Brittle Fracture Mode Research Articles

Stress Rupture Studies of V-notched Grade 92 Steel for High Temperature Applications

In the present study, grade 92 steel blanks were subjected to normalizing (1080 °C / 1 hr.) and tempering (780 °C / 1 hr.) followed by air cooling. Stress rupture tests were performed using start testing systems’ uni-axial creep testing machines. The specimens used were circumferential 60° V-notch specimens with notch depth of 0.5, 1.0 and 2.0 mm. The test temperature was 650 °C and net applied stress level was 195 MPa. Notch strengthening effect was observed in grade 92 steel due to generation of tri-axial state of stress at the notch root. Post-creep characterization included scanning electron micrography of creep fractured samples which showed mixed mode of fracture. The fracture at area near the notch root was dominated by brittle fracture whereas the one near the center of the specimen was dominated by ductile fracture. Brittle mode of fracture was attributed to the mechanical constraint at the notch root and ductile fracture developed following creep cavitations and micro-void coalescence.

Rotary ultrasonic machining of ZrO2-NbC and ZrO2-WC ceramics

ZrO2-based ceramics are extremely difficult to manufacture, rotary ultrasonic machining (RUM) being an interesting machining process for these materials. In this work, ZrO2-NbC and ZrO2-WC ceramic composites have been RUM-processed and the differences in cutting forces, surface quality and dominant machining mechanisms under different machining operations and conditions have been studied. In ZrO2-NbC, the presence of the ductile fracture material removal mode is more evident than in ZrO2-WC, where the brittle fracture mode seems to be dominant. This contributes to a slightly better machinability of ZrO2-NbC compared to ZrO2-WC, what results in slightly higher material removal rates (MRR) and lower surface roughness.

The effect of platinum-aluminide coating features on high-temperature fatigue life of nickel-based superalloy Rene®80

Low cycle fatigue is the most important failure mode in Aviation/Industrial engine rotary turbine parts. In this paper, the influence of Pt-aluminide coating parameters on high temperature low cycle fatigue behavior of superalloy Rene?80 which is used to manufacture turbine blades, has been investigated. For this purpose, initial platinum layers of different thicknesses (2?m and 8?m) were coated on fatigue specimens. Then the aluminizing process was performed with two conditions of low temperature-high activity and high temperature-low activity. The results of microstructure investigations performed by scanning electron microscope and X-ray diffraction phase analysis indicated a three-layer structure for the coating (bi-phase (Ni,Pt)Al+PtAl2, single-phase (Ni,Pt)Al and interdiffusion zone) with different chemical compositions at both thicknesses of the platinum layer and using both aluminizing methods. Also, the results of low cycle fatigue tests at 871?C, R=0 and strain rate of 2?10 -3 s-1 showed a decline in fatigue properties in coated specimens as compared to uncoated sample, at total strains of 0.4, 0.8, and 1.2%. This reduction was lower in the low temperature-high activity with platinum layer thickness of 2?m, while it was more significant in the high temperature-low activity with the platinum layer thickness of 8?m. The fractography studies on coated and uncoated specimens indicated a mixed mode of ductile and brittle fracture.

Microcrack Growth Properties of Granite under Ultrasonic High‐Frequency Excitation

The failure of most rock materials is essentially a process of crack initiation and propagation. It is of great significance to study the microcrack growth characteristics of granite under ultrasonic high‐frequency excitation for understanding the failure mechanism of rock under ultrasonic vibration. In this paper, the experimental and numerical simulation methods are used to study the propagation characteristics of rock cracks under ultrasonic vibration. Scanning electron microscopy (SEM) was used to observe the growth of microcracks in granite samples after ultrasonic vibration for 0 min, 2 min, and 4 min. A discrete element software PFC2D was used to simulate and solve the cracking mechanism of rock cracks under ultrasonic vibration. Also, it is found that the action of ultrasonic vibration can effectively promote the development of microcracks in the granite samples. The main three cracks causing the failure of quartz under the ultrasonic high frequency are intragranular cracks, transgranular cracks, and grain boundary cracks. The breakage of transgranular cracks usually contributes a shell‐like fracture, that is, a regular curved surface with a concentric circular pattern appears on the fracture surface, which is a typical quartz brittle fracture mode. In addition, the feldspar grain failure is mainly caused by intragranular crack and transgranular crack. Microcracks are wavy expansion in feldspar grain. Mica failure is mainly caused by grain boundary crack, and the effect of lamellar cleavage on the failure of mica is significant. Moreover, it is also found that the mechanism of microcrack propagation is tensile failure. The failure of feldspar grains is mainly contributed to the failure of granite.

Mechanical and Microstructural Characterization of an AZ91⁻Activated Carbon Syntactic Foam.

In this study, activated carbon (AC) particles were combined with AZ91 alloy to manufacture a magnesium syntactic foam. This novel lightweight foam has a very low density, in the range of 1.12–1.18 gcm−3. The results show that no chemical reaction occurred between the AZ91 matrix and the activated carbon particles. The mechanical properties of the foam were evaluated under quasi-static compression loading conditions, and showed a consistent trend for the energy absorption of the fabricated AZ91–AC syntactic foams. The deformation mechanism of samples was a brittle fracture mode with the formation of shear bands during the fracture of all samples.

Evaluation of cavitation erosion resistance of arc-sprayed Fe-based amorphous/nanocrystalline coatings in NaCl solution

Abstract The cavitation erosion behavior of Fe-based amorphous/nanocrystalline coatings has been investigated in specimens prepared by arc spraying process. The cavitation erosion test was performed in a vibratory cavitation apparatus using 3.5 wt% NaCl solution. The results showed that the FeNiCrBSiNbW coating with an amorphous/nanocrystalline structure exhibited higher cavitation erosion resistance compared to the 316L coating in 3.5 wt% NaCl solution. The spray formed defects such as pores, micro-cracks, intersplat regions and oxides in the FeNiCrBSiNbW coating had a negative effect on the cavitation erosion and corrosion behavior. The cavitation erosion failure of the FeNiCrBSiNbW coating was attributed to a brittle fracture mode. The primary cavitation erosion mechanism involved erosion of the intersplats regions, formation of pitting corrosion products and brittle detachment of sprayed layers.

Effects of Thermo-Mechanical Fatigue and Low Cycle Fatigue Interaction on Performance of Solder Joints

This paper demonstrates the feasibility of low cycle fatigue effects on the thermo-mechanical fatigue evolution in the solder joints of a power module. To achieve this goal, finite-element method (FEM) simulation and experimental studies have been carried out. One of the prepared samples was exposed to the sole thermal cycling while another one underwent frequent drop impacts, as an indicator of low cycle fatigue event, during the thermal cycling. The FEM results indicate that the thermal cycling leads to the accumulated creep strain in the solder joints. The amount of creep strain considerably increases when the drop impacts are coupled to the thermal cycling. This phenomenon is due to the additional peeling stress applied by drop impacts to the solder joints. It is also revealed that with an increase in the number of drop impacts, the rate of strain accumulation declines which may be due to the restriction of changes led by stress triaxiality. The fractography also validates that the sample exposed to both drop and thermal loadings shows an entire brittle fracture mode. Moreover, the EDS map shows that the elemental distribution heterogeneity and intermetallic growth were created under mutual effects of drop impacts and thermal cycle fatigue.

Effect of epoxy resin on thermal, dynamic, and mechanical properties of rigid poly(vinyl chloride)

This work is concerned with the effect of an epoxy resin on the properties of rigid poly(vinyl chloride) (PVC). The epoxy resin concentrations of 0, 1, 2, 4, and 6 phr were used to prepare PVC/epoxy polymer blends and the viscoelastic behavior of the blends was investigated by dynamic mechanical thermal analysis and rheometry test. The results revealed that the low molecular weight epoxy resin did not greatly affect the viscoelastic properties of PVC. From the morphological point of view, the smallest droplet size of epoxy dispersed in the polymer blends was found in the sample with 1 phr epoxy resin, and the largest one was for the sample with 6 phr epoxy. The thermal properties of PVC/epoxy blends were investigated using differential scanning calorimetry and thermogravimetric analysis, as well. According to our research, the initial decomposition temperature of PVC was increased about 6°C by the incorporation of epoxy resin. The results of tensile test showed that the addition of epoxy resin decreased the elongation‐at‐break of PVC about 50% in the samples without calcium carbonate and about 25% in the samples containing calcium carbonate. Moreover, the failure mode of PVC was changed from a ductile fracture mode to a brittle fracture mode with the addition of epoxy resin. J. VINYL ADDIT. TECHNOL., 25:E72–E79, 2019. © 2018 Society of Plastics Engineers

Effect of unstable crack growth on mode II interlaminar fracture toughness of a thermoplastic PEEK composite

An end notched flexure (ENF) test was used to study the effect of growth stability on mode II interlaminar fracture toughness (GIIC) of a carbon/PEEK composite. The instability of the ENF test was used to induce both stable and unstable crack growth with a static loading. Two techniques were used to evaluate mode II fracture toughness: the traditional compliance method and the infra-red thermography technique. The compliance technique has the advantage of being simple and it has already been confirmed for standard tests. Nevertheless its results may be inaccurate for unstable crack growth propagation. The infra-red thermography technique enables fracture toughness to be accurately measured for non-standard tests in which the crack growth may be unstable.Both methods are complementary and this study came to the conclusion that GIIC is sensitive to crack growth velocity. Fracture toughness showed a low value for crack growth start and unstable propagation, and a high value for stable crack growth. The transition between a ductile fracture mode (for low crack growth velocity) and a brittle fracture mode (for high crack growth velocity) explained the important variability in GIIC values.

Effect of Ni Addition to Sn0.7Cu Solder Alloy on Thermal Behavior, Microstructure, and Mechanical Properties

The addition of Ni on lead-free solder alloys Sn0.7Cu was tested in this study. A small amount of Ni was added to the solder alloys to evaluate the thermal behavior, microstructure, and mechanical properties of the composite solders after sintering and isothermal aging for 3 days and then compared with the results of the binary Sn0.7Cu solder. The results indicated that the ultimate tensile strength and yield tensile strength changed when adding 0.25 wt.% of Ni. The tested results of the differential scanning calorimeter showed that the addition of Ni (such as 0.5 and 1 wt.%) could obviously increase the solidification temperature of Sn0.7Cu alloys in the cooling process. When the Ni content was increased to 0.25 wt.% in the ternary condition, which includes the Sn0.7Cu-xNi (x = 0, 0.1, 0.25, 0.5, and 1 wt.%) solder alloys for the indentation creep test, the minimum creep rate reached the maximum; however, the trend was reversed as the Ni content was higher than this level. In addition, the microstructure of Sn-0.7Cu-xNi solder alloys was obviously different with the eutectic Sn-0.7Cu solder, such that the Ni gradually accumulated in the (Cu, Ni)6Sn5 IMCs within the Ni-containing solder alloys. Additionally, this process refined the microstructure of the Sn-0.7Cu solder. The fracture surface of the eutectic Sn-0.7Cu solder revealed ductile fracture modes; however, there was no mixed ductile–brittle fracture mode occurred when the Ni content was in the range of 0-1 wt.%.

Fundamental understanding of the deformation mechanism and corresponding behavior of RB-SiC ceramics subjected to nano-scratch in ambient temperature

To get insight into nano-scale deformation behavior and material removal mechanism of RB-SiC ceramic, nanoscratch experiments were performed using a Berkovich indenter. Structure changes in chips and subsurface deformation were characterized by means of Raman spectroscopy. The result shows that the SiC phase underwent amorphization in ductile chips, while no amorphous feature can be observed in brittle chips and substrate within scratch groove. The following estimated stress surround the indenter reveals that amorphous deformation in ductile chips is governed by tangential stress (above 95 GPa), whereas the dislocations-based substrate deformation mechanism was dominated by normal stress. In the end, the effects of normal load and scratching velocity on the scratch behavior including scratch residual depth, elastic recovery and friction coefficient that related to RB-SiC ceramic deformation mechanism were also analyzed. With the increase of normal load, the deformation mechanism transfers from ductile to brittle fracture mode and cause the decrease of elastic recovery and the increase of residual depth and friction coefficient. Furthermore, the increased high density of dislocations as a result of the increased scratching velocity give rise to the increase of scratch hardness, which finally result in the increase of elastic recovery and decrease of residual depth and friction coefficients. This study contributes a new understanding of the brittle material deformation mechanism during a nano-scale scratching process.

Effect of solid fraction on microstructures and mechanical properties of a Mg-Al-La-Ca alloy processed by rheocasting

Morphological evolution of microstructures and tensile properties in Mg-Al alloy containing La and Ca (Mg-6Al-3La-1Ca) processed by rheocasting with different solid fractions (fs) were investigated at this paper. An apparatus for semisolid metal alloy processing was used to melt and to obtain the rheocast material. Isothermal mechanical stirring experiments were carried out with fs = 0.30, 0.49 and 0.61 at 950 rpm for 10 min. A conventional casting (no stirring) was also obtained for comparisons. Results showed that microstructure in conventional casting is composed by α-Mg dendritic matrix and Al11La3, (Al,Mg)2Ca and Mg2Ca compounds. Non-dendritic and globular microstructures were observed in the rheocasting. For fs = 0.30 the acicular Al11La3 phase precipitated at globule boundaries, while for the fs = 0.49 and fs = 0.61 the acicular phase precipitated both inside and at globule boundaries. The ideal rheocasting condition was achieved fs = 0.30 due to the improvement of 76% for the ultimate tensile strength (UTS) and 80% in the strain to fracture when compared to the conventional casting. Fracture surface analysis pointed to a mixed ductile-brittle fracture mode containing dimples and quasi-cleavage facets for the lowest solid fraction (fs = 0.3), and a fully brittle fracture mode consisting exclusively of cleavage planes for the highest solid fraction (fs = 0.61).

Stress corrosion cracking of new 2001 lean–duplex stainless steel reinforcements in chloride contained concrete pore solution: An electrochemical study

An experimental SCC tests has been performed on 2001 lean–duplex stainless steel (LDSS) and 2205 duplex stainless steel (DSS) reinforcements. Specimens were tested by loads up to 85% of their yield strength and simultaneously exposed to 8% chloride in simulated concrete pore solution. Although the maximum corrosion current density value obtained for 2001 LDSS was three times higher than 2205 DSS, specimens demonstrated passivity. Cycling polarization curve was performed which shows that 2205 DSS does not develop any pit but 2001 LDSS does. Finally, specimens were loaded until failure; fracture surface analysis detected a combination of ductile and brittle fracture mode in 2001 LDSS. Even if 2001 LDSS shows worse SCC performance, it is enough to ensure the nominal service life of reinforced concrete structures with lower cost.

The role of vacuum degree in the bonding of Al/Mg bimetal prepared by a compound casting process

Lost foam casting (LFC) solid-liquid compound process has been regarded as an attractive method for preparing the Al/Mg bimetal, and the vacuum degree is an extremely important factor during the LFC process. In this work, the effects of the vacuum degree on the interfacial microstructures, mechanical properties as well as fracture behavior of the Al/Mg bimetal were systematically studied. The results show that the vacuum degree had a significant effect on the defect fraction and interface layer fraction of the Al/Mg bimetal, while the phase compositions of the interface layer were almost invariant. The defect fraction initially reduced with increasing vacuum degree and then increased when the vacuum degree exceeded 0.03 MPa, and the change rule of the interface layer fraction was opposite to that of the defect fraction. The phase distribution in the interface layer was heterogeneous, and the interface layer was divided into three regions: Al12Mg17+δ(Mg) eutectic, Al12Mg17+Mg2Si and Al3Mg2+Mg2Si. The microhardnesses of the interface layers of the Al/Mg bimetal obtained with different vacuum degrees were much higher than those of the Mg and Al matrixes. An optimal bonding was obtained with a vacuum degree of 0.03 MPa. The fracture of the Al/Mg bimetal mainly occurred in the junction zone of the Al12Mg17+Mg2Si region and the Al3Mg2+Mg2Si region, exhibiting a brittle fracture mode.

Microstructure and Mechanical Properties of Cast Al-5Zn-2Mg Alloy Subjected to Equal-Channel Angular Pressing

In the present work, cast Al-5Zn-2Mg alloy was processed through equal-channel angular pressing (ECAP) in route BC up to four number of passes. Microstructure and mechanical properties were investigated on processed and unprocessed materials. In cast condition, the material was composed of dendritic structure. After homogenization treatment, large-sized grains were observed. After ECAP processing, significant grain refinement was observed. After ECAP processing, high-density dislocations and high degree of misorientation between the grains were observed. In cast material, rod-shaped precipitates were observed, while, after ECAP processing, spherical-shaped precipitates were observed. ECAP processing leads to a noticeable improvement in the mechanical properties of the material. After four passes, 122% improvement in the microhardness and 135% improvement in the ultimate tensile strength of the material were observed. After three passes, a slight decrease in the mechanical properties was observed. This is attributed to the dissolution of the metastable η′ phase, annihilation of dislocations, dynamic recrystallization and texturing during ECAP processing. Brittle fracture mode was observed in tensile testing cast and homogenized samples. After ECAP processing, fracture mode was changed into shear fracture mode.

Effect of Partial Substitution of Mn for Ni on Mechanical Properties of Friction Stir Processed Hypoeutectic Al-Ni Alloys

In this study, the mechanical properties of as-cast and FSPed Al-2Ni-xMn alloys (x = 1, 2, and 4 wt pct) were investigated and compared with those of the as-cast and FSPed Al-4Ni alloy. According to the results, the substitution of 2 wt pct Mn for 2 wt pct Ni leads to the formation of fine Mn-rich intermetallics in the microstructure increasing the tensile strength, microhardness, fracture toughness, and specific strength of alloy by 22, 56, 45, and 35 pct, respectively. At higher Mn concentrations, the formation of large Mn-rich platelets in the microstructure reduces the tensile properties. Friction stir processing at 12 mm/min and 1600 rpm significantly enhances both the strength and ductility of the alloy. The tensile strength, yield strength, fracture strain, fracture toughness, microhardness, and specific strength of FSPed Al-2Ni-4Mn alloy improved by 97, 83, 30, 380, 152, and 110 pct, respectively, as compared to those of the as-cast Al-4Ni alloy. This can be attributed to dispersion strengthening of Ni- and Mn-rich dispersoids, formation of ultrafine grains, and elimination of casting defects. The fractography results also show that the brittle fracture mode of the as-cast Mn-rich alloys turns to a more ductile mode, comprising fine and equiaxed dimples in FSPed samples.

Stress corrosion cracking and brittle failure in a fiber-reinforced plastic (FRP) insulator from a 400 kV transmission line in humid environment

Abstract A fiber-reinforced plastic (FRP) suspension insulator belonging to the Israel Electric Corporation (IECo) failed catastrophically on a 400 kV transmission line during service at the humid south Israeli coastline. The failure occurred during the summer of 2016 after 15 years of service. The suspension had a declared life expectancy of 30 years. The failure resulted in a local outage at a high cost. An investigation was commissioned. The insulator includes a FRP rod with a housing and weather-sheds made of silicon rubber (SiR); the FRP rod is fitted into a metal connector at its end. The fracture occurred 30–40 mm above the metal fitting/grading ring area near the first weather-shed. Macroscopically, the fracture surface was partially characterized by the features of brittle fracture mode in FRP, while the rest of the fracture surface featured severe fiber pullout and debonding. It was also possible to observe that a large crack in the SiR housing led directly to the fracture surface area that included stained and crushed fibers. EDS analysis revealed that the housing around the crack and the stained fibers themselves were polluted with a significant amount of nitrogen, in addition to other foreign pollutants. After surveying similar cases, it was strongly suggested that the source of nitrogen was nitric acid formed as a result of corona discharges on the insulator's surface. The acid caused the housing, fiber and resin to undergo corrosion. SEM imaging revealed fiber fracture surfaces with a morphology typical of stress corrosion cracking, showing that stress was part of the failure mechanism. The fracture of fibers and loss of resin led to a decrease in FRP rod strength and to failure.

Effect of Uniaxial Tension on the Microstructure and Texture of High Mn Steel

Cold‐rolled to 42% thickness reduction and annealed at 500, 625, and 700 °C, an Fe–17Mn–3Al–2Si–1Ni–0.06C wt% steel is subjected to uniaxial tension and characterized via digital image correlation and electron back‐scattering diffraction. The cold‐rolled and 500 °C samples return similar microstructures comprising predominantly α′‐martensite and remnant ϵ‐martensite fractions and a trace fraction of untransformed austenite (γ) before and after uniaxial tension. For the 625 and 700 °C samples, uniaxial tension results in the transformation of the initially reverted and recrystallized γ into ϵ and α′‐martensite via strain localization. The γ shows the formation of , double‐fiber texture, whereas the ϵ and α′‐martensite show the development of the {hkil}ϵ and || ND fibers, respectively. extension twinning is also observed in ϵ‐martensite upon uniaxial tension. The cold‐rolled sample exhibits a mixed brittle and ductile fracture mode. The fracture of the 500 °C sample is similar to that of the cold‐rolled sample, whereas the 625 and 700 °C samples display a ductile fracture mode.

Effect of Mn and Cr on structure and mechanical properties of Al-10%Mg-0.1%Ti alloy

This study examines the effect of different concentrations of Cr and Mn on the microstructure and mechanical properties of Al-10%Mg-0.1%Ti Alloy by using Al-10Cr and Al-10Mn master alloys. The microstructures were studied via the aid of an optical microscope (OM), X-ray diffraction (XRD) and energy dispersive X-ray (EDS) integrated with scanning electron microscopy (SEM). Cr and Mn contents promote the formation of Al7Cr and Al11Cr2, and Al6Mn phases in the structures of the alloy. The mechanical properties of the alloy increase with Mn content (0.5–3 wt%) due to grain refining and dispersion hardening effect of Al6Mn while higher Cr content precipitates deleterious secondary brittle phase of Al11Cr2. Cr and Mn contents beyond 1 and 3 wt% deteriorate the ultimate tensile strength (UTS) and percentage elongation of the as-cast alloy due to the presence of higher area fraction of brittle intermetallic phases. A direct relationship exists between hardness values and the addition of Cr and Mn contents. Low and high Cr and Mn contents show ductile and brittle fracture modes.

Experimental and numerical investigation of mechanical behaviors of 2.5D woven composites at ambient and un-ambient temperatures

An angle-interlock 2.5D woven composites (2.5D-WC), as a man-made new structural layout in the family of woven composites, have recently attracted increasing interest in its promising applications in the fields of astronautics and aeronautics. Nevertheless, published results are rarely associated with the temperature-dependent mechanical behaviors of this material. The current work emphasizes on the evaluation of the thermo-mechanical behaviors, damage propagation and failure mechanisms of 2.5D-WC at ambient and un-ambient temperatures by using experimental and numerical approaches. Experimental results show warp mechanical modulus and strength experience a decrease trend when the temperature increases. However, the weft strength is not sensitive to temperature, giving an indication of anisotropic behavior. There is no necking phenomenon near the fracture surface regardless of temperature and loading direction, indicating a brittle fracture mode. Temperature and fiber yarn arrangement direction play a critical role in alternating the damage propagation and failure mechanisms. Additionally, simulation results quantitatively elaborate the corresponding experimental findings and proposed mechanisms, which can be used to evaluate the temperature-dependent mechanical behaviors of 2.5D-WC. Ultimately, we hope these findings could further promote the engineering applications of 2.5D-WC, especially in aero-engine fields.