Our safety, comfort and peace of mind are heavily dependent upon our capability to prevent, predict or postpone the failure of components and structures on the basis of sound physical principles While the external loadings acting on a material or component are clearly important, There are other contributory factors including unfavourable materials microstructure, pre-existing defects and residual stresses Residual stresses can add to, or subtract from, the applied stresses and so when unexpected failure occurs it is often because residual stresses have combined critically with the applied stresses, or because together with the presence of undetected defects they have dangerously lowered the applied stress at which failure will occur Consequently it is important that the origins of residual stress are understood, opportunities for removing harmful or introducing beneficial residual stresses recognized, their evolution in-service predicted, their influence on failure processes understood and safe structural integrity assessments made, so as to either remove the part prior to failure, or to take corrective action to extend life This paper reviews the progress in these aspects in the light of the basic failure mechanisms

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Residual stress and its role in failure

Roles of microstructure in fatigue crack initiation

Investigation of residual stress relaxation under cyclic load

Many manufacturing processes can induce residual stresses in components. These residual stresses influence the mean stress during cyclic loading and so can influence the fatigue life. However, the initial residual stresses induced during manufacturing may not remain stable during the fatigue life. This paper provides a broad and extensive literature survey addressing the stability of surface and near-surface residual stress fields during fatigue, including redistribution and relaxation due to static mechanical load, repeated cyclic loads, thermal exposure and crack extension. The implications of the initial and evolving residual stress state for fatigue behaviour and life prediction are addressed, with special attention to fatigue crack growth. This survey is not a critical analysis; no detailed attempt is made to evaluate the relative merits of the different explanations and models proposed, to propose new explanations or models or to provide quantitative conclusions. Primary attention is given to the residual stresses resulting from four major classes of manufacturing operations: shot peening and related surface treatments, cold expansion of holes, welding and machining.

A literature survey on the stability and significance of residual stresses during fatigue

This paper presents an overview of the past research on Surface Integrity (SI) studies in the context of machined components from a range of work materials including stainless steels, Ni and Ti alloys, hardened steels for dies and moulds, bearings and automotive applications. Typical surface alterations such as phase transformations, microhardness and residual stress are discussed and correlated with the functional performance of the machined products. A summary of past and current modelling efforts is then presented along with projections for developing predictive models for SI and means for enhancing product sustainability in terms of its functional performance. Reference to this paper should be made as follows: M'Saoubi, R., Outeiro, J.C., Chandrasekaran, H., Dillon Jr., O.W. and Jawahir, I.S. (2008) 'A review of surface integrity in machining and its impact on functional performance and life of machined products', Int. J. Sustainable Manufacturing, Vol. 1, Nos. 1/2, pp.203-236.

A review of surface integrity in machining and its impact on functional performance and life of machined products

The Influence of Surface Enhancement by Low Plasticity Burnishing on the Corrosion Fatigue Performance of AA7075-T6

Surface enhancement, the creation of a layer of residual compression at the surface of a component, is widely used to improve the fatigue life in the automotive and aerospace industries. The compressive layer delays fatigue crack initiation and retards small crack propagation. The benefits of surface enhancement are lost if the compressive layer relaxes at the operating temperature of the component. Surface enhancement methods producing minimal cold work are shown to produce the most thermally stable compression. The residual stress and cold work distributions developed in IN718 by shot peening, gravity peening, laser shock peening (LSP) and low plasticity burnishing (LPB) are compared. Estimation of cold work (equivalent true plastic strain) from x-ray diffraction line broadening is described. Thermal relaxation at temperatures ranging from 525C to 670C is correlated to the degree of cold working of the surface, independent of the method of surface enhancement. Highly cold worked (> 15%) shot peened surfaces are found to relax to half the initial level of compression in minutes at all temperatures investigated. The rapid initial relaxation is shown to be virtually independent of either time or temperature from 525C to 670C. The LPB process is described with application to IN718. High cycle fatigue performance after elevated temperature exposure is compared for surfaces treated by LPB and conventional (8A intensity) shot peening.

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The Effect of Cold Work on the Thermal Stability of Residual Compression in Surface Enhanced IN718

Surface enhancement methods induce a layer of residual compressive stress to improve fatigue life. Shot peening is inexpensive and widely used, but the associated cold work accelerates relaxation of the compressive layer at turbine temperatures and increases sensitivity to overload relaxation. “Deep rolling” burnishing methods produce deep compression, but with cold work comparable to shot peening. Laser shock peening (LSP) produces deep compression with minimal cold work and impressive FOD resistance, but is costly and presents logistical problems in manufacturing. Low Plasticity Burnishing (LPB) has been investigated as a rapid, inexpensive surface enhancement method. Preliminary results indicate depth and magnitude of compression comparable to LSP. Compression reaching the alloy yield strength and extending to a depth of 1.2 mm (0.047 in.) is achievable with cold work of less than 4%. Excellent surface finish can be achieved with no detectable metallurgical damage. Ease of adaptation to CNC machine tools allows LPB processing at costs and speeds comparable to machining operations. The LPB process is described with application to IN718. Thermal stability at engine temperatures is compared to conventional shot peening. Resistance to 0.13 and 0.25 mm (0.005 and .010 in.) deep sharp notch FOD was achieved, even after exposure to engine temperatures. Elevated temperature crack growth data are presented showing the arrest of existing 0.46 mm x 0.91 mm (0.018 x 0. 036 in.) fatigue cracks by the deep compressive layer.

Fod resistance and fatigue crack arrest in low plasticity burnished in718

Low plasticity burnishing (LPB) has been investigated as a surface enhancement process and corrosion mitigation method for aging aircraft structural applications. Compressive residual stresses reaching the alloy yield strength and extending to a depth of 1.25 mm (0.050 in.) deeper than typical corrosion damage is achievable. Excellent surface finish can be achieved with no detectable metallurgical damage to surface and subsurface material. Salt fog exposures of 100 and 500 h reduced the fatigue strength at 2×106 cycles by 50%. The LPB of the corroded surface, without removal of the corrosion product or pitted material, restored the 2×106 fatigue strength to greater than that of the original machined surface. The fatigue strength of the corroded material in the finite life regime (104 to 106 cycles) after LPB was 140 MPa (20 ksi) higher than the original uncorroded alloy and increased the life by an order of magnitude. Ease of adaptation to computer numerical control (CNC) machine tools allows LPB processing at costs and speeds comparable to machining operations. Low plasticity burnishing offers a promising new technology for mitigation of corrosion damage and improved fatigue life of aircraft structural components with significant cost and time savings over current practices.

Low cost corrosion damage mitigation and improved fatigue performance of low plasticity burnished 7075-T6

Low Plasticity Burnishing (LPB) has been developed as a rapid, inexpensive surface enhancement method adaptable to existing CNC machine tools. LPB produces a deep layer of compression with minimal cold work of the surface, comparable to laser shock peening (LSP), but can be incorporated into manufacturing operations at lower cost. Minimizing cold work during surface enhancement has been shown to improve both thermal stability at engine temperatures and resistance to overload relaxation accompanying foreign object damage (FOD). Recent research leading to the development of a practical LPB demonstration facility and tooling is described. The mechanism for compressive residual stress development during LPB has been studied with elastic-plastic finite element modeling. DOE methods have been utilized to optimize compressive magnitude and depth with minimum cold work. Using optimum burnishing parameters, compression on the order of the material yield strength can be achieved to depths exceeding 0.040 in. (1mm) with low cold work. Residual stress and cold work distributions developed by LPB in Ti-6Al-4V are compared to traditional shot peening and LSP. The compressive layer produced by LPB is shown to be resistant to both thermal and overload relaxation. After exposure to engine temperatures, the high cycle fatigue (HCF) strength at 2x10

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Paul S. Prevey

Papers

The Influence of Surface Enhancement by Low Plasticity Burnishing on the Corrosion Fatigue Performance of AA7075-T6

The Effect of Cold Work on the Thermal Stability of Residual Compression in Surface Enhanced IN718

Fod resistance and fatigue crack arrest in low plasticity burnished in718

Low cost corrosion damage mitigation and improved fatigue performance of low plasticity burnished 7075-T6

The Effect of Low Plasticity Burnishing (LPB) on the HCF Performance and FOD Resistance of Ti-6AI-4V