Internal Friction Response Of Fatigued 40Cr Steel And Remaining Life Estimation

Internal friction,the phenomenon of vibration energy loss due to internal reasons,which is also called 'damping capacity' in engineering,is an inherent attributes of materials.Due to the change of internal structure organization and the fatigue damage accumulation caused by fatigue loading,the intrinsic internal friction of material or components would become different.In this way the fatigue damage degree and remaining life status of material can be characterized via internal friction testing.The overview of relevant researches about internal friction of metal as well as prediction of fatigue life and damage is shown in this article,followed with the survey of how internal friction theory and technology is applied to the study of metal's fatigue.Based on these,the study of internal friction property of 40Cr steel after tension-tension fatigue was conducted on the multi-function material testing machine MTS809 and the DMA testing instrument TA-Q800,and the variation of intrinsic internal friction related parameter of the specimens was obtained.In this research conclusions arrived are as followings:(1)As an charactering quality of internal friction,the loss modulus measured under a certain steady temperature condition could be regarded as an indicator of the damage accumulation and remaining life status of 40Cr due to the tension-tension fatigue.(2)Internal friction response analyzing of 40Cr steel's tension-tension fatigued specimens at the temperature condition of 50? showed a result that the change of loss modulus caused by cyclic loading could be divided into three phases.The fluctuation of loss modulus at the first stage could be attributed to the evolution of microdefects inside the specimens at the early edge of fatigue progress,and later the loss modulus reached its first peak when the cyclic loading level arrived at 20%;In the second period,the loss modulus showed a monotonically increasing trend,which could be connected with the extension of fatigue crack;And later in the third period,when the loss modulus decreased again,this could be attributed to the beginning of fatigue crack's rapid extending.(3)For 40Cr steel which was fatigued under tension-tension cyclic loading,when its loss modulus varies in the early stage or increases at a low rate later,it means the damage degree of specimens were around 80%,and could be regarded as in a safe service life state;While the rate increases,and even the modulus shows an decrease after a peak,it means the damage degree was going up to 90%,which could be a sign of the coming of fatigue crack's instability propagation.