The influence of different welding speeds and rotary speeds on the formation and mechanical properties of friction stir weld joints of armor grade aluminum alloy was presented. The developed weld ...joints were characterized by bend tests, micro-hardness tests, tensile tests, optical and scanning electron microscopies. Mechanical properties (i.e., micro-hardness, ultimate tensile strength and elongation to fracture) increased with the increase in rotary speed or decrease in welding speed. The effect of welding speed on micro-hardness of heat affected zones was more profound than the rotary speeds. The welding speeds and rotary speeds influenced the mechanical properties and their effects on various mechanical properties of the friction stir welded joints can be predicted with the help of regression models.
This study aimed to compare the four-point flexural strength of CAM-milled and sintered (as-sintered, AS) specimens with those of high-polished (HP) specimens using chairside polishing systems to ...simulate clinical surface conditions.
Seven full-contour zirconia CAM/CAM blanks with various yttria contents (3, 4, 5 mol%) including three high-translucent groups (5Y) were selected to prepare flexural specimens. The bend bar specimens (2.0 × 4.0 × 25.0 mm3) were fabricated by using STL file and dental CAM machine with the respective zirconia blanks (98 mm ϕ and 10–14 mm in height). Twelve bar specimens were machined from one zirconia puck and a total of 24 specimens were prepared from each group. The pre-sintered bar specimens were sintered by using a dental zirconia furnace at 1530–1550 °C for 2 h according to the instructions. All sintered specimens were divided into two groups: as-sintered (AS) group and high-polished (HP) groups (n = 12). HP groups were subjected to polishing one surface of specimens using a three-step polishing system and finally finished with diamond polishing. After cleaning and drying, the flexural strength of all specimens was determined by a fully articulating four-point flexure fixture consisting of a 1/4-point test configuration with an inner/outer span of 10/20 mm. Statistical differences between AS and HP groups were conducted with Weibull analysis. The fractured surfaces of zirconia specimens were observed using a field emission SEM and EDS to detect failure origins.
The mean AS flexural strength values were significantly lower than those of HP counterparts. However, Weibull moduli expressing the reliability of HP groups were generally decreased although not significantly in comparison to their AS. The fracture of the AS specimens mostly originated from extrinsic CAM-milling defects, while the HP specimens were fractured from intrinsic subsurface or volume defects including pores, large grain clusters, inclusions, and corner-located critical flaws. Two high-translucent (5Y) zirconia groups were not affected in their strength and reliability after polishing, whereas one 5Y zirconia significantly increased its strength but significantly lowered its reliability.
The extrinsic and intrinsic strength-limiting defects should be considered in evaluating the flexural strength and reliability of dental CAD/CAM zirconia ceramics for full-contour restorations. For the materials tested in this study, more optimized processing of blanks and milling protocols of pre-sintered zirconia blanks should be developed including post-sintering surface finishing to reduce the flaw population regulating strength and reliability which will affect the survivability of dental zirconia prostheses.
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The investigation results of the thermal shock resistance of the TiC-VC/NbC/WC-Ni-Cr alloys, produced by the powder metallurgy method using original nano and fine grained powders of tungsten carbide ...and nickel at different heating temperatures under thermocycle loading are presented. The number of heating-cooling cycles ranging from 600, 700, and 800°C till the room temperature before the microcracks initiation on the surface of the sample was determined. It was found that alloys obtained using nano powders possess higher thermal shock resistance and higher fracture threshold than those compared with fine grained alloys. The alloys fracture macro- and microstructures with nano additives contain a greater of ductile fracture elements than those compared with fine grained ones.
This paper dealt with the combined processing of roll-casting and hot-rolling to fabricate high-performance Cu/Al8011/Al5052 composite with ultrathin copper thickness. It was shown that the ...intermetallic compounds formed at the Cu/Al interface include Al4Cu9, AlCu, and Al2Cu from the copper side, respectively, in which the CuAl phase has the lowest thickness due to its the high diffusion activation energy. Also, Al2Cu, due to having the lowest amount of Gibbs free energy, was initially formed at the interface, followed by the Al4Cu9 and AlCu phases. Although the room-temperature tensile strength of the trilayered composite is close to each other in both directions, the elongation to failure in the rolling direction is about 37% better than that of the transversal direction. Furthermore, no clear slip bands were observed in the uniform plastic deformation zone of the stress-strain curve during the high-temperature tensile tests compared to the room-temperature counterparts. Despite the decrease in tensile strength with increasing test temperature, this composite shows good tensile behavior at temperatures below 200 °C due to the pure shear fracture mode.
•Cu/Al8011/Al5052 composite with ultrathin copper thickness was fabricated by roll-casting and hot-rolling.•Both interfaces of the manufactured composite were metallurgically well-bonded.•Copper is the first layer to fracture during tensile deformation.•The uniform interface of Cu and Al8011 is split into several parts in the trilayered composite.
•The fracture cause of a fuel supply tube of an aircraft engine was determined.•This fracture was produced by a fatigue mechanism.•The material, a stainless steel X6CrNiTi18-10, was microstructurally ...characterized.•The origin was located in defects in the outer wall of the tube.
Multiple fracture cases of fuel control pressure tubes from aircraft engines have been reported. The aim of the present study is to determine the cause of this tube fracture. The studied set was composed by the mentioned tube, a welded connecting pipe, where the fracture has been produced, and a union nut. The fracture has been produced in one most critical zones of the tube, in a region next to the supporting body of the union nut to the connector.
A visual examination and with a stereo microscope of the tube fracture surface has been carried out in order to determine the macrofractographic features. The results revealed a plastic macrodeformation of the tube, a damaged surface and signs of a possible corrosion process.
The material has been chemical, mechanical and microstructural characterized. The results confirmed that it was within specifications. Fracture surface was also inspected by scanning electron microscopy to determine the microfractographic characteristics in order to find out the failure mechanism involved in the fracture. Fatigue striations, which are typical from a progressive fracture by a fatigue mechanism, have been observed. The origin of the fracture has been placed in defects located in the outer wall of the tube, leading to a final overload fracture.
•Fractal dimension for fatigued samples of aluminium and steel alloys was investigated.•Correlation between fractal dimension and loading type was studied via entire fracture area method.•Average ...values of the fractal dimension exhibited fixed behaviour regardless of the material type.•Fractal dimension based on extreme values effectively correlated data using linear functions.
Fracture surfaces after biaxial fatigue tests were compared using fractal dimension for three types of metallic materials in smooth and notched specimens made of S355J2 and 10HNAP steels and 2017-T4 aluminium alloy, considering both proportional and nonproportional cyclic loading. High-resolution optical 3D measurement studies were performed on the entire fracture surface. A direct correlation between fractal dimension and fatigue loading was established. This systematic relationship can serve as a basis for obtaining information about fatigue loading from the fracture surfaces of failed materials and structures. Moreover, measurements of the fracture surface with an optical profilometer, quantitative analysis, and fractography contribute to a better comprehension of the fatigue failure processes. Differences in individual zones of fatigue fractures were identified while demonstrating the correctness of the total fracture surface method.
The effects of cold expansion on fatigue crack growth from fastener holes in aircraft structure are a well‐studied problem, with many contributions focusing on both residual stresses and “long crack” ...experimental data. In this study, the authors have used quantitative fractography techniques to measure fatigue crack growth from naturally occurring discontinuities on an engineering “crack initiation” scale. Fatigue crack growth rates were measured through the full range of crack depths, allowing the effects of beneficial residual stresses to be directly observed. The almost identical early crack growth rates in cold expanded and non‐cold expanded specimens were demonstrated up to crack depths of approximately 0.1–0.2 mm. Beyond this depth, fatigue crack growth in cold expanded specimens was markedly slowed to the point at which failure ultimately arose from cracks away from, or growing into, the fastener hole. The available life improvement factor for cold expansion is therefore maximized through a crack growth‐based lifing approach.
Highlights
Sub‐millimeter fatigue crack growth rates from cold‐expanded fastener holes were measured.
Service lives for cold‐expanded holes under crack initiation and durability crack growth fatigue criteria were compared.
Fatigue crack growth rate data from natural cracks suitable for correlation of analytical models is presented.
The microstructure and fatigue performance of 24CrNiMo low alloy steel by selective laser melting (SLM) were investigated through SEM, EBSD and fatigue tester. The results indicated that the SLM ...sample with relatively high fatigue strength (580 MPa) was obtained. The excellent fatigue behavior was mainly contributed to the excellent combination of strength and plasticity as indicated by EBSD analysis showing fine grain sizes, high kernel average misorientation (KAM) and Schmid factor (SF) values. Both manufacturing defects (e.g., lack of fusion, pore) and the matrix can act as crack initiation sites, meanwhile, the former can be divided into surface and interior defects considering the defect location at the origin of fatigue micro-cracks. According to the scatter degree of the fatigue data in the S–N curve, the difference in the location of original cracks showed a greater impact on the fatigue lifetime than that in the type of original cracks. The fracture mechanism of the SLM sample evolved from brittle fracture in the fatigue crack propagation zone to ductile fracture in the instantaneous fracture zone.
These past decades, the ability of thermoplastic composites to be welded has been of great interest, with numerous studies investigating the process and mechanical strength of joints for quasi-static ...loadings. The present study aims to determine the loading speed influence on the behaviour of these structures, as crashworthiness is of tremendous interest in the transport industry for safety issues. The polyamide-based composite used in this work is sensitive to strain rate, which may influence the interface and the entire structure's behaviour. Tensile tests were performed on single lap joint specimens for loading speeds from 1.2 mm min−1 to 7 m s−1, considering several substrate orientations. Loading speed influences the lap shear strength for the different configurations: increasing loading speed induces a performance enhancement of 11.2%–22.9%. This phenomenon is caused by the matrix strain rate sensitivity, which stiffens and strengthens the substrate shear behaviour and the welded joint behaviour. Finally, the fractography analysis confirms that fracture mechanisms observed are similar for each loading speed tested with primarily fibre/matrix debonding. Therefore, improvement in the welded structures' performance is due to the mechanical performance of its constituents and their influence on the interfacial strength.
•Single lap shear tests conducted up to 7 m s−1 for three cross-ply configurations.•Lap shear strength enhances with the increase in loading speed.•Stiffening of 0/45S/454 and 454/454 with the increase in loading speed.•Cohesive fracture leads to high performances of ultrasonically welded joints.
This study covers a low-velocity impact property analysis of Polyethylene self-reinforced composites (PET-HDPE/HDPE SRC)through experimental and numerical methods emphasizing the influence of ...interfacial properties. Self-reinforcing a polymer material has gained a lot of attention due to maximum exploration of their light density characteristics with better chemical compatibility and interfacial adhesion of matrix and reinforcement, resulting in higher fuel efficiency. Reusability of self-reinforced thermoplastic composites even without separating their constituent materials has been observed to be aiding the circular economy strategies. The study aimed to analyze the peak load characteristics, energy-absorbing characteristics, and damage response of these materials processed through the hot compaction technique and subjected to a drop mass impact test. Fractographic visual inspection, optical macrographs, Scanning electron microscopy analysis, and through transmission ultrasonic C-scan were employed to study the damage evolution and failure mechanisms under the low-velocity impact. Three levels of impact energy were included in the test matrix, and initiation of the damage, nature of the matrix, interface debonding, matrix and fibre failures could be identified through the inspection of the damage cross-section at the impact eye. A finite element model developed through commercial FEA packages (Abaqus) was utilized for the complex mechanisms of damage and failure. Weak interfacial adhesion due to the self-lubricating nature of the polyethylene fibre surface contributed to the fibre pull out during the drop weight impact. A qualitative analysis and correlation of the nature of surface damage and internal damage were also studied, which are the factors that are necessary for the development of cargo containers and marine/aerospace ultralight luggage applications.
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