THE NEED FOR XEW MATERIALS IN AGING AIRCR4FT STRUCTURES

SUMMARY The end of the Cold W-ar and political and economic considerations has resulted in an effort to extend the life of many aircraft that are the backbone of X.\TO operational forces. Although some are designated to be replaced with new aircraft. the replacement schedule of many often requires an unprecedented life span of bet\\-een JO to 60 years before retirement. Many of the older aircraft have encountered, or can be expected to encounter, aging problems such as fatigue cracking, stress corrosion cracking, corrosion and wear. In order to ensure continued airworthiness and flight safety the structural components undergoing these problems will have to be repaired or replaced. Alloy development that has taken place since a large percentage of the older aircraft were put into service has resulted in several new materials, heat treatments and processing technology that can be used for appropriate longer lasting and higher performing airframe components thus reducing life-cycle costs. This paper describes some of these materials and their advantages ov-er those suffering from “aging problems.” 1. INTRODUCTIOS In 1996 the U.S. Air Force requested the National Research Council (IiRC) to identify research and development needs and opportunities to support the continued operations of their aging aircraft. The results of this study, which was undertaken by a committee selected by the National Materials Advisory Board of the NRC, were published in the Committee’s final report in 1997.’ Among the many recommendations made by the Committee, one was to develop guidelines to broaden the application of improved materials as substitutes for incumbents with low damage tolerance and corrosion resistance. Such substitutions must make good business sense with respect to reduction in life-cycle costs and materials availability. Examples of reducing the life-cycle cost by implementing new materials on aging aircraft structure are given in the paper by Austin et al in this proceedings2 The U.S. Air Force, as well as the air forces of other NATO countries, has many old aircraft that form the backbone of the total operational force structure. >lany of these, e.g. the KC-135, the B-52, and the C-141were introduced into service in the 1950s and 1960s. Even the F- 15 air superiority fighter became operational 20 to 25 years ago and the F- 16 and KC- 10 jet trainer at least 15 years ago. The extended use of these aircraft,