Kinetics of small fatigue cracks in steel during cyclic loading

The kinetics of development of fatigue microcracks in a high-strength 08Kh14AN4MD steel is studied when a cantilever specimen with several notches is subjected to rotational bending tests. The specific feature of the tests consists in the fact that, when the specimen is loaded at a constant load, different stress amplitudes are realized in different notches. As a result, after the sample has failed across the section with the maximum stress, a longitudinal polished section of the specimen contains fatigue cracks having nucleated in the mouths of the other notches at lower stress amplitudes. A relation between inflection points in the fatigue curve and the conditions of local and developed yield in a notch has been established. Fatigue fracture mechanisms at a superhigh number of loading cycles are studied, and their relation to the structure near a crack is found. A rapid method for estimating the fatigue limit at a superhigh number of loading cycles is proposed.     

面内双轴循环载荷下304不锈钢的力学行为

为研究面内双轴载荷下304不锈钢材料的力学行为,运用有限元方法对十字形试样尺寸进行了设计与优化,在自主设计的面内双轴疲劳试验系统上对304不锈钢进行了单轴拉伸、双轴比例加载和非比例圆路径下的力学试验。结果表明,比例加载条件下304不锈钢的棘轮应变累积最小,圆路径下材料的棘轮应变最大,而单轴加载的棘轮应变累积介于比例载荷与圆路径之间。进入棘轮应变稳定增长阶段,圆路径对应的棘轮应变率高于单轴与比例加载,说明圆路径使304不锈钢的损伤失效进程加速。     

The growth of short fatigue cracks under compressive and/or tensile cyclic loading

Under cyclic compressive loading, fatigue cracks can initiate and propagate a short distance before being arrested. Recent studies have shown that the process of crack closure is importantly involved in the arrest process. An analytical means for dealing with this type of crack propagation is presented. Calculations based on the method of analysis compare favorably with experimental results. The method is general in nature and can also deal with short crack propagation under tension-compression and tension-tension types of loading.     

Damage to thick steel sheet in cyclic plastic flexure

The laminar development of damage in thick 10Г2С1 steel sheet during cold plastic deformation by cyclic flexure is investigated. The steel is subjected to pure flexure in a symmetric cycle, with strain of amplitude 5.5%. The variation in steel strength indicates the damage kinetics in the plastic deformation. In the region of reversible damage, the rate of change in the strength increases to a maximum and then declines. In the region of irreversible damage there is no further change in strength of steel before failure occurs. An upper limit is proposed for the damage of the steel: 20–30% of N_f, where N_f is the number of cycles to failure in cyclic extension–compression. In structural terms, the upper limit of reversible damage is assumed to correspond to the transformation of cellular dislocation structure to band structure.     

The nucleation of fatigue cracks in a low-alloy steel under high-cycle fatigue conditions and uniaxial loading

A basic understanding of the mechanism of fatigue-crack nucleation in a high-strength steel, under true high-cycle conditions, is developed from microstructural observations of rotating-bending fatigue specimens and theoretical results for the stress concentrations at inclusions and holes. The results show that for true high-cycle fatigue (fatigue life > 10⁶ cycles), where the applied stresses are too low to cause localised cyclic-plasticity at the site of inclusions, crack nucleation does not occur while the inclusions remain undamaged and firmly bonded to the matrix. Fatigue-crack nucleation is found to occur only after either the progressive debonding or local fatigue damage of alumina inclusions has led to the formation of holes at the specimen surface: the fatigue cracks being nucleated by a highly localised cyclic plasticity effect at points of maximum stress intensity on the hole boundary. The inclusion/matrix debonding mechanism is therefore not an essential preliminary to the nucleation of fatigue cracks, under true high-cycle conditions.Manganese sulphide inclusions and cementite particles are shown to have virtually no influence on the mechanism of fatigue-crack nucleation under the present high-cycle conditions. For the case of manganese sulphide this is a direct result of the high axial-elongation of the inclusions and the consequent low stress concentration under axial loading: under more general states of fatigue loading manganese sulphide inclusions are unlikely to remain ineffective.     

Thermal gradients,strain rate,and ductility in sheet steel tensile specimens

The effect on ductility of strain rate and thermal gradients arising from deformation is examined in tensile specimens of 1008 AK steel. The total elongatione _tot is taken as the measure of ductility, since it reflects changes in the strain hardeningn and strain-rate sensitivitym. Tensile specimens are pulled to failure in 23 °C air, at initial strain rates from 10⁻³ to 10⁻¹ s⁻¹, with thermocouples recording temperature along the 50.8 mm gauge section. The maximum temperature is ∼110 °C just prior to failure at the highest rate. Thee _tot, however, remains fairly constant with rate at ∼40 pct. When thermal gradients are prevented by immersing the specimens in circulating water at 23 °C,e _tot, increases with rate to a maximum of ∼54 pct at 10⁻¹ s⁻¹. Direct measurements of isothermal values ofm at 23, 60, and 90 °C show thatm increases with rate.e _tot, therefore, would be expected to increase with rate. Since under nonisothermal conditionse _tot does not change, it appears thatm and thermal gradients are competing influences on ductility at higher rates. Enhanced ductility in stampings should be possible by suppressing gradients, either by controlling die temperature or by heat transfer properties of a lubricant.     

Crack propagation in ceramic materials under cyclic loading conditions

An analysis is presented which enables crack propagation rates under cyclic loading condiditions to be predicted from static slow crack growth parameters. A comparison of the predicted times to failure under cyclic conditions with available measured failure times, for several ceramic materials at ambient temperatures, suggests that there is no significant enhancement of the slow crack growth rate due to cycling. This is verified in a series of measurements of slow crack growth rates under static and cyclic conditions.     

Mercury embrittlement of Cu−Al alloys under cyclic loading

The effect of mercury on the room temperature, high cycle fatigue properties of three alloys: Cu-5.5 pct al, Cu-7.3 pct Al, and Cu-6.3 pct Al-2.5 pct Fe has been determined. Severe embrittlement under cyclic loading in mercury is associated with rapid crack propagation in the presence of the liquid metal. A pronounced grain size effect is noted under mercury, while fatigue properties in air are insensitive to grain size. The fatigue results are discussed in relation to theories of adsorption-induced liquid metal embrittlement. N. S. STOLOFF is Professor of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12181, where T. M. Regan was formerly Research Assistant.     

加载方向对TRIP双相不锈钢板带拉伸性能的影响

 为了研究加载方向对一种TRIP型双相不锈钢板带力学性能的影响,利用拉伸试验机研究了加载方向与轧制方向分别成0°、45°和90°条件下试验钢板带的拉伸变形行为。利用EBSD、TEM、XRD等分析手段对比研究了不同加载方向下形变组织演化的特点及形变诱导马氏体相变动力学规律,探讨了作用机理。结果表明,试验钢表现出明显的各向异性,其中各方向塑性和抗拉强度(由高到低)的变化规律为0°＞45°＞90°,但屈服强度对加载方向不敏感。钢中奥氏体相发生了形变诱导马氏体相变,主要演化机制为γ→ε→α′,从而形成TRIP效应。0°加载有助于TRIP效应的发生与发展,而90°加载时,两相间的应变配分延迟了马氏体相变的进程,抑制了TRIP效应。通过回归分析分别建立了不同加载方向下形变诱导马氏体相变动力学模型,可实现各加载方向下不同变形阶段马氏体转变量的预测。     

Crack propagation in a pipe steel subjected to cyclic loading after long-term operation

The mechanical properties (under static and cyclic loading) and damage of a product pipeline made of 3G steel have been studied after its long-term operation. The ultrasound velocity in a damaged steel and the fatigue crack length on the sample surface are determined at various stages of macrocrack initiation. At the stage of main-crack initiation, the cumulative curves of fatigue microcrack distribution are well described by both exponential and power functions, with their exponents decreasing with increasing stress amplitude. The majority of fatigue microcracks have a length close to the average grain size.     

中碳钢化学成分对CSP薄板坯表面纵裂的影响

从钢水化学成分影响高温力学性能的角度,研究了钢水主要成分对CSP薄板坯表面纵裂的影响。研究表明：在钢中其它成分相近的条件下,随着钢中锰含量的提高,钢的高温强度和塑性不断提高,裂纹敏感性逐渐下降;提高钢的高温强度和塑性的化学成分对减轻表面纵裂的发生有利。     

The effect of cyclic loading on the dislocation structure of fully pearlitic steel

The fatigue behavior and associated crack inititation of pearlite has been correlated with developing dislocation configurations in a cyclically deformed fully pearlitic steel. Under fatigue conditions at low stress amplitudes, dislocations are found to be generated and largely confined to the cementite/ferrite interfaces, most likely due to the development of elastic incompatibility stresses between the cementite and ferrite. This deformation mode encourages fatigue crack initiation parallel to the cementite lamellae. The fatigue limit of pearlite appears not to be influenced by the interlamellar spacing, a result basically different from that found in monotonic deformation where yielding and flow are strongly affected by the spacing. The apparent differences are discussed in terms of the different dislocation configurations formed during monotonic and cyclic deformation.     

Non-linear elastic behaviour of the roller bearing steel SAE 52100 during cyclic loading

When subjected to uniaxial symmetrical push-pull tests, the hardened roller bearing steel SAE 52100 (100Cr6) exhibits unusual sickle-shaped hysteresis loops. This observation can be attributed to the fact that the elastic modulus is stress-dependent as a consequence of non-linear elastic effects. Elastic unloading experiments performed in tension and in compression during a cycle permitted the stress dependence of the elastic modulus to be determined quantitatively. The experimental observations can be described in good approximation by a quadratic extension of Hooke's law in accord with an anharmonic lattice potential. Applying this concept, the originally bent hysteresis loops can be converted into hyseteresis loops of the usual shape in the representation of stress vs plastic strain. A comparison of the results with measurements on α-iron whiskers reported in the literature indicated that the departures from linear elasticity observed in high-strength steels are in the same order of magnitude as in the case of pure iron.     

Fe-19Cr-0.2Ni-5Mn-0.2Si在循环加载下的马氏体相变规律及其影响

 相变诱导塑性(TRIP)双相不锈钢具有优良的强度和塑性,且兼顾经济性,因此工业应用潜力很大。而厘清TRIP型双相不锈钢在循环加载下产生的马氏体相变对其循环力学性能的影响规律,是促进其进一步开发及工业化应用的基础。以TRIP型双相不锈钢Fe-19Cr-0.2 Ni-5Mn-0.2Si为研究对象,开展循环性能及相变特征研究。应用INSTRON试验机,分别进行拉伸试验和应变幅为0.6%的对称循环加载试验,测定试验钢的拉伸力学性能及循环软硬化性能。在循环加载过程中,应用铁素体测量仪测量不同循环周次下的马氏体转变量,分析马氏体相变特征。利用透射电镜,观测典型循环周次下的微观结构,分析马氏体相变和位错结构演化规律。进而,研究马氏体相变和位错结构演化对循环软硬化性能的作用机制。结果表明,试验钢在拉伸条件下,表现出明显的TRIP效应;循环初期马氏体转变速率较快,之后转变速率逐渐降低并且逐渐趋于零;循环软硬化特征可分为3个阶段,初始循环硬化、循环软化和二次循环硬化阶段;初始循环硬化由两相中位错的增殖引起的硬化效应起主导作用;随后的循环软化,由铁素体中低能位错结构所引起的软化效应起主导作用;在二次循环硬化阶段,相变马氏体对材料的硬化起主导作用。总的来说,马氏体相变对试验钢循环加载初期的循环软硬化性能影响较小,但对循环后期的性能影响较大。     

Load sharing between austenite and ferrite in a duplex stainless steel during cyclic loading

The load sharing between phases and the evolution of micro- and macrostresses during cyclic loading has been investigated in a 1.5-mm cold-rolled sheet of the duplex stainless steel SAF 2304. X-ray diffraction (XRD) stress analysis and transmission electron microscopy (TEM) show that even if the hardness and yield strength are higher in the austenitic phase, more plastic deformation will occur in this phase due to the residual microstresses present in the material. The origin of the microstresses is the difference in coefficients of thermal expansion between the two phases, which leads to tensile microstresses in the austenite and compressive microstresses in the ferrite. The microstresses were also found to increase from 50 to 140 MPa in the austenite during the first 100 cycles when cycled in tension fatigue with a maximum load of 500 MPa. The cyclic loading response of the material was, thus, mainly controlled by the plastic properties of the austenitic phase. It was also found that initial compressive macrostresses on the surface increased from −40 to 50 MPa during the first 10³ cycles. After the initial increase of microstresses and macrostresses, no fading of residual stresses was found to occur for the following cycles. A good correlation was found between the internal stress state and the microstructure evolution. The change in texture during cyclic fatigue showed a sharpening of the deformation texture in the ferritic phase, while no significant changes were found in the austenitic phase.     

FTSC中碳硼微合金钢热轧板卷边部裂纹分析

唐钢FTSC薄板连铸机生产中碳硼微合金钢热轧板卷存在边部裂纹缺陷。通过对SS400B钢进行高温热塑性研究,发现弯曲矫直温度为750℃时热塑性最差;由于铸坯边角部冷却速度快,温度低,塑性较差,边角部振痕处存在可见裂纹;在对存在边部裂纹的铸坯取样分析时发现晶界间有非金属元素析出,同时晶界间存在裂纹。轧制时形成边部裂纹缺陷。