R. Fathallah

Effect of surface properties on high cycle fatigue behaviour of shot peened ductile steel

Abstract Experimental investigations into shot peened ductile steel have been carried out, applying three surface finishing conditions: as machined, standard shot peening using 100% coverage and severe shot peening with 1000% coverage (high exposure time). The properties of the shot peened surfaces were examined and characterised, and specimens were then submitted to three point bending tests. The fatigue limit was determined for each case. In this way, the dependence of fatigue behaviour on initial surface finishing properties was determined, and a relationship is suggested to describe and correlate fatigue limits with initial surface properties. A phenomenological approach is proposed to characterise and to correlate qualitatively and quantitatively the influence of local shot peened surface properties on fatigue limit of treated specimens. The Crossland multiaxial failure high cycle fatigue criterion is used in this approach to model the influence of each surface property.

Determination of Shot Peening Coefficient of Restitution

Abstract To characterise an impact between two solids, a coefficient of restitution is generally used er equal to 1 when the impact is perfectly elastic, 0 when the impact is totally plastic and between 0 and 1 when the impact is elastic - plastic. For the shot peening surface treatment process, this coefficient, which is linked to the shot hardness/ material hardness ratio, is of interest for two reasons. First, to characterise the distribution of the stream energy produced by an impact (stream energy = energy of the induced plastic deformations in the used shot and in the treated material + elastic energy + dissipated energy into vibrations etc.). Two, to compare the efficiency of different types (material, hardness, size and shape) of shot. The coefficient of restitution er can be introduced to an existing shot peening model. The present paper gives a theoretical analysis of the distribution of the different energies during the shot peening process and describes an experimental investigation to determine the values of the coefficient of restitution for four principal types of shot; three in cast steel: cut wire Fil Coupé (F.C.) 0.6 mm and WS280 with two hardness values: 48 and 58 HRC, and one in low carbon cast steel:F16. The target was 13% manganese steel sheet with a hardness of 60 HRC. Results of the different coefficients of restitution were noted for each type of shot and the values obtained were presented and discussed. They show reasonable agreement with the theoretical analysis. It has been observed that the coefficient of restitution is principally linked to the shot hardness, material hardness ratio and the shot shape.

Optimizing Residual Stress Profile Induced by Laser Shock Peening Using DOE Technique

The control of residual stress is crucial in ensuring the integrity of engineering components and Laser Shock Peening (LSP) process can be used to good effect to introduce the beneficial compressive residual stress levels required. It is, however, difficult to use normal laser peening control systems to establish the ideal peening conditions that will result in the best component performance. This paper presents results from a study to optimise the laser peening parameters for a typical titanium super alloy used in high performance turbine blade by investigating how the main peening process parameters influence residual stress profiles resulted by numerical simulations. Statistical Design of Experiments (DoE) was used to limit the number of experiments required for optimisation to be possible. Using this technique and numerical depth profiling methods for residual stress analysis, the maximum compressive residual stresses in Ti-6Al-4V were measured for a range of peening conditions. The results of the detailed process characterisation investigations have shown that, by using careful DoE, it is possible to fully optimise the laser shock peening process to obtain greater benefits than would be possible with traditional control processes.

Finite Element Prediction of Laser Shock Peened Surface Modifications in Ti-6Al-4V Alloy

This paper presents a numerical simulation of the Laser Shock Peening process (LSP) using finite element method. The majority of the controlling parameters of the process have been taken into account. The laser loading has been characterised by using a repetitive time Gaussian increment pressure applied uniformly at circular impacted zone. The behavior of the subjected material is supposed to be elasto-visco-plastic coupled with damage using the Johnson Cook law with his shear failure model. The proposed model leads to obtain the surface inducing modifications, which are classified in this work into three categories: (i) the in-depth residual stress profile, (ii) the induced plastic strains profile and (iii) also the superficial damage which can be induced in few cases where the operating conditions are not well chosen. An application on a laser shock peened super alloy Ti-6Al-4V has been carried out. The comparison of the residual stresses, obtained by X-ray diffraction method and by finite element calculation, shows a good correlation.

Modeling of the Superficial Laser Shock Peening Treatment Process: Application on a Titanium Aircraft Turbine Engine Blade

This paper presents a numerical simulation of the Laser Shock Peening (LSP) process using Finite Element Method (FEM). The majority of the controlling parameters of the process have been taken into account. The LSP loading has been characterized by using a repetitive time Gaussian increment pressure applied uniformly at the impacted zone. The used behavior law of the treated material is supposed Johnson Cook elastic-viscous-plastic coupled with damage. The proposed model leads to obtain the surface modifications (i) the in-depth residual stresses profile, (ii) the induced plastic strains profile, (iii) the geometrical surface modification of the impacted zone and (iv) the superficial damage which can be induced in few cases, where the operating conditions are not well chosen and optimized. An aeronautical application of LSP has been carried out on aircraft turbine engine blade made by Ti-6Al-4V super alloy. This mechanical treatment is applied in order to increase the durability of titanium fan blades and decrease their sensitivity to foreign object damage (FOD). The resulting surface compressive residual stress significantly improves the high-cycle-fatigue properties of the component and greatly increases resistance to blade failure. Finally, we studied the feasibility of the influence of LSP treatment on the phenomenon of crack propagation by introducing a superficial crack defect on the edge of the studied blade structure. This is physically consistent and leads to optimize the operating conditions in order to limit the damage risks.

Effect of Artificial Defect and Mean Shear Stress on Torsional Fatigue Behaviour

The aim of this work is to study the influence of artificial defect and mean stress on fatigue strength under torsion loading. Spherical artificial defects have been machined at the surface of gauge length of fatigue samples. Experimental investigations conducted on both defective and defect free materials. The crack initiation mechanisms have been identified based on several observations on Scanning Electron Microscope (SEM) at different stage of fatigue life. It is observed that the defect free material subjected to torsion loading allows relatively earlier initiation. Experimental results show that mean shear stress has no effect on fatigue behaviour under torsion loading. It is also concluded that defects are much more damaging in high cycle fatigue regime than in low cycle fatigue one.