Non-destructive evaluation and quality control of surface treatments

The significance of residual stresses in relation to the integrity of LWR pressure vessels

High-efficiency green machining of single crystal 4H–SiC based on tribo-oxidation

Shot peening is a cold-working process that induces a small indentation on a metallic surface by impacting shots onto it causing residual compressive stresses which delays the initiation of fatigue cracking. The experimental assessment of shot-peening process parameters is not only very complex but costly as well. The most promising technique is the explicit dynamics finite-element (FE) analysis capable of determining the shot-peening process parameters subject to the selection of a suitable material’s constitutive model and numerical technique. In this work, Ansys/LS-Dyna code was employed to simulate the impact of shots of various sizes made of steel and glass on an aluminium alloy plate defined with strain-rate-sensitive elasto-plastic material model. The impacts were carried out at various incident velocities. The effects of velocity, size, and material of the shots on the compressive residual stress, indentation, and force–time response were investigated. The results showed that impact with the steel shots caused more indentation and larger area of plastic deformation in target material than the glass shots due to higher kinetic energy. Furthermore, increasing the shot velocity and size resulted in an increase in indentation. However, a negligible effect of the shot velocity, size, and material was observed on target’s surface and subsurface residual stress. The obtained results demonstrated that the proposed FE models simulated the shot-peening process in a realistic way. The study can be used to determine and optimise the shot-peening parameters such as shot size, material and velocity, and amount indentation and residual stresses in the aluminium alloy plate.

Abrasive waterjet peening with elastic prestress: A parametric evaluation

Barkhausen noise (BN) measurements are commonly used for surface characterisation. However, often there is also a need to verify the sub-surface region because detrimental tensile stresses may be present after different manufacturing steps. Especially in a grinding burn, the surface stress may be compressive, but it changes quickly into tensile stress below the surface. The aim of this study was to find out whether regular surface-sensitive BN measurement is also sensitive to the stresses below the surface caused by grinding burns. More specifically, the aim was to study the relationship between BN features and sub-surface stresses and to identify a model that estimates sub-surface stresses. Real samples were collected from an actual process. The samples were cylindrical samples manufactured from commercial alloyed AISI/SAE L6 steel that was through-hardened prior to grinding. Barkhausen noise measurements were carried out for 42 grinding burn locations followed by X-ray diffraction-based residual stress surface measurements and residual stress depth profiles. Depth information was obtained through step-by-step electrolytic removal of thin layers. The stress profiles were pre-processed through interpolation and averaged stress was calculated as a function of depth below the surface. Correlation analysis was carried out to evaluate the relationships between BN features and stress at different depths and among BN features. The main outcome of the analysis was that BN measurement is dominated by the sub-surface tensile stresses rather than the compressive stress at the surface. It was also noticed that BN features form two groups, corresponding to average Barkhausen activity and magnetising field strength leading to maximum Barkhausen activity. Models for stress at different steps were identified systematically. The performance of the models for sub-surface stresses was reasonable with R2 values of around 0.85 and root mean squared error (RMSE) values of around 95 MPa. Based on the results, it is concluded that BN measurement provides information about sub-surface stresses and that stress can be evaluated through straightforward modelling, allowing fast detection of grinding burns.

The surface and near surface residual stress state induced by machining may have a significant effect on the structural integrity of some important mechanical components. This article describes an experimental investigation of this stress state during high speed machining of Grade 5 titanium alloy (Ti6Al4V). The size and depth of the stress field is evaluated by non-contact probing measurement techniques as a function of cutting speed (70–200 m/min) and depth of cut (0.25–1.0 mm). X-ray diffraction and synchrotron energy dispersive diffraction are used to evaluate the surface (X-ray diffraction) and sub-surface (energy dispersive diffraction) residual stress fields. The results indicate that both techniques may be successfully employed to evaluated machining induced residual stress fields and that the energy dispersive diffraction method is eminently suitable for probe depths up to 100 µm in Grade 5. The results clearly indicated that significant residual stresses are introduced during machining both at conventional and high cutting speeds. The stresses are largely compressive and aligned with the main cutting direction for both rough and finish cuts. The results also show that the residual stress level is a strong function of the cutting speed. The overall stress level effectively becomes more tensile but also decreases with increasing cutting speed.

Machining-induced residual stress significantly affects the fatigue life, geometric stability and service performance of machined components. This study presents a fully coupled thermo-mechanical two-dimensional (2D) Finite Element (FE) model to investigate residual stress distributions induced by High-Speed Milling (HSM) of TC4 titanium alloy. The model’s accuracy was validated by comparing the simulated outcomes with experimental data on chip segmentation, cutting forces and temperatures, with relative errors ranging from 6.85% to 12.40%. Beyond model verification, periodic fluctuations of plastic strain and residual stress in the machined surface and subsurface were simulated. Results showed tensile stress dominance at lower cutting speed (150 m/min), while compressive stress prevailed at higher speed (450 m/min), particularly for σ11. Subsurface stress profiles exhibited hook-shaped distributions, with compressive stress peaking at depths of 7–10 μm and diminishing beyond 22 μm. A good correlation was observed between experimental and simulated results, with subsurface stress consistently compressive and increasing with cutting parameters. The effect of thermo-mechanical loads plays a critical role in shaping residual stress distributions; thermal loads primarily influence surface stress, whereas mechanical loads govern subsurface stress. This research provides a comprehensive understanding of residual stress behavior and its impact on fatigue life and service performance.

ABSTRACTShot peening is a widely applied surface treatment in a number of manufacturing processes in several industries including automotive, mechanical and aeronautical. This surface treatment is used with the aim of increasing surface toughness and extending fatigue life. The increased performance during fatigue testing of the peened components is mainly the result of the sub-surface compressive residual stress field resulting from the plastic deformation of the surface layers of the target material, caused by the high-velocity impact of the shot. This compressive residual stress field hinders the propagation and coalescence of cracks during the second stage of fatigue testing, effectively increasing the fatigue life well beyond the expected life of a non-peened component.This paper describes a 3D computational model of spherical projectiles impacting simultaneously upon a flat surface. The multi-impact model was developed in ABAQUS/Explicit using finite element method (FEM) and taking into account controlling parameters such as the velocity of the projectiles, their incidence angle and different impact locations in the target surface. Additionally, a parametric study of the physical properties of the target material was carried out in order to assess the effect of temperature on the residual stress field.The simulation has been able to successfully represent a multi-impact processing scenario, showing the indentation caused by each individual shot, as well as the residual stress field for each impact and the interaction between each one of them. It has been found that there is a beneficial effect on the residual stress field magnitude when shot peening is carried out at a relatively high temperature. The results are discussed in terms of the current shot-peening practice in the local industry and the leading edge developments of new peening technologies. Finally, an improved and affordable processing route to increase the fatigue life of automotive components is suggested.

2D and 3D numerical investigation on contact stress behaviour on different combinations of gear pair

We propose a novel point grinding wheel (NPGW) with coarse grinding zone angle θ. According to earlier studies, this type of grinding wheel has high grinding efficiency, long service life, and generates very little surface roughness. The contact zone between the grinding wheel and workpiece is modified by the addition of the NPGW with the coarse grinding zone angle θ and the inclined angle α of point grinding process, and as a result, grinding force and grinding zone temperature are also altered. The grinding force theoretical model of NPGW was established and the grinding zone temperature was simulated by the finite element method, based on the theory of grinding heat generation and distribution and triangular heat source distribution model. At the same time, seven pieces of NPGW with different angles θ used for grinding experiments were fabricated. The theoretical model of grinding force and the simulation of grinding temperature coincided with experimental results. Therefore, the grinding force theoretical model and the grinding temperature simulation provide an auxiliary and predictive method for the actual process. Moreover, the influence of parameters on grinding force and grinding zone temperature can be obtained, and the results indicate that the NPGW and point grinding process can reduce grinding force and grinding zone temperature. Finally, plastic deformation is attributed to the production of compressive stress and grinding heat is attributed to the production of tensile stress. Therefore, the residual stress is influenced by the grinding force and the grinding temperature coupled. The residual stress of workpiece surface and subsurface was studied. In conclusion, the NPGW and point grinding process can reduce residual stress of workpiece surface and subsurface, in the case of tensile and compressive stress.

The influence of surface elasticity and surface residual stress on the elastic field of an isotropic nanoscale elastic layer of finite thickness bonded to a rigid material base is considered by employing the Gurtin-Murdoch continuum theory of elastic material surfaces. The fundamental solutions corresponding to buried vertical and horizontal line loads are obtained by using Fourier integral transform techniques. Selected numerical results are presented for the cases of a finite elastic layer and a semiinfinite elastic medium to portray the influence of surface elasticity and residual surface stress on the bulk stress field. It is found that the bulk stress field depends significantly on both surface elastic constants and residual surface stress. The consideration of out-of-plane terms of the surface stress yields significantly different solutions compared to previous studies. The solutions presented in this study can be used to examine a variety of practical problems involving nanoscale/soft material systems and to develop boundary integral equations methods for such systems.

When a ceramic material creeps under a localized stress and then cools under load, a portion of the creep flow stress is retained as a residual compressive stress due to elastic rebound being constrained by the creep zone. Localized contact-creep was used to generate residual compressive surface stress in soda-lime glass and two sintered aluminas. The Vickers indentation technique was used to measure the residual stress within the contact-creep area. Alumina with a higher elastic modulus than glass retained higher residual compressive surface stress. The results were in reasonable agreement with the predicted stress distribution given by finite element analysis.

Mechanical finishing of ferrite materials used in magnetic storage systems are found to have a shallow magnetically inactive layer due to residual elastic stress in the surface. The residual stresses in ground and lapped Ni-Zn surfaces are measured and compared with comparable values for steel. Whereas the residual stress in a diamond ground ferrite surface is of predominantly mechanical origin (compressive) that in a conventionally ground steel surface is predominantly of thermal origin (tensile). Methods of reducing the residual stress in a lapped ferrite surface are discussed.

Effect of residual surface stress on the fatigue behavior of a low-alloy powder metallurgy steel

On the invariance of hardness at sharp indention of materials with general biaxial residual stress fields