Finite Element Modelling For Prediction Of Residual Stress Pattern From Single Shot Impacting On Aluminium (LM 13)

Finite Element Modelling For Prediction Of Residual Stress Pattern From Single Shot Impacting On Aluminium (LM 13)

Finite Element Modelling For Prediction Of Residual Stress Pattern From Single Shot Impacting On Aluminium (LM 13)

Santosh P. More1, Dr. V. V. Kulkarni2

1PG student KIT College of engineering Kolhapur,

2Head Of Production Department KIT College of engineering Kolhapur

Abstract

Shot peening is a complex and random process which is controlled by many input parameters. Numerical methods, which are normally used for impact problems will prohibitively put strain on the computing resources since a large number of impacts are involved in the computations. Shot peening is a process in which a stream of shot is blasted against an engineering component to generate a high compressive residual stress regime at the surface of the component. This paper describes a 3D finite element dynamic analysis of single shot impacting on a aluminium (LM

13) component. A study is conducted to investigate the effect of shot impact on component surface and the resulting residual stress profile. The effect of strain- hardening parameter is more complex as it depends on the relative magnitude of the strainhardening yield stress to the initial yield stress and the impact energy. So such modelling is used to predict residual stress after shot peening.

Keywords: Shot peening, Residual stress, Finite element modelling.

1. Introduction

   Shot peening is used in numerous engineering applications. It is a cold-work process in which a stream of spherical shot is blasted against an engineering component. Each shot impacts on the target surface, causing plastic deformation. After contact between the shot and the component ceased, a high compressive residual stress is generated at the surface of the component. Compressive residual stress in the surface layers of the component greatly improves the fatigue strength. It is therefore very useful to be able to predict the pattern and magnitude of the residual stress distribution near the surface after shot peening

   Shot peening is a very complex process to model numerically, it involves dynamic analysis of fast moving shot impacting on a metallic component which can often has complex geometry. There are some significant numbers of parameters involved in shot peening which need to be controlled and regulated in order to produce a more beneficial compressive residual stress distribution within the component. These parameters can be categorized into three groups relating to the shot, the component and the process. The shot parameters include size, density, shape, impact velocity, rotary inertia, incident angle and hardness. The component parameters include geometry, initial yield stress, work hardening characteristics and hardness. The process parameters include mass flow rate, air pressure, angle of attack, distance between nozzle and component and percentage coverage. In order to control the resulting residual stress pattern in peened components, it would be highly beneficial to establish quantitative relationships between these parameters and residual stress characteristics.

   Modelling the entire shot peening process is both extremely difficult and very expensive and would not allow a careful examination of the effect of key parameters on the residual stress pattern within the component. This paper describes a three-dimensional dynamic finite element (FE) study of single shot impacting on a metallic component. A parametric study is then performed with the aim of gaining a better understanding of the effect of shot impact on the resultant residual stress pattern.

2. Literature review

   Several analytical and numerical studies of single shot impacts on components have been reported. Some progress has been made in recent years but the understanding of single shot impact is still far from complete. The interrelationships of the parameters and

   the residual stress characteristics are not clearly determined yet. Study of the contact problem between elastic and elastic plastic materials resulting from the loading of two bodies was pioneered by Hertz [1]. A comprehensive source of reference material can be found in the classic works of Timoshenko and Goodier [2]. Further sources of references on indentation and allied subjects are given by Goldsmith [3] and Johnson [4]. More recent analytical models [56] have been developed that predict the residual stress distribution and the plastically deformed region in single shot impacts on components. Because of the complexity of the shot peening process, simplifying assumptions were adopted. These assumptions make these analytic approaches unsuitable for dealing with practical applications with, for instance, complex geometry and non-linear material properties.

   There have been numerous experimental studies attempting to measure the residual stress distribution, fatigue life and the influence of the shot, component and process parameters. Some of these experiments have focused on single shot impacts on components [5, 78]. Single steel ball static indentation tests and dynamic tests were done by Al-Hassani [5]. The dynamic tests were performed using a specially designed Tornedo cartridge gun and 4.8 mm diameter steel balls with impact velocities in the range 50150 m/s. In this paper measurement of residual stress is carried out as the function of time.

   The FE modeling provides a powerful method for simulating the shot peening process. The dynamic impacting of single or multiple shots with high velocity and the double non-linearity of the problem due to the contact of two bodies and the elasticplastic behaviour of the component can all be taken into account in an appropriate FE analysis. A dynamic elasticplastic FE method to predict the residual stress distribution in shot peening was developed by Al-Obaid [10] Hardy et al.

   [9] was the first to solve the contact problem of a rigid sphere indenting an elasticperfect plastic half-space using the FE method. These studies were developed when computer power was limited. Computer time was emphasized and simplifying assumptions were often necessary. The accuracy of the results was not always goodAl-Hassani et al. [11] presented a numerical simulation of single shot impact on a component and examined single shot impacting with an oblique angle but very limited results were presented. Guagliano and their co-workers [12], Baragetti [13] simulated shot peening by using the well-known FE program ABAQUS Explicit but no numerical results of single shot impacts, except several results of multiple shots impacts, were described in those papers. Deslaef and

   Rouhaud [14, 15] presented a FE simulation of single and multiple shot impacting a component and examined the effect of rigid and deformable shot. The numerical results were compared with those experimental measurements obtained for multiple shot impacts and showed significant differences.

   A more systematic study of shot peening process using FE was presented by Meguid and his co-workers [16, 17]. They conducted a dynamic FE analysis of single and multiple shot impacts. The effect of some parameters was investigated but not comprehensively. The majority of the numerical studies have used specific values of the model parameters so it is not easy to assimilate the effect of each parameter on the resulting residual stress distribution from the simulation. In this study, an attempt is made to conduct a careful parametric study of single shot impact on a component to gain a sound understanding of the effect of the key parameters concering shot peening. It is useful for connecting with the numerical studies of entire shot peening process.

3. Finite element model

   The model used for shot peening simulation is generated in LS Dyna, consist of three dimensional circular body of Aluminium (LM 13) material having following boundary condition and geometrical properties and act as target in impact analysis. Target have radius R = 8dshot, height H = 3dshot where d shot is the shot diameter.

   Deck Setup

   Shot hits target normal to surface under

   initial velocity.

   Outer side of target is constrained in all direction. Considering target fixed along the

   periphery.

   Fig.1 Boundary conditions Target properties:

   Material	Tensile strength	Youngs modulus	Poissons ratio	Density
   Aluminiu m LM 13	180 Mpa	73.1 Gpa	0.35	2070G m/ Cm 3

   Shotmesh modelas rigid elements (materialtype mat20)

   Massapplied to all nodes equally.

   Shot Properties:

   Youngs modulus	Poissons ratio	Density	Hardness HRC	Shot size
   2.1*10E5 Mpa	0.265	7200 Kg / M 3	45	1mm

   S320 steel shot

   Fig.2 b mesh model

4. Parametric study of single shot impact model:

   The three-dimensional FE model was developed using the commercial finite element code Ls Dyna. Fig. 2a and b shows the FE mesh that was used to investigate single shot impact on a component in the present paper.

   Targetmesh model

   Globalelementlength 0.2 mm and min 0.1 mm

   Full integration element considered.

   Mesh Model

   Fig.2 a Mesh model

   In LS Dyna, rigid bodies can be defined with an analytical rigid surface. So, a fully spherical surface with a mass positioned at its centre was used to model a shot as shown in Fig. 1 Convergence tests were conducted using different meshes and element types to ensure the numerical results presented in this paper were not affected by the choice of mesh or element types.

   Experimental measurement of single shot impact was very rare. No experimental data is found in the literature for comparison with this study. Fig 2 shows impact zone of metallic shot on aluminium LM13 target with a velocity and at perfectly at normal to the surface of circular target.

   Fig.3 Shot impact zone.

   Following are the results of shot peening simulation with different speeds of metallic shot. In this parametric study only one parameter is varied & rest all parameters are maintained constant. In this study other process parameter like shot size maintained as 1mm, distance between nozzle & target is 150mm & angle of impact taken as 90degree.First iteration is carried at a speed of

   15 meters per second whose result of stress & displacement variation are shown below.

   Fig.4 Stress contour at shot speed 15 m/s

   Fig.6 Displacement contour at shot speed 15m/s

   Fig.5 Stress plot at shot speed 15m/s

   Figure 4 & 5 shows stress contour and stress plot at a shot speed of 15 m/s. Stress plot shows stresses developed in circular plate. Stress plot should be drawn for time interval from point of contact of shot up to rebounding of shot. Stress plot shows maximum stress developed at the time of contact is 275 Mpa. After rebounding of shot due to stress relaxation some part of stress is released. After 0.00014 second 27 Mpa residual compressive stress retained in material. This residual stress forms compressive stress layer on material surface.

   Fig.7 Displacement plot at shot speed 15m/s

   From figure 6& 7 it is clear that at the time of contact material deform 0.008 mm & due to elastic recovery, material contracts and final displacement is 0.0005 mm.

   Following are the result of second iteration which is carried out at a shot speed of 20 m/s. Von misses diagram shown below is at 20 m/s.

   Fig.8 Stress contour at shot speed 20 m/s

   Figure 8 & 9 shows stress contour and stress plot at a shot speed of 20 m/s. Stress plot shows stresses developed in circular plate. Stress plot shows maximum stress developed at the time of contact is 427 Mpa. After rebounding of shot due to stress relaxation some part of stress is released. After 0.00015 second 50Mpa residual compressive stress is retained in material. This residual stress forms compressive stress layer on material surface.

   Fig.9 Stress plot at shot speed 20m/s

   Following figure no.10 shows displacement plot of impact analysis at shot speed of 20 m/s. From the displacement plot it is clear that plate deformation is about 0.005mm which is negligible.

   Fig.10 Displacement plot at shot speed 20m/s

5. Conclusion:

   This study presents a 3D finite element dynamic analysis of single shot impacting on a component. This parametric study has been conducted to investigate the effect of the single shot impact on the residual stress distribution in surface layer. The parameters which are investigated include shot velocity.

   This study makes clear that as speed of shot increases residual stress intensity increases

   .Comparatively displacement of material is negligible. Also from stress contour it is clear that near the surface, value of compressive stress is more and as we see along depth its value decreases as we move downward.

   Thus we can create FEA models as per various process parameters of shot peening i.e. Shot incident angle, Distance between shot & target, Shot size, Shot material, Shot velocity, Shot hardness etc. By doing such parametric study we can find out optimized process parameter of shot peening process which will lead good result in fatigue life improvement and reduces cost experimental iteration.

6. References:

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