Christopher E Truman

Novel Applications of the Deep-Hole Drilling Technique for Measuring Through-Thickness Residual Stress Distributions

This paper presents novel applications of the deep-hole drilling technique for residual stress measurement in components. The results outline the versatility of this technique for application in complex situations such as residual stress measurement in very thick components, components with difficult access to the measurement location, very large components, components requiring high spatial resolution, and components requiring detailed near surface residual stress distribution. The deep-hole drilling measurements were compared with results obtained using other measurement techniques. Excellent agreement was found between the results.

Contact mechanics of wedge and cone indenters

Abstract Several aspects of the mechanics of indentation of a half-space by an elastic indenter which is either conical or wedge-shaped are addressed. These include elucidation of the contact law, the state of stress induced when the indenter is either pressed normally or sliding with Coulomb friction, the strength of the contact, and the influence of shearing forces less than those necessary to cause sliding, including those induced by elastic mismatch.

A shrink-fit shaft subject to torsion

The problem studied is an elastic, circular shaft, fitted into a cavity normal to the free surface of a half-space. The cavity is smaller than the shaft, so that there is a residual radial stress. A torque is applied to the shaft, giving rise to a region of slip between the shaft and the socket. Its extent is determined by forming an integral equation whose kernel is given by a circular ring dislocation, which has a Burgers vector whose magnitude is constant, oriented in the tangential direction. The problem has direct application to the study of shrink fitted shafts in wheels, whose diameter is large compared with the shaft.

The tilted punch under normal and shear load (with application to fretting tests)

The use of a tilted fretting fatigue pad in experimental investigations of fretting fatigue is discussed, and a concise, closed-form analysis of the resulting contact law, pressure distribution and partial slip regime found. The salient physical quantities controlling the size and characteristics of the contact are deduced. Preliminary tests using pads of this form have shown them to be very useful in applying carefully graded slip damage in a controlled way.

An integrated approach for measuring near-surface and subsurface residual stress in engineering components

This paper presents measurements performed using an integrated centre hole drilling-deep hole drilling technique on two components: a butt-welded plate and a rail. The results were compared with those available from other measurement techniques and with finite element (FE) simulations of the residual stresses. It was found that incremental centre hole drilling and deep hole drilling worked well as complementary techniques. Subsurface deep hole drilling measurements performed in the welded plate at two locations produced very similar results and evidence of good repeatability. The present experimental results were found to be in reasonable agreement with other measurements and FE predictions of residual stresses.

Impact of Residual Stress and Elastic Follow-Up on Fracture

The presence of tensile residual stress in cracked structures combined with external loading leads to circumstances where a structure may fail at a lower applied load than when residual stresses are not present. This is taken into consideration in the fracture assessment codes which are usually invoked to determine whether a cracked structure is fit-for-purpose. These codes typically attempt to decompose the stresses present in the structure under consideration into either “secondary” or “primary” components, in order to simplify the assessment and avoid the need for detailed numerical modeling. It is acknowledged that whether a given residual stress field should be classified as “secondary” or as “primary” is dependent on the level of associated elastic follow-up (EFU). However, although there is a significant body of work related to the influence of EFU on the high temperature creep behavior of uncracked structures, the EFU concept has not yet been rigorously applied to the fracture assessment of cracked structures. This paper represents a first step towards a more rigorous application of the EFU concept to the fracture assessment of cracked structures containing residual stresses. Insight is provided into the influence of residual stress and EFU on fracture by considering the behavior of a simple three-bar assembly. Having introduced the concept, a three-bar type test rig capable of generating fit-up residual stresses with varying levels of EFU in a compact-tension fracture-specimen is presented. Results, produced using this test rig, from two cases with identical levels of initial residual stress but different levels of associated EFU are considered. It is concluded that EFU is important in determining how the residual force in the specimen changes (and therefore how the component of crack driving force associated with the residual force changes) as damage accumulates in the specimen subsequent to fracture initiation.

A statistical study of the coefficient of friction under different loading regimes

Shrink-fits are common engineering fastening systems that comprise a cylindrical hub and shaft locked together by a radial pressure due to interference in size at their interface diameter. Frictional forces at the interface allow the transmission of a torque or resistance to axial movement in the assembly. The measurement of the coefficient of friction under simulated shrink-fit conditions is difficult and time consuming. A flexible experimental approach is presented that can test over a range of pressures, sizes, loading directions, i.e. torsion, compression and tension, and which has the ability to detect changes in the normal load applied due to loading direction. A series of statistical validation studies are conducted on 15 mm diameter low carbon steel specimens to verify theoretical formulations that suggest the coefficient of friction is different for the different loading regimes, and also identifies the surface roughness measured axially or radially as being contributory to the final coefficient of friction value. The experimental approach is applied to a novel laminate sleeve for an electrical machine in order that high confidence is achieved in measuring the coefficient of friction under realistic service conditions.

PREDICTING HOW CRACK TIP RESIDUAL STRESSES INFLUENCE BRITTLE FRACTURE

A probability distribution model, based on the local approach to fracture, has been developed and used for estimating cleavage fracture following prior loading (or warm pre-stressing) in two ferritic steels. Although there are many experimental studies it is not clear from these studies whether the generation of local residual stress and/or crack tip blunting as a result of prior loading contribute to the enhancement in toughness. We first identify the Weibull parameters required to match the experimental scatter in lower shelf toughness of the candidate steels. Second we use these parameters in finite element simulations of prior loading on the upper shelf followed by unloading and cooling to lower shelf temperatures to determine the probability of failure. The predictions are consistent with experimental scatter in toughness following WPS and provide a means of determining the relative importance of the crack tip residual stresses and crack tip blunting. We demonstrate that for our steels the crack tip residual stress is the pivotal feature in improving the fracture toughness following WPS. The paper finally discusses these results in the context of the non-uniqueness and the sensitivity of the Weibull parameters.Copyright

Diffraction measurements for evaluating plastic strain in A533B ferritic steel—a feasibility study

It is known that the physical properties of many engineering materials may be strongly affected by previous loading, in particular prior plastic deformation. Most obviously, work hardening will alter subsequent yielding behaviour. Plastic deformation may also preferentially align the material microstructure, resulting in anisotropy of subsequent behaviour and a change in material fracture resistance.When physical characterization is undertaken by experimental testing it is, therefore, important to have some knowledge of the current state of the material. As a result, it is desirable to have methods of quantitatively evaluating the level of plastic deformation which specimen material may have experienced prior to testing.This paper presents the results of a feasibility study, using a ferritic reactor pressure vessel steel, into the use of diffractive methods for plastic strain evaluation. Using neutron diffraction, changes in diffraction peak width and anisotropy of peak response were correlated with plastic deformation in a tensile test. The relationships produced were then used to evaluate permanent deformation levels in large samples, representative of standard fracture toughness test specimens.

Characterizing residual stresses in rectangular beam specimens following thermomechanical loading

Edge welding and partial quenching are explored as two processes capable of generating well-defined residual stress fields in beam specimens. The purpose of introducing residual stress fields into test specimens is to allow the influence of pre-existing residual stresses on the fracture behaviour of metallic components under applied loads to be studied in a systematic manner. Three materials are considered in this paper: two stainless steels (type 316H and Esshete 1250) and one ferritic steel (A533B). The paper presents both numerical and experimental results. The numerical results were obtained using finite element analysis, and the experimental measurements made primarily with the neutron diffraction technique, and also with X-ray synchrotron diffraction and incremental centre hole drilling. There was, in many cases, good agreement between predictions and measurements; however, there were several instances where finite element predictions differed significantly from measurements. This difference was often most pronounced close to the edge-welded or partially quenched specimen edge where the sensitivity of the numerical models to the assumed thermal and mechanical boundary conditions was greatest. The results presented confirm the usefulness of these two processes as a means of introducing residual stress fields into test specimens but highlight the need for experimental validation of numerical models. It is also demonstrated that, if a crack is subsequently introduced into the specimens, it is possible to generate both tensile and compressive crack-tip residual stress states.