Characterisation of fatigue fracture surfaces of friction stir channelling specimens tested at different temperatures

The fatigue fracture surfaces of friction stir channelling specimens tested at room temperature, 120 °C and 200 °C were observed in a scanning electron microscope (SEM) in order to analyse their morphology and the crack propagation mechanisms. Three different friction stir channelling conditions were tested and analysed. For all specimens tested the developing fatigue-crack has always initiated at the advancing side, namely on the boundary between the nugget and the thermo mechanically affected zone (TMAZ) into the interior of the specimen. The crack has propagated through the channel nugget with a path tangential to the advancing side. After the crack has reached the processed surface, a second crack initiated at the channel bottom. The fracture surfaces have shown a semi-elliptical shape crack front. This second crack has propagated uniformly through the base material. Fatigue crack propagation on the TMAZ was mainly characterised by fatigue striations. It was found, on most of the surfaces observed, a clear coexistence of the intergranular fracture mode and the transgranular fracture mode. A relationship between the fatigue testing temperature and the roughness of the fracture surfaces was found. The fracture surfaces roughness was considerably lower at a testing temperature of 200 °C for the three friction stir channelling conditions analysed.     

Geometrical analysis and pattern recognition using mapping technologies of three-dimensional fracture surfaces in materials

Geometrical analysis of fracture surfaces in materials was made using newly developed computer programs on the three-dimensional images reconstructed by the stereo matching method. The global value of the fractal dimension of the fracture surface was estimated by the box-counting method on a fatigue fracture surface of a Cu-Be alloy and impact fracture surfaces of a SiC and an alumina. The results of the present analysis were well correlated with those of the two-dimensional fractal analysis. The fractal dimension map (FDM) by the box-counting method and the surface roughness map (SRM) proposed in this study can give important information about the local fracture mechanisms, the crack growth direction or the fracture origin in materials. FDM and SRM have interesting characteristics by which one can discriminate the flat regions, the regions of complex geometry or the steeply inclined areas on a given fracture surface. Pattern recognition using mapping technologies of FDM and SRM is also applicable to the extraction of “hidden patterns” on fracture surfaces, which cannot be observed only by microscopes.     

Casting defects induced fatigue damage in aircraft frames of ZL205A aluminum alloy – A failure analysis

Two different types of aircraft frame components, which had collapsed respectively in their former vibration-fatigue performance tests, were submitted for failure analysis. The two failed frames were both made of aerial material ZL205A, a high-strength cast Al–Cu–Mn–Ti alloy. According to a series of experimental procedures including visual observations, X-ray detections, fractography inspections, microstructure examinations, mechanical tensile tests, hardness measurements and fluorescent penetrating inspections, it was indicated that the fracture was attributed to fatigue cracks which were induced by casting porosity defects at the external surfaces of frames. Numerous fine fatigue striations presented in the vicinity of casting porosities. Especially, it was observed of a special appearance of latitude–longitude crossed fatigue striations on the fracture surface due to the coupled stresses supplied by the former multi-directional vibration tests. The overload fast-rupture regions on fracture surfaces suggested the typical cleavage fracture mode, which was characterized by a number of river patterns and cleavage steps. The intergranular spatial dendrite-shaped casting porosities largely contributed to the local stress-concentrations in matrix materials. Triangular grain boundaries induced by the former casting burnt implied that the intergranular melting phenomenon had occurred. Furthermore, the effect of groove-shaped structure at roots of spatial convex-bodies on the edge of casting porosity was especially analyzed. And the influence of the casting porosity size on fatigue cracks was briefly discussed.     

Fatigue crack growth behaviour of A533B steel in pressurized water at 288 and 28 °C

Fatigue crack growth behaviour of A533B steel was investigated in pressurized water at 288 °C using specimens machined from four different orientations. When inclusions were oriented along the direction of crack propagation, fatigue crack growth rate (FCGR) was enhanced compared to when they were perpendicular to the direction of crack propagation. At low ΔK levels FCGR in ambient water was slightly higher than that in 288 °C water. This may be attributed to the occurrence of intergranular cracking in ambient water tested specimen. Though mainly ductile striations were observed on the fracture surfaces, isolated intergranular facets (in a specimen tested in ambient water) and fan shaped features were also present. Hydrogen induced damage was clearly evident in the ambient water tested specimen in the form of isolated intergranular facets.     

利用无线局域网遥测的3-D表面轮廓重构

提出一种基于IEEE820.11无线局域网(WLAN)遥测捕获图像的三维恢复算法,通过无线网络中的现场计算机获取远端物体双目图像,采用立体视觉法对双目图像的精确匹配,得到三维场景的精确重构恢复。实验结果表明,该算法能有效地实现双目图像匹配和恢复远端物体的三维表面信息。     

A fast pixel matching method based on phase feature extraction in online phase-measuring profilometry

A fast pixel matching method based on phase feature extraction in online phase-measuring profilometry is proposed, which is used to solve the problem where the object motion is along a straight line. The movement of the object results in the displacement of images in the captured deformed patterns, so pixel matching is carried out to modify the positions of images, which not only realizes the point-to-point correspondence of the object, but extracts the equivalent phase-shifting deformed patterns. So, the 3D surface shape of the object moving on the pipeline can be reconstructed successfully with an equivalent phase-shifting algorithm. The equiphase surface phase feature extracted from the phase distributions of the object obtained by Fourier transform profilometry is used to assist pixel matching in this paper. Normalization and further binarization of the phase feature extraction improve pixel matching’s accuracy and speed. The simulation and experiments verify the feasibility and validity of the proposed method.     

MIXED FATIGUE FRACTURE MORPHOLOGY OF FERRITIC DUCTILE IRON

Abstract— This paper presents the results of the fractographical investigation of ferritic ductile iron samples, which fractured under three-point pulsating bending at 140 Hz and R = 0.09. The fracture morphology was of mixed inter-transgranular character and the striations were observed on both trans-and intergranular facets. This allowed comparisons to be made between inter-and transgranular crack growth rates. The general conditions necessary for the creation of intergranular striations are formulated with respect to preceding sporadically published works.     

Role of Poisson’s ratio mismatch on the crack path in glass matrix particulate composites

The hydrogen-related fracture propagation process in martensitic steel was investigated through crystallographic orientation and fracture surface topography analyses. The hydrogen-related fracture surface consisted of three typical surfaces, namely smooth surfaces, surfaces with serrated markings, and surfaces with dimples. Crystallographic orientation analysis suggested that the smooth surface was generated by intergranular fracture at prior austenite grain boundaries, and the surface with serrated markings originated from quasi-cleavage fracture propagated along \(\{011\}\) planes. According to the reconstructed fracture propagation process by fracture surface topography analysis, the intergranular fracture at prior austenite grain boundaries initiated and propagated suddenly at the early stages of fracture. The quasi-cleavage fracture along \(\{011\}\) planes then gradually propagated within the prior austenite grains. At the final stages of fracture, ductile fracture accompanied by dimples occurred around the edge of the specimen. The results clearly indicated that the fracture propagation path changed with the proceeding fracture from the prior austenite grain boundaries to along \(\{011\}\) planes within the prior austenite grains.     

Intergranular and transgranular cracking of Cu–30Zn brass during fatigue loading

Abstract

Fatigue tests have been performed on annealed α–brass to examine the dependence of fracture morphology on ?K. The cracking was predominantly intergranular at low values of ?K_Q (the stress intensity factor range when the specimen does not comply with plane strain conditions) and changed progressively to transgranular cracking at high ?K_Q values. Detailed scanning electron microscopy has been performed on the fracture surfaces of the specimens, especially from matching areas on opposite faces. It has been shown that matching extrusion and intrusion pairs as well as one–to–one matching of fine slip lines occurred on the intergranular facets indicating that plastic deformation causes the intergranular cracking. Intergranular cracking persists at low ?K_Q values even though the crack growth rate is smaller than for transgranular cracking because the latter is difficult to initiate. Transgranular cracks form only at regions of localized strain, e.g. coarse slip bands, or at cold–worked surfaces but such transgranular cracking cannot be maintained at low ?KQ values.

MST/209     

An unusual failure of a nickel-aluminium bronze (NAB) hydraulic valve

Investigation of a hydraulic leak from a nickel-aluminium bronze (NAB) valve in a naval vessel revealed that two corners of a square flange had fractured in a macroscopically brittle fashion such that the valve was in three pieces – an unusual occurrence for a ductile material. Moreover, fracture surfaces were almost entirely intergranular, and there were no signs of dimpled fracture surfaces or shear lips typical of fast fracture in NAB. A few macroscopic progression markings were evident and, at high magnifications, intergranular facets exhibited slip lines and (probably) fatigue striations. Fracture surfaces produced by fatigue of specimens cut from the valve were similar to the in-service fractures for fatigue at very low (near-threshold) crack-growth rates, but not for fatigue at higher crack-growth rates. Data in the literature for NAB, especially in regard to previous failure analyses of NAB hydraulic couplings, indicated that environmentally assisted cracking modes such as stress-corrosion cracking were not involved. All the above (and other) observations indicated that failure of the valve occurred entirely by high-cycle fatigue, probably in the gigacycle regime, at very low stress amplitudes. The cyclic stresses are most probably associated with high-frequency vibration/resonance, but the source of the excitation causing this problem was not identified. High mean stresses, due to torquing bolts to specified levels on the square flange (which is free to deflect) probably facilitated fatigue crack initiation from machining grooves. Various recommendations were made in regard to preventing further failures.     

Failure Analysis of High-Speed Pinion Gear Shaft

The failure of a high-speed pinion gear shaft was investigated. The pinion gear was an integral part of a system used to compress the natural gas. It was a high-speed gear mounted on two roller bearings. An abnormal wear pattern was observed on the shaft surface, beneath the inner race of the bearing. The material from shaft was observed to be dislodged and stuck to the surface of the inner race. This transfer of material imparted an imbalance to the assembly, and abnormal sounds and fumes were observed two days before failure. The macrofeatures of the fracture surface resemble these of fatigue but electron microscopy showed brittle and mostly intergranular fracture. Fatigue features such as striations were not found on the fracture surfaces. Fatigue samples made from the same material and heat-treated to the same hardness were tested under uniaxial fatigue and the fracture features were compared with the original crack surface. The microfeatures of fracture surfaces were almost identical. The root cause of failure was determined to be fatigue, and cracks on the pinion shaft in the region beneath the inner bearing race lead to the transfer of material.     

Identification of modes of fracture in a 2618-T6 aluminum alloy using stereophotogrammetry

The identification and the development of a quantification technique of the modes of fracture in fatigue fracture surfaces of a 2618-T6 aluminum alloy were developed during this research. Fatigue tests at room and high temperature (230 °C) were carried out to be able to compare the microscopic fractographic features developed by this material under these testing conditions. The overall observations by scanning electron microscopy (SEM) of the fracture surfaces showed a mixture of transgranular and ductile intergranular fracture. The ductile intergranular fracture contribution appears to be more significant at room temperature than at 230 °C. A quantitative methodology was developed to identify and to measure the contribution of these microscopic fractographic features. The technique consisted of a combination of stereophotogrammetry and image analysis. Stereo-pairs were randomly taken along the crack paths and were then analyzed using the profile module of MeX software. The analysis involved the 3-D surface reconstruction, the trace of primary profile lines in both vertical and horizontal directions within the stereo-pair area, the measurements of the contribution of the modes of fracture in each profile, and finally, the calculation of the average contribution in each stereo-pair. The technique results confirmed a higher contribution of ductile intergranular fracture at room temperature than at 230 °C. Moreover, there was no indication of a direct relationship between this contribution and the strain amplitudes range applied during the fatigue testing.     

Secondary cracking process during fatigue crack propagation

Using the scanning electron microscope, fatigue striations in secondary microcracks normal to the main fatigue cracks have been observed on fracture surfaces of fatigue specimens made from 2024 and 2618 aluminium alloys, a Ti-6A1-V titanium alloy, and an Incolloy 901 nickel-base refractory alloy. Tentative explanations of these observations are presented in the paper together with a short discussion.     

EFFECT OF TEMPERATURE ON THE FATIGUE-CRACK PROPAGATION BEHAVIOUR OF INCONEL X-750

Abstract— Linear-elastic fracture mechanics techniques were used to characterize the effect of temperature on the fatigue-crack propagation behaviour of precipitation heat-treated. Inconel X-750 in an air environment over the temperature range 24 to 649°C. In general, crack growth rates were found to increase with increasing temperature, particularly at the highest test temperature (649°C). The effect of stress ratio on the fatigue-crack growth behaviour of Inconel X-750 was examined at 538°C, and results indicated that the elevated temperature fatigue response of this nickel-base superalloy was relatively insensitive to stress ratio level at the growth rate levels studied. Metallographic and electron fractographic examination of Inconel X-750 fatigue fracture surfaces revealed operative crack growth mechanisms to be a function of temperature and prevailing stress intensity factor. Under room temperature and intermediate temperature conditions (up to 538°C), all fatigue fracture surfaces exhibited a faceted crystallographic morphology at low crack growth rates followed by striations in the higher growth rate regime. At the highest test temperature (649°C), the fatigue crack was found to propagate by an intergranular mechanism.     

Fracture surface topography and fracture mechanism in austenitic SUS316 steel plates fatigued by repeated bending

Fatigue experiments were carried out using the austenitic SUS316 steel plates (the average grain diameter is about 1.3 × 10⁻⁵ m) by repeated bending. The three-dimensional fatigue fracture surfaces were then reconstructed using stereo pairs of scanning electron micrographs by the stereo matching method. Striations were observed on the stage II fatigue fracture surface, while fine slip steps were found on the stage I fatigue fracture surface. The averaged value of the fractal dimension of stage I fracture surface was about 2.2 and was almost the same as that of stage II fracture surface when the fractal dimension was measured in the length scale range smaller than about one grain-boundary length (about 8 × 10⁻⁶ m). This may be attributed to the fact that both fracture surfaces were formed by the same mechanism, namely, slipping-off. According to the two-dimensional fractal analysis, both stage I and stage II fatigue fracture surfaces did not exhibit anisotropy in the length scale range of the fractal analysis smaller than about one grain-boundary length in the SUS316 steel. The fractal dimension of the fatigue fracture surface increased with decreasing the magnification of images when the maximum length scale of the fractal analysis was extended to the size of analyzed area. Magnification dependence of the fractal dimension was associated with large steps and ledges, which were not “typical” fractals.     

Failure study of connecting shafts of a plug screw feeder in a paper production plant

The connecting shafts of two plug screw feeder units of a paper production plant failed within a relatively short period after the production start of the plant. To investigate the reasons for these failures, material characterizations of the broken shafts were performed. The program included tensile, impact and hardness tests and metallographical and fractographical investigations. Fracture surfaces of the failed shafts were studied using light and electron microscopy. Obvious striations and beach marks could be observed by fractographic examination. The fracture features observed on the fracture surfaces reveal that (a) the shafts had experienced fatigue crack growth before the final failure; (b) the crack growth initiated from a sharp undercut at the threaded part of the shaft; (c) the fatigue driving forces have combined torsional and bending loading, having torsional as the governing load.     

Shear and tensile plastic behavior of austenitic steel TRIP-120 compared with martensitic steel HSLA-100

The mechanical performance of TRIP-120 a novel transformation induced plasticity steel alloy, is evaluated for different loading cases and strain rates. The performance is compared with HSLA-100, a low-alloy steel developed by the United States Navy and currently used in naval hulls. The response of these materials under uniaxial tension and shear was investigated to the point of fracture at isothermal and adiabatic conditions. TRIP-120 shows a significant improvement in dissipated energy at fracture compared to HSLA-100. SEM images of ductile fracture surfaces for tensile and torsion samples of both TRIP-120 and HSLA-100 are compared, and the presence of transformed martensite in the TRIP-120 dynamic torsion specimens is confirmed with optical microscopy and magnetometry.     

不确定车数的随机车辆路径问题模型及算法

研究了一类有时间约束、车辆数量不确定的随机车辆路径问题;建立了该类问题的随机规划数学模型;设计了模型求解的遗传算法、禁忌搜索算法和遗传-禁忌混合算法.禁忌算法采用了对当前解的车辆-顾客分配结构和解的路径顺序分别禁忌的双层禁忌算法,使算法全局性更好,同时也降低了搜索时间.把禁忌算法作为变异算子应用于遗传算法形成了混合算法.最后给出了计算示例,对算法进行了比较分析.     

Schwingbruchstrukturen an faserverstärkten Kunststoffen

Structures of Fatigue Fractures in Fibre-Reinforced Plastics The report deals with macrofractographic and microfractographic structures in non-reinforced epoxide resins as well as in glass-fibre reinforced plastics and carbon-fibre reinforced plastics after fatigue stresses. Epoxide resins show fatigue striations and fracture lines similar to those in metals. The distance between the fatigue striations amounts to between 0.70 and 150 m?m and increases continuously in the direction of fracture propagation. The instantaneous fractures in non-reinforced resin show parabolic cusps besides Wallner-lines and lance-shaped structures. Surfaces of delamination fractures of glass-fibre reinforced plastics show fatigue striations which are characteristic for fibre-reinforced composites, but their morphology differs from the fatigue striations in non-reinforced resin. This was the first time that fatigue striations could be discovered in fibre-reinforced composites of a fibre content of 70 per cent by volume. These fatigue striations are strongly marked at the positions of the fibres and will probably arise only in case of a sufficiently high local stress intensity. Moreover, the amounts of the local slicing and shearing stress components, of which is composed the stress causing the fracture, could be partly responsible for the formation of fatigue striations. The formation of the fatigue striations allows to draw conclusions with respect to the fracture-causing local stress distribution and the local crack propagation in fibre-reinforced plastics. The surfaces of instantaneous fractures in glass-fibre reinforced plastics and carbon-fibre reinforced plastics show parabolic cusps. This is another distinguishing characteristic as compared with the surfaces of fatigue fractures.     

Affine invariant matching of broken boundaries based on an enhanced genetic algorithm and distance transform

Past research work has shown that the process of shape matching can be rendered into an optimisation problem that determines, based on evolutionary algorithms, the best matching score between pairs of object boundaries. This important finding has enabled near planar objects to be identified efficiently when they are captured under different camera viewpoints. Among other evolutionary techniques, the genetic algorithm (GA) has demonstrated its feasibility in matching silhouette images of objects that are captured under a wellcontrolled environment. As the latter is not guaranteed in practice, the method has also been extended to match fragmented and incomplete contours. Despite the moderate success achieved, the overall performance is rather inconsistent and also varies significantly among different geometries. To overcome this problem, two variants of a novel approach based on the integration of a simple GA, the distance transform and the migrant principle are developed and presented. Experimental results reveal that the proposed methods are capable of matching incomplete and broken contours with a high success rate and exhibit good stability in performance.