Fracture Toughness KIC Research Articles

Stress corrosion cracking behaviors of FV520B stainless steel used in a failed compressor impeller

Stress corrosion cracking (SCC) in compressor impellers made of FV520B stainless steel frequently occurred, and hence was investigated by slow strain rate tests (SSRTs) and fracture toughness tests, combined with finite element analysis (FEA) based on ABAQUS and ZENCRACK software. SSRTs revealed that the FV520B stainless steel had slight susceptibility to aqueous brine solution, which increased with increasing temperature, and fracture surface exhibited ductile feature with dimples similar to that in nitrogen; however, this material showed significant susceptibility to NACE solution A at various temperatures, and fracture surface consisted of initial quasi-cleavage region and subsequent dimple region; both tempered martensite matrix and localized δ ferrites were susceptible to SCC. Fracture toughness KIC (JIC) of this steel was 150MPam1/2 (102KJ/m2) in air, but threshold stress intensity factor (KISCC) dramatically reduced to 48MPam1/2 in NACE solution A, and Boltzman equation was modified to describe SCC propagation rate (da/dt-K) curves. Comparison with FEA results indicated that the fracture of SSRT specimen was initially affected by environmental factor and then mainly attributed to mechanical factor, whereas the whole fracture process of impeller was closely related to corrosive environment. This study was useful for SCC evaluation of materials and failure analysis of compressor impellers.

Environmentally induced crack (EIC) initiation, propagation, and failure: A 3D in-situ time-lapse study of AA5083 H131

Environment Induced Cracking (EIC) has been observed in three dimensions (3D) at high spatial and temporal resolution for AA5083 H131 alloy using in situ synchrotron X-ray CT. The initiation of EIC cracks occurred in humid air at ∼0.5σy and started from corrosion sites associated with intermetallic particle clusters. Time-resolved monitoring of the crack growth confirmed cracks grow below threshold KIEIC and are susceptible to arrest under fixed displacement conditions. Even when the cracks exceed threshold KIEIC and enter sustained propagation they exhibit very uneven growth rates. Failure occurs when the K of the largest crack exceeds the fracture toughness KIC.

Fracture and fatigue behaviour of a laser additive manufactured Zr-based bulk metallic glass

Laser additive manufacturing of bulk metallic glass (BMG) provides an effective bypassing of the critical casting thickness constraints that limit the size of components that can be produced; however, open questions remain regarding the resulting mechanical properties. In this work, a Zr-based BMG known as AMZ4 with composition Zr59.3Cu28.8Nb1.5Al10.4 was printed using a laser powder bed fusion (LPBF) technique. Micro X-ray computed tomography results together with electron microscopy imaging revealed porous processing defects in LPBF produced AMZ4 that led to a loss in tensile strength. Fatigue crack growth studies revealed a fatigue threshold, ΔKth., of ∼1.33 MPa√m and a Paris law exponent of m = 1.14, which are relatively low values for metallic materials. A KIC fracture toughness of 24−29 MPa√m was found for the LPBF BMG samples, which is much lower than the KQ of 97−138 MPa√m and KJIC of 158−253 MPa√m measured for the cast alloy with the same composition. The lower fracture toughness of the laser processed AMZ4 was attributed to ∼7.5× higher dissolved oxygen in the structure when compared to the cast AMZ4. Despite the higher level of oxygen, the formation of oxide nanocrystals was not observed by transmission electron microscopy. Oxygen induced toughness loss was confirmed by dissolving elevated concentrations of oxygen into cast AMZ4 rods, which led to a reduction in bending ductility and changes in the short-range order of the glass structure, as revealed by synchrotron X-ray diffraction.

Microstructure, mechanical and thermo-physical properties of CVD TiCxN1-x coatings on cemented carbide substrates grown with C2H6 as C feeding precursor

The established industrial processes used for the growth of TiCxN1-x coatings by chemical vapor deposition (CVD) suffer from substantial limitations, either in regard of brittle phase formation or restriction in the C/(C + N) ratio. Using the alternative C precursor C2H6 allows to overcome these issues. Thus, within this work, the microstructure, phase composition, micro-mechanical and thermo-physical properties of CVD TiCxN1-x coatings grown with C2H6 were investigated. Through adjustment of the C2H6 and N2 flow in the feed gas, the C/(C + N) ratio in the coatings was varied between pure TiN and TiC0.80N0.20. All coatings are characterized by a single-phase face centered cubic structure. The 〈110〉 fiber texture present in all coatings becomes more pronounced with increasing C content. None of the investigated coatings showed thermal cracks on the surface. The thermal conductivity decreases with addition of C from 45 ± 5 W/mK in TiN to 32 ± 3 W/mK in all ternary TiCxN1-x coatings. TiC0.47N0.53 exhibits the highest hardness (30.0 ± 1.4 GPa), while TiC0.63N0.36 turned out as the stiffest coating with a Young's modulus of 576 ± 23 GPa. The fracture stress σF and toughness KIC are superior in coatings with moderate C and N content, with TiC0.63N0.37 being the strongest (σF = 7.7 ± 0.4 GPa) and TiC0.47N0.53 (KIC = 4.4 ± 0.3 MPa m1/2) the toughest within this series. Coatings with moderate to high C content were found to exhibit a microstructure provoking a lower thermal conductivity and improved mechanical properties compared to those with a low C/(C + N) ratio.

Preparation of glass–ceramic materials from coal ash and rice husk ash: Microstructural, physical and mechanical properties

Wastes such as coal and rice husk ashes, which are widely available in Colombia, were successfully used to synthesize glass-ceramics in the (Na2O)–CaO–Al2O3–SiO2 system, which are obtained from thermally treating the parent glasses. The raw materials were mechanically conditioned, and the glasses were designed based on the CaO/SiO2 molar ratio, which was varied between 0.25 and 0.39. The glasses were obtained by melting the powders at 1450°C for 2h, and the melted powder was then poured into water. To obtain the glass–ceramic material, the temperature of the glass thermal treatment, which was generally lower than 1000°C in all cases, was determined by differential thermal analysis. The glass-ceramics obtained were microstructurally, physically and mechanically characterized. In addition, the durability was determined in acidic and alkaline environments (HCl and NaOH solutions). Glass–ceramics with densities of 2607–2739kg/m3, water absorption below 0.1%, Vickers hardness above 600MPa and elastic modulus of ∼96GPa were obtained. The fracture toughness Kic was in the range of 0.39–0.59MPam1/2. The chemical durability was considered excellent (with mass losses of ∼0.5mg/cm2), therefore these glass–ceramics can be good candidates for different applications in the construction sector.

Laboratory investigation of the mode-I fracture of sandstone caused by a combination of freeze-thaw cycles and chemical solutions

The mechanical parameters and deterioration mechanisms of sandstone specimens obtained from the hydrofluctuation belt in the Three Gorges Reservoir Region of China were investigated under a combination of freeze-thaw cycles and different chemical solutions. The degradation in several mechanical characteristics was discussed, and the degree of freeze-thaw deterioration of the mechanical behaviors was quantitatively analyzed based on the crack propagation radii and a damage variable that reflects the changes in porosity. The results showed that the mechanical parameters of sandstone had a significant damage deteriorating trend under freeze-thaw cycles and different chemical solutions. And the degree of damage deterioration of fracture toughness KIC was the greatest, followed by that of tensile strength; that of compression strength was the smallest. There was obvious consistency among fracture toughness KIC, tensile strength, and compression strength of sandstone under a combination of freeze-thaw cycles and different chemical solutions; those were obvious linear relationship. The crack propagation radii for the various effects of freeze-thaw cycles and chemical solutions were smaller than those of the natural state and the radii generally decreased with increasing freeze-thaw cycles.

Comparison of laser-MIG hybrid and autogenous laser welding of M250 maraging steel thick sections—understanding the role of filler wire addition

The present work is directed towards understanding the role of filler addition in laser-MIG hybrid welding (LHW) process as compared with that in autogenous laser welding (ALW) process during welding of 10-mm-thick maraging steel plates. Addition of filler wire is beneficial in improving the edge bridging between the parts to be welded and also in improving the fusion zone microstructure. Single-pass LHW of the 10-mm-thick maraging steel plates was performed using a combination of 3.5-kW CO2 laser and synergic pulse MIG welding power source at a welding speed of 1 m/min. MIG filler wire with reduced solutes such as Mo and Ti contents was used for the LHW process. Simultaneously, double-sided single-pass ALW of similar plates was performed. The role of filler wire addition on the microstructure and mechanical properties of the LHW fusion zone (FZ) was studied and compared with that of ALW FZ. The usage of filler wire resulted in less solutes in the LHW FZ after welding, and hence, the volume fraction of “reverted austenite (RA)” formed during aging was minimized when compared with ALW FZ. During transverse weld tensile testing, the ALW welds yielded higher tensile properties when compared with the LHW weld due to their fine grain structure and high number density of precipitates. Whereas the KIc fracture toughness of the ALW FZ was low with the KIc value of 49.5 MPa√m due to the presence of the high amount of RA as compared with LHW which exhibited the KIc value of 77.5 MPa√m. The study vividly brings out the advantages of LHW process in improving the microstructure of the fusion zone due to the addition of filler wire.

Effects of solid-solution temperature on microstructure and mechanical properties of a novel 2000 MPa grade ultra-high-strength steel

A novel 2000 MPa grade ultra-high-strength steel AIR0509 with high fracture toughness and low cost has recently been developed. The effects of solid-solution temperature on the microstructure and mechanical properties of this steel were investigated. The increase in solid-solution temperature first increased and then decreased the values of ultimate strength (UTS) and Charpy U-notch (CUN) energy. The increase in the UTS and CUN values was caused by the dissolution of the primary carbides M6C and MC, while the decrease in both strength and toughness was due to the increase in the prior austenite grain size. Samples that were solid-solution treated at 1000 °C exhibited an optimal combination of strength and toughness with a UTS of 2020 MPa, yield strength of 1780 MPa, and CUN energy of 68 J, as well as a correlative fracture toughness KIC value of about 105 MPa m1/2.

Fracture mechanics behavior of TiNbSn alloys as a function of alloy content, cold working and aging

Mechanical properties such as tensile strength, ductility and elastic modulus, which are directly related to fracture mechanics characteristics, can be controlled during termo-mechanical manufacture of TiNbSn alloys. Cold working the alloys reduces its ductility and elastic modulus. Aging reduces their ductility further, but increases their elastic modulus. The present study aims to evaluate the effect of Nb (35% and 42% wt.) and Sn (0% and 2.5% wt.) contents on fracture toughness and fatigue crack growth of TiNbSn alloys of both cold worked and aged at 400 °C. The fracture toughness KIC was performed, and fracture analysis allowed to evaluate micromechanics changes. Crack growth rate(da/dN–ΔK) tests were also performed. Aging was beneficial to fracture threshold ΔKth and crack growth rate of the alloys. However, aging did not interfere with fracture toughness KIC. The increase in Nb content is beneficial to both fracture toughness and crack growth rate of the aged alloys. The addition of Sn in the aged 42% Nb alloy decreases the crack growth rate at higher ΔK. For alloys of similar elastic modulus, the size of dimples is proportional to fracture toughness.

High-pressure synthesis of high-performance submicron-sized polycrystalline β-Si3N4 bulk without additives

Dense and submicron-sized polycrystalline β-Si3N4 bulk samples with average grain size of 263 ± 68 nm have been synthesized using micron-sized α-Si3N4 powder (2 μm) as the starting material by simultaneously tuning the pressure and temperature conditions. The measured Vickers hardness HV of the submicron-sized polycrystalline β-Si3N4 could reach up to about 20 GPa, approaching the upper limit of the single crystal β-Si3N4. The measured fracture toughness KIC of the polycrystalline β-Si3N4 has been enhanced from 3.0 to 4.1 MPa m1/2, as much as 36%, while without sacrificing the hardness. This study demonstrates that using micron-sized powder as the starting material to synthesize submicron-sized polycrystalline bulks by phase transformation under extreme pressure conditions is an efficient pathway to break through the old paradigm—the higher the fracture toughness of a material, the lower the hardness.

Failure Prediction of AISI 420 Martensitic Stainless Steel Using the Theory of Critical Distances

This work aims to evaluate the capability of the theory of critical distances (TCD) to predict the static failure of U-notched AISI 420 martensitic stainless steel specimens with different geometric features under pure bending loading. Theoretical estimates of the stress intensity factor during fracture onset were calculated according to the line (LM) and point methods (PM), which consider the characteristic length L, inherent strength σ0, and notch tip radius ρ. Initially, L and σ0 were determined on the basis of the material’s properties (i.e., fracture toughness KIc and ultimate tensile strength σu,t), resulting in imprecise estimates. Conversely, L and σ0 determined using the appropriate analysis of linear–elastic stress fields ahead of notches with different sharpness provided highly accurate predictions. The microscopic study of fractured specimens ensured better comprehension of the results. Moreover, the accurate values of L and σ0 were used to predict the failure of V-notched specimens.

Fracture toughness and crack resistance curves of acrylic bone cements

The fracture toughness KIc of 11 clinically used acrylic bone cements was studied in air at room temperature with single edge V-notched beam specimens. By driving the crack step-wise through the specimens, crack resistance curves ("R-curves") were recorded. One group of bone cements showed an increase of the fracture toughness with increasing crack length (including CMW1+G and several Palacos bone cements) whereas another group (including Simplex, SmartSet, Copal and some Palacos bone cements) did not exhibit an R-curve behavior. The plateau values for KIc ranged from 0.93 MPa√m (Simplex P) to 1.98 MPa√m (Palacos R+G). The observation of the crack growth with an optical microscope revealed some mechanisms influencing the crack growth like the formation of microcracks in the extended damage zone of the crack tip, the attraction of the crack by inclusions or the shielding of the crack tip by bridges in the wake of the crack. Furthermore, bone cements could be distinguished by the pattern of the path the crack followed during propagation. The crack pattern of CMW1+G provides a possible explanation of the distinct R-curve behavior of this cement.

Experimental study on fracture toughness of a compacted clay using semi-circular bend specimen

Single-edge notched beam specimens have been used widely to determine the fracture toughness KIC of compacted clays under three-point bending loading. However, this specimen type is not very suitable for the material with low strength. In this study, notched semi-circular bend specimens were employed to determine the KIC of a compacted clay, the effects of initial notch length, specimen thickness, moisture content and dry density on KIC were investigated. Based on the testing results, the ratio of initial notch length to specimen radius a0/R = 0.3–0.5 was suggested. There was no remarkable thickness-dependent size effect for the compacted clay. To investigate the relationship between the KIC and tensile strength σt of the compacted clay, unnotched semi-circular bend specimens were employed for determining the σt. It is concluded that there is a good linear relationship between these two parameters. According to the discussion on the testing results of different compacted clays, an empirical formula KIC = 0.3283σt was obtained, which can be used to estimate the KIC with σt of directly compacted clays. Additionally, the size of fracture process zone of the compacted clay was calculated by an estimation method.

Characterization of nano ceramic composite of Mullite-Magnesia-Yttria stabilized Zirconia system

The oxide and nano ceramic system of Mullite-Magnesia-Yttria Stabilized Zirconia were studied. Some fracture toughness models were examined to evaluate the crack length parameter to comply with Palmqvist and median crack criteria at c/a ratio equal to 3.0. The mullite crystal structure was observed using SEM and looks likely needle, whereas 3YSZ crystal appears smoother. Through EDS analysis, it detected the oxides in Mullite system, i.e., Al2O3, and SiO2. With a similar method, some oxides observed in the Mullite-Magnesia-3YSZ system such as MgO, Al2O3, SiO2, Y2O3, and ZrO2. With XRD analysis, nano MMZ system shows containing Al6Si2O13, Corundum (Al2O3), Zirconia (ZrO2), and Quartz (SiO2). Fracture toughness KIC of the ceramic composite of nano mullite-magnesia-zirconia complies with Palmqvist and median crack criteria. Nano ceramic particle shows a higher value of hardness and fracture toughness while compared with oxide ceramic system.

Properties of TiC and TiN Reinforced Alumina–Zirconia Composites Sintered with Spark Plasma Technique

In this paper, Al2O3–ZrO2 composites with an addition of 20 wt% TiN and 10 wt% TiC were modified. The addition of zirconia in a range from 2 to 5 wt% of the monoclinic phase and 10 wt% of Y2O3 stabilised ZrO2 affected the mechanical properties of the composites. A new type of sintering technique—the spark plasma sintering (SPS) method—within a temperature range from 1575 °C to 1675 °C, was used. Vickers hardness, apparent density, wear resistance and indentation fracture toughness KIC(HV) were evaluated at room temperature. An increase of the sintering temperature resulted in an improvement of Vickers hardness and an increase of the fracture toughness of the tested composites. The tribological properties of the samples were tested using the ball-on-disc method. The friction coefficient was in a range from 0.31 to 0.55, depending on the sintering temperature. An enhancement of the specific wear rate was dependent on the sintering temperature. The mechanical properties of the samples sintered by pressureless sintering (PS) were compared. X-ray diffraction patterns were presented in order to determine the phase composition. SEM microstructure of the tested composites sintered at different temperatures was observed.

Numerical Analysis of Stress Causing Fracture Failure in the Gear Transmission System Applied on an Agricultural Machinery Equipment

In general, failures often occur in gear transmission systems, even this case also occurs in an agricultural machinery equipment for example in a hand tractor. This study aimed to determine the cause of the spur gear fracture through numerical studies. The fracture surface was observed using a scanning electron microscope (SEM) in order to identify crack initiation. The results of the chemical composition test indicated that the material used was according to the AISI 8620 standard. It is necessary to create a finite element analysis (FEA) modeling. The stress, strain and stress intensity factor was next analysed near the crack tip using the FEA. The results of the FEA pointed out that the value of the maximum stress intensity factor KI occurs in the center of the tooth which experiences an initial crack due to friction. The KI value obtained from the FEA indicates that the value is higher than the fracture toughness KIC. Finally, crack propagation occurs in the teeth of the spur gear that causes the tractor to stop working.

Influence of compaction and degassing on the properties of submicron WCCo produced by the PPS method

Influence of compaction and degassing on the properties of submicron WCCo produced by the PPS method. The present study is concerned with the effect of the parameters of the degassing operation (temperature, load and heating rate) conducted at the initial stage of the Pulse Plasma Sintering (PPS) process and the sintering temperature at the final stage of the process, on the properties and microstructure of WCCo with a 6wt% cobalt content sintered by this method. The results of the study have shown that when the heating rate is too high, the material obtained is porous. In most experiments, the sintering temperature of 1050°C appeared to be too low to obtain WCCo composites with density close to the theoretical value (GT). Sintering at the temperature increased to 1070°C yielded sinters with density above 99%GT, with hardness of about 1900 HV30 and fracture toughness KIC=9.3 MNm-3/2.

Improved mechanical performance and wear resistance of Ti-coated cBN–WC–Ni composites

Abstract

Hard yet tough CrN/Si3N4 multilayer coatings deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering

Hard yet tough coatings with simultaneously high hardness and toughness represent a new class of high-performance coatings, which are vital for tribological applications due to the enhanced surface/interface properties. In this work, CrN/Si3N4 multilayer coatings with various Si content were reactively deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering. The as-deposited coatings exhibit smooth surface morphology, well-defined nc-CrN/a-Si3N4 interfaces, uniform thickness and highly dense architecture. Microstructure evolution is induced by Si content changing from scaly nanocolumnar to densely packed fibrous grains to tapered crystallites. These features allow the achievement of the increased properties in terms of high hardness of 30.6 GPa, H/E* of 0.099, H3/E*2 of 0.387, elastic recover We of 58.5% and fracture toughness KIC of 3.69 MPa·m1/2. Hard yet tough CrN/Si3N4 multilayer coatings show excellent tribological properties by enhanced cooperative deformability through energy dissipation and stress relaxation at well-arranged nc-CrN/a-Si3N4 interfaces to reduce crack initiation and propagation, and adhesion failure, thanks to the enhanced H, H/E*, H3/E*2, We and KIC. Meanwhile, under limited normal load conditions, uniform tribolayers (Cr2O3, FeO(OH), Fe2O3 and Fe3O4) caused by tribochemical reaction on the worn surface also act to make contributions towards reducing friction and wear as a lubricant.

High performance epoxy-based composites for cryogenic use: A approach based on synergetic strengthening effects of epoxy grafted polyurethane and MWCNTs-NH2

Synergetic strengthening effects of epoxy grafted polyurethane (EP-PU) and aminated MWCNTs (MWCNTs-NH2) on mechanical performance of EP-based composites at 77 K were systematically investigated using the tested results at room temperature (RT) as comparisons. EP-PU was successfully synthesized by controlling the reaction between –OH in EP and –NCO in PU prepolymer, which showed the advantages of relatively low viscosity and outstanding chemical miscibility with EP matrix. The most obvious enhancements were achieved with contents of 30 wt% EP-PU and 0.2 wt% MWCNTs-NH2 in tensile, flexural, impact and KIC fracture toughness properties of EP composites as well as flexural properties and interlaminar shear strength (ILSS) of carbon fiber (CF)/EP composites at both RT and 77 K, which confirmed the synergetic strengthening effects of EP-PU and MWCNTs-NH2. Specifically, KIC fracture toughness of EP composites and ILSS of CF/EP composites at 77 K were enhanced by 26.0 and 31.4%, respectively, compared with the corresponding neat composites. The EP-PU component showed β or γ relaxation behavior at cryogenic condition and existed in form of scattered micro-spheres in cured EP matrix due to the phase separation. The riveting and bridging of MWCNTs-NH2 around EP-PU micro-spheres were considered to provide extraordinary synergy on the effective stress diversion and crack propagation inhibition at cryogenic condition through establishing the chemically bonded multi-scale interface.