MODELLING THE INFLUENCE OF STRAIN HARDENING AND PLASTIC CONSTRAINT ON CRACK CLOSURE OF ARBITRARILY THICK CCT-SPECIMENS

The triaxial stress constraint and the effective yield stress distribution in the plastic zone for strain hardening materials are studied, and then a modified strip-yield model is proposed to investigate the thickness effect of CCT specimens. Consequently, a plastic constraint factor α is defined and analysed in detail. The results show that the factor α can comprehensively account for the influence of thickness, crack length, loading level and hardening exponent. A simple expression for the plastic zone length and a fitting expression involving α are obtained. Application of the modified strip model to Newman’s crack closure model, and comparison with FEM results, show that the model can account for the influence of thickness on crack closure.     

Self-healing behavior of strain hardening cementitious composites incorporating local waste materials

The self-healing behavior of a series of pre-cracked fiber reinforced strain hardening cementitious composites incorporating blast furnace slag (BFS) and limestone powder (LP) with relatively high water/binder ratio is investigated in this paper, focusing on the recovery of its deflection capacity. Four-point bending tests are used to precrack the beam at 28 days. For specimens submerged in water the deflection capacity can recover about 65–105% from virgin specimens, which is significantly higher compared with specimens cured in air. Similar conclusion applies to the stiffness recovery in water cured specimens. The observations under ESEM and XEDS confirmed that the microcracks in the specimens submerged in water were healed with significant amount of calcium carbonate, very likely due to the continuous hydration of cementitious materials. The self-healing cementitious composites developed in this research can potentially reduce or even eliminate the maintenance needs of civil infrastructure, especially when repeatable high deformation capacity is desirable, e.g. bridge deck link slabs and jointless pavements.     

Effect of loading rate on crack velocities in HSC

This paper presents the recent results of an experimental program aimed at disclosing the loading rate (loading-point-displacement rate) effect on the crack velocity in high-strength concrete (HSC). Eighteen three-point-bend tests were conducted using either a servo-hydraulic machine or a self-designed drop-weight impact device. Four strain gauges mounted along the ligament of the specimen were used to measure the crack velocity. Six different loading rates were applied, from 10⁻⁴ mm/s to 10³ mm/s (average strain rate from 10⁻⁶ to 10⁻¹ s⁻¹), i.e., a low loading-rate range (5.50 × 10⁻⁴ mm/s, 0.55 mm/s and 17.4 mm/s) and a high loading-rate range (8.81 × 10² mm/s, 1.76 × 10³ mm/s and 2.64 × 10³ mm/s). At low loading rates, the crack propagates with increasing velocity. Under high loading rates, the crack propagates with slightly decreasing velocity, though the maximum crack speed reached up to 20.6% of the Rayleigh wave speed of the tested HSC. In addition, the loading-rate effect on crack velocities is pronounced within the low loading-rate regime, whereas it is minor under the high loading-rate range.     

Low-cycle fatigue model of damage accumulation - The strain approach

The paper presents a model of damage accumulation designed to analyse fatigue life of structural elements exploited in multiaxial, non-proportional, low-cycle loading conditions. The discussed approach consists of two calculation blocs. In the first bloc the components of stress and strain tensor are determined. This module, in which Mroz’s multisurface model was used, contains constitutive relations and the law of kinematic hardening. The second bloc contains the dependencies which determine the growth of anisotropic measure of damage accumulation (associated with the physical plane) and crack initiation criterion. The growth of the damage accumulation measure was associated with the loading damage accumulation function and the increment of non-dilatational plastic strain on the physical plane. It was assumed that crack initiation occurs when stress or a measure of damage accumulation on any physical plane reaches critical values.     

Characterization of Tensile Strain Hardening Behaviors for 32Mn-7Cr-1Mo-0.3N Cryogenic Austenitic Steel

The strain hardening behaviors of 32Mn-TCr-1Mo-0.3N austenitic steel were characterized by a simple and effective method. The results show that Hollomon relationship is not applicable during total uniform deformation stage. The flow equation was proposed, Inσ=αexp（lnε/b）＋c. The variation rates of strain hardening exponents with true strain at 77 K are obviously higher than that at other temperatures and the value of d^2σ/dε^2 becomes positive during high strain stage. The characters of this variation are principal reasons for increasing elongation at 77 K. The forming of mechanical twin as well as ε-martensite leads to a high elongation at 77 K.     

一种新的钝化线形式

分析了裂纹钝化过程和J积分测试中的钝化线形式。研究认为,对于高韧性金属,现行J积分测试标准对有效数据的选取,因规定的钝化线斜率太低而误判率较高。因此建议,对钝化过程的描述应更多地考虑形变强化的贡献。在此基础上,给出了一种新的钝化线形式,即J=2K(0.5ε)n·Δa,该方程与实测结果吻合较好。     

Strain hardening and orientation hardening/softening in cold rolled AA 5052 aluminum alloy

The tensile properties of cold rolled sheets were measured for the hot band and annealed hot band of AA 5052 aluminum alloy. The variation in yield strength with rolling true strain was used to represent the hardening rate of cold rolled sheets. The Taylor factor (M?) of cold rolled sheets in tension along the rolling direction was calculated based on the measured orientation distribution functions. The strain hardening and orientation hardening/softening produced by cold deformation were analyzed. The results show that the contribution to the hardening rate of cold rolled sheets comes largely from the deformed microstructure and partly from the texture change. For the annealed hot band the orientation softening occurs at strains below 0.5, while the orientation hardening occurs at strains over 0.5. For the hot band the dM?/dε value is always positive, indicating that orientation softening does not occur.     

Creep behaviour and creep microstructures of a high-temperature titanium alloy Ti–5.8Al–4.0Sn–3.5Zr–0.7Nb–0.35Si–0.06C (Timetal 834): Part I. Primary and steady-state creep

The tensile creep behaviour of the high-temperature near -Ti alloy Ti–5.8Al–4.0Sn–3.5Zr–0.7Nb–0.35Si–0.06C (Timetal 834) with a duplex microstructure has been extensively investigated in the temperature range from 500°C to 625°C and the stress range from 100 to 550 MPa. Both primary and secondary creep are being considered. The results of the primary creep are analysed in terms of the dependencies of stress on strain (strain hardening) and on strain rate (strain rate sensitivity). It is shown that the strain-hardening exponent depends on temperature, and takes values between 0.5 for 500°C and 0.33 for higher temperatures; this would give a dependence of the primary creep strain of σ² and σ³. The strain rate exponents obtained in both primary and secondary creep have been found to be similar; this is also the case for the activation energies. It is thought that, in the stress and temperature range investigated, creep is controlled by bow-out and climb of dislocation segments pinned at lath boundaries and second-phase particle. Analysis of the dislocation substructure is presented to give some support for this mechanism.     

静电除尘器星形线材的组织与性能

研究了国产仿YUS110及进口SUS316L两类静电除尘器不锈钢星形线材的显微组织、静拉伸性能与疲劳特性.结果表明,两类线材的屈服强度相同,但进口线材的疲劳强度明显高于国产线材;进口线材的疲劳扩展辉纹较国产线材明显细密,且二次裂纹较多较深.分析认为这与两类星形线材的材质及其成形加工造成的形变强化效应有直接关系.     

不偶合装药爆破的试验研究

在试验的基础上,探讨了不偶合装药爆破效果与不偶合系数之间的关系。试验中发现,当不偶合系数为2．0时,平均块度与偶合装药时几乎完全相同,面粉矿率及大块率则较小。当不偶合系数大于等于2．0时,随着不偶合系数的增大,粉矿率减小,大块率上升．应变波的测试结果表明,随着不偶合系数的减小,应变波幅值逐渐增大．通过裂缝扩展试验,发现裂缝扩展速度随着不偶合系数的增大逐渐减小．以上表明,爆破效果最终由应力波和爆生气体两者的共同作用决定．     

Effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel

The effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel have been investigated. Yield strength is reduced by sensitisation, but ultimate tensile strength is nearly unaffected. Strain-hardening behaviour is changed by sensitisation, too. Although sensitisation may induce martensite formation near grain boundary, twin boundary, and austenite/martensite interface, the sensitisation-induced martensite does not exert a great influence on tensile behaviour in the 304 steel. In the unsensitised condition, martensitic transformation in the steel bulk induced by prior deformation and liquid-nitrogen immersion also does not change strain-hardening behaviour in the present steel.     

Comparison between the Methods of Determining the Critical Stress for Initiation of Dynamic Recrystallization in 316 Stainless Steel

Several methods have been proposed to calculate the critical stress for initiation of dynamic recrystallization （σc） on the basis of mathematical methods. One＇ of them is proposed by Stewart et al. in which this critical point appears as a distinct minimum in the （-dθ/dσ vs σ） through differentiating from θ vs σ. Another one is presented by Najafizadeh and Jonas by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σc was attained numerically from the coefficients of the third-order equation that was the best fit from the experimental θ-σ data. Hot compression tests were used in the range of 1000 to 1100℃ with strain rates of 0.01^-1 s^-1 and strain of I on 316 stainless steel. The result shows that Najafizadeh and Jonas method is simpler than the previous one, and has a good agreement with microstructures. Furthermore, the value of normalized critical stress for this steel was obtained uc=σc/σp=0.92.     

Strain hardening and softening in a nanocrystalline Ni-Fe alloy induced by severe plastic deformation

The strain response of an electrochemically deposited nanocrystalline Ni-20 wt.% Fe alloy processed by high-pressure torsion (HPT) was investigated by monitoring changes in hardness. Strain hardening was observed in the very early stage of HPT, followed by strain softening before the onset of a second strain hardening stage. Structural investigations revealed that the two hardening stages were associated with an increase in dislocation density, whereas the strain softening stage was accompanied by a reduction in the dislocation and twin densities, thereby demonstrating the main dependence of hardness on the dislocation density in this material. Grain growth occurred during HPT and its role in the hardness evolution is also discussed.     

4t/h锅炉简体开裂分析

对运行两年即产生开裂的4t／h锅炉筒体进行了化学分析、金相检验、超声波测厚及力学性能检测。检验结果表明,水质不良引起应力腐蚀开裂是造成筒体产生裂纹的主要原因,提出了改进意见。     

超高韧性水泥基复合材料多缝开裂特性及其自生愈合

应变硬化水泥基复合材料(Strain hardening cement-based composites,SHCC)是超高性能水泥基材料的一种。通过三点弯曲加载试验,分别对普通砂浆和SHCC试件诱导开裂,对两种材料的开裂特性及其裂缝分布规律进行了研究。结果表明,SHCC材料试件与普通水泥砂浆试件相比具有更高的承载力和应变能力,初始开裂荷载约为普通砂浆试件的2.5倍。SHCC材料中的PVA纤维的桥接作用有效延迟了裂缝的产生与扩展,将普通砂浆试件中数量少而大的裂缝转化为多而细的微小裂缝,呈现多缝开裂特性,产生的裂缝宽度符合正态分布规律,且90%的微小裂缝小于30μm。这些微小裂缝在潮湿环境中产生一定程度的自生愈合,在水中的愈合程度和速度均高于在潮湿空气中。     

Crack initiation and growth path under multiaxial fatigue loading in structural steels

In real engineering components and structures many accidental failures occur due to unexpected or additional loadings, such as additional bending or torsion. There are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path. It is widely believed that fatigue crack nucleation and early crack growth are caused by cyclic plasticity. This paper studies the effects of multiaxial loading paths on the cyclic deformation behaviour, crack initiation and crack path. Three types of structural steels are studied: Ck45, medium carbon steel, 42CrMo4, low alloy steel and the AISI 303 stainless steel. Four biaxial loading paths were applied in the tests to observe the effects of multiaxial loading paths on the additional hardening, fatigue crack initiation and crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It is shown that these materials have different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were conducted using the critical plane approaches, either based on stress analysis or strain analysis (Findley, Smith–Watson–Topper, Fatemi–Socie, Wang–Brown–Miller, etc). Comparisons of the predicted crack orientation based on the critical plane approaches with the experimental observations for the wide range of loading paths and the three structural materials are shown and discussed. Results show the applicability of the critical plane approaches to predict the fatigue life and crack initial orientation in structural steels.