Fatigue Life Calculation Methods Research Articles

New fatigue life calculation method for quenched and tempered steel SAE 4140

In stress-controlled constant amplitude and service loading tests at ambient temperature mechanical stress-strain hysteresis, temperature and electrical resistance measurements were performed to characterize the fatigue behavior of the quenched and tempered steel SAE 4140. The applied measurement methods use deformation-induced changes of the microstructure in the bulk material and represent the actual fatigue state. A new test procedure combines any kind of load spectra with periodically inserted constant amplitude sequences to measure the plastic strain amplitude, the change in temperature and the change in electrical resistance at the same time. The average values of the measuring sequences are plotted as function of the number of cycles in cyclic ‘deformation’ curves and represent the summation of microstructural changes caused by service loading. On the basis of generalized Morrow and Basquin equations the physically based fatigue life calculation method “PHYBAL” was developed for constant amplitude and service loading. With only three fatigue tests, Woehler (S–N) and fatigue life curves can be calculated in very good agreement with experimental ones determined in a conventional manner. The application of “PHYBAL” provides an enormous saving of experimental time and costs.

Short-Time Procedure for the Determination of Woehler and Fatigue Life Curves Using Mechanical, Thermal and Electrical Data

Mechanical stress-strain hysteresis, temperature and electrical resistance measurements were performed to characterize the fatigue behavior and to calculate the lifetime of metals under constant amplitude loading and random loading. Constant amplitude sequences were periodically inserted in random load tests to measure the plastic strain amplitude as well as the deformation-induced changes in specimen temperature and electrical resistance. These data are plotted versus the number of cycles for the fatigue assessment under random loading, similar as commonly practiced under constant amplitude loading. On the basis of Morrow and Basquin equations in generalized formulations, to be applicable for mechanical, thermal and electrical measurement techniques, a physically based fatigue life calculation method “PHYBAL” was developed. This new short-time procedure requires data of only three fatigue tests for a rapid and nevertheless precise determination of Woehler curves for constant amplitude loading or fatigue life curves for random loading.

Influence of elevated temperatures on the cyclic deformation behaviour of the magnesium die-cast alloys AZ91D and MRI 230D

The magnesium alloys AZ91D and MRI 230D were investigated in form of die-cast specimens with a cast skin. The fine-grained microstructure consists of a dendritic magnesium solid solution and interdentritic precipitates. The cyclic deformation behaviour was characterised in stress-controlled load increase tests and constant amplitude tests by means of mechanical stress–strain hysteresis measurements at room temperature and at T = 150 °C. The MRI alloy leads to higher plastic strain amplitudes and nevertheless higher lifetimes for both temperatures. Load increase tests allow a reliable short-time estimation of the endurance limit under both, room and elevated temperatures. With the physically based fatigue life calculation method “PHYBAL” the lifetime of the magnesium alloys can be calculated on the basis of cyclic deformation data determined in one load increase test and two constant amplitude tests in excellent agreement with the conventionally determined S– N curve.

Fatigue assessment and fatigue life calculation of quenched and tempered SAE 4140 steel based on stress–strain hysteresis, temperature and electrical resistance measurements

ABSTRACTIn this paper, mechanical stress–strain‐hysteresis, temperature and electrical resistance measurements are performed for the detailed characterization of the fatigue behaviour of quenched and tempered SAE 4140 steel used for many applications in the automotive industry. Stress‐controlled load increase and constant amplitude tests (CATs) were carried out at ambient temperature on servo‐hydraulic testing systems. The applied measurement methods depend on deformation‐induced changes of the microstructure in the bulk material and represent the actual fatigue state. The plastic strain amplitude, the change in temperature and the change in electrical resistance can be equally used for an assessment of baseline fatigue properties in generalized cyclic deformation curves as well as in generalized Morrow and Coffin–Manson curves. On the basis of comprehensive experimental fatigue data, the physically based fatigue life calculation method ‘PHYBAL’ based on the generalized Basquin equation was developed. S–N (Woehler) curves calculated with ‘PHYBAL’ using data from only three fatigue tests agree very well with the conventionally determined ones.

Fatigue strength of a typical ship structural detail: tests and calculation methods

In previous years, several large-scale specimens have been fatigue tested in the Ship Structures laboratory of the Dipartimento di Ingegneria Navale e Tecnologie Marine (DINAV) of the University of Genova (Italy). They represent the crossing of a T-shaped beam into deep web plates, simulating either a primary supporting member or bulkhead plating. Each specimen is characterized by two joints, both watertight but with different geometry. The purpose of this paper is to review all the available test results using calculation methods for fatigue strength of welded structural details, namely: nominal stress approach, local stress approaches and the fracture mechanics approach. Test results allow SN curves to be drawn which can be used for these and similar details, quite common in shipbuilding. Moreover, calibration of stress concentration factors and coefficients in the various fatigue life calculation methods is possible using the experimental data.

Fatigue Life of a SG Cast Iron under Real Loading Spectra: Effect of the Correlation Factor Between Bending and Torsion

Abstract This paper deals with the effect on life of the desynchronism between two variable amplitude load sequences in combined bending and torsion. Experiments were carried out on smooth specimens made of the EN-GJS800-2 cast iron. The comparison between experimental lives and predicted ones with the following fatigue life calculation methods is presented: Smith-Watson-Topper, Fatemi and Socie (method proposed by Bannantine), Wang and Brown, Socie's proposal for high cycle fatigue, and Morel. If the scatter of experiments is considered, these experiments show a low effect of the correlation factor on life. All the simulated fatigue life calculation methods give good results for proportional loads, but their predictions are not good for non-proportional loads. Morel's proposal seems to be the best to predict life of the tested material with our non-proportional fatigue test conditions.

Investigations on a statistical measure of the non-proportionality of stresses

In mechanical engineering fatigue assessment of components, where complex non-proportional loads are acting on, plays an important role to identify critical spots and to predict the lifetime till crack initiation. For this case, a measure for the complexity of local stress states would be very useful to select appropriate criteria or fatigue life calculation methods. In this contribution, a degree of non-proportionality of stresses is investigated, which has been originally proposed by Chu et al. This degree of non-proportionality of stresses is a statistical measure for the variation of principal stress directions. It is shown, how this parameter can be determined for general triaxial stress states, and conditions are derived for mathematical consistency. An example shows that the degree of non-proportionality of stresses can be used even for the compensation of methodical weaknesses of the critical plane criterion, when assessing fatigue effects of local rotating principal stresses for ductile materials such as tempered steel. The critical plane criterion has been used in conjunction with a stress based method (concept of synthetic S/ N-curves, which is quite popular in Germany) and linear damage accumulation according Miner’s rule.

On the Accuracy of Rolling Bearing Fatigue Life Prediction

Ball and roller bearings are designed to meet endurance requirements basically determined according to the Standard fatigue life calculation method. This method is based on the Lundberg-Palmgren fatigue life theory as modified by reliability, material, and lubrication factors. As application load and spied requirements have increased, the Lundberg-Palmgren method has resulted in bearings of increased size, adding unnecessarily to the size and weight of mechanisms. This is a critical design situation for weight and size-sensitive components such as aircraft gas turbine engines and helicopter power transmissions. The bearing life prediction method developed by Ioannides and Harris recognizes the existence of a fatigue limit stress. If the stresses an operating bearing experiences do not exceed the limit stress, the bearing can achieve infinite life. In any case, the method tends to predict longer lives than the Lundberg-Palmgren method. This paper evaluates the life prediction accuracies of the Lundberg-Palmgren and Ioannides-Harris methods by comparing lives calculated according to these methods and to those actually experienced in 62 different applications. As a result of the investigation, the Ioannides-Harris method is shown to more accurately predict bearing fatigue endurance.

Application and Verification of Fatigue Life Calculation Methods for AZ91E-T6 Cast Magnesium Alloy Under Variable Amplitude Loading

The goal of this research was to determine the applicability of commonly used fatigue life calculation methods for variable amplitude loading with sand cast AZ91E-T6 magnesium alloy using notched “keyhole” specimens. Commercially available fatigue life calculation software packages were used to calculate the fatigue crack “initiation” life (1 mm crack) and fatigue crack growth life corresponding to the experiments. The resulting correlation between experimental and calculated fatigue lives ranged from good to poor, and from conservative to nonconservative, depending on the analysis method used. It was shown that these fatigue life calculation methods can be used with this sand cast magnesium alloy but with certain apprehension.

Fracture analysis of thick-wall cylinder pressure vessels

Fracture mechanics analysis of cylindrical pressure vessels is described in a brief fracture case study. Also included are additional examples of crack growth in pressure vessels and cannons. The case study is of an early brittle failure and subsequent redesign of a cannon tube. Fracture mechanics test methods are described which were developed specifically for testing of cylindrical geometries. Three examples of current fracture analysis of cylindrical pressure vessels are presented. Fast fracture of a vessel is described, including effects of tension residual stress and crack shape. Evidence of environmentally assisted fracture of a cannon tube is presented. Fatigue crack growth and life calculation methods for cylindrical pressure vessels are discussed including effects of compressive residual stress due to overstrain.