Research On Effect Of Pre-stress On Fatigue Crack Growth Behavior Of Al-alloy

The influence of cold work processing technology on fatigue properties of metal structures is complex and changeable,which is one of the hot topics at present.In this paper,the effect of preset stress on fatigue crack growth and fracture mechanism of sheet metal specimens is studied with 5052 aluminum alloy as the object.The specific research contents are as follows:1 The influence of the prestress level(120 MPa,137 MPa,158 MPa,183 MPa)on the fatigue crack growth rate(da/dN-Delta K)drift and life of the sheet metal specimens was studied.The fatigue crack propagation test results show that the fatigue life of the pretreated specimen increases with different amplitude,and the 158 Mpa prestress increases the fatigue life of the specimen to the maximum,and the fatigue life can be increased by 294.6% compared with the original specimen without the preset stress.The range of the influence of different preset stress levels on the fatigue crack growth rate is also different.By comparing the da/dN-?K with non stress treatment samples,it is found that the 158 Mpa prestress treatment has the greatest impact on the fatigue crack properties of the sample,and the impact range continues to 17 mm;The influence of 183 Mpa preset stress treatment on the fatigue crack performance of samples is smaller than that of 158 MPa specimen.The value reaches 15mm;The influence of 120 MPa and 137 MPa prestress treatment on the fatigue crack performance is the smallest.The value reaches 11 mm.2 The hardness distribution and change rule of pre-stressed stress treated specimen and non stress treated specimen on different paths of fracture and fracture side were explored.The microhardness of the micro Vivtorinox shows that the hardness of the prestress treated samples and the non stress treated samples at the fracture surface is distributed between 70HV-85HV;The hardness of the same pretreated specimens on different paths is basically the same,and the hardness is equal.The hardness of different preset stress treated samples on the same path is the same.On the two hardness measurement paths on the side of the fracture surface,120 MPa prestress treatment specimens,137 MPa preset stress treatment specimens,and 158 MPa preset stress treatment specimens have abnormal hardness points at the gap 7 mm,16 mm and 6 mm respectively.After analyzing the photoelectric spectrum and other hardness data,the author thinks that the main reason for this phenomenon is element aggregation or material point defects.3 The fracture morphology and fracture characteristics of pre-stressed stress treated and non stress treated specimens were studied.SEM showed that all specimens showed obvious high cycle fatigue failure characteristics.The fatigue stripe is clearly visible during the steady growth of cracks.There are many tearing toughened fossa in the microstructures,and a very thin tearing edge around the dimple,which shows a typical characteristic of the ductile fracture of the dimple.