Advanced Materials Research Vols. 891-892

Abstract: For engineering structures subjected to cyclic load, fatigue failure normally occurs at geometrical discontinuities such as holes and notches. In aircraft structures, such locations may experience occasional severe loading that can cause appreciable local plastic deformation. This poses a significant challenge to fatigue life modelling. For such locations subjected to variable amplitude loading of a large number of cycles, the numerical analysis of fatigue life requires an accurate and robust model for cyclic plasticity, in order to reliably determine the stress and strain response. In this paper, we explore the potential of a nonlinear kinematic hardening model in improving fatigue life analysis. The work is motivated by the inability of an existing strain-life model to capture the difference in fatigue damages caused by an unclipped and clipped service load spectrum. Preliminary results show that the strain-life method based on the nonlinear kinematic hardening model was able to qualitatively demonstrate the trend in fatigue life for two critical locations analysed, and it was able to give much improved quantitative prediction for one location. Further work is under way to verify the model against more test data and to improve its capability in dealing with material cyclic softening or hardening behaviour.

Abstract: Some fundamental studies carried in a synthetic rubber - Chloroprene CR29 are presented in the first part of the paper. A critical analysis of test results, shows that an energy based approach permits the determination of fatigue lives in this material. This aspect is further enhanced by biaxial fatigue tests in the same material. These tests covering a life range from 10000 to 1000000 cycles show that the energy based model is very efficient to describe the fatigue behavior. Some evidence of strain induced crystallization (previously observed in natural rubber) with associated life enhancement at high load ratios is also presented. A comprehensive model based on the determination of the constitutive laws taking into account the viscoelastic behavior is developed showing excellent correlation with experimental data.

Abstract: Atomic force microscopy (AFM) and focused ion beam technique (FIB) were adopted to study the early stages of surface relief evolution in 316L steel and polycrystalline copper fatigued with constant plastic strain amplitudes at different temperatures (316L steel at 93, 173 and 573 K; copper at 83, 173 and 295 K). Qualitative and quantitative data on the morphology and shape of persistent slip markings (PSMs), occurrence of extrusions and intrusions and the kinetics of extrusion growth are reported. They are discussed in relation with recent physically based theories of surface relief formation leading to fatigue crack initiation.

Abstract: In-situ Low Cycle Fatigue test (LCF) at temperature 635 °C have been performed in SEM on flat specimen ofcast Inconel 713LC superalloy. The aim of the investigation was to studymechanisms of the fatigue damage during elastic-plastic cycling by theobservations of the characteristic surface relief evolution and theaccompanying internal dislocation structures. The selected locations on thesurface were systematically studied in-situ and documented by SEM and usingAFM. The surface relief in the first tensile half-cycle was formed by numerousslip steps on the primary slip planes (111). In the following compressionhalf-cycle additional opposite slip were formed. The relief was modified in thenext cycles but without forming additionally new slip traces in the primarysystem. The reorientation of two grains in the gauge area was measured usingEBSD. At the end of cyclic loading the relation between surface persistent slipmarkings and persistent slip bands in the interior of the material wasdocumented by TEM on lamella prepared by FIB. The early stages of extrusion andintrusion formation were documented. The damage mechanism evolution is closelyconnected with the cyclic strain localization to the persistent slip bands thatare also places of fatigue crack initiation.

Abstract: Very high cycle fatigue degradation of type 316L austenitic stainless steel, which is used as the structural material of neutron spallation sources under intensive neutron irradiation environment, is investigated by using an ultrasonic fatigue testing machine. The strain rate imposed on the structure of neutron spallation source is almost equivalent to that produced in the testing machine. The temperature on the surface was controlled by the air-cooling. The effect of strain rate on the fatigue strength is recognized to increase the fatigue limit.

Abstract: Crack initiation along surface persistent slip bands (PSBs) has been widely observed and modelled. Nevertheless, from our knowledge, no physically-based fracture modelling has been proposed and validated with respect to the numerous recent experimental data showing the strong relationship between extrusion and microcrack initiation.

Abstract: Most of the engineering failures especially in mechanical and aerospace industry are due to the fatigue. Fatigue cracks and their propagation can be monitored by observing changes in the structural stiffness resulting from strength reduction as a function of the number of loading cycles. This monitoring can be observed using piezoceramic (PZT) transducer based electromechanical impedance (EMI) technique, and digital image correlation (DIC) system which uses variations on structural surface. The fatigue load usually aggravates the fracture if any defects pre-exist in the structure. The present work monitors multiple-crack emanating from electrode sparked multiple plane defects which are induced in to the specimens prior to the fatigue test, using EMI technique and DIC system. The fracture occurs much below the ultimate stress for the structures which already have defects as in the present case. EMI technique uses signature comparison of healthy and cracked state of the structure to depict crack growth. Initial detection of surface hair-line cracks from the corners of induced defects and their propagation till merging and subsequent failure were explained by signature variations and DIC techniques. Thus, a signal processing technique EMI, and image processing technology DIC were found to complement each other in prediction of early crack and their appearance on surface of the specimens.

Abstract: The aim of this study is to discuss an effect of stress ratio and loading mode on high cycle fatigue performances of extruded magnesium alloys. Axial loading fatigue tests under three conditions of stress ratio, R, of 0, -1 and-1.5, and also rotating bending fatigue tests have been performed in laboratory air at room temperature using hourglass shaped specimens of AZ31, AZ61, AZ80 and T5-treated AZ80 alloy. From the experimental results, some materials showed a specific stepwise S-N curve on which two knees appear. The shape of S-N diagram depended on a kind of tested materials, applied stress ratio and loading mode. It was suggested from the detail observation of fracture surface that fatigue crack initiation mechanism changed from a twin-induced failure mode at high stress amplitude level to a slip-induced one at low stress amplitude level. This transition was determined with the relation between the minimum stress during a fatigue cycle and the compressive yield stress at which deformation twin occurs.

Abstract: There are different prescriptions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed (i) to determine fatigue crack propagation limit curves for high strength steels and their welded joints, based on the Paris-Erdogan law; (ii) to use the determined limit curves for engineering critical assessment (ECA) calculations. Experiments were performed on different high strength steels and their welded joints; and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed by ΔK-decreasing and constant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the limit curves. Engineering critical assessment calculations were performed on a welded structural element having crack like defects.

Abstract: Alloy 104 is a novel high strength, α+β titanium alloy primarily aimed at aero-engine fan disc applications. Two microstructural variants of Alloy 104 have been assessed. Room temperature tensile strength and elongation have been investigated alongside a more detailed study of low and high cycle fatigue behaviour. The alloy clearly demonstrated an improved fatigue resistance in both microstructural conditions, whilst maintaining forgeability and a comparable density to Ti-6Al-4V. Furthermore, the alloy has been subjected to a load regime with a hold period at peak loads and proven to be insensitive to dwell fatigue.