The Development Of A Synchrotron Radiation X-ray 3d Microtomography Based In Situ Fatigue Machine And The Applications

The fatigue behavior of the materials has always been a crucial issue in the engineering field.However,traditional methods of material research and its internal damage evolution in terms of surface measurement or the destructive fracture have an ultimate inadequacy which cannot obtain the synchronous tomography observation of internal damages initiation and accumulation before its final fracture.Such destructive experiment cannot afford the big amount of material consuming as specimen.High energy synchrotron radiation X-ray is currently the only applicable mean to penetrate the bulk of the material that makes the damage evolution of fatigue to be visualized.This paper developed one type of in situ observation tensile and fatigue test machine based on the SR-μCT technology,and illustrated the details of some key problems during the development.The main advantage of this machine is that without any destruction of material or much radiation,the evolution process of internal fatigue crack initiation and growth can be observed simultaneously.Such advanced experiment will have a great impact in establishing the fatigue life model of a material.This paper can be divided into three sections:the designation of the machine and its key parts structure,the signal processing system,debug and experiment application.The first part was accomplished by a cam-rod periodic structure driven via servo motor.The reliability of specimen clamping was guaranteed by using screw fastening.Except for the designation,finite element analysis were simulated to ensure the safety when the structure is working,which including the static strength,fatigue strength,and modal analysis.The signal processing system used multilevel low-noise amplifier,and furthermore,there were also all kinds of analog and digital signal filters to reduce the signal interference.Moreover,the high quality power source were used,along with the EMI design,the signal interference were eliminated.The access of digital signal was done by the A/D converter,therefore the whole system can output analog signal and digital signal.Debugging and experiment were accomplished in SSRF,and two materials of 7020 aluminum alloy and porous fiber stainless steel were employed under the fatigue load and static load,the results were extracted.Furthermore,in order to characterize the in situ fatigue test results based on SR-μCT and attempted to establish one kind of experiment-theory relationship,a modified fatigue crack growth rate model was proposed.Verification of the new model was carried out by integrating the fatigue crack growth rate of several engineering materials.It was found that the predicted results were well agreement with the experiment data.