A comparison of microcrack closure load development for stage I and II cracking events for Al 7075-T651

Closure stress measurements for both Stage I and II surface microcracks of lengths comparable to grain size are made using a compliance technique for Al 7075-T651 specimens fatigued in air at 30 pct relative humidity. As previously reported for Stage II cracking in Al 2219-T851, residual crack openings are observed for zero load which can be empirically related to the crack closure stress. This functional dependence is subsequently used to calculate closure stress values for both Stage I and II events for microcracks in Al 7075-T651 on the basis of residual crack opening values. Stage I crack interaction with grain boundaries is found to induce closure stresses close to the maximum applied stress, explaining the effective crack retardation of Stage I cracks by grain boundaries observed for Al 7075. In these cases the closure load is found to slowly fall as cracking continues by a Stage II mode into a second grain. For Stage n cracking it is possible to estimate the closure load from the cracking path as observed from the surface.     

焊接中残余应力的大小与分布及减少措施

熔化焊是通过高温熔化金属以实现连接的一种连接工艺。该焊接过程中金属材料受局部加热，并随之快速冷却，在焊剂与基体金属之间产生了较大的残余应力。其产生大大地降低该焊接结构的抗弯强度，并且容易导致脆性断裂，因此有必要对这种残余应力的大小和分布，以及其受焊接条件影响的规律进行探讨。该文运用ANSYS分析软件，分析了薄板单层电弧焊后残余应力的大小，并且讨论了焊接速度、焊件尺寸大小、焊件外部约束和预热等因素对其大小的影响及其减少的措施。     

Residual stress relaxation of welded steel components under cyclic load

It is a well known fact that the fatigue strength and the life of welded steel components are affected, to a considerable extent, by residual stresses distributed around their weldments. When externally applied load is superimposed on residual stresses, unexpected deformations and failure of the components can occur. These residual stresses are not constant, but are relaxed or redistributed during in‐service. Under monotonic load relaxation takes place when the sum of external and residual stress locally exceeds the yield stress of material used. It is noteworthy that under cyclic load the residual stress is considerably relieved by the first or the early cycles of load, and then is gradually relaxed with increasing loading cycles. Although many investigations in this field have been carried out, the phenomenon of and mechanism of stress relaxation are still not clear, and only a few comprehensive models have been proposed to predict amount of relaxed residual stress. In this study, the characteristics of residual stress relaxation under monotonic and cyclic load were investigated, and a model for quantitatively predicting the amount of residual stress relaxation is proposed.     

Rehabilitation of Cracked Aluminum Connections with GFRP Composites for Fatigue Stresses

The original design of existing aluminum overhead sign structures (OSS) did not consider fatigue as a limit state. Cracks propagate in the welds of the connection between the main chord and branches of OSS due to fatigue stresses caused by wind-induced vibration, which occasionally lead to complete fracture of the welds. A rehabilitation method for cracked aluminum welded connections using glass fiber-reinforced Polymer (GFRP) composites is investigated for its effectiveness under fatigue stresses. The results of constant amplitude fatigue tests for three types of aluminum connections from actual OSS are presented: (1) connections with no known cracks; (2) cracked connections rehabilitated with GFRP composites; and (3) connections with 90% of the weld removed and subsequently repaired with GFRP composites. The fatigue limits of the three connection types are established for four stress ranges including the constant amplitude fatigue limit threshold. The rehabilitated connections from OSS exceeded the fatigue limit of the aluminum welded connections with no known cracks. The repaired connections with 90% of the weld removed satisfied the constant amplitude fatigue limit threshold. A cumulative damage index is established which leads to a fatigue reduction factor for the rehabilitation design of cracked aluminum connections using the GFRP composites.     

Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-T7451

Aluminum alloy 7050 was friction-stir welded (FSW) in a T7451 temper to investigate the effects on the microstructure and mechanical properties. Results are discussed for the as-welded condition (as-FSW) and for a postweld heat-treated condition consisting of 121 °C for 24 hours (as-FSW + T6). Optical microscopy and transmission electron microscopy (TEM) examination of the weld-nugget region show that the FS welding process transforms the initial millimeter-sized pancake-shaped grains in the parent material to fine 1 to 5 μm dynamically recrystallized grains; also, the FS welding process redissolves the strengthening precipitates in the weld-nugget region. In the heat-affected zone (HAZ), the initial grain size is retained, while the size of the strengthening precipitates and of the precipitatefree zone (PFZ) is coarsened by a factor of 5. Tensile specimens tested transverse to the weld show that there is a 25 to 30 pct reduction in the strength level, a 60 pct reduction in the elongation in the as-FSW condition, and that the fracture path is in the HAZ. The postweld heat treatment of 121 °C for 24 hours did not result in an improvement either in the strength or the ductility of the welded material. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW + T6 condition, at a stress ratio of R = 0.33, shows that the FCG resistance of the weldnugget region is decreased, while the FCG resistance of the HAZ is increased. Differences in FCGRs, however, are substantially reduced at a stress ratio of R = 0.70. Analysis of residual stresses, fatigue-crack closure, and fatigue fracture surfaces suggests that decrease in fatigue crack growth resistance in the weld-nugget region is due to an intergranular failure mechanism; in the HAZ region, residual stresses are more dominant than the microstructure improving the fatigue crack growth resistance.     

Numerical Analysis of Stake Mark Induced Failure of a Rotor Shaft

In this paper, the stake mark induced failure of a generator rotor forging is analysed using finite element analyses in combination with a Cockroft & Latham failure model. Evidence is found from damage analysis that the rotor failed due to micro cracks and residual stresses induced by a double stake mark, which was located at a position subject to concentrated rotating‐bending stress amplitudes and torsional mean stress. The analysis presented here verifies the results of the damage analysis by showing that the stake mark configuration of the failed rotor was the most detrimental among a number of similar configurations. According to the finite element analysis, this stake mark configuration combines a high probability of inducing propagating sub‐surface micro‐cracks with considerable tensile residual stresses.     

Residual Stress Measurement and Fatigue Crack Growth Prediction after Cold Expansion of Cracked Fastener Holes

The cold expansion technique is often used to introduce beneficial compressive stress at fastener holes, and can be used for remedial work where cracks already exist. In this paper, results are presented showing the effect of preexisting cracks on the residual stress field produced by cold expanding a fastener hole, and on subsequent fatigue crack growth. The effect on the residual stresses was experimentally evaluated in two ways: indirectly, in terms of retained expansion and directly, by measurement of the stresses using the X-ray and neutron diffraction techniques. The retained expansion ratio showed that cold expansion is more sensitive to the existence of precracks at lower levels of applied interference, and the inlet and outlet faces have different behavior. The stress measurements showed that preexisting cracks reduce the compressive residual stresses more on the mandrel inlet face than on the outlet face and in the middle of the specimen. The effect on fatigue crack growth rates was modeled using a linear-elastic fracture mechanics approach. It was found that cold expansion of a hole containing a preexisting crack longer than 1 mm introduces little benefit for subsequent fatigue crack growth behavior.     

共线多孔边裂纹的应力强度因子研究(英文)

An analytical method for calculating the stress intensity factor of multiple cracks at collinear holes in an infinite plate was introduced. Complex functions are used to calculate the stress functions. Approximate superposition method is applied to solve SIF problems of multi-hole plate. Numerical examples of symmetrical cracks arising from collinear holes are examined by this method. By comparing the analytical results and finite element results, it was realized that the analytical method is valid but has limitations. Use multi-stage calculation method, SIF problems on symmetrical cracks arising from collinear holes can be solved.     

Thermal microstresses in beryllium and other HCP materials

The thermal stresses that may develop due to crystal anisotropy within the microstructure of beryllium and certain other hcp materials are described. The magnitude of the unrelaxed thermal stress in polycrystalline beryllium is estimated numerically. The calculated values for the local thermal stresses that might occur in polycrystalline beryllium are comparable to the bulk stresses that are measured for yielding and fracture. The possible influence of these stresses on the initiation of cleavage cracks in rolled beryllium sheet and in extruded beryllium plate is described. N. R. Borch, formerly with Lawrence Radiatior. Laboratory, University of California, Livermore, Calif.     

The effect of residual stress on the fatigue crack growth behavior of Al-Si-Mg cast alloys—Mechanisms and corrective mathematical models

The fatigue crack growth (FCG) behavior of various types of alloys is significantly affected by the presence of residual stress induced by manufacturing and post-manufacturing processes. There is a qualitative understanding of the effects of residual stress on fatigue behavior, but the effects are not comprehensively quantified or accounted for. The difficulty in quantifying these effects is largely due to the complexity of residual-stress measurements (especially considering that parts produced in similar conditions can have different residual-stress levels) and the lack of mathematical models able to convert experimental data with residual stress into residual-stress-free data. This article provides experimental, testing, and mathematical techniques to account for residual-stress effects on crack growth rate data, together with two methods for eliminating residual stresses in crack growth test specimens. Fracture-mechanics concepts are used to calculate, in simple and convenient ways, stress-intensity factors caused by residual stresses. The method is advantageous, considering that stress-intensity factors are determined before the actual test is conducted. Further on, residual-stress-intensity factors are used to predict the residual-stress distribution in compact tension (CT) specimens prior to testing. Five cast Al-Si-Mg alloys with three Si levels (in unmodified (UM) as well as Sr-modified (M) conditions) were analyzed both with and without residual stress. Fatigue cracks are grown under both constant stress ratio, R=0.1, and constant maximum stress-intensity factor, K _max = const., conditions. The mechanisms involved in crack growth through residual-stress fields are presented.     

Structural Integrity Assessment of Welded Pressure Vessel Produced of HSLA Steel

Welded joint is a critical region of a welded structure.The operational safety of welded pressure equipment mostly depends on the behaviour of loaded weldments.Safety of welded structure is dependent on the properties of welded joint as whole and of its constituents (parent metal,heat affected zone and weld metal).In this paper the behaviour of welded joint cracked constituents is considered.Structural integrity assessment procedure is applied to welded pressure vessel produced of high-strength low-alloyed steel,operating at-40°C,comparing crack driving force and material crack resistance,using path-independent contour J-integral as fracture mechanics parameter.The comparison of crack driving force,expressed by J-integral and material resistant curve,J-R curve,provide the possibility to determine the extent of the stable crack as well as the critical crack size for its final fast propagation and also to assess the structural integrity of a cracked pressure vessel.     

A Benchmark Study on Casting Residual Stress

A benchmark study was undertaken for casting residual stress measurements through neutron diffraction, which was subsequently used to validate the accuracy of simulation prediction. The “stress lattice” specimen geometry was designed such that subsequent castings would generate adequate residual stresses during solidification and cooling of ductile cast iron, without any cracks. The residual stresses in the cast specimen were measured using neutron diffraction. Considering the difficulty in accessing the neutron diffraction facility, these measurements can be considered as a benchmark for casting simulation validations. Simulations were performed using the identical specimen geometry and casting conditions for predictions of residual stresses. The simulation predictions were found to agree well with the experimentally measured residual stresses. The experimentally validated model can be subsequently used to predict residual stresses in different cast components. This enables incorporation of the residual stresses at the design phase along with external loads for accurate predictions of fatigue and fracture performance of the cast components.     

Fatigue crack propagation in carburized high alloy bearing steels

Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, K_i where K_i, = σ_id _i ^1/2 (σ_i = internal stress, d_i = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes K_e = K_a + K_i where K_a is the applied stress intensity. Values of K_i were calculated as a function of distance from the surface using experimental measurements of σ_i and a value of d_i = 11 mm (0.43 inch). The resultant values of K_e were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with K_lc = 50 MPa · m^1/2 (45 Ksi · in^1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive stresses, and fracture is resisted by the relatively tough core.     

LEFM Analysis and Service Simulation Testing of a Welded Frame Structure

In this paper the results from fatigue analysis and testing of a welded frame are discussed. The structure contains some typical welds, which can be found on a frame to a wheel loader. The test was carried out with a service load spectrum with an overall stress ratio, R, of about ‐1. The fatigue cracks started from the root side of the welds in all cases. The test results were correlated with linear elastic fracture mechanics (LEFM) including different assumptions of residual stress distributions.     

Residual stresses in rails due to roll straightening

The knowledge of residual stresses in rails due to roll straightening has become increasingly important for the evaluation of the fatigue behaviour of rails. The residual stresses are induced both by the bending process and the roll contact leading to a very complicated distribution especially near those parts of the surface where the roll contact had taken place. So far, no clear picture has emerged with respect to the residual stress field, either from experiments or from simulations. This study intends to clarify the residual stress distribution by simulation with a fully three-dimensional model taking into account both the bending and contact process. This work shows that especially in those areas where the roll contact happens compressive residual longitudinal stresses appear on the surface. Furthermore, a very steep gradient of the residual longitudinal stress distribution near the surface is observed which is reported also in very recent X-ray and neutron diffraction experimental investigations.     

Finite element simulation of the influence of TiC inclusions on the fatigue behavior of NiTi shape-memory alloys

In fatigue experiments of NiTi shape-memory alloys (SMAs), TiC inclusions have been found to cause cracks. Based on bending-rotation fatigue (BRF) experiments, which have evolved as one standard method to study the structural fatigue of superelastic NiTi wires, the influence of TiC inclusions on the fatigue behavior of NiTi SMAs has been analyzed quantitatively in this article. Aurichio’s superelastic model was implemented into the finite element (FE) code ABAQUS. One specimen without inclusion and seven specimens with inclusions, at different distances with respect to the neutral axis of the wire specimens, have been analyzed. The stress distributions at the cross sections are nonlinear, and there is a stress plateau in the cross section when the phase transformation occurs. The stress distribution in the cross section of the specimen without inclusion is not only dependent on the load, but also dependent on the loading path and loading history. On the other hand, the maximum stress of the specimen without inclusion is not always at the surface, which is due to the phase transformation behavior of SMAs. The existence of the inclusions changes the stress distributions in the cross section. The maximum stress is dependent on the position of the inclusions, the load, and the loading path. It has been found that the maximum stresses increase as the distance from the inclusion to the neutral axis increases. When the inclusion is at the specimen surface, the maximum stress is the highest among all the studied cases. Such high stresses caused by the inclusions can easily induce fatigue cracks. The simulation can explain the fatigue behavior of BRF experiments and provide a deep insight into the fatigue fracture mechanism of SMAs.     

Analysis of strip edge cracks during cold rolling of thin strip

In this paper,the mechanics of strip edge cracks and its propagation has been studied,and the effects of strip edge drop and stress intensity factor(SIF) on edge crack defections during cold rolling of thin strip have been discussed.An experimental investigation was presented into the effect of strip edge drop on edge cracks during cold rolling of thin strip.The edge crack increases significantly due to more inhomogeneous deformation and work hardening at the strip edge.The effective stress intensity fac...     

Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and K _max effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK–K _max approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications.     

Thermal Relaxation of Residual Stresses in Nickel-Based Superalloy Inertia Friction Welds

This article describes an experimental study aimed at characterizing the extent of residual stress relaxation during thermal treatment of inertia friction-welded alloy 720Li nickel-based superalloy welded tubular rings. In the as-welded condition, yield level tensile hoop stresses were found by neutron diffraction in the weld region along with axial bending stresses (tensile toward the inner diameter (ID)/compressive toward the outer). The evolution of these residual stress levels during postweld heat treatment (PWHT) was mapped experimentally over the weld cross section. After 8 hours of PWHT, the axial stresses relaxed by 70 pct, whereas the hoop stresses reduced by only 50 pct. Some scatter of residual stress evolution was found between samples, particularly for the axial stress direction. This was attributed to substandard tooling to grip the rings. The results on subscale samples were transferred to a full-scale aeroengine (650-mm diameter) compressor drum assembly that was postweld heat treated for 8 hours. It was found that the residual stresses, particularly in the axial direction, were noticeably lower in this full-scale weld component compared to the subscale weld heat treated for the same time. The differences seem to be best rationalized by the different standards of jigging used during joining these two types of welds.