Fatigue Lifetime Prediction Research Articles

The use of strain range partitioning in thermo-mechanical fatigue

The strain range partitioning (SRP) method was developed as an aid to fatigue lifetime prediction and works on the assumption that inelastic strain can be divided into two different types of strain, viz. plasticity and creep. The effects of creep depend upon where in the cycle it occurs and whether it is reversed by plasticity or creep. In practice the SRP method is seldom used because of the large amount of experimental data required to determine the basic relationships and methods which can be applied directly to a fatigue loop with a hold-time (for example, with the incorporation of a frequency factor) are more often preferred. However, the SRP method is well suited to describing types of strain occurring in fatigue at varying temperatures as shown by the examples of bithermal fatigue.

Fatigue lifetime prediction methods: An analysis of the different approximations involved in local approaches

This paper deals with different aspects included in fatigue life prediction algorithms of metallic components under complex uniaxial loadings: plastic, damage and loading analysis. It attempts to identify and analyse the various models generally used at each of these steps. Some comparisons are made with experimental results.

Surface damage accumulation in low-cycle fatigue: Physical analysis and numerical modelling

The physical low-cycle fatigue damage of smooth 316 L stainless steel specimens is described at a mesoscopic scale (i.e.the grain size). More than 80% of the low-cycle fatique lifetime consists of the development of multiple surface short cracks, the surface lengths of which are less than ten grain sizes. High densities of surface cracks lower than a grain size favour statistical aspects of their mutual interactions and their developments. This gives rise to an approach of low-cycle fatigue damage based on statistical physics. The fatigue damage description consists of accumulation of elementary independent damage events: the cracking of one grain. The statistical aspect of fatigue damage accumulation led to the development of numerical Monte Carlo type modelling. Principles of the modelling are presented. Low-cycle fatigue lifetime predictions concern in-air experiments and corrosion-fatigue experiments on 316 L stainless steel.

Low cycle fatigue and creep fatigue behaviour of a 316L stainless steel: Simon, S., Ibabe, J.M.R. and Fuentes, M. Z. Metallk. (Oct. 1993) 84(10), 708–715

The high temperature low cycle fatigue behaviour of cast IN-738 nickel-base superalloy was studied at 850 °C. The effect of environmental damage on the low cycle fatigue life of this alloy was found to be significant with prior exposure at 1000 °C for 240 h in air. It was also found that the environmental damage caused the mode of crack initiation and propagation to change from the transgranular mode to the intergranular mode. The results were explained on the basis of the effect of the environment on the modes of crack initiation and propagation. Since different reductions in the fatigue life of this alloy were observed as a result of different thermal exposures, the effect of environment has to be accounted for in any high temperature low cycle fatigue lifetime prediction method.

欠陥を有するセラミックスの疲労寿命推定法

A new method of predicting the fatigue lifetime of ceramics with various sizes of defects has been developed. In this method, first, specimens each of which has an artificial crack of constant size are used to estimate fatigue crack propagation parameters. In the vicinity of the tip of an artificial crack induced by Knoop indentation, a tensile stress field is generated. Then, the stress intensity factor acting as the driving force of fatigue crack growth was considered the sum of two terms associated with the applied stress and residual stress field. By numerical computation inserting the stress intensity factor into the power law of crack propagation, crack propagation parameters are estimated. Subsequently, using the obtained parameters, the fatigue lifetimes of the smooth specimens with fracture originating from a small defect, are predicted.This method was applied to three types of silicon nitride ceramics. As a result, the cyclic fatigue lifetimes of precracked specimens, which were set free from residual stress field by surface removal, were precisely predicted using the obtained parameters. Thus, the values of these parameters were proved to be adequate. Furthermore, the predicted fatigue lifetimes of the smooth specimens were in good agreement with the experimental results, considering the scatter of static strength.

Cyclic fatigue lifetime predictions of partially stabilized zirconia with crack resistance curve characteristics

Abstract Predictions of static and cyclic fatigue crack growth of indentation-initiated strength-limiting flaws in a Mg-PSZ material are compared with observations. The extent of environmental effects on cyclic loading lifetime prediction is determined by integrating the cyclic stress cycle following determination of crack growth parameters from static fatigue data. The effect of crack tip shielding as a result of the crack resistance curve is also included. The true cyclic fatigue effect is then determined. Crack growth dependence on the maximum stress intensity, K max , and ΔK and possible mechanistic explanations of cyclic fatigue effect are also considered.

Fatigue of Yttria‐Stabilized Zirconia: II, Crack Propagation, Fatigue Striations, and Short‐Crack Behavior

Fatigue crack propagation in 3Y‐TZP was investigated using controlled surface flaws. A unique growth law strongly dependent on the maximum stress intensity factor and quadratically dependent on the amplitude of the range of stress intensity factor was established. This growth law was found to apply for both surface flaws and internal flaws and could be used to predict fatigue lifetime. The presence of residual stress altered the growth mechanics so that an inverse growth rate dependence on the applied stress, reminiscent of the so‐called “short‐crack behavior,” was manifested. Fatigue striations resulting from alternate overload fracture and fatigue fracture during stress cycling were observed. The appearance of striations varied with the R ratio and was very sensitive to the loading condition and crack geometry.

LOW CYCLE FATIGUE DAMAGE ACCUMULATION IN ARMCO‐IRON

AbstractA study has been performed relating to fatigue damage accumulation in Armco‐iron with a mean ferrite grain size of 40 μ. Two constant strain amplitudes, εa= 1 × 10−3 (Nf= 250,000 cycles) and εa= 6 × 10−3 (Nf= 3050 cycles) were employed to derive base data for cumulative fatigue loading where unit blocks of low (1000 cycles) and high (10 cycles) strains were mixed until failure occurred. Observations in a scanning electron microscope permitted a periodic evaluation of surface damage to be recorded. Two regimes of damage were noted. During the early crack initiation phase, crack density increased and crack growth rate was constant. During the later phase, or crack propagation stage, new cracks did not nucleate, the smallest cracks arrested, crack density decreased, and the largest cracks grew in an exponential manner, which included linkage with smaller cracks. A damage accumulation model is presented which permits a prediction of fatigue lifetime under variable amplitude loading.

The effect of environment on the high temperature low cycle fatigue behaviour of cast nickel-base IN-738 alloy

The high temperature low cycle fatigue behaviour of cast IN-738 nickel-base superalloy was studied at 850 °C. The effect of environmental damage on the low cycle fatigue life of this alloy was found to be significant with prior exposure at 1000 °C for 240 h in air. It was also found that the environmental damage caused the mode of crack initiation and propagation to change from the transgranular mode to the intergranular mode. The results were explained on the basis of the effect of the environment on the modes of crack initiation and propagation. Since different reductions in the fatigue life of this alloy were observed as a result of different thermal exposures, the effect of environment has to be accounted for in any high temperature low cycle fatigue lifetime prediction method.

Fatigue lifetime prediction with the aid of SAW NDE

The present studies concentrated on predicting the remaining fatigue life for single fatigue cracks in the Paris regime of macrocrack propagation. Acoustic surface waves were used to interrogate the crack during cyclic fatigue. The inversion of the obtained scattering data provided crack depth and crack length as a function of the number of cycles applied in tension-tension fatigue. Auxiliary experiments were conducted to study the acoustic response of the crack to tensile and compressive loads, thought to open and close the crack. The technique may allow for new insights into the physics of the “crack closure” effect.

Stress calculations for lifetime prediction in turbine blades

In order to predict fatigue lifetime in cooled turbine blades it is necessary to know stabilized cyclic stress and strain fields for typical ground-to-ground cycle.Previous works permit to obtain the viscoplastic temperature dependent behavior and the classical one-dimensional plane cross-section method gives the stress and strain fields on the blade.However this classical method does not take into account the restraining effects of moments due to action of centrifugal forces on the deflected blade. This paper presents an extension of the plane cross-section method in which the coupled problem induced by these restraining effects is solved without iteration. p]Results are given for a turbine blade made of a refractory alloy and compared with the case in which restraining effects are neglected.