In Situ Measurement of the Strain Field at the Fatigue Crack Tip Based on Sub-Image Stitching and Matching DIC

Abstract

Studying the in situ measurement and evolution of the strain field at the crack tip during fatigue crack growth (FCG) is of great significance for understanding the fracture characteristics of materials and predicting fatigue life. Herein, a new method is proposed for the in-situ measurement of the strain field at the fatigue crack tip based on microscopic digital image correlation (DIC). The method proposed solves the problem of the existing in situ strain field measurement method being unable to dynamically track the crack tip and take the crack tip image due to the limitation of the field of view of the microscopic camera. A macroscopic camera is used to capture the global crack images on one side of the compact tension (CT) specimen. Meanwhile, a microscopic camera is used to track and capture the crack propagation speckle image on the other side of the CT specimen. The proposed method was verified by experiments with Quenching and Partitioning 980 (Q&P980) steel, and the results showed that the method has high accuracy, with the average measurement error being less than 5% and the maximum error being less than 10%. A butterfly shape of the measured strain field and the strain concentration near the crack tip were observed. The success of this method will help to obtain better insight into and understanding of the fracture behavior of metal materials as well as precise prediction of the fatigue life of metal materials.

In Situ Measurement of Cyclic Plastic Zone and Internal Strain Response of Q&P Steel near Fatigue Crack Tip Region Based on Micro-DIC

“…In order to obtain the mechanical properties of Q&P980, the uniaxial tensile test was performed, and the test specimen was a dog-bone specimen made of Q&P980 steel. The mechanical properties as shown in Table 1 were obtained by fitting and modifying the stress–strain curve combined with the Ramberg–Osgood (R-O) model [ 32 ]; the data processing details were introduced in our previous work [ 29 ].…”

“…As shown in Figure 3 and Figure 4 , the hardware components of the in situ measurement system and experimental set-up for CPZ in situ measurement are the same as those in our previous work [ 29 ]. However, in addition to image calibration software, multi-scale image synchronous acquisition software, image stitching and matching software, and VIC-2D software, the in situ CPZ measurement system also includes software for measuring the CPZ and internal response at the crack tip.…”

“…The measurement flow of CPZ based on DIC is shown in Figure 8 , in which the core technologies are DIC and the load–strain curve characteristic judgment algorithm. In order to calculate the in situ strain field at the different crack lengths [ 29 ], it was necessary to use the image stitching and matching algorithm to obtain the full-field reference image and the reference images corresponding to the target images, as shown in Figure 8 . Our previous work [ 29 ] introduced the specific implementation and the accuracy verification details of the sub-reference image stitching and target image matching algorithm.…”

“…This method combines micro-DIC with the reference image stitching and target image matching algorithms, as well as the load–strain loop curve characteristic judgment algorithm to measure the fatigue crack tip CPZ at any length in the stable crack propagation stage. The reference image stitching and target image matching algorithms, which were introduced in our previous work [ 29 ] in detail, are mainly used to solve the measurement problem, that is, that the micro-camera cannot be directly used to measure the micro crack tip strain fields of the long crack. In our previous work [ 29 ], we mainly obtained in situ online strain fields data near the crack tip region of short and long cracks.…”

“…The reference image stitching and target image matching algorithms, which were introduced in our previous work [ 29 ] in detail, are mainly used to solve the measurement problem, that is, that the micro-camera cannot be directly used to measure the micro crack tip strain fields of the long crack. In our previous work [ 29 ], we mainly obtained in situ online strain fields data near the crack tip region of short and long cracks. The obtained strain fields data are an important basis for the current research carried out in this paper.…”