Finite Element Analysis of Laser Cut Edge Beam Section for High Stress Intensity Structural Assessment

This research has analyzed the stability of laser cut-edges in the location of high stress intensity of a structural beam. The cut-edge characteristic properties formed during laser-cutting processing have been over prolonged periods determined to have beneficial effects on fatigue life. During this study two high strength steel grades S355MC and DP600 have been examined. Optimal fatigue lives were attained by minimizing the laser cut-edge surface damage, maintaining the formation of shallow striations and by controlling the near edge microstructural deformations during the cutting process. This was validated using a bespoke component in which was tested under four-point loading in which there is a area of stress concentration forming a localized plastic zone. The importance of this region is critical due to the fact that it is the area that influences the fatigue life of the structure. Predicting the lives to crack initiation was determined through FE analysis which is based on the use of E–N cut-edge fatigue data.     

Life Assessment and Crack Growth Properties of Laser Cut Dual-Phase Steel

The effect of laser cut-edges has been studied as a method for producing an optimum fatigue life performance of advanced high-strength steel. During this study, DP600 high-strength steel laser cut-edges have been fatigue tested under S-N and E-N fatigue loading regimes. The cut-edge surface characteristic properties and internal metallurgical alterations have been observed to directly influence fatigue life of the steel. This paper has investigated the crack initiation and growth properties of the initial crack to mode two. It is shown that alterations in the surface properties can be harnessed so that beneficial properties can be produced to retard crack initiation. It was determined that the laser power and cutting speed can be used independently to produce the appropriate balance between microstructure and optimum surface properties. Optimal fatigue lives were attained by minimizing the laser cut-edge surface damage, maintaining the formation of wide area striations and by forming a uniform layer of martensitic material close to the cut-edge. These results suggest that laser cutting can be used to enhance the fatigue life to failure of fracture-sensitive steel grades.     

Understanding the Effects of Mechanical and Laser Cut-Edges to Prevent Formability Ruptures During Automotive Manufacturing

The cut-edge condition has an important influence on the formability capacity of high-strength steel (HSS) automotive structures. XF350 and DP600 under examination were observed to display a decreased level of formability in the surface regions because mechanical punched edge hole-flanging capacity is dependent on ductility and the surface quality of the cut-edge produced. Formability was observed to be highly dependent not purely based on the properties of the steel but also the cut-edge properties. Hole expansion capacity (HEC)-forming experiments were performed on flat circular plates with mechanical and laser-cut holes to investigate the fracture and forming limits of HSS. The HEC properties of the cut-edges were determined using mechanical and laser-cutting processes using various cutting process parameters in which certain edge types displayed decreases in edge ductility. It was found that, by altering the processing parameters during the cutting process, the edge quality can be improved, and this has a positive effect on the formability capacity of steel components.     

Optimizing the Prestrain Fatigue Performance of Transformation-Induced Plasticity-Aided Steel

The cut-edge properties of automotive structures formed during the manufacturing processes significantly influence fatigue and formability performance of high-strength steels. This factor is becoming increasingly important as advanced high-strength transformation-induced plasticity TRiP-aided DP600 steels under examination exhibit an increased sensitivity to fatigue cracks initiating from mechanical cut-edges. It was determined that under prestraining, the effects of plastic deformation of the microstructure can be used to optimize fatigue life. This was particularly the case where the prestraining significantly improved the fatigue lives of mechanical cut-edges up to a prestrain level of 5%. It is proposed that the effect of prestraining can be used to optimize the fatigue lives of even damaged mechanical cut-edges. These parameters can be used in the manufacture of structures with both optimum formability and fatigue lives.     

Effect of Mechanical Cut-Edges on the Fatigue and Formability Performance of Advanced High-Strength Steels

Mechanical cut-edge properties influence the fatigue lives and formability capacity of advanced high-strength steels. This factor is critical as S355MC and DP600 exhibited an increased sensitivity to fatigue cracks initiating from defects on the cut-edge fracture zone. Mechanical cut-edges as a result displayed a decreased level of formability that was highly dependent on the cut-edge surface and internal microstructure of the cut-edge produced. It was determined that, by controlling the mechanical clearance, optimized mechanical cut-edges were produced. This was achieved through minimizing surface damage and by controlling the internal and topographical properties of the cut-edge zones.     

A Review of Fatigue Failure Properties from Edge Defects

Journal of Failure Analysis and Prevention - The cut-edge surface and internal properties formed during component manufacturing are critical to the durability of steel structures with exposed...     

Durability of automotive jounce bumper

This study was carried out to predict the durability of automotive car jounce bumper using Finite Element Analysis (FEA). Fatigue life correlations were taken from literatures and it was incorporated into FEA codes. The simulated results were validated with experimental work. The FEA results showed good agreement with the experiment conducted on the jounce bumper in term of load–displacement response. In term of the durability of the component, the fatigue life predicted shows agreement at lower fatigue strains. However, the error becomes larger as the fatigue strains become higher. The differences between the predicted fatigue life and the experimental fatigue life were discussed. Finally, the predicted crack initiation side was also validated in the experiment.     

Finite element and experimental study on multiaxial fatigue analysis of rail clip failures

The rail clip fastening system is an important structural component of railway track systems providing flexibility and turnover resistance for running rails. High replacement frequency of fasteners was observed compared with other components because of fatigue failures of rail clips. In this study, implicit and explicit finite element (FE) models were developed for E‐clip and Fast‐clip with material and fatigue properties obtained from experimental testing. The fatigue loading experiments were conducted to determine the strain‐life relationship. The assessments of the fatigue damage and fatigue life were analysed using the FE results for the rail clip strain/stress components with the Fatemi‐Socie multiaxial fatigue criterion. A time‐efficient smallest enclosing circle algorithm was developed to search the critical plane orientation and the maximum shear strain amplitude for fatigue analysis. This work provides a method for FE and experimental study of multiaxial fatigue analysis of rail clip failures subjected to dynamic loading.     

The influence of porosity on the fatigue life for sand and permanent mould cast aluminium

The automotive industry always strives to achieve light weight components to reduce fuel consumption and to meet environmental requirements. One way to obtain weight reduction is to replace steel components with components made of aluminium or other light weight materials. Aluminium has good corrosion properties and a high strength to weight ratio which makes it favourable in many applications. The increased use of aluminium castings in the automotive industry does also imply that the need for design data for aluminium increases. Especially for castings, the influence of casting defects are always an issue. For this reason fatigue properties for as-cast sand and permanent mould specimens with different contents of porosity have been studied.

Sand cast and permanent mould cast aluminium specimens of two different geometries were fatigue tested in cyclic bending at R = −1. Prior to fatigue test specimens were examined by X-ray and sorted into three quality groups depending on the porosity level. The aim of this work was to investigate the fatigue life for sand cast and permanent mould cast AlSi10Mg with different amounts of porosity. An additional aim was to predict the largest defect contained in a specified volume of a component, by using a statistical analysis of extreme values, and relate it to the fatigue life.

The results showed that fatigue strength for a smooth specimen geometry decreases by up to 15% with increased porosity. For specimens with a notched geometry, no influence of porosity on the fatigue strength was found. This is believed to be due to a much smaller volume subject to high stress than for specimens with low stress concentration.     

Analysis of manufacturing effects on fatigue failure of an automotive component using finite element methods

In this study, the fatigue life of an automotive suspension component was analysed using finite element methods with regard to stamping and welding effects. Because automotive suspension components are produced by forming and welding sheet metal, there are various effects on the final product, such as uneven thickness distribution, residual stresses and weld notches. Manufacturing effects may change the mechanical performance of the automotive components; therefore, it is desirable to consider these effects in the early design stage. Residual stresses due to work hardening and thermal deformation were investigated through process simulation. The redistribution and relaxation of residual stresses in a component were investigated in fatigue life analysis under a cyclic loading condition. Various equivalent relaxation curves were investigated and one was selected after comparisons with test results. The fatigue simulation results were compared to the test results; a good correlation between the two was achieved for the residual stress effects in terms of life cycles and failure locations. The simulation results also show that welding produces more detrimental effects than stamping with regard to the fatigue life of a component.     

High cycle fatigue analysis in presence of residual stresses by using a continuum damage mechanics model

This study attempts to predict the high cycle fatigue life of steel butt welds by numerical method. At first, FE simulation of plate butt welding is carried out to obtain the weld-induced residual stresses employing sequentially coupled three-dimensional (3-D) thermo-mechanical FE formulation. Then, a nonlinear damage cumulative model for multiaxial high cycle fatigue based on continuum damage mechanics (CDM), which can incorporate the effect of welding residual stresses, is derived using FE technique. The high cycle fatigue damage model is applied to the butt welds subjected to cyclic fatigue loading to calculate the fatigue life considering the residual stresses, and the computed total fatigue life which takes into account the fatigue crack initiation and the propagation is compared with the test result. In addition, the fatigue life prediction of the welds without considering the residual stresses is implemented to investigate the influence of welding residual stresses on the fatigue performance. The FE results show that the high cycle fatigue damage model proposed in this work can predict the fatigue life of steel butt welds with high accuracy, and welding residual stresses should be taken into account in assessing the fatigue life of the welds.     

Influence of shot peening on high-temperature corrosion and corrosion-fatigue of nickel based superalloy 720Li

High-temperature corrosion fatigue, a combination of corrosion with a fatigue cycle, is an emerging generic issue affecting power generation and aero gas turbine engines and has the potential to limit component life. Historically, surface treatments, such as shot peening have been used to improve component life and have been optimised for fatigue response. Research into optimisation of shot peening techniques for hot corrosion and high-temperature corrosion fatigue has shown 6–8A 230H 200% coverage to provide overall optimum performance for nickel-based superalloy 720Li based on the limited data within this study. Utilisation of electron backscatter diffraction techniques, in combination with detailed assessment of corrosion products have been undertaken as part of this work. The resultant cold-work visualisation technique provides a novel method of determining the variation in material properties due to the shot peening process and the interaction with hot corrosion. Through this work it has been shown that all three shot peening outputs must be considered to minimise the effect of corrosion fatigue, the cold work, residual stress and surface roughness. Further opportunity for optimisation has also been identified based on this work.     

Fatigue characterization and modeling of 30CrNiMo8HH under multiaxial loading

Fatigue of 30CrNiMo8HH steel alloy has been studied thoroughly. Uniaxial cyclic tension-compression, cyclic torsion, proportional tension-torsion, and non-proportional tension-torsion at various strain ratios have been considered. Tests were performed at standard laboratory conditions on solid and tubular specimens machined from an actual driveline component. Fractography was conducted on the tested samples to investigate the fatigue mechanisms involved. Under torsion, large numbers of early micro cracks were found to emanate from the sample's surface, with a few propagating into very long longitudinal cracks. In biaxial tests, cracks tend to propagate into the gauge reducing the cross section area. A strain energy density fatigue parameter has been employed for life prediction of the material under uniaxial and biaxial loading. The life prediction method is based on two different cracking mechanisms that agree with the observed cracking mechanisms in torsion and biaxial loading of 30CrNiMo8HH steel alloy studied here. Energy-based properties are obtained and the predicted lives are compared to experimental results. The results obtained agree well with experiments.     

Fatigue resistance of steel and titanium PVD coated spur gears

The aim of this work is to investigate into the possibility of enhancing the fatigue resistance of CrN-PVD coated components. In particular, PVD coated spur gears were tested and a numerical simulation of crack propagation was carried out. The coating layer microhardness and the residual stresses characterising the surface film were measured. The results obtained were then introduced in a numerical model for predicting the fatigue life procedure of coated gears used in gearboxes for automotive applications. The number of cycles necessary to reach specified crack depths in coated and uncoated steel and titanium spur gears was numerically determined. This represents a powerful tool to predict the fatigue life of coated gears. Benefits induced by the presence of the coating were pointed out. A sensitivity analysis was also carried out: Furthermore, the effects on the fatigue crack propagation of the residual stress gradient (evaluated by means of X-ray measurements), the elastic properties of the bulk material and coating were evaluated.     

Performance of steel beams strengthened with CFRP laminate – Part 2: FE analyses

A new method for repairing and strengthening steel is under development and consists of using CFRP (carbon-fibre-reinforced-polymer) laminates bonded to the steel substrate. Research on this method has been conducted by a few research groups in recent years. The idea is to let the CFRP laminate carry a large part of the stresses and thereby reduce the load on the steel, which may have had its capacity lowered due to deterioration or fatigue. The present paper presents the results of FE analyses of steel beams strengthened with bonded CFRP laminates. The interfacial shear and peeling stresses that appear in the bond line between the steel and CFRP laminate are studied in both the elastic and plastic phase of the steel beam. Comparisons with the results obtained from laboratory tests conducted on steel beams strengthened with bonded CFRP laminates show that the behaviour of the strengthened beams can be captured using FE analyses. The distribution of the shear and peeling stresses near the end of the bond line were obtained from the FE analyses, together with the interfacial stresses that develop near beam mid-span due to the yielding of the steel. These stresses may exceed the capacity of the adhesive and cause debonding in this region.     

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.     

Cause of failure and optimization of a V-belt pulley considering fatigue life uncertainty in automotive applications

High accuracy of dimensions and strength in design requirements are required to produce reliable automotive components with consistent strength distribution. For example, a V-belt pulley is widely used to transmit power between rotational mechanical elements. However, due to defects from the manufacturing process and heterogeneity of materials, different kinds of failure damage may occur in pulleys of identical shape and material. Common applications in the automotive industry include crankshafts, water pumps, air-conditioner compressors and power steering pumps. Although the shape and the usage of pulleys are very simple, evaluating the pulley design is difficult because the loading conditions and installation environment are complicated. This paper focuses on the clutch pulley in the A/C compressor system of automotives and cause of failure was investigated. The applied stress distribution of the pulley under high-tension and torque was obtained by using finite element analysis (FEA) and based on theses results, the life of the pulley with variation in fatigue strength was estimated with a durability analysis simulator. The results for failure probabilities of 50% and 1% were compared with the fatigue life. Incidentally, the purpose of this study was to optimize the fatigue life of vehicle components from the stochastic point of view. The fatigue life was obtained by an approximation function, and the optimum design was verified by fatigue tests considering durability and validity. The design optimization of a V-belt pulley was performed using an approximation function, which improved the fatigue life. A new shape optimization procedure was presented to improve the fatigue life of the pulley in automotive applications and the shape control concept was introduced to reduce the shape design variables. Design of experiment (DOE) was employed to evaluate the design sensitivity of fatigue life with respect to shape design variables.     

Bithermal low-cycle fatigue evaluation of automotive exhaust system alloy SS409

This investigation provides thermomechanical fatigue data for the ferritic stainless steel alloy SS409, used extensively in automotive exhaust system components. The data were generated to assess the total strain version of the strain range partitioning (TS-SRP) method for the design and durability assessment of automotive exhaust systems. The cyclic lifetime and the cyclic stress–strain–temperature–time behaviour for alloy SS409 were measured using bithermal tests with extreme temperatures of 400 and 800 °C. Fatigue lives ranged up to 10 000 cycles with hold-times of 0.33–2.0 min. The bithermal fatigue behaviour was compared to isothermal, strain-controlled fatigue behaviour at both 400 and 800 °C. Thermomechanical cycling was found to have a profound detrimental influence on the fatigue resistance of SS409 compared to isothermal cycling. Supplementary bithermal tests with hold-times ranging from 40 s to 1.5 h were conducted to calibrate the TS-SRP equation for extrapolation to longer lifetimes. The observed thermomechanical (bithermal) fatigue lives correlated well with estimated lives using the TS-SRP equations: 70% of the bithermal fatigue data fall within a factor of 1.2 of calculated life; 85% within a factor of 1.4; and 100% within a factor of 1.8.     

Finite element simulation of fretting wear and fatigue in thin steel wires

This paper studies the effect of fretting on fatigue life reduction of thin steel wires, using the frictionally-induced multiaxial contact stresses obtained from a finite element wear model, validated in previous work. The fatigue life prediction model uses a critical-plane SWT approach in a 3D crossed cylinder problem. A new damage accumulation methodology for the adaptive mesh simulation, based on the cyclic material removal, has been developed. Four methods (Manson’s universal slope, Muralidharan modified universal slopes, medians and fatigue S–N curves) for estimation of the fatigue coefficients of the wire have been used. Manson’s method and medians method give lives closer to those obtained from fretting wear tests in thin steel wires. The other methods are more conservative. The methodology predicts correctly the life reduction of this component due to the increase of normal load (contact pressure), while it is not clearly predicted that an increase of the stroke reduces the life of these components as shown in the experimental testing. Guidelines for developing a more robust methodology are proposed.     

Stress-Based Uniaxial Fatigue Analysis Using Methods Described in FKM-Guideline

The process of prevention of failure from structural fatigue is a process that should take place during the early development and design phases of a structure. In the ground vehicle industry, for example, the durability specifications of a new product are directly interweaved with the desired performance characteristics, materials selection, manufacturing methods, and safety characteristics of the vehicle. In the field of fatigue and durability analysis of materials, three main techniques have emerged: nominal stress-based analysis, local strain-based analysis, and fracture mechanics analysis. Each of these methods has their own strengths and domain of applicability??for example, if an initial crack or flaw size is known to exist in a structure, a fracture mechanics approach can give a meaningful estimate of the number of cycles it takes to propagate the initial flaw to failure. The development of the local strain-based fatigue analysis approach has been used to great success in the automotive industry, particularly for the analysis of measured strain time histories gathered during proving ground testing or customer usage. However, the strain life approach is dependent on specific material properties data and the ability to measure (or calculate) a local strain history. Historically, the stress-based fatigue analysis approach was developed first??and is sometimes considered an ??old?? approach??but the stress-based fatigue analysis methods have been continued to be developed. The major strengths of this approach include the ability to give both quantitative and qualitative estimates of fatigue life with minimal estimates on stress levels and material properties, thus making the stress-based approach very relevant in the early design phase of structures where uncertainties regarding material selection, manufacturing processes, and final design specifications may cause numerous design iterations. This article explains the FKM-Guideline approach to stress-based uniaxial fatigue analysis. The Forschungskuratorium Maschinenbau (FKM) was developed in 1994 in Germany and has since continued to be updated. The guideline was developed for the use of the mechanical engineering community involved in the design of machine components, welded joints, and related areas. It is our desire to make the failure prevention and design community aware of these guidelines through a thorough explanation of the method and the application of the method to detailed examples.