Papers by Author: Thomas H. Hyde

Abstract: In service components such as steam pipes, pipe branches, gas and steam turbine blades, etc. which operate in engineering applications such as power plant, aero-engines, chemical plant etc., can operate at temperatures which are high enough for creep to occur. Often, only nominal operating conditions (i.e. pressure, temperatures, system load, etc.) are known and hence precise life predictions for these components, which may be complex in terms of geometry or weld characteristics, are not possible. Within complex components it can also be the case that the proportion of the material creep life consumed may vary from position to position within the component. It is therefore important that non-destructive techniques are available for assisting in the making of decisions on whether to repair, continue operating or replace certain components. Small specimen creep testing is a technique which can allow such analyses to be performed. Small samples of material are removed from the component to make small creep test specimens. These specimens can then be tested to give information on the remaining creep life of the component. This paper presents the results of small ring specimens tested under creep conditions and shows the comparison to standard (full size) creep testing for materials used under high temperature in industry.

Abstract: A fatigue crack growth test program has been carried out on hollow, SuperCMV shaft specimens, with transverse holes, under combined torsional and axial loading. The experimental results show that fatigue cracks always initiated in the stress concentration areas, i.e., in the transverse holes. Up to four cracks were observed to be initiated at different positions near the holes in the shafts. The fatigue crack propagation was initially found to occur under Mode I conditions, followed by a period of mixed Mode II/III crack growth. Three dimensional, finite element, elastic-plastic analyses have also been conducted, in an attempt to predict the crack-initiation locations and lives. The predicted crack-initiation sites agree with the experimental observations, for a range of loading conditions. The initiation sites were found to be approximately on the planes of maximum principal stress. The predicted, torque-dominated, fatigue lives of the shafts, obtained by use of a stress-life (S-N) approach, correlate reasonably well with the experimental results.

Abstract: The prevention of fatigue failure is an important consideration in the design of aeroengine shafts. In this paper, based on the elastic and elstic-plastic finite element analyses, for a series of small-scale hollow shafts with transverse holes, the maximum princple stress and von Mises stresss around the hole have been obtained. Hence, using McDiarmid and Von Mises criteria, the fatigue crack initiation site and life can be determined. The hoop stress, which is the driving force for the Mode I growth of short cracks initiated around the hole, has been characterised as well.

Abstract: This paper describes high temperature cyclic and creep relaxation testing and modelling of a high nickel-chromium material (XN40F) for application to life prediction of superplastic forming (SPF) tools. An experimental test programme to (i) characterise the high temperature cyclic elastic-plastic-creep behaviour of the material over a range of temperatures between 20oC and 900oC, including cyclic controlled strain-range tests at different strain-rates and creep relaxation tests, and (ii) identify the material constants relevant to thermo-mechanical fatigue (TMF) life prediction, is described. The objective of the material testing is the development of high temperature material and failure models for cyclic analyses and life prediction of SPF and diffusion bonding (DB) dies for titanium aerospace components.

Abstract: The testing setups, results and analysis of constant load creep and low-cycle high temperature fatigue tests of tungsten inert gas (TIG) butt-welded, thin-section INCONEL 718 (IN718) specimens are presented. The main objectives were to determine the effect the welds have on failure time and analyse any differences in their failure behaviour. It was found that although welded IN718 may exhibit comparatively little loss of tensile strength, its ductility and creep and high temperature fatigue properties are severely compromised due to its changed microstructure.

Abstract: An experimental programme of cyclic mechanical testing of a 316 stainless steel, at temperatures up to 600°C, under isothermal conditions, for the identification of material constitutive constants, has been carried out using a thermo-mechanical fatigue (TMF) test machine with induction coil heating. The constitutive model adopted is a modified Chaboche unified viscoplasticity model, which can deal with both cyclic effects, such as combined isotropic and kinematic hardening, and rate-dependent effects, associated with viscoplasticity. The characterisation of 316 stainless steel is presented and compared to results from cyclic isothermal tests. A least squares optimisation algorithm has been developed and implemented for determining the material constants in order to further improve the general fit of the model to experimental data, using the initially obtained material constants as the starting point in this optimisation process. The model predictions using both the initial and optimised material constants are compared to experimental data.

Abstract: The development of aeroengines with increasing thrust capabilities requires the development of shaft technology to deal with this greater power transmission, whilst still restricting their dimensions and weight. Modern aerospace drive shafts are predominantly of a single-alloy design and significant benefits could be obtained from using a dual alloy shaft, where a high temperature alloy is used at the turbine, i.e. hot, end of the shaft and a high strength alloy is used for the spline end of the shaft, where high strength is required, rather than high temperature performance. Whilst the processes of joining dissimilar materials are widely used the evolution of the joint and its strength characteristics are not fully understood. A program of research has been instigated to lead to an improved understanding of friction welds and their behaviour under monotonic and cyclic loadings with the overall objective to establish confidence in the welding parameters for these material combinations and the associated post-weld heat treatments. This paper presents an overview of the mechanical testing program and the aims of this work, illustrated with some examples from the monotonic and cyclic test work carried out on inertia friction welded dual alloy shaft components.

Abstract: This paper reviews a range of simplified and approximate techniques developed and implemented by the authors in the context of elastic-plastic, elastic-creep and elastic-plastic-creep, monotonic loading of both simple components and complex structures. The primary motivation for the work is to provide more rapid methods for estimating failure variables for damage assessment of structures. The context of the work has included both aerospace structures and tubular frameworks, such as offshore jacket-type structures, both of which belong to a class of structures, which contain repeated units where localised plastic damage may be concentrated.

Abstract: This paper describes some recent work on the stress analyses and failure prediction of some typical pressurised high temperature components under creep conditions, including plain pipes, pipe bends, branch connections and welds etc, in the main steam pipework of power plant. The materials used are typically low alloy ferritic CrMoV steels. Experimental creep testing methods, and the procedures used for generating the material properties in creep and damage constitutive equations, are briefly described. Some typical numerical results are presented to illustrate the main characteristics of the behaviour of these components and to demonstrate the effects of geometry, material properties and loading modes on stress distributions and failure life predictions. The emphasis of the paper is on welded components.