SURFACE AND IN-DEPTH CRACK PROPAGATION IN HIGH TEMPERATURE LOW-CYCLE FATIGUE OF SUS 316 STAINLESS STEEL

Abstract— The low-cycle fatigue crack propagation behaviour of surface cracks in SUS316 stainless steel at 700°C, in both the surface direction and the in-depth direction, has been studied with special emphasis on the role of oxidation. The coalescence behaviour of surface cracks is essential for the process of crack propagation in high temperature low cycle fatigue, irrespective of the existence of oxidation effects. For sub-surface cracks the process of crack propagation is divided into two stages characterized by differences in fracture mode. In both stages, the in-depth crack propagation rate in air is higher than that in vacuum. This difference in crack propagation rate is the main reason for the decrease of fatigue life in air compared with that in vacuum. The crack propagation behaviour in the in-depth direction can be estimated from the conversion of the surface crack length into the subsurface depth by the use of an aspect ratio.     

4.5Ni钢表面裂纹的低周疲劳扩展行为研究

采用悬臂弯曲加载方式，以总应变范围作为受检和控制参数，分析了高强度４．５Ｎｉ钢表面裂纹的低周疲劳扩展行为，给出了裂纹扩展速率ｄ（２ａ）／ｄＮ与总应变范围ΔεＴ的关系式及关系曲线。同时对弯曲加载条件下低周疲劳损伤断口微观形貌进行了观察分析。指出４．５Ｎｉ钢的低周疲劳裂纹扩展方式主要是穿晶，疲劳辉纹为晶体学延性辉纹，疲劳裂纹扩展属于塑性钝化模型机制。     

Nature of failure of ML8 magnesium alloy in low-cycle fatigue

Fractographic analysis of fracture surfaces of specimens of ML8 high-strength cast magnesium alloy, tested under low-cycle pulsed tensile loading conditions, showed that in both quasistatic and fatigue failure, secondary cracks form and propagate at high rates. In this case, the resistance to crack propagation is high as a result of adhesion bonds between the grains. This reflected in the form of traces of plastic microfailure directly on the surface of the grains in the form of pits and traces of slit deformation. It is shown that failure can take place regardless of the relationship with the determining deformation mechanism as a result of the presence of a large number of inclusions and metallurgical defects of different sizes.Translated from Problemy Prochnosti, No. 3, pp. 25–28, March, 1990.     

Thermo-mechanical fatigue testing of a rhenium-free single-crystalline super alloy

Abstract

Due to high temperatures and mechanical loads, cracks are initiated in aero engine turbine blades which limit the cyclic life of these components. The materials used for components which underlie high thermal and mechanical load are single crystalline (SX) nickel based super alloys that in most cases contain a certain amount of rhenium. Dramatically increasing Re prices lead to the development of Re-free alloys.

In this work, low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests were carried out on the Re-free single crystal M-247LC SX. The test results are shown and a model based on crack propagation was used to predict LCF and TMF life. It was shown, that the modeling results fit properly for out-of-phase TMF and LCF life while for in-phase TMF differences between calculated life and experiments occur due to a different mechanism of fracture.     

Effect of beta flecks on mechanical properties of Ti–10V–2Fe–3Al alloy

The effects of beta flecks on tensile properties and low-cycle fatigue life were investigated at room temperature for Ti–10V–2Fe–3Al alloy. It was found that beta flecks had a significant influence on tensile ductility and low-cycle fatigue life. The greater the volume fraction of beta flecks (P_A) or maximum area of beta flecks (S_max), the lower the tensile ductility and low-cycle fatigue life. Extensive scanning electron microscopy (SEM) and light microscopy (LM) observation showed that under tensile load, cracks preferentially nucleated at β grain boundaries of beta flecks, then grew, connected and propagated along grain boundaries to form characteristics of intergranular fracture and quasi-cleavage fracture. While under an alternating load, beta flecks acted as sites for low-cycle fatigue crack nucleation due to inhomogeneous alternating strains between soft GB and aged beta matrix. The presence of beta flecks accelerates both the crack nucleation and early crack propagation.     

Fatigue Life Estimation of Structural Components Using Macrofibre Composite Sensors

Abstract: Recently, a number of different structural health monitoring (SHM) techniques have been developed for the online inspection of air, land and sea engineering structures. Various smart materials are employed for detecting eminent damage in situ. Fatigue cracks in structural components are the most common cause of structural failure when exposed to fatigue loading. Fatigue design of structural components is typically accomplished either using a set of stress cycle (S‐N) data obtained from prior fatigue tests or using the fracture mechanics approach. The fracture mechanics approach considers the fatigue life of structures as a summation of crack initiation life and crack propagation life. The stress intensity factor (SIF) is required for the estimation of fatigue crack propagation life from the linear elastic fracture mechanics (LEFM) perspective. However, the accurate prediction of the SIF is difficult especially when the geometry or the boundary conditions of a structure becomes complex. In this study, a SHM application of macrofibre composite (MFC) sensors is presented. A set of MFC sensors is used for the real‐time measurement of the SIF. The measured values of the SIF are later used for the prediction of the crack propagation life. The impedance‐based SHM technique using the same set of MFC sensors is employed for the detection of crack initiation life.     

Mechanical properties of Al–Li alloys Part 2 Fatigue crack propagation

Abstract

Mechanisms influencing the ambient temperature mechanical properties of commercial Al–Li alloys 2090, 2091, 8090, and 8091 are examined, with specific emphasis on the role of microstructure. In Part 2, results on fatigue crack propagation behaviour are presented for both ‘long’ (≥ 5 mm) and ‘microstructurally small’ (~1–1000 μm) cracks and compared with behaviour in traditional high strength aluminium alloys. In general, it is found that the growth rates of long fatigue cracks in Al–Li alloys are up to two to three orders of magnitude lower than in traditional 7000 and 2000 series alloys, when compared at an equivalent stress intensity range ?K. By contrast, corresponding growth rates of microstructurally small fatigue cracks were up to two to three orders of magnitude higher than the long crack results. Such observations are attributed to the prominent role of crack tip shielding in Al–Li alloys resulting from the tortuous and deflected nature of the crack paths which results in a reduced crack tip ‘driving force’ from crack deflection and, more importantly, from the consequent crack closure induced by the wedging of fracture surface asperities. Since microstructurally small cracks are unable to develop the same level of shielding from crack closure by virtue of their limited wake, small crack growth rates are significantly accelerated. Unlike fracture toughness behaviour, artificial aging of commercial Al–Li alloys to peak strength has a mixed influence on the (long crack) resistance. Although behaviour at higher growth rates is relatively unaffected, in 2091 nominal threshold ?K_TH values are increased by 17%, whereas in 8090 and 8091 they are decreased by 16–17%. However, all alloys show reduced effective fatigue thresholds at peak strength after correcting for crack closure.

MST/926b     

基于Beach Marking方法的钢结构疲劳裂纹检测研究

研究裂纹的形成和扩展规律是钢结构疲劳断裂的一个重要课题。该文讨论了一种十分经济有效的检测钢结构疲劳裂纹萌生与扩展的方法,即Beach Marking方法。首先简要总结了该方法的发展历史和研究现状,并系统归纳了其基本原理和关键问题。同时,通过焊接H型钢梁的试验案例,探索该方法应用在大尺寸构件上的实际效果,并具体展示该方法的设计步骤和应用过程。试验表明,Beach Marking方法简单实用,关键在于设计合理的疲劳荷载序列;通过分析疲劳断口上留下的疲劳弧线,并结合断裂力学理论,可以获得疲劳裂纹的扩展规律和参数,为钢结构疲劳寿命数值分析提供科学依据。     

Low-cycle fatigue in Ni3Al-based alloys

The low-cycle fatigue properties of hot-extruded powders of a Ni₃Al-based alloy, IC 218, with nominal composition Ni-16.5Al-8.0Cr-0.4Zr-0.1B (at %) have been evaluated at room temperature. Tests were conducted under total strain conditions in a laboratory air environment. Results indicate that the low-cycle fatigue performance of the PM processed IC 218 nickel aluminide is superior to other structural alloys especially at higher strain amplitudes. These results are explained in terms of the high ductility of the fine-grain material and good crack growth propagation resistance in these alloys. Stress response curves for annealed IC 218 alloys indicate considerable cyclic hardening followed by cyclic softening. The onset of cyclic softening is found to occur at a constant cumulative plastic strain. The critical cumulative plastic strain criteria are verified for step-loaded IC 218 nickel aluminide coupons.     

SMALL CRACK GROWTH AND FATIGUE LIFE PREDICTIONS FOR HIGH-STRENGTH ALUMINIUM ALLOYS: PART I—EXPERIMENTAL AND FRACTURE MECHANICS ANALYSIS

The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests were conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plasticity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions. Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminium alloys was developed and demonstrated using the crack-closure model.     

Two methods of investigation of the influence of hydrogen on the propagation rate of a crack and behavior of fracture of high-strength steels

We present two methods for the investigation of the influence of hydrogen on the propagation rate of a crack and behavior of fracture of high-strength steels. The method for investigation of the influence of electrolytic hydrogenation on a subcritical growth of a crack in high-strength steels is based on the use of simple beam specimens of a certain geometry and on the application of lateral loading in such a way that the stress intensity factor can be constant at the tip of a preliminary induced crack. The method is of great importance for the performance of comparative experiments in evaluating the influence of active media and structural anisotropy of specimens made of high-strength steels with limited sizes on their corrosion crack resistance. Typical examples of the application of the method to investigation of the role of electrolytic hydrogenation in subcritical propagation of cracks and their branching in highstrength steels are given. The method for investigation of heat release under strain and fracture of hydrogenated specimens involves the use of microcalorimetric devices, which allow one to study the influence of hydrogenation on peculiarities of the kinetics of elastic and plastic strains of high-strength steels. We illustrate the efficiency of the method proposed by plotting the “load-elongation” curves and corresponding (in time) characteristics of heat release power in the process of strain and fracture of specimens made of a high-strength steel. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 113–120, July–August, 1998     

Observations on fatigue crack paths in the corners of cold-formed high-strength steel tubes

Fatigue crack propagation in cold-formed corners of high-strength structural steel plate-type structures has been investigated. Large- and small-scale test specimens having complex residual stress states and subject to multi-axial cyclic local stresses have been investigated using both laboratory tests and numerical simulations. The combinations of alternating bending stress, alternating shear stress and static mean stress producing complex multi-axial stress states have been found to influence the fatigue crack path behaviour. Straight, zig-zag and “S” shaped cracks were observed depending on the material strength, range of cyclic loading, residual stress field and multi-axiality of the local stresses. Numerical simulations of residual stresses and linear elastic fracture mechanics were used to help understand the alternate crack paths. Mode I cracks propagating into a static compressive stress field did not arrest, but, due to the multi-axial stresses, combinations of mixed mode I, II and III crack growth with distinct paths were observed. The crack paths depend on the type and range of cyclic loading, material properties and residual stress conditions of the specimens.     

Mikrofraktographische Analyse des Delaminationswachstums in zyklisch belasteten Kohlenstoffaser/Duroplastharz-Verbundwerkstoffen

Microfractographic Analysis of Delamination Growth in Fatique Loaded – Carbon Fibre/Thermosetting Matrix Composites Carbon-fibre-reinforced plastics (CFRP) are known to be considerably less sensitive to fatigue loading than aluminium (Al) alloys, for instance. However, even in the presence of small delaminations, the damage tolerance of structural components may be considerably reduced. The scope of the present contribution is to investigate fatigue phenomena in CFRP materials (with thermosetting matrix) by means of microfractography. The microgfractographic features of the fracture surfaces mirror the processes of deformation and fracture at the delamination front. The fatigue fracture behaviour of a CFRP laminate subjected to cyclic mixed-mode loading is determined by matrix-controlled failure mechanisms. Under pure mode-II loading conditions, rollers in addition to fatigue striations appear in the fibre imprints whose formation mechanism was explained by means of high-resolution field-emission scanning electron microscopy (FE-SEM). The ratio between the local tensile and shear stress components influences the propagation direction of secondary cracks originating at the fibres. The local fracture propagations in these secondary cracks can be recognised through the fatigue striations appearing on the surface of the matrix. A comparison with static mixed-mode loading reveals that in both cases the crack propagation follows the path of the local maximum main stress. Applying mathematical relationships derived from the theory of elasticity permitted developing a mixed-mode loading model which makes it possible to predict the crack processes and hence to explain the formation of typical fracture-morphological features.     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

1240MPa级Ti-38644高锁螺栓的拉伸疲劳增寿机理

用扫描电镜和能谱观测分析Ti-38644高强钛合金高锁螺栓的拉伸疲劳断口,揭示了高锁螺栓的疲劳裂纹萌生和扩展的微观特征和疲劳增寿机理。结果表明,Ti-38644高强钛合金高锁螺栓的疲劳断口包括疲劳裂纹萌生区、扩展区和瞬断区:疲劳裂纹从螺栓头下圆角滚压薄弱部位表面萌生,随后在基体中呈放射性扩展;进入扩展区后裂纹的尺寸由微观扩展至宏观,以疲劳条带扩展机制为主,同时也存在解理断裂。头下圆角处的变形层对Ti-38644高锁螺栓的疲劳寿命有显著的影响,变形层使Ti-38644钛合金高锁螺栓的疲劳寿命明显提高。通过微观组织与疲劳寿命的对比,探讨了Ti-38644钛合金高锁螺栓疲劳强化的作用机理。     

铸造、锻造和粉末冶金TC4钛合金损伤容限行为对比研究

目的 研究影响铸造、锻造和粉末冶金TC4钛合金的损伤容限行为差异的主要因素。方法 分别从裂纹尖端塑性变形行为、二次裂纹及断口表面粗糙度3个方面对比,分析造成3种成形方法制备的TC4钛合金的断裂韧性和疲劳裂纹扩展速率差异的原因。结果 铸造TC4钛合金断裂韧性优于锻造和粉末冶金TC4钛合金,主要是因为新产生的裂纹面积大,消耗更多断裂能量。铸造TC4钛合金疲劳裂纹扩展速率低于锻造、粉末冶金TC4钛合金,其主要原因为曲折的裂纹路径和断面粗糙度诱发裂纹闭合效应以及长而深的二次裂纹。结论 3种成形方法制备TC4钛合金损伤容限行为差异的主要原因是断裂形成了不同裂纹路径形貌。     

钛合金双片层组织对性能的影响

研究了通过热处理制度调整,在合金α片层之间形成细小的条状次生α相,形成一种新型的钛合金显微组织——双片层组织.通过对比等轴组织、双态组织、片层组织和双片层组织的性能,结果表明,在合金的强度和塑性不损失的条件下,双片层组织进一步提高了裂纹在合金中的扩展阻抗,使得合金的断裂韧性得到改善,疲劳裂纹扩展速率得到降低.双片层组织...     

Statistical description of the effect of Zr addition on the behavior of microcracks in Cu–6Ni–2Mn–2Sn–2Al Alloy

As possible substitutes for high-strength Cu–Be alloys, Cu–6Ni–2Mn–2Sn–2Al alloys have been developed. To clarify the physical background of the effect of trace Zr on the fatigue strength of such alloys, the initiation and propagation behavior of a major crack that led to the fracture of the tested specimens was monitored. When the stress amplitude was less than σ _a = 350 MPa, the fatigue life of the alloys with Zr was about 2–2.5 times larger than that of the alloy without Zr. When σ _a > 350 MPa, the effect of Zr addition on the fatigue life dramatically decreases as the stress amplitude increases. The increased fatigue life due to Zr addition resulted from an enhancement of the crack initiation life and microcrack growth life. The enhanced crack initiation life was mainly attributed to the strengthening of grain boundaries due to the precipitation of SnZr compounds. A statistical analysis of the behavior of multiple cracks was made to quantitatively evaluate the scatter in fatigue behavior. The statistical analysis supported the conclusions obtained from the behavior of a major crack.     

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments

Fatigue tests under rotating bending and reversed torsion were carried out in air, distilled water and 3% saltwater, using smooth specimens of high-strength low alloy steel (Cr-Mo steel). The initiation and growth behavior of small fatigue cracks in each environment were evaluated based on detailed observations, and the effects of corrosive environment were also discussed. The fatigue strength decreased with increasing aggressiveness of test environment. The decreases in corrosive environment were due to earlier fatigue crack initiation. From the observed locations at which small fatigue cracks began, it was considered that the crack initiation was primarily governed by hydrogen embrittlement in distilled water and also affected by corrosive dissolution in 3% saltwater. The validity of the application of linear fracture mechanics for small fatigue cracks was established. The growth rates of small fatigue cracks were higher than for large through cracks, and not accelerated by the corrosive environment. Moreover, fatigue life in the corrosive environment was estimated by using the crack growth characteristics in air.