Two experimental methods to determining stress–strain behavior of work-hardened surface layers of metallic components
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
References (28)
- et al.
Fatigue limits prediction of surface treated materials
J. Mater. Process. Technol.
(2006) - et al.
Resistance of a ductile steel surface to spherical normal impact indentation: use of a pendulum machine
Wear
(1997) - et al.
A dynamic indentation technique for the characterization of the high strain rate plastic flow behaviour of ductile metals and alloys
J. Mech. Phys. Solids
(1991) - et al.
Characterisation of elasto-plastic behaviour for contact purposes on surface hardened materials
Contact fatigue of automotive gears: evolution and effects of residual stresses introduced by surface treatments
Fatigue Fract. Eng. Mater. Struct.
(2000)- et al.
Characterization of mechanical properties in surface-treated materials
J. Test. Eval.
(2000) - et al.
Numerical simulation of residual stress relaxation in shot peened high-strength aluminum alloys under reverse bending fatigue
J. Eng. Mater. Tech. (ASME)
(2010) - et al.
The determination of yield strength from hardness measurements
Metall. Trans.
(1971) - et al.
Determination of stress–strain relation of the shot peened surface layers with the help of X-ray diffraction half-width
- Desvignes, M., 1987. Influence du grenaillage de précontrainte sur la tenue en fatigue de ĺacier 35CD4. Docteur en...
Etude du Comportment d’Aciers Cémentés Sollicités en Fatigue de Contact
X-ray diffraction method
A micro-mechanism based analysis for size-dependent indentation hardness
J. Mater. Sci.
Estimating yield strength from hardness data
Metal Prog.
Cited by (19)
Surface mechanical property and residual stress stability of shot-peened Mg-8Gd-3Y alloy by in-situ X-ray diffraction
2023, Journal of Materials Research and TechnologyDetermination of surface mechanical property and residual stress stability for shot-peened SAF2507 duplex stainless steel by in situ X-ray diffraction stress analysis
2020, Journal of Materials Research and TechnologyCitation Excerpt :However, one of the challenges in characterizing the yield strength and stress–strain relation of the superficial layer processed by SP is the small thickness of the hardened layer, which is subjected to residual stresses. The classical mechanical tests cannot be implemented owing to the difficulty of preparing representative layers with homogeneous cross-section [23]. Recently, a new approach with an explicit physical meaning has been put forward to determine the mechanical characteristics of the thin layer under a biaxial residual stress state [24].
Evolution of surface integrity and fatigue properties after milling, polishing, and shot peening of TC17 alloy blades
2020, International Journal of FatigueCitation Excerpt :iii) A work-hardened layer of 80 µm induced by shot peening improves the yield strength of the material. Moreover, the value of fatigue crack propagation threshold is increased, leading to an enhanced fatigue limit [57]. ( iv) Microstructural refinements in the subsurface layer restrain the generation of slip bands during the HCF vibration fatigue tests, which prevents fatigue crack initiation.