Einfluß einer Laseroberflächenhärtung auf die Dauerlestigkeit von glatten Proben mit kurzem Riß aus dem Stahl C70W2

The influence of laser transformation hardening on fatigue limit of smooth C70W2 steel specimens with small crack Laser transformation hardening as a local heat treatment technology can be used to restore the fatigue strength of crack-damaged components. The investigations have been carried out on smooth steel specimens containing small surface cracks. The obtained 80% fatigue limit improvement of the laser hardened specimens in relation to the crack-damaged initial condition is explained by the fatigue crack growth threshold increase due to martensitic transformation hardening, the action of residual stresses on the crack front and possible crack tip blunting.     

Behaviour of sub-clad and through-clad cracks under consideration of the residual stress field

Aim of the present study is an assessment of the behaviour of cracks in the ferritic base metal of components supplied with an austenitic welded cladding under special consideration of the residual stress field caused by the welding and heat treatment processes. For this purpose, an experimental and numerical investigation has been performed. The experimental study consisted of two component tests at low temperature using large-scale specimens with sub-clad and surface cracks. Subsequently, the experiments were analyzed numerically where the residual stress field was determined in an advanced numerical simulation of the welding and heat treatment processes. Based on the results, a fracture mechanics assessment of the ferritic and austenitic material zones on crack initiation and arrest was performed. The experimental and numerical results reveal that fracture was initiated in the ferritic range whereas the austenitic cladding remained intact even in the case of a limited crack extension in the base metal underneath the cladding.     

Different R-Curves for Two- and Three-Dimensional Cracks

R-curves caused by crack face interactions are important for the strength assessment of components with small cracks. Whereas an influence of the initial crack length on the R-curve is well understood on the basis of fracture mechanics, it will be shown here that also the type of the crack affects the crack growth resistance. For a given crack-bridging relation it can be shown that 3-dimensional cracks as circular internal cracks and semi-circular surface cracks must exhibit a lower R-curve than obtainable from 2-dimensional edge cracks.     

Fracture strength of flawed cylindrical pressure vessels under cryogenic temperatures

Damage tolerant and fail-safe approaches have been employed increasingly in the design of critical engineering components. In these approaches, one has to assess the residual strength of a component with an assumed pre-existing crack. In other cases, cracks may be detected during service. Then, there is a need to evaluate the residual strength of the cracked components in order to decide whether they can be continued safely or repair and replacement are imperative. A three-parameter fracture criterion is applied to correlate the fracture data on aluminium, titanium and steel materials from test results on cylindrical tanks/pressure vessels at cryogenic temperatures. Fracture parameters to generate the failure assessment diagram are determined for the materials considered in the present study. Failure pressure estimates were found to be in good agreement with test results.     

Measurement and modelling of residual stress effects on cracks

Defects that form by mechanisms such as fatigue and stress corrosion cracking are influenced both by external loads on engineering structures and internal, residual stresses that are generated during the manufacture and operation. This paper describes a programme of experimental and analytical work on a high‐strength, low‐toughness aluminium alloy (AL2024‐T351) to assess the influence of residual stress on crack opening displacement (COD) and crack‐driving force (CDF) for a range of fatigue crack lengths in compact tension (CT) specimens containing a mechanically induced residual stress field. Comparison of experimentally measured and numerically predicted CODs, at the mid‐plane and surface of CT specimens, show generally good agreement for cracks introduced into the finite‐element model in a progressive, element‐by‐element manner. Cracks introduced in a simultaneous manner give larger than observed CODs. The CDFs for the progressively introduced crack are always smaller than for simultaneously introduced. These results have implications for the assessment of initiation for slowly growing cracks.     

The residual strength characteristics of stiffened panels containing fatigue cracks

In heavy structural members, where plane strain conditions prevail, linear fracture mechanics can be used for predicting residual strength. Aircraft structures consist largely of sheet structures with plane stress conditions where linear fracture mechanics do not seem to apply. Yet it is in the aircraft main structure that large fatigue cracks can develop and that has to be designed fail-safe. The present paper describes a method to predict the residual strength of a cracked sheet structure.Contrary to an unstiffened sheet, the sheet structure contains stiffening elements that can act as crack stoppers. This crack arresting action and its consequences for the residual strength are considered in the analysis.The paper proposes a method that relates the crack resistance of a stiffened panel to that of an unstiffened sheet. It takes full account of sheet-stringer interaction in the cracked region. A criterion for crack arrest is put forward. Ultimate panel failure after crack arrest is initiated either by subsequent unstable crack growth or by stiffener failure. Critical load conditions for both failure modes are presented. In case crack arrest does not occur, the residual strength of the unstiffened panel constitutes a safe lower bound.Computational results of the interacting rivet forces by both analytical and numerical (finite element) methods are presented. From these the load concentration in the stiffener and the reduction of the stress intensity at the crack tip can be determined. This enables the complete residual strength characteristics to be predicted.The results of residual strength tests on bonded and riveted panels with symmetric strip stiffeners or eccentric Z-stringers fully substantiate the method proposed for residual strength calculations.     

Fracture mechanics and scale effects in the fatigue of railway axles

Fatigue of railway axles is one of the basic problems of fatigue. However, in spite of the criticality of this component, modern approaches have not been used for addressing a critical revision of traditional design. The scope of this paper is to study the scale effects in fatigue limit and in crack growth rate for a high strength steel used for high speed railway axles.Fatigue limit tests on micro-notched specimens led to the determination of fatigue thresholds for small cracks of the examined steel. This allowed us to successfully analyse the `scale effect' and the fatigue strength of full-scale axles in terms of threshold stress for short cracks emanating from small non-metallic inclusions.A series of crack propagation tests on small scale specimens lead to the definition of an EPFM crack propagation model which has been successfully compared with propagation data on full-scale components. These results support the application of the crack propagation model for the determination of axle inspection intervals.     

Dynamic shear plugging of beams and plates with an advancing crack

The perforation response of a thin or intermediately thick beam/plate struck by a rigid, heavy, flat-nosed projectile traveling at a high velocity is studied analytically. Based on a newly developed expression relating indentation depth to crack length, a shear-plugging model including two stages: indentation and crack growth, is proposed. In this model, shear resistance in the ligament decreases as the cracks propagate through the target thickness. The closed-form solutions for residual velocities, plastic energy, shear zone width, crack propagation speeds, and temperature rise are derived. A coupled shear–tension solution that takes into account global deformation of the plate is also developed. The coupled solution improves prediction for the case with the initial impact velocity near the ballistic limit. Comparisons with experimental results presented in the open literature are made showing rather good correlation, which validates the accuracy of the present solution.     

Weight functions for edge cracks in thin surface layers

A procedure is given for the calculation of the weight function for a surface crack located in a thin surface layer perpendicular to the interface. The procedure is demonstrated for cracks approaching the interface and cracks going beyond the interface.     

Correlating cryogenic fracture strength using a modified two-parameter method

Damage tolerant approaches have been employed increasingly in the design of critical engineering components. In these approaches, one has to assess the residual strength of a component with an assumed pre-existing crack. In other cases, cracks may be detected during service. Then, there is a need to evaluate the residual strength of the cracked components in order to decide whether they can be continued safely or repair and replacement are imperative. A modified two-parameter fracture criterion is applied to correlate fracture data from tensile cracked plates made of aluminium alloys, titanium alloys and stainless steel at cryogenic temperatures. Fracture parameters to generate the failure assessment diagram are determined for the materials considered in the present study. An attempt is made to correlate the fracture data on pressure vessels tested at cryogenic temperatures. The fracture parameters obtained from the fracture data of tensile cracked plates and pressure vessels were found to be closely matching with each other. Failure pressure estimates were found to be in good agreement with test results. This study indicates that the failure assessment diagram of a material generated from tensile fracture plate configurations can be applied for failure pressure estimations of flawed pressure vessels.     

Simulation of fatigue crack growth in components with random defects

The paper presents a probabilistic method for the simulation of fatigue crack growth from crack-like defects in the combined operating and residual stress fields of an arbitrary component. The component geometry and stress distribution are taken from a standard finite element stress analysis. Number, size and location of crack-like defects are ‘drawn’ from probability distributions. The presented fatigue assessment methodology has been implemented in a newly developed finite-element post-processor, P • FAT, and is useful for the reliability assessment of fatigue critical components. General features of the finite element post-processor have been presented. Important features, such as (i) the determination of the life-controlling defect, (ii) growth of short and long cracks, (iii) fatigue strength and fatigue life distribution and (iv) probability of component fatigue failure, have been treated and discussed. Short and long crack growth measurements have been presented and used for verification of the crack growth model presented.     

On damage tolerance design of fuselage structure (longitudinal cracks)

Aircraft structure is the most obvious example where functional requirements demand light weight and, therefore, high operating stresses. An efficient structural component must have three primary attributes; namely, the ability to perform its intended function, adequate service life and the capability of being produced at reasonable cost. To ensure the safety of aircraft structures, the Air Force requires damage tolerance analysis. This paper focuses its attention on designing a fail-safe fuselage structure. Two types of damage most frequently associated with the structural integrity of the fuselage are longitudinal cracks under high hoop stresses induced by cabin pressurization and circumferential cracks under stresses from vertical bending of the fuselage. The analysis of these types of cracks is complex, first due to the complex structural configuration (i.e. frames, skin longeron and crack stopper straps) and secondly due to the influence of the curvature of the shell. Various analytical and empirical approaches have been used to study the damage tolerance capability of the fuselage structure. Due to the lack of a comprehensive model to calculate the stress intensity factors for the complex structure, experiments usually have been performed to measure the crack growth rates and to demonstrate the residual strength of fuselage-type structural components containing circumferential and longitudinal cracks.

In this paper various analytical and empirical approaches used in evaluating the damage tolerance capability of the fuselage structure are critically evaluated and compared. A model which accounts for the influence of frames, straps and curvature is developed. This model is then used in an example problem having typical military cargo aircraft fuselage structural elements. The Air Force damage tolerance requirements are discussed in detail.     

Microstructural effects on crack closure and propagation thresholds of small fatigue cracks

The crack closure behaviour of microstructurally small fatigue cracks was numerically simulated by combining the crack-tip slip band model with the plasticity-induced crack closure model. A Stage II crack started to propagate from an initiated Stage I crack. When the plastic zone was constrained by the grain boundary or the adjacent grain with higher yield stresses, the crack opening stress increased with crack extension, and the effective component of the stress range decreased. The crack-tip opening displacement range (Δ CTOD ), first decreased with crack extension due to the development of the residual stretch, then increased until the tip of the plastic zone reached the neighbouring grain boundary. When the plastic zone was blocked by the grain boundary, Δ CTOD began to decrease. The arrest condition of cracks was given by the threshold value of Δ CTOD . At the fatigue limit, the arrest of small cracks takes place just after the Stage II crack crosses the grain boundary when the grain boundary does not act as a barrier. Only when the grain boundary has a blocking strength and the yield stress of adjacent grains is not so high, the arrest of Stage II cracks takes place before the crack reaches the grain boundary. The fatigue limit decreases with the mean stress. The predicted relation between the fatigue limit and the mean stress is close to the modified Goodman relation.     

Dauerfestigkeitsberechnung und Bemessung autofrettierter innendruckbeanspruchter Bauteile

Calculation of the endurance limit and design of autofrettaged pressurized components The authors present a design conception for autofrettaged (initially overloaded), pressurized components, allowing to calculate the residual stresses at the fatigue critical point and the endurance limit for crack initiation for components of arbitrary shape. The numerical investigations and the experimental verification as well show, that autofrettage significantly improves the endurance limit, particularly when applied to thickwalled notched components. If nonpropagatting cracks can be accepted, further improvement is possible. The endurance limit of the tested components with nonpropogating cracks is increased by 130% compared to the nonautofrettaged parts.     

A MODEL FOR THE FATIGUE LIMIT AND SHORT CRACK BEHAVIOUR RELATED TO SURFACE STRAIN REDISTRIBUTION

A model based on surface strain redistribution and the reduced closure stress of short cracks is shown to successfully predict the fatigue limit and short crack growth behaviour for aluminium alloy 2024-T351. Using this approach, the length of non-propagating cracks can be anticipated. The local stress intensity range may be resolved into two components (first the linear elastic fracture mechanics component and the second is due to surface strain concentration). Consequently, the local stress intensity range of aluminium alloy 2024-T351 is a maximum at a depth of approximately half a grain diameter and a minimum at a depth slightly in excess of three grain diameters. The reduced closure stress for short cracks coupled with the increased applied stress intensity caused by surface strain redistribution accounts for the variation of the effective stress intensity parameter as a function of crack depth. This parameter is a maximum for the smallest possible crack (3 μm) and decreases as crack length increases.     

Determination of Remnant Residual Stresses in Fracture Toughness Specimens Extracted From Large Components

Abstract: Material fracture toughness data are required to undertake fitness‐for‐service assessments of engineering components containing cracks. Calculations of crack driving force in the component are compared with material fracture toughness values to assess the likelihood of subsequent failure. Experimental measurements of fracture toughness are usually made on small specimens extracted from a larger ‘parent’ component following strict experimental guidelines, formulated to ensure measured toughness values in the fracture specimens are appropriate for use in the full‐size component. Implicit in this procedure is the assumption that the extracted fracture specimens contain no residual stresses, with any residual stresses in the full‐size component being accounted for in the crack driving force calculation. This paper considers a recent conjecture within the structural integrity community that the extracted fracture specimens may themselves contain a residual stress field which may influence measurements of fracture toughness. This could potentially lead to a degree of ‘double accounting’, i.e. the effect of residual stresses may be included in both the material toughness and the crack driving force. This, in turn, could lead to unnecessary conservatism in safety assessments. To explore this conjecture, the results of numerical modelling and neutron diffraction measurements of residual stresses in fracture specimens extracted from two different welded parent components are presented. One of the components is significantly larger than the extracted specimens, with the other being marginally larger than the extracted specimens. Results confirm the intuitive expectation that the residual stresses in specimens extracted from much larger components are negligible, whereas if the dimensions of the extracted specimens are comparable with the larger component then significant residual stresses may remain.     

THE FAILURE OF LIGAMENTS DUE TO MULTIPLE-SITE DAMAGE USING INTERACTIONS OF DUGDALE-TYPE CRACKS

A self-similar, narrow-strip Dugdale model is invoked to determine interactive crack tip plastic zones. Symmetric collinear multiple-site damage (MSD) cracks are considered for the coalescence of plastic zones between adjacent cracks. The Swift's ligament failure criterion is employed to predict the residual strength with or without considering the interaction between Dugdale-type plastic zones. Better agreement is achieved between the calculated results and experimental data if the interaction between the plastic zones is implemented in a computing scheme.     

Mechanics of strength-degrading contact flaws in silicon

The micromechanics of indentation-induced flaws in monocrystalline silicon have been studied in relation to strength determination. In the first part, the evolution of the deformation-fracture pattern during contact with a Vickers pyramid is described. Emphasis is thereby placed on the vital role of the residual component of the elastic-plastic contact field in driving the cracks. In the second part, the response of the cracks in subsequent strength testing is followed. A precursor stage of stable growth is observed prior to attaining a failure configuration, consistent with augmentation of the applied tensile (flexural) loading by the residual contact component. No detectable slow crack growth due to environmental influence is found. Nevertheless, silicon is revealed as a material of extreme susceptibility to brittle fracture, with significant strength degradation from contacts on the microscale. The relevance of this brittleness to the mechanical behaviour of silicon components as a function of fabrication and prospective service conditions is discussed.     

Fatigue limit prediction of notched components using short crack growth theory and an asymptotic interpolation method

Mechanical components have stress risers, such as notchs, corners, welding toes and holes. These geometries cause stress concentrations in the component and reduce the fatigue strength and life of the structure. Fatigue crack usually initiates at and propagates from these locations. Traditional fatigue analysis of notched specimens is done using an empirical formula and a fitted fatigue notch factor, which is experimentally expensive and lacks physical meaning. A general methodology for fatigue limit prediction of notched specimens is proposed in this paper. First, an asymptotic interpolation method is proposed to estimate the stress intensity factor (SIF) for cracks at the notch root. Both edge notched and center notched components with finite dimension correction are included into the proposed method. The small crack correction is included in the proposed asymptotic solution using El Haddad’s fictitious crack length. Fatigue limit of the notched specimen is estimated using the proposed stress intensity factor solution when the realistic crack length is approaching zero. A wide range of experimental data are collected and used to validate the proposed methodology. The relationship between the proposed methodology and the traditionally used fatigue notch factor approach is discussed.     

基于SINTAP-FAD方法的含裂纹缺陷钢结构构件安全性评定研究

在施工和运营过程中,钢结构构件上出现的裂纹缺陷可能会扩展而导致其发生断裂破坏,因此对含裂纹缺陷钢结构构件的安全性进行评估具有重要意义。首先,引入断裂力学K_IC断裂判据和失效评定图（FAD）概念,并介绍了结构整体性评估程序（SINTAP）的原理;给出了裂纹缺陷评定所需材料参数的确定方法,以及含裂纹缺陷构件的断裂力学参数和塑性屈服荷载的计算方法;然后,提出了基于SINTAP-FAD的含裂纹缺陷钢结构构件的安全性评估流程。最后,以华南和华东两个电厂锅炉钢结构大板梁的焊接缺陷为例,进行了基于K_IC断裂判据和SINTAP-FAD的裂纹缺陷安全性评定研究。结果表明,基于SINTAP-FAD的方法为含裂纹缺陷工作的钢结构构件的安全性评估提供了一种行之有效的方法,考虑多裂纹耦合作用的安全性评定方法有待进一步研究。