A crack-growth arresting technique in aluminum alloy
Introduction
Many failures in engineering applications or machine components have been caused by a fatigue crack initiated from points at which stress was concentrated. Where the stress concentration level is higher than a critical value, continuous crack-growth results in failure in the machine components. As for the fatigue process in many service conditions, the crack-growth process is a large percentage of the fatigue life. Therefore, it is a useful idea to arrest the crack-growth by some method before the failure.
A number of investigations have been performed to prolong the fatigue life of defective machine components [1], [2], [3], [4], [5], [6], [7], [8]. The hole-related method of arresting fatigue crack arresting fatigue crack propagation [1], [2], [3], [4], [5], [6], [7] was developed by some practical ideas. One of the most popular techniques is drilling a hole at the crack tip [1], [2] to remove the crack tip singularity. This hole is called a stop-drilled hole or fastener hole. As a result, the crack-growth is delayed or stopped until a new crack is initiated. The influence of hole size on the number of cycles for a new crack initiation was also studied by some researchers [2], [4], [5]. It was revealed rigorously that the stress concentration was the dominant contribution in the comparison. Since the bigger diameter may affect the dimensional property of a machine component, the strength of the structural component has to be considered [9]. However, it is expected that the bigger diameter of the hole makes other problems for a machine component under some practical conditions.
Following the basic method of drilling a hole at the crack tip, cold-working related processes [3], [4], [5] and drilling holes surrounding crack tip [6], [7] were also studied. However, a more practical, feasible method of arresting a crack is still necessary. A simple method, which can more effectively arrest crack-growth, is expected.
The focus of the present study is to propose a new technique to arrest crack initiation at stop-drilled holes with additional hole arrangement. This method reduces the stress concentration at the edges of the stop-drilled holes. The effectiveness of this method was confirmed by the calculation of the stress concentration level at the edge of stop-drilled hole using the body-force method [10], [11], [12] and by some experiments.
Section snippets
Method of arresting crack-growth and analysis model
Fig. 1 shows the shape of the calculation model for the specimen with a height of 2H and width of 2W, which was employed to investigate the method of arresting crack-growth. The stress concentration factor Kt at point A in Fig. 1 (bottom of the stop-drilled hole) was calculated by software for two-dimensional elastic problems based on the body-force method. The related program analysis of the software was presented by Nisitani and Saimoto [10]. As a well-known formula, stress concentration
Calculation of the stress concentration factor at the edge of a stop-drilled hole
Fig. 2, Fig. 3 show the results of the calculations of the stress concentration factor Kt at point A in various angles and hole distances. The horizontally dotted line is the stress concentration factor Kt for the base type. The diameters of stop-drilled holes d1 were 2 mm and 3.2 mm. The stress concentration factor Kt was accomplished by calculating the arrangement of the ancillary holes and was compared to that of the base type. The distances between the edge of the ancillary holes and the
Material and experimental method
To confirm that crack initiation is arrested at point A of a stop-drilled hole, fatigue tests in cases of different hole arrangement were performed. The material used was 2024-T3 aluminum alloy. Chemical composition (wt.%) and mechanical properties of the material are shown in Table 1, Table 2, respectively. Specimens were cut from plates so that the rolling direction was along the loading axis.
Fig. 7 shows the geometry of the specimen. The shape of the specimen was rectangular. The specimen
Conclusions
A method of extending the fatigue life of a cracked plate after machining stop-drilled holes was proposed. In this method, to reduce the stress concentration factor Kt at the edges of stop-drilled holes, the ancillary holes were machined along with the stop drilled holes. For examining the usefulness of the method, the calculation of Kt at the edges of stop-drilled holes and certain experiments were performed. The main results obtained are as follows:
- 1.
The values of the stress concentration
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