Fatigue behaviour of re-worked fastener holes in aluminum alloys

本研究的目的是研究航空铝材试样返工后紧固件孔的疲劳裂纹和裂纹扩展行为。目的是了解所涉及的疲劳开裂机制，并开发分析技术来模拟和设计老化飞机的维修方案。传统铝制机身结构的疲劳裂纹大多来自紧固件孔，在达到临界尺寸之前就被检测到。这些孔被重新加工，通常通过增大孔的尺寸来消除疲劳损伤；在飞机重新投入使用之前，要安装新的紧固件。孔的再加工还可以包括以下一种或多种：冷膨胀以产生有益的压残余应力，过盈配合紧固件以提高疲劳寿命，过盈配合衬套以提高刚度，恢复净截面并提高疲劳寿命。在这项研究中，将采用一步一步的方法，通过在实验室样品上收集的同时测量的实验疲劳数据，为返工紧固件细节开发一个有限元模型。将检查加工变量，以确定哪些技术对延长寿命最有利，哪些技术对返工孔修复方案有害。加工变量包括：边缘距离、冷膨胀量和过盈配合量。这对军事和民用航空航天工业的好处是巨大的。传统的铝制机身继续大量生产；例如，波音737和747的产量仍然非常高。此外，加拿大军队继续操作和获得新版本的飞机，如CC-130（大力神）和CH-148（支奴干），并且必须延长平台的使用寿命，如CF-188（大黄蜂）。该应用的寿命延长技术对于确保加拿大航空运输、搜救和国防系统在未来几十年的安全和经济至关重要。

This proposal is to study fatigue cracking and crack growth behaviour in re-worked fastener holes in aircraft aluminum specimens. The objective is to understand the fatigue cracking mechanisms involved and to develop analytical techniques to model and design repair schemes for aging aircraft.The majority of fatigue cracks in conventional aluminum airframe structures emanate from fastener holes and are detected before reaching critical size. These holes are re-worked, generally by over sizing the hole to remove the fatigue damage; new fasteners are installed before the aircraft is returned to service. Re-working of a hole may also include one or more of the following: cold expansion to impart beneficial compressive residual stresses, interference fit fasteners to improve fatigue life, and interference fit bushings to improve stiffness, restore net section and improve the fatigue life. In this research, a step by step approach will be used to develop a finite element model for reworked fastener details supported by simultaneously measured experimental fatigue data collected on laboratory specimens. Processing variables will be examined to determine which techniques are most beneficial to life extension and which techniques are detrimental to re-worked hole repair schemes. Processing variables will include: edge distance, amount of cold expansion, and amount of interference fit.The benefits to the aerospace industry, both military and civilian, are tremendous.Conventional aluminum airframes continue to be produced in great numbers; for example, Boeing's production of the 737 and 747 remains exceptionally high. Furthermore, the Canadian Forces continues to operate and acquire new versions of aircraft such as the CC-130 (Hercules) and CH-148 (Chinook), and have to extend the lifetime of platforms such as the CF-188 (Hornet). Life extension technologies for this application are vital to ensure the safety and economy of Canada's air transport, search and rescue, and defence systems for many decades to come.