Showing papers on "Fractography published in 2020"

Anisotropic mechanical properties and deformation behavior of low-carbon high-strength steel component fabricated by wire and arc additive manufacturing

Abstract: Wire and arc additive manufacturing (WAAM) is an efficient technique for fabricating large and complex components that are applied in the manufacturing industry. In this study, anisotropic mechanical properties of a low-carbon high-strength steel component fabricated by WAAM were investigated via mechanical testing, and the transversal and longitudinal deformation behavior of the component were studied using the digital image correlation (DIC) method. Additionally, the features of microstructure, texture, and fracture mode of the inter-layer area and deposited area were also investigated to reveal the mechanism of anisotropy. The results showed the mechanical properties of longitudinal specimens were inferior to that of the transversal specimens. Several strain concentration zones in the longitudinal specimen were relevant to the inter-layer characteristics observed from the fracture surface and macrostructure, which was confirmed by the strain evolution recorded by DIC. The inter-layer areas were proved to be the weak link in the deposited component by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) analysis results, including various phase composition, phase morphology, misorientation angle, grain size, Schmid factor, and texture. Finally, based on the fractography analysis, anisotropy resulted from inter-layer zones is also confirmed via the comparison of transversal and longitudinal fracture morphology.

An experimental analysis of tensile, hardness and wear properties of aluminium metal matrix composite through stir casting process

Abstract: Among several methods to fabricate Aluminium metal matrix composites, stir-casting technique is used for large-scale production due to its being less expensive. This research is concerned with the fabrication of Aluminium composites with SiC and Jute Ash Particles, for sake of producing a cost-effective composite with enhanced properties. Wear behaviour was studied using pin on disc tribometer and it was found that wear resistance increases with-addition of reinforcement particles. Fabricated composite samples showed almost 4 times better wear resistance compared to base material. Matrix metal reinforced with SiC showed the best wear resistance of all fabricated samples. Coefficient of friction significantly decreased with the presence of reinforcement due to formation of mechanical mixed layer. Tensile Strength was escalated with the introduction of reinforcement as load is transferred to strongly bonded reinforcement particles. Microhardness was enhanced with the introduction of reinforcement and Sample 2 with SiC as reinforcement showed the best microhardness. Microstructure and fractography of the base metal and fabricated composites was analyzed by Scanning Electron Microscope (SEM).

Mechanical properties of Invar 36 alloy additively manufactured by selective laser melting

Abstract: Invar 36 alloy is widely used in aerospace engineering, owing to its extremely low coefficient of thermal expansion. This work aims to explore the underlying influence of process parameters on the microstructures and mechanical properties of Invar 36 additively manufactured by selective laser melting (SLM) with island scanning strategy. A full factorial design for a wide range of SLM process parameter sequence was established. Density and the mechanical properties including hardness, tensile strength as well as microstructures, fractography were characterized. The results confirm that the increment of exposure time and the reduction of point distance remarkably induce stable melting and achieve superior density and hardness. The laser energy density shows a pronounced effect on the microstructures, as extremely low laser energy density induces considerable lack-of fusion pores, unmelted powder particles and blurry boundaries of islands. While, very high laser energy density induces a few keyhole pores, and makes the laser scanning tracks and island boundaries transfer to be irregular. The optimal laser energy density is recommended to be 99.2 J/mm3, and the as-fabricated Invar 36 shows the relative density of 99.5%, Vickers hardness of 1.8 GPa and ultimate tensile strength of 480 MPa those are very comparable to conventionally fabricated one. The testing temperatures of 200 °C and 600 °C induce the coarsening of γ phase, and result in a significant degeneration of tensile properties. The correlations between the SLM process and mechanical properties provide experimental basis of the additive manufacturing Invar 36 alloy.

Mechanical behavior and fractography of graphite and boron carbide particulates reinforced A356 alloy hybrid metal matrix composites

Abstract: The present work shows the mechanical behavior of as-cast A356 alloy and A356-4 wt% graphite (Gr)-12 wt% boron carbide (B4C) hybrid composites. The hybrid composite samples were developed by two stage stir casting technique. The developed hybrid composites were subjected to scanning electron microscope (SEM) and energy dispersive spectroscope to analyze its microstructure followed by physical and mechanical tests like density, hardness, and tensile tests. The SEM analysis showed almost uniform distribution of particulates; also, hardness and tensile properties improved upon addition of reinforcement viz., graphite and B4C. The density of hybrid composite decreased upon addition of reinforcement when compared with base A356 alloy. Fractography of tensile specimens were carried out for analysis of fractured surfaces.

Understanding interlaminar toughening of unidirectional CFRP laminates with carbon nanotube veils

Abstract: The introduction of nanostructured interlayers is one of the most promising strategies for interlaminar reinforcement in structural composites. In this work, we study the failure mechanism and interlayer microstructure of aerospace-grade structural composites reinforced with thin veils of carbon nanotube produced using an industrialised spinning process. Samples of unidirectional carbon fiber/epoxy matrix composites interleaved with different composition CNT veils were prepared using hot press method and tested for interlaminar fracture toughness (IFT), measured in Mode-I (opening) and Mode-II (in-plane shear), and for interlaminar shear strength (ILSS), evaluated by the short beam shear (SBS) test. The crack propagation mode could be directly determined through fractography analysis by electron microscopy and resin/CNT spatial discrimination by Raman spectroscopy, showing a clear correlation between interlaminar reinforcement and the balance between cohesive/adhesive failure mode at the interlayer region. Composites with full resin infiltration of the CNT veils give a large increase of Mode II IFT (88%) to 1500 J/m2 and a slight enhancement of apparent interlaminar shear strength (6.5%), but a decrease of Mode I IFT (−21%). A toughening factor, defined as the relative increase in toughness per interleaf-to-ply thickness ratio, give a record-high value of 15.8 for Mode II IFT. The results help establish the role of interlayer infiltration, interlaminar crossings and formation of a carbon fiber bridgings, for interlaminar reinforcement with interleaves.

Experimental and numerical study of failure behavior and mechanism of coal under dynamic compressive loads

Abstract: A comprehensive understanding of the failure behavior and mechanism of coal is a prerequisite for dealing with dynamic problems in mining space. In this study, the failure behavior and mechanism of coal under uniaxial dynamic compressive loads were experimentally and numerically investigated. The experiments were conducted using a split Hopkinson pressure bar (SHPB) system. The results indicated that the typical failure of coal is lateral and axial at lower loading rates and totally smashed at higher loading rates. The further fractography analysis of lateral and axial fracture fragments indicated that the coal failure under dynamic compressive load is caused by tensile brittle fracture. In addition, the typical failure modes of coal under dynamic load were numerically reproduced. The numerical results indicated that the axial fracture is caused directly by the incident compressive stress wave and the lateral fracture is caused by the tensile stress wave reflected from the interface between coal specimen and transmitted bar. Potential application was further conducted to interpret dynamic problems in underground coal mine and it manifested that the lateral and axial fractures of coal constitute the parallel cracks in the coal mass under roof fall and blasting in mining space.

Microstructural analysis and fatigue fracture behavior of rail steel

Abstract: The purpose of this paper is an experimental study on fatigue fracture performance of rail steel which is used in Iran railway. For this purpose, the fatigue, fracture toughness and fatigue...

Effect of thermomechanical processing on fatigue behavior in solid-state additive manufacturing of Al-Mg-Si alloy

Abstract: This work presents, for the first time, an in-depth investigation of the structure–property–fatigue relationships of an Al-Mg-Si alloy (AA6061) processed via additive friction stir-deposition (AFS-D). As industry focus continues to shift for more efficient and lightweight structures, quantitative studies on the cyclic performance of additively manufactured materials are needed. In this study, the AFS-D processed AA6061-T6 was machined into specimens in two orthogonal orientations and subjected to monotonic and strain-controlled fatigue testing. The microstructural features of as-deposited AA6061 exhibited evidence of dynamic recrystallization and grain refinement. In addition, significant reduction in the intermetallic particles was observed after AFS-D processing. The fatigue results demonstrate that the as-deposited material, particularly the longitudinal direction, exhibited similar fatigue performance to wrought AA6061-T6 in both low-cycle and high-cycle fatigue regimes, which is a promising result for additively manufactured material in the as-deposited condition. By contrast, the as-deposited build direction orientation possessed slightly lower fatigue resistance than the wrought feedstock material. The AFS-D material was observed to exhibit different damage mechanisms from porosity-based damage mechanisms observed in fusion-based additively manufactured materials. Lastly, a microstructure-sensitive fatigue model was employed to capture the fatigue effects of the AFS-D processing on the AA6061.

Effect of interfacial intermetallic compounds evolution on the mechanical response and fracture of layered Ti/Cu/Ti clad materials

Abstract: Three-layered Ti/Cu/Ti clad materials fabricated by cold roll-bonding process and in the following underwent heat treatment cycles. Microstructure evolution and mechanical properties of as-rolled and annealed samples studied by SEM, XRD, microhardness and tensile examinations. According to the results, as-rolled and treated samples contained several IMCs at their Ti/Cu interfaces while thickness of the reaction layer enhanced noticeably by annealing. Examined average IMC layers' thickness were 1.34, 1.61, 2.39 and 5.39 μm, for as-rolled, 300, 500 and 700 °C annealed specimens, respectively, consisting four different phases of CuTi2, CuTi, Cu3Ti2, and Cu4Ti. However, continuity, uniformity and thickness variation as well as the growth rate during treating for each layer was distinct. From SEM and XRD results that acquired from separated surfaces, major debonding occurred at CuTi2/CuTi and CuTi/Cu3Ti2 interfaces suggesting that bonding strength for these IMC layers were weaker than those for the other IMC/IMC interfaces and the parent-metal/IMCs. The stress-strain curves demonstrated that the strength decreased and subsequently ductility improved by annealing up to 700 °C. Meanwhile, SEM analyses revealed that cracks within IMC layers during tensile deformation emerged only in specimen annealed at 700 °C; however, it did not lead debonding or layer separation similar to other clad metals reported in literature. By fractography, it is indicated that dimples formed in the whole fracture surface of as-rolled and annealed specimens at 300 and 500 °C. For the 700 °C annealed specimen, however fracture mode was ductile, dimples hardly noticed since the cracks propagation and extension path was along the weak IMC interfaces. In addition, this sample unlike others exhibited limited necking extension before failure. According to the obtained results and as the current research's novelty, it is claimable that intermetallics formed in Ti/Cu clad materials would be more ductile comparing with other compounds reported for talented cladding metals in literature.

Evaluation of grit-blasting as a pre-treatment for carbon-fibre thermoplastic composite to aluminium bonded joints tested at static and dynamic loading rates

Abstract: Light-weighting of transportation structures necessitates multi-material design employing composites and aluminium, with thermoplastic composites being of increasing interest to the industry. Adhesive bonding is a viable solution for joining dissimilar materials, but joint performance can be considerably affected by surface preparation. In this paper, alumina grit-blasting is investigated as a surface preparation technique for thermoplastic-matrix composites to be bonded to aluminium alloys. Grit-blasting is performed on composite adherends for varying durations, and the resulting chemical and morphological modifications are analysed using goniometry, profilometry, scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Adhesively-bonded single-lap joints are tested at quasi-static and dynamic (0.5 m/s) loading rates, and fractography analysis is performed at macro and micro scales. It is found that high lap shear strength and work-to-failure can be achieved through optimisation of the grit-blasting parameters. The optimised process produces a composite surface with plasticised matrix, minimal fibre exposure, and favourable surface chemistry for adhesive bonding. Grit-blasting can thus be a simple, yet effective surface preparation technique for composites to be bonded to aluminium.

Vat Polymerization-Printed Partially Stabilized Zirconia: Mechanical Properties, Reliability and Structural defects.

Abstract: Additive manufacturing (AM) of ceramics, particularly of zirconia, is becoming of increasing interest due to the substantial freedom available in the design and fabrication process. However, due to the novelty of the field and the challenges associated with printing dense bulk ceramics suitable for structural applications, thorough investigations that explore the effects of printing on the mechanical performance are limited. Previous work has identified anisotropy in the mechanical properties and attributed it to the layer-by-layer deposition. However, substantiated fractographic evidence detailing the origins and effects of layer lines on the probability of failure are limited. This study investigates the mechanical properties of a dense (>99 %TD), partially stabilized zirconia fabricated by a digital light projection printing method following ASTM standards. Hardness and strength evaluations were conducted, followed by a Weibull analysis and fractography. The investigation entailed five unique build directions and a conventionally manufactured reference material that was used as a control. Although the strengths were comparable to the reference material for some orientations, fracture frequently initiated at layer lines and related defects in all orientations. The findings indicate that if the layer lines can be prevented or engineered, the strength of vat printed ceramics can be improved substantially.

Influence of processing temperature on formability of thin-rolled DP590 steel sheet

Abstract: DP590 steel is used as an alternative material in automobile applications due to its high strength and considerably good other mechanical properties. However, major concern of DP590 steel forming i...

Fatigue assessment of as-built and heat-treated Inconel 718 specimens produced by additive manufacturing including notch effects

Abstract: The fatigue behaviour of notched and unnotched specimens produced by additively manufactured Inconel 718 are analysed in the as-built and heat-treated conditions. The surfaces display high roughness and defects acting as fatigue initiation sites. In the as-built condition, fine sub-grains were found, while in the heat-treated state, the sub-grains were removed and the dislocation density recovered. SN-curves are predicted based on tensile properties, hardness and defects obtained by fractography, using the √area-method.

Effect of low-temperature surface hardening by carburization on the fatigue behavior of AISI 316L austenitic stainless steel

Abstract: The influence of low-temperature gaseous carburization on the fatigue behavior of AISI 316 L austenitic stainless steel was investigated. Tension-compression axial fatigue tests were performed under ambient conditions on untreated and carburized AISI 316 L. The results show that the carburized AISI 316 L has a 22% higher endurance limit compared to untreated AISI 316 L. Fractography investigations with scanning electron microscope (SEM) reveal that for the untreated AISI 316 L fatigue cracks initiate at the surface regardless of the applied stress level. For the carburized AISI 316 L fatigue cracks initiate at the surface for relatively high-level stresses; for relatively low-level stresses fatigue cracks initiate at inclusions beyond the carburized case. After carburization, the ductility in the outmost 10 μm of the carburized case has significantly reduced, leading to micro-crack occurrence during fatigue tests and associated relaxation of compressive residual stress in this region. Beyond this surface-adjacent region, no evident stress relaxation occurs due to the enhanced yield strength of the carburized case. The enhanced fatigue performance is mainly ascribed to the compressive residual compressive stress profile introduced by the carbon-concentration profile over the case. Moreover, solid solution strengthening by interstitially dissolved carbon contributes to improve the fatigue performance.

Microstructure and mechanical properties of rene 41 alloy manufactured by laser powder bed fusion

Abstract: Precipitation hardenable Ni-based superalloy Rene 41, considered to have moderate weldability, was produced by laser powder bed fusion (LPBF). Crack-free and dense parts were obtained directly from processing. Parts were characterized in both as-built condition and after aging heat treatment. In the as-deposited condition, scanning electron microscopy and electron back scattered diffraction analysis revealed a fine columnar dendritic microstructure with a preferred orientation along . Transmission electron microscopy analysis revealed the absence of γ′ precipitates in the as fabricated state. After heat treatment, the columnar grain morphology was maintained and formation of distinct carbide particles at the grain and cell boundaries were observed along with the precipitation of fine and homogeneously distributed γ’. The size of these γ′ precipitates were measured as 8–30 nm which is smaller compared to the reported values in the literature. Significant increase in the microhardness value from 347 HV to 548 HV is observed after the heat treatment. Increased room temperature yield and ultimate tensile strength values reaching 857 and 1165 MPa were measured for the samples built parallel to the base plate in the as built state. These values increased to 1263 and 1580 MPa after the heat treatment, ascribed to the presence of fine γ’ precipitates. Fractography analysis revealed that failure occurred in a ductile manner for both as-fabricated and heat-treated samples.

Fracture Toughness Analysis of Epoxy-Recycled Rubber-Based Composite Reinforced with Graphene Nanoplatelets for Structural Applications in Automotive and Aeronautics

Abstract: This study proposes a new design of lightweight and cost-efficient composite materials for the aeronautic industry utilizing recycled fresh scrap rubber, epoxy resin, and graphene nanoplatelets (GnPs). After manufacturing the composites, their bending strength and fracture characteristics were investigated by three-point bending (3PB) tests. Halpin-Tsai homogenization adapted to composites containing GnPs was used to estimate the moduli of the composites, and satisfactory agreement with the 3PB test results was observed. In addition, 3PB tests were simulated by finite element method incorporating the Halpin-Tsai homogenization, and the resulting stress-strain curves were compared with the experimental results. Mechanical test results showed that the reinforcement with GnPs generally increased the modulus of elasticity as well as the fracture toughness of these novel composites. Toughening mechanisms were evaluated by SEM fractography. The typical toughening mechanisms observed were crack deflection and cavity formation. Considering the advantageous effects of GnPs on these novel composites and cost efficiency gained by the use of recycled rubber, these composites have the potential to be used to manufacture various components in the automotive and aeronautic industries as well as smart building materials in civil engineering applications.

Tension and fatigue behavior of Al-2124A/SiC-particulate metal matrix composites

Abstract: The tension and fatigue properties of SupremEX® 225XE composites (Al-2124A/25%/SiCp/3 μm) were determined for extruded samples tested in the longitudinal orientation in the T4 condition. Tension testing was conducted at 0.001/sec on as-machined cylindrical samples tested with high alignment fixtures according to ASTM E−8. The fatigue tests were conducted on polished hourglass fatigue samples with stress ratio R = 0.1 and test frequency of 20 Hz using hydraulic grips on a Model 810 MTS servo-hydraulic testing machine according to ASTM E466-15. A total of 26 samples were tested with stress levels between 448 MPa and 557 MPa. The resulting stress vs. cycles to failure (i.e. S-N) curve was compared to the unreinforced matrix alloy, other unreinforced high strength aerospace aluminum alloys, as well as other conventionally processed MMCs. The SupremEX® 225XE MMC exhibited improvements to both the low cycle fatigue (LCF) and high cycle fatigue (HCF) performance compared to these materials. The fatigue strength at 107 cycles was 448 MPa, well in excess of those for monolithic aluminum alloys as well as conventionally processed MMCs. Detailed fractography revealed the presence of a unique cone-shaped fracture feature for samples failing in HCF. Calculations based on fracture morphologies suggested catastrophic fatigue fracture occurred as the material reached its critical fracture toughness. Initial modeling of this fatigue performance used a variant of the Universal Slopes Criterion proposed in early work at Case Western Reserve University by S. M. Manson.

Study on coal fractography under dynamic impact loading based on multifractal method

Abstract: Coal fractography is a powerful tool for interpreting coal fracture behaviors, which is significant for dealing with failure issues encountered in deep coal mining. However, the accuracy of coal fr...

Characterization and Analysis of Porosities in High Pressure Die Cast Aluminum by Using Metallography, X-Ray Radiography, and Micro-Computed Tomography

Abstract: Mechanical performance of cast aluminum alloys is strongly affected by the defects formed during solidification. For example, fractography studies of the fatigue specimens have shown that fatigue failure in aluminum castings containing defects is almost always initiated from defects, among which pores are most detrimental. However, elimination of these pores is neither always economically nor technically possible. This work characterizes defects in high pressure die cast aluminum alloy as an illustrative material, but the methods used can be applicable to other types of castings and defects. The defects were evaluated using metallography as well as micro-computed tomography techniques. The variability of defects between the specimens of two sizes as well as different porosity levels are studied statistically. The distributions of defects based on location within the specimens are also analyzed. Moreover, the maximum defect size within the specimens are estimated using extreme value statistics, which can be used as an input to fatigue life prediction models. Extreme value statistics is applied on both 2D and 3D defect data. The accuracy of each approach is verified by comparing the estimated maximum defect size within the specimens with the maximum observed defects on fracture surfaces of fatigue specimens.