Research On The On-line Measurement Method Of Fatigue Crack-tip Deformation Field Based On Stroboscopic Illumination And DSCM

Fatigue crack is the main and the most dangerous type of failure in engineering components.It is significant to study the fatigue crack expanding and fracture for safety of mechanical products.At present,the fatigue crack growth test of metallic materials has a single measurement content,which can not reveal the fatigue crack initiation,propagation mechanism from a perspective of microcosmic and macroscopic deformation.The digital speckle correlation method combined with stroboscopic illumination technology,was applied to fatigue crack expansion test.The compact tension specimens in the high-frequency sinusoidal alternating loads were studied at the region of the crack tip deformation field variation.The main work of this paper includes the following four aspects:1.Analyzing the structure and working principle of electromagnetic resonant fatigue test machine,established the three degrees of freedom vibration system mechanical model.The phase difference between load acting on the specimen and electromagnetic force was obtained by the dynamics analysis.2.According to the high vibration frequency of the electromagnetic resonant fatigue test machine,in order to acquisite speckle image of load characteristics point,stroboscopic imaging sine curve fitting time compensation secondary image acquisition method was proposed in this paper,and the equipment was selected after analyzing hardware devices.3.To realize the on-line measurement of the deformation field in fatigue crack tip,the dual-correlation function DSCM fast algorithm was proposed to improve the search efficiency.Aiming at the characteristic of the large displacement gradient of crack tip,the reliability guided approach was adopted to calculate the whole field displacement,and the calculation program is compiled by using Lab VIEW and C language.Finally,compared with the traditional DSCM algorithm,the results show that the calculation accuracy of the two algorithms is consistent,but the calculation time of proposed algorithm in this paper is shorter.4.The speckle images of different crack length were captured by using the stroboscopic imaging system,combining with programs written before,the displacement field and strain amplitude in the region of interest were calculated on-line and its variety law was also studied.Finally,the causes of error are analyzed.The reliability of proposed crack tip deformation field on-line measurement method was verified through theoretic study and testing.On one hand it provided new tools for real-time monitoring of engineering components;on other hand it provided a new method for on-line measurement of fracture mechanics parameters,which is of great significance on deepening understanding of the material nature and researching the nature of fatigue deformation process.