**1. Introduction**

Shot peening (SP) is an effective surface strengthening method and widely used in industry, which can effectively improve the surface performance of material. The effect of SP depends on the material and parameters during SP process. The small SP intensity will lead to the strengthening effect unobvious, but the excessive SP intensity may result in the formation of micro-cracks and reduce the strength. So, a proper SP intensity should be conducted. Additionally, the improper shot balls will increase the surface roughness of material, which is not benefit to the surface properties. During SP, the shot balls impact the surface of material at a high speed, which transforms the kinetic energy of shot balls into the elastic energy of internal stress. So after SP, the internal energy of material is increased, and it is in the metastable state. While the external environment changes, the material with high energy may be transferred to the low energy state spontaneously, leading to the stress relaxation. For instance, in the high temperature environment, the residual stress will promote the local creep of the material, resulting in reducing the residual stress, which will weaken the effect of SP and be not conducive to the improvement of fatigue properties [1].

models mentioned above lead to the introduction and popular application of 3D models. Because the 3D SP models can show the effect of SP coverage rate on residual stress distribution, they are approaching to the practical work more and become the main choice in recent years, especially 3D models with dynamic analysis. In the overview of SP simulation, some 3D models are adopted with different model descriptions, kinds of material models, kinds of analysis, number of shot balls, and so on [8, 13–20]. On these 3D models, most studies focus on homogeneous material and few work involves 3D inhomogeneous inclusions while establishing models. Even though some researchers have established 3D inhomogeneous models for metal matrix composites using other methods [21–25], few investigation focused

Finite Element Dynamic Analysis on Residual Stress Distribution of Titanium Alloy and Titanium…

Composite material is composed of two or more kinds of materials with different chemical and physical properties, and also with different size such as micro or macro. In metal matrix composites, the difference in the properties of metal matrix and reinforcement makes the presence of interface. Due to the mismatch between matrix and reinforcement, the material properties in the vicinity of the interface are not continuous, so that the material properties and microstructure in the vicinity of the interface will vary obviously. The variation of properties has a serious effect on the macroscopic properties of the composites [26]. The size of reinforcement in the composite is typically between several μm and several tens of μm, but

stress distribution around reinforcement in the composite by experimental method. It is a feasible method to carry out numerical simulation using finite element analysis to solve this

As one kind of important metal matrix composites, titanium matrix composites have wide application prospects in the field of aerospace, automobile, and other industries because of their good properties such as high specific strength, good ductility, and excellent fatigue properties, etc. [27–29]. About the residual stress distribution of titanium matrix composites after SP, the experimental investigation has been carried out in our previous work by XRD method [30–33]. However, the measured residual stress by experiments only reveals the average stress of matrix and reinforcements, because the beam size of X-ray is much bigger than the dimension of reinforcement. So, it is hard to directly test the residual stress distribution in and around the reinforcements by experiments, which depends on the method of simulation. In our current work, 3D finite element dynamic analysis of multiple shot impacting is performed on Ti-6Al-4V alloy and titanium matrix composite (TiB+TiC)/Ti-6Al-4V (TiB:TiC = 1:1 (vol%)). The program of ANSYS/LS-DYNA [34] is utilized in the 3D finite element dynamic analysis, and the 3D homogeneous and inhomogeneous models are set up. The systematic study is conducted using this 3D dynamic model to investigate the effect of coverage rate, shot balls' radius, and shot velocity on the residual stress distribution after SP. Moreover, TiC and TiB reinforcements in the composite are constructed in a composite method by the simplified inhomogeneous model. The influence of reinforcements on the stress distribution is analyzed, and the residual stresses in and around reinforcements are obtained and discussed in detail. Moreover, the experimental results by XRD method are compared with the simula-

, therefore, it is difficult to determine the

http://dx.doi.org/10.5772/intechopen.73120

25

the irradiation area of X-ray analysis is about 1 mm2

on SP.

problem.

tion results finally.

Usually, the fatigue strength and fatigue life of cyclically loaded metallic components can be considerably improved due to the compressive residual stress (CRS) and work hardening induced on the surface layer after SP [2]. The distribution of CRS is mainly affected by the parameters of SP and the materials' condition. A significant number of parameters are needed to be regulated and controlled in order to obtain a more beneficial CRS distribution. Therefore, in practical application of SP, the empirical knowledge should be accumulated for getting the appropriate processing parameters, which usually requires time and money consuming. For obtaining the suitable SP parameters and minimizing these trails, the numerical simulation of SP is conducted, building a better understanding of SP process with the aim of study, analyzing and predicting the relationship between the influencing factors and simulation results.

SP simulation has been developed since more than four decades. Some references have indicated that finite element method is a suitable and useful method to predict residual stress distribution after SP [3–7]. The finite element models for SP include 2D and 3D models. Usually, 2D models are adopted to simulate a single impact on a semi-infinite target body, and the simulated accuracy is verified by comparing the simulated stress distribution with the results measured by neutron diffraction and X-ray diffraction (XRD) methods. [8–12]. However, based on the direct comparison of residual stress distribution along the symmetry axis of a 2D model with the experimental results, it is questionable because the measurement area by experiment is different from the impact area of 2D simulation. Generally, the measurement area by some techniques, like XRD, neutron diffraction, and hole drilling, is larger than the diameter of dimple utilized in 2D models, and these techniques just provide macroscopic stress values. Moreover, a very important parameter, the coverage rate, cannot be considered in 2D simulation during the multiple SP process. The shortcomings of 2D models mentioned above lead to the introduction and popular application of 3D models. Because the 3D SP models can show the effect of SP coverage rate on residual stress distribution, they are approaching to the practical work more and become the main choice in recent years, especially 3D models with dynamic analysis. In the overview of SP simulation, some 3D models are adopted with different model descriptions, kinds of material models, kinds of analysis, number of shot balls, and so on [8, 13–20]. On these 3D models, most studies focus on homogeneous material and few work involves 3D inhomogeneous inclusions while establishing models. Even though some researchers have established 3D inhomogeneous models for metal matrix composites using other methods [21–25], few investigation focused on SP.

**1. Introduction**

fatigue properties [1].

tion results.

Shot peening (SP) is an effective surface strengthening method and widely used in industry, which can effectively improve the surface performance of material. The effect of SP depends on the material and parameters during SP process. The small SP intensity will lead to the strengthening effect unobvious, but the excessive SP intensity may result in the formation of micro-cracks and reduce the strength. So, a proper SP intensity should be conducted. Additionally, the improper shot balls will increase the surface roughness of material, which is not benefit to the surface properties. During SP, the shot balls impact the surface of material at a high speed, which transforms the kinetic energy of shot balls into the elastic energy of internal stress. So after SP, the internal energy of material is increased, and it is in the metastable state. While the external environment changes, the material with high energy may be transferred to the low energy state spontaneously, leading to the stress relaxation. For instance, in the high temperature environment, the residual stress will promote the local creep of the material, resulting in reducing the residual stress, which will weaken the effect of SP and be not conducive to the improvement of

24 Finite Element Method - Simulation, Numerical Analysis and Solution Techniques

Usually, the fatigue strength and fatigue life of cyclically loaded metallic components can be considerably improved due to the compressive residual stress (CRS) and work hardening induced on the surface layer after SP [2]. The distribution of CRS is mainly affected by the parameters of SP and the materials' condition. A significant number of parameters are needed to be regulated and controlled in order to obtain a more beneficial CRS distribution. Therefore, in practical application of SP, the empirical knowledge should be accumulated for getting the appropriate processing parameters, which usually requires time and money consuming. For obtaining the suitable SP parameters and minimizing these trails, the numerical simulation of SP is conducted, building a better understanding of SP process with the aim of study, analyzing and predicting the relationship between the influencing factors and simula-

SP simulation has been developed since more than four decades. Some references have indicated that finite element method is a suitable and useful method to predict residual stress distribution after SP [3–7]. The finite element models for SP include 2D and 3D models. Usually, 2D models are adopted to simulate a single impact on a semi-infinite target body, and the simulated accuracy is verified by comparing the simulated stress distribution with the results measured by neutron diffraction and X-ray diffraction (XRD) methods. [8–12]. However, based on the direct comparison of residual stress distribution along the symmetry axis of a 2D model with the experimental results, it is questionable because the measurement area by experiment is different from the impact area of 2D simulation. Generally, the measurement area by some techniques, like XRD, neutron diffraction, and hole drilling, is larger than the diameter of dimple utilized in 2D models, and these techniques just provide macroscopic stress values. Moreover, a very important parameter, the coverage rate, cannot be considered in 2D simulation during the multiple SP process. The shortcomings of 2D Composite material is composed of two or more kinds of materials with different chemical and physical properties, and also with different size such as micro or macro. In metal matrix composites, the difference in the properties of metal matrix and reinforcement makes the presence of interface. Due to the mismatch between matrix and reinforcement, the material properties in the vicinity of the interface are not continuous, so that the material properties and microstructure in the vicinity of the interface will vary obviously. The variation of properties has a serious effect on the macroscopic properties of the composites [26]. The size of reinforcement in the composite is typically between several μm and several tens of μm, but the irradiation area of X-ray analysis is about 1 mm2 , therefore, it is difficult to determine the stress distribution around reinforcement in the composite by experimental method. It is a feasible method to carry out numerical simulation using finite element analysis to solve this problem.

As one kind of important metal matrix composites, titanium matrix composites have wide application prospects in the field of aerospace, automobile, and other industries because of their good properties such as high specific strength, good ductility, and excellent fatigue properties, etc. [27–29]. About the residual stress distribution of titanium matrix composites after SP, the experimental investigation has been carried out in our previous work by XRD method [30–33]. However, the measured residual stress by experiments only reveals the average stress of matrix and reinforcements, because the beam size of X-ray is much bigger than the dimension of reinforcement. So, it is hard to directly test the residual stress distribution in and around the reinforcements by experiments, which depends on the method of simulation. In our current work, 3D finite element dynamic analysis of multiple shot impacting is performed on Ti-6Al-4V alloy and titanium matrix composite (TiB+TiC)/Ti-6Al-4V (TiB:TiC = 1:1 (vol%)). The program of ANSYS/LS-DYNA [34] is utilized in the 3D finite element dynamic analysis, and the 3D homogeneous and inhomogeneous models are set up. The systematic study is conducted using this 3D dynamic model to investigate the effect of coverage rate, shot balls' radius, and shot velocity on the residual stress distribution after SP. Moreover, TiC and TiB reinforcements in the composite are constructed in a composite method by the simplified inhomogeneous model. The influence of reinforcements on the stress distribution is analyzed, and the residual stresses in and around reinforcements are obtained and discussed in detail. Moreover, the experimental results by XRD method are compared with the simulation results finally.
