Correction Method for Calculating Critical Plane Position of Geometric Discontinuity Steel Structure Under Multiaxial Loading

Critical plane method is one of the most promising approaches to predict the fatigue life when the structure is subjected to the multiaxial loading.The stress-strain status and the critical plane position for smooth specimens are calculated using theoretical approaches when the loading mode is a continuous function.However,because of the existence of stress concentration and the characteristic of multiaxial non-proportion,it is difficult to calculate the stress-strain status and the critical plane position of geometric discontinuity structure by theory method.In this paper,a new numerical simulation method is proposed to determine the critical plane of geometric discontinuity structure under multiaxial loading.Firstly,the strain status of dangerous point is analyzed by finite element method.Secondly,the maximum shear strain amplitude of arbitrary plane is calculated using coordinate transformation principle.Finally,the plane which has the maximum shear strain amplitude is defined as the critical plane.The critical plane positions are analyzed when loading mode and notch parameters are different.Meanwhile,the relationship between notch depth and associated parameters on critical plane as well as that between loading amplitude and associated parameters on critical plane are given quantitatively.

LIFE PREDICTION APPROACH FOR RANDOM MULTIAXIAL FATIGUE

According to the concept of critical plane, a life prediction approach forrandom multiaxial fatigue is presented. First, the critical plane under the multiaxial randomloading is determined based on the concept of the weight-averaged maximum shear strain direction.Then the shear and normal strain histories on the determined critical plane are calculated and takenas the subject of multiaxial load simplifying and multiaxial cycle counting. Furthermore, amultiaxial fatigue life prediction model including the parameters resulted from multiaxial cyclecounting is presented and applied to calculating the fatigue damage generated from each cycle.Finally, the cumulative damage is added up using Miner's linear rule, and the fatigue predictionlife is given. The experiments under multiaxial loading blocks are used for the verification of theproposed method. The prediction has a good correction with the experimental results.

The Effect of Surface Pit Treatment on Fretting Fatigue Crack Initiation

This paper analyses the effect of surface treatment on fretting fatigue specimen by numerical simulations using Finite Element Analysis.The processed specimen refers to artificially adding a cylindrical pit to its contact surface.Then,the contact radius between the pad and the specimen is controlled by adjusting the radius of the pit.The stress distribution and slip amplitude of the contact surface under different contact geometries are compared.The critical plane approach is used to predict the crack initiation life and to evaluate the effect of processed specimen on its fretting fatigue performance.Both crack initiation life and angle can be predicted by the critical plane approach.Ruiz parameter is used to consider the effect of contact slip.It is shown that the crack initial position is dependent on the tensile stress.For same type of model,three kinds of critical plane parameters and Ruiz method provide very similar position of crack initiation.Moreover,the improved sample is much safer than the flat-specimen.

A UNIFIED MULTIAXIAL FATIGUE DAMAGE PARAMETER

According to the critical plane principle, a unified multiaxialfatigue damage parameter is presented based on the varying behaviourof the strains on the critical plane. Both the parameters of themaximum shear strain amplitude and normal strain excursion betweenadjacent turning points of the maximum shear strain on the criticalplane are considered in the multiaxial fatigue damage parme- terpresented. An equivalent strain amplitude is made with bothparameters of the maximun shear strain amplitude and normal strainexcursion by means of von Mises criterion. Thus a new multiaxialfatigue damage model is given based on the critical plane approach.

Effect of wear debris on fretting fatigue crack initiation

Both wear and fatigue occur in fretting condition,and they interact with one another during the whole process.Fretting fatigue is commonly analysed without considering the effect of wear in partial slip regime,although wear affects the lifetime of crack initiation.This paper investigates,for the first time,the effect of wear debris on fretting fatigue crack initiation.To investigate the effect of debris,first fretting wear characteristics in partial slip regime are analysed for loading conditions.Then,the effect of wear on fretting fatigue crack initiation is investigated using Ruiz parameters and critical plane methods without considering the debris effect.Through the results,we can see that loading conditions affect the wear profiles in different ways.Moreover,wear has a significant effect on the fatigue in partial slip regime without considering debris especially on the crack initiation location.Finally,considering wear debris in the analysis,its effect on critical plane parameters is investigated.It is found that by considering the wear debris effect,the frtting fatigue crack initiation location is shifted towards the trailing edge.The predictions of both crack initiation location and lifetime show a good agreement with the experimental data.