Showing papers on "Tensile testing published in 2021"

Hierarchical crack buffering triples ductility in eutectic herringbone high-entropy alloys

Abstract: In human-made malleable materials, microdamage such as cracking usually limits material lifetime. Some biological composites, such as bone, have hierarchical microstructures that tolerate cracks but cannot withstand high elongation. We demonstrate a directionally solidified eutectic high-entropy alloy (EHEA) that successfully reconciles crack tolerance and high elongation. The solidified alloy has a hierarchically organized herringbone structure that enables bionic-inspired hierarchical crack buffering. This effect guides stable, persistent crystallographic nucleation and growth of multiple microcracks in abundant poor-deformability microstructures. Hierarchical buffering by adjacent dynamic strain-hardened features helps the cracks to avoid catastrophic growth and percolation. Our self-buffering herringbone material yields an ultrahigh uniform tensile elongation (~50%), three times that of conventional nonbuffering EHEAs, without sacrificing strength.

Bifunctional nanoprecipitates strengthen and ductilize a medium-entropy alloy.

Abstract: Single-phase high- and medium-entropy alloys with face-centred cubic (fcc) structure can exhibit high tensile ductility1,2 and excellent toughness2,3, but their room-temperature strengths are low1–3. Dislocation obstacles such as grain boundaries4, twin boundaries5, solute atoms6 and precipitates7–9 can increase strength. However, with few exceptions8–11, such obstacles tend to decrease ductility. Interestingly, precipitates can also hinder phase transformations12,13. Here, using a model, precipitate-strengthened, Fe–Ni–Al–Ti medium-entropy alloy, we demonstrate a strategy that combines these dual functions in a single alloy. The nanoprecipitates in our alloy, in addition to providing conventional strengthening of the matrix, also modulate its transformation from fcc-austenite to body-centred cubic (bcc) martensite, constraining it to remain as metastable fcc after quenching through the transformation temperature. During subsequent tensile testing, the matrix progressively transforms to bcc-martensite, enabling substantial increases in strength, work hardening and ductility. This use of nanoprecipitates exploits synergies between precipitation strengthening and transformation-induced plasticity, resulting in simultaneous enhancement of tensile strength and uniform elongation. Our findings demonstrate how synergistic deformation mechanisms can be deliberately activated, exactly when needed, by altering precipitate characteristics (such as size, spacing, and so on), along with the chemical driving force for phase transformation, to optimize strength and ductility. Increased strength and ductility in a medium-entropy alloy of Fe, Ni, Al and Ti is demonstrated using nanoprecipitates that simultaneously hinder phase transformation and block dislocation motion.

Microstructure evolution and mechanical properties of in-situ Ti6Al4V–TiB composites manufactured by selective laser melting

Abstract: In-situ TiB reinforced titanium matrix composites (TMCs) were fabricated by selective laser melting (SLM) of ball-milled Ti6Al4V–TiB2 powders. Optimized SLM processing and stress relief annealing were applied to obtain crack-free and fully dense composites. TiB reinforcement is mainly present in the form of whisker clusters and exhibits a quasi-continuous distribution in TMC1 (2 vol%TiB) while a full-continuous distribution in TMC2 (5 vol%TiB). The distribution of TiB whisker clusters in primary β-Ti grain is not consistent with the complete dissolution mechanism proposed previously. As a result, a dual mechanism of direct reaction and precipitation has been put forward to describe the formation of TiB phase. The microhardness, compressive strength and tensile strength of TMC1 are improved by 14%, 36%, 25% respectively, compared with those of Ti6Al4V alloy. These enhancements can be mainly attributed to Hall-Petch strengthening and load-bearing transformation strengthening. The fracture surface of TMC1 after tensile testing shows a mixture of regions of cleavage facets with regions of small dimples.

The influence of austenitization temperature on microstructural developments, mechanical properties, fracture mode and wear mechanism of Hadfield high manganese steel

Abstract: In this investigation, the effect of austenitization temperature on microstructural evolution, mechanical properties, fracture mode, and wear mechanism of a high carbon Hadfield steel was studied. Four blocks of the Hadfield steel were cast in an induction furnace. One hour austenitization was performed at 1000 °C, 1075 °C, 1150 °C, and 1225 °C on the cast samples followed by quenching in water. Uniaxial tensile test, pin on disk wear test and Vickers hardness measurements were employed on the processed samples. An optical microscope and a field emission scanning electron microscope were used to study the microstructural evolution. Transmission electron microscope was employed to observe the carbides that were formed. Moreover, scanning electron microscopy technique was used to define the mode of fracture on the tensile test samples. Results showed that increasing austenitization temperature reduced carbides and increased austenite grain size. Mechanical properties measurements also showed that increasing austenitization temperature increased yield/tensile strengths, hardness, and wear resistance of this steel. However, these increments were made at the expense of ductility. Fractography results showed a very ductile mode of fracture. The share of the ductile fracture mode was further increased by reducing the austenitization temperature from 1225 °C to 1000 °C.

Impact of process parameters of resistance spot welding on mechanical properties and micro hardness of stainless steel 304 weldments

Abstract: Resistance spot welding (RSW) is an essential process in the automobile sector to join the components. The steel is the principal material utilized in car generation because of its high obstruction against erosion, toughness, ease of support and its recuperation potential. Due to this, it was planned to study the mechanical properties, hardness and microstructure characteristics of RSW of Stainless steel 304.,In the present research, RSW of 304 stainless steel plates with 1 mm thickness and effect of current intensity, welding time, electrode pressure and holding time on nugget diameter, tensile strength microhardness and microstructure of the joints was investigated. The specimens were prepared according to the dimensions of 30 × 100 mm with 30 mm overlaps joint through the RSW machine. The tensile test of the specimen was carried out on a universal testing machine and microhardness of specimens measured using Vickers’s hardness tester. Taguchi L16 orthogonal array was used to scrutinize the significant parameters for each output.,It has been observed that the tensile strength of the specimen is affected by the current intensity and nugget diameter, and the weld time has a significant effect on the tensile strength. Microhardness is highly influenced by electrode pressure and holding time, as the increase in both these parameters resulted in the increase of microhardness. This is due to rapid cooling, which is done by the cooling water flowing through the copper electrodes.,This study was carried out using a copper electrode with a flat face with selected parameters and response factors. The study can be useful for researchers working on optimization of welding parameters on stainless steel.

Mechanical properties and self-healing capacity of ultra high performance Fibre Reinforced Concrete with alumina nano-fibres: Tailoring Ultra High Durability Concrete for aggressive exposure scenarios

Abstract: The effects of alumina nano-fibres are investigated in this paper on the mechanical performance of Ultra High Performance Fibre Reinforced Cementitious Concrete and their efficacy in enhancing the durability of the cementitious composite when exposed to extremely aggressive conditions, with main reference to the stimulated autogenous crack sealing and self-healing capacity. A tailored characterization of the flexural and tensile behaviour of the composite has been first of all performed, also with the purpose of validating an experimental and analytical approach for the identification of the tensile stress vs. strain/crack opening constitutive relationship, which makes use of a purposely conceived indirect tensile test methodology, called Double Edge Wedge Splitting test. Secondly the crack sealing and self-healing capacity have been investigated, considering the recovery of both mechanical flexural performance and durability properties (water permeability) and cross analysing the results for a thorough validation. Microstructural investigations have complemented the aforementioned experimental programme to confirm the efficacy of alumina nano-fibres in enhancing the durability performance of the investigated composites. Superior performance of the mix with alumina nano-fibres with respect to parent companion ones has been highlighted and explained through both a nano-scale reinforcing effects which helps in controlling the cracking process since its very onset as well as through their hydrophilic nature which is likely to foster cement and binder hydration reactions, which can usefully stimulate crack sealing and performance healing recovery at both the macroscopic and mesoscopic fibre-matrix interface) level.

Tensile strength evaluation of glass/jute fibers reinforced composites: An experimental and numerical approach

Abstract: Polymer-based composites have an exceptional perspective to replace traditional structural materials like steel and aluminium, owing to their low weight, high strength, and outstanding performance at elevated temperatures. However, the utilization of natural reinforcements for functional polymer composites is still in infancy. In this study, the tensile properties of natural and synthetic fiber-reinforced hybrid composites are reported. Glass-jute hybrid composites, prepared through hand layup technique, were used with different glass and jute fiber stacking sequences. The experimental results stipulate that the tensile properties of glass fiber reinforced polymer (GFRP) were merely affected at lower jute fiber concentration. The strength of composites consisting of single jute fabric lamina and four glass-fiber laminas were comparable with five-laminas GFRP composites. For validation of the experimental tensile testing results, a numerical simulation was also executed. Errors between experimental and numerical simulations were found for different stacking sequences due to non-uniformity in jute fiber diameter and the manufacturing process adopted for these hybrid composites. Fractographic analysis revealed the micro voids and adhesive failure at different joining layers of fibers as the primary cause of delamination.

Flammability, Tensile, and Morphological Properties of Oil Palm Empty Fruit Bunches Fiber/Pet Yarn-Reinforced Epoxy Fire Retardant Hybrid Polymer Composites.

Abstract: Oil palm empty fruit bunches (OPEFB) fiber is a natural fiber that possesses many advantages, such as biodegradability, eco-friendly, and renewable nature. The effect of the OPEFB fiber loading reinforced fire retardant epoxy composites on flammability and tensile properties of the polymer biocomposites were investigated. The tests were carried out with four parameters, which were specimen A (constant), specimen B (20% of fiber), specimen C (35% of fiber), and specimen D (50% of fiber). The PET yarn and magnesium hydroxide were used as the reinforcement material and fire retardant agent, respectively. The results were obtained from several tests, which were the horizontal burning test, tensile test, and scanning electron microscopy (SEM). The result for the burning test showed that specimen B exhibited better flammability properties, which had the lowest average burning rate (11.47 mm/min). From the tensile strength, specimen A revealed the highest value of 10.79 N/mm2. For the SEM morphological test, increasing defects on the surface ruptured were observed that resulted in decreased tensile properties of the composites. It can be summarized that the flammability and tensile properties of OPEFB fiber reinforced fire retardant epoxy composites were reduced when the fiber volume contents were increased at the optimal loading of 20%, with the values of 11.47 mm/min and 4.29 KPa, respectively.

Overcoming the strength-ductility trade-off of an aluminum matrix composite by novel core-shell structured reinforcing particulates

Abstract: The trade-off between strength and ductility of particulate reinforced metal matrix composites (PRMMCs) has been a longstanding puzzle. Here we propose an effective strategy to surmount the inverse relationship between strength and ductility of an A356 Al alloy based PRMMC by in situ synthesizing novel reinforcing particulates with a special core-shell (CS) structure. Such structure features a Ti core inside a dual-layer shell: the inner layer has a nano-grained (~130 nm) heterogeneous structure, and the outer layer possesses a composite structure composed of a (Al,Si)3Ti substrate with dense dispersion of nanoparticles. As a result, the obtained composite reinforced with such CS reinforcing particulates (CS composite) achieves an unprecedented tensile elongation to failure of 8.3 ± 0.8% and a uniform elongation of 7.1 ± 0.6%, which nearly triples that of the same alloy based composite reinforced with monolithic (Al,Si)3Ti particulates (monolithic composite) and equivalent to corresponding matrix alloy while maintaining high ultimate tensile strength of 373 ± 8.8 MPa and yield strength of 268 ± 7.9 MPa, equivalent to monolithic composite simultaneously. This special architecture of shell renders itself a high capability of stress bearing and good toughness, and the nanoparticles in outer layer further slower crack development, which significantly postpone crack formation in shell. Subsequent propagation of cracks in Ti core is also constrained remarkably by the transformation-induced plasticity effect occurred ahead of crack tips resulting from stress-induced phase transformation of hcp-Ti into fcc-Ti. These factors lead to highest work hardening rate that undergoes a long plateau and thus overcome the strength-ductility trade-off of A356 alloy based PRMMC.

Comparing the effect of continuous and pulsed current in the GTAW process of AISI 316L stainless steel welded joint: microstructural evolution, phase equilibrium, mechanical properties and fracture mode

Abstract: In this paper, the effect of continuous and pulsed current in the gas tungsten arc welding (GTAW) on the various properties of an AISI 316L stainless steel joints was investigated. 316L austenitic stainless steel sheets with a thickness of 10 mm were used together with ER309L filler. The sheets were welded together by the GTAW technique in two modes of continuous and pulsed currents. Microstructural characterization and phase equilibria were done using optical microscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with electron backscatter diffraction (EBSD) detector, techniques. Charpy impact, uniaxial tensile, and microhardness tests were used to investigate the effect of the type of the welding current on mechanical properties of the joints. The fracture surfaces were evaluated by FE-SEM after tensile and Charpy impact tests. Results showed that the weld metal (WM) microstructure is austenitic-ferritic (AF). It was also consisted of columnar and coaxial structures, in a way that varying the welding current from continuous to the pulsed mode changed the morphology of the grains from elongated columnar to a fine coaxial morphology. In addition, such a change in the welding current reduced the size of the grains in the WM, and the width of the unmixed zone (UMZ) as well. XRD analysis showed that the predominant phase and the preferred crystal plane of the WM are austenite, and (111), respectively. Both joints were broken from the base metal (BM) during the tensile test. Also, the above change of the welding current mode increased hardness and fracture toughness of the WM. Finally, fractography of the joints indicated that both joints experienced a completely ductile fracture.