regions. Fractography, acoustic emission monitoring and cyclic strain recording have elucidated damage mechanisms in each region. In region I (high cyclic stress) damage is caused by matrix creep, that leads to fibre failure. In region III (low cyclic stress), the predominant damage mechanism is matrix crack growth. Cracks initiate at surface machining damage and grow, bridged by intact fibres, into the bulk. The matrix crack growth transfers stress to fibres, eventually causing them to fail in overload, resulting in specimen failure. In region II (intermediate cyclic stress) damage is by a combination of the mechanisms observed in regions I and III. Comparison of Ti-6-4/SM1140+ with Ti-6-4/SCS-6 shows that fatigue lives are similar in regions II and III. In region I it is possible that Ti-6-4/SM1140+ has inferior lives to Ti-6-4/SCS-6.     

Inspection of intrinsic critical currents for spin-transfer magnetization switching

We examined the relationships between critical current, I/sub c/, and switching time, /spl tau//sub p/, for spin-transfer switching in two regions: (region I) /spl tau//sub p//spl Gt//spl tau//sub 0/, where thermal switching is accompanied and (region II) /spl tau//sub p/< several tens times /spl tau//sub 0/, where /spl tau//sub 0/ is the attempt time for thermal switching (/spl ap/1 ns). We estimated I/sub c0/, defined as the intrinsic I/sub c/ at 0 K, for both regions and confirmed experimentally that those I/sub c0/ coincided with each other at room temperature (RT). The value of I/sub c/ at /spl tau//sub p/=1 ns, measured with microwaves, was approximately 1.6 times the I/sub c0/. This suggested that we use at least two times I/sub c0/ as the writing currents of magnetic memory devices for nsec spin-transfer switching at RT. Although I/sub c0/ for both regions were defined as I/sub c/ at 0 K (I/sub c//sup 0K/) in theory, they showed temperature dependence at low temperatures; |I/sub c0/| for region I increased with decreasing temperature, and the estimated I/sub c//sup 0K/ was approximately three times I/sub c0/ for RT. This temperature dependence was quite different from that for region II.     

Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

混凝土/花岗岩界面Ⅰ-Ⅱ复合型动态断裂试验研究

为研究混凝土/岩石界面在复合型应力条件下的动态断裂性能,考虑四种应变率(10-5~10-2 s-1)及三种模态(21.8°,41.7°和60.3°)工况,对混凝土/花岗岩复合试件进行了四点剪切试验,获得了荷载与裂缝张开位移及裂缝剪切位移的关系曲线;结合界面力学理论和结构动力分析得到了界面Ⅰ型和Ⅱ型动态应力强度因子,据此得到并分析了断裂韧度、应变能释放率的率相关性及模态比相关性。结果表明:在所研究的应变率和模态角范围内,同一时刻的裂缝张开位移均大于裂缝剪切位移;Ⅰ型和Ⅱ型断裂韧度均随应变率的提高而增加,Ⅰ型断裂韧度随模态角的增大而减小,Ⅱ型断裂韧度随模态角的增大而增加;应变能释放率随应变率和模态角的增加均呈现出增长趋势。     

A multiscale approach for modeling crystalline solids

In this paper we present a modeling approach to bridge the atomistic with macroscopic scales in crystalline materials. The methodology combines identification and modeling of the controlling unit processes at microscopic level with the direct atomistic determination of fundamental material properties. These properties are computed using a many body Force Field derived from ab initio quantum-mechanical calculations. This approach is exercised to describe the mechanical response of high-purity Tantalum single crystals, including the effect of temperature and strain-rate on the hardening rate. The resulting atomistically informed model is found to capture salient features of the behavior of these crystals such as: the dependence of the initial yield point on temperature and strain rate; the presence of a marked stage I of easy glide, specially at low temperatures and high strain rates; the sharp onset of stage II hardening and its tendency to shift towards lower strains, and eventually disappear, as the temperature increases or the strain rate decreases; the parabolic stage II hardening at low strain rates or high temperatures; the stage II softening at high strain rates or low temperatures; the trend towards saturation at high strains; the temperature and strain-rate dependence of the saturation stress; and the orientation dependence of the hardening rate. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fatigue damage in nickel and copper single crystals at nanoscale

Nanoscale fatigue damage simulations using molecular dynamics were performed in nickel and copper single crystals. Cyclic stress–strain curves and fatigue crack growth were investigated using a middle-tension (MT) specimen with the lateral sides allowing periodic boundary conditions to simulate a small region of material as a part of a larger component. The specimen dimensions were in the range of nanometers, and the fatigue loading was strain controlled under constant and variable amplitude. Four crystal orientations, [111], [100], [110] and [101] were analyzed, and the results indicated that the plastic deformation and fatigue crack growth rates vary widely from one orientation to another. Under increasing strain amplitude loading, nickel nanocrystals experienced a large amount of plastic deformation causing at least in one orientation, [101], out-of-plane crack deviation in a mixed mode I+ II growth. Under constant amplitude loading, the fatigue cracks were a planar mode I type. Double slip is observed for some orientations, while for others, many more slip systems were activated causing a more evenly distributed plastic region around the crack tip. A comparative analysis revealed that small cracks grow more rapidly in copper than in nickel single crystals.     

THE INFLUENCE OF NOTCH PLASTICITY ON SHORT FATIGUE CRACK BEHAVIOUR

Abstract— A study has been made of fatigue crack formation and growth at the root of different notch profiles in a structural steel subjected to fully reversed tension-compression loading. The scale of stage I microstructural crack growth at notches decreased with increasing notch root strain and was comparable to the size of stage I cracks in shallow hourglass profile specimens at the same strain. Stage II crack growth rates were faster within the notch plastic field than in the elastic stress field of the bulk material.     

Microstructural evolution and threshold stress for superplastic flow in the Zn-Al eutectoid

The microstructural evolution and the stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy were investigated by prestraining specimens at two strain rates corresponding to Regions I and II. Even though the scale of microstructure was similar, the stress-strain rate curves of differently prestrained specimens were distinctly different in the lower strain-rate regime. While Region I of low rate sensitivity was more prominent when prestrained at a lower strain rate of Region I, it was less distinct because of prestrain in Region II. The threshold stress for superplastic flow, as assessed by an extrapolation procedure, varied with the nature of prestrain. The interphase boundaries were more rounded (higher mean curvature) on prestraining on Region II, compared to Region I. The correlation between the changes in the mean curvature of phase boundaries and the threshold stress arising from the nature of prestrain was consistent with the boundary-migration controlled sliding mechanism to interpret the threshold stress for superplastic flow.     

Effect of electroslag refining on the fracture toughness and fatigue crack propagation rates in heat treated AISI 4340 steel

The AISI 4340 steel has been electroslag refined and the improvement in mechanical properties has been assessed. Electroslag refining (ESR) has improved tensile ductility, plane strain fracture toughness, Charpy fracture energy, and has decreased fatigue crack growth rates. The K_IC values for the ESR steel are nearly twice those estimated in the unrefined steel and higher than those obtained in the vacuum arc remelted steel. Fatigue crack growth rates in region I and in region III are found to be decreased considerably in the ESR steel, while they are unaffected in region II. Measurements on heat treated samples have shown that the ESR steel has a better response to heat treatment. Both the suggested heat treatments namely austenitizing at 1140–1470 K as well as the conventional heat treatment of austenitizing at 1140 K have been followed. The improvement in the mechanical properties of ESR steel has been explained on the basis of removal of nonmetallic inclusions and reduction in sulfur content in the steel.     

Effect of aging on work hardening behaviour of cold rolled Nimonic C-263 alloy

Abstract

Experimental true stress–true strain data of Nimonic C-263 alloy in solution treated as well as aged condition have been analysed using different flow relationships. Ludwigson relationship provides the best fit of the data for all the conditions investigated. The transition in macroscopic flow behaviour of the alloy with plastic strain, in solution treated condition, can be correlated with the transition in deformation mode from low strain regime to high strain regime. Although aging does not appear to alter the macroscopic flow behaviour, it causes a considerable change in flow parameters of the Ludwigson relationship and substructural evolution. On the other hand, the effect of sheet thickness is marginal. The flow data of the aged alloys fitted according to Ludwigson model not only yield a unique set of flow parameters for each aging condition but also exhibit a systematic trend with aging time. The transition in macroscopic flow behaviour of the alloy with strain, in aged conditions, can be correlated with a change in dislocation mechanism from dislocation–precipitate interaction at lower strains to dislocation–dislocation interaction at higher strains leading to formation of a dense dislocation tangled networks in the matrix regions surrounding the precipitates. The alloy in both solution treated and aged conditions exhibits three fairly distinct stages of strain hardening. The strain hardening rate decreases in regime I, remains constant in regime II and begins to fall again in regime III. Furthermore, it is observed that the alloy specimen with longitudinal orientation (L, i.e. parallel to rolling direction), exhibits marginally highest strain hardening rates, while specimens with long transverse orientation exhibit lowest strain hardening rates both in solution treated and aged conditions. However, for all other in-plane orientations (i.e. L+30°, L+45° and L+60°), the strain hardening rate data are fairly very close and lie in between those of longitudinal and long transverse orientations.     

The near-tip field at high crack velocities

Several velocity regions, distinctly different as regards crack edge propagation characteristics, can be distinguished. The simplest case is mode III with only the subsonic and the supersonic regions. For modes I and II four different regions can be recognized. When analyzing the near-tip field at leading and trailing edges it is found that some velocity regions are forbidden. The most important field characteristic is the energy flow to or from the edge. A clear difference exists between modes I and II: for mode I the whole region between Rayleigh and irrotational wave velocities is forbidden, for mode II only the subsonic super-Rayleigh region.In attempts to provoke crack edge propagation at a velocity in a forbidden region, the result appears to be edge propagation at velocities alternating between velocities in non-forbidden regions, above and below the attempted velocity.A study of the stresses ahead of the edge region of a mode II crack expanding in both directions indicates that the edge might accelerate (by a jump) spontaneously from a sub-Rayleigh to an intersonic velocity.

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Résumé Selon les caractéristiques de propagation du bord d'une fissure, on peut distinguer divers registres distincts de vitesses de fissuration. Le cas le plus simple est celui du Mode III, où l'on ne rencontre que les vitesses subsoniques et supersoniques. Les Modes I et II conduisent à distinguer quatre vitesses différentes. Lorsqu'on analyse le champ du voisinage de l'extrémité de la fissure en ses bords moteurs, on trouve que certaines vitesses sont proscrites. La caractéristique la plus importante du champ est le transfert d'énergie depuis ou vers le bord. Il existe une distinction claire entre les Modes I et II: en Mode I, toute la gamme comprise entre les vitesses des ondes de Rayleigh et d'une onde irrotationnelle sont prescrites; en Mode II, seule est proscrite la gamme subsonique supérieure à l'onde de Rayleigh.Lorsqu'on tente de provoquer une propagation des bords d'une fissure à une vitesse dans une gamme proscrite, le résultat semble être une propagation de bord à des vitesses oscillant entre deux gammes non proscrites, de part et d'autre de la vitesse visée.En étudiant les contraintes en avant du bord d'une fissure de Mode II se développant selon les deux directions imposées, on constate que le bord peut spontanément s'accélérer suivant un ressaut, en passant d'une vitesse inférieure à une onde de Rayleigh à une une vitesse transonique.