Uniaxial ratcheting and fatigue failure of tempered 42CrMo steel: Damage evolution and damage-coupled visco-plastic constitutive model

Uniaxial ratcheting and fatigue failure of tempered 42CrMo steel were observed by the tests under the uniaxial stress-controlled cyclic loading with non-zero mean stress [G.Z. Kang, Y.J. Liu, Mater. Sci. Eng. A 472 (2008) 258–268]. Based on the obtained experimental results, the evolution features of whole-life ratcheting behavior and low-cycle fatigue (LCF) damage of the material were discussed first. Then, in the framework of unified visco-plasticity and continuum damage mechanics, a damage-coupled visco-plastic cyclic constitutive model was proposed to simulate the whole-life ratcheting and predict the fatigue failure life of the material presented in the uniaxial stress cycling with non-zero mean stress. In the proposed model, the damage was divided into two parts, i.e., elastic damage and plastic damage, which were described by the evolution equations with the same form but different constants, since the maximum applied stresses in most of loading cases were lower than the nominal yielding strength of the material. The ratcheting of the material was still described by employing a nonlinear kinematic hardening rule based on the Abdel-Karim–Ohno combined kinematic hardening model [M. Abdel Karim, N. Ohno, Int. J. Plast. 16 (2000) 225–240] but extended by considering the effect of damage. The maximum strain criterion combined with an elastic damage threshold was employed to determine the failure life of the material caused by two different failure modes, i.e., fatigue failure (caused by low-cycle fatigue due to plastic shakedown) and ductile failure (caused by large ratcheting strain). The simulated whole-life ratcheting behavior and predicted failure life of tempered 42CrMo steel are in a fairly good agreement with the experimental ones.     

The torsional fatigue properties of titanium-alloy spring wire

The absence of any torsional fatigue-test data on currently used titanium-alloy spring wire along with inconsistent fatigue-test data derived from earlier test programs prompted this investigation. The torsional fatigue testing was performed on straight lengths of 13V?11Cr?3Al titanium-alloy spring wire in diameters of 0.148 in. (0.376 cm), 0.225 in. (0.572 cm) and 0.374 in. (0.950 cm) considered as representative. A second material selected as a possible alternate for 13V?11Cr?3Al spring was 3Al?8V?6Cr?4Mo?4Zr which was tested in diameters of 0.225 in. (0.572 cm) and 0.376 in. (0.955 cm). Testing as straight-wire lengths eliminated the manufacturing variables introduced by fabricating a helical-spring geometry. Two methods were considered for improving fatigue life, namely abrasive cleaning and shot peening. At a torsional stress of 100 ksi (689.5 MPa), the test data indicate that an improvement in fatigue life of at least one order of magnitude may be realized by shot peening 0.225-in. (0.572-cm)-diam 13V?11Cr?3Al wire to an intensity of 0.015A. A similar improvement of fatigue life may be gained by shot peening 0.148-in. (0.376-cm)-diam wire to a similar intensity. A substantial fatigue-life improvement was obtained by an obrasive-cleaning operation on all wire diameters tested. The fatigue life of the 3Al?8V?6Cr?4Mo?4Zr titanium-alloy wire in the cold-worked and aged condition was lower at all stress levels and diameters tested than for similar diameters of 13V?11Cr?3Al titanium-alloy wire.     

A numerical study on the deformation and fracture modes of steel projectiles during Taylor bar impact tests

A heterogeneous material model based on macro-mechanical observations is proposed for simulation of fracture in steel projectiles during impact. A previous experimental study on the deformation and fracture of steel projectiles during Taylor bar impact tests resulted in a variety of failure modes. The accompanying material investigation showed that the materials used in the impact tests were heterogeneous on scales ranging from microstructure as investigated with SEM to variation in fracture strains from tensile tests. A normal distribution is employed to achieve a heterogeneous numerical model with respect to the fracture properties. The proposed material model is calibrated based on the tensile tests, and then used to independently simulate the Taylor bar impact tests. A preliminary investigation showed that the simulations are sensitive to assumptions regarding the anvil behaviour and friction properties. A flexible anvil and a yield-limited friction law are shown to be necessary to correctly reproduce the experimental behaviour. The proposed model is then shown to be capable of correctly reproducing all fracture modes but one, and also predict critical impact velocities for projectile fracture with reasonable accuracy. Fragmentation at velocities above the critical velocity is not well reproduced due to excessive element erosion. Measures to make the element erosion process more physical are proposed and discussed with their respective drawbacks. The use of a simple fracture criterion in combination with an element erosion technique accentuates the effect of distributing the fracture parameter.     

Nonequilibrium statistical foundation of fatigue fracture

The physical foundation of statistical law of fatigue fracture is discussed. The universal forms of the microcrack growth rate, fluctuation growth coefficient and distribution function and fatigue life distribution function have been given. The project is supported by the National Natural Science Foundation of China     

Mesomechanics of fatigue fracture for polycrystals with macroconcentrators

Fatigue fracture of aluminum alloys is analyzed from analysis of time-spatial mesoscale substructures. The behavior involves many stages at the mesoscale and translational–rotational movement of the bulk structure element in addition to fragmentation. Mesofragments on a trajectory of the fatigue crack were also studied.     

The shear fracture of dual-phase steel

Unexpected fractures at high-curvature die radii in sheet forming operations limit the adoption of advanced high strength steels (AHSS) that otherwise offer remarkable combinations of high strength and tensile ductility. Identified as “shear fractures” or “shear failures,” these often show little sign of through-thickness localization and are not predicted by standard industrial simulations and forming limit diagrams. To understand the origins of shear failure and improve its prediction, a new displacement-controlled draw-bending test was developed, carried out, and simulated using a coupled thermo-mechanical finite element model. The model incorporates 3D solid elements and a novel constitutive law taking into account the effects of strain, strain rate, and temperature on flow stress. The simulation results were compared with companion draw-bend tests for three grades of dual-phase (DP) steel over a range of process conditions. Shear failures were accurately predicted without resorting to damage mechanics, but less satisfactorily for DP 980 steel. Deformation-induced heating has a dominant effect on the occurrence of shear failure in these alloys because of the large energy dissipated and the sensitivity of strain hardening to temperature increases of the order of 75 °C. Isothermal simulations greatly overestimated the formability and the critical bending ratio for shear failures, thus accounting for the dominant effect leading to the inability of current industrial methods to predict forming performance accurately. Use of shell elements (similar to industrial practice) contributes to the prediction error, and fracture (as opposed to strain localization) contributes for higher-strength alloys, particularly for transverse direction tests. The results illustrate the pitfall of using low-rate, isothermal, small-curvature forming limit measurements and simulations to predict the failure of high-rate, quasi-adiabatic, large-curvature industrial forming operations of AHSS.     

16Mn钢在不同条件下的疲劳行为研究

对截面为3 mm×3mm的16Mn钢试件在空气和3.5%NaCl溶液中分别进行疲劳试验,获得了S-N曲线,并对疲劳试样表面和断口形貌进行了观察.结果表明:与空气相比,3.5%NaCl腐蚀溶液使16Mn钢的疲劳强度显著降低;在空气中疲劳试样只有一个萌生于试样表面基体的裂纹源,而在3.5%NaCl溶液中一般有多个裂纹源,而...     

Applied research on rail fatigue and fracture in the United States

An overview is presented of the U.S. rail fatigue and fracture research program. This program is a joint government/industry effort to establish a fracture control plan for U.S. freight railroad track. Results of the first two phases of basic investigation are summarized and current progress on the third phase is reported. Phase three is devoted to applied fracture mechanics, involving experiments to measure the rate of growth of actual rail cracks and attempts to organize the results as empirical life-estimation procedures.     

45钢的J-C损伤失效参量研究

为了在结构碰撞效应的有限元分析中描述材料行为,通过开展45钢在不同应力状态和温度下的准静态材料力学性能实验及拉伸SHB实验,考察了应力状态三轴度、温度和应变率对材料失效应变的影响。由实验数据得到了Johnson-Cook失效模型参量,并通过出现失效的Taylor撞击实验和数值模拟进行了一定的验证,表明模型描述与实验结果的趋势一致。     

Recent advances in notch analysis of fracture and fatigue

Summary Neuber's micro- and macrosupport effect theories are applied to obtain on expression for the plastic work required for crack extension. On this basis analytical predictions for fracture toughness, thickness effects and fatigue crack propagation threshold are made and found in very good agreement with experimental results.

---

Übersicht Mit Hilfe der Neuberschen Theorie der Mikro- und Makrostützwirkung wird eine allgemeine Formulierung für die Rißerweiterungskraft bei nichtlinearem Werkstoffverhalten hergeleitet. Darauf aufbauend werden die Rißzähigkeit, Dickeneffekte und Rißstop bei Schwingbeanspruchung theoretisch behandelt und eine sehr gute Übereinstimmung mit Versuchsergebnissen festgestellt.

---

---

Dedicated to o. Prof. Dr.-Ing. Dr. rer. nat. h. c. H. Neuber.     

Brush-like modes of fatigue fracture in unidirectional fibre composites

Fatigue fracture of unidirectional fibre composites under tension along the fibres is discussed with account of the interaction between various mechanisms of damage such as single and multiple fibre ruptures, matrix cracking, and matrix-fibre debonding. The case of brittle fibres and a comparatively weak and ductile matrix is considered that exposes non-conventional modes of fracture, named “brush-like” cracks. Growth of such cracks under cyclic quasistatic loading is studied, and the effect of various factors on the crack growth rate is investigated.     

Mesoscale fatigue failure of welded joints of low-alloy steel

The fatigue failure kinetics for welded joints of low-alloyed steel is investigated by analyzing the displacement vector field under high-cyclic loading. Failure accumulation in the heat-affected zone at the mesoscale is found to possess a multi-stage character that is associated with the formation and evolution of mesostructure deformation and fragmentation. Identified are the pertinent parameters that described the mechanical state of welded joints under load.     

Transient stress of steel phase transformation and transformation-induced fatigue

Expansion changes the size of any machine element and can thus generate severe stress or distortion failures during its operation if this phenomenon is not taken into account during the design process. On the other hand, most steels contract during heating within a short temperature range as a result of microstructural phase transformations. This transient contraction may generate further short- and long-term dysfunction of a metallic structure. The current paper focuses on this issue; investigating the effects of the microstructural transformation on the transient stress response of a metallic structure. Analysis is carried out by means of finite element simulation employing material data from typical constructional steel. The paper emphasizes the necessity of considering material phase changes when dealing with the transient stress or displacement response of a metallic structure and demonstrates the differences obtained when the transformation effects are ignored or accounted for in the design process. The mechanism of transformation-induced fatigue is thus discussed and proposed to the engineering community for establishment.     

Low-cycle fatigue loading of BDS 25G pipeline steel

The behavior of BDS 25G pipeline steel, is studied for constant amplitude low-cycle fatigue. The same is done for the weld metal. Results are compared for temperatures of 20°C and 300°C, and tabulated for the estimate of different fatigue parameters.     

加载速率对船用钢断裂韧性的影响

在加载速率为100～106MPam1/2/s的范围内,分别采用准静态、示波冲击、Hopkinson杆型试验装置对某船用钢进行了断裂韧性测试。试验结果表明,此钢的断裂韧性对加载速率敏感,即随着加载速率的升高,断裂韧性下降;应用位错动力学对实验现象作出了解释。     

Numerical study of a model of fatigue wear and fracture

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 134–141, May–June, 1991.     

拉伸应变率对20钢层裂特性的影响

采用轻气炮加载技术和激光速度干涉(VISAR)测速技术相结合,对不同拉伸应变率条件下20钢的层裂特性进行了实验研究。通过改变飞片和样品的几何尺寸来调整拉伸应变率的大小,研究了拉伸应变率对20钢层裂强度的影响。实验的拉伸应变率的变化范围为104~106 s-1,最大拉伸应变率接近激光加载所能产生的拉伸应变率,相比激光加载,薄飞片技术容易保证一维应变条件。实验结果显示20钢的层裂特性明显依赖着拉伸应变率的大小,106 s-1条件下层裂强度比104 s-1时提高近70%。基于对数值计算结果的分析,讨论了影响层裂强度的主要外载荷因素。     

含钢率对GFRP管-钢骨混凝土组合构件抗冲击性能的影响

为了研究含钢率对GFRP（glass fiber reinforced polymer）管-钢骨混凝土组合构件抗冲击性能的影响,建立15个组合构件的数值模型,在验证模型正确的基础上,通过分析典型构件的冲击全过程、截面弯矩发展以及破坏时应变分布规律,研究构件在侧向冲击荷载作用下的破坏模式;通过分析构件冲击力时程曲线、侧移时程曲线以及能量变化情况,探究含钢率对不同长细比构件抗冲击性能的影响。结果表明：与未配置钢骨的构件相比,GFRP管-钢骨混凝土构件的抗冲击承载力提高了7%~134%,侧向位移减小了13%~68%。在侧向冲击荷载作用下,构件的破坏模式以弯曲破坏为主,同时伴随着GFRP管和混凝土冲击区域的局部破坏,抗弯刚度是影响构件抗冲击性能的主要因素之一。构件的抗冲击承载力随着含钢率的增高而提高,随着构件长细比的增大而降低。含钢率相差1.5%时,截面惯性矩较大的窄翼缘型钢对构件的抗冲击性能更有利。对于长细比大于20的构件,钢骨耗能是组合构件总能耗的主要组成部分。