Papers by Author: Christopher E. Truman

Abstract: The residual stresses in heat treated 7075 aluminium alloy blocks have been characterised using two neutron diffraction strain scanning instruments. The influence of uniaxial cold compression (1-10%) on relieving the residual stress has been determined. Increasing the magnitude of cold compression from 1 to 10% has been shown to have a beneficial effect on the residual stress distribution by reducing the range between the maximum and minimum residual stresses. The effect of over aging 7075 on residual stress has also been characterised using neutron diffraction and this was found to reduce the residual stress by 25-40%. A relationship between {311} peaks widths and amount of cold compression was also observed.

Abstract: Nosing is a cold metal-forming process used in the manufacture of plain spherical bearings. This process ensures the outer bearing race conforms to the shape of the inner race (a ball), with a composite liner in-between to provide a low frictional moment. These bearings must be precision engineered due to the large forces and demanding environments they operate within in service. The manufacture of these bearings and related process settings is very much an experiential route, although increasingly Finite Element simulations are used to predict and characterise complex material behaviour. It is imperative the numerical nosing models are validated against experimental measurements due to uncertainties in material properties, process variables and part manufacture variations. In this paper, neutron diffraction is used to determine the residual stresses in a large nosed bearing. Measurements were made on the POLDI instrument at PSI, Switzerland. This paper compares the predicted stresses with measurement results, and draws conclusions concerning the validity and usability of the models.

Abstract: The main objective of the present study is to validate a simple over-coring deep-hole drilling (oDHD) residual stress measurement technique by utilising finite element simulations of the technique. A number of three dimensional (3D) finite element analyses (FEA) were carried out to explore the influence of material removal and the cutting sequence during the deep-hole drilling (DHD) residual stress measurement process on the initial residual stress field. Two models were considered in the study. First, the residual stress field predicted in a rapid spray water quenched solid cylinder was used as the initial stress field for the DHD FE model. The DHD reconstructed residual stresses were compared with the initial FE predicted stresses. Different cutting sequences and different dimensions were systematically simulated before arriving at an optimum solution for the oDHD technique. The oDHD technique significantly improved the spatial resolution and was applied in a second model consisting of a 40mm thick butt-welded pipe. The DHD reconstructed residual stresses compared very well with the initial FE predicted weld residual stress thereby validating the oDHD technique.

Abstract: Residual stress measurement techniques using mechanical strain relaxation depend on a number of physical quantities and are therefore sensitive to errors associated with the measured data. The resulting stress uncertainties can easily become significant and compromise the usefulness of the results or lead to misinterpretation of the behaviour of the residual stress distributions. It is therefore essential to develop an error analysis procedure for the measurements undertaken. Error analysis procedures for the deep hole drilling (DHD) method are developed to consider triaxial residual stresses. A modified deep hole drilling method, called the incremental deep-hole drilling (iDHD), was applied to measure the near yield residual stress distributions in a cold water quenched aluminium 7010 alloy forged block. The experimental results are used to illustrate the errors.

Abstract: In common with all mechanical strain relief residual stress measurement methods, extra care must be taken when making measurements on components containing highly triaxial residual stress fields which are close to yield. The introduction of a free surface, created as part of the measurement procedure, can lead to plastic redistribution of the residual stress field. Usually, this is not accounted for in the elastic inversion algorithms of the experimental procedure. This paper demon­strates the usefulness and accuracy of deep-hole drilling (DHD) method [1] in a component predicted to contain a triaxial residual stress field. Previous measurements [2] are compared with the results of a DHD simulation on a type 316H stainless steel pipe containing a repair weld offset from an original girth weld. The influence of different material models was also studied.

Abstract: The most critical stage in the heat treatment of high strength aluminium alloys is the rapid cooling necessary to form a supersaturated solid solution. During cold water quenching of thick sections, the thermal gradients are sufficient to cause inhomogeneous plastic deformation which in turn leads to the development of large residual stresses. Two 215 mm thick rectilinear forgings made from 7075 and 7010 were heat treated, and the through thickness residual stresses measured by neutron diffraction and deep hole drilling. The distribution of residual stresses was found to be similar for both alloys varying from highly triaxial and tensile in the core to a state of biaxial compression in the surface. The 7010 forging exhibited significantly larger tensile stresses in the core. 7075 is a much more quench sensitive alloy when compared to 7010. This results in loss of supersaturation by second phase precipitation during quenching in the core of the 7075 forging.

Abstract: The paper presents the results of residual stress measurements on a ferritic steel plate containing a repair weld. The repair was considered representative of that found in the secondary circuit piping in power plant. The paper primarily uses the deep hole drilling (DHD) technique, but compares results found by this technique with those obtained using neutron diffraction. Both sets of measurements confirmed that highly tensile residual stresses exist in the repair weld. The two measurement techniques produced results that were in acceptable agreement, but the neutron diffraction results were consistently higher than the deep hole drilling results. It was thought this was due to the use of a constant stress-free lattice parameter d0 .

Abstract: Assessment of the integrity of structures such as reactor pressure vessels is a critical issue in relevant industries. In a full integrity assessment, the presence of initial residual stresses (RS) needs to be taken into account. An initial RS field is introduced into a type 316 stainless steel cylindrical vessel with no defects and to one with a partial circumferential crack on its outer surface. Relaxation of RS following several proof load cycles, in form of internal pressure, applied to the vessel is explored using finite element simulations. It is found that the proof loading process generally relaxes the RS and is proved to be beneficial to both cracked and un-cracked vessels with or without the presence of initial RS. Interaction of residual stresses with warm pre-stressing is further investigated using A533B steel at room and low temperature subjected to axial loading. The results are compared with similar analyses but with no introduction of an initial RS field to explore the interaction effects on fracture resistance, as well as the role of partial crack on the RS distribution / redistribution. The differences are discussed and illustrated.

Abstract: This paper describes initial investigations into the use of frozen-stress photoelasticity to study the development of slippage within shrink-fitted shaft/hub pairs, with the aim of studying the residual stresses remaining after partial slippage has occurred. Results to date show the feasibility of measuring the shear stresses along the partially-slipping interface between shaft and hub.

Abstract: This paper presents the results of an experimental and numerical study carried out to investigate the effect of warm pre-stressing on cleavage fracture in ferritic steels using cracked and notched specimens. It is shown that the local approach based on Weibull theory predicts the increase in toughness following warm pre-stressing in highly constrained geometries. The observed effect of pre-loading in low constraint specimens such as round notched bars is less. The local approach could not predict the differences and it is suggested that the variation of triaxiality factor, the ratio of hydrostatic stress to Von Mises, in the plastic zone, is a contributing factor.