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.     

Festwalzen und Schwingfestigkeit

Deep Rolling and Fatigue Strength The fatigue properties of specimens and components are largely increased by deep rolling. Depending on geometrical shape of components and material strength the compressive residual stresses and the increased surface hardness made by deep rolling have a different effect on the improvement of fatigue strength. The fatigue properties of smooth specimens and components with a sufficient toughness can be raised by increase of surface hardness, whereas in case of notched parts the influence of permanent compressive residual stresses is dominant. The application of deep rolling in case of crankshafts shows a clear superiority of mechanical strengthening procedure to thermal surface strengthening. If there are some reasons to improve the wear behaviour beside the fatigue strength it is commendable to combine thermal and mechanical surface strengthening.     

Procedure and Results of Investigation into Fatigue Strength Characteristics of Motorcycle Engine Crankshafts

We present a procedure of studying the fatigue strength of crankshafts of a two-cylinder motorcycle engine. The use of a unique loading scheme provided obtaining patterns of in-service fracture of the tested crankshafts. The results of comparative fatigue tests of some series of crankshafts of various design and technological versions, namely, nonhardened crankshafts with a fillet and hardened and nonhardened crankshafts without a fillet (zero fillet) are given and analyzed.     

Fatigue strength of rolled JIS SM50A beams

The fatigue strength of rolled beams with stiffeners welded to the web was examined. The state of residual stress in these beams was varied by different cooling and straightening processes. Parallel series of tests were carried out both on plate specimens with corner-notches, simulating the edge notches in the beam-flange tip, and on fillet welded specimens. These test results were analysed using the fracture mechanics concepts of stable crack growth. A new fracture mechanics model for cracks originating from notches is proposed. This is based on the concept that the cyclic plastic zone size at the root of a notch determines the equivalent size of the notch as a fatigue crack. With this model, the severity of notches as well as their size can be taken into account to describe the initial flaw conditions. With the initial crack size estimated, a theoretical crack-growth equation was derived from the fatigue test data of rolled beams and notched plates. The analysing method together with the derived relationship was applied for the evaluation of the fatigue strength of welded beams and transverse fillet welds. The effect of residual stresses on fatigue behavior of these beams and plate specimens was also estimated by assuming an additional effect of the maximum stress on the theoretical crack-growth equation.     

Numerical modelling of deep rolling influence over crankshaft bending and correlation with fatigue behaviour

The requirements for mechanical reliability of automotive crankshafts are continuously increasing, thus pushing the demand for an optimized processing. Nonetheless, the manufacturing‐induced residual stresses at critical sites for fatigue enhancement are not clarified in the state‐of‐the‐art on the topic. In particular, there is a lack of information on the effect of final manufacturing stages to improve the component life endurance, such as deep rolling, in the overall stress state while the component is under operational loads. This study deepens the validation of a finite element deep rolling model under development with the aid of an in‐house developed crankshaft resonance fatigue test rig. The stress state obtained from the deep rolling simulation was input as a predefined stress field for the simulation of operational conditions experimented at the test rig. Test results produced cracks at the fillet radii of the cast iron crankshafts as anticipated. Overlapping the fractography with the simulation's final stress field yielded interesting correlation with the crack morphology. This contributed with a strong indication of the model correctness. Moreover, it can be further implemented to indicate whether the process parameters such as roller force and angle are fully optimized for each particular crankshaft application.     

Numerical simulation of fatigue crack propagation in compressive residual stress fields of notched round bars

In this paper, the influence of the residual compressive stresses induced by roller burnishing on fatigue crack propagation in the fillet of notched round bar is investigated. A 3D finite element simulation model of rolling has allowed to introduce a residual stress profile as an initial condition. After the rolling process, fatigue loading has been applied to three‐point bending specimens in which an initial crack has been introduced. A numerical predictive method of crack propagation in roller burnished specimens has also been implemented. It is based on a step‐by‐step process of stress intensity factor calculations by elastic finite element analyses. These stress intensity factor results are combined with the Paris law to estimate the fatigue crack growth rate. In the case of roller burnished specimens, a numerical modification concerning experimental crack closure has to be considered. This method is applied to three specimens: without roller burnishing, and with two levels of roller burnishing (type A and type B). In all these cases, the computational finite element predictions of fatigue crack growth rate agree well with the experimental measurements. The developed model can be easily extended to crankshafts in real operating conditions.     

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.     

Simulations of stress distributions in crankshaft sections under fillet rolling and bending fatigue tests

The residual stresses due to fillet rolling and the bending stresses near the fillets of crankshaft sections under bending fatigue tests are important driving forces to determine the bending fatigue limits of crankshafts. In this paper, the residual stresses and the bending stresses near the fillet of a crankshaft section under fillet rolling and subsequent bending fatigue tests are investigated by a two-dimensional plane strain finite element analysis based on the anisotropic hardening rule of Choi and Pan [Choi KS, Pan J. A generalized anisotropic hardening rule based on the Mroz multi-yield-surface model for pressure insensitive and sensitive materials (in preparation)]. The evolution equation for the active yield surface during the unloading/reloading process is first presented based on the anisotropic hardening rule of Choi and Pan (in preparation) and the Mises yield function. The tangent modulus procedure of Peirce et al. [Peirce D, Shih CF, Needleman A. A tangent modulus method for rate dependent solids. Comput Struct 1984;18:875–87] for rate-sensitive materials is adopted to derive the constitutive relation. A user material subroutine based on the anisotropic hardening rule and the constitutive relation was written and implemented into ABAQUS. Computations were first conducted for a simple plane strain finite element model under uniaxial monotonic and cyclic loading conditions based on the anisotropic hardening rule, the isotropic and nonlinear kinematic hardening rules of ABAQUS. The results indicate that the plastic response of the material follows the intended input stress–strain data for the anisotropic hardening rule whereas the plastic response depends upon the input strain ranges of the stress–strain data for the nonlinear kinematic hardening rule. Then, a two-dimensional plane-strain finite element analysis of a crankshaft section under fillet rolling and subsequent bending was conducted based on the anisotropic hardening rule of Choi and Pan (in preparation) and the nonlinear kinematic hardening rule of ABAQUS. In general, the trends of the stress distributions based on the two hardening rules are quite similar after the release of roller and under bending. However, the compressive hoop stress based on the anisotropic hardening rule is larger than that based on the nonlinear kinematic hardening rule within the depth of 2 mm from the fillet surface under bending with consideration of the residual stresses of fillet rolling. The critical locations for fatigue crack initiation according to the stress distributions based on the anisotropic hardening rule appear to agree with the experimental observations in bending fatigue tests of crankshaft sections.     

Lebensdauerberechnung festgewalzter Kerbproben unter Axialbelastung auf der Basis eines Rißfortschrittskonzepts

Axial fatigue life calculation of fillet rolled specimens by means of a crack growth model Fillet rolling is a method which significantly improves the fatigue strength of members. Residual compressive stresses induced in the surface layer during the fillet rolling process are able to retard or prevent crack propagation. An elastic‐plastic on the J‐integral based crack growth model considering the crack opening and closure phenomenon in nonhomogeneous plastic stress fields is described. Experimentally determined crack growth curves and fracture fatigue life curves at constant amplitude loading were used to verify the developed model.     

Fatigue crack initiation prediction of cope hole details in orthotropic steel deck using the theory of critical distances

Orthotropic steel decks are vulnerable to fatigue cracking in welded connections and complex geometrical details. A total of three fatigue tests were conducted on segments of orthotropic steel deck to evaluate the fatigue performance of trough‐to‐crossbeam connections with various cut‐out configurations. In the tests, the specimens were subjected to cyclic three‐point bending load and the fatigue cracks were more likely to initiate from the cope holes in the crossbeam web rather than the trough‐to‐crossbeam fillet welds. Three‐dimensional finite element models (FEM) of the specimens were built and validated by the measured deflections and stresses. Using the validated FEM, the characteristic stresses based on the theory of critical distances (TCD) were calculated for the stress concentrations along the cope holes. The fatigue crack initiation life, predicted by the TCD‐based stress combined with the plain material S–N curve, agreed reasonably with the fatigue test results. The TCD method could further form a basis of fatigue crack propagation analysis using the fracture mechanics approaches.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

Failure Analysis of a Vehicle Engine Crankshaft

An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.     

Zur Steigerung der Schwingfestigkeit durch mechanisch erzeugte Druckeigenspannungen

About the Fatigue Strength Improvement due to Compressive Residual Stresses Induced by Coldworking The fatigue strength improvement due to compressive residual stresses induced by coldworking depends essentially on the material, the type of load sequence, and the load level. The estimation of the fatigue life improvement is very difficult, because the load sequence can also induce residual stresses due to plastification in the notch root. In this case it gives complicate interaction effects between the two types of residual stresses. By tests the fatigue strength improvement due to stress coining was determined for two types of materials (an Al-alloy AlCuMg₂, and a fine grained structural steel FeE 47) under three different load sequences, at two load levels and on notched specimens of four different sizes. Besides the residual stresses induced by stress coining also those caused by the load sequences were evaluated. The results show, that the fatigue strength improvement of higher strength materials is superior to that of lower strength materials. In the case of higher strength materials the main effect results from the reduction of the mean stress by the compressive residual stresses while for the lower strength materials also the strain hardening by coldworking is important. Possibilities for the estimation of fatigue strength improvement will be presented.     

Einfluß von Druckvorspannungen auf die Dauerfestigkeit metallischer Werkstoffe bei ein‐ und mehrachsiger Beanspruchung

Effect of compression stresses on the fatigue strength of metals at uniaxial and multiaxial state of loading The calculation of the effect of multiaxial mean stresses on the fatigue strength required to consider the relative direction between the constant and cyclic parts of stresses. For this a new concept for the assessment of stresses has been presented for any fracture behaviour of materials. Experimental investigations on the effect of compression stresses on the fatigue strength of hollow cylinder samples from the steel 20MnCr5 has been used to support the parameter determination of the strength model     

Truck Diesel Engine Crankshaft Failure Analysis

A diesel engine crankshaft fractured in service after 76010 km of operation. The fracture took place on the first crankpin, and the fracture surface has a 45° inclination with respect to the axial. The results indicate that fatigue is the dominant failure mechanism of the crankshaft. It was observed that the fatigue crack initiated at the fillet region of the first crankpin-web. This crankpin is the one among the six crankpins which bear operational load. Absence of the induction hardening case in the fillet region decreased the fatigue strength and led to fatigue initiation and propagation in the weakened region. Although hard-rolling process was conducted in the fillet region, the depth of hard-rolling layer was insufficient to produce the desired residual compressive stress in the fillet region, and therefore the fillet could not offer resistance to the applied load. In addition, the presence of network-like ferrite in the microstructure facilitated the fatigue crack to be initiated and propagated.     

FATIGUE STRENGTH OF NON-LOAD-CARRYING FILLET WELDED JOINTS: EFFECTS OF WELD RESIDUAL STRESSES AND STRESS CONCENTRATION

The fatigue strength of non-load-carrying fillet welded joints of KE36(TMCP) steel was studied. Both residual stress measurements and fatigue tests were carried out, with the plate thickness, the plate width and the heat input being varied. Specimens given a Post Weld Heat Treatment (PWHT) were also prepared. The plate width had no effect on the fatigue strength, because it hardly affected the transverse residual stresses at the weld toe. However, the heat input influenced the transverse residual stress distribution, and a significant difference in fatigue strength due to the heat input was observed, especially when N≥ 10⁶ cycles. It was also found that PWHT removed almost all the residual stresses at the weld toe, improving the fatigue strength drastically. In this study, the values of stress concentration factor K₂ were estimated by Machida's method and it was concluded that the thickness effect resulted from a combination of both stress concentration and residual stresses with the contribution of the latter being particularly significant for N≥ 10⁶ cycles.     

Computation of fracture mechanics parameters for structures with residual stresses

Most manufacturing processes induce residual stresses in metallic parts, which will influence the in-service behaviour of the part with respect to fatigue and damage tolerance resistance, corrosion resistance and strength. In the particular case of a compressive residual stress field, the fatigue and damage tolerance behaviour is actually improved. The objective of this work is to develop a numerical method, based on the boundary element method and the principle of superposition, to assess the influence of residual stresses on fracture mechanics parameters. The numerical procedures are detailed and applied on an actual toughness test specimen, for which failure loads with and without residual stresses where measured. In spite on the complexity of the failure process with internal stresses, the method captures well the magnitude of the toughness variation.     

Effect of substrate pre-treatment on the low cycle fatigue performance of tungsten carbide-cobalt coated additive manufactured 316 L substrates

Numerous studies already identified that the fatigue strength of 316 L parts processed by laser beam melting (LBM) is distinctly affected by the surface integrity. Among others, surface defects as well as residual stresses are of crucial importance. Despite new findings in the field of surface engineering of laser beam melting (LBM) parts, the low cycle fatigue strength of thermally sprayed additively manufactured substrates has not been in the focus of research to date. This study aims at evaluating the effect of different pre-treatments onto 316 L substrates processed by laser beam melting (LBM) prior to the deposition of a high velocity oxy-fuel (HVOF) sprayed tungsten carbide-cobalt coating and their effect on the low cycle fatigue strength. Therefore, 316 L substrates were examined in their as-built state as well as after grit blasting with regards to the surface roughness, strain hardening effects, and residual stresses. To differentiate between topographical effects and residual stress related phenomena, stress-relieved 316 L substrates served as reference throughout the investigations. The tungsten carbide-cobalt coated and differently pre-treated 316 L substrates were mechanically tested under quasi-static and dynamic load conditions. Besides the low cycle fatigue strength, the fracture toughness as well as the fracture mechanism were identified based on fracture surface analysis.     

Fatigue assessment of high‐frequency mechanical impact (HFMI)‐treated welded joints subjected to high mean stresses and spectrum loading

The fatigue strength of welded joints can be improved with various post‐weld treatment methods. High‐frequency mechanical impact treatment is a residual stress modification technique that creates compressive residual stresses at the weld toe. However, these beneficial residual stresses may relax under certain loading conditions. In this paper, previously published fatigue data for butt and fillet welded joints subjected to high stress ratios and variable amplitude cyclic stresses were evaluated in relation to the current International Institute of Welding (IIW) recommendations on fatigue strength improvement and a proposed IIW design guideline for high‐frequency mechanical impact‐treated welded joints. The evaluation showed that the current IIW recommendations resulted in both non‐conservative and overly conservative fatigue strength estimations depending on the applied stress level, whereas the proposed fatigue assessment guideline fitted the current data well.     

Optimization of a crankshaft rolling process for durability

A crankshaft is often designed with a small fillet radius. The crankshaft fillet rolling process is one of the commonly adopted methods in engineering to improve fatigue life of the crankshaft. Compressive residual stresses on and below the fillet radius surface are induced through the fillet rolling operation. Consequently, fatigue life of the crankshaft is improved. An analytical technique is used to optimize the crankshaft rolling process to comply with a crankshaft design criterion for durability. A nonlinear finite element analysis is implemented to approximate the stress distributions induced by the crankshaft rolling process, and a crack modeling technique is developed to calculate the equivalent stress intensity factor ranges based on the combined residual and operational stress distributions along various crack growth planes. The threshold equivalent stress intensity factor range is obtained from previous staircase testing on crankshaft sections. The durability design criterion is met if the threshold equivalent stress intensity factor range exceeds the largest calculated equivalent stress intensity factor range. Due to the complexity of the modeling techniques in simulating the rolling process and calculating the equivalent stress intensity factors, a meta-model is generated based on the uniform design method for the choice of sample points and the quadratic polynomial fitting technique for a response surface generation. In the meta-model optimization process, rolling force, rolling angle, and fillet radius are the control factors, while the variations of the threshold equivalent stress intensity factor range, rolling force, rolling angle, and fillet radius are considered as the noise factors. By using the Hooke–Jeeves direct pattern search method and the Monte Carlo simulation technique, the optimal design is obtained for the highest reliability and the smallest coefficient of variation (COV).