| Small punch fatigue test that has emerged in recent years is a new approach for testing fatigue properties of materials.It can obtain fatigue properties of materials by micro specimens.Therefore,developing the small punch fatigue test is of great significance to the fatigue properties testing of in-service equipment materials,new materials and local micro-zone materials.This paper takes 316 L stainless steel as the research object.Firstly,the clamp of small punch fatigue test is designed and manufactured.Then the small punch fatigue test results and fracture mechanism of 316 L stainless steel are studied.Finally,combining experiments,theoretical derivation and finite element analysis,a method that can use small punch fatigue test and one uniaxial fatigue test to obtain strain-life curve of materials is proposed.And the influence of test variables on small punch fatigue test results is studied.The main contents and conclusions of this paper are as follows:(1)The small punch fatigue test clamp has been designed and manufactured by SUS630 stainless steel.Through the checking calculation and experiment verification,it is proved that the clamp has sufficient strength and stability,and small punch fatigue test results can be obtained by the clamp reliably.(2)The small punch fatigue test of 316 L stainless steel was carried out successfully.It was found that the experimental curve has fatigue characteristics.The maximum load-life formula was fitted by power law relationship.Through fracture observation of the broken specimens,we can find that cracks of specimen initiate on the outer surface with a distance from the center of the specimen firstly and propagate along the thickness direction,and then evolve into circumferential cracks.After circumferential cracks occurred,radial cracks initiated and caused fracture when load is relatively small.When the load is relatively large,the circumferential cracks propagate to fracture.In general,the fracture of the specimen is dominated by the circumferential cracks.The analysis of experimental results proves that the small punch fatigue test has the ability to evaluate fatigue properties of materials.(3)Combining experiments,theoretical derivation and finite element analysis,a correlation method for obtaining 316 L stainless steel strain-life curve through small punch fatigue test and one uniaxial fatigue test is proposed.This method combines Chen model and proposes energy parameter based on critical plane.It is believed that when the energy parameters of the small punch fatigue and uniaxial fatigue are the same,they have the same fatigue life.By comparing the strain-life curves obtained by the correlation method and the uniaxial fatigue test,it is found that the error of their fatigue life is within the factor of 2.(4)Combining with finite element analysis and correlation method,the influence of key test variables on small punch fatigue test life is analyzed.In the range of 0.5 Hz-2 Hz,the frequency has little effect on fatigue life.However,the fatigue life of material increases with the increase of friction coefficients,specimen thicknesses,punch diameters,lower die diameters and chamfers. |
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