Hill Engineering Blog

Aircraft Airworthiness & Sustainment 2018

Hill Engineering is presenting about residual stress evaluation in legacy aircraft cold expanded fastener holes at the upcoming 2018 Aircraft Airworthiness & Sustainment Conference (AA&S) in Jacksonville, FL. The mission of this conference is to identify the challenges and the collaborative work being done within the aerospace community in improving the safety, airworthiness, availability and cost effectiveness of both our military and commercial aircraft fleets. Hill Engineering’s presentation will include a summary of recent work in airframe structures, demonstrating the effect of aircraft service on the residual stress at cold expanded fastener holes. The abstract text is presented below.

It is well established that compressive residual stresses provide improved fatigue performance and damage tolerance enhancement. Compressive residual stresses act to slow the growth of fatigue cracks, which can provide substantial benefits in terms of performance, safety, cost, and inspection intervals. To take advantage of this concept, cold working processes are used to induce compressive residual stress near many critical aircraft fastener holes. A typical cold working expansion process involves pulling a tapered mandrel through a hole. The material near the hole is plastically deformed resulting in a state of compressive residual stress near the hole.

Since the residual stress from cold hole expansion has an important positive effect on the fatigue and damage tolerance of the structure, the stability and presence of residual stress over the service life is important in terms of aircraft sustainment. This presentation provides a summary of recent experiments to quantify the level of residual stress remaining in post-service aircraft structure. As part of this effort, lower wing skins from A-10 and T-38 aircraft were torn down and residual stress measurements were performed using the contour method, which provides a two-dimensional map of the residual stress distribution. Element specimens representing specific structural details were also manufactured and tested to estimate the original (pre-service) residual stress from cold expansion. A comparison between the original and remaining residual stress is provided.

If you are planning to attend the conference please stop by to discuss Hill Engineering’s capabilities in fatigue analysis and design and residual stress measurement. Please contact us for more information.