13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Short/Small Crack Model Development for Aircraft Structural Life Assessment Min Liao 1*, Guillaume Renaud 1, Yan Bombardier 1 1 Aerospace Structures, National Research Council Canada (NRC) 1200 Montreal Rd, Ottawa, Ontario, Canada K1A 0R6 * Corresponding author: min.liao@nrc-cnrc.gc.ca Abstract This paper presents the testing and modeling results from an NRC project on short/small ① crack model development for aircraft structures. Fatigue tests were conducted for 7075-T73 hand forging aluminium material using precracked compact tension (C(T)) and naturally cracked single edge-notch tension (SENT) coupons, under various stress ratios. The adjusted compliance ratio (ACR) method was used for the C(T) coupon tests aiming to quantify remote closure due to plasticity and forging-induced residual stresses. For the SENT coupons, three-dimensional StressCheck finite element (FE) models were developed to accurately calculate the stress intensity factors of surface and corner cracks. Both C(T) and SENT test data were combined to develop short-long fatigue crack growth rate models. These models were then used in crack growth life analyses for coupon and component cases, taken from transport aircraft under spectrum loadings. It was shown that the newly developed models resulted in more accurate fatigue life estimations. Keywords Short/small crack growth, 7075-T73 , Adjusted compliance ratio, Marker bands, CanGROW. 1. Introduction Currently, the practical life prediction technologies for metallic aircraft structures are mainly safe-life (SL), damage-tolerance (DT, including fail-safe), and flaw-tolerance (including flaw-tolerance safe-life for helicopter). In addition, a combined SL and DT (two-stage) total life approach is used to overcome the limitations of these two approaches taken separately. However, the transition between SL and DT is not clearly defined and justified. One of the reasons is that the small crack region remains a “grey zone” for which a robust/practical approach and test database are still missing for many aircraft designers/manufacturers, although extensive research has been carried out in the past decades. In collaboration with other organizations, NRC is developing the Holistic Structural Integrity Process (HOLSIP) to augment the traditional SL and DT approaches with the ultimate goal to evolve HOLSIP into a new paradigm for both the design and sustainment stages. One of the current HOLSIP development efforts is to further develop the short/small crack database and the physics-based models. Recently, an NRC project was completed with testing and modeling results for 7075-T73 aluminum forging material, which is used in Royal Canadian Air Force (RCAF) large transport aircraft [1]. A literature review revealed that short/small crack data are scarce for 7075-T73 aluminum alloys for fatigue durability analysis of aircraft structures. The short/small crack growth rate data have significant scatter due to material microstructures, specimen types and geometries, testing and measuring techniques, which lead to extensive testing time and costs. As such, some researchers perform long precrack crack tests and use the near-threshold data to represent the average small crack growth rate, which has two issues: 1) the long crack data produce overly high stress intensity threshold factor Kth ( at da/dN=3.94E-9 inch/cycle or 1E-10 m/cycle in ASTM E647) which would result in un-conservative life prediction; 2) the scatter is different from that of naturally small cracks, which are more affected by intrinsic microstructures effects. ① As per ATSM E647 (Appendix X3), a small crack is defined as being small when all physical dimensions (in particular, both length and depth of a surface crack) are small in comparison to a relevant microstructural scale, continuum mechanics scale, or physical size scale. While a short crack is defined as being short when only one physical dimension (typically, the length of a through-crack) is small according to the description of ‘small crack’.
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