Multiaxial Stress in the Fatigue Life of Mechanical Parts (original) (raw)
Related papers
A notch multiaxial-fatigue approach based on damage mechanics
International Journal of Fatigue, 2012
The fatigue assessment of structural components under complex multiaxial stresses (cyclic or random stress histories) can be conveniently tackled by means of damage mechanics concepts. In the present paper, a model for notch fatigue damage evaluation in the case of an arbitrary multiaxial loading history is proposed by using an endurance function which quantifies the damage accumulation in the material up to the final failure. The material collapse can be assumed to occur when the damage is complete, that is, when the parameter D reaches the unity. In the case of notched structural components, such a damage parameter D must be evaluated by taking into account the stress value as well as the gradient effect at the notch root. The proposed model, which also employs the stress invariants and the deviatoric stress invariants to quantify the damage phenomenon, is calibrated through a Genetic Algorithm once experimental data on the fatigue behaviour of the material being examined are known for some uniaxial or complex stress histories. The model presents the advantages to be mechanically based and to not require any evaluation of a critical plane and any loading cycle counting algorithm to determine the fatigue life.
Multiaxial Fatigue Criterions Applied to Mechanical Components
In many practical situations, mechanical components are subjected to multiaxial loading and the required design lifetime often exceeds 10 8 cycles. For example, the expected lifetime of engine components, railroad wheels, crankshafts, turbine blades, etc. is more than 10 9 cycles. The traditional fatigue criterions assumed a hyperbolic relationship between stress and fatigue life, but experimental results in steels show that the fatigue fracture can occur beyond 10 7 cycles. This means that for very high number of cycles the fatigue limit does not an asymptotic behaviour and the concept of infinite fatigue life is not correct. So, the fatigue in metal components with design lifetimes greater than 10 7 cycles is an interesting topic for the development of advanced technologies. Taking into account that experimental fatigue tests are very expensive and time-consuming, the development of numerical fatigue models capable of predicting the durability of components, is a key task to carry out mechanical components design in shorter times. Multiaxial fatigue criterions can be classified in the following way: 1) Empirical, 2) Stress Invariants, 3) Critical Plane, 4) Strain Energy, 5) Combined Energy/Critical Plane and 6) Mesoscopic. In this paper, we present results from numerical models using the finite element method (FEM) analyzing mechanical components subjected to high number of impact cycles with an invariant criterion.
Computers & Structures, 2002
This paper presents an elastic-plastic finite element (FE) analysis of an axisymmetric circular cylinder with a cir-13 cumferential notch subject to multiaxial nonproportional fatigue loading with variable amplitudes. The von Mises yield 14 criterion and the linear kinematic hardening rule of Prager-Ziegler are applied to describe the elastic-plastic material 15 behavior. Two different loading combinations are considered: (1) constant tension with variable amplitude torsion; (2) 16 variable amplitude tension with variable amplitude torsion. Numerical results for the local stress-strain curves at the 17 notch-root are presented and discussed. 18 52 kind of fatigue life prediction model is concerned, a 53 through understanding of the local stress-strain re-54 sponses at the notch-root and a deep insight into the 55 damage evolution process induced by multiaxial variable 56 amplitude fatigue loading are required. With this moti-57 vation, an elastic-plastic FE analysis for a notched ax-58 isymmetric circular cylinder subjected to multiaxial 59 nonproportional synchronous loading with variable 60 amplitudes is performed in this investigation. 61 The von Mises [31] yield criterion and the linear ki-62 nematic hardening rule of Prager/Ziegler [32,33] are 63 applied for describing the elastic-plastic material prop-64 erties. The FE program package ABAQUS ([34,35]) is 65 used to solve the corresponding boundary value prob-66 lem. In particular, two different loading combinations 67 are investigated, namely, constant tension combined 68 with variable amplitude torsion, and variable amplitude 69 tension superposed by variable amplitude torsion. De-70 tailed numerical results are presented and discussed. 71 Special attention of the analysis is given to investigate 72 the local elastic-plastic stress-strain responses at a 157
Physical Mesomechanics, 2020
Real mechanical assemblies favor the initiation and propagation of fatigue cracks due to stress concentration phenomena arising from the geometrical features such as notches, corners, holes, welding toes, etc. Classical fatigue analysis of notched specimens is done using an empirical formula and a fitted fatigue strength reduction factor, which is experimentally expensive and lacks physical scene. In the present paper, a simple and meaningful methodology is proposed to assess notched components against multiaxial fatigue. In this method, by precisely defining a finite-size volume surrounding the fatigue crack initiation site (notch tip), over which the strain energy is averaged, the morphological effect on the process zone is fully addressed. Such a method takes into account the effect of combination of different modes (I, II, III) and the load ratio. In order to implement it for components with a sharp or blunt notch, it is enough to analyze a linear elastic finite element model and to know only the properties of materials obtained from simple uniaxial tests. New relationships for determining an effective (tensile-type) stress and fatigue strength reduction factors are derived for notched specimens. The accuracy of the proposed model is validated by experimental data available in the literature, related to tubular specimens weakened with sharp/blunt notches under combined bending-torsion loading. Such a situation widely appears in equipment used for various branches of industry such as piping, automotive, power plant, drilling, etc.
This paper describes three new models that can be used (1) to determine the stress-strain response in stress concentration zones for components submitted to complex multiaxial loading paths, (2) to extract relevant cycle sequences from a general threedimensional loadin path, (3) to find the hypersphere enclosing any type of cyclic load history. These three models can be combined to post-process an elastic Finite Element Analysis, and provide a fast estimation of the fatigue life of the components.
KnE Engineering, 2020
This paper presents a methodology to predict the fatigue lifetime in notched geometries subjected to multiaxial loading on the basis of the cumulated strain energy density. The modus operandi consists of defining an energy-based fatigue master curve that relates the cumulated strain energy density with the number of cycles to failure using standard cylindrical specimens tested under low-cycle fatigue conditions. After that, an elastic-plastic finite-element model representative of the material behaviour, notched geometry and multiaxial loading scenario is developed and used to account for the strain energy density at the crack initiation site. This energy is then averaged using the Theory of Critical Distances and inserted into the energy- based fatigue master curve to estimate the lifetime expectancy. Overall, the comparison between the experimental and predicted fatigue lives has shown a very good agreement. Keywords: Multiaxial fatigue, Fatigue life prediction, Strain energy density
Use of elastic stresses for a multiaxial fatigue prediction
2007
A new computational method derived from Papuga PCr multiaxial criterion is presented in the paper. While the PCr criterion is suitable for a comparison of a local multiaxial loading with a fatigue limit, the PCF criterion derived is focused on computation within a finite life. Its use is intended for a variable amplitude multiaxial loading, where the Palmgren-Miner damage cumulation law is applied. The PCF method is based on local elastic stresses and their action within the standard S-N curves of smooth specimens. No arrangement concerning the stress gradient effect was applied, since the experiments serving for comparison were carried on smooth and unnotched specimens. The experimental set covers different load paths applied to specimens manufactured of CSN 41 1523 structural steel. Computational results are promising for cases with load paths formed from single unclosed lines, but for the cases with load paths related to closed constructs it provides too conservative solution. A ...
The present paper is concerned with the formulation of an elasto-plastic strain based approach suitable for assessing fatigue strength of notched components subjected to in-service variable amplitude cyclic loading. The hypothesis is formed that the crack initiation plane is closely aligned with the plane of maximum shear strain amplitude, its orientation and the associated stress/strain quantities being determined using the Maximum Variance Method. Fatigue damage is estimated by applying the Modified Manson-Coffin Curve Method (MMCCM) along with the Point Method (PM). In the proposed approach, the required critical distance is treated as a material property whose value is not affected either by the sharpness of the notch being assessed or by the profile of the load spectrum being applied. The detrimental effect of non-zero mean stresses and degree of multiaxiality of the local stress/strain histories is also considered. The accuracy and reliability of the proposed design methodology was checked against several experimental data taken from the literature and generated under different uniaxial variable amplitude load histories. In order to determine the required local stress/strain states, refined elasto-plastic finite element models were solved using commercial software ANSYS®. This preliminary validation exercise allowed us to prove that the proposed approach is capable of estimates laying within an error factor of about 2. These preliminary results are certainly promising, strongly supporting the idea that the proposed design strategy can successfully be used to assess the fatigue lifetime of notched metallic components subjected to in-service multiaxial variable amplitude loading sequences.
Fatigue strength of welded joints under multiaxial loading: experiments and calculations
Fatigue & Fracture of …, 2001
In reality most welded components are loaded with a combination of different variable forces and moments that often cause a state of multiaxial stress in the fatigue-critical areas. If the multiaxial loading is non-proportional, traditional deformation-based hypotheses are not able to give a reliable lifetime prediction. This investigation is a cooperation between three German research institutes to build an experimental database for the verification of different concepts of lifetime prediction. In accordance with former investigations, a flange-tube connection made of steel P460 was used. The test program was divided into constant amplitude and variable amplitude tests. The ratio between the nominal bending and shear stress is 1. For the variable amplitude tests, a Gaussian-standard is used. A lifetime prediction software for multiaxial state of cyclic stress was developed. The software has a modular structure and allows calculations with different hypotheses and methods. The calculations are based on the local elastic stresses. This is an acceptable method for high-cycle fatigue. In this work, two general types of calculation, the Integral Approach and Critical Plane Approach and a local stress-based modification of the von Mises Criterion, the hypothesis of effective equivalent stress (EESH) are shown. The damage accumulation is performed with the elementary Miner's rule (S-N curve without fatigue limit). The statistical distributions of the damage sums are also shown.
A non-linear model for the fatigue assessment of notched components under fatigue loadings
International Journal of Fatigue, 2016
This paper presents a general theory for the estimations of an entire fatigue curve in ductile materials based on the implicit gradient approach. In order to modify the slope of the Woehler curves, the material was considered non-linear. The average stress of the hysteresis loop was taken into account by means of Walker's model. Subsequently, the implicit gradient method was adopted for the numerical evaluation of the effective stress and strain at low-and medium-cycle fatigue life and was then related to the fatigue strength of the material. The characteristic length, relating to the fatigue behaviour of the material, was considered constant for the fatigue lifetime. In order to confirm the proposed method, new experimental data were obtained, relating to axisymmetric notched specimens loaded with nominal stress ratio R=-1 and R=0. In terms of the effective strain amplitude, evaluated by means of the implicit gradient approach, the different Woehler curves of notched specimens were summarised in a unique fatigue curve as a function of Walker's cycle parameter.