13:40
Session 2: Fatigue crack growth and life prediction methods I
Chair: Elke Hombergsmeier
13:40
20 mins
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The outcomes of research into nucleation and small fatigue crack growth in aluminium alloy 7085-T7452
Ben Main, Ben Dixon, Michael Jones, Simon Barter
Abstract: Aluminium alloy (AA) 7085-T7452 is a recent addition to the 7XXX series of aluminium (Al), zinc (Zn), magnesium (Mg) and copper (Cu) high strength aerospace alloys which has applications in primary airframe structure of the Airbus A380 and Lockheed Martin F-35. AA7085-T7452 was developed for large unitized, lightweight airframe structures since its low quench sensitivity and good through-thickness fracture toughness enables forgings of up to 12 inches (305mm) thick. The Defence Science and Technology Group (DSTG), working with RMIT University, have completed several studies concerning small fatigue crack nucleation and growth in this material using specimens with representative production surface finishes and loaded with service representative spectra.
This paper presents an overview of observations concerning fatigue crack nucleation and small fatigue crack growth rates (FCGR) in AA7085-T7X in contrast with other 7XXX-T7X alloys. Two influences on the fatigue durability of this alloy were investigated. Firstly, the ‘fatigue crack like effectiveness’ of surface etch pitting arising from the commonly used Type 1C anodising process was assessed by deriving equivalent initial discontinuity size (EIDS) values via a fractography-based method. This work showed etch pitting associated with Type 1C anodising surface treatments is less effective in nucleating fatigue cracks in AA7085-T7X compared to AA7050-T7X. Secondly, FCGRs for physically small or near-threshold fatigue cracks were quantified for AA7085 using fractography-based measurements and these were compared with equivalent measurements for AA7050 and AA7075 in the T7X condition. Here, small crack and near-threshold FCGRs in AA7085 were largely consistent with those for AA7050 and AA7075 T7X materials. These results highlight the authors’ current progress toward their goal of understanding the fatigue properties of AA7085-T7452 well enough to allow accurate fatigue life predictions for structural certification and sustainment of AA7085-T7452 airframe components.
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14:00
20 mins
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Back to basics for the fatigue crack growth rate in metallic alloys
Emiel Amsterdam, Jan Willem Wiegman, Marco Nawijn, Jeff De Hosson
Abstract: The field of fracture mechanics started with Griffith’s energy concept for brittle fracture in 1920. In 1963, Paris et al. used a fracture mechanics’ parameter to introduce an equation for the fatigue crack growth rate in ductile materials and this equation is now commonly known as the ‘Paris law’. However, the Paris law and the semi-empirical models that followed ever since do not fully account for the main intrinsic and extrinsic properties involved with fatigue crack growth in metallic alloys. In contrast, here we introduce a dimensionally correct fatigue crack growth rate equation that is based on the original crack driving force as introduced by Griffith and the presence of plasticity in a metal to withstand crack propagation. In particular we found that the fatigue crack growth rate shows a power law relationship with the cyclic strain energy release rate over the maximum stress intensity factor. The new description corrects for the ratio between the minimum and maximum stress during a loading cycle at constant amplitude and for crack growth retardation under variable amplitude loading. It is shown that the outcome of this study allows for reliable predictions of variable amplitude fatigue crack growth life in civil and aerospace structures that currently still heavily rely on testing to ensure safety against catastrophic failure. It is argued that the novel concept presented is an essential step in describing ab initio the fatigue crack growth rate phenomenon in metallic systems.
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14:20
20 mins
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Past, present, and future stress intensity factor solutions for crack at holes
James Sobotka, Craig McClung, Yi-Der Lee, Joseph Cardinal
Abstract: This presentation summarizes advances in stress intensity factor (SIF) solutions for cracks at holes, set in the historical context of the past forty years and current problems that demand new, novel solutions to support damage tolerance (DT) assessments. Special attention is given to corner cracks, since they are usually the initial crack state and often dominate total DT lifetime.
Classic solutions by Raju and Newman (1980s) and Fawaz and Andersson (2000s) modelled wide plates, requiring separate finite-width correction factors for practical application. While the Newman-Raju correction factors were state of the art for their time, they have significant limitations, and we developed new equations with improved accuracy for tension, bend, and pin-loading for single or dissimilar double cracks. Post-transition scenarios led us to develop novel compounding solutions for corner-through or dissimilar through-through crack combinations, building on formulations from NRC-Canada.
Weight function (WF) solutions address the practical challenge of finite geometry effects in a different way. Our WF formulation employs analytical basis functions coupled with large matrices of reference solutions over the range of finite widths and offsets. More importantly, the WF approach handles additional stress states besides uniform remote loading and permits explicit treatment of residual stress (RS), including shakedown RS from local plasticity. Most WF solutions are based on stress gradients in a single direction, but because stresses at the corner of a hole are inherently bivariant, we have also developed a WF solution that accommodates in-plane and out-of-plane stresses for a corner (or a surface) crack.
Current DT challenges involve physical issues that may not be tractable using traditional approaches. These challenges include cold expansion, interference/clearance fits, manufacturing-induced RS, nonlinear material response, out-of-plane bending, hole interaction, and crack interaction. Our recent efforts exploit advances in curved through-crack formulations, principal component analysis, automatic generation of crack fronts, and machine learning via Gaussian Process models. These tools are leading to incremental advances to support cracks at a row of holes, interference fit for through cracks, and tractable calculations of bivariant stresses near stress concentrations under remote loading.
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14:40
20 mins
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Research for thermal load and procedure to predict fatigue life up to form a fatigue crack on CFRP/Aluminum alloy hybrid joints
Takao Okada, Hisashi Kumazawa, Takafumi Toyosawa, Tomo Takeda, Toshiyuk Kasahara, Kouich Yamada, Kasumi Nagao, Yuichiro Aoki, Hirokazu Shoji
Abstract: In ICAF2015, JAXA and NRC presented the life distribution up to first linkup of adjacent fatigue cracks formed in the riveted Aluminum lap joint under constant amplitude fatigue load. The fatigue life up to 0.5mm crack formation was predicted by SWT equation and the life up to first linkup was predicted using in house code.
Currently, JAXA have been conducted the research to evaluate the fatigue life up to form a fatigue crack in a metal/composite hybrid joint. Thermal stress at high and low temperature is occurred in metal/composite hybrid joints due to the difference of coefficient of thermal expansion between metal and composite materials. For application of SWT equation to predict the fatigue life of the metal/composite hybrid joints, accurate calculation of stress and strain in the joint including thermal effect is very important. In this study, thermal load in a mechanically fastened hybrid joint under temperature cycle was investigated experimentally and numerically. In addition, material constants of aluminum sheets were obtained with the strain-controlled fatigue tests for precise prediction by SWT equation.
For the investigation of the thermal load in the hybrid joint, the mechanically fastened hybrid joint specimens composed of two aluminum plates and a composite plate were prepared. Experimental results indicated that relationships between temperature and elastic strain on specimen surface in temperature cycle exhibit hysteresis loop. Finite element analysis for the hybrid joint was also conducted and captured the hysteresis loop obtained by the experiment.
In the material data measurements, the fatigue life for the aluminum alloy in relation to the product of the strain amplitude and the maximum stress has been obtained by the strain-controlled fatigue tests. The strain-controlled fatigue tests are terminated when applied load is reduced by the fatigue crack. The obtained cycles are planned to evaluate the life for formation of certain crack size by fracture surface observation and the relationship between the crack formation cycles and the product of the maximum stress and the strain amplitude are obtained. The obtained relationship would be used to preliminary predict a fatigue crack formation life of the hybrid joint using FEM result.
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