Zum Mechanismus der Wechselfestigkeitssteigerung durch Werkstoffverfestigung und Druckeigenspannungen nach einer Oberflächenbehandlung

The Improvement of Fatigue Limit as a Result of Hardening and Macrostresses Due to a Surface Treatment Surface treatments, that increase the hardness as well as induce surface residual macrostresses, are universaly able to improve the fatigue limit. It is shown, that depending on the shape of specimens both effects together are responsible for the raise of the fatigue strength, which is in contrast to former opinions. The increase of hardness increases the stress required for crack initiation and is thus decident for unnotched specimens, whereas in this case the influence of permanent residual stresses is relatively smaller. Notched specimens of sufficient stress concentration factor k_t are determined by the crack propagation conditions, which can be controlled decisively by mean loads. The increase of hardness improves the resistance against crack initiation proportional to the 1/k_t portion of the unnotched fatigue limit, but cracks remain nonpropagating as long as a certain minimum alternative stress, which can be raised by compressive residual stresses, is not exceeded. Depending upon concentration factor, mean compressive load and hardness the transition from crack initiation to crack propagation as the criterion for fatigue fracture can be estimated by several fatigue-strength-diagrams, which are evaluated for specimens of constant hardness but are valid for surface hardened specimens as well.     

Größeneinfluß und Dauerfestigkeitseigenschaften unter Besonderer Berücksichtigung optimierter Oberflächenbehandlung

Size Effect and Fatigue Properties with Respect to Optimized Surface-Treatment. A hyperbolic function describes the geometrical size effect of notched specimens made from heat treated steel. An estimation of fatigue properties of components under one level fatigue tests is possible, if there are comparable materials and surface properties. The fatigue properties of specimens are well described by standardized stress-N graphs. The slope of the stress-N graphs in the range of load cycle depends on the concentration factor and not on the size effect. The fatigue properties of components are largely increased by thermal and mechanical surface strengthening. For the determination of the improvement of fatigue properties it is important to known the initiation of cracking. The improved fatigue properties of inductive surface hardened smooth specimens can be explained by the initiation of cracking below the surface. Mechanically strengthened notched specimens start cracking on the surface. The increase of fatigue properties for these specimens is explained by compressive residual stresses. The fatigue properties of notched specimens can be improved by the optimisation of mechanical strengthening, to higher values than for smooth surface strengthened specimens. This is due to compressive residual stresses. They decrease the tensile stresses which are responsible for crack propagation. If the tensile stress is below fatigue limit for initiation of cracking the crack arrests immediately.     

Assessment of specific contribution of residual stress generated near surface anomalies in the high temperature fatigue life of a René 65 superalloy

This study evaluates the influence of residual stresses induced by the fabrication of surface anomalies on the fatigue crack growth in a nickel based superalloy. To separate the notch effect of the geometry from the residual stress field induced by fabrication of the surface flaws, two V-type anomalies are considered: scratches and dents with equivalent morphology and size. A specially designed heat treatment has been used to reduce the magnitude of residual stresses around these anomalies in order to highlight their effects on the different stages of the crack propagation, under low cycle fatigue conditions at 400 °C. The crack initiation life is short for both anomalies but in the presence of compressive residual stresses, a decrease of the fatigue crack growth rate has been observed during the first stages of the crack propagation. Furthermore, the results showed that without residual stresses, scratches and dents exhibit the same behaviour. Thus, the residual stress field below surface anomalies is the main parameter controlling the fatigue life from surface anomalies.     

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.     

Fatigue Resistance of an Anodized and Hardanodized 6082 Aluminum Alloy Depending on the Coating Thickness in the High Cycle Regime

In this study, the high cycle fatigue behavior of an anodized 6082 aluminum alloy is investigated. Main focus is on the most relevant influencing factors for crack initiation and propagation under cyclic loading and damage mechanisms considering coating type, thickness, and residual stresses. The bare substrate is compared to anodized and hardanodized specimens with three coating thicknesses, for each coating type, in the range from 20 to 70 μm. Coating hardness and microstructure as well as residual stresses are analyzed. Fatigue and fracture behavior under alternating tension–compression loading is determined. Dependent on the coating thickness, the fatigue strength is reduced by 8%–50% after anodizing and by 50%–62% after hardanodizing. As the coating thickness is equal to the initial crack length from a fracture mechanical point of view, stress intensities at the crack tips are higher for thicker coatings respectively longer initial crack lengths. Therefore, propagation of fatigue-induced cracks from the coating into the substrate is promoted for a higher coating thickness resulting in premature failure. A significant correlation between the coating thickness and tensile residual stresses induced by both coatings in the subjacent substrate is not found and residual stress influence on the overall fatigue strength is only minor.     

An experimental and analytical study of fatigue crack shape control by cold working

This paper presents an experimental and analytical study of crack shape evolution in steel specimens under cyclic loading. It is widely known that the introduction of compressive residual stresses by cold working the surface can be highly beneficial in improving the fatigue performance of structural components. Although it is recognised that relaxation of surface compressive residual stress can reduce the potential benefits, the effects of residual stress on crack shape evolution are often overlooked. A recently developed technique termed controlled stitch cold working, which applies differing intensities of compressive residual stress at specific regions in a structure, is shown in the paper to considerably influence fatigue crack propagation by containing crack propagation in one primary direction.     

Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

Fatigue testing under fully reversed axial loading (R=?1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.     

FE‐Festwalzsimulation und Dauerfestigkeitsberechnung festgewalzter Bauteile mit Konzepten der Schwingbruchmechanik

FE‐Simulation of Fillet Rolling and Fatigue life Calculation based on Fracture Mechanics Concepts Fillet rolling is a method which significantly improves the fatigue strength of members. Residual stresses induced in the surface layer during the fillet rolling process are able to retard or prevent crack propagation. For fatigue strength prediction of fillet rolled notched members a fracture mechanics based concept is described. It consists of three parts: • Finite element simulation of the fillet rolling process to calculate the residual stresses • Simulation of residual stress redistribution due to cyclic load • Assessment of fatigue cracks starting from notch roots and propagating under compressive residual stresses by means of fracture mechanics.     

Effects of residual stresses on the fatigue behaviour of welded steel structures

Residual stresses due to the welding process in steel structures can significantly affect the fatigue behaviour. Usually, high tensile residual stresses up to the yield strength are conservatively assumed at the weld toes. This conservative assumption can result in misleading fatigue assessments. Areas with compressive residual stresses may be present in complex structures, where the details are less critical than predicted. This is shown in the paper by the example of fillet‐welded stiffener ends, where beneficial compressive residual stresses cause the initiation of fatigue cracks at other locations in less‐strained areas. Another example for the effects of residual stresses concerns the stress initiation and propagation at a structural detail under fully compressive load cycles. Fatigue cracks are possible here due to high tensile residual stress fields. The conclusion is that the welding‐induced residual stresses should be known in advance for a reliable fatigue assessment, which becomes possible to an increasing extent by numerical welding simulation.     

Fatigue crack propagation into the residual stress field along and perpendicular to laser beam butt‐weld in aluminium alloy AA6056

Laser beam butt welds in Al‐alloys are very narrow and are accompanied by steep residual stress gradients. In such a case, how the initial crack orientation and the distance of the notch tip relative to the weld affect fatigue crack propagation has not been investigated. Therefore, this investigation was undertaken with two different crack orientations: along the mid‐weld and perpendicular to the weld. Fatigue crack propagation ‘along the mid‐weld’ was found to be faster in middle crack tension specimens than in compact tension specimens. For the crack orientation ‘perpendicular to the weld’, the relative distance between the notch tip and the weld was varied using compact tension specimens to generate either tensile or compressive residual stresses near the notch tip. When tensile residual stresses were generated near the notch tip, fatigue crack propagation was found to be faster than that in the base material, irrespective of the difference in the initial residual stress level and whether the crack propagated along the mid‐weld or perpendicular to the weld. In contrast, when compressive weld residual stresses were generated near the notch tip, fatigue crack arrest, slow crack propagation, multiple crack branching and out of plane deviation occurred. The results are discussed by considering the superposition principle and possible practical implications are mentioned.     

Festigkeit und Spannungsabbau bei der dynamischen Belastung von Stahl- und Aluminiumproben mit bekannten Eigenspannungen

Strength and Relaxation of Stress During Dynamic Loading of Steel and Aluminium Specimens with known Residual Stress Distributions Residual stresses of the first kind can be introduced in multiply – connected bodies without accompanying changes in the material. The circular ring represents the simplest example in this group. For the circular ring the residual and applied stresses in a fatigue test can be calculated accurately with relations from the theory of elasticity. Circular rings with and without residual stresses were subjected to fatigue testing. It was established that tensile residual stresses reduce the fatigue life and compressive residual stresses have the opposite effect, with the stresses referred to the point of crack initiation. The fatigue behaviour is altered appreciably even by small magnitudes of residual stress. The effect on fatigue life is the same, whether at the point of crack initiation a residual stress or a mean stress of the same magnitude and direction acts. The stress relaxation depends primarily on the difference between the maximum superposed stress and the yield strength or the 0.2% proof stress. The degree of relaxation is particularly high when the yield strength is reached. The first few cycles in a fatigue test are important for the fading of stress. The state of residual stress changes only slightly thereafter. The stress relaxation increases again somewhat only after a very large number of cycles. The changes observed when the sum of residual and loading stresses is below the yield strength may be attributed to the small plastic deformations in favourably oriented crystallites.     

Behaviour of small cracks during their propagation from Vickers indentations in coarse-grain steel: An experimental investigation

In this investigation we look at the influence of the local residual stresses caused by Vickers-pyramid indenting on the initiation and early propagation of small cracks from indentations in coarse-grain martensitic steel. The size of these indentations is comparable to the grain size. Specimens with and without a local residual stress field were tested on a rotary bending machine. A focused ion beam and a scanning electron microscope were used to reveal the influence of those stresses on the location of the cracks’ initiation and the mechanism of the small-crack propagation. The existing local residual stresses assist in the initiation of two cracks at a level lower than the fatigue limit. The early small-crack propagation is gradually obstructed by the residual stress-field configuration until the cracks become non-propagating cracks. At levels higher than the fatigue limit, both cracks succeed in breaking through the compressive stressed domain and link together. From that moment the crack begins to behave as a long crack, penetrating outside the indentation into the tensile-stressed domains.     

Fatigue of fillet welded A515 steel

A study has been made of the fatigue behavior of fillet welded ASTM A515 steel. As-welded and stress-relieved skip fillet weld specimens were tested under pulsed tension and altering cyclic load to determine stress-life and crack propagation behavior. Crack initiation and propagation features were determined from sectioned surfaces. All fatigue cracks were semi-elliptic and initiated from weld end toes. The length/depth ratio was approximately constant during propagation. There was no consistent effect of tensile residual stress on fatigue life under pulsed tension but there was a detrimental effect under alternating loads. An equivalent crack model has been proposed to quantify the stress concentration effect at the crack initiation site based on the application of the Paris equation. The test results show that the equivalent crack can give a reasonable prediction of the fatigue life of a welded structure and is a potentially convenient tool in fatigue design.     

Modellierung des Ermüdungsrisswachstums in Schweißverbindungen unter Berücksichtigung von Schweißeigenspannungen

The present paper contains research results determined within the framework of a project called IBESS (?Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“) by the Materials Mechanics Group of the Technische Universität Darmstadt [1]. Aim is to calculate the fatigue life of welded joints by taking into account the effect of residual stresses and the influence of the weld toe geometry. Here, the fatigue life is regarded as period of short fatigue crack growth. Two and three dimensional finite element models, with cracks as initial defects, are constructed for this purpose. Fatigue crack growth analyses are performed by using the node release technique together with the finite element program ABAQUS. The welding residual stresses as well as the plasticity induced crack closure effects are considered. Structural calculations are performed in order to introduce residual stress fields in finite element models. The calculated compressive residual stress field matches the measured one especially in the weld notch area. The effective cyclic J‐integral (ΔJ_eff) is used as crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a Python code was written for the determination of ΔJ_eff at every crack length phase. The calculated fatigue lives were compared with experimental data and a good accordance between both results was achieved. The impact of welding residual stresses on ΔJ_eff as well as on the fatigue life during short crack growth was investigated. As expected, results revealed that at lower stress amplitude, a compressive residual stress field is favorable to the fatigue life, whilst a tensile residual stress field is unfavorable. The influence of residual stresses can be neglected only for large load amplitudes.     

Effects of Shot‐Peening on Fretting Fatigue Crack Growth Behavior in Ti‐6Al‐4V

Abstract: The effects of shot‐peening on fretting fatigue crack growth behaviour in titanium alloy, Ti‐6A1‐4V were investigated. Three shot‐peening intensities: 4A, 7A and 10A were considered. The analysis involved the fracture mechanics and finite element sub‐modelling technique to estimate crack propagation lives. These computations were supplemented with the experimentally measured total fretting fatigue lives of laboratory specimens to assess the crack initiation lives. Shot‐peening has significant effect on the initiation/propagation phases of fretting fatigue cracks; however this effect depends upon the shot‐peening intensity. The ratio of crack initiation and total life increased while the ratio of the crack propagation and total life decreased with an increase of shot‐peening intensity. Effects of residual compressive stress from shot‐peening on the crack growth behaviour were also investigated. The fretting fatigue crack propagation component of the total life with relaxation increased in comparison to its counterpart without relaxation in each shot‐peened intensity case while the initiation component decreased. Improvement in the fretting fatigue life from the shot‐peening and also with an increase in the shot‐peening intensity appears to be not always due to increase in the crack initiation resistance from shot‐peened induced residual compressive stress.     

The influence of roller burnishing on the fatigue crack propagation in notched round bars—Experimental observations under three-point bending

Roller burnishing involves a local plastic deformation on the surface that permits the fatigue strength of structures to be increased. Crack propagation is delayed by the introduction of compressive residual stresses. In this way, the process is particularly useful in the presence of stress concentrators, for example in the fillets of crankshafts or in notched shafts. Crack propagation in round bars has been widely investigated, experimentally and numerically. However, the aim of the present work is to study roller burnished notched shafts (whose groove approaches the fillets of crankshafts) loaded in three‐point bending. Experimentally, a sequence of destructive interrupted tests has been performed to obtain the crack kinetics and shapes during fatigue life. Thus, the beneficial influence of roller burnishing has been confirmed on both crack initiation and propagation. Some cracks were even stopped by compressive residual stresses. Otherwise, the crack shape observed was very original : cracks propagated first at the edge of the coupon and then, into the depth of the shaft. Multicracking and crack closure have been pointed out and appear to be of great importance.     

关于残余应力的静载松弛与最佳喷丸残余应力场的研究

本文研究了喷丸残余应力场在疲劳加载初期的静载松弛现象及机理,并对最佳喷丸残余应力场进行了探讨。试验结界表明,残余应力在疲劳过程中的静载松弛是工件表层材料剧烈塑性变形的结果。残余应力的静载松弛会使晶界、相界等障碍物处形成一定数量的微裂纹,给工作表层材料带来损伤,降低疲劳裂纹的形核寿命。为了避免疲劳初期由于残余应力的静载松弛所造成的损伤,在喷丸后采用应力松弛低温回火工艺,预先降低残余应力场中的最大残余压应力值,建立最佳残余应力场。这种通过热激活的方式使残余应力发生的松弛属子非损伤性松弛,因而能够有效地提高材料在S—N曲线上的较高交变应力区的疲劳寿命。     

Investigation of residual stresses in a repair‐welded rail head considering solid‐state phase transformation

Repair welding for recovery from local damage of a rail head surface is known to cause high residual stress and can accelerate fatigue in the rail. This study examines repair‐welded rails by applying experimental and numerical approaches. In the former approach, two newly manufactured rail specimens and four repair‐welded rail specimens with two different weld depths were prepared, and their residual stresses were measured with a sectioning method. In the latter approach, a finite element repair welding simulation model was developed that adopted a prescribed temperature method with a moving block as an input heat source, and the thermal strain caused by the volume change due to solid‐state phase transformation was considered. Overall, the residual stresses correlated well between the experimental and numerical approaches. The measured high compressive residual stress of ?290 MPa seems to be beneficial to prevent a crack initiation in the rail surface.     

残余应力对金属疲劳强度的影响

残余应力对光滑试样高周疲劳极限的影响可以用Goodman关系来描述，但必须要得到残余应力作用系数m、合理地提取残余应力的表征值和区分开其它因素的影响。残余应力对缺口疲劳极限的作用大于对光滑试样的作用，是由于残余应力也存在应力集中现象，而且不易衰减。残余应力的应力集中系数不仅与缺口几何因素有关，还与材料特性有关。试验研究还表明，表层残余压应力对于承受轴向载荷且疲劳残纹萌生于表面的零件也十分有益。     

SUBSURFACE CRACK INITIATION DURING FATIGUE AS A RESULT OF RESIDUAL STRESSES

Abstract— The influence of shot peening on the bending fatigue strength of hardened specimens of a carbon steel is reported. Effects of residual compressive stresses after shot peening, as a function of distance from the surface, are discussed along with the evidence of scanning electron micrographs from fractured specimens. Subsurface crack initiation is reported at all stress amplitudes below a threshold value of 1100 N/mm². Assuming that the fatigue strength is enhanced locally due to compressive residual stresses the experimental results can be explained with the aid of the Goodman relationship.