薄钢板低周疲劳测试技术研究

Study on Low-cycle Fatigue Testing Technology for Thin Sheet Metal

汽车钢板材料由于厚度较薄,在低周应变疲劳性能测试中,试样容易发生屈曲,并且受限于试验段的标距,无法使用传统机械式引伸计进行应变的测量,在工程中,其低周疲劳性能的测试是一项技术难点.论文基于数字图像相关技术,提出了一种非接触式的光学引伸计,结合光学显微镜头和防屈曲装置,实现材料表面应变的实时监测.该动态应变测量系统使用普通的CCD相机,采用了优化的计算方法,应变测量的频率可以高达60 Hz.该动态应变测试技术可以广泛地应用于各个工程领域的动态变形测量.

Thin sheet steel components with a thickness less than 2.5 mm are commonly used in automobile manufacture.Due to the small thickness,the specimens are easy to buckle when subjected to tensile and compressive loads in low-cycle fatigue tests.To improve their stability,the specimens are designed with short gage length.Restrained by the length of the specimen,the conventional mechanical and electrical extensometers are not applicable for strain measurement,which is a technical difficulty in engineering practice.In this paper,a non-contact optical extensometer based on a new digital image correlation technique combined with a microscopic imaging lens and an anti-buckling restraint device is developed to achieve real-time monitoring of the surface strain of material.Through the two-step integer-pixel strategy including particle swarm optimization（PSO）and block-based gradient descent search（BBGDS）algorithm combined with inverse compositional Gauss-Newton（IC-GN）,the real-time optical imaging system can reach a sampling rate of 60 Hz for strain data acquisition incorporated with parallel computing technology.This makes it possible to monitor dynamic strain during low-cycle fatigue tests.In our study,the fatigue properties of SPFH540 are used to verify the universality and reliability of the real-time strain measurement system.A Manson-Coffin life prediction model is then accepted to evaluate the fatigue strain-life properties of thin sheet SPFH540.The fatigue test results for a steel sheet show that the real-time strain measurement system provides an effective approach for conducting strain-life fatigue tests on thin sheet metals.The special microscopic imaging system is appropriate for characterizing the strain-life fatigue properties and provides an affordable way for low-frequency dynamic studies without using the high-speed cameras.