Life Evaluation Model Of Stress Fatigue For Metals

Fatigue is a frequently encountered failure of mechanical components,but its evaluation is still dependent on experimental works.However,experience criteria or evaluation methods are strongly limited by test conditions.When the geometry,loading styles are different,experience may be no longer applicable.Therefore,it is useful and meaningful to propose a new fatigue damage evolution model,which is suitable for any cyclic stress states,and can be applied to problems with stress concentration or singular behaviors.Stress fatigue is induced by damage accumulation.In order to propose a unified life evaluation model,the dominative factor of fatigue damage accumulation should be established.From the view of cyclic motion of atom's balance position,cyclic deformation was regarded as the first dominative factor of damage accumulation,while static deformation was regarded as the second one because it was the base of cyclic deformation.Therefore,a new multi-axial fatigue damage evolution law has been proposed from the view of cyclic motion of atom's balance position.And life evaluation formula has been deduced for arbitrary cyclic stress state,including uni-axial loading,proportional multi-axial loading and non-proportional multi-axial loading.The life evaluation formula has been examined by various uni-axial and muliti-axial experimental results.It is found that the formula could estimate fatigue life well for any cyclic stress state with the use of material's fatigue constants which can be determined by uni-axial tests only.It has been found that both fatigue limit and proportional coefficient of life should be dependent on mean stress.The relationship between fatigue limit and mean stress,and the relationship between the proportional coefficient of life and fatigue limit have been proposed from uni-axial experimental results in the literature.Different stress ratios or mean stresses may lead to different S-N curve shapes even for uni-axial fatigue problems.It has been found that the equivalent symmetrical cyclic stress and cycles can give the expression of S-N curves in the same form.By introducing the concept of characteristic length,fatigue problem with stress concentration or singularity can be evaluated in a unified way.And characteristic length is a material constant.Considering the fatigue failure of material within characteristic length,the evaluation of fatigue strength and life evaluation for problems with stress concentration or singularity has been studied by theory and experiment.It is proposed that the material failure occurs when the equivalent stress amplitude within the characteristic length is larger than fatigue strength of material.To evaluate the whole fatigue life of stress concentration or stress singularity problems,the multi-axial fatigue damage evaluation formula for smooth material is necessary,because the multiple characteristic lengths should be considered step by step to follow the fatigue process.The formula of crack propagation rate has been proposed in this paper.The determination of critical damage is the other major problem to predict fatigue life with the methods of damage mechanics,brittle failure will occur when the value of fatigue damage reaches to critical damage.By adopting the concept of average tangential stress within the characteristic length,a unified evaluation method for brittle fracture has been proposed,which called “the field stress theory”.According to this method,no matter what a singular point or what kind of singularity it is,the fracture can be evaluated in a unified way.Moreover,only the tensile strength and the fracture toughness are enough to evaluate the fracture from a singular point with arbitrary singularities,no additional new material property is required.The validity of the theory is examined by brittle fracture tests of specimens with macro-crack,short cracks,V-shaped notch,etc.,and also verified by experimental results in the literature.