An electron microscopy study of worn ceramic surfaces

An electron microscopy investigation of worn surfaces of monolithic alumina, whisker-reinforced alumina, monolithic silicon carbide and silicon-silicon carbide has been performed. The emphasis has been to explore the mechanisms of wear debris generation and tribofilm formation and their influence on the tribological behaviour in closed contacts of the flat-on-flat category.     

The role of stacking fault energy and induced residual stresses on the sliding wear of aluminum bronze

An analysis was made of the residual stresses induced during sliding contact of a series of Cu-Al alloys of various stacking fault energies (SFEs) using the conventional two-exposure X-ray technique. Compressive residual stresses were found in all alloys which increased with decreasing SFE in a manner analogous to the strain-hardening behavior. The macroscopic wear rate was found to be linearly related to the compressive stresses induced during the wear process. The increase in wear rate associated with increasing hardness and residual compressive stresses is attributed to the presence of a residual tensile stress normal to the surface.     

X-ray diffraction investigation of white layer development in hard-turned surfaces

Hard turning process is widely used for finishing the hardened surfaces of machine parts exposed to high load. Such parts can be gears, bearings, or short shafts. As in each machining process, hard turning may change the properties of the surface layer. In this research work, we deal with material structure changes developing in the bore of gears made of a material of 20MnCr5. Microscope images show that the structure of the material is altered on the surface of the bores due to the turning process under different conditions. In certain cases, a certain layer that cannot be etched develops, called a white layer. Surface layer modification may take place using different technological data. In this paper, we will prove that proper selection of these technological data (cutting speed, depth of cut, feed rate) may eliminate the formation of a white layer or may decrease its thickness. The fine structure of this surface layer was investigated using the X-ray diffraction method, whilst the thickness of the layer was measured by a computer image analysis method. In this paper, the different conditions for the development of the white layer are described, its composition and some of its properties are determined, and the relationship between technological data and the thickness of the white layer is investigated.     

Application of quenching to create highly triaxial residual stresses in type 316H stainless steels

It has been proposed that highly triaxial residual stress fields may be sufficient to promote creep damage in thermally aged components, even in the absence of in-service loads. To test this proposal, it is necessary to create test specimens containing highly triaxial residual stress fields over a significant volume of the specimen. This paper presents results from an experimental and numerical study on the generation of triaxial residual stresses in stainless steel test specimens. Spray water quenching was used to generate residual stress fields in solid cylinders and spheres made from type 316H stainless steel. A series of finite element simulations and measurements were carried out to determine how process conditions and specimen dimensions influenced the resulting residual stress distributions. The results showed that highly compressive residual stresses occurred around the surfaces of the cylinders and spheres and tensile residual stresses occurred near the centre. Surface residual stresses were measured using the incremental centre hole-drilling technique, while internal residual stresses were measured using neutron diffraction. Overall there was good agreement between the predicted and measured residual stresses. The level of triaxiality was found to be very sensitive to the heat transfer coefficient, and could be controlled by adjusting the cooling conditions and changing the dimensions of the steel samples. This differed from other processes, such as welding and shot-peening, where the magnitudes and distributions of residual stresses are ill-defined and the volume of material subjected to a triaxial residual stress state is relatively small.     

High-temperature residual stresses in thin films characterized by x-ray diffraction substrate curvature method

A new x-ray technique to determine temperature dependencies of macroscopic stresses in thin films by characterizing the substrate curvature is introduced. The technique is demonstrated on polycrystalline TiN and Al thin films deposited on Si(100) wafers. The structures are thermally cycled in the temperature range of 25-400 degrees C using a newly developed heating chamber attached to a commercial x-ray diffractometer. The curvature of the freestanding samples was determined by the rocking curve measurement of substrate Si 400 reflections at different lateral positions of the samples, and the stresses are calculated using Stoney's formula. The results show that the magnitude of the stress is in good agreement with the results obtained by other techniques. For the practical application of the technique, the sample mounting and the temperature control are of great importance.     

Different finite element techniques to predict welding residual stresses in aluminum alloy plates

This study is a 3D thermomechanical finite element (FE) analysis of a single-pass and butt-welded work-hardened aluminum (Al) 5456 plates. It aims to validate the use of FE welding simulations to predict residual stress states in assessing the integrity of welded components. The predicted final residual stresses in the plate from the FE simulations are verified through comparison with experimental measurements. Three techniques are used to simulate the welding process. In the first two approaches, welding deposition is applied by using element birth and interaction techniques. In the third approach, the entire weld zone is simultaneously deposited. Results show a value at approximately the yield strength for longitudinal residual stresses of the welded center of the butt-welded Al alloy plates with a thickness of 2 mm. Considering the application of a comprehensive heat source, along with heat loss modeling and the temperature-dependent properties of the material, the approach without deposition predicts a reasonable distribution of residual stresses. However, the element birth and interaction techniques, compared with the no-deposit technique, provide more accurate results in calculating residual stresses. Furthermore, the element interaction technique, compared with the element birth technique, exhibits higher efficiency and flexibility in modeling the deposition of welded metals as well as less modeling cost.     

An investigation on worn surfaces of chopped glass fibre reinforced polyester through SEM observations

The wear mechanisms of chopped strand mat (CSM) glass fibre reinforced polyester (CGRP) composite subjected to dry sliding against smooth stainless steel counterface (R_a=0.06 μm) were studied using a pin-on-disc technique. The effects of normal load (30-90 N), sliding velocity (2.8-3.9 m/s) and sliding distance (0.7-3.5 km) on friction and wear behaviour of the CGRP composite in two different CSM orientations (parallel and anti-parallel) were measured. The worn surfaces of the CGRP composite specimens for each specific test condition were examined using scanning electron microscopy (SEM).Sliding in P-orientation exhibited lower friction coefficient at lower load and higher speed compared to AP-orientation. Meanwhile, sliding in AP-orientation exhibited (15%) less friction coefficient at higher load compared to P-orientation. At higher range of all tested parameters, AP-orientation exhibited less mass loss (16%) compared to the P-orientation.Interestingly, SEM observations showed various wear mechanisms that predominated by abrasive nature. Damage of different features in the matrix and CSM glass fibre associated with higher values of load, speed, and sliding distance such as micro- and macro-cracks in the matrix, interface separation, fibre debonding and fracture, and different sizes of fractured fibres were evident.     

Modeling of residual stresses in milling

A model to predict residual stresses produced from milling is presented. It uses process conditions as inputs and predicts surface and subsurface residual stress profiles due to milling. The model formulation incorporates cutting force and cutting temperature predictions and utilizes those parameters to define the thermomechanical loading experienced by the workpiece. Model predictions are compared with published experimental data for both cutting forces and residual stress profiles. The results show that the model performs well in predicting residual stress trends for various milling conditions. Residual stress magnitudes as well as profiles are well predicted with the modeling approach.     

Freezing of aqueous specimens: an X-ray diffraction study

The effects on water of two cooling methods, immersion in a liquid cryogen and high-pressure freezing, were studied by X-ray cryodiffraction on different sucrose solutions. The nature of the ice formed by each method depends on both the sucrose concentration and the specimen thickness. In order to compare the two methods, we mainly studied specimens having a thickness of 0.2 mm. Under these conditions, freezing by immersion gives rise to hexagonal (IH), cubic (IC) and amorphous (IV) ices when the sucrose concentration (weight/weight) has a value within the range 0–30%, 30–60%, 60% and higher, respectively. The temperature of the phase transitions IV–IC, IC–IH depends on the sucrose concentration. High-pressure freezing gives rise to two specific forms of ice: an amorphous and a crystalline ice (ice III). Ice III is observed when pure water samples are high-pressure frozen provided that the sample temperature does not rise above −150 °C. Above this temperature, ice III transforms into hexagonal ice. Amorphous ice is formed when the sucrose concentration is higher than 20%. The amorphous ice formed under high pressure has a similar, but not identical, X-ray diffraction pattern to that of amorphous ice formed at atmospheric pressure. While the X-ray diffraction pattern of amorphous ice formed at atmospheric pressure (IV) shows a broad ring at a position corresponding to 0.37 nm, that of high-pressure amorphous ice (IVHP) shows a broader ring, located at 0.35 nm. IVHP presents a phase transition (IVHP–IV) at temperatures that depend on the sucrose concentration. We also observed that some precautions have to be taken in order to minimize the alcohol contamination of high-pressure frozen samples. The ice-phase diagram presented in this paper should be taken into account in all methods dedicated to the structural study of frozen biological specimens.     

图像处理软件在铝青铜显微组织分析中的应用

采用通用图像处理软件对铝青铜显微组织做定量分析,该方法不需特殊的设备,对所分析的材料没有严格的要求,有较高的准确性。结果表明,利用计算机图像分析技术分析铝青铜的显微组织方便、实用。该项技术将为不同材料的研究和应用提供分析的依据,计算机图像技术在材料分析领域具有实用性、先进性和广阔的应用前景。     

Experimental study of residual stresses and interference of locomotive wheels by the acoustoelasticity method

ltrasound method to evaluate residual stresses in locomotive wheels using the electromagnetic–acoustic excitation and reception of elastic waves is described. The results of residual stress distribution throughout the wheel thickness before and after fitting of wheel centers are presented. A method for controlling the interference of locomotive wheels by the measured values of residual stresses using the acoustoelasticity method is proposed.     

Kinetics of edge crack in the field of residual stresses

A methodical approach and program for calculating the kinetics of flat crack extension with an arbitrary front configuration in the spatial elements of constructions have been developed. The technique creates possibilities to estimate cyclical growth and changes in the front configuration at an arbitrary distribution of stresses. The experimentally calculated method of estimating the crack dimensions has been stated. The example of calculating a crack in the pipeline in the presence of residual stresses and taking into account T stresses is presented.     

Investigation of the influence of specimen geometry on quench behaviour of steels by X-ray determination of surface residual stresses

Considerable residual stresses may form during quenching due to the differential cooling and the increase in volume accompanying the phase transformations. The design of a part may be entirely responsible for the formation of residual stresses at a critical level and even cracking during quenching. Furthermore, a certain design may be perfectly safe for one type of steel, or one type cooling conditions, and unsafe for another.In this study, an experimental procedure to investigate the influence of specimen geometry on the evolution residual stresses is proposed. The cylindrical specimens with 30 mm outer diameter were prepared from C60 and 90MnCrV8 steel bars. First, solid cylinders were quenched according to different procedures. Then, the treatment giving the minimum residual stress was applied to the hollow cylinders having various hole diameter and degree of eccentricity. By changing the position of holes in the cross-section of the specimens, a thickness gradient as a function of the eccentricity ratio was obtained. Thus, for a given transformation behaviour and quenching conditions, the effect of shape becomes more discriminating on the eccentrically drilled holes. The tangential residual stresses were determined at the specified points along the circumference of the cylinders by X-ray diffraction and d-sin² Ψ technique. The microstructures of the specimens were determined by metallographic investigation also using hardness values and respective CCT-diagrams. The results were discussed considering the microstructural evaluation of the specimens.     

Evaluating the level of residual stresses in anisotropic electric steel

The level of residual stresses (RS) in anisotropic electric steel (AES) is its most important functional characteristic that is responsible for losses of electric power in magnetic circuits of transformers during their service life. Reasons are given for the emergence of RS in AES and the known estimation methods are provided, the main shortcoming of the latter being the lack of quantitative estimates of RS. A simple and practically feasible method is suggested for estimating RS based on measurement of the anisotropy of magnetic properties of AES.     

A case study on the effect of thermal residual stresses on the crack-driving force in linear-elastic bimaterials

The effect of thermal residual stresses in bimaterial structure with initial crack located near a sharp interface is discussed in this paper. Bimaterial compact tension (CT) specimen is used in the analysis, and the residual stresses are introduced by cooling of the specimen. The residual stresses affect the stress and strain fields near the crack tip, and the crack-driving force is different compared with that in the homogeneous material without residual stresses. This difference can be quantitatively expressed through an additional crack-driving force term—the material inhomogeneity term, Cinh. In this paper, it is evaluated using the post-processing procedure based on the concept of configurational forces, following a finite-element analysis. The results indicate that accurate numerical analysis of pre-cracked bimaterials should include the effect of thermally induced residual stresses. This effect cannot be neglected, even for bimaterials with homogeneous mechanical properties and inhomogeneity in thermal properties only (e.g. welded joints of ferritic and austenitic steel). Based on the obtained results, data from this study can be used in engineering practice to improve integrity and work safety of various inhomogeneous structures.     

聚乙烯醇包覆的纳米晶硫化亚铁的X射线衍射研究

利用液相沉淀法制备出20nm以下不同粒度的聚乙烯醇(PVA)包覆的硫化亚铁(FeS)纳米颗粒样品,对纳米晶feS进行常规X射线衍射研究。通过所测FeS的X射线衍射谱,探讨晶粒尺寸与谱线宽化的变化情况,同时发现纳米晶FeS与Taylor＆Mao报道的120kbar±40kbar,22℃与50kbar±20kbar,120℃±30℃状态下体材料的FeS的高压相具有相同的晶体结构。常温常压下制备出具有高压相的纳米晶FeS,为进一步研究凝聚态高压新相的结构与性质提供新的实验数据。     

A study on wear and worn surfaces of grey cast iron affected by a novel silicate additive

A novel aluminium silicate hydroxide additive has shown excellent anti‐wear effect in practical applications and has been mainly studied on steels in laboratory conditions. In this paper, the pin‐on‐disk sliding wear tests were carried out to investigate the wear and worn surfaces of grey cast iron with additives in different concentration. It was found that the silicate additive showed an obvious anti‐wear effect and a reliable duration, reduced the number of pits and cracks on the worn surfaces and improved the nanohardness of the worn surfaces of grey cast iron disks by 72%. Furthermore, Raman spectroscopy displayed that the carbon structure of the worn surface of grey cast iron disks with the additive had an obvious transformation from nanocrystalline graphite to amorphous carbon. Copyright © 2015 John Wiley & Sons, Ltd.     

An inverse approach to determination of residual stresses induced by shot peening in round bars

In this paper an analysis is presented to determine the distribution of a residual stress field from a limited incomplete set of measurements obtained from shot-peened round steel bars. Using an Airy stress function as the primary unknown the axisymmetric stress equations are solved directly. In this new method there exists the flexibility to impose physical conditions that govern the behavior of residual stress to achieve a meaningful complete stress field. This new method aims to achieve the best agreement between the model prediction and limited measured stress components in the sense of least squares approximation. The power of this new method is demonstrated by analyzing experimental data and achieving a good agreement between the model prediction and the results obtained from residual stress measurement.     

On the effect of residual stresses in quasi-static cracking of materials

The quasi-static cracking technique has been extended to include fracture toughness measurement of materials with residual stresses. Theoretical analyses are presented which show that the effective fracture toughness and residual strain energy could be readily evaluated using this approach. Experiments on adhesive joints with prior-introduced residual stresses gave results in good agreement with theory. A simple graphical method to measure the genuine fracture toughness of materials with residual stresses is proposed and verified. Additionally, conditions for the fracture of slender beams glued to rigid substrates due to swelling were given and discussed.