Showing papers on "Fractography published in 2017"

Microstructure and mechanical properties of cold sprayed 6061 Al in As-sprayed and heat treated condition

Abstract: Microstructural and mechanical properties of cold sprayed 6061 aluminum deposits on 6061-T6 aluminum alloy substrates are investigated under various heat treatment conditions, i.e. as-deposited, stress relieved and T6. The local mechanical property variation in the as-deposited material are explored using nanoindentation technique, and correlated with microstructural characterization conducted via electron back-scattered diffraction. It is found that the prior particle boundaries have ~ 0.4 GPa higher hardness than particle interiors, which is attributed to grain refinement in these regions promoted by local dynamic recrystallization. Also, the bulk-scale mechanical properties of the deposits are evaluated by microtensile testing in various post-heat treatment conditions and compared to those of conventionally processed 6061-T6 aluminum. The as-deposited material showed markedly higher ultimate strength (~ 460 MPa) and lower ductility (~ 3%) compared to conventionally processed material and this is attributed to significant cold working during the cold spray deposition process and associated grain boundary strengthening and dislocation strengthening mechanisms. Heat treated specimens showed a slight improvement in both ultimate strength and ductility compared to the as-deposited condition. These improvements are attributed to an improvement in metallurgical bonding at prior particle boundaries and a modest increase in the density of strengthening precipitates. Fractography of the specimens revealed that the heat treatment also changes the fracture characteristics of the cold sprayed 6061 aluminum deposit. The residual stress profiles and bond strength of the deposits are also studied using x-ray diffraction, tensile pull-off and three lug shear testing, respectively.

Fractography of zirconia-specimens made using additive manufacturing (LCM) technology

Abstract: A relatively new method to manufacture complex ceramic prototypes and components is additive manufacturing (AM). With the LCM (Lithography-based Ceramic Manufacturing)-technology the green body is manufactured layer-by-layer using selective curing of light-sensitive ceramic slurry by a mask exposure process. After curing by blue light the component is removed from the building platform and the green body is sintered to a ceramic component. The aim of this work is to investigate the influence of processing and layer architecture on the mechanical properties of an Yttria-stabilized zirconia ceramic. Strength tests were performed by uniaxial bending tests and by biaxial Ball-on-three Balls (B3B) tests. To identify typical fracture initiating flaws a systematic fractographic investigation was performed on different batches of Ball-on-three Balls-test and bending test specimens, respectively. Through additional investigations it was found that hardness and fracture toughness were independent on the layer architecture. But an extensive fractographic analysis showed that the strength was limited by flaws, which were introduced by processing and handling. If these flaws can be avoided by optimisation of the process the strength should be equal to that of conventional processed ceramics.

Mechanical properties of adhesively single lap-bonded joints reinforced with multi-walled carbon nanotubes and silica nanoparticles

Abstract: This paper aims to investigate the effect of adding nanoparticles to the adhesive layer on the shear strength and elongation at failure of adhesively bonded single lap joints (SLJs) Two different toughening particles including the silica nanoparticles (SNPs) and the multi-walled carbon nanotubes (MWCNTs) were considered for reinforcing the adhesive joints The experimental results showed that the highest improvements in the SLJ shear strength and elongation at failure were obtained for 02 and 08 wt% of MWCNTs and SNPs, respectively The fractography results indicated that adding nanoparticles improved the failure mode from adhesive to dominant cohesive representing improved adhesion between the adhesive and adherends Moreover, different damage mechanisms were observed for the adhesives reinforced with different toughening particles Several mechanisms including crack growth deviation, shear yielding, plastic deformation, and pull out phenomena were observed from scanning electron microscope (S

Towards understanding the influence of porosity on mechanical and fracture behaviour of quasi-brittle materials: experiments and modelling

Abstract: In this work, porosity-property relationships of quasi-brittle materials are explored through a combined experimental and numerical approach. In the experimental part, hemihyrate gypsum plaster powder (CaSO 4 ⋅1/2H 2 O CaSO4⋅1/2H2O) and expanded spherical polystyrene beads (1.5–2.0 mm dia.) have been mixed to form a model material with controlled additions of porosity. The expanded polystyrene beads represent pores within the bulk due to their light weight and low strength compared with plaster. Varying the addition of infill allows the production of a material with different percentages of porosity: 0, 10, 20, 30 and 31 vol%. The size and location of these pores have been characterised by 3D X-ray computed tomography. Beams of the size of 20×20×150 20×20×150 mm were cast and loaded under four-point bending to obtain the mechanical characteristics of each porosity level. The elastic modulus and flexural strength are found to decrease with increased porosity. Fractography studies have been undertaken to identify the role of the pores on the fracture path. Based on the known porosity, a 3D model of each microstructure has been built and the deformation and fracture was computed using a lattice-based multi-scale finite element model. This model predicted similar trends as the experimental results and was able to quantify the fractured sites. The results from this model material experimental data and the lattice model predictions are discussed with respect to the role of porosity on the deformation and fracture of quasi-brittle materials.

Mechanical bonding properties and interfacial morphologies of austenitic stainless steel clad plates

Abstract: The current paper focuses on the correlation between microstructure and mechanical properties in low-carbon steel/austenitic stainless steel clad composite fabricated by hot-roll bonding. For this reason, the morphology and interfacial characteristics, tensile properties shear strength and fracture toughness of the cladded material were evaluated. From the main results, it was found that carbon element diffusion caused the forming of a decarburized ferrite zone (DFZ) of the parent metal and a carburized austenite zone (CAZ) of the clad layer, and between these two area, a thin diffusion layer with rapid element component change are formed in the hot-roll cladding process. Stress-strain curves obtained from tensile testing of parent metal and clad layer can predict the bi-material tensile behavior. The shear test proved that the stainless steel clad plate presents an acceptable shear bond strength at the interface joint. Impact test toughness results confirm that fracture took place only in the parent metal side of cladded specimens; the clad layer was bent but without obvious fracture. Fractography was carried out using scanning electron microscope in the tensile, shear bond test specimens and Charpy impact ones. It reveals the presence of predominantly dimpled fracture. Charpy impact specimens of the interface failed in mixed mode while impact specimens of the base plate failed in ductile mode.

Effects of Process Conditions on the Mechanical Behavior of Aluminium Wrought Alloy EN AW-2219 (AlCu6Mn) Additively Manufactured by Laser Beam Melting in Powder Bed

Abstract: Additive manufacturing is especially suitable for complex-shaped 3D parts with integrated and optimized functionality realized by filigree geometries. Such designs benefit from low safety factors in mechanical layout. This demands ductile materials that reduce stress peaks by predictable plastic deformation instead of failure. Al–Cu wrought alloys are established materials meeting this requirement. Additionally, they provide high specific strengths. As the designation “Wrought Alloys” implies, they are intended for manufacturing by hot or cold working. When cast or welded, they are prone to solidification cracks. Al–Si fillers can alleviate this, but impair ductility. Being closely related to welding, Laser Beam Melting in Powder Bed (LBM) of Al–Cu wrought alloys like EN AW-2219 can be considered challenging. In LBM of aluminium alloys, only easily-weldable Al–Si casting alloys have succeeded commercially today. This article discusses the influences of boundary conditions during LBM of EN AW-2219 on sample porosity and tensile test results, supported by metallographic microsections and fractography. Load direction was varied relative to LBM build-up direction. T6 heat treatment was applied to half of the samples. Pronounced anisotropy was observed. Remarkably, elongation at break of T6 specimens loaded along the build-up direction exceeded the values from literature for conventionally manufactured EN AW-2219 by a factor of two.

Mechanical behaviour of thermoplastic composites spot-welded and mechanically fastened joints: A preliminary comparison

Abstract: The in-plane and out-of-plane mechanical behaviour of both ultrasonically spot-welded and mechanically fastened joints was investigated by double-lap shear and pull-through tests, respectively. Spot-welded specimens showed comparable onset failure load and significantly higher joint stiffness compared to mechanical fasteners when carrying shear load. The failure modes and the damage within specimens were analysed after mechanical tests. Intralaminar failure and very limited damage on the out-most ply were found for welded specimens, whereas catastrophic through-the-thickness failure was observed for mechanically fastened joints. Based on the experimental outcomes, the mechanical performance and failure mechanisms of spot-welded joints were critically assessed in comparison to the mechanical fasteners .

Tensile and bending behaviors and characteristics of laminated Ti-(TiBw/Ti) composites with different interface status

Abstract: A series of laminated Ti-(TiBw/Ti) composites fabricated at different diffusion welding temperatures of 1100 °C, 1200 °C and 1300 °C were subjected to tensile and bending tests. The results showed that the interfacial bonding strength, interfacial toughness and residual stress gradually increased with the increasing fabrication temperature. Therefore, along the transversal direction, the laminated composites fabricated at 1300 °C revealed the highest tensile strength and fracture elongation. However, the highest tensile strength (694 MPa) and elongation (22.7%) along the longitudinal direction, was recorded with the laminated composites fabricated at 1200 °C due to reasonable interfacial bonding and residual stress. Interestingly, the laminated composites fabricated at 1100 °C exhibited the highest bending fracture toughness and fracture work along arrester orientation. Due to the weak interfaces, the crack propagation path was displaced by delamination cracks and re-nucleated multiple tunnel cracks, which in turn lead to reduction in stress intensity of the main crack. This was beneficial to the toughening of the laminated Ti-(TiBw/Ti) composites.

Mechanical characterization and modeling of the deformation and failure of the highly crosslinked RTM6 epoxy resin

Abstract: The nonlinear deformation and fracture of RTM6 epoxy resin is characterized as a function of strain rate and temperature under various loading conditions involving uniaxial tension, notched tension, uniaxial compression, torsion, and shear. The parameters of the hardening law depend on the strain-rate and temperature. The pressure-dependency and hardening law, as well as four different phenomenological failure criteria, are identified using a subset of the experimental results. Detailed fractography analysis provides insight into the competition between shear yielding and maximum principal stress driven brittle failure. The constitutive model and a stress-triaxiality dependent effective plastic strain based failure criterion are readily introduced in the standard version of Abaqus, without the need for coding user subroutines, and can thus be directly used as an input in multi-scale modeling of fibre-reinforced composite material. The model is successfully validated against data not used for the identification and through the full simulation of the crack propagation process in the V-notched beam shear test.

Effect of tempering temperature and inclusions on hydrogen-assisted fracture behaviors of a low alloy steel

Abstract: Abstact The tensile properties and fracture behaviors of the pre-charged hydrogen low alloy steel subjected to various tempering temperatures were investigated by slow strain rate tensile tests. Hydrogen was introduced into the samples by electrochemical method in 0.5 mol L−1 NaOH solution with 1 g L−1 CH4N2S for 24 h at room temperature (298 K). The results show that with an increase in tempering temperature, the resistance to hydrogen embrittlement increases. For the sample tempered at 200 °C, hydrogen-assisted microcracks initiate at the lath boundaries and the interfaces between inclusions and matrix. Moreover, the void around the inclusion initiates at the matrix-inclusion interface and grows in the form of the interface decohesion for hydrogen free sample, while the void also starts at the interfaces and propagates into the steel matrix along vertical tensile stress direction in the presence of hydrogen. Fractography reveals that for hydrogen-charged samples, the crack originates from mixed O-Si-Al-Ca inclusions in the samples tempered at 200 °C and 400 °C, while it is not located at inclusions in the sample tempered at 650 °C.

Creep deformation behavior of 9Cr1MoVNb (ASME Grade 91) steel

Abstract: Detailed investigations have been performed to examine the creep deformation behavior and microstructural evolution of modified 9Cr-1Mo steel which is widely used in high temperature power plant component. The creep data were analysed in terms of the temperature compensated power law and Monkman-Grant relation. The creep activation energy of Grade 91 steel was determined with 543±30 kJ/mol without threshold stress compensation, while after correcting the threshold stress; the activation energy is decreased to 303±15 kJ/mol. This value is close to the activation energy of creep in α-Fe. The calculated threshold stress showed a strong dependence on temperature. The creep behavior of the steel was described by the modified Bird-Mukherjee-Dorn relation. The rate controlling creep deformation mechanism was identified as the edge dislocation climb with stress exponent of n=5. Further, the value of creep damage tolerance factor (λ) and stress exponent was used to identify the cause of creep damage, showed significant difference in the high and low stress regimes. The fracture surface morphology of the ruptured specimens was studied by scanning electron microscopy to further elucidate the failure mechanisms. Whereas deformed microstructure was examined by transmission electron microscopy. The significant decrease in creep strength in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure.

Study of tensile properties, fractography and morphology of aluminium (1xxx)/coconut shell micro particle composites

Abstract: Aluminium (1xxx)/coconut shell micro particle (Al/CMP) composites have been developed using a compo cast technique. Coconut shells (CSs) were processed with the aid of mortar/pestle and disc grinder and then classified with a set of sieves vibrated with a sine shaker. The CMP additions increased from 2% to 10% at 2% interval. Microstructural/chemical composition analyses were carried out with the aid of scanning electron microscopes (ASPEX 3020) with attached energy dispersive X-ray spectroscopy. Phases were identified using an X-ray diffractometer (XRD). The tensile properties and mode of fracture were studied using Instron extensometer and Avery Denison Impact Testing Machine respectively. Results revealed 99.3% purity of aluminium matrix. The presence of new phases in the aluminium matrix is attributable to chemical interaction between Al and CMPs. The fine grained structure of Al/CMPs composites was confirmed by SEM and optical micrographs. The enhancement in the tensile properties is attributable to the presence of hard phases in the Al matrix and good interfacing bonding between Al matrix and CMP reinforcements. The cone and cup surface appearance with fibrous, dull, dimple and goose grain microstructure of the fracture surfaces of the composites is an indication of ductile fracture. Hence low cost metal matrix aluminium based composites have been developed.

Fractography of self-glazed zirconia with improved reliability

Abstract: The fractography of a new grade of zirconia ceramics, known as self-glazed zirconia, was investigated. The as-sintered intact top surface was made with superior smoothness that mimicked the optical appearances of the natural teeth enamel. The beneath surface opposite to this was made hierarchically rough with microscopic pits of the size up to 60 μm together with grain-level roughness of about 2 μm. The three-point bending test of the samples made with the hierarchically rough surface being tensile one demonstrated an average bending strength of 1120 ± 70 MPa and a Weibull modulus of as high as 18 ascribed to the improved structural homogeneity. Surface topography was found the main origins of crack initiation leading to fracture. The observed unusually predominant transgranular fracture mode of submicron-sized grains disclosed a possible toughening mechanism of disassembling of mesocrystalline grains that differs significantly from the commonly quoted phase transformation toughening of this category of ceramics.

Microstructures, Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components

Abstract: Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (σ-FFLD) and effective plastic strain-stress triaxiality space (ηEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using above-mentioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal.

A study on fracture toughness and strain rate sensitivity of severely deformed Al 6063 alloys processed by multiaxial forging and rolling at cryogenic temperature

Abstract: In the present work, strain rate sensitivity, using tensile and compression test data at different strain rates, fracture toughness using three-point bend test on multi axially forged (MAF) 6063 Al alloys at liquid nitrogen temperature have been investigated. The hardness, tensile strength and strain rate sensitivity (SRS) of the deformed alloy have increased to 116 HV, 337 MPa, and 0.02, from 60 HV, 220 MPa and 0.004 respectively, due to accumulation of high dislocation density in the alloy processed multi axial forging at cryo temperature. Large plastic strain induced during cryo forging of 6063 Al alloy led to effective fragmentation of grains, facilitating grain refinement in the alloy. The cryoforged alloy exhibits multimodal microstructure composed of nanocrystalline, ultrafine and coarse grains. The fracture toughness of deformed (MAF and CR) 6063 Al alloy has been increased from 8.32 MPa m1/2 to 13.78 MPa m1/2 due to grain refinement and high fraction of grain boundaries. The reduction in dimple size with increasing strain showed the presence of fine grains in multiaxial forged 6063 Al as evident from fractography studies.

The onset and evolution of fatigue-induced abnormal grain growth in nanocrystalline Ni–Fe

Abstract: Conventional structural metals suffer from fatigue-crack initiation through dislocation activity which forms persistent slip bands leading to notch-like extrusions and intrusions. Ultrafine-grained and nanocrystalline metals can potentially exhibit superior fatigue-crack initiation resistance by suppressing these cumulative dislocation activities. Prior studies on these metals have confirmed improved high-cycle fatigue performance. In the case of nano-grained metals, analyses of subsurface crack initiation sites have indicated that the crack nucleation is associated with abnormally large grains. However, these post-mortem analyses have led to only speculation about when abnormal grain growth occurs (e.g., during fatigue, after crack initiation, or during crack growth). In this study, a recently developed synchrotron X-ray diffraction technique was used to detect the onset and progression of abnormal grain growth during stress-controlled fatigue loading. This study provides the first direct evidence that the grain coarsening is cyclically induced and occurs well before final fatigue failure—our results indicate that the first half of the fatigue life was spent prior to the detectable onset of abnormal grain growth, while the second half was spent coarsening the nanocrystalline structure and cyclically deforming the abnormally large grains until crack initiation. Post-mortem fractography, coupled with cycle-dependent diffraction data, provides the first details regarding the kinetics of this abnormal grain growth process during high-cycle fatigue testing. Precession electron diffraction images collected in a transmission electron microscope after the in situ fatigue experiment also confirm the X-ray evidence that the abnormally large grains contain substantial misorientation gradients and sub-grain boundaries.