Bastien Weber

A Stress-Based Approach for Fatigue Assessment Under Multiaxial Variable Amplitude Loading

ABSTRACT This paper presents a new multiaxial variable amplitude fatigue life prediction method. The main steps of this stress-based approach are the definition of a so-called counting variable to identify and extract cycles from the multiaxial random stress history, the use of two multiaxial criteria which are extended from endurance to finite fatigue lives, in order to calculate the life of each extracted cycle by Rainflow counting and then the use of damage rules to assess the life of the material submitted to the multiaxial stress sequence. The different steps are detailed: the chosen counting variable is justified, the global approach and critical plane approach criteria are presented and both linear Miner and non linear Lemaitre & Chaboche damage laws may be used. Several biaxial variable amplitude fatigue tests that were carried out at the Technical University of Opole (Poland) allow validation the proposed method with both damage rules.

Improved iron loss modelling approach for advanced electrical steels operating at high frequencies and high inductions in automotive machines

ArcelorMittal recently developed a specific new range of electrical steel grades for automotive applications, as an answer to the specific requirements for electrical traction machines (requirements such as excellent efficiency over a wide speed range, high power density, high reliability and robustness). These electrical steel grades are optimised over a wide frequency range to achieve the specific material property compromise of magnetic, electric, mechanical and thermal performance. Moreover, to maximise the potential of these advanced electrical steel grades, careful machine design is needed: when optimising the machines power density for instance, its crucial not to over-dimension the machine, neither in magnetic nor in mechanical aspects of the electrical steel laminations. Therefore more precise modelling tools are becoming more essential: ArcelorMittal developed an improved approach for the iron loss modelling of machines based on a better model description and appropriate material characterisation of the relevant phenomena influencing the machines iron losses (such as lamination punching and pressing of the stacks). Furthermore, ArcelorMittal has also in-house advanced mechanical characterisation tools such as fatigue testing on electrical steels, for tighter mechanical machine design. When such advanced magnetic and mechanical design tools are used, on the optimised electrical steel grades of the ArcelorMittal iCAReTM offer for automotive applications, the maximum potential of these electrical steels can be exploited.

On the efficiency of the integral approach in multiaxial fatigue

Abstract The design of structures or mechanical components against multiaxial fatigue generally requires the use of one multiaxial fatigue criterion. Many models have been proposed. The aim of the present paper is to focus on the most recent concept that is named the integral approach. Its main feature is to assess the fatigue damage at the analysed point of the structure from stress averages realised over all the material sectional planes passing through this point. The different propositions are first presented and their efficiency is then discussed and compared with the fatigue prediction obtained by criteria based upon the critical plane concept. It definately seems that the contribution of several damaged physical planes agrees with the actual fatigue behaviour of metals in a better manner than by considering the stresses acting on one single sectional plane.

Comparison of Variance and Damage Indicator Methods for Prediction of the Fracture Plane Orientation in Multiaxial Fatigue

ABSTRACT Two methods that enable prediction of the fracture plane orientation are presented and compared in this paper. The first one is a statistical approach, which is based on the variance of an equivalent stress. It is assumed that the fracture plane is the one where the variance of a linear combination of the shear and normal stresses acting on this plane is maximum. The second one uses the so-called damage indicator of a multiaxial fatigue criterion, which is based on the research of the critical plane. The formulation of the criterion involves shear and normal stress amplitudes and mean normal stress. The fracture plane is the critical plane; That is to say the one where the damage indicator is the highest. A comparison of the two methods against experimental results is made for biaxial cyclic and random stress states.

Fatigue behavior of spot-welded joints in air and under corrosive environments

A bstractThe main aim of the project was to evaluate the influence of combined effects of fatigue loading and exposure to cyclic corrosion testing on the corrosion and the fatigue resistances of coated steel-based materials joined by resistance spot welding. Seven steel-based materials including cold rolled mild steels, high-strength steels, and press-hardened steels (PHS) were selected and provided by steel suppliers with different surface coatings for resistance spot welding. Panels were joined using conventional resistance spot welding in both lap-shear and T-peel designs. Joined samples were painted by e-coating following the industrial process. Metallographic characterization of the steel materials revealed that microstructures and metallic coating composition and thickness were as expected. Cross section of spot welds showed good quality and typical evolutions of hardness. Tensile tests and fatigue tests were performed on reference samples (fatigue in “air,” i.e., nonexposed to corrosion) leading to typical SN-lines as described in part I of this paper. The results were used to evaluate the influence of corrosion on tensile strength of the joined samples. Fatigue tests performed on combined and alternating corrosion and fatigue revealed that fatigue life is affected by extent of corrosion near the spot weld, with a reduction of the fatigue life at higher applied load and slight increase for lower load, compared to tests “in air.” This phenomenon was not observed for lap-shear configuration in alternated fatigue-corrosion mode.

A Multiaxial Variable Amplitude Fatigue Life Prediction Method Based on a Plane Per Plane Damage Assessment

A multiaxial variable amplitude fatigue life prediction method is proposed in this paper. Three main steps are distinguished. The first one concerns the counting of multiaxial cycles and uses the normal stress to a physical plane as the counting parameter. Then a multiaxial finite fatigue life criterion allows one to assess the material life corresponding to each cycle on any physical plane. A damage law and its cumulation rule describe the damage induced by each cycle plane per plane. By this way the critical plane for a given multiaxial stress history is found out. It is assumed to be the fracture plane and the fatigue life of the material is traduced as the number of repetitions of the sequence up to crack initiation. At this stage, material fatigue criteria and linear and nonlinear damage laws assume that the material is damaged. One distinguishes among these criteria critical plan type whose formalism can identify the crack initiation plan. An application is given for each load. In the context of multiaxial solicitations of variable amplitude, a validation of the estimation of the orientations of the priming planes is carried out based on experimental results on cruciform test pieces; the estimated orientations are close to those observed experimentally.

Contribution of Kinematical and Thermal Full-field Measurements for Identification of High Cycle Fatigue Properties of Steels

Using kinematical and thermal full-field measurements for identification of mechanical parameters has become a very promising area of experimental mechanics. The purpose of this work is to extend the use of non-conventional tests and full field measurements (kinematical and thermal) to the identification of the fatigue properties of a dual-phase steel. A particular attention is paid to the influence of plastic pre-strain on the fatigue limit. Indeed, an analytical approach is proposed to define the geometry of the specimen permitting to obtain a constant gradient of plastic strain within the zone of interest after a monotonic pre-strain. Then, a self-heating test under cyclic loading is carried out on the pre-strained specimen. During this cyclic test, the thermal field is measured using an infrared camera. Finally, a suitable numerical strategy is proposed to identify a given thermal source model taking into account the influence of a plastic pre-strain. The results show that, with the non-conventional test and the procedure developed in this work, the influence of a range of plastic pre-strain on fatigue properties can be identified by using only one specimen. It is worth noting that a great number of specimens is required to determine this effect by using classical fatigue campaign.

Prévision de l’orientation du plan d’amorçage en fatigue à l’aide d’un critère multiaxial de fatigue de type plan critique

Les criteres de fatigue multiaxiaux permettent de determiner la duree de vie d’un materiau soumis a des sollicitations multiaxiales variables au cours du temps. On distingue parmi ceux-ci les criteres de type plan critique dont le formalisme permet d’identifier le plan d’amorcage de fissure. Le critere de Robert utilise pour cette etude est applique a des chargements multiaxiaux en contraintes d’amplitude soit constante, soit variable. Qu’il s’agisse de l’un ou de l’autre de ces chargements, le plan de dommage maximal constitue le plan critique, c’est-a-dire le plan d’amorcage de fissure au sens du critere. Une application est donnee pour chaque type de chargement. Dans le cadre de sollicitations d’amplitude variable, l’estimation par calcul de l’orientation du plan d’amorcage est comparee aux resultats experimentaux.