On the assessment of fatigue life of marine diesel engine crankshafts

The fatigue strength and its correct assessment play an important role in design and maintenance of marine crankshafts to obtain operational safety and reliability. Crankshafts are under alternating bending on crankpins and rotating bending combined with torsion on main journals, which mostly are responsible for fatigue failure. The commercial management success substantially depends on the main engine in service and of its design crankshaft, in particular. The crankshaft design strictly follows the rules of classification societies. The present study provides an overview on the assessment of fatigue life of marine engine crankshafts and its maintenance taking into account the design improving in the last decades, considering that accurate estimation of fatigue life is very important to ensure safety of components and its reliability. An example of a semi-built crankshaft failure is also presented and the probable root case of damage, and at the end some final remarks are presented.     

Berechnung der Schwingfestigkeit festgewalzter Kurbelwellen

Predicting the fatigue strength of fillet-rolled crankshafts Since three years Darmstadt University of Technology uses finite element method for simulation of fillet rolling process. Now, together with Daimler-Benz AG, a fracture mechanics based concept has been successfully applied predicting the fatigue strength of fillet-rolled crankshafts. For these parts conventional assessment of fatigue behaviour shows several disadvantages. The new concept reduces time and costs for development and design. It consists of three parts:

• calculation of residual stresses induced by fillet rolling and affected by crankshaft and roller geometry, rolling load and work hardening data of material
• simulation of residual stress redistribution due to cyclic load
• assessment of fatigue cracks starting from notch root and propagating under compressive residual stresses by means of linearelastic fracture mechanics.

     

Dual metal crankshaft for heavy‐duty engines by cast‐joining of DCI and forged steel

An alternative design of a crankshaft applicable for heavy‐duty diesel engines up to 5000 horsepower has been introduced to achieve cost effectiveness and manufacturing efficiency simultaneously not sacrificing its reliability. Because it is expensive and time consuming to make full‐scale prototype and run a test, analytical feasibility study has been made in this paper. From the point of material, cast‐joined dual metal for a crankshaft is totally unprecedented and will be realized by virtue of the latest dual metal cast joining of ductile cast iron (DCI) and forged steel, which consequently reduces the manufacturing lead time dramatically. The strength of the dual metal interface is experimentally investigated to make sure its possibility and soundness. The key features of the new crankshaft include crankpins and journals made of forged steel and crankwebs of DCI. Prior to actual manufacturing of the new conceptual crankshaft, the design is analytically examined to verify its reliability in an existing heavy‐duty engine. Both conventional and the new dual metal crankshafts are investigated and compared to each other in terms of the stress and fatigue using comprehensive multi‐body dynamic analysis. The results demonstrate that the new dual metal approach is likely to provide higher reliability than the conventional monometallic crankshafts. In addition, it is shown that the inferior mechanical properties of DCI for a crankweb are effectively countered by the superior characteristics of forged steel for a crankpin and journal in the dual metal crankshaft.     

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).     

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.     

Measurement and analysis of wheel loads for design and fatigue evaluation of vehicle chassis components

Using a multiaxial‐randomly loaded air‐suspension rear‐axle for commercial vehicles as an example, this paper gives an insight in the mechanics and interactions of wheel forces and moments, forces acting in the suspension components and the stress response of the axle casing. Taking load‐time and stress‐time data measured on a prototype vehicle on a test track as a basis, fatigue, frequency and correlation analyses for all relevant manoeuvres and straight‐ahead driving situations are performed. Special focus is given to the study and determination of appropriate correlations between the mentioned properties for manoeuvres and straight ahead driving on rough roads. The results point out all decisive load situations which may affect the fatigue behaviour of the axle under operational conditions and were taken into account for the derivation of optimized design solutions, as well as for the experimental verification of the final solution in the test rig.     

Influence of the forming angle in cross wedge rolling on the multi-directional forging of crankshafts

In each industrial process for the production of crankshafts a low energy demand and a fast processing time are required. Crankshafts have a very complex geometry and are forged with a high percentage of flash compared to other forging parts. Recent research showed the feasibility of a flashless forging of crankshafts. One way to forge a flashless crankshaft within three steps is to use cross wedge rolling, multi-directional forging and final forging. This paper presents the investigation results regarding the influence of the forming angle α in cross wedge rolling on different parameters at multi-directional forging. First, the state of the art is presented. As a basis for the investigations the process development and tool design of cross wedge rolling and multi-directional forging are described. Afterwards the paper’s results regarding the influence of the forming angle α on flash generation, billet temperatures, forming degree, forming forces and effective strain will be presented as a parameter study. Generally, flash is generated because a rotation-symmetric billet is forced into an asymmetric movement. The influence of an increasing forming angle leads to a higher amount of flash at the bottom of the crankwebs.     

感应淬火对曲轴扭转疲劳性能的影响

目的研究感应淬火对曲轴扭转疲劳性能的影响,为曲轴的设计和制造工艺调整提供技术参考。方法开展淬火曲轴和未淬火曲轴的扭转疲劳强度试验,利用升降法得到疲劳试验结果,从试验数据和微观组织等方面开展分析和讨论。结果未经过淬火的曲轴在99.9%存活率下的扭转疲劳极限为967.6N·m,经过感应淬火的曲轴在99.9%存活率下的扭转疲劳极限为1361.2N·m。感应淬火后曲轴的表面形成深度约3.5 mm的淬火层,平均硬度为HV0.5600,金相组织为细针状马氏体。曲轴的失效情况均为连杆颈油孔处开裂。结论 38MnVS6非调质钢曲轴在感应淬火后的扭转疲劳极限提升了约41%,曲轴油孔内壁的加工缺陷是形成裂纹源的主要原因,对曲轴淬火层区域的油孔内壁进行一定的表面处理,可进一步提高曲轴的扭转疲劳强度。     

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.     

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.     

Verhalten laserschockverfestigter und festgewalzter Randschichten der Ti‐Legierung Ti‐6Al‐4V bei schwingender Beanspruchung unter erhöhten Temperaturen

Residual stress stability and near‐surface microstructures in high temperature fatigued mechanically surface treated Ti‐6Al‐4V It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly‐stressed metallic components. Deep rolling is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of deep rolling on the low‐cycle and high‐cycle fatigue behavior of a Ti‐6Al‐4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near‐surface microstructures. Preliminary results on laser shock peened Ti‐6Al‐4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ～450 °C, i.e., at an homologous temperature of ～0.4 T/T_m (where T_m is the melting temperature). Based on cyclic deformation and stress/life (S/N) fatigue behavior, together with the X‐ray diffraction and in situ transmission electron microscopy observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti‐6Al‐4V at such higher temperatures, despite the almost complete relaxation of the near‐surface residual stresses. In the absence of such stresses, it is shown that the near‐surface microstructures, which in Ti‐6Al‐4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.     

A single parameter to evaluate stress state in rail head for rolling contact fatigue analysis

Based on the Smith‐Watson‐Topper (SWT) method, a phenomenological approach for multiaxial fatigue analysis, the maximum SWT parameter is proposed as a single parameter to evaluate the stress state in the rail head for assessing the fatigue integrity of the structure. A numerical procedure to calculate the maximum SWT parameter from a finite element analysis is presented and applied in a case study, where the stress and strain fields due to wheel/rail rolling contact are obtained from a three‐dimensional finite element simulation with the steady‐state transport analysis technique. The capability of the SWT method to predict fatigue crack initiation in the rail head is confirmed in the case study. Analogous to von Mises stress for strength analysis, the maximum SWT parameter can be applied to evaluate the fatigue loading state not only in rail head due to rolling contact fatigue but also in a generic structure subjected to a cyclic loading.     

The needs of understanding stochastic fatigue failure for the automobile crankshaft: A review

This paper reviews the fatigue failure mechanisms for the automobile crankshaft under service loading through the stochastic point of view. Fatigue failure of crankshafts are reviewed in general, as it is a major concern due to the uncertainties that arise i.e. randomness in structural materials, the geometric shape of the component and randomness of service loads. There has been very little research carried out in assessing the fatigue failure using the stochastic process in predicting the fatigue life of crankshafts. This review paper discusses the durability aspects of the component and is followed by a review of the characteristics of loading and the stochastic fatigue failure effect on the components. In addition, the stochastic approach from empirical model aspect using a safe-life approach from the more recent advances in computational methods to assess stochastic fatigue failure was discussed and reviewed in the context of this paper. The integration between the empirical and probabilistic methods can be quantified using statistical models, which evaluate the damage that leads to fatigue and eventually fatigue failure. Hence, this review provides a platform for understanding the stochastic fatigue failure for an accurate predictive prediction on the structural integrity of components, especially in the automobile industry.     

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.     

PREDICTION OF FATIGUE FAILURE IN A CRANKSHAFT USING THE TECHNIQUE OF CRACK MODELLING

Abstract— A new technique, known as crack modelling, is used here to predict fatigue failure in a crankshaft component. The technique uses a linear elastic finite element (FE) analysis to derive a stress intensity factor ( K ) for the component under load. The novel feature of the technique is that K is calculated without introducing a crack into a component; the stress field around the maximum stress point is examined and compared to that for a standard centre-cracked plate. The fatigue limit for a crankshaft was successfully predicted, when compared to experimental data. The only material parameter required for this prediction was the threshold stress intensity range, ΔK_th.     

A Study of the Fatigue Behaviour of an Aluminium‐Clad Steel Material Compound

Abstract: In this study we dealt with the characterisation of an aluminium‐clad steel material compound and the influence of the manufacturing process on its fatigue behaviour. As components of the Al6016‐T4/FeP06 material compound are manufactured predominantly by rolling and deep drawing, the influence of the plastic deformation during manufacturing on the fatigue behaviour must be investigated. Particularly, the cyclic hardening behaviour of FeP06 has a marked effect on the fatigue life of the aluminium–steel compound. Simulation models for fatigue life estimation of this material compound must therefore consider the influence of the plastic deformation which occurs, e.g. during deep drawing, on the fatigue behaviour. This study focused on the technological influences of the rolling process on the fatigue behaviour of an AA6016‐T4/FeP06 compound. The influence of the load sequence on the S/N‐curve of FeP06 was also studied.     

On the investigation of the biaxial fatigue behaviour of unidirectional composites

The paper illustrates the preliminary activity of an extensive research program oriented to investigate the multiaxial fatigue behaviour of unidirectional composite laminates, with particular attention to the analysis of the damage mechanisms and their correlation with the local multiaxial stress state to be used then as the basis for the development of multiaxial fatigue criterion. The definition of an effective experimental procedure for multiaxial fatigue testing is carefully discussed in terms of specimen geometry, specimen manufacturing and local stress state. Once identified in the thin-walled tubular specimens under tension–torsion loading the best test configuration for the aims of the research, the results of comparative fatigue tests investigating the influence of the tubes geometry (wall thickness to diameter ratio) on the transverse fatigue response are presented. In the final part of the paper the effects of an increasing shear stress component (σ₆) on the transverse (σ₂) fatigue strength and damage evolution in UD glass–epoxy tubes are illustrated.     

QT700-2球墨铸铁发动机曲轴失效分析

目的某厂生产的QT700-2球墨铸铁曲轴在路试过程中出现断裂,需寻找失效原因并提出解决措施。方法通过应用金相组织分析、化学成分分析、表面残余应力测试和力学性能测试等方法,对该曲轴的失效原因进行了分析。结果测试后分析结果表明,曲轴是在较大扭转循环载荷下,在第四连杆颈滚压圆角边缘多点萌生裂纹而导致疲劳断裂是其失效的主要原因。结论建议改进热处理工艺,保证组织的均匀性;改进加工工艺,减少应力集中;并强化表面残余应力以提高曲轴的疲劳寿命。     

Study of component structural equivalent fatigue based on a combined stress gradient approach and the theory of critical distance

Fatigue limit load is one of the most important factors and concerns in designing and manufacturing critical mechanical parts such as crankshafts. Traditionally, this governing parameter is obtained via a time and money-consuming experiment and analysis of a simple structure while it becomes theoretically complicated for sophisticated cases. In this paper, proper extrapolation methods to calculate the stress gradient in the stress concentration area are first chosen to obtain the material parameter using the theory of critical distance (TCD) indirectly. Then the fatigue limit load of crankshafts with the same material properties but different structures are computed. Validation between the prediction and the experimental results shows that this combined approach may provide a more satisfactory result in terms of fatigue limit for quick engineering prediction.     

Einfluss von Randschichtzustand und Haltezeit auf die thermische Ermüdung der Magnesiumbasislegierung AZ31

In this paper thermal fatigue of magnesium base alloy AZ31 in the temperature range between +50°C and +290°C is investigated. Experiments were carried out under constant total deformation (out‐of‐phase loading) and the resulting stress amplitudes as well as the plastic strain amplitudes were recorded as a function of the number of thermal loading cycles. In particular the consequences of mechanical surface treatments (deep rolling) and of hold‐times were investigated. In both cases no particular influence compared with untreated specimens loaded without hold‐times was observed, which is due to the interaction of deformation and recrystallization processes during thermal fatigue.