Advanced Materials Research Vols. 891-892

Abstract: Despite corrosion prevention or protection schemes/treatments and corrosion prevention and control plans, in-service corrosion does occur and has the potential to impact the structural integrity of aircraft. Whilst the fatigue management of the aircraft is generally well understood as reflected in typical Aircraft Structural Integrity Management Plans (ASIMP), which in some cases contain environmental degradation plans, limited provision beyond find and fix exists for corrosion repair. Thus the repair of corrosion can be a major through life cost driver as well as an aircraft availability degrader. This find and fix policy exists largely because tools are currently considered too immature to accurately assess the structural significance of corrosion when it is detected. In this paper a process is described which should allow an alternative to the current find (corrosion) and fix philosophy for pitting corrosion. The method is intended to maintain a probability of failure consistent with ASIMP structural certification requirements for fatigue cracks initiating from corrosion pits for a specific period. Unanticipated maintenance costs significantly more than planned maintenance. Thus delaying the repair of pitting corrosion until the next scheduled maintenance, should save considerable resources and improve aircraft availability. The development of analytical tools capable of accurately assessing the effect of corrosion on the durability of a structure would be considered a major advance for the ASIMP.

Abstract: Use of Magnesium alloys as body implants are breaking into a new paradigm of biomedical engineering as they are biocompatible, biodegradable and have mechanical properties close to that of bone. Even though corrosion fatigue (CF) and stress corrosion cracking (SCC) failures are among the most common concerns for metallic implants, CF behaviour of magnesium alloys in physiological environments has received little attention. This article reports the CF results of a common cast magnesium alloy (AZ91D) in modified simulated body fluid (m-SBF). Results showed that there was a remarkable difference in fatigue strength of Mg alloys when tests were performed in m-SBF.

Abstract: This paper describes the fatigue properties of the steel P92. This material is widely used in the energy industry, especially for pipes and pipe bends of supercritical steam turbines. Steel P92 is alloyed with 2 % of tungsten compared to steel P91. This increases a creep strenght of the material. It is possible to reduce wall thickness of the P92 pipe up to about 20%. Fatigue tests were carried out on standard samples and compared with SFT samples (Small Fatigue Test). Using the device SSam 2 made by company Rolce Royce, it is possible to gently remove a samples from energy component without power plant shutdowns. Consider these correlations, i tis possible to determine mechanical properties of the material from small amount of removed experimental material.

Abstract: Legacy 7XXX series aluminum alloys were developed primarily for their high strength with less regard for their fatigue properties, corrosion resistance and fracture toughness. The constituent alloying elements in these materials (used to achieve high strengths) markedly increased their corrosion susceptibility. Consequently, aircraft structures made from these alloys have exhibited fatigue and corrosion damage. In the present work, we have investigated a crack finding in a fuselage skin of AA7XXX series alloy. This investigation revealed the crack propagated by a combination of fatigue and corrosion. Through the use of extensive metallography, mechanical analysis and laboratory experiments, we have separated the contributions to the damage growth due to corrosion and fatigue. We have also confirmed that in-service mixed-mode failures like this, observed in these alloys, can be reproduced reliably in the laboratory. Furthermore, it was observed that the presence of corrosion can actually change the propagation of a fatigue crack from mode I, the preferable orientation for fatigue crack propagation, to mode II, the preferable orientation for corrosion propagation. Even though the mechanical driving force is enough to grow the crack in mode I, the presence of corrosion can change it to mode II by electrochemical degradation of the material. Using electrochemical measurements, we relate the change in failure mode to the frequency of cyclic loading. At slow enough cyclic frequency the electrochemical energy released due to galvanic corrosion degrades the material such that the crack turns and propagates in the orientation which has only one third the mechanical driving force as compared to the original crack propagation path. This is the first time such phenomenon has been successfully replicated in the laboratory and modeled with finite element analysis.

Abstract: This paper discusses the physical meaning of crack opening corrections in fatigue crack growth analyses proposed in the literature. To provide an explanation concerning the physical relevance of these corrections, an energy approach is proposed to evaluate fatigue loading conditions. Considering cyclic loading in terms of cyclic energy rather than in stress cycle, explains the origin of the crack opening corrections.

Abstract: The plasticity-induced crack closure of through-thickness cracks, artificially obtained from short cracks grown in CT specimens of 304L austenitic stainless steel, is numerically simulated using finite elements. Crack advance is incremented step by step, by applying constant ΔK amplitude so as to limit the loading history influence to that of crack length and crack wake. The calculation of the effective stress intensity factor range, ΔK_eff, along curved shaped crack fronts simulating real crack fronts, are compared to calculation previously performed for through-thickness straight cracks. The results for the curved crack fronts support that the front curvature is associated to constant ΔK_eff amplitude, thus assumed to be the propagation driving force of the crack all along its front.

Abstract: Hard phases such as martensite regions affect micro-crack extension by blocking the plastic zone ahead of the crack tip, but also by changing the crack opening which can be taken as loading quantity for cracks. This paper deals with the measurement of crack opening for microcracks in a ferrite/martensite dual phase steel. The methods used are in-situ testing in the SEM, X-ray tomography, and digital image correlation. It was found that martensite regions affect the relative displacement of the crack phases both at the crack tip and in the crack wake.

Abstract: In this study, fatigue crack closure behavior was investigated in the aluminum alloy 6061-T6 and the carbon steel JIS. S25C. It was found that crack closure in the aluminum alloy 6061-T6 showed the characteristics of plasticity-induced fatigue crack closure (PIFCC), whereas the carbon steel JIS. S25C showed the characteristics of roughness-induced fatigue crack closure (RIFCC). The experiments included the determination of the crack-opening levels K_op as a function of stress intensity factor range ΔK and the effect of surface removal on the crack-opening level. In order to simulate the behaviors of the plasticity-and the roughness-induced fatigue crack closure, the finite element method was adopted. The results of FEM were in good agreement with the experimental results. It was cconcluded that at a given yield strength level , a low Youngs modulus and a low work-hardening coefficient will favor PIFCC, whereas a high Youngs modulus and a high work-hardening coefficient will favor RIFCC.

Abstract: Fatigue crack propagation behaviors in a duplex stainless steel have been studied using an in-situ SEM/EBSD fatigue test and a conventional da/dN test. Crack propagation behaviors in grain, effect of Schmid factor, propagation cross the grain or phase boundaries have been discussed. Crack propagation occurs mainly in the grains with a high Schmid factor, but the crack can also propagate in the grains with very small Schmid factor. Crack deflection occurs mainly at the phase boundaries, but crack branching occurs mainly in the grains due to the dislocation slip. In-situ SEM/EBSD fatigue test confirms that crack propagation deflection can lead to a decrease in crack propagation rate. Formation of crack branches can significantly reduce the crack propagation rate, which can cause crack growth retardation in the main crack path in the worst case. The crack branches formed are usually not ideal. They can propagate almost transversely to the main crack direction with a mode II stress intensity factor, SIF, and a rate that is much higher than that of the main crack.

Abstract: Crack closure is an important factor affecting fatigue crack growth in high strength alloy materials. Plasticity is known to be the main driver of closure, but in some materials and at some stages other mechanisms such as fracture surface roughness and debris accumulation are also important. Analytical models based on the plasticity induced closure concept have been very successful in correlating fatigue crack growth rates and lives for a range of materials under constant amplitude and spectrum loading. However, extreme values of plastic constraint factors, significantly lower than those determined from three dimensional finite element studies on similar geometry, are needed to achieve good correlation with experimental results, particularly for materials which exhibit rough and tortuous fatigue surfaces. One such material investigated here is β-annealed Ti-6Al-4V titanium alloy. The aim of this paper is to further develop, apply and evaluate a crack closure model which combines roughness and plasticity induced closure, incorporating experimental measurements of fracture surface roughness from tests on Eccentrically Loaded Single Edge-Notch Tension (ESE(T)) specimens. The model was first proposed by Zhang et al. 2002 for short cracks in 2000 series Aluminium Alloy. The model was evaluated here for physically longer cracks in the Titanium Alloy material. Accurate surface profile and roughness measurements were made using an optical 3-D profiler with a vertical resolution of better than 0.15 μm. Verification of the proposed model was carried out by comparing the model prediction with the closure measurements by back-face strain compliance using the pin-loaded ESE(T) specimens. Results from the roughness model were then compared with results from the FASTRAN analytical crack closure code. Analysis using this new approach, with plastic constraint factors more closely aligned with the values determined from independent classical and three dimensional finite element studies, provide a solid basis from which to implement the approach in FASTRAN.