Changes in mechanical properties of vehicle components after strengthening under low-amplitude loads below the fatigue limit

Changes in the mechanical properties of vehicle components were investigated after strengthening under low-amplitude loads below the fatigue limit (SLAL). The strengths of vehicle components including fatigue strength, static strength and yield strength were significantly increased by SLAL. The surface hardness of the front axle, composed of a kind of low strength steel without heat treatment, was enhanced with increased strengthening and damaging. The change of surface hardness was reversed completely for the transmission gear, composed of a kind of high strength steel with surface heat treatment. The natural frequency of the transmission gear was appreciably increased by SLAL.     

EURAXLES – A global approach for design,production and maintenance of railway axles: WP1 – Advances in fatigue load analysis and reliability assessment of railway axles

The work presented in this paper was led within the collaborative project “Euraxles” of the FP7 program of the European Commission. It aimed at developing processes and methods that contribute to the minimization of the risk of fatigue failure of railway axles in service. This paper focuses on the development of a method to assess the reliability of axles according to fatigue damage. The proposed approach is mainly based on the stress strength interference analysis (SSIA) and the fatigue‐equivalent‐load (FEL) methods. It aims at calculating the axles’ probability of fatigue failure, by characterizing the variability of real in‐service loads and the scatter of the axles fatigue strength, and at evaluating more accurately the actual design margins. First of all, the main lines of the stress strength interference analysis method are recalled. This method aims at evaluating the in‐service reliability of components for their design or their homologation. It is used in many industries for various applications (mechanical components or systems, electronic elements, etc.). In the second part, the fatigue load analysis method that is proposed for railway axles is described. It starts with a post‐processing of an axle load measurement: from a time signal of forces applied to both wheels fitted on the axle, fatigue cycles of bending moment applied to the axle are identified and transformed into a cyclic equivalent load, the M_eq, which is a measurement of the severity of the initial variable load. Then, virtual but realistic load spectra are generated, thanks to a classification operation followed by a random draw of elementary load data that considers the operation and maintenance conditions of the axle. All the spectra are then analysed thanks to the fatigue‐equivalent‐load method in order to build the distribution of in‐service load severities that gives a picture of the stress to which the axles are submitted. In the third and last part of the paper, the methods are applied to real data of “Société nationale des chemins de fer français” (SNCF), the French national railway operator. Sensitivity analyses are performed in order to quantify the effect on the M_eq of variations of parameters and to verify the convergence and robustness of the process. Finally, results obtained for a passenger coach are given. The comparison between the distribution of load severities and the normative load, defined as according to european standards EN13103, shows that, for the studied axle, the normative load is very conservative. Using the axles fatigue limits identified on full‐scale tests, a stress strength interference analysis is performed to calculate the probability of failure of the axle.     

A model of cumulative fatigue strengthening effect due to coaxing

In this paper, a fatigue experiment of auto drive shaft smooth specimen is studied in details. Through the analysis of load amplitude and load cycles, which influenced on the residual fatigue strength of structure, we find a mathematical model of strengthening effect due to coaxing. Based on the auto drive shaft work load spectrum, a cumulative fatigue strengthening model is suggested. Then, a new method of lightweight design that can maximise cumulative strengthening effect is proposed. Our model and method can provide a theoretical basis for lightweight design on structural parts of automobiles.     

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.     

Review of the fatigue damage tolerance of high-speed railway axles in Japan

Railway axles are one of the most important components in railway systems since a fail-safe design is not available. In the present paper, the fatigue tolerance of the high-speed railway axle in Japan is reviewed. To maintain the safety, the fatigue strength of the axle has been extensively studied. Theses case histories and consequent improvements in manufacturing process are presented. The crack propagation behavior of the induction hardened axle is studied based on the fracture mechanics. Concerning the powered railway axles, the fatigue design method in Japan is compared with that in Europe and the effect of the train velocity on the allowable load is discussed.     

Festwalzen – eine Technologie für effizienten Leichtbau

Deep Rolling – a technology for efficient lightweight design Deep rolling is a proven method for increasing the fatigue strength of dynamically loaded parts. Although this application has been used for many years, procedures that induce work hardening and residual stresses are not well known. This article concerns both deep rolling and the potential for considering the resultant residual stresses in the design phase of components to achieve efficient lightweight design. In addition, the article includes new research results regarding the numerical modelling of deep rolling.     

Procedure for reliable durability validation of train axles

The structural durability of wheelset axles depends on the operational loading as well as on the fatigue strength of axles. To validate an axle design the time‐varying stresses in individual critical areas of the axle, represented by their stress spectra, must be taken into account. The allowable stresses in individual axle areas depend both on the fatigue strength and on the spectrum of operational stresses. The procedure for the axle structural durability validation, presented in this paper, includes the numerical fatigue life estimation as well as the experimental durability approval. Also the influences of axle design, material properties and manufacturing technology on structural durability are discussed.     

Fatigue properties of transformation-induced plasticity and dual-phase steels for auto-body lightweight: Experiment,modeling and application

The substitution of conventional high strength steels (HSS) with advanced high strength steels (AHSS), e.g., low-alloy multiphase transformation-induced plasticity steel (TRIP steel) or dual-phase steel (DP steel), for body lightweight brings about increased stress of notched components. Thus the fatigue properties of TRIP and DP steels and the fatigue life of notched lightweight design are important considerations for reasonable material selection during the design stage of auto-body. For the mentioned issue, cyclic strain-controlled fatigue properties of TRIP and DP steels with equivalent grade and lightweight result were investigated experimentally. Different cyclic behaviors of TRIP and DP steels were observed due to different interior microstructures. The cyclic stress behavior of TRIP steel is characterized by cyclic hardening followed by stable at lower strain amplitudes, and softening at higher strain amplitudes; however cyclic softening followed by stable occurs consistently for DP steel throughout entire strain amplitude range of test. TRIP steel possesses enhanced fatigue life and cyclic stress at the same strain amplitude than DP steel. Furthermore, local strain-life models of two steels were developed by linear regression of experimental data, to predict and compare the fatigue life of notched body structures made of them by finite element method. The simulation result illustrates that TRIP steel can provide more beneficial potential than DP steel for the lightweight design of notched body structures from the viewpoint of fatigue resistance.     

Structural durability criteria for commercial vehicle components from the self strengthening cast ausferrite nodular iron EN‐GJS‐800–8 (ADI) in comparison to the ferritic EN‐GJS‐400–15

The structural durability of safety components in the chassis comprises not only the fatigue behaviour under cyclic variable amplitude service loading, but also its interaction with prestrains caused by special events and the rupture behaviour under impact loading due to misuse . From this background, the structural durability behaviour of Panhard rods made from ferritic cast nodular iron EN‐GJS‐400–15 was compared with the behaviour of rods made from the austempered EN‐GJS‐800–8. The components investigated, Panhard rods and cast plugs, made from the austempered material revealed a higher impact resistance than the components made from the ferritic cast nodular iron. Due to their ausferrite microstructure, Panhard rods made from EN‐GJS‐800–8 display a significantly superior fatigue strength behaviour, especially under spectrum loading, and offer a potential for lightweight design. Prestrains do not affect the fatigue behaviour under variable amplitude loading and the plastic deformation of the component under impact loading can be increased by appropriate design reducing the stiffness in the shaft area and achieving a weight reduction by 15 %.     

Cumulative fatigue damage evaluations on spot‐welded joints using 590 MPa‐class automobile steel

This study focused on local strain behaviour near the slit edge of spot‐welded joints, where the fatigue crack initiated, and investigated methods of evaluating cumulative fatigue damage. A method of evaluating local strain amplitude by following modified Goodman's law gave almost the same result as an evaluation approach based on the external force and provided reasonable result on general strength design. An approach based on Smith–Watson–Topper's equation was easy to evaluate cumulative fatigue damage compared with the method based on modified Goodman's law and gave a good agreement with a criterion of the modified Miner's rule.     

Multiaxial fatigue behaviour of a short‐fibre reinforced polyamide – experiments and calculations

This paper deals with the fatigue behaviour of a short fibre reinforced thermoplastic under multi‐axial cyclic stress. Based on experimental results on notched and plain specimens, limits of existing methods for the fatigue life estimation in the design process of components exposed to complex multi‐axial loads were investigated. During the manufacturing process of short fibre reinforced thermoplastic components, a moderately anisotropic behaviour in stiffness and strength arises. Because of the material's anisotropy, classical failure hypotheses for the assessment of multiaxial load cases do not apply. In this study, a fatigue failure hypothesis was implemented that assesses the stress components in accordance with the correlating fatigue strengths in the material coordinate system, considering potential interaction between the stress components. Striving for a verified multi‐usable fatigue life assessment method, multiaxial load cases were examined experimentally. The experimental results on unnotched and notched specimens and the fatigue life estimation on the basis of the Tsai‐Wu‐failure hypothesis will be presented.     

The lifetime prediction of a rotary screw compressor of a liquid cooler subjected to high pressure and high frequency loads

The estimation of the lifetime of high pressure and high frequency loaded components by testing is very costly and time-consuming. The simulation of fatigue life is an important design step in the fast moving and competitive automotive industry, where the steady rise of engineering components and the demand for lightweight construction concurred with enhanced reliability require an optimized dimensioning process. The paper presents the chosen problems of 3D numerical strength and fatigue simulation of a rotary screw compressor of a liquid cooler. The presented results of the FEM mechanical calculation based on a real components. The fatigue analysis were executed using the criterion of critical plane. Local multi-axial state of stresses and deformation has been calculated based on Hooke’s law with simultaneous consideration of kinematics hardening rule of material according to Jiang’s model.     

Fatigue properties of multifunctional metal‐ and carbon‐fibre‐reinforced polymers and intrinsic capabilities for damage monitoring

Carbon‐fibre‐reinforced polymers (CFRP) structures offer enhanced lightweight potential in comparison with monolithic metallic concepts. Brittle failure behaviour and the insufficient level of electrical conductivity limit the lightweight potential of composites. One promising new approach to solve these issues is the additional integration of metal fibres. Structural components are subjected to cyclic loads during their lifetime. Therefore, the present study focuses on the influence of additional steel fibre reinforcement on the fatigue behaviour of CFRP laminates. Magnetic properties are determined because of the deformation‐induced phase transformation of the chosen austenitic steel fibres, which are also applied as intrinsic damage sensors. Interrupted fatigue tests are carried out accompanied by scanning electron microscopy to obtain differences in failure mechanisms. Beside a detailed overview of the steel fibre influence on the fatigue properties of conventional CFRP structures, the functional evidence of a new method for nondestructive testing by a magnet inductive measuring device is shown.     

Digital Image Correlation Used for Experimental Investigations of Al/Mg Compounds

Al/Mg compounds produced by hydrostatic extrusion exhibit unique characteristics in terms of high strength and low weight, which are required in lightweight safety parts. The compound interface between the core and sleeve is a brittle intermetallic phase consisting of Al₂Mg₃ and Al₁₂Mg₁₇. However, a certain plastic deformability is required for subsequent processing of the semi‐finished product by forming. To obtain a basic understanding of the deformation behaviour of the interface, the Digital Image Correlation is used together with a new analysis procedure based on the approximation of the displacement field with polynomial functions. The procedure is first tested with a bending test. The specimens exhibit a remarkable deformation behaviour under bending at high temperatures, which is confirmed by the results of radial upsetting tests. Even under a multi‐axle load, the interface maintains a full material joint, though fragmentation occurs, and a new secondary interface between the fragments can be observed. In addition, an evaluation of light microscopy images and Eulerian Hencky strain values at the interface, which are based on the Digital Image Correlation results, imply a connection between the strain and the boundary layer's appearance.     

汽车零部件(轴)的可靠性稳健优化设计

应用汽车零部件可靠性稳健优化设计的理论方法,对汽车典型轴系零部件,如半轴、前轴和后桥进行了可靠性稳健优化设计,给出了计算仿真分析结果,为工程实际的汽车零部件的可靠性稳健优化设计提供了理论依据。     

Neue Methoden zur Beurteilung der Betriebsfestigkeit im Fahrzeugauslegungs‐ und ‐absicherungsprozess

Advanced durability evaluation in vehicle design and validation process The modern process of evaluation and validation conducted in the automotive industry uses experimental, metrological, and calculation‐based methods. Offering various examples, the present paper describes new developments in the determination and evaluation of operating strength, particularly in terms of virtual methods and their application in practice. The first point considered is the virtual determination of load data, the second is the improvement of calculated fatigue life. Two current examples in the development of methods are presented in this context: The first example examines the inhomogeneity of materials in calculating aluminium castings. The second example describes the approach taken in the configuration of components made of short‐fibre‐reinforced polymers, applying a new method of calculation.     

Investigating the effects of loading,mechanical properties and layers geometry on fatigue life of asphalt pavements

In this paper, fatigue life, crack growth trajectory, and stress intensity factors of top‐down and bottom‐up cracks have been investigated in a multilayered asphalt pavement using numerical method. Finite element models, Paris law, and maximum tangential stress criteria were used to determine the effects of vehicle position, thickness, and stiffness of layers on fatigue life of pavements. The results show that the minimum fatigue life of pavements with top‐down crack occurs when the wheels are symmetrically located relative to the crack plane. On the other side, in pavements with bottom‐up crack, it occurs when the front tire located on top of the crack. Moreover, the result reveals that the top‐down crack grows slower than bottom‐up crack. Also, the surface layer's stiffness has the most effect on fatigue life of pavement.     

Schwingfestigkeit und Korrosionsverhalten hochfester Parabelfedern

Fatigue Strength and Corrosion Behavior of Parabolic Leaf Springs In the past, by means of constructive, material and production engineering measures, high‐tensile utility vehicleparts, for example suspension and guide springs for the axle unit of lorries and good trailers, were able to achieve considerable improvements in their fatigue strength qualities. This applies especially to parabolic leaf springs with lower weights, reduced space and improved roadability, which in comparison to trapezoidal leaf springs used previously, make a higher fatigue strength value possible. The relationships between material, strength, surface condition and fatigue strength quality of high‐tensile utility vehicle parts without corrosion effects, were determined in an extensive investigation on single leaf parabolic springs [1, 2]. These results are summarized in in the first part of the following report, followed by the presentation of a research work which investigated the corrosion behavior of single layer parabolic leaf springs with different coating systems. This investigation was initiated by the fact, that a large proportion of vehicle spring failures are caused by corrosion up to now.     

Multiaxial notch fatigue life prediction based on pseudo stress correction and finite element analysis under variable amplitude loading

A new computational methodology is proposed for fatigue life prediction of notched components subjected to variable amplitude multiaxial loading. In the proposed methodology, an estimation method of non‐proportionality factor (F) proposed by authors in the case of constant amplitude multiaxial loading is extended and applied to variable amplitude multiaxial loading by using Wang‐Brown's reversal counting approach. The pseudo stress correction method integrated with linear elastic finite element analysis is utilized to calculate the local elastic‐plastic stress and strain responses at the notch root. For whole local strain history, the plane with weight‐averaged maximum shear strain range is defined as the critical plane in this study. Based on the defined critical plane, a multiaxial fatigue damage model combined with Miner's linear cumulative damage law is used to predict fatigue life. The experimentally obtained fatigue data for 7050‐T7451 aluminium alloy notched shaft specimens under constant and variable amplitude multiaxial loadings are used to verify the proposed methodology and equivalent strain‐based methodology. The results show that the proposed methodology is superior to equivalent strain‐based methodology.     

Prediction methods of fatigue critical point for notched components under multiaxial fatigue loading

Two methods based on local stress responses are proposed to locate fatigue critical point of metallic notched components under non‐proportional loading. The points on the notch edge maintain a state of uniaxial stress even when the far‐field fatigue loading is multiaxial. The point bearing the maximum stress amplitude is recognized as fatigue critical point under the condition of non‐mean stress; otherwise, the Goodman's empirical formula is adopted to amend mean stress effect prior to the determination of fatigue critical point. Furthermore, the uniaxial stress state can be treated as a special multiaxial stress state. The Susmel's fatigue damage parameter is employed to evaluate the fatigue damage of these points on the notch edge. Multiaxial fatigue tests on thin‐walled round tube notched specimens made of GH4169 nickel‐base alloy and 2297 aluminium‐lithium alloy are carried out to verify the two methods. The prediction results show that both the stress amplitude method and the Susmel's parameter method can accurately locate the fatigue critical point of metallic notched components under multiaxial fatigue loading.