Phil E. Irving

State-of-the-art in product service-systems

Abstract A Product-Service System (PSS) is an integrated combination of products and services. This Western concept embraces a service-led competitive strategy, environmental sustainability, and the basis to differentiate from competitors who simply offer lower priced products. This paper aims to report the state-of-the-art of PSS research by presenting a clinical review of literature currently available on this topic. The literature is classified and the major outcomes of each study are addressed and analysed. On this basis, this paper defines the PSS concept, reports on its origin and features, gives examples of applications along with potential benefits and barriers to adoption, summarizes available tools and methodologies, and identifies future research challenges.

The role of residual stress and heat affected zone properties on fatigue crack propagation in friction stir welded 2024-T351 aluminium joints

The effects of weld residual stress and heat affected zone on the fatigue propagation of cracks parallel and orthogonal to the weld direction in friction stir welded (FSW) 2024-T351 joints were investigated. Crack propagation behaviour was sensitive to both weld orientation and the distance of the crack from the weld line. Growth rates both faster and slower than in the parent material were observed, depending on the crack orientation and distance from the weld. Weld residual stress was mechanically relieved and the effects on crack propagation observed. A comparative analysis of the results indicated that crack growth behaviour in the FSW joints was generally dominated by the weld residual stress and that microstructure and hardness changes in FSWs had a minor influence.

Fatigue damage characterization in carbon fibre composite materials using an electrical potential technique

Changes in electrical resistance during static and fatigue loading of unidirectional and cross ply carbon fibre reinforced polymer composites have been studied. The carbon fibres in the study were T300 and the matrix resins were Hexcel 914 and 920. It was found that changes in resistance during static tensile testing were about three per cent of the original resistance of the samples, while fatigue testing caused resistance changes of up to 10% of the original resistance, immediately prior to final failure. The initial linear portion of the resistance increase on static testing was reversible and could be attributed to reversible elastic strains in the fibres; later non-linear changes were a consequence of fibre fracture and were irreversible. Changes in resistance during fatigue also contained both reversible and non-reversible components. It was found that during fatigue testing the initial changes in resistance caused by the first few thousand cycles could be correlated with the eventual life. Samples with large initial resistance change had reduced lives compared with those with small changes in resistance. Fatigue lives of composite laminates may thus be predicted from monitoring of initial resistance changes. Many of the results could be explained via the parallel resistance model of conduction in composite laminates.

Effect of resin and fibre properties on impact and compression after impact performance of CFRP

Abstract The effect of resin and fibre properties on composite impact, compression after impact (CAI) and mode II energy release rate (GIIC) performance has been studied. Impact events were instrumented to record values of Pc, the critical load for initiation of impact damage. Impact response of the laminates was strongly influenced by the fracture toughness of the resin. In contrast, use of high strength and high stiffness fibres did not improve the resistance to impact. The differences in impact and CAI response of the laminates were largely a consequence of the impact damage created at the damage threshold, Pc, rather than of the differences in delamination growth. As a strong correlation was found between GIIC values measured by delamination tests, and those calculated from measurements of Pc, it is suggested that instrumented impact testing may be a more convenient way of determining GIIC in CFRP laminates than delamination tests.

Experimental and finite element study of the electrical potential technique for damage detection in CFRP laminates

The electrical potential technique was applied to detect and locate impact damage in carbon fibre reinforced polymer (CFRP) plates of Hexcel T300/914 composite. The potential field across the surface of the laminates was measured using arrays of electrical contacts. A constant current of 100 mA was applied to the plate, and the changes in the potential distribution resulting from impact damage documented. These results have been compared with those calculated using 3D finite element simulation of current flow in CFRP laminates. The results show good agreement between simulation and experimental results and will permit detailed analytical studies of optimum network and current input configurations for the practical realization of a damage sensing system. A comparison of potential distribution on the top surface for damaged and undamaged laminates shows a substantial difference in the potential field around the impact damaged area. This could form the basis of a damage detection system.

Corrosion pit size distributions and fatigue lives: a study of the EIFS technique for fatigue design in the presence of corrosion

Abstract Samples of Alclad 7475 aluminium alloy containing a 3.18 mm radius semi-circular edge notch were subjected to artificial and natural corroding environments, producing a distribution of pit depths in the notch from 20 to 150 μm. After corrosion exposure, 20 samples were subjected to constant and variable amplitude loading. The pits reduced fatigue life by 40–50% when compared with lives of uncorroded samples. Measurements of fatigue crack growth rates on cracks greater than 50 μm in length showed similar rates in both corroded and uncorroded samples. The effect of the pits was to greatly reduce the number of cycles to grow the crack to 100 μm. Multiple crack initiation at pits occurred in almost all tests, resulting in irregular growth as either crack shielding or crack coalescence occurred. Equivalent initial flaw size distributions (EIFSD) were calculated from the life data. Life predictions made from the EIFS distributions were non-conservative. The errors are shown to result from multiple initiation and crack coalescence behaviour. The applicability of EIFS approaches to design against pitting corrosion is discussed.

An Integrated Approach to the Determination and Consequences of Residual Stress on the Fatigue Performance of Welded Aircraft Structures

Although residual stress in welded structures and components has long been known to have an effect on their fatigue performance, access to reliable, spatially accurate residual stress field data has been limited. Recent advances in neutron and synchrotron X-ray diffraction allow a far more detailed picture of weld residual stress fields to be obtained that permits the development and use of predictive models that can be used for accurate design against fatigue in aircraft structures. This paper describes a fully integrated study of the three-dimensional residual stress distribution accompanying state-of-the-art fusion welds in 2024-T4 aluminum alloy, and how it is affected by subsequent machining and service loading. A particular feature of this work has been the development of techniques allowing the nondestructive evaluation of the residual stress field in the full range of specimens used to provide the design data required for welded aircraft structures and the integration of this information into all aspects of damage tolerant design.

The influence of hydrogen loading and the fabrication process on the mechanical strength of optical fibre Bragg gratings

Abstract This paper investigates the influence of hydrogen loading and other stages of the fabrication process on the mechanical strength of fibre Bragg gratings. Following UV irradiation, tensile tests were carried out on Ge–B codoped photosensitive fibres with and without hydrogen loading. Fibre Bragg gratings (FBGs) were written using a range of UV wavelengths, namely 246, 255 and 266 nm. The tensile strength of the optical fibres was determined in their as-received status and following the various stages of FBG fabrication. The mechanical strength was assessed using Weibull statistics. The results indicate that the strength of FBGs is influenced by the UV irradiation parameters and by the hydrogen-loading process. FBGs fabricated using shorter UV wavelengths and low pulse power intensity exhibit a high mechanical strength. The FBGs written in hydrogen loaded fibres have less than 50% of the strength of FBGs that have not been hydrogen loaded. Fibre fracture morphology observed by scanning electron microscope reveals fracture mechanisms of FBGs, which are correlated with the structural change of the silica fibres induced during the FBG inscription process. Combined with surface information gathered by atomic force microscopy, fracture mechanics is applied to understand the mechanisms of strength degradation caused by the FBG inscription process.

Full-field Laser Shearography Instrumentation for the Detection and Characterization of Fatigue Cracks in Titanium 10-2-3

A multi-component shearography instrument is used to investigate the in-plane surface strain fields in titanium 10-2-3 alloy specimens subjected to tensile loads. Titanium 10-2-3 alloy samples containing fatigue cracks with lengths from 0.7 to 5 mm were investigated using the shearography instrumentation. In-plane and out-of-plane measurements of surface strain are presented, along with comparative data from resistance strain gages. Factors determining the detection sensitivity, the limit of detection for crack lengths, and the accuracy of the strain field were established.

Design for diagnostics and prognostics: A physical-functional approach

This paper describes an end-to-end Integrated Vehicle Health Management (IVHM) development process with a strong emphasis on the COTS software tools employed for the implementation of this process. A mix of physical simulation and functional failure analysis was chosen as a route for early assessment of degradation in complex systems as capturing system failure modes and their symptoms facilitates the assessment of health management solutions for a complex asset. The method chosen for the IVHM development is closely correlated to the generic engineering cycle. The concepts employed by this method are further demonstrated on a laboratory fuel system test rig, but they can also be applied to both new and legacy hi-tech high-value systems. Another objective of the study is to identify the relations between the different types of knowledge supporting the health management development process when using together physical and functional models. The conclusion of this lead is that functional modeling and physical simulation should not be done in isolation. The functional model requires permanent feedback from a physical system simulator in order to be able to build a functional model that will accurately represent the real system. This paper will therefore also describe the steps required to correctly develop a functional model that will reflect the physical knowledge inherently known about a given system.