Flaw IdeNtification Through The Application Of Loading (FINAL)

The teardown and inspection of aircraft, which have completed a significant period of service, is a central requirement of many Aircraft Structural Integrity Management Plans (ASIMP). The reasons for this include a need to inspect for the potential onset of wide spread fatigue damage and to assess the impact of corrosion and in-service mechanical damage. Furthermore, service life data from fleet aircraft are required to confirm laboratory fatigue test results and substantiate the assumptions made during safe-life calculations or probabilistic risk and reliability studies. A teardown and inspection of the fracture critical F/A-18 wing attachment bulkheads (or centre barrel - CB) has been initiated to achieve these goals for the RAAF's F/A-18 fleet. Use is being made of ex-service CBs supplied from the Canadian Forces and U.S. Navy (USN) CB replacement programs. Investigations suggest that the largest 'likely' cracks in the critical bulkheads will be less than 1 mm deep at the time a CB is replaced. Since the detectable crack depth threshold for current NDI (using high frequency eddy current (HFEC) detection) is 1.0 mm or greater, these cracks may not be found. To significantly improve the probability of detecting cracks that are below the lower threshold of NDI, an increase in their size by accelerated fatigue testing of the CBs has been implemented. Cyclic loads (using the mini-FALSTAFF spectrum) are applied to the wing attachment lugs of ex-service CBs in a test rig to simulate in-flight wing loads. The loading is of sufficient magnitude and duration to ensure that any existing cracks will be grown to a size that ensures their detection under laboratory conditions. Quantitative fractography has been performed on observed cracking to obtain crack growth data and to determine the size, nature and cause of discontinuities that initiate fatigue cracking. This paper will provide a summary of the teardown philosophy, methodology and preliminary results

A computational study of the influence of surface roughness on material strength

In machine component stress analysis, it usually assumed that the geometry specified in CAD provides a fair representation of the geometry of the real component. While in particular circumstances, tolerance information, such as minimum thickness of a highly stressed region, might be taken into consideration, there is no standard practice for the representation of surface quality. It is known that surface roughness significantly influences fatigue life, but for this to be useful in the context of life prediction, there is a need to examine the nature of surface roughness and determine how best to characterise it. Non-smooth geometry can be represented in mathematics by fractals or other methods, but for a representation to have a practical value for a manufactured component, it is necessary to accept that there is a lower limit to surface profile measurement resolution. Resolution and mesh refinement also play a part in any computational analysis undertaken to assess surface profile effects: in the analyses presented, a nominal axi-symmetric geometry has been taken, with a finite non-smooth region on the boundary. Various surface roughness representations are modelled, and the significance of the characterized surface roughness type is investigated. It is shown that the applied load gives rise to a nominally uni-axial stress state of 90% of the yield, although surface roughness features have the effect of modifying the load path, and give rise to localized regions of plasticity near to the surface. The material of the test model is assumed to be elasto-plastic, and the development and evolution of plastic zones formed within the geometry are shown for multiple load cycles

A review of composite product data interoperability and product life-cycle management challenges in the composites industry

A review of composite product data interoperability and product life-cycle management challenges is presented, which addresses “Product Life-cycle Management”, developments in materials. The urgent need for this is illustrated by the life-cycle management issues faced in modern military aircraft, where in-service failure of composite parts is a problem, not just in terms of engineering understanding, but also in terms of the process for managing and maintaining the fleet. A demonstration of the use of ISO 10303-235 for a range of through-life composite product data is reported. The standardization of the digital representation of data can help businesses to automate data processing. With the development of new materials, the requirements for data information models for materials properties are evolving, and standardization drives transparency, improves the efficiency of data analysis, and enhances data accuracy. Current developments in Information Technology, such as big data analytics methodologies, have the potential to be highly transformative

Determination of material constants for crack size dependence crack growth model

A crack size-dependence crack growth model was used to characterise the fatigue crack growth in AL 7075-T6 and AL 2024-T351 alloys. It is shown that the crack growth parameters K and a can be used to linearise the crack growth in regions I (elastic) and II (plastic) by plotting fatigue data linear-linear da/dN × a versus K3, where cubic stress dependency is assumed. A theoretical attempt was made to relate this crack size-dependence fatigue crack growth parameters to the strain-life relationship constants. A reasonably good agreement was achieved when comparing between the theoretical predicted and experimental determined material constants