Advanced Materials Research Vols. 891-892

Abstract: The high-cycle fatigue properties of 0.35% carbon steel and work-hardened oxygen-free copper in 10MPa hydrogen were studied. The fatigue limit of the carbon steel in hydrogen was almost the same as that in air. The fatigue strength at 10⁷ cycles of the copper was higher in hydrogen than in air. The fatigue life of both materials is longer in hydrogen than in air. The reason was the delays in the crack initiation and the early propagation of the cracks in hydrogen. For both materials, the detrimental effect on the fatigue strength due to the hydrogen environment was small, however, it was determined that hydrogen participates in the slip deformation. The morphology of the slip bands was specific in hydrogen. In the copper, the slip bands, which are non-viable in air, developed in hydrogen.

Abstract: There are currently serious doubts about the accuracy of Miners rule for designing welded structures subjected to variable amplitude loading, particularly under spectra cycling down from a constant maximum stress. The deficiency has been attributed primarily to stress interaction effects that cause crack growth acceleration. In this paper the crack growth response of a structural steel and an aluminium alloy to a loading spectrum designed to promote fatigue crack acceleration is studied and the potential mechanisms responsible are evaluated.

Abstract: Increasingly broader application of composite materials in commercial airplane primary structure has led to a renewed, more in-depth assessment of the effect composite elements in mechanical joints have on the durability of the metal structure to which they are mated. Thermal stresses, clamp-up loss due to composite creep, composite joint properties, and lightning and electrical grounding effects have been found to be some of the parameters most significantly affecting or limiting the fatigue performance of the metallic constituents within these hybrid joints, influencing the way joint design and analysis are approached.

Abstract: Marine composite structures subject to dynamic loading typically incorporate more than one material type, and consist of laminate sections up to hundreds of millimetres in thickness. These solid hybrid laminates exhibit different behaviour in static and fatigue loading from thin aerospace composite laminates and sandwich structures. There is therefore a need to better understand the likely damage and degradation mechanisms that will occur in these thick structures and to concurrently develop nondestructive evaluation (NDE) technology to meet the consequent inspection problems. In this paper we present details of an ongoing fatigue program on marine composite blades. The challenges for ultrasonic NDE of thick composites, and emerging inspection methods using state-of-the-art inspection systems and analysis tools will be discussed.

Abstract: The three dimensional grain mapping technique for polycrystalline material, which is called X-ray diffraction contrast tomography (DCT) has proposed. In the present study, the measurement of DCT was conducted in SPring-8, which is the brightest synchrotron radiation facility in Japan, and the condition of measurement and data procedure are discussed. Developed technique was applied to aluminium alloy and stainless steel. The shape and location of grain could be determined by the developed three-dimensional mapping technique using the apparatus in a bending beam line of SPring-8. To evaluate plastic deformation, the grain orientation spreads of individual grains were measured. The grain orientation spread is caused by the mosaicity, which relates to the dislocation structure in a grain. The grain orientation spread was found to increase with increasing plastic strain. Fatigue damage also could be evaluated by the grain orientation spread in the DCT measurement.

Abstract: In recent years, fatigue limit estimation based on dissipated energy has been introduced in various industries because of its time and cost effectiveness. However, the mechanism of energy dissipation and the relationship between energy dissipation and fatigue damage have not been investigated well. In this study, mechanism of energy dissipation is investigated in relation with formulation of slip bands for JIS type 316L stainless steel through observation of slip bands by optical microscope and atomic force microscope.

Abstract: Greatly reduced inspection intervals of skin damage repairs pose a significant financial problem for aging commercial aircraft fleets. Such intervals for visual inspections are the result of simplified conservative repair substantiation analyses, based on the same crack propagation scenarios and curves that were established in the initial project development. These neglect the structural role of external repair (“doubler”) and consider only the increase in hidden crack path. A more refined approach to reassessing inspection intervals after a repair may keep maintenance jobs in accordance with common C-check routines in most cases. This approach, based on new crack growth simulations for worst case scenarios that could occur at the region of repair, uses respective kinetic equation and new geometric stress intensity factor functions, obtained in additional FEM (Finite Element Method) analyses. In particular, for standard rectangular repairs, the number of possible geometric configurations is astonishing considering length, width, skin and “doubler” thickness, reinforced panel dimensions, and frame and stringer cross sections. This investigation deals mainly with defining a minimum sufficient number of intermediate crack length values for FEM analyses in each propagation scenario. A conservative but efficient definition of most relevant parameters for a new numerical analysis campaign is another important issue. The results obtained are helpful for the improvement of the operational efficiency and safety of an aging fleet.

Abstract: This paper reports on investigations on the residual fatigue resistance of a 2024 aluminium alloy of an A320 aircraft at the end of life. The fatigue data (S-N and da/dN curves) are compared with data obtained on a pristine alloy using a similar procedure. The results are analysed on the basis of fracture surfaces observations and of AFGROW fatigue life computations.

Abstract: As known, friction stir welded (FSWed) joints are mature to be applied in aircraft structure. However, the weld creates local discontinuity in property and local tensile residual stress, which harm the damage tolerance of welded panels. So crack retarders were bonded in the weld zone to improve damage tolerance. Finite element method was used to calculate stress intensity factors by ABAQUS software. Finite element models were built to simulate the function of bonded retarder. Cohesive elements were used between the substrate panel and bonded retarder. A Fortran program was made to input residual stress to finite element models. Stress intensity factors from residual stress with and without bonded retarders were calculated and compared. Effects of residual stress on stress intensity factors and redistribution of residual stress were taken into considered. Effective R ratios were calculated with crack growing through the weld. Effects of bonded retarder on stress intensity factors were calculated. The results were compared with the experimental findings.

Abstract: The damage behavior of cellular composites under creep and cyclic tensile loading conditions is investigated. Tensile cyclic tests at 1 Hz and 20 Hz test frequency and creep tests are performed and compared to previous investigations under compressive loading. In contrast to the compressive loading, no frequency effect is identified in tensile loading and the fatigue curves have steeper slopes. The damage behavior is analyzed by means of scanning electron microscope (SEM) and high resolution 3D X-ray tomography. It is found that microcracks are forming in the glass foam granules first; the final failure is caused by a macrocrack passing through the glass foam granules and the matrix. The damage evolution is measured in terms of stiffness loss of the specimen and is found to have different behavior than in compression.