Fatigue Strength Criterion Research Articles

Thermomechanical design in the automotive industry

ABSTRACTIn this paper thermomechanical fatigue assessment in the automotive industry is discussed. The design strategy is based upon a consistent approach of the thermomechanical loading, the mechanical constitutive law of the material, the damage parameters and the fatigue strength criteria. The good understanding of these different steps allows one to perform predictive calculations of automotive parts subjected to thermomechanical loading. The main hypotheses and modelling choices are presented and results are illustrated by a series of computations on real 3D structures. Cracked area and lifetime prediction are described in the case of aluminium alloy cylinder heads subjected to transient thermal loadings.

Fatigue of cantilevered pipe fittings subjected to vibration

ABSTRACTVibration fatigue failures in piping systems often occur at cantilevered small‐bore fittings (SBF) such as pressure tappings and drain valves. Piping vibration has often been assessed by measuring vibration displacement or velocity. However, in the case of straight cantilevered fittings with a concentrated mass, a better method exists. This paper presents a simple robust method of calculating vibration induced stress for cantilevered fittings at both the fitting branch neck and the main pipe. The method is based on measurement of acceleration at the concentrated mass (e.g., valve) and simple mechanics calculations. Extensive laboratory and field verification of the technique is presented. The choices of a fatigue strength criterion, such as ASME BPVC VIII Div. 2, BS7608, or a fracture mechanics approach such as BSI PD6493, are discussed. Several case studies are presented as well as simple ‘good engineering practice’ rules‐of‐thumb for SBF design. These ‘rules‐of‐thumb’ are now incorporated in the piping specifications of the author's company.

Fatigue strength CAE system for three-dimensional welded structures

ABSTRACT Fatigue strength assessment methods are applied to practical arc-welded structural features that have complex three-dimensional geometry. Two methods are investigated in the present research. The criterion of the first method is cyclic plastic zone size, which is calculated from stress singularity parameters. The criterion of the second method is hot spot stress. These fatigue strength criteria are proved to be applicable to the fatigue strength assessment of practical arc-welded structural features. These criteria enable a critical point at which fatigue cracks are most likely to initiate to be specified for each type of welded joint and load. Consequently, the critical point simplifies the classification of various welded joints. Furthermore, a fatigue strength computer-aided engineering system, which includes functional engines for the evaluation of the fatigue strength of a designed shape, is proposed.

Complex stress state effect on fatigue life of GRP laminates. Part II, Theoretical formulation

The synergistic effect of in-plane stress tensor components on fatigue strength is not traditionally considered in the design of thin-wall box-beam structures, e.g. composite rotor blades in general. Fatigue life calculations account only for the normal stresses due to bending and centrifugal forces, neglecting the contribution of shear and transverse normal stresses. The theoretical formulation of a life prediction methodology accounting for all in-plane stress tensor components, through the use of a multiaxial fatigue strength criterion, is presented here. Comparison of theoretical predictions with experimental results from constant amplitude, uniaxial, off-axis tests demonstrates the drastic effect of shear and transverse normal stresses, besides that of axial normal stress, in reducing operational life of a GRP structural laminate.

Energy Criterion for Fatigue Strength of Wood Structural Members

This report describes a mathematical model for fatigue strength of cellulosic materials under sinusoidal loading. The model is based on the Reiner-Weissenberg thermodynamic theory of strength in conjunction with a nonlinear Eyring’s three-element model. This theory states that failure depends on a maximum value of the intrinsic free energy that can be stored elastically in a volume element of the material. The three-element mechanical model, which consists of a linear spring in series with a parallel array of another linear spring and an Eyring dashpot, provides a good description of rheological material properties. The strength model system was able to predict rupture occurrence of polymers and wood structural members under constant and ramp loading with satisfactory results. For sinusoidal loading, the present study shows that the strength model system can predict time at fracture as a function of applied mean stress, amplitude of cyclic stress, and stress frequency. Numerical examples with model parameters evaluated for small Douglas-fir beams are presented.

Fatigue strength of working blades of turbines and compressors repaired by welding

A method of determining the welding repair zones of working blades of gas turbine engines is proposed based on the principle of equal strength of the component using the fatigue strength criterion. The method takes into account the difference of the mechanical properties of the deposit and the parent metal. Investigations were carried out on new and repaired working blades of turbines and compressors of different gas-pumping systems. The experimental results are used to prepare recommendations for repairing the working blades subjected to pilot-plant service conditions.

Statistical evaluation of the service life of a complex shaped, large-scale part

1. A method is proposed for calculating the service life of actual parts according to the parameter of failure probability with the use of fatigue resistance characteristics of large-scale models, which is suitable for calculations concerning parts of arbitrary complex shape given representative statistical data about loading. The method is recommended for engineering calculations of service life according to fatigue strength criteria on a probability basis for different unique complex shaped parts for which direct fatigue tests are impossible. 2. The basic stages of the calculation are: statistical evaluation of loading on the part; fatigue tests on large-scale models of the actual part, making possible integral consideration of the effect of stress state on fatigue resistance of the complex shaped gauge section; determination of the fatigue limit distribution function for the actual part; calculation of the service life distribution function of the actual part. 3. Experimental verification was obtained for the high reliability of calculated stress state parameters in complex-shaped parts under torsion by the proposed method based on fatigue test results for large-scale specimens. Application to the wobbler spindle of the Pilger mill at the 220 plant in the CTRW gave a service life distribution function L with a median value L=1.8 years of rolling mill operating time, agreeing with data from VNIImetmash on the service life of this spindle. 4. Numerical evaluation of fatigue failure probability for critical large-scale, complex-shaped parts, carried out by this method in the working design stage, is necessary to provide the specific γ-percent service life of the machine as a whole.

Criteria relating to the fatigue life of steels subjected to alternating loads under conditions of uniaxial and biaxial static strain

1. We have analyzed existing criteria relating to fatigue strength for asymmetrical loading cycles under conditions of uniaxial and biaxial strain. We have shown that at present there are no criteria really suitable for describing the behavior of all materials. 2. We have proposed a fatigue-strength criterion which gives an excellent description of the behavior of ductile steels for asymmetrical loading cycles under conditions of uniaxial and biaxial strain. 3. We have shown that the proposed criterion may be modified to describe the behavior of materials sensitive to biaxial strain.

組合繰返應力を受ける金属材料の破壊法則

組合繰返應力を受ける金属材料の破壊法則

A Criterion for Fatigue Strength of Metals under Combined Stresses

可鍛性材料に対する松村博士の破損説即ち「可鍛性材料に於ては最大剪断應力が変容の程度によつて変化する定数値に達するとき彈性的破損を起す」といふ考へは延性の大なる材料に関する限り実驗結果に非常に良く當嵌まるものである。併し、脆性材料に対しては當嵌まらない。よつて著者は次の如き考へ方を提案致し度い。即ち延性の大なる材料に対しては松村博士の説をそのまゝ用ひるが、脆性材料及び延性の比較的乏しい材料に対しては松村博士の説に於て最大剪断應力の代りに最大主應力を以て置き代へる。且これを繰返應力の場合に當嵌めて「延性の大なる材料に於ては繰返應力による最大剪断應力が変容の程度によつて変化する定数値に達するとき疲労破壊を起し、脆性材料及び延性の比較的乏しい材料に於ては繰返應力による最大主應力が変容の程度によつて変化する定数値に達するとき疲労破壊を起す」と考へる。而してこの考へ方より出発して種々の組合應力状態における疲労強さを表はすべき条件式を導き、これが従耒の実驗結果と良く一致することを知つた。