Main Fracture Mode Research Articles

Effects of irradiation and stress on crack initiation behavior of 316SS flux thimble tubes in PWR environment

Irradiation assisted stress corrosion cracking (IASCC) behavior and irradiated microstructural features of cold-worked 316 stainless steel flux thimble tube specimens were investigated in 340 °C simulated PWR primary water. It covers three dose levels (∼40, ∼60 and ∼100 dpa) and five stress levels (35%, 40%, 50%, 60% and 70% of irradiated yield strength at high temperature. Crack initiation in highly irradiated materials occurred rapidly in 33 failed specimens at high-stress level. A clear dose dependence for IASCC initiation was observed. The susceptibility to IASCC initiation reached saturation around 40–60 dpa. Fracture surface of different dose level O-ring specimens showed that intergranular cracking was the main fracture mode. Discrete slip steps were observed on intergranular fractured regions of each dose level specimens, which is associated as evidence of dislocation channeling. A threshold stress level which was close to 30% yield strength was determined on various testing results from literature and this study. Data scatter was observed for those results from different studies but all agreed on that the failure rate increases with stress level and neutron dose.

Solder joint reliability performance study and shear characterization of low-Ag SAC lead-free solders for handheld application

As handheld electronic products move toward smaller and thinner form factor, the demand of high reliability under stringent field usage also increasing. Drop test, based on the JEDEC JESD22-B111A, accompanied with temperature cycling (TC) test, based on JEDEC JESD22-A104F, are two of the most critical board-level reliability tests for handheld makers. In terms of solder joint reliability (SJR), total 7 candidates of LF35-liked lead-free solder alloys are investigated for its drop performance, and TC reliability evaluation. Among 7 solders, leg 4 and 5 exhibit both decent drop and TC behavior, which are likely owing to the silver (Ag) precipitation hardening and bismuth (Bi) solid solution hardening effect respectively. In addition, the shearing mechanical characterization of 7 solder alloys are being studied. Shear strength of leg 4 and 5 shows higher strength value than others. In terms of failure analysis under drop test, three main fracture modes are observed, including via cracking at PCB side, solder/intermetallic compound (IMC) cracking at package side, and Cu trace broken at package side. Furthermore, in terms of failure modes of TC test, solder/IMC cracking at package side are the main fracture modes. In summary, this study provides an overall reliability performance mapping for 7 solders, and list out a table comprising of shear strength, void% analysis, TC reliability, and drop reliability performance for each LF35 solders which lays out potential future solder alloy to meet higher handheld phone drop reliability margin.

Microstructure, tribological property and high temperature tensile property of Co-Cr-Ni-W alloy parts fabricated by laser directed energy deposition

In this study, Co-Cr-Ni-W alloy parts were fabricated on the surface of 304 stainless steel (SS) substrate by laser directed energy deposition (LDED). Microstructure, microhardness and tribological behavior of as-deposited samples and 750 °C–950 °C aging-treated samples were tested. Carbides precipitated from Co-Cr-Ni-W alloy, and martensite transformation from γ-Co (FCC) to ε-Co (HCP) occurred in high temperature. The changes of microstructure caused by aging treatment could increase the microhardness and change wear mechanism. Tensile property of Co-Cr-Ni-W alloy at 750 °C–950 °C was also investigated. Co-Cr-Ni-W alloy formed a hard-sliding structure under high temperature, leading to high tensile strength. The main fracture mode of Co-Cr-Ni-W alloy was brittle cleavage fracture.

Ultrastretchable Helical Carbon Nanotube-Woven Film.

Helical carbon nanotube (HCNT) is regarded as one of the most promising nanomaterials due to its excellent tensile strength and superhigh stretchability. Here, a novel HCNT-woven film (HWF) is proposed, and its in-plane and out-of-plane mechanical properties are systematically investigated via molecular dynamics (MD) simulation. The MD results show that HWF possesses highly stretchable capability resulting from sliding and straightening of CNT segments, and the maximum tensile strain can reach 2113%. Furthermore, the HWF presents an obvious tensile mechanical anisotropy. The torsion failure is the main fracture mode when the HWF is stretched along the longitudinal direction. However, when the HWF is stretched along the transverse direction, the fracture is mainly caused by intertube compression. On the other hand, the HWF can dissipate large amount of kinetic energy of projectile via sliding and fracture of HCNTs, leading to high specific penetration energy. This work provides a theoretical guidance for designing and fabricating next-generation superstrong two-dimensional CNT-based nanomaterials.

Damage evolution and failure characteristics of a cemented gangue backfill considering direct shear and dynamic tests

Backfill mining is an effective technique that can prevent and control the impact of dynamic disasters which can occur in deep underground coal mines. This technique allows to reduce the surface subsidence and achieve a greener mining. The backfill shear mechanical properties under mining disturbances, are of great significance for ensuring the surrounding rock stability in deep mining. Therefore, studying the shear mechanical properties of the cemented gangue backfill body (CGBB) under coupled dynamic and static loads actions in deep coal mining is of great importance. In this paper, shear mechanical tests and the acoustic emission (AE) technology were used to study the shear mechanical response characteristics of CGBB under low-frequency dynamic normal load conditions. The results show that a dynamic normal load with an amplitude of 5 kN improves the shear CGBB strength, while a dynamic load with an amplitude greater than 10 kN will cause a shear strength deterioration, and the degree of deterioration is positively correlated with the load amplitude. Under low-frequency dynamic normal load conditions, the shear CGBB stress-strain curve exhibits classical characteristics. AE indicate that the main CGBB damage changes from a nonlinear deformation to an elastic stage under low-frequency dynamic normal load conditions. During the shear failure process, the cumulative AE counts are positively correlated with the load amplitude. The crack type identification based on RA-AF shows that the low-frequency dynamic normal load affects the CGBB crack type. Under constant normal load (CNL) conditions, tensile or mixed cracks account for 63.95% and are the main fracture mode. Conversely, under low-frequency dynamic normal load conditions, shear cracks account for more than 60% and are the main failure mode. This research provides a reference for the CGBB stability analysis under complex stress conditions for the case of a deep underground coal mining.

Effect of Al Content on the Microstructure and Mechanical Properties of the Sintered Fe–Mn–Al Porous Steel

Al content is of great importance for the evolution of the microstructure and compressive performance of the high‐Mn and high‐Al porous steels. Herein, Fe–Mn–Al porous steels with different Al contents are prepared via powder sintering at different temperatures. The results show that the main phases of the sintered samples at 640 °C are α‐Fe, α‐Mn, and Al, while a small amount of Fe2Al5 and Al8Mn5 exists in the samples with 15 and 20 wt% Al. At 1200 °C, the sintered sample with 5 wt% Al is composed of γ‐Fe, while those with 15 and 20 wt% Al are mainly α‐Fe. The length of Mn depletion region and porosity of the samples increase with the increase of Al content. The largest total and open porosity are observed in the 1200 °C‐sintered sample with 20 wt% Al, which are 63.1 and 60.7 vol%, respectively. The compressive strength decreases sharply from 770 MPa for the sample with 5 wt% Al to 7.5 MPa with 15 wt% Al, after sintering at 1200 °C. Ductile fracture occurs in the sample with 5 wt% Al, while brittle fracture is the main fracture mode in those with 15 and 20 wt% Al.

Dynamic shear behavior of forged CrMnFeCoNi high entropy alloy using hat-shaped specimens over a wide range of temperatures and strain rates

Dynamic shear deformation generally occurs in machining, punching and other manufacturing processes, so it is of key significance to study the shear properties of materials under high strain rate for guiding machining and realizing the application of materials. In this work, the dynamic shear behavior of forged CrMnFeCoNi high entropy alloys at different temperatures and strain rates was studied by using hat-shaped specimens. Dynamic loading at a wide temperature range (-100–650 °C) and ultra-high strain rate (5 × 105 s−1) was achieved. To investigate the influence of specimen geometry on shear properties, the shear widths (the horizontal width between the upper hat portion and the lower brim portion of the specimen) were set as 50 μm,100 μm, 200 μm and 400 μm, respectively. The results showed that the adiabatic shear was a precursor to failure, and the resistance to adiabatic shear localization of this alloy was related to the strain rate and geometric effect. The shear strength decreased gradually with the increase of temperature. The microstructure of the shear zone was characterized. The central ASB was composed of nanograins with the size of 200–300 nm, containing nano-twins and dislocations. The hardness of the central ASB was 70 % higher than that of the matrix due to the strengthening effect of fine grains. The grains in the transition region were deformed along the shear direction to form slender sub grains containing a large number of dislocations and nano-twins, resulting in strain hardening. The cracks propagated along the ASB, and the ductile fracture characterized by dimples was the main fracture mode.

Three-dimensional visualization structural characteristics and performance of discontinuous coarse fiber bamboo nodes

Bamboo nodes play a crucial role in ensuring the strength of hollow-structured bamboo, but compared with the internodes of unidirectional fiber structures, their structural which has discontinuous fiber characteristics are less understood. Here, we studied the structure-performance-fracture relationship between the internodes and nodes of bamboo. We examined the spatial heterogeneity of bamboo nodes through a three-dimensional reconstruction imaging strategy. We combined pendulum impact and high-speed photography with scanning electron microscopy to explore the regular fracture mechanism of bamboo nodes and compare it with internodes. An increase in slightly convex and ridged bamboo nodes occurred because there were thicker fiber bundles at the bamboo nodes. This caused fiber bundles to be deflected towards both sides instead of following a straight line. The bamboo nodes were more prone to regular fracture due to the discontinuous characteristics of fibers at bamboo nodes and the large void defects in the cross-section of node ridges, which destroyed the longitudinal continuity of thin-walled cells and fibers. In addition, under an impact, the main fracture mode of bamboo nodes was surface-penetrating fracture, and the main fracture modes between nodes included fiber pulling, fiber pulling out, and fiber bridging. This study provides a more in-depth understanding of bamboo and practical guidance for its development and applications in engineering structure.

Study on the effect of Cu addition on fatigue crack growth behavior of high-strength Al-1.2Mg-1.2Si alloy

This work investigated effect of Cu addition on fatigue crack growth rate and fatigue fracture morphology of Al-1.2Mg-1.2Si alloy by fatigue test machine and Scanning Electron Microscope. It revealed that effect of Cu on the fatigue crack growth rate of these alloys was related to Cu content. When the Cu content was 0.18, there did no significant influence on fatigue crack growth ability. But when Cu addition was raised to 0.37, the fatigue cracks growth resistance was significantly reduced. However, Cu addition did not significantly affect the fatigue fracture mode of the alloys. The ductile fracture was main fracture mode on this alloy.

Comparative study on microstructure and mechanical properties of a novel nano-composite strengthening heat-resistant steel and two typical heat-resistant steels

In this study, a new nano-composite strengthening austenitic heat-resistant steel designed for ultra-super critical (USC) thermal power plants was analyzed in comparison with the traditional H3RC and S30432 heat-resistant steels to assess its reliability. The microstructure, precipitates, hardness, tensile property and low cycle fatigue behaviors of the three steels at room temperature and service temperature were characterized. The results show that the SP2215 has finer grains, higher twin density, more primary MX, NbCrN and M23C6 precipitates compared with the HR3C and S30432 steels. The main precipitates in the steels after solution treatment are secondary (Nb, V)(C, N) phase (MX). The subsequent aging treatment of the SP2215 steel results in precipitation of the M23C6 carbides at the grain boundaries and Cu-rich phase inside the grains. As a result, the SP2215 steel shows higher Vickers hardness, tensile and fatigue strength (400 MPa) at room temperature than the other two steels. Transgranular fracture is the dominant tensile fracture mechanism of the three steels at room temperature, while intergranular cracking becomes the main fracture mode at high temperature due to precipitation of the continuous M23C6 at the grain boundaries. At the temperature of 700 ºC, fatigue strength of the SP2215 under a peak stress lower than 450 MPa decreases only a little.

Tensile Deformation and Fracture of Unreinforced AZ91 and Reinforced AZ91-C at Temperatures up to 300 °C.

Magnesium alloys are still attractive materials for applications that necessitate light weight due to their low density, moderate strength, and good corrosion resistance. AZ91 is one of the widely applied magnesium alloys due to its very good castability and strength. However, one of the drawbacks of magnesium alloys is the low elastic modulus. So, reinforcing AZ91 with carbon short fibers with the aim of further increasing the strength and improving the elastic modulus is investigated in this study. Squeeze cast AZ91-23 vol.% carbon short carbon (AZ91-C) and the unreinforced AZ91 are deeply examined by tensile testing at different temperatures (20, 100, 150, 200, 250, and 300 °C). Tensile stress-strain curves are measured and the tensile parameters (yield stress, ultimate tensile strength and strain) are defined and presented against the test temperature. Yield stress of AZ91 at 20 °C (109 MPa) is doubled (226 MPa) in the reinforced AZ91-C. Yield stress is found to slightly decrease with increasing the test temperature. Ultimate tensile strength of AZ91 at 20 °C (198 MPa) is increased (262 MPa) in the reinforced AZ91-C. The improvement of the ultimate tensile strength due to reinforcing increases with increasing the test temperature. Flow curves are determined and described by a modified Mecking-Kocks relationship and the flow parameters are determined and described as a function of the test temperature. Microstructure investigation was undertaken of the fractured tensile specimens at the grain boundaries rich in eutectic structure formed at the grain boundaries. Mixed brittle/ductile fracture mode is detected on the fracture surface of unreinforced AZ91, while the SEM investigations show matrix/carbon fiber detachment and fiber fracture as main fracture modes.

Effects of thermal cycling on bonding properties of novel low-shrinkage resin adhesive.

The current study aimed to investigate the bonding properties of a novel low-shrinkage resin adhesive containing expanding monomer and epoxy resin monomer after thermal cycling aging treatment. Expanding monomer of 3,9-diethyl-3,9-dimethylol-1,5,7,11-tetraoxaspiro-[5,5] undecane (DDTU) as an anti-shrinkage additive and unsaturated epoxy monomer of diallyl bisphenol A diglycidyl ether (DBDE) as a coupling agent were synthesized. A blend of DDTU and DBDE at a mass ratio of 1∶1, referred to as "UE", was added into the resin matrix at the mass fraction of 20% to prepare a novel low-shrinkage resin adhesive.Then, the methacrylate resin adhesive without UE was used as the blank control group, and a commercial resin adhesive system was selected as the commercial control group. Moreover, the resin-dentin bonding and micro-leakage testing specimens were prepared for the thermal cycling aging treatment. The bonding strength was tested, the fracture modes were calculated, the bonding fracture surface was observed by scanning electron microscope (SEM), and the dye penetration was used to evaluate the tooth-restoration marginal interface micro-leakage. All the data were analyzed statistically. After aging, the dentin bonding strength of the experimental group was (19.20±1.03) MPa without a significant decrease (P>0.05), that of the blank control group was (11.22±1.48) MPa with a significant decrease (P<0.05) and that of the commercial control group was (19.16±1.68) MPa without a significant decrease (P>0.05). The interface fracture was observed as the main fracture mode in each group after thermal cycling by SEM. The fractured bonding surfaces of the experimental group often occurred on the top of the hybrid layer, whereas those of the blank and commercial control groups mostly occurred on the bottom of the hybrid layer. Micro-leakage rating counts of specimens before and after thermal cycling were as follows: the experimental group was primarily 0 grade, thereby indicating that a relatively ideal marginal sealing effect could be achieved (P>0.05); meanwhile, the blank control group was primarily 1 grade, and the penetration depth of dye significantly increased after thermal cycling (P<0.05); the commercial control group was primarily 0 grade without statistical difference before and after thermal cycling (P>0.05), while a significant difference was observed between the commercial control group and experimental group after thermal cycling (P<0.05). The novel low-shrinkage resin adhesive containing 20%UE exhibited excellent bonding properties even after thermal cycling aging treatment, thereby showing a promising prospect for dental application.

Numerical investigation of rock breaking by a dish-shape cutter with advance slotting

A numerical model of rock breaking by dish-shape cutter wedge cracking assisted by advanced slotting was established by coupling the finite element method (FEM) and cohesive element method (CEM). The accuracy of the model was verified by uniaxial compression and cutter cutting rock experiments. The rock-breaking process of cutter wedge cracking was analyzed by combining the characteristics of thrust load and the ratio of tensile damage. The effects of the slotting feature and tool geometric parameters on the rock-breaking performance were studied. The results showed that advanced slotting can weaken the strength of rock, reduce the load of the cutter, and improve the rock-breaking efficiency. Shear failure was the main fracture mode of uncut rock extruded by the cutter, while tensile failure was the main fracture mode of extruded rock advanced slotting. The propulsion load of the cutter and the size of the rock fragment increased with increasing cutting thickness and inclination angle. The optimal values of cutting thickness and inclination angle of cutter were 30 mm and 40°, respectively. The load and specific energy consumption of cutter rock breaking decrease with increasing advanced slotting depth. However, the downward trend was no longer obvious when the slotting depth was more than 15 mm. The rock-breaking performance of the cutter increased with increasing radius, but the increasing amplitude tended to be constant when the cutter radius was greater than 70 mm.

Mechanism of accelerating creep rupture under high stress level of pack cementation aluminide coatings deposited onto T92 steel tube

An aluminide coating was deposited on commercial T92 steel by pack cementation. The coating exhibits a typical bilayer structure composed of an outer FeAl layer and an inner interdiffusion layer. The creep rupture life of the coated and uncoated T92 was investigated. Results show that the rupture life of the coated T92 decreases significantly, i.e., decreases by 46 % at the creep stress of 110 MPa compared with the uncoated T92. By examining the microstructure and fractography of the coated specimen, the mechanism of aluminide coating deteriorating creep rupture life is clarified. The main fracture mode of T92 substrate with and without coating is microvoid accumulation fracture. The characteristic microstructure of the aluminide coating is responsible for the premature creep rupture. The formation of columnar grains and brittle nature of FeAl phase make it tend to crack during creep. The phase transformation and needle-like AlN precipitates in the inner interdiffusion layer contribute to the inward propagation of cracks and accumulation of creep damage.

Effect of Grain Boundary Morphology on the High Temperature Tensile Properties of Developed Ni-Based Superalloy (LESS)

In order to investigate the mechanical properties of the developed wrought γ'-hardened Ni-based superalloy, LESS alloy (Low Eta Sigma Superalloy), and to understand the deformation behavior, tensile tests were carried out from room temperature to 800oC in comparison with IN 740H alloy. Like other superalloys, the tensile properties of the LESS alloy were found to be dependent on temperature. As the temperature rose between room temperature and 700oC, the ultimate tensile strength (UTS) decreased slightly, and then decreased sharply. The yield strength (YS) of the LESS alloy was found to be about 40% higher than that of IN 740H alloy from room temperature to 700oC. On the basis of microstructure observations, the high yield strength of the LESS alloy is thought to result from the dense grain boundary, where the fine carbides (MC, M23C6) and the γ' phase form a continuous film structure, which more effectively suppressed dislocation movements during deformation. It was found that the deformation mode of the LESS alloy changed from γ'- shearing to mixed mode (γ'-shearing + climb) at around 700~800oC. Although the main fracture mode of the LESS alloy was ductile dimple fracture, elongation decreased with temperature due to the local brittle region caused by micro-twins.

High Temperature Fatigue Behavior and Failure Mechanism of Ti-45Al-4Nb-1Mo-0.15B Alloy

Strain-controlled low cycle fatigue experiments were carried out on the TiAl alloy Ti-45Al-4Nb-1Mo-0.15B at 400 °C and 750 °C to reveal the cyclic mechanical behavior and failure mechanism. The TiAl alloy presents stable cyclic characteristics under fatigue loading at elevated temperatures. No obvious cyclic softening or cyclic hardening was manifested during experiments. The cyclic stress–strain relationship is well described by the Ramberg–Osgood equation. The fatigue lifetime at different temperatures has a log-linear relationship with the total strain ranges. The fracture morphology indicates the main fracture mode of fatigue specimens at 400 °C is a brittle fracture, while there is a ductile fracture at 750 °C. Meanwhile, the trans-lamellar fracture is dominant for the lamellar microstructure and the percentages of the inter-lamellar fracture decreases with the strain amplitude.

Effect of micro‐arc oxidation treatment on laser joint microstructures of Ti‐6Al‐4V alloy/CFRP

Resin‐based carbon fibre reinforced composite (CFRP) with excellent properties has been widely applied in aerospace industry. In this study, polyether ether ketone enforced composite (PEEK‐CFRP) and Ti‐6V‐4V titanium alloy had been joined by laser welding process. Micro‐arc oxidation (MAO) process was conducted on the surface of titanium alloy to improve the joint property. Fractures and mechanical properties of joints were analysed. Interfacial microstructures of the joints had been investigated, and the formation mechanism of joint had been figured out. The results showed that after MAO treatment, the joint strength had been greatly improved and the shear strength reached to 42.3 MPa compared with pretreated sample. The wettability of CFRP on titanium alloy was rising with the contact angle of 68.8°. Fracture showed that cohesive failure contributed to the main fracture mode. As joints were formed by both mechanical bonding and chemical bonding, ‘anchor‐shaped’ structures were found in the joint to enhance the mechanical bonding effects due to the flowing of the melted resin. (−OH) bonds were identified at the surface of MAO‐treated titanium alloy that provided conditions for hydrogen bond interaction. TiO2, TiO and Ti–V–C phase were found at interface, whereas Ti–F and Ti–O–F bonds were generated during the welding process, which made great contribution to the chemical bonds between titanium alloy and CFRP.

Effect of oxygen content of tantalum powders on the characteristics of parts processed by laser powder bed fusion

In this study, spherical tantalum (Ta) powders, having different oxygen contents, were fabricated by a radio frequency plasma spheroidization technique. A laser powder bed fusion (LPBF) technique was then used to consolidate these powders and the effects of different LPBF processing parameters on the densification, microstructure and mechanical properties of parts were investigated. The results indicated that the oxygen content of the used Ta powder has significant influences on the densification, microstructure and the consequent mechanical properties of printed parts. The relative density of the parts printed from the powders with a low oxygen content of ∼150 ppm was insensitive to the applied laser energy density and was higher than the ones prepared from the powders with higher oxygen contents. The grain refinement was observed and the texture in the side-view of the as-built samples changed from {100}∥BD to {111}∥BD with the increase in the oxygen content of the used powders. The microhardness and ultimate tensile strength of samples increased and the ductility of parts decreased with the increase in the oxygen content of the used raw powders. The main fracture mode of the LPBF consolidated Ta samples changed from a typical ductile rupture to the combination of cleavage and ductile rupture modes with the increase of the oxygen content in the processed parts.

Temperature effects on tensile behaviors and relevant deformation mechanisms of a low-cost nickel-based single crystal superalloy containing 1.5% Re

In this work, tensile properties and relevant deformation mechanisms were investigated particularly in a novel low-cost second-generation nickel-based single crystal superalloy containing 1.5 wt% Re. The results showed that the experimental alloy exhibited an anomalous yielding phenomenon. The yield strength of alloy reached a peak value of 1120 MPa but the corresponding elongation was only 8.2% at 760 °C. Furthermore, the double yield phenomenon occurred at 900 °C and above whereas the strain softening behavior appeared at high temperatures. With increasing temperature, the main fracture mode gradually evolved from pure shear to microvoid accumulation fracture. The dislocations sheared into γ′ particles and led to the formation of stacking faults, which was the dominant deformation mechanism below 760 °C. Particularly, the formation of both K-W locks and L-C locks is closely related to the peak yield strength of the alloy at 760 °C. At 900 °C, the tensile deformation was controlled by the combination of APBs shearing and Orowan bypassing γ′ particles. As the temperature further increased to 1000 °C and 1100 °C, the dislocation networks formed on γ/γ′ interface could hinder the dislocations shearing into γ′ particles. Meanwhile, Orowan bypassing and dislocations climbing became the dominant deformation mechanisms. The results of this work could provide vital support for development and application of low-cost SX superalloys.

Preparation of multicomponent A2Zr2O7 transparent ceramics by vacuum sintering

Recently, high-entropy oxide ceramics have become a hot topic in the field of high entropy materials. In this paper, multicomponent pyrochlore A2Zr2O7 transparent ceramics were prepared via vacuum sintering using combustion synthesized nanopowders. The phase analysis results indicate that the powders exhibit defective fluorite structure and the ceramics are in pyrochlore structure. The structural order degree of ceramics varies with the increase of incorporated components. It is found that the grain size of A2Zr2O7 ceramics is related with the component of A-site. The main fracture mode of final ceramics exhibit typical transgranular fracture. The multicomponent A2Zr2O7 ceramics exhibit excellent optical transmittance, and the highest in-line transmittance reaches to 80% for #A2ZO ceramic at 1880 nm.