Development of residual strength evaluation tool based on stress-strain approximation

To allow for a safe design of metallic structures, it must be considered that, as a result of the manufacturing and processing operations, cracks or flaws below the NDI (Non-Destructive Inspection) detectability level always exist in the component after inspection. Further problems originate from the defect geometry. Failure of engineering structural components and structures have been mostly traced to surface cracks. Especially for a surface crack containing structure that is thin, the limit collapse of a ligament is usually the main cause of structure rupture. Study of the evaluation procedure relating to the limit load of the surface crack ligament is, therefore, an important project for conventional fracture assessment, particularly for pressure vessel (LBB) assessment. For this purpose, novel more accurate residual strength prediction method based on the Elasto-Plastic Fracture Mechanics (EPFM) has been developed and tested. Laboratory tests on tensile plate specimens with surface cracks were performed considering two different materials. In the final part of the work, effort was directed toward the verification and justification of selected analytical methods by adequate component testing. The most significant results of this work deal with residual strength evaluation for the thin wall pressure containing components. The important finding is that there is a potential for improvement in comparison to the current methods that may be used to increase payoff of the lightweight structures. The presented very robust analysis method and the useful structure integrity evaluation procedure should significantly contribute to the state-of-the-art structure optimisation and being applied to the design of the light-weight structures should ease the effort of the structure engineer to develop the successful and reliable hardware and to keep in place with advancing technologies. This revised version was published online in August 2006 with corrections to the Cover Date.     

A continuum damage model applied to high-temperature fatigue lifetime prediction of a martensitic tool steel

High‐temperature operational conditions of hot work tool steels induce several thermomechanical loads. Depending on the processes, (i.e. forging, die casting or extrusion), stress, strain, strain rate and temperature levels applied on the material are nevertheless very different. Thus, lifetime prediction models need to be able to take into account a broad range of working conditions. In this paper, a non‐isothermal continuum damage model is identified for a widely used hot work tool steel AISI H11 (X38CrMoV5) with a nominal hardness of 47 HRc. This investigation is based on an extensive high‐temperature, low‐cycle fatigue database performed under strain rate controlled conditions with and without dwell times in the temperature range 300–600°C . As analysis of experimental results does not reveal significant time‐dependent damage mechanisms, only a fatigue damage component was activated in the model formulation. After normalization, all fatigue results are defined on a master Woehler curve defined by a nonlinear damage model, which allows the parameter identification. Last, a validation stage of the model is performed from thermomechanical fatigue tests.     

Prediction of very high cycle fatigue failure for high strength steels, based on the inclusion geometrical properties

This article deals with the experimental and predicted fatigue endurance of the high strength steels, European 100C6 (martensitic and bainitic) and the Japanese SUJ2 in the gigacycle regime. Tests were carried out with stress ratio R = −1 in tension–compression condition at room temperature. To attain the high number of cycles required in a reasonable period of time, an ultrasonic test machine working at 20 KHz was used to obtaining 1.7 × 10⁹ cycles in approximately 24 h. The relationship between the geometrical properties of inclusions associated with fatigue failure and the fatigue life of these steels was studied. Thereafter, with basis on a simplified evaluation of the highest stress in the elliptical inclusion for fatigue Mode I, three models to predict the fatigue life for these high strength steels were proposed adjusting non-linear regression curves to the corresponding experimental results.     

各向异性复合材料开孔板拉伸强度预测及模型验证

基于有限断裂力学方法建立了一种预测多向复合材料开孔板拉伸强度的通用和半经验模型。该模型同时采用基于应力形式的失效准则和基于能量形式的失效准则预测失效。模型仅需铺层弹性常数、无缺口层合板的强度以及0°铺层的断裂韧性等参数。基于线弹性断裂力学建立了多向复合材料层合板的断裂韧性与0°铺层断裂韧性之间的关系, 进而预测了任意铺层复合材料开孔板发生纤维主导拉伸失效时的强度。将模型预测结果与开孔板拉伸强度的试验数据进行了对比验证, 预测误差最大为9.7%, 与点应力和平均应力等方法的对比表明, 该模型的预测精度高于传统的特征长度方法。      

A high-cycle fatigue accumulation model based on electrical resistance for structural steels

For high-cycle fatigue of metals, the DC electrical resistance is a more sensitive parameter to the initiation of micro-cracks during the irreversible fatigue damage accumulation process. This implies that the electrical resistance is a suitable parameter that can be consistent with the fatigue damage physical mechanism. The relation between the ratio of electrical resistance changes and the cyclic fraction of the fatigue specimen may reasonably represent deterioration in mechanical properties of structural steels during the high-cycle fatigue process. The high-cycle fatigue damage accumulation model based on electrical resistance for structural steels was proposed. The model was verified by some experimental data for three structural steels; normalized 45C steel, 20 Mn steel and 16 Mn steel, and good agreement was obtained. The corresponding fatigue lifetime on the basis of the electrical resistance model was also performed. The results show that the approach to fatigue lifetime prediction and failure based on the electrical resistance is a good non-destructive technique.     

On effect of hydrogen on very high cycle fatigue behaviours of high strength steels

Abstract

In order to analyse the effect of hydrogen on very high cycle fatigue properties, hydrogen was precharged into two high strength steels. The applied stress intensity factor range at the periphery of inclusions before and after being precharged is approximately proportional to the cubic root of inclusion size. In addition, the applied stress intensity factor range at the periphery of inclusions after being precharged was lower compared with uncharged specimens. The additional stress intensity factor range generated by hydrogen ΔK_H is raised after the hydrogen was precharged. A simple prediction equation of S–N curve was proposed by introducing the hydrogen influence factor. The proposed prediction equation can reasonably describe the S–N curves for precharged specimens.     

From a local stress approach to fracture mechanics: a comprehensive evaluation of the fatigue strength of welded joints

This paper investigates the possibility of unifying different criteria concerned with the fatigue strength of welded joints. In particular, it compares estimates based on local stress fields due to geometry (evaluated without any crack-like defect) and residual life predictions in the presence of a crack, according to LEFM. Fatigue strength results already reported in the literature for transverse non-load-carrying fillet welds are used as an experimental database. Nominal stress ranges were largely scattered, due to large variations of joint geometrical parameters. The scatter band greatly reduces as soon as a 0.3-mm virtual crack is introduced at the weld toe, and the behaviour of the joints is given in terms of Δ K _I versus total life fatigue. Such calculations, not different from residual life predictions, are easily performed by using the local stress distributions determined near the weld toes in the absence of crack-like defects. More precisely, the analytical expressions for K _I are based on a simple combination of the notch stress intensity factors K ₁^N and K ₂^N for opening and sliding modes. Then, fatigue strength predictions, as accurate as those based on fracture mechanics, are performed by the local stress analysis in a simpler way.     

On the design and test of equivalent configurations for notch and fretting fatigue

This work investigates the possibility of designing fretting and notch fatigue experiments that are nominally equivalent in terms of damage evaluated by a multiaxial fatigue model. The methodology adopted to carry out this search considered a cylinder‐on‐flat contact geometry and a V‐notched plate. The loading conditions and geometries of the experimental configurations were adjusted to obtain the same decay of the multiaxial fatigue index from the hot spot up to a critical distance. Aluminium alloy 7050‐T7451 was used in the experimental evaluation of the methodology. Considering the well‐known scatter of fatigue data and the limited number of specimens available, the obtained results suggest that the use of the notch analogy in fretting fatigue is appropriate.     

Influence of casting surfaces on fatigue strength of ductile cast iron

Quantitatively evaluating the fatigue strength of ductile iron (DI) with casting surfaces involves several complicated factors such as surface roughness, transition of microstructures from surface to interior, several types of defects and residual stresses. Tension–compression fatigue tests have been performed using DI having casting surfaces composed of a ferritic structure, a ferrite‐pearlitic structure and a pearlitic structure. Residual stresses were relieved by annealing in order to separately evaluate each factor. The parameter model was applied for quantitative evaluation of fatigue strength. Surface roughness was considered to be mechanically equivalent to a defect, and the effective defect size due to the interaction between the surface roughness and a defect was defined. The present study proposes a method of evaluating the maximum defect size using statistics of extremes and the lower bound of the scatter of fatigue strength, for practical design.     

Improvement in the fatigue strength of chromium electroplated AISI 4340 steel by shot peening

In landing gear, an important mechanical component for high responsible applications, wear and corrosion control is currently accomplished by chrome plating or hard anodising. However, some problems are associated with these operations. Experimental results have also shown that chrome‐plated specimens have fatigue strength lower than those of uncoated parts, attributed to high residual tensile stress and microcracks density contained into the coating. Under fatigue conditions these microcracks propagate and will cross the interface coating‐substrate and penetrate base metal without impediment. Shot peening is a surface process used to improve fatigue strength of metal components due to compressive residual stresses induced in the surface layers of the material, making the nucleation and propagation of fatigue cracks difficult. This investigation is concerned with analysis of the shot peening influence on the rotating bending fatigue strength of hard chromium electroplated AISI 4340 steel. Specimens were submitted to shot peening treatment with steel and ceramic shots and, in both cases, experimental results show increase in the fatigue life of AISI 4340 steel hard chromium electroplated, up to level of base metal without chromium. Peening using ceramic shot resulted in lower scatter in rotating bending fatigue data than steel shots.     

A model of corrosion fatigue crack growth in ship and offshore steels

A model describing corrosion fatigue crack growth rate da/dN has been proposed. The crack growth rate is assumed to be proportional to current flowing through the electrolyte within the crack during a loading cycle. The Shoji formula for the crack tip strain rate has been assumed in the model. The obtained formula for the corrosion fatigue crack growth rate is formally similar to the author's empirical formulae established previously. The different effects of ΔK and the fatigue loading frequency f on da/dN, in region I as compared to region II of the corrosion fatigue crack growth rate characteristics can be described by a change of one parameter only: the crack tip repassivation rate exponent.     

An effect of strength of railway axle steels on fatigue resistance under press fit

High-cycle fatigue tests with an evaluation of fatigue limit were carried out on large model components of bars with press fitted hubs of diameter 63/59 mm. Bars were made of three railway axle steels EA1N, EA4T and 34CrNiMo6 with considerable different strength from 586 MPa to 1041 MPa, respectively. Detection and measurement of crack growth under hubs by ultrasonic method was performed during the tests. In spite of the differences in strength and alloying of tested bars, differences in mean value of fatigue limit were not significant. This result was connected with specific damage mechanism and microcracks initiation under hubs with fretting effects. Short fatigue crack growth under hubs occurred at stress intensity factor range ΔK considerably bellow threshold value ΔK_th of long cracks. Simultaneous growth of main cracks from more than one point of surface circumferential area under hub was quite frequently observed.     

3D fracture mechanics investigation on surface fatigue crack propagation

Surface fatigue crack propagation is the typical failure mode of engineering structures. In this study, the experiment on surface fatigue crack propagation in 15MnVN steel plate is carried out, and the crack shape and propagation life are obtained. With the concept of ‘equivalent thickness’ brought into the latest three‐dimensional (3D) fracture mechanics theory, one closure model applicable to 3D fatigue crack is put forward. By using the above 3D crack‐closure model, the shape and propagation life of surface fatigue crack in 15MnVN plates are predicted. The simulative results show that the 3D fracture mechanics‐based closure model for 3D fatigue crack is effective.     

On the notch sensitivity of flax fibre metal laminates under static and fatigue loading

Damage progression and failure characteristics of open‐hole flax fibre aluminium laminate (flax‐FML) specimens subjected to quasi‐static tensile or tension‐tension fatigue loading were experimentally investigated. Notched and unnotched flax‐FML composites exhibited brittle fracture with little or no fibre pull‐out and minimal delamination at the aluminium/adhesive interface. The flax‐FMLs were tested to failure under tension‐tension fatigue loading conditions (R ratio of 0.1; frequency of 10 Hz; applied fatigue stresses ranging between 30% and 80% of the respective ultimate tensile strength values). The fatigue cycles to failure decreased with the increase in the applied fatigue stress and hole diameter. A phenomenological modelling technique was developed to evaluate the fatigue life of an open‐hole flax‐FML composite. Fatigue tests on specimens subjected to a maximum load equivalent to 35% of the respective tensile failure strength were interrupted at around 85% of the corresponding fatigue life. The accumulated fatigue damage in these specimens was characterised using X‐ray computed tomography. For benchmarking purposes, the fatigue performance and related damage progression in the flax‐FML composite were compared with those of the glass‐FMLs.     

Mean stress and plasticity effect prediction on notch fatigue and crack growth threshold,combining the theory of critical distances and multiaxial fatigue criteria

In the present work, we propose a robust calibration of some bi‐parametric multiaxial fatigue criteria applied in conjunction with the theory of critical distances (TCD). This is based on least‐square fitting fatigue data generated using plain and sharp‐notched specimens tested at two different load ratios and allows for the estimation of the critical distance according to the point and line method formulation of TCD. It is shown that this combination permits to incorporate the mean stress effect into the fatigue strength calculation, which is not accounted for in the classical formulation of TCD based on the range of the maximum principal stress. It is also shown that for those materials exhibiting a low fatigue‐strength‐to‐yield‐stress ratio σ_fl,R = ?1/σ_YS, such as 7075‐T6 (σ_fl,R = ?1/σ_YS = 0.30), satisfactorily accurate predictions are obtained assuming a linear‐elastic stress distribution, even at the tip of sharp notches and cracks. Conversely, for any materials characterized by higher values of this ratio, as quenched and tempered 42CrMo4 (σ_fl,R = ?1/σ_YS = 0.54), it is recommended to consider the stabilized elastic‐plastic stress/strain distribution, also for plain and blunt‐notched samples and even in the high cycle fatigue regime still with the application of the TCD.     

坦克扭力轴表面缺陷对疲劳寿命的影响

为探求坦克扭力轴过早产生疲劳断裂的原因,应用弹性有限元方法模拟了坦克扭力轴表面各种形状、尺寸的凹坑处产生的应力集中现象,结合坦克在某一训练科目下的载荷谱,使用疲劳分析软件计算了具有不同形状和尺寸缺陷的扭力轴的疲劳寿命(存活率为99%).结果表明:一旦扭力轴表面出现了凹坑,尤其是出现了狭长的裂纹状凹坑时,其疲劳寿命会急剧下降;当出现狭长的裂纹状凹坑时,将其扩展成等径凹坑(不增大凹坑深度),可使凹坑对扭力轴疲劳寿命的影响降至最低.     

Tensile tests and analyses of notched specimens fabricated from high strength steels using a generalized yield model

This article summarises the uniaxial tension tests for 20 notched bars fabricated from high strength steel Q345 specified in Chinese National Standards. The effects of the notch radius, r, and that of the notch depth ratio, d/D , on the ductility and fracture resistance of this high strength steel are examined. The experimental data are further analyzed using a generalized yield model together with an elliptical fracture stress envelope originally proposed by the first author. The experimental results demonstrate that cracks initiate at the notched section, with the fracture surface filled with many dimples and shearing marks. Specimens with a sharper notch radius (a smaller r) and a larger notch depth (a smaller d/D ratio) show poor ductility, but high fracture strength. The stress field computed from the numerical procedure including the generalized yield model indicates that the crack initiation occurs at the centre of the notched section which experiences the highest stress triaxiality ratio (σ_m/σ_seq) . As the stresses at the notched section reach the limiting values determined from the elliptical fracture criterion, macroscopic fracture failure in the notched bar occurs.     

复合材料螺接修理结构参数化建模分析工具开发及验证

螺接修理在复合材料结构的临时性修理,尤其是战伤修理中应用较广泛。然而其修理设计过程较复杂,建模分析难度较大,难以满足工程快速定参的需要。本文针对含穿透损伤复合材料层合板的螺接修理问题,采用VB.NET结合p型有限元技术,开发了一套参数化的建模分析工具。分析工具通过界面读取修理结构的几何参数、螺栓布局、螺栓大小、载荷、材料属性等参数,自动创建有限元模型并进行求解。根据求解结果,分析工具可为用户提供修理结构的螺栓载荷、钉载比例、危险孔孔边应力等。另外,通过引入有限断裂力学,结合两级模型分析技术可预测得到修理结构的失效强度和失效位置。最后,采用典型算例对分析工具的有效性进行了验证。     

Effect of notch on fatigue behaviour of a directionally solidified superalloy at high temperature

The effect of notch types and stress concentration factors (K_t) on low cycle fatigue life and cracking of the DZ125 directionally solidified superalloy has been experimentally investigated. Single‐edge notched specimens with V and U type geometries were tested at 850 °C with stress ratio R = 0.1. High temperature in situ optical method was used to observe crack initiation and short crack propagation. Scanning electron microscope observation of fracture was used to analyse the failure mechanism. The results reveal that fatigue resistance decreases with K_t increasing from 1.76 to 4.35. The ratcheting is found to be affected by both K_t and the nominal stress from the displacement–force curve. In situ observations indicate that the cracking does not occur at the notch apex but at the location where the max principal stress or Hill's stress is the highest. According to the scanning electron microscope observations, the failure of the notched specimens strongly depends on the anisotropy microstructures.