Dominant Fracture Mode Research Articles

Optimization of the grain boundary network and tensile properties of powder metallurgy-hot isostatic pressed Ni-based alloy Inconel 625

Powder metallurgy-hot isostatic pressing (PM-HIPing) is an advanced manufacturing technology for large-sized and complex-shaped components for nuclear applications. In the present study, a novel PM-HIPing strategy was proposed to optimize the grain boundary network and tensile properties of Ni-based alloy Inconel 625, and it was verified by a series of experiments. Samples with 69 % and 74 % of coincidence site lattice (CSL) grain boundaries were obtained when the narrow-size ranged powder was consolidated at 1140 °C and 1180 °C, respectively. The fraction of CSL grain boundaries was further increased to ∼78.5 % after a high temperature solution treatment. Room temperature yield strength and ultimate tensile strength of PM-HIPed alloy 625 were comparable to that of the wrought specimens, elongation of the specimens was ∼56–60 %, the samples all featured with dimple ductile dominant fracture mode.

Evolution of the mixed mode FPZ and fracture roughness of Beishan granite after coupled thermo-hydro-mechanical treatment

Deep surrounding rocks contain a network of longitudinally intersecting fissures. Exploration of their fracture process zone (FPZ) size evolution during mixed loading and fracture surface roughness after failure has always been a hot topic, especially for coupled thermo–hydro–mechanical (THM) treatment. This study takes Beishan granite as the research object. The fracture surface roughness and FPZ length were obtained using Grasselli statistical and mixed strain–displacement methods. Finally, an electron microscope was employed to observe the fracture surface after failure at different temperatures and loading angles. Results reveal a notable temperature and load dependence of the fracture surface roughness and FPZ length evolution. In the low-temperature stage (25°C–300°C), the mineral primarily exhibited transgranular fractures and the average fracture surface roughness showed a slightly decrease with increasing temperature. In the high-temperature stage (500℃–650℃), the degree of internal damage to the prepared specimens substantially increased, with intergranular fractures being the dominant fracture mode, and rapid roughness growth occurred. Under the same temperature condition, the roughness value was in the order of mode I > mixed mode > mode II. The FPZ length increased, stabilized, and decreased as the load increased. In the low-temperature stage, the FPZ length reached its maximum value at the maximum load, whereas in the high-temperature stage, the FPZ length was fully developed before the peak load. Under the same loading angle condition, the FPZ lengths corresponding to the three loading modes followed the order of mode I < mixed mode < mode II.

Irradiation assisted stress corrosion cracking of 347 stainless steel in simulated PWR primary water containing lithium hydroxide or potassium hydroxide

The susceptibility of type 347 stainless steel to irradiation assisted stress corrosion cracking (IASCC) in pressurized water reactor (PWR) primary water with two water chemistries was evaluated. Miniature four-point bend tests were used to assess the effect of potassium hydroxide (KOH) in comparison to lithium hydroxide (LiOH) on the stress to initiate IASCC. Four-point bend specimens were machined from baffle-former bolts removed from the Salem unit 1 and D.C. Cook unit 2 reactors with accumulative doses ranging from 7.8 to 26.4 dpa. It was found that the pseudo-threshold stress to initiate IASCC in this alloy was 50–60% of the irradiated yield strength irrespective of water chemistry. Intergranular cracking was the dominant fracture mode in all specimens and the fracture morphology was characterized by a tortuous fracture path through the oxide and was common to both water chemistries. Grain boundary oxidation combined with localized stress enhancement at deformation band – grain boundary sites were precursors for IASCC initiation in both water chemistries. The pseudo-threshold stress for IASCC initiation is in good agreement with O-ring test results on both 316SS and 304SS. Overall, the stress to initiate cracks in neutron-irradiated 347SS was indistinguishable between simulated primary water containing LiOH or KOH.

Failure mechanisms of hybrid metal–composite joints with different protrusion densities under a tensile load

The mechanical behaviors of hybrid metal–composite joints with different protrusion densities were investigated by combining numerical and experimental methods. High-fidelity finite element models that considered the failure modes of all components were developed, and specimens based on metal additive manufacturing technology were tested under quasi-static tensile load to verify the numerical calculations. The results showed that the load capacity and the dominant fracture mode of joints were significantly affected by the metal protrusion density. The failure mechanisms of joints under different protrusion conditions exhibited a clear difference, which proved the possibility of an optimal and functional joint design.

Investigation on the microstructure and mechanical properties of gum metal TNTZO titanium alloy prepared by selective laser melting

In this study, a high-dense multi-functional gum metal Ti36Nb2Ta2.7Zr0.3O (mass%, TNTZO) alloy was successfully produced using selective laser melting (SLM). The as-built components exhibited epitaxially grown β grains with a slight deviation from the building direction, resulting in a cubic-like texture where (001)β was oriented along the building direction. Additionally, nanoscale ω precipitates were found embedded in the β matrix. The uniaxial tensile tests revealed that the as-built TNTZO alloy had repeatable strength with a yield strength of 756 MPa. Dislocation slip was the dominant mechanism responsible for the deformation of the as-built TNTZO alloy, with no phase transformation or deformation twin observed. The interaction between the ω variants with the dislocations during plastic deformation led to the formation of “ω-free channels” due to the different orientation relationship of ω variants with β matrix. The post-mortem analysis revealed that ductile fracture is the dominant fracture mode.

Effect of Mo2C on the microstructure and properties of (W, Mo)C-10Co cemented carbides

Mo2C doped WC-10Co alloys were prepared via an in-situ synthesis method composed of steps of spray drying, roasting, carbothermal pre-reduction and carbonization-vacuum sintering. The effects of Mo2C content on the microstructure and mechanical properties of cemented carbides were studied in detail. The results showed that WC and Mo2C formed a complete solid solution by the current method. With the increase of Mo2C content from 0 wt% to 0.5 wt%, the effects of solution strengthening and fine grain strengthening were obvious, and the grain size was decreased from 0.72 ± 0.32 μm to 0.62 ± 0.27 μm, while the hardness was improved from 1316 ± 20 HV30 to 1549 ± 14 HV30. However, the (W, Mo)C solid solution would deteriorate the fracture toughness of the alloys. With the further increase of Mo2C content to 9 wt%, the lattice distortion in the solid solution became more serious. Meanwhile, the dissolution of more Mo2C in the liquid Co further weakened the dissolution-precipitation process of the solid solution, thus inhibiting the growth of grains and increasing the hardness of the alloys. In addition, with the increase of Mo2C content, the dominant fracture mode of the alloy changed from transgranular fracture to intergranular fracture.

Effect of ceramic-binder interface on the mechanical properties of TiB2-HEA composites

A non-equiatomic CoCrFeNiMo high-entropy alloy (HEA) was used as a binder for the TiB2-HEA composites. The TiB2–6 vol% HEA and TiB2-10 vol% HEA composites were successfully fabricated by mechanical alloying and spark plasma sintering. Phase analysis revealed the presence of FCC and TiB2 phases in the composites. A detailed microstructural study using scanning electron microscopy and transmission electron microscopy helped with the understanding of the impact of carbide/binder interface on fracture toughness and strength of the TiB2-HEA composites. TEM revealed a few nanometer-thick intermediate layer between the TiB2 and HEA phases that facilitates the enhanced wettability between ceramic and binder phase. The fracture mode was affected by the HEA binder concentration, i.e., the dominant mode of fracture changed from intergranular to transgranular as binder content increased from 6 vol% to 10 vol%. The relative density, Vickers hardness and fracture toughness of TiB2-10 vol% HEA composite reached up to 98.50 ± 0.30%, 22.00 ± 0.62 GPa and 9.23 ± 0.45 MPa m1/2, respectively. The high fracture toughness of the TiB2-10 vol% HEA composite was the result of the synergism of high densification, excellent wettability, crack deflection, crack bridging, and transgranular fracture of the TiB2 particles.

Resistance spot welding of NiTi shape memory alloy sheets: Microstructural evolution and mechanical properties

Resistance spot welding was used for welding superelastic NiTi shape memory alloy sheets. The microstructure of the welded sample showed a fully austenitic NiTi nugget without any visible weld defect due to the insignificant change in the transformation temperature during the welding process. Additionally, the fusion zone has a homogenous microhardness distribution with an average hardness value of ~340 HV, which is below the average hardness value for NiTi base material (~410 HV) because of the large grain size in the weld nugget. The perseverance of the superelastic response as a main functional property of NiTi alloy after the RSW procedure is significant. The welded sample failed via pullout failure mode during tensile shear loading conditions. Face digital image correlation imaging confirmed that strain is localized in the area around the nugget. The dominant fracture mode of the sample during tension-shear loading was ductile, and the EBSD results of the fracture area revealed the excessive deformation around the HAZ resulted in a transgranular fracture. Therefore, resistance spot welding is highly capable of providing a sound, straightforward and reliable joint between NiTi sheets.

Optimization for solderability of large-size single-crystal diamond with heterogeneity alloys by tantalum coating

To ameliorate the solderability of single-crystal diamond with heterogeneity metals/alloys, tantalum (Ta) coatings were prepared on the diamond by utilizing the double glow plasma surface metallurgy (DGPSM) technique. Using Ag-Cu-Ti solder, the Ta-coated single-crystal diamonds were soldered onto cemented carbide (WC-Co) matrixes. The effects of metallization temperatures on the microstructure, phase, and solderability of the Ta-coated diamond were investigated. The results show that uniform and compact Ta coatings with increased thicknesses are obtained with increasing metallization temperatures. Besides, under all the temperatures, the diffusion layers composed of Ta2C and TaC could be formed at the Ta/diamond interface, which is beneficial to enhance the coating adhesion. The average shear strength of the soldered joint with Ta-coating increases first and then decreases with the increase of temperature. The maximum value is 118.7 MPa on the soldered joint with 850 °C Ta coating, which is approximately 57% higher than without Ta coating. Three failure modes occur during the shear process, including the ductile fracture of the soldered seam, brittle fracture in the interior of Ta carbides at the Ta/diamond interface, and brittle fracture of the single-crystal diamond itself. Among them, the ductile fracture of the soldered seam is the dominant fracture mode.

Numerical Study about the Influence of Superimposed Hydrostatic Pressure on Shear Damage Mechanism in Sheet Metals

It is generally accepted that the superimposed hydrostatic pressure increases fracture strain in sheet metal and mode of fracture changes with applying pressure. Void growth is delayed or completely eliminated under pressure and the shear damage mechanism becomes the dominant mode of fracture. In this study, the effect of superimposed hydrostatic pressure on the ductility of sheet metal under tension is investigated using the finite element (FE) method employing the modified Gurson–Tvergaard–Needleman (GTN) model. The shear damage mechanism is considered as an increment in the total void volume fraction and the model is implemented using the VUMAT subroutine in the ABAQUS/Explicit. It is shown that ductility and fracture strain increase significantly by imposing hydrostatic pressure as it suppresses the damage mechanisms of microvoid growth and shear damage. When hydrostatic pressure is applied, it is observed that although the shear damage mechanism is delayed, the shear damage mechanism is dominant over the growth of microvoids. These numerical findings are consistent with those experimental results published in the previous studies about the effect of superimposed hydrostatic pressure on fracture strain. The numerical results clearly show that the dominant mode of failure changes from microvoid growth to shear damage under pressure. Numerical studies in the literature explain the effect of pressure on fracture strain using the conventional GTN model available in the ABAQUS material behavior library when the mode of fracture does not change. However, in this study, the shear modified GTN model is used to understand the effect of pressure on the shear damage mechanism as one of the individual void volume fraction increments and change in mode of fracture is explained numerically.

The effects of TGO growth stress and creep rate on TC/TGO interface cracking in APS thermal barrier coatings

To design reliable thermal barrier coatings (TBCs), it is essential to elucidate the interface failure mechanism and establish a better life-prediction model. Up to now, how the variations of thermally grown oxide (TGO) growth stresses and creep rates affect the interfacial cracking behavior between top coat (TC) and TGO has not been clearly studied. In this work, the thermos-mechanical model with simplified cosine curve interface morphology in TBCs is developed utilizing numerical simulation software (ABAQUS). According to the calculated stress states of TC/TGO interface, the possible damage scenarios are inferred. On this basis, the interface crack nucleation and propagation is simulated by the surface-based cohesive behavior. General features of the interface crack are in good coincidence with previous conjectures. Furthermore, large TGO growth stresses will trigger off premature cracking, conversely, the formation of TC/TGO interface crack can be postponed due to large TGO creep rates. In addition, the dominant fracture mode of TC/TGO interface is impacted by the variations of TGO growth stresses as well as its creep rate. The position of interface fracture mode transition has been shifted to a certain extent.

Investigation of the Effect of Bedding Layer Angle and Tunnel Number on the Stability of Tunnel under Uniaxial Compression Using PFC2D

In this paper the effect of bedding layer angle on the stability of tunnel under uniaxial compression have been investigated using particle flow code in two dimensions (PFC2D). For this purpose, numerical rectangle models with dimension of 100*100 mm have been prepared. These models consist of layers with different mechanical properties i.e., concrete layer and gypsum layer. The angle of these layers related to horizontal axis change from 0° to 90° with increment of 15°. These models are consisting of one, two and three tunnel. The diameter of tunnel change based on the tunnel number. The tunnel diameter was 6 m, when one tunnel exists in the model. The tunnel diameter was 3 m, when two tunnels exist in the model. The tunnel diameter was 2 m, when three tunnels exist in the model. These models were subjected to uniaxial compression. The results show that tensile cracks are dominant mode of fracture occurred in the models. The joint angle and tunnel number have important effect on the failure pattern and failure strength. Also, the mechanical properties of beddings control the crack growth path. The crack grows through the weak layers when bedding angle was equal to 45° and 60°, but it intersects the layer for any other bedding angels.

Effect of Hot Extrusion on Microstructure and Mechanical Properties of Mg–5Sn–xZr Alloys

The effects of Zr addition (0, 0.5 and 1 wt%) and extrusion ratio (6:1 and 12:1) on microstructure and mechanical properties of the Mg–5Sn alloy were studied. Results showed that the dendritic structure of the as-cast M–5Sn alloy gradually transformed into a nearly globular structure with Zr addition. The effect of Zr addition was evident through refining the microstructure, promoting the tendency to form partially divorced eutectic and increasing the Mg2Sn volume fraction, which improved the thermal stability of the alloy during homogenizing treatment at high temperatures. Hot extrusion was found to be very effective for enhancing both strength and ductility of the Mg–5Sn–xZr alloys. Grain refinement induced by twin dynamic recrystallization and dynamic precipitation of fine particles were realized as the main strengthening mechanisms of the extruded alloys. It was found that an increase of the extrusion ratio led to relatively coarser grains, lower yield strength, and higher hardening capacities. However, the Mg–5Sn–1Zr alloy extruded with the area-reduction ratio of 12 exhibited the best combination of tensile strength and elongation of 243 MPa and 10.6%, respectively. Moreover, tensile fracture studies revealed the intergranular cracking and quasi-cleavage fracture as the dominant fracture modes of the as-cast and extruded alloys, respectively.

In-situ visualisation of dynamic fracture and fragmentation of an L-type ordinary chondrite by combined synchrotron X-ray radiography and microtomography

The relationship between the dynamic mechanical properties of stony meteorites and their microstructures was investigated in-situ for an L-type ordinary chondrite using a split-Hopkinson pressure bar apparatus and ultra-high speed phase-contrast X-ray radiography at the European Synchrotron Radiation Facility (ESRF). Synchrotron X-ray microtomography (μCT) was performed both prior to and immediately following dynamic compression to correlate key structural features between the initial microstructure and recovered fragments as well as to identify the leading mechanisms for fracture and fragmentation. Real-time visualisation of damage evolution in the specimens revealed the very first cracks to be initiated at the sites of FeNi-metal nodules. These cracks propagated rapidly through the largest group of chondrules (the porphyritic olivine type chondrules) along the loading direction, which led to the formation of column-like fragments. μCT analysis of the collected fragments confirmed the dominant mode of fracture to be transgranular with a clear link between FeNi-metal nodule statistics and the size distribution of fragments, emphasising their role in mechanical failure and fragmentation process. The resulting fragmentation was used to validate the predictions of brittle fragmentation models, and found to be in good agreement with the laboratory-scale impacts. In turn, these models can help unravel the consequences of impact-induced fragmentation processes that have helped shape the solar system.

Thermal Shock Behavior of Twill Woven Carbon Fiber Reinforced Polymer Composites

In the current research, the effect of cyclic temperature variation on the mechanical and thermal properties of woven carbon-fiber-reinforced polymer (CFRP) composites was investigated. To this, carbon fiber textiles in twill 2/2 pattern were used as reinforced phase in epoxy, and CFRPs were fabricated by vacuum-assisted resin-infusion molding (VARIM) method. Thermal cycling process was carried out between −40 and +120 °C for 20, 40, 60 and 80 cycles, in order to evaluate the effect of thermal cycling on mechanical and thermal properties of CFRP specimens. In this regard, tensile, bending and short beam shear (SBS) experiments were carried out, to obtain modulus of elasticity, tensile strength, flexural modulus, flexural strength and inter-laminar shear strength (ILSS) at room temperature (RT), and then thermal treated composites were compared. A dynamic mechanical analysis (DMA) test was carried out to obtain thermal properties, and viscoelastic properties, such as storage modulus (E’), loss modulus (E”) and loss factors (tan δ), were evaluated. It was observed that the characteristics of composites were affected by thermal cycling due to post-curing at a high temperature. This process worked to crosslink and improve the composite behavior or degrade it due to the different coefficients of thermal expansion (CTEs) of composite components. The response of composites to the thermal cycling process was determined by the interaction of these phenomena. Based on SEM observations, the delamination, fiber pull-out and bundle breakage were the dominant fracture modes in tensile-tested specimens.

Biofilm Adhesion to Surfaces is Modulated by Biofilm Wettability and Stiffness

AbstractAlthough many surfaces in industry and medicine are colonized by bacterial biofilms, little is known about the physical principles that govern the adhesion properties of such bacterial communities. In part, this is due to the technical challenge associated with the characterization of a biofilm directly on the substrate it is grown on. Moreover, distinguishing between the cohesive and adhesive properties of a (bio)material requires information on the amount of material transferred between the interacting surfaces, which is not easily possible in existing measurement techniques applied in biofilm research. Here, a new method is introduced which allows for characterizing the detachment process of biofilms in situ and makes it possible to identify the dominant mode of fracture. As a countersurface in those detachment tests with biofilm layers, either a synthetic/inorganic material surface or another biofilm layer can be used. By comparing results obtained with different biofilms generated at distinct cultivation conditions, how two selected material properties, i.e., the biofilm wettability and the biofilm stiffness, contribute to the detachment process can be shown. The novel measurement approach demonstrated here can easily be adapted further to enable adhesion/detachment measurements with a broad range of other biofilms including those grown at submerged conditions.

Characteristics and mechanisms of hydrogen-induced quasi-cleavage fracture of lath martensitic steel

This study presents an in-depth characterization of the microstructures, crystallographic orientations, and dislocation characteristics beneath hydrogen-induced quasi-cleavage fracture features of an as-quenched, lath martensitic (α') 22MnB5 steel. The fracture surfaces of gaseous hydrogen-embrittled martensitic specimens were analyzed by a combination of multiple analytical tools: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). The dominant fracture mode in the hydrogen-affected zones was quasi-cleavage fracture, which involved significant plasticity, evidenced by plastic zones near tear ridges and a high density of dislocations beneath the quasi-cleavage facets. The martensite constituent sizes, variant orientations, and boundaries influenced the quasi-cleavage surface morphologies. Fractography revealed the occurrence of {100}α'-type cleavage across martensitic laths, developing relatively “flat” quasi-cleavage surfaces, in addition to {110}α'-type cracking likely along lath and block boundaries, and fracture along non-cleavage planes. The likelihood of the formation of the relatively “flat” quasi-cleavage surfaces increased with increasing martensitic constituent sizes. Substantial dislocation bands formed on intersecting {112}α' slip planes within a martensite block below the hydrogen-induced cleavage fractures on {100}α' planes. River markings on the quasi-cleavage surfaces were found to originate from the complex, hierarchical lath martensitic microstructure. Steps and ridges on the quasi-cleavage facets generally linked with the various martensitic boundaries, suggesting that they were produced as a result of crack deviations at those boundaries. The fracture paths and martensite quasi-cleavage mechanisms are discussed in the context of the role of hydrogen and Cottrell cleavage model.

Microstructure and mechanical properties of hot-pressed Ti–Co–Si compounds reinforced by intermetallic phases

Monolithic TiCo and Ti5Si3 phases and Co(ss)/Ti5Si3–Ti3Co2Si nanocomposite were fabricated via mechanical alloying followed by hot-pressing (HP). The XRD patterns of bulk samples showed a general decrease in peak widths after hot-pressing owing to an increase in crystal size caused by exposure to high temperature during the HP process. Nevertheless, the nano-crystallinity of the structure was retained after HP and no new phase was formed. As a dominant fracture mode, this material exhibited an intergranular fracture feature with low dimpled ruptures. Dry-sliding wear behavior was evaluated at room and high temperatures. The mean coefficient of friction and wear rate gradually decreased with increasing temperature due to the presence of discontinuous thin tribo-oxide and tribo-nitride layers on the worn surface. The wear rate obtained in this work was ~10 times lower than that previously reported for Ti compounds.

Tensile Notch Sensitivity of Additively Manufactured IN 625 Superalloy

The notch sensitivity of additively manufactured IN 625 superalloy produces by laser powder bed fusion (LPBF) has been investigated by tensile testing of cylindrical test pieces. Smooth and V-notched test pieces with four different radii were tested both in as-built state and after a stress relief heat treatment for 1 h at 900 °C. Regardless of the notch root radius, the investigated alloy exhibits notch strength ratios higher than unity in both as-built and in stress-relieved states, showing that the additive manufactured IN 625 alloy is not prone to brittleness induced by the presence of V-notches. Higher values of notch strength ratios were recorded for the as-built material as a result of the higher internal stress level induced by the manufacturing process. Due to the higher triaxiality of stresses induced by notches, for both as-built and stress-relieved states, the proof strength of the notched test pieces is even higher than the tensile strength of the smooth test pieces tested in the same conditions. SEM fractographic analysis revealed a mixed mode of ductile and brittle fracture morphology of the V-notched specimens regardless the notch root radius. A more dominant ductile mode of fracture was encountered for stress-relieved test pieces than in the case of the as-built state. However, future research is needed to better understand the influence of notches on additive manufactured IN 625 alloy behaviour under more complex stresses.

TEM characterization of hot-pressed ZrB2-SiC-AlN composites

The impact of the AlN additive on the microstructural features and consolidation behavior of the ZrB2-SiC system was studied. For this objective, different weight percentages of AlN were added to the ZrB2-30 vol% SiC ceramics, in which 4 wt% phenolic resin was used as a binder. All specimens were hot-pressed at sintering circumstances of 1900 °C under 10 MPa for a dwelling time of 120 min. The addition of AlN had a remarkable effect on the sintering behavior of ZrB2-SiC, attaining fully dense composite samples. Although the thermodynamic study suggested the formation and consumption of several compounds over the hot-pressing, only the peaks related to carbon was detected in the XRD patterns. The thermodynamic assessment also implied the generation of a liquid phase over the sintering as the main factor in boosting the sinterability of specimens. The microstructure of as-produced ceramics was also inspected using FESEM and HRTEM images. Such micrographs endorsed the beneficial role of liquid phase sintering mechanism in forming coherent and strong interfaces among the available phases. Fractographical evaluation manifested the transgranular type as the dominant fracture mode for all hot-pressed composites.