Two experimental methods to determining stress–strain behavior of work-hardened surface layers of metallic components

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Abstract

Two experimental methods to characterize the mechanical behavior of work-hardened surface layers of metallic materials, induced by mechanical surface treatments, are described and discussed. The study was conducted using two steels with different mechanical properties, subjected to a similar shot-peening treatment. To characterize the elastic–plastic behavior of the surface layers affected by shot-peening, the X-ray diffraction assisted four-point bending method (XRDABM) and the normalized hardness variation method (NHVM) were applied. The results were compared and discussed taking into account the advantages and disadvantages of both methods. Similar yield strength values of the treated surfaces were found by both proposed methods. Taking into account their measuring characteristics, these can be considered complementary methods.

Introduction

Mechanical components are usually subjected to different kinds of loading, which can induce fatigue, corrosion and wear failure. The lifetime of these components depends essentially on the final properties of the material's surface layers, resulting from production processes. To improve their service lives, these components often undergo surface hardening treatments, such as surface quenching, carburizing, nitriding, shot-peening, etc., which generate compressive residual stresses in the surface layers and modify their chemical composition, microstructure and mechanical properties when compared to the bulk material. The origin of these residual stresses is related to the heterogeneity of plastic deformation in the surface layers, due to a local irreversible elongation of the layers, which is incompatible with the component's remaining material. These deformations can be of thermo-chemical, metallurgical and mechanical origin. The surface treatments are usually classified according to the main process involved in the treatment (François et al., 1996).

Mechanical surface treatments, like shot-peening for example, use local plastic deformation to introduce compressive residual stresses. This local plastic deformation is induced by the successive impacts of the peening medium on the material's surface. Two main mechanisms are involved (Wohlfahrt, 1984): a direct plastic elongation at the surface, induced by the tangential forces (hamming effect), which implies maximum compressive residual stress at the surface, and a plastic strain in the sub-layers, induced by the Hertz pressure (Hertz effect), which implies maximum compressive residual stresses within the material, below the surface. Furthermore, heat dissipation could imply plastic compression and the appearance of tensile residual stresses at the surface. Depending on the mechanical characteristics of the materials, the combination of these effects produces, by overlapping, the final in-depth compressive residual stress profile. Mechanical properties of the shot-peened layers, however, depend on how the material behaves under cyclic plastic deformation at high plastic strain rates. Therefore, the elastic–plastic behavior of the material near the surface should be completely different from that found for the bulk material (Cao and Castex, 1989). In addition, remembering only the case of two damage mechanisms, the study of the contact fatigue or wear needs valid behavior laws for surface-treated material layers. Knowledge of their mechanical behavior is important because it allows one to estimate the evolution of the treated parts under service. This is required for studies of very different kinds, such as the study of the reliability of residual stress measurement techniques, like the incremental hole-drilling technique (Nobre et al., 2000, Gibmeier et al., 2004), or the numerical prediction models of residual stress relaxation due to dynamic loading (Batista, 2000, Benedetti et al., 2010).

The particular nature of the hardened layers, which do not exist independently and are subjected to residual stresses due to the production processes, means that their mechanical properties cannot be determined by the classical mechanical tests, owing to the difficulty of obtaining samples with a homogeneous cross-section, representative of those layers. In general applications, the hardness test is currently used to estimate changes in the mechanical properties of hardened layers, in particular to verify its work-hardening behavior. However, for numerical analysis applications, the actual elastic–plastic behavior laws have to be known, especially the variation of the yield strength along the treated layers.

Section snippets

Brief description of some previous studies and importance of the present study

Different procedures and methodologies have been proposed for evaluating the mechanical properties of surface-treated materials (Cao and Castex, 1989, Virmoux et al., 1994, Batista and Dias, 2000, Nobre et al., 2004). Desvignes (1987) used the elastic–plastic calculation method of Zarka and Casier (1979) to determine the yield strength of a shot-peened steel as a function of the treated depth, based on the elastic–plastic analysis of residual stress relaxation during fatigue tests. Cao and

Stress determination by X-ray diffraction (XRD)—principles

To determine stresses using XRD it has to be borne in mind that three kinds of stresses can be conventionally defined, i.e., first, second and third order (Macherauch et al., 1973). First order stresses are those on the scale of a few grains, those of the second order are on the scale of one grain, and those of the third order are on the scale of a few interatomic distances. In general, the measured stresses are the superposition of these three kinds of stresses. The three orders of stresses

Materials and experimental procedures

The experimental study was performed using two steels with different mechanical properties. Table 1 shows the mechanical properties and chemical composition of both steels. The yield strength, the tensile strength and the strain-hardening exponent of the bulk material were obtained using standard tensile specimens according to the standard ASTM E 8 (2009), which were machined from the as-received plates. Tensile tests were carried out on the materials without surface treatment. The tests were

Tensile tests

The monotonic mechanical properties of the bulk material that correspond to the steels used in this study can be inferred from the stress–strain curves obtained during the tensile tests. Fig. 4 (left) shows the engineering stress–strain curves obtained for both steels, and Fig. 4 (right) shows the corresponding true stress–plastic strain curves for the whole range of strain, i.e., until fracture occurs. These figures allow the comparison of the monotonic mechanical properties of both steels.

Micro-hardness and X-ray analysis of the shot-peened surfaces

The

Conclusions

In this work, two experimental methods for the characterization of the elastic–plastic behavior of the surface layers of metallic materials, subjected to mechanical surface treatments, such as shot-peening, were proposed and discussed. The obtained results have been compared considering the main advantages and disadvantages of each one. The normalized hardness variation method (NHVM) can be used for local yield stress estimation along the whole treated depth of a material. With the yield stress

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