Application of digital-image-correlation techniques in analysing cracked cylindrical pipes

Cracks induced by external excitation on a material that has defects may generate the stress concentration phenomenon. The stress concentration behaviour causes local buckling, which will induce the damage of the members made of this material. Thus, developing techniques to monitor the strain variation of a cracked member is an important study. The traditional technique (such as strain gauge) can only measure the average strain of a region. The strain variation within this region cannot be determined. Therefore, it cannot sufficiently reflect the mechanical behaviour surrounding the crack. The Digital image correlation technique recently developed is an image identification technique to be applied for measuring the object deformation. This technique is capable of correlating the digital images of an object before and after deformation and further determining the displacement and strain field of an object based on the corresponding position on the image. In this work, this technique is applied to analyse the mechanics of a cylindrical pipe experiencing crack destruction. The fixing device is used to avoid shaking the specimen during the pressurizing process. The image capture instruments are fixed on the stable frame to measure the deformation of specimen accurately. Through the cylindrical pipe cracking test, the capacity of the digital image correlation technique for surveying the strain variation in a tiny region is validated. Then, the experimental results obtained using the digital image correlation analysis is used to demonstrate the crack development tendency in defect materials and the stress concentration zone.