An evaluation of shot peening, residual stress and stress relaxation on the fatigue life of AISI 4340 steel

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Abstract

Shot peening is a method widely used to improve the fatigue strength of materials, through the creation of a compressive residual stress field (CRSF) in their surface layers. In the present research the gain in fatigue life of AISI 4340 steel, used in landing gear, is evaluated under four shot peening conditions. Rotating bending fatigue tests were conducted and the CRSF was measured by an X-ray tensometry prior and during fatigue tests. It was observed that relaxation of the CRSF occurred due to the fatigue process. In addition, the fractured fatigue specimens were investigated using a scanning electron microscope in order to obtain information about the crack initiation points. The evaluation of fatigue life, relaxation of CRSF and crack sources are discussed.

Introduction

Fatigue is an important parameter to be considered in the behavior of mechanical components subjected to constant and variable amplitude loading. Mechanical, metallurgical and environmental variables can influence the fatigue resistance of a structural component [1]. In structural engineering applications, nucleation and propagation of fatigue cracks are some of the most important considerations in the mechanical properties of metals. In high strength steels, surface and subsurface defects play an important role in the reduction of the fatigue limit [2]. AISI 4340 steel is widely used in the aircraft industry for fabrication of structural components, in which strength and toughness are fundamental design requirements.

One of the known ways to improve fatigue resistance is by using the shot peening process to induce a compressive residual stress in the surface layers of the material, making the nucleation and propagation of fatigue cracks more difficult [3], [4].

The shot peening results depend on various parameters. These parameters can be grouped in three different classes according to Fathallah et al. [5]: parameters describing the treated part, parameters of stream energy produced by the process and parameters describing the contact conditions. Kobayashi et al. [6] presented a mechanism of creation of compressive residual stress by shot peening through an experiment by dropping a large steel ball on a thick plate specimen. In industries shot peening is controlled with the help of Almen plates. They are standardized thin plates that are placed in parallel to the treated material, receiving therefore, the same treatment. This treatment induces residual stresses in small plates that become deformed. This deflected shape caused by the process is called Almen intensity, and its value is appropriate to adjust the shot peening parameters [7], [8].

There are basically two ways of modeling the Compressive Residual Stress Field (CRSF) caused by shot peening. The first is through finite element analysis [7], [9], [10] and the second is through empiric models based on experimental data [11], [12]. However, relaxation of the CRSF induced by shot peening has been observed over the fatigue life [13], [14], [15]. Kodama [16] shows that the residual stress relaxation of annealed carbon steel varied with fatigue cycles. This relaxation during the fatigue cycles was divided into two stages: the surface yielding in the first cycle and gradual change in the following cycles. In these two stages the residual stress of shot peened specimens decrease considerably when compared to those with no fatigue cycles. Furthermore, Farrahi et al. [17] worked with the AFNOR 60SC7 spring steel and showed that fatigue life improvement, resulting from shot peening, can be attributed to the maximum residual stress and also to the depth of the plastically deformed layer. A correlation was found between the fatigue life and the area under the residual stress curve.

In general, fatigue crack initiation occurs on the specimen surface. Wang et al. [18] demonstrated that fatigue crack can be initiated from the interior of many materials in the case of high-cycle fatigue (HCF). In the high-cycle regime (>107 cycles), all crack sources were found at non-metallic inclusions located in the interior of the specimen. Shengping et al. [19] showed that the shot peening process pushes the crack initiation points beneath the compressive residual stress zone in all the cases studied except for 0.45% carbon steel. In this research, the possibility of relaxation of CRSF during the fatigue process was not considered. Yet Zeller [20] demonstrates the residual stress relaxation on the aforementioned steel caused by fatigue loading.

In the present research, the rotating bending fatigue strength of AISI 4340 steel is evaluated as a function of shot peening in the conditions used in industries. In order to study the shot peening influence on fatigue life, the behavior of CRSF during the fatigue process and the crack initiation points of fatigued specimens were studied.

Section snippets

Experimental work

The chemical composition of AISI 4340 used was 0.41 C–0.73 Mn–0.8 Cr–1.74 Ni–0.25 Mo–0.25 Si, wt%. The mechanical properties of this alloy are: (50–53)HRC, yield strength of 1511 MPa, ultimate tensile of 1864 MPa, and fatigue limit of 800 MPa (54% of yield strength). These properties were obtained by means of quenching from 815°C followed by tempering in the range (230±5)°C for 2 h. This material was treated with four intensities of shot peening: 0.0027 A (8 psi), 0.0063 A (13 psi), 0.0083 A (18 psi),

Fatigue

The S/N curves for the base material and for the four peening conditions analyzed are shown in Fig. 2. Although 20 tests have been done for each curve, only the average points are presented for each stress level. The five stress levels indicated in Fig. 2 are not all of the levels tested, but they will be references of stress in the studies done throughout the text. It is possible to observe in Fig. 2 an improvement in the fatigue resistance of the specimens shot peened, compared to the base

Conclusions

An increase in shot peening intensity resulted in an increase in the maximum compressive residual stress and the width of the CRSF. However, the surface residual stress was nearly independent of the peening conditions.

An increase in the shot peening intensity, and consequently, the increase of the original CRSF size, does not necessarily increase the fatigue life of AISI 4340 steel with 53HRC hardness.

The shot peening treatment pushes the crack sources beneath the surface in most of medium and

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