Evaluating surface deformation and near surface strain hardening resulting from shot peening a tempered martensitic steel and application to low cycle fatigue

The plastic deformation resulting from shot peening treatments applied to the ferritic heat resistant steel FV448 has been investigated. Two important effects have been quantified: surface roughness and strain hardening. 2D and 3D tactile and optical techniques for determining surface roughness amplitude parameters have been investigated; it was found that whilst R_a and S_a were consistent, S_z was generally higher than R_z due to the increased probability of finding the worst case surface feature. Three different methods for evaluating the plastic strain profile have been evaluated with a view to establishing the variation in yield strength near the surface of a shot peened component. Microhardness, X-ray diffraction (XRD) line broadening and electron backscatter diffraction (EBSD) local misorientation techniques were applied to both uniaxially deformed calibration samples of known plastic strain and samples shot peened at intensities varying from 4A to 18A to establish the variation in plastic strain and hence the variation in yield strength. The results from the three methods were compared; XRD and EBSD profiles were found to be the most similar with microhardness profiles extending much deeper into the sample. Changes in the measured plastic strain profile after exposure to low cycle fatigue and the correlation of these changes with the cyclic stress–strain behaviour of the material are also discussed with a view to assessing the importance of the dislocation profile in component life assessment procedures.     

Characterization and density control of GaN nanodots on Si (111) by droplet epitaxy using plasma-assisted molecular beam epitaxy

In this report, self-organized GaN nanodots have been grown on Si (111) by droplet epitaxy method, and their density can be controlled from 1.1 × 10¹⁰ to 1.1 × 10¹¹ cm^-2 by various growth parameters, such as substrate temperatures for Ga droplet formation, the pre-nitridation treatment of Si substrate, the nitridation duration for GaN crystallization, and in situ annealing after GaN formation. Based on the characterization of in situ RHEED, we can observe the surface condition of Si and the formation of GaN nanodots on Si. The surface nitridaiton treatment at 600°C provides a-SiNx layer which makes higher density of GaN nanodots. Crystal GaN nanodots can be observed by the HRTEM. The surface composition of GaN nanodots can be analyzed by SPEM and μ-XPS with a synchrotron x-ray source. We can find GaN nanodots form by droplet epitaxy and then in situ annealing make higher-degree nitridation of GaN nanodots.     

Damage phenomena in lubricated rolling-sliding wear of a gas carburised 0.85%Mo low-alloyed sintered steel: theoretical analysis and experimental verification

The resistance to surface and subsurface damage during lubricated rolling-sliding wear of a carburised low-alloy sintered steel and the effect of shot peening were investigated. The formation of both contact fatigue cracks and of brittle tensile cracks may be predicted by a theoretical model that was experimentally validated. Carburising is effective in increasing the resistance to contact fatigue, but pores in a hard and brittle matrix may act as pre-existing cracks. Shot peening increases the contact fatigue resistance since compressive residual stresses oppose the nucleation of surface cracks.     

从转角枪的发明看军事技术集成创新

分析转角枪的历史发展、技术组成,指出:军事技术集成创新的主要优势在于找准需求后相对容易实现;TRIZ(Theory of Inventive Problem Soving)理论是适用于集成创新的高效方法;集成创新应大力依赖的主体是用户。集成创新虽没有根本性的技术变革但也可能引发重大军事变革。中国引进转角枪后,引进消化吸收再创新却迎头赶上,从反面印证了我国集成创新能力相对较弱。     

Evaluating the effect of SiC content on iron-based nanocomposite

Iron has wide application in manufacturing industries and the objective of this study is to investigate the effect of silicon carbide (SiC) particles on iron-based nanocomposites. SiC is effectively reinforced into iron matrix by mechanical alloying process using high-energy ball mill. The Fe–SiC nanocomposites with various compositions of SiC viz., 15%, 20% and 25% are characterised using X-ray diffraction and atomic force microscopy. The nanocomposite powders are compacted and sintered into pellets. Their elastic properties, hardness and Poisson's ratio are determined using the pulse echo technique. Moreover, the densities of the pellets are measured using Archimedes’ principle by the water immersion method.     

Detecting recrystallization in a single crystal Ni-base alloy using resonant ultrasound spectroscopy

The use of resonant ultrasound spectroscopy (RUS) as a nondestructive evaluation (NDE) technique for Ni-base single crystal superalloys has been investigated. Manufacture of single crystal superalloys can be challenging due to the prevalence of defects induced during single crystal growth or subsequent processing. Common defects involve the presence of misoriented (non-single crystal) material that change the bulk elastic properties and, as a result, are detectable by RUS. To control the extent of misoriented material, recrystallization induced by shot peening the surface of the single crystal has been studied. RUS was then used to determine the presence and depth of misoriented material due to recrystallization. Recrystallization of shot peened cylindrical single crystal specimens occurred to a depth of 80 μm and 178  μm during subsequent heat treatments. Experimental average resonance frequency shifts of 1.835% ±1.704% and 2.380% ±2.910%, respectively, were measured over a frequency range from 20–200 kHz when compared to the baseline shot peened condition. Finite element (FE) models using the ABAQUS Lanczos Eigen frequency solver assessed the influence of recrystallization as a function of depth from the surface and showed good agreement with the measured resonance frequency shifts. For the greatest NDE sensitivity on production-scale turbine blades and other gas turbine components, a coupled RUS measurement and FE modeling approach is essential, and has the potential to improve single crystal processing approaches and manufacturing yields.     

Surface layer condition and the fatigue strength of an AZ91 alloy after ball peening

The present work compares the states of the surface textures and the oscillatory fatigue strengths of specimens made of an AZ91 alloy after turning and dynamic bearing ball peening. It was found that the specimens ball peened at different time have similar surface geometric structure parameters but different strengthening intensity. It was shown that fatigue strength follows greater peening intensity and random and isotropic surface. It was also ascertained that improving (by peening) the condition of the surface layer means enhancing fatigue strength by 20–87% compared with turning.     

Analysis of the S–N curves of welded joints enhanced by ultrasonic peening treatment

Contrast fatigue tests were carried out on T-shape tubular joints of 20 steel in three conditions: as welded, treated by ultrasonic peening treatment (UPT) before loading and UPT under loading. Results are: (1) Dispersity of test results measured by nominal stress is much larger than that measured by hot spot stress. After UPT before loading, fatigue strength of 20 steel tubular joints measured by hot spot stress increases by 67% and fatigue life is prolonged by 22–45 times. (2) Under low stress ratio R, UPT before loading can improve the fatigue performance of welded tubular joints significantly. (3) Under high stress ratio R, UPT under loading (static load) is recommended to improve the fatigue performance of welded tubular joints. UPT under loading not only enhances the fatigue properties at low stress level, but also at high stress level. (4) The general rule of S–N curves of welded joints treated by UPT is commonly effected by external load (static load) and self release of residual stress.     

A new effective method to estimate the effect of laser shock peening

The paper focuses on the influencing parameters to the plastically affected depth and maximum residual stress of the metallic target after laser shock peening. Firstly, by using a new coupling analysis method, the shock pressure characteristics including peak pressure and pressure duration are given. Secondly, based on the deduced pressure profile, dimensional analysis method is employed to find the controlling parameters, and the relationships of plastically affected depth, maximum residual stress versus peak pressure, pressure duration and laser spot size are given. Thirdly, a two dimension axisymmetric finite element model based on LS-DYNA package is built, and the dynamic responses of metallic target subject to laser shock processing are computed with different input parameters. The result shows that the plastically affected depth is proportional to pressure duration, whereas the maximum residual stress is independent with it; the plastically affected depth and the maximum residual stress are not affected by laser spot size within a certain range, whereas have approximate linear relationships with peak pressure after reaching to a certain level; maximum residual stress and plastically affected depth increase significantly for a thin target configuration in laser shock peening system.     

A simple but effective FE model with plastic shot for evaluation of peening residual stress and its experimental validation

We propose a practical finite element (FE) model for evaluation of peening residual stress. The model aims to produce a solution approaching the endeavored 3D FE solution. We investigate the effect of physical factors including material damping, dynamic friction and strain rate. The kinematical factors including shot diameter and impact velocity are also considered. Integrating those factors and plastic shots, we set up an effective FE model. Based on the arc height and coverage matching with the Almen saturation curve, impact velocity needed for FE analysis is determined. The model is found to provide the solution comparable with the 3D multi-impact FE solution and the experimental XRD result.