Estimation Of Fatigue Strength Research Articles

Fatigue Tests for Automotive Design: Optimization of the Test Protocol and Improvement of the Fatigue Strength Parameters Estimation

Despite of continuous improvement of numerical computations, experimental fatigue tests are still of prior concern for high safety level parts, in particular for the automotive industry. Destructive fatigue tests are required in order to prove to authorities an effective fatigue design by highlighting the weakest link and its failure mode, and possibly taking account for the manufacturing process influence. Actually, experimental tests need for expensive means (i.e. rigs and specimens) and fatigue phenomenon is expected to take long time to get a crack issue. Thus, cost and delay time reduction always calls for more effective methods.Within this framework, we address here some recent improvements dealing with, respectively: a) the optimization of the fatigue test protocol, i.e. how the load time history is applied to the specimen in order to minimize the overall fatigue test duration; b) the reliability of the fatigue strength estimation, i.e. how the experimental data are considered in a reliable statistical model. The first point let us focus on a so-called Locati fatigue test protocol, whereas the second point let us make use of Maximum Likelihood and Bayesian techniques.These improvements are currently and successfully applied to chassis system parts at PSA Peugeot Citroën.

Influence of temperature on the delamination process under mode I fracture and dynamic loading of two carbon–epoxy composites

This paper experimentally analyzes the influence of temperature and type of matrix on the delamination process of two composites subjected to fatigue loading through the study of their fracture under mode I behavior. The materials were manufactured with the same AS4 unidirectional carbon reinforcement and two epoxy matrices with different fracture behavior. The chosen temperatures for the experiments were 20 (room temperature), 50 and 90°C.The experimental study carried out under dynamic loading enabled the authors to determine the influence that temperature has on the onset of delamination for the entire range of fatigue life of the material, from the low number of cycles zone to the high number of cycles zone. That is, it enabled the plotting of fatigue curves, represented as GImax–N (number of cycles required for the onset of delamination given a certain energy release rate) for an asymmetry coefficient of 0.2 (the ratio between the maximum and minimum fracture energies applied during the dynamic tests).The experimental data obtained were treated with a probabilistic model based on a Weibull distribution which allowed the identification of relevant aspects of the fatigue behavior of the materials such as the estimation of fatigue strength for periods greater than the tested values and the analysis of the reliability of the results.

The Uniform Statistical Model of Fatigue Strength

The uniform statistical model of fatigue strength is provided in the article. This model combines parameters of the probability distribution functions (PDF) of fatigue endurance limit and fatigue life time, S-N curve and uses only four parameters. The offered model describes the area of multi-cycle fatigue, characteristic of a local zone of dependent accumulation of fatigue damage (the high level of survivability). For this local zone, the two-parametric double-e xponential probability distribution function with logarithmic fatigue life time (DEDF) is fair. By estimating fatigue strength parameters based on the results of tests, more than 600 specimens are given in the paper.

An alternative definition of the shear stress amplitude based on the Maximum Rectangular Hull method and application to the C‐S (Carpinteri‐Spagnoli) criterion

ABSTRACTIn the present paper, the fatigue strength estimation capabilities of the modified C‐S (Carpinteri‐Spagnoli) criterion are improved by employing the Maximum Rectangular Hull (MRH) method proposed by the first author. The C–S criterion is a multiaxial high‐cycle fatigue criterion based on the critical plane approach and takes into account both shear stress (Mode II) and normal stress (Mode I) mechanisms to evaluate the orientation of the critical plane. The fatigue damage parameter used is given by a nonlinear combination of the equivalent normal stress amplitude, Na,eq, and the shear stress amplitude, Ca, acting on the critical plane. In the present paper, the shear stress amplitude is evaluated through the MRH method. Some experimental data available in the literature are compared with the theoretical estimations, concluding that the multiaxial fatigue strength evaluations provided by the C–S criterion are improved when Ca is computed applying the MRH method instead of the Minimum Bounding Circle (MBC) method.

Lifetime of semi-ductile materials through the critical plane approach

This paper presents the results of fatigue strength estimation depending on the variable orientation of the critical plane for proportional and non-proportional bending and torsion with regard to specimens made of aluminium alloy 2017A. The algorithm applied for the fatigue strength evaluation is based on the Carpinteri–Spagnoli proposal and its subsequent modifications in accordance with the ideas developed by the authors. The objective of this paper is to search for a model which offers the best possible results of fatigue strength estimation for materials with properties which are intermediate between elastic-brittle and elastic–plastic, such as aluminium alloys, and a further insight into them with regard to their fatigue strength.

Development of the positive mean stress diagrams using genetic algorithm approach

ABSTRACTIn this numerical study, a new optimum positive mean stress fatigue failure equation is developed using previous experimental data and genetic algorithm. Two independent curve fitting coefficients are implemented in the equation to supply better correlations with experimental data of the failure strength envelope. In the literature, Gerber, Goodman, Soderberg, Morrow, Bagci, ASME (elliptic line), Clemson, Sekercioglu and so on suggested different equations for estimating fatigue strength envelope under mean stress condition. In these models, the effect of the materials was not considered in details. Some of these mean stress linear expressions are very conservative or have stress area bigger than the yield limit. The yield strength and the effect of materials are considered in the proposed model. The values of the positive mean stress, which correspond to fatigue failure, are obtained, and a minimum average absolute error among the models presented in the literature is remarked. The proposed model, which has less conservative structure, can be effectively used in the mechanical design process.

On the estimation of the material fatigue properties required to perform the multiaxial fatigue assessment

ABSTRACTTo accurately perform the fatigue assessment of engineering components subjected to in‐service multiaxial fatigue loading, the adopted design criterion must properly be calibrated, the used information usually being the fatigue strength under both pure uniaxial and pure torsional fatigue loading. Because of the complex fatigue response of metallic materials to multiaxial loading paths, the only reliable way to generate the necessary pieces of calibration information is by running appropriate experiments. Unfortunately, because of a lack of both time and resources, very often, structural engineers are requested to perform the multiaxial fatigue assessment by guessing the necessary fatigue properties. In this complex scenario, initially, the available empirical rules suitable for estimating fatigue strength under both pure axial and pure torsional fatigue loading are reviewed in detail. Subsequently, several experimental results taken from the literature and generated by testing metallic materials under a variety of proportional and non‐proportional multiaxial loading paths are used to investigate the way such empirical rules affect the accuracy in estimating fatigue strength, the damage extent being evaluated according to the modified Wöhler curve method. Such a systematic validation exercise allowed us to prove that under proportional loading (with both zero and non‐zero mean stresses), an adequate margin of safety can be reached even when the necessary calibration information is directly estimated from the material ultimate tensile strength. On the contrary, in the presence of non‐proportional loading, the use of the empirical rules reviewed in the present paper can result, under particular circumstances, in a non‐conservative fatigue design.

Kinematic Motion Analysis and Structural Analysis of Bellcrank Structures Using FEM

The results of kinematic motion analysis were used for the structural analysis based on data that the load applied to each part. The problem of the fatigue strength estimation of materials or components containing natural defects, inclusions or in homogeneities is of great importance for both a scientifically or industrial point of view. Fatigue behavior in components is often affected by the presence of residual stresses introduced by processes such as actuator system. Analysis can provide the estimation of the crack growth curves with sufficient accuracy, even in case of complicated bell crank structures which are crucial for preserving aileron integrity and which participate in transfer of load. Probability of crack detection or any other damage detection is a result of many factors. An endurance life prediction of bell crank is used finite element analyses. Endurance test data for slim test specimens were compared with the predicted fatigue life for verification.

Estimations of mechanical properties and fatigue life in pre-strained conditions

In this paper, the effects of tensile pre-deformation on material properties and fatigue life are investigated. For this purpose, a series of tensile fracture tests and fully reversed cyclic mode load-controlled tests have been performed on the tested 430 Stainless Steel in as-received condition and for the same tested material pre-strained at room temperature. Tensile pre-strain, , conditions were induced at three levels: 5%, 8%, and 12%. Experimental observations on tensile properties, fatigue strength coefficient, and exponent were done and are represented in this study. Empirical formulas were developed in polynomial form to estimate the tensile properties in pre-strained condition. Meanwhile, in this study, two empirical relationships for estimating fatigue strength coefficient and exponent using ultimate strength have been proposed and the corresponding approximation of S–N curves based on the Basquin equation form have been developed. The approximation of the S–N curve has provided good predictions with most predicted lives within the factor 4 bound, but 73% of the predicted lives were still within the factor 2 bound. In consideration of the pre-strain effect on material behavior, a modified energy parameter was chosen and provided a more accurate prediction of reversals to failure for all pre-strained samples.

Fatigue strength estimation of adhesively bonded tongue and groove joint of thick woven composite sandwich structures using genetic algorithm approach

In the present work, the fatigue behavior of tongue and groove joints bonded by a toughened epoxy adhesive was investigated. Axial cyclic tests were performed by different design configuration conditions and the effects of design parameters were evaluated. The bonding strength of adhesives under fatigue loading is influenced by many factors such as, the length of bondline, adhesive thickness, traverse pre-stress on near the free edges of bond line and material of the joining parts. Since all these factors affect the fatigue strength of the adhesively joined parts, the effects of these parameters need to be investigated. The present paper describes the use of the stochastic search process that is the basis of a Genetic Algorithm, in developing fatigue strength estimation of adhesively bonded thick woven E-glass/vinyl ester laminates. Non-linear estimation models were developed using genetic algorithm. Developed models are validated with experimental data. Genetic Algorithm Fatigue Strength Estimation Model for Tongue and Groove Joints was developed to estimate the fatigue strength of the adhesively bonded joint. The strongest adhesively bonded joints can be achieved by selecting optimum design parameters obtained from the models. The logarithmic number of cycles was increased 2.46 times by selecting aluminum EN AW 5083 insert instead of composite insert materials. The joint fatigue strength was significantly improved by selecting appropriate design parameter values.

On the characterization of the critical plane with a simple and fast alternative measure of the shear stress amplitude in multiaxial fatigue

The goal of this work is not only to characterize the critical plane determination as a well-posed problem, but also to propose an alternative measure for the shear stress amplitude. Usually such an amplitude is characterized by the radius of the Minimum Circumscribed Circle (MCC) to the shear stress vector history in a material plane. In the present paper, a measure which considers a Maximum Rectangular Hull (MRH) is discussed. Available experimental data involving proportional and nonproportional stress paths and a critical plane criterion are invoked to compare the fatigue strength estimates based on both methods (MCC and MRH). It is shown that the multiaxial fatigue estimates are improved for most data evaluated when the equivalent shear stress amplitude is computed in terms of the MRH.

OS0521 時間強度によるアルミ合金の高温疲労限度線図 : その3:破壊形態に応じた線図の定式化(OS5-5 寿命予測,OS-5 材料の疲労挙動と損傷評価2)

A fatigue strength diagram, applied to the estimation of fatigue strength under mean stress or residual stress, is not obvious in case of aluminum alloys because they do not have endurance limit-especially at high temperature. Thus, we make up the generalized fatigue strength diagram at N cycles of aluminum alloy at high temperature, using the strength at N cycles instead of the endurance limit, and divide the single line into the multiple ones by fracture feature; typical fatigue failure, failure with plastic deformation and the one with creep. In this study, we formulate the constitutive equations for this set of the fatigue strength curves at N cycles. Both the line for typical fatigue failure and the one for failure with plastic deformation are expressed as straight lines. The one for failure with creep is expressed as straight line or curve, which is dependent on creep mode.

Fatigue strength and failure mechanisms of wrought aluminium alloys in the VHCF-region considering material and component relevant influencing factors

Investigations at room temperature were carried out with smooth specimens made of the aluminium alloys EN AW 6056 and EN AW 6082. Different failure mechanisms were investigated for instance according to material and number of cycles. The influence of mean stress on fatigue strength of smooth specimens made of EN AW 6056 and transition of failure mechanism from the high cycle fatigue (HCF) to the very high cycle fatigue regime (VHCF) are discussed. Fracture surface and microstructure of the crack initiation sites were observed by means of scanning electron microscope (SEM), electron back scatter diffraction analysis (EBSD analysis) and metallographic methods. Transmission electron microscopy (TEM) was used to find differences between unstressed and tested material of EN AW 6082. The results reveal for both aluminium alloys that the fatigue strength decreases with increasing number of cycles, which is known for materials with face-centred cubic lattice. In the very high cycle regime the crack initiation sites of the smooth specimens were internally. The crack initiation sites of the notched specimens were always at the root of the notch independent of the number of cycles. The microstructures of fracture surfaces were different for both aluminium alloys. Slightly notched and sharp notched (screws M10, thread rolled before heat treatment) specimens made of EN AW 6056 were investigated. Fatigue tests were done with a servohydraulic testing machine and a resonant frequency machine. Tests run up to a maximum number of N = 2 × 10 8 cycles (EN AW 6056) or N = 10 9 cycles (EN AW 6082). A comparison of the experimental results with estimated fatigue strength of a design code (FKM-Guideline, 2003 [1]) is presented.

Application of quantitave image analysis of graphite structures for the fatigue strength estimation of cast iron materials

The characteristic formation of graphite microstructures defines the physical and mechanical properties of cast irons. The graphite morphology of cast iron structures can be classified according to their shape, arrangement and size using as reference the structure micrographs specified in the standards ASTM A 247-67 and DIN EN ISO 945. These specific values do not provide any mathematical correlation between microstructure and mechanical properties. The application of quantitative image analysis in microstructure investigations allows a statistically reproducible evaluation method. Although cast irons were quite often focus of research works in this sector, the graphite morphology classification has been mainly handled devoid of any quantitative correlation to material properties.In this article an interrelationship between the fatigue strength of cast iron materials and a quantitative characteristic of the graphite microstructure was established. Thereby it is necessary to identify the largest graphite precipitates, which can be statistically considered as potential crack initiation sites. In this context three different cast iron materials were investigated: ENGJL-270, EN-GJV-450 and EN-GJS-700.

316 発電水車用ステンレス鋳鋼の疲労き裂進展挙動の解明および疲労強度評価(機械材料・材料加工III)

316 発電水車用ステンレス鋳鋼の疲労き裂進展挙動の解明および疲労強度評価(機械材料・材料加工III)

1130 時間強度によるアルミ合金の高温疲労限度線図 : その2 : 種々のアルミ合金への適用結果(OS11-7 材料の疲労挙動と損傷評価-疲労損傷・寿命予測-)

A fatigue strength diagram, applied to the estimation of fatigue strength under mean stress or residual stress, is not obvious in case of aluminum alloys because they do not have endurance limit-especially at high temperature. Thus, we make up the generalized fatigue strength diagram at N cycles of aluminum alloy at high temperature, using the strength at N cycles instead of the endurance limit, and divide the straight line by fracture feature; plastic deformation and creep. In this study, we apply this fatigue strength diagram at N cycles to some aluminum alloys. As the results, we clarify the validity of this diagram. And, this diagram shows a possibility of proper fatigue design of aluminum alloys compared to conventional fatigue strength diagrams.

Fatigue behaviour and life predictions of case‐hardened steels

ABSTRACTThis paper presents experimental and analytical results on fatigue behaviour of case‐hardened steel. Fully reversed strain‐controlled constant amplitude axial fatigue tests were performed on through‐hardened case, through‐hardened core and case‐hardened steel specimens. Surface versus sub‐surface cracking and the role of residual stresses and their relaxation are discussed. Multi‐layer models of the case‐hardened specimens were used to predict crack nucleation sites as well as fatigue lives, and the predictions corresponded well with the experimental results. Linear elastic fracture mechanics (LEFM) was also used to conduct fatigue crack growth analysis to further explain the experimental observations from the fracture surfaces of the case‐hardened specimens. A fatigue strength estimation method based on hardness and inclusion size was used to estimate the fatigue limit of the materials investigated. Fractography of fracture surfaces and crack nucleation location are also presented.

Endurance Analysis of Automotive Vehicle’s Door W/H System Using Finite Element Analysis

In the automotive electronics industry, demand for low-cost, high strength-to in-service performance for electronic components continues to drive the development of vehicles’ door Wiring Harness (W/H) system for new applications. The problem of the fatigue strength estimation of materials or components containing natural defects, inclusions or in homogeneities is of great importance from both a scientific and industrial point of view. This article gives some insight into the dimensioning process, with special focusing on fatigue analysis of W/H in a vehicle’s door structures. An en-durance life prediction of door W/H was calculated using finite element analyses. Endurance test data for slim test specimens were compared with the predicted fatigue life for verification. The final life expectancy of the component combines the effects of these microstructural features with the complex stress state arising from the combined service loading and residual stresses.

SM45C 환봉 용접재의 피로거동에 관한 실험적 연구

Abstract For this study, SM45C steel rods using generally for power transmission shafts and machine components was selected and welded by butt-GMAW method. And then it was studied about estimation of fatigue strength and the region of infinite life by Haigh diagram using Goodman's equation. Fatigue strength in weld zone presents highly in order of the boundary between deposited metal zone and heat affected zone, deposited metal zone, heat affected zone. This result agrees with distribution of hardness in weld zone. Fatigue strength in base metal zone presents highly compared with weld zone in low cycles between 10  cycles and 10  cycles, but it presents the lowest fatigue strength on the order of heat affected zone in the vicinity of 10  cycles. It is the result that the first high compressive residual stress distributed by drawing process of the steel rods is released and the base metal is softened by alternating stresses. The region of infinite life by Haigh diagram presents highly in order of the boundary between deposited metal zone and heat affected zone, deposited metal zone, heat affected zone. From this results, it is demanded that the stress for safety design of machine components using SM45C butt-welded steel rods must be selected in the region of the lowest infinite life of heat affected zone.

The Modified Wöhler Curve Method calibrated by using standard fatigue curves and applied in conjunction with the Theory of Critical Distances to estimate fatigue lifetime of aluminium weldments

This paper is concerned with the application of a novel engineering method we have recently devised to estimate fatigue lifetime of aluminium welded joints subjected to constant-amplitude uniaxial and multiaxial fatigue loading. The assessment technique employed in the present study is based on the use of the so-called Modified Wöhler Curve Method (MWCM), a conventional critical plane approach, applied in conjunction with the theory of critical distances (TCD). In more detail, the MWCM was initially calibrated by using two standard curves: the first one, stated by Eurocode 9, suitable for assessing ground butt welds subjected to uniaxial loading, whereas the second one, suggested by the International Institute of Welding (IIW), suitable for estimating fatigue strength of aluminium welded details loaded in torsion. Subsequently, a unifying critical distance value to be performed to assess aluminium welded joints was calculated by taking full advantage of the master curve supplied by the notch-stress intensity factor (N-SIF) approach and obtained by summarising the uniaxial fatigue strength of cruciform aluminium welded details characterised by different absolute dimensions. Finally, the accuracy and reliability of the devised method was systematically checked by means of several experimental results taken from the literature and generated by testing a variety of welded geometries subjected to uniaxial as well as to multiaxial fatigue loading. Such an extensive validation exercise allowed us to prove that our approach is successful in estimating fatigue damage in aluminium welded details, resulting in predictions mainly falling within the two reference scatter bands adopted to calibrate the method itself. Such a high accuracy level is very promising, especially in light of the fact that our engineering approach can be applied to assess real aluminium welded components by directly post-processing simple linear-elastic finite element (FE) models.