User:Kandregula prasad

ROLE OF DAMAGE TOLERANCE APPROACH IN AEROSPACE INDUSTRY

In Aerospace engineering domain, the natural trend is to improve the products towards increased durability, reliability while preserving functionality, high strength-to-weight ratio, and, limiting costs. Most of the components are typically designed for fatigue using the Palmgren/Miner method in SAFE-LIFE/FAIL-SAFE Approach by usage of load-spectrum, using S-N/E-N curves. However, the inability to quantify reliability, the cost of retiring parts that probably have no damage, and the fact that the crack that usually results in aircraft part failure is not modeled, have all led toward the Damage-Tolerance(DT) design approach being implemented. The fatigue damages develop in stages where defects nucleate in initially undamaged regions and then propagate in a stable manner until, if the cracks are not detected in time, catastrophic failure occurs. The understanding of the mechanisms for both defect nucleation and evolution is therefore a key issue.

Figure 1: Crack growth in response to cyclic loads In DT approach, it is assumed that components have a pre-existing flaw, from which a crack will grow under dynamic loads. This assumption makes it possible to account for in-service or manufacturing defects in determining dynamic life. As per FAR-25.571 statement “An evaluation of the strength, detail design, and fabrication must show that catastrophic failure due to fatigue, corrosion, or accidental damage, will be avoided throughout the operational life of the airplane”. The main idea is that the components should be periodically examined for the presence of cracks. If cracks are identified, then the useful fatigue life is calculated in time/load-cycles/blocks. In DT crack growth rates are used to determine inspection intervals to insure a crack is detected before catastrophic failure. To detect small cracks, NDT is used. It is important to know, the limitations of available NDT-techniques in crack-detection ability. If no cracks are detected, in fact, fatigue propagation lives should be based on the largest crack size that can be missed during inspection. In static loading case, failure will occur at only one load. But in dynamic loading condition, a crack in structure will propagate in response to application of cyclic loads. As shown schematically in Figure 1. Crack-growth is negligible when the crack is very small. Since these effects are nearly impossible to observe, it can be argued that some tiny flaws are always present in a structure. An alternative interpretation is that a small crack is initiated in perhaps 5% of the time range of the diagram due to a manufacturing flaw or material inclusion and then grows during the greater part of the time range to failure. As the crack increases in size, increments of extension get larger until a critical dimension is attained at which the structure fractures in the course of a single cycle of loading. In DTA, the crack growth life is greater than any in-service life that could drive a crack to a dangerous size. This objective can be achieved with an inspection program that detects cracking initiated by fatigue, accident, or corrosion before propagation to failure. Inspection frequencies must be at intervals that are fractions of expected growth life to afford the opportunity for corrective action that maintains structural safety if cracks are found. The economic feasibility of an inspection plan must consider the cost trade-off between inspection methods and intervals. Some components can not be designed for DT due to the limitations of geometry, inspectability or good design practice. Such structure’s evaluation choice between DT and SAFE-LIFE lies with applicant's preference. Prevention is better than cure. Preventive methods like, Shot peening is used primarily to increase the fatigue life. Hence DT refers to the ability of the design to prevent structural cracks from precipitating catastrophic fracture when the airframe is subjected to flight or ground loads. The fail-safe design did not prevent a failure.