Experimental and Numerical Study of the Reinforced Panels Subjected to Tensile Loading: Crack Stoppers
Abstract
The present study concerns with the experimental and numerical investigation of crack stoppers ahead of an edge crack in panels subjected to tensile loading. Two different patches (rectangular and semi-annular patches) have been analyzed. The patches (aluminium and steel) are placed at different distance from the crack, symmetrically on both sides of the panel and at a finite distance ahead of the crack tip. A finite distance ahead of the crack tip reveals that depending on the distance, the crack tip could remain straight or curve. In such cases, the crack could either be arrested, run through or run around the reinforcements. Moreover, the degree of instability is reflected by an index parameter that accounts for the effect of load, geometry and material properties. Moreover, a geometrically nonlinear, two-dimensional (2D) finite element analysis (Comsol Multiphysics) has been employed to determine the local energy intensity. It would be of special interest to know whether the crack would run straight and arrest at the patch regardless of the other variables. The ultimate goal for straight crack path is to produce sufficient low local energy intensity. This gives a significant advantage because as the local energy intensity is increased, crack would tend to curve and lead to complete fracture of the patched specimens. It is equivalent of moving the patch closer to the crack tip. The predictions made from the strain energy density theory, as well as, there is a good agreement between finite element results and experimental findings.
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Introduction
A challenging problem which arises during production are the defects in structural components. A practice often used is to repair the cracked members by bonding patches to redirect the load path [1-8] to reduce the local stress or energy intensity level below critical. However, it is well known, that unlike the repair patches [9–15], which are used when fatigue cracks are detected, crack retarders will be part of the original aircraft structure and subjected to operational loads and environments throughout the entire service life. It means that aircraft structures could be safer, lighter, and cheaper. Especially, by using adhesively bonded composite patches, which are more efficient and much less damaging to the parent structure than standard repairs based on mechanically fastened metallic patches [16].
The development of bonded composite repair technology has been accelerated by many researches in the aerospace industry in the interests of increasing the service life and reducing the repair cost [17–19]. Composite repair has taken numerous roles on military aircraft, such as reinforcement in areas where stress corrosion cracking can occur, as on the lower surface of the wing pivot on a U.S. Air Force F-111, or fatigue cracking around a fuel decant hole on a Royal Australian Air Force (RAAF) Mirage III. On commercial aircraft, the use of composite repairs is in early stages [20]. On the other hand, one of the key issues in aircraft failure is the presence of multiple cracks in a local area, known as multisite damage (MSD). Analysis of the repair of MSD has been proposed by Park et al., [21] by placing a patch over a cracked pin-loaded fastener hole.
In addition to knowledge of the detailed stresses near the defects or cracks, a suitable failure criterion is needed to determine the effectiveness of the repair process. Improper repair could cause more harm than good. This has been known in the aircraft repair industry. The classical failure criteria are not adequate because they could not correct local and global failure in a constant manner to include the combined effects of loading rate, geometry and material.
Conclusion
This work has conclusively proven that the instability index can be used as an indicator of the effectiveness of reinforcement to arrest a crack. Several parameters, such as geometric and materials of the patch alters effectively is the intensity of the load transferred to the crack tip region and hence the crack growth characteristics. Broadly translated our findings indicate that the edge crack specimen gives rise to unstable fracture under uniform tensile loading. However, the kind of the crack propagation, depends on the rate and the way of loading of the specimen from the test machine, namely, whether the increase of the imposed displacement or the force respectively, will be controlled.
The evidence from this study that, for low local energy intensity of the rectangular patch specimens, the initial crack runs straight and arrests at the patch edge regardless of the initial crack length and position. However, as the patch is moved closer to the crack tip crack path deviated from the straight line (critical point). This occurred for the rectangular patches regardless of the other variables. A temporary arrest of extension is presented at point G, but with furthermore increasing of the loading, leaded to catholic fracture. Moreover, the crack trajectory was straight for the semi-annular patch even when patch distance from the crack tip is increased, a new crack trajectory was observed. This is independent of the geometrical characteristics of the specimen, as well as, the initial crack length. Nevertheless, the crack tip is still trapped inside the semi-annular patch for a certain load. Our work has led us to conclude that, we can direct to a certain point outside the patch zone. That means, a further investigation of the method of Hypothesis of the Maximum Gradient of the Strain Energy Density (HMG of SED) must be investigated. The most important limitation lies in the fact that this method provides great accuracy only in a smallscale specimen. It is also recommended that further research should be undertaken in the area of ductile materials.