Nature Of Brittle Fracture Research Articles

Fabrication of structurally graded material (pure PLA/WFPC): Mechanical and microscopic aspects

The aim of this study is to provide first-hand knowledge on additively manufactured structurally graded material (SGM) (polylactic acid / wood-fiber polymer composites) wall. Polylactic acid or polylactide is most effectively used in manufacturing plastic films, bottles, biodegradable medical devices, textile, and agricultural goods. Wood-fiber polymer composites are essentially used in furniture, particleboard, oriented strand board, medium-density fibreboard, plywood, and constructions. SGM is characterized through the change in material composition that is applicable to specific functions. Fabricated SGM exhibited mechanical property such as tensile strength (15.78 ± 0.05 MPa), compressive strength (46.8 ± 2.7 MPa), flexural strength (88.2 ± 0.15 MPa), and impact energy (1.64 ± 0.07 J). Hardness value of SGM is 74.5 shore-D. The coupon studies revealed that SGM possesses better mechanical characteristics than wood-fiber polymer composites and is on par with polylactic acid. The observed morphologies on fracture surface indicate the brittle nature of fracture. This SGM come under advanced materials that are utilized in biomedical, defence, energy, aviation, and nuclear sector, for integrated structural and thermal application.

Microstructure, Mechanical Properties, and Corrosion Behavior of Co‐Based Stellite 6 Multilayer Overlays Deposited on ASTM A36 Steel by Gas Metal Arc Welding Process

Hardfacing is an effective method for restoring the damaged part. Herein, cobalt‐based superalloy Stellite‐6 is deposited on A36 substrate using gas metal arc welding process. The mechanical properties, microstructure, and corrosion behavior of Stellite‐6 overlays on A36 substrate are examined. Defect‐free hard overlays are obtained, and a dilution ratio of 19.9% is achieved with weaving technique. X‐ray diffraction analysis reveals the presence of M7C3, M23C6, M6C, and martensite in the Stellite‐6 hard overlays. The microstructure of the deposit contains martensite, carbides, and interdendritic precipitates while the mean hardness is 471 HV0.5. Tensile testing with specimens extracted on the longitudinal (LD) and transverse (TD) orientation of the stellite‐6 deposit shows a strength above 950 MPa along with the brittle nature of fracture. Grain growth is epitaxial and shows dendritic morphologies. The higher fraction of high‐angle grain boundaries improves the tensile strength of the deposit and interface region. The localized corrosion behavior of stellite‐6 hard overlays is found to be excellent, and the corrosion analysis exhibits a corrosion rate of 0.10 mils per year (mpy). The outcomes of the study highlight the feasibility of Stellite‐6 hard overlays for extending the service life of A36 steel in chloride environments.

Experimental Fatigue Studies of Additively Manufactured Fiber Reinforced Thermoplastic

Additive manufacturing (AM) is a field of study that is gaining importance due to quick manufacturing techniques while providing viable flexibility in design modifications. In comparison with conventional manufacturing methods, AM uses easier processes to manufacture intricate components. AM products maintain consistency as it is constructed by robust universal AM machines. In the present study, a continuous carbon fiber thermoplastics (CFRTP) as a replacement to aluminum alloy (Al6061-T6) material is used for additively manufacturing bicycle crank components. The results from the tensile test and fatigue behavior of the component indicate a strong dependence of percentage concentration of the reinforcement in the CFRTP. A finite element analysis is performed to ascertain the stress distribution for a static load condition as mimicked in the real world situations. To explore the brittle fracture nature of the CFRTP specimen, a SEM image depicting the microstructural nature is provided. It is imperative to note that the comparative studies indicate the light weight CFRTP crank specimen stands out to be a viable replacement to the Al 6061-T6 material.

Studies on mechanical behaviour and tensile fractography of boron carbide particles reinforced Al8081 alloy advanced metal composites

Metal composites are advanced materials in the field of automotive and aero applications like wing root fittings of an aircraft. Al8081 alloy with 3 and 6 wt% of micro B4C composites were produced with vortex process. The prepared Al8081 with micro B4C composites were microstructural characterized by SEM micrographs. Further, mechanical behaviors of these composites were studied to know the influence of boron carbide particles on hardness and tensile properties of Al8081 alloy. SEM micrographs revealed the uniform distribution of particles in the Al8081 matrix. The hardness of Al8081 alloy was enhanced with the addition of B4C particles. Ultimate and yield strengths were improved with the incorporation of hard boron carbide particles with slight decrease in ductility. Tensile fractured surfaces revealed the ductile mode of fracture in Al8081 alloy and B4C added composites exhibited both ductile and brittle nature of fracture.

Microstructure, Mechanical Properties and Fracture Behavior of Magnesium/Steel Bimetal Using Compound Casting Assisted with Hot-Dip Aluminizing

In this work, microstructure, mechanical properties and fracture behavior of the magnesium/steel bimetal using compound casting assisted with hot-dip aluminizing were investigated, and the interface bonding mechanism of the magnesium/steel bimetal were also analyzed. The results indicate that the magnesium/steel bimetal obtained without hot-dip aluminizing had larger gaps through the whole interface without reaction layers between magnesium and steel, leading to a poor mechanical bonding. After the steel substrate was hot-dip aluminized, an intermetallic layer along with an Al topcoat layer were formed on the surface of the steel substrate, and the intermetallic layer was constituted by Fe2Al5, τ10-Al9Fe4Si3, FeAl3 and τ6-Al4.5FeSi phases. In the case of the magnesium/steel bimetal obtained with hot-dip aluminizing, a compact and uniform interface layer with an average thickness of about 17 μm that consisted of Fe2Al5, τ10-Al9Fe4Si3, FeAl3 and Al12Mg17 intermetallic compounds was formed between the magnesium and the steel, obtaining a superior metallurgical bonding. The interface layer had much higher nano-hardnesses compared to the magnesium and steel matrixes, and its average nano-hardness was up to 11.1 GPa, while there were respectively 1.1 and 4.2 GPa for the magnesium and steel matrixes. The shear strength of the magnesium/steel bimetal with hot-dip aluminizing reached to 23.3 MPa, which increased by 8.59 times than that of the composites without hot-dip aluminizing. The fracture of the magnesium/steel bimetal with hot-dip aluminizing represented a brittle fracture nature, initiating from the interface layer.

Interfacial bonding mechanism and pouring temperature effect on Al/Cu bimetal prepared by a novel compound casting process

In this work, the Al/Cu bimetal was prepared by a novel lost foam casting (LFC) compound process, and the effect of pouring temperature on microstructure and mechanical properties of the Al/Cu bimetal was studied. The formation mechanism of the Al/Cu interface was also discussed. Results show that the Al/Cu interface consisted of intermetallic compounds layers (IMCs layers) and eutectic layer. The IMCs layers were identified as Al2Cu, AlCu, and Al4Cu9 phase layers, and the eutectic layer was composed of Al2Cu, α(Al) and Si phases. The pouring temperature primarily influenced the morphology and size of the IMCs layers and eutectic layer as well as the pore defect in the interface. Based on the cooling curve and the morphology of the IMCs layers, a new formation mechanism of the Al/Cu interface was proposed. The Al/Cu bimetal obtained with a pouring temperature of 800 °C had the relatively highest bonding strength, up to 81 MPa, resulting from its improvement of the IMCs layers. Shear fracture exhibited a brittle fracture nature, mainly initiating with the IMCs layers, and the Si phase also played an important role in the fracture process.

Characteristics and Formation Mechanism of the Interface of Mg/Al Bimetallic Composites Prepared by Lost Foam Casting

In the present work, the Mg/Al bimetallic composites were successfully prepared by the lost foam casting (LFC) process, and the characteristics and formation mechanism of the interface of the Mg/Al bimetallic composites were investigated. The results show that a uniform and compact metallurgical interface with an average thickness of about 1400 μm was formed between magnesium alloy and aluminum alloy. The interface layer of the Mg/Al bimetallic composites was composed of three different reaction layers, namely the Al12Mg17+δ(Mg) eutectic layer adjacent to the magnesium matrix, the Al12Mg17+Mg2Si interlayer and the Al3Mg2+Mg2Si layer close to the aluminum matrix. The microhardnesses of the interface layer were remarkably higher than those of the magnesium and aluminum matrixes. The stress strength of the Mg/Al bimetallic composites was up to 47.67 MPa. The fractograph of the push out sample mainly showed a brittle fracture nature. The formation of the interface of the Mg/Al bimetallic composites was attributed to the fusion and diffusion bonding. With the variations of the concentrations of the different elements at the interface, the Al3Mg2 intermetallic phase first formed near to the aluminum matrix, and then the Al12Mg17 and Mg2Si successively generated toward the magnesium matrix, finally obtaining the interface layer of the Mg/Al bimetallic composites.

Interfacial microstructures and mechanical properties of Mg/Al bimetal produced by a novel liquid-liquid compound casting process

In the present work, a novel method was proposed to produce Mg/Al bimetal using the lost foam casting (LFC) liquid-liquid compound process with a Zn interlayer, and the interfacial microstructures and mechanical properties of the Mg/Al bimetal were investigated for the first time. Obtained results reveal that a compact metallurgical interface was formed between the AZ91D magnesium alloy substrate and the A356 aluminum alloy substrate. The metallurgical interface included two reaction layers. The reaction layer adjacent to the AZ91D substrate was composed of α-Mg, Al5Mg11Zn4 and MgZn phases, and the reaction layer adjacent to the A356 substrate consisted of α-Al, Mg32(Al, Zn)49 and Mg2Si phases. The addition of the Zn interlayer not only prevented different liquid metals from directly mixing, but also restrained the formation of Mg-Al intermetallics. The microhardnesses of the reaction layers were much higher than those of the substrates, and the reaction layer close to the A356 substrate had the highest microhardness. The shear strength of the Mg/Al bimetal was about 10.91 MPa, and the fracture surface of the Mg/Al bimetal exhibited a brittle fracture nature.

Hydrogen Susceptibility of Pre-strained Type 316L Austenitic Stainless Steels in Aqueous Solutions

This paper presents the observations made as a result of hydrogen effects in austenitic stainless steels which led to reduction of its original mechanical properties. This paper therefore seeks to understand the mechanisms and effects induced by hydrogen leading to embrittlement. The samples of Type 316L austenitic stainless steel with 20% pre-strain were charged with hydrogen through galvanostastic cathodic loading for 12, 24, 36, and 48 hours respectively, with sample as Cathode and the platinum wire as Anode. Afterwards, they were fractured under tensile test at a slow strain rates. The loss of ductility was evident as observed by the brittle nature of fracture. Small pieces were cut-off near the fracture region and examined through XRD and SEM analysis. The peak widening and slight shifting of the peak positions was observed. The surface cracking was also observed, an indication of surface-induced stresses. The severity of the effects increased with the hydrogen loading time, which is predictive of a real service conditions for austenite steels in aqueous environments.

A unified and universal Griffith-based criterion for brittle fracture

Within the framework of continuum fracture mechanics, brittle fracture of a notch is conventionally distinguished from that of a crack due to the absence of stress singularity near the notch root. It is revealed that the fracture stress, which is equal to the ideal strength, effectively describes the brittle fracture from the root of a nano-notch; however, it fails below a critical dimensional limit (i.e., an extremely confined stress concentration field of 5 nm) due to the continuum assumption for the discrete atomic systems. On the basis of atomic fracture mechanics accounting for atomic discreteness, surprisingly, the Griffith-based criterion which is originally proposed for cracks describes brittle fracture well in non-crack systems (notches) even below the critical size. We further demonstrate an ultimate unification of the proposed Griffith-based criterion for brittle fracture in both non-crack and crack systems even with different shapes for all scales. These results provide an in-depth understanding of the inherent nature of brittle fracture, and greatly benefit the safety evaluation and design of brittle failure in wide range of engineering applications.

Effects of Melt-to-Solid Insert Volume Ratio on the Microstructures and Mechanical Properties of Al/Mg Bimetallic Castings Produced by Lost Foam Casting

A356 aluminum and AZ91D magnesium alloys, which act as a solid insert and a melt, respectively, were used to prepare Al/Mg bimetallic castings using the lost-foam casting process, and the effects of the melt-to-solid volume ratio (VR) on the microstructures, mechanical properties, and fractographies of the Al/Mg bimetallic castings were investigated in this paper. Obtained results show that the average thickness of the reaction layer between aluminum and magnesium significantly increased with increasing VR, and a compact and uniform interface was obtained with a VR of 14.6. The reaction layers of all the bimetallic castings obtained by different VRs mainly consisted of the Al12Mg17 + δ eutectic layer close to the magnesium matrix, the Al12Mg17 + Mg2Si intermediate layer as well as the Al3Mg2 + Mg2Si layer next to the aluminum base. The microhardnesses of reaction layers of all the bimetallic castings with different VRs were considerably higher than those of the magnesium and aluminum matrix alloys, particularly the Al3Mg2 layer. Excessive thick reaction layer and pore defects remarkably weakened the bonding strength of the bimetallic castings, especially pore defects, and a relative maximum shear strength was obtained with a VR of 14.6. The fractographs of the push-out samples showed a change in the fracture surface from a mixed brittle and ductile fracture nature to that of a brittle fracture nature with the increase of VR.

Effect of Nanoclay, Cloisite15A on the Mechanical Properties and Thermal Behavior of Glass Fiber Reinforced Polyester

A series of glass fiber reinforced polyester nanocomposites have been prepared with varying compositions viz., 0, 0.5, 1, 1.5 and 2 wt % Cloisite15A as nanofiller.. The effects of nanoclay content on the mechanical, dynamic mechanical and thermal properties of nanocomposites were studied. Tensile testing, Impact testing, Dynamic Mechanical Analysis and Thermogravimetric analysis are employed to investigate mechanical and thermal characteristics. Tensile and impact test results are pointing to the optimum percentage of clay loading as 1%. Also the Tensile modulus attains the optimum status at 1% filled samples for different Cross head speed (CHS). Dynamic mechanical behavior does not indicate any consistent behavior. However the storage modulus showed an improved performance for 1% nano filled sample. The glass Transition Temperature, Tg falls in the range 95 to 110 oC for all samples. Thermogravimetric Analysis indicates the first phase of degradation starts around 200 oC for all samples. The fractured surface analyzed by Scanning Electron Microscopy(SEM) indicate brittle nature of fracture for pure polyester reinforced with glass fiber mat, Whereas the fracture changed to ductile nature by the addition of nano clay.

Effects of Be additions on microstructure, hardness and tensile properties of A380 aluminum alloy

The effects of beryllium (Be) on the microstructure, hardness and tensile properties of A380 aluminum alloy were investigated. The base and Be-containing A380 alloys were conventionally cast in a ductile iron mold. The microstructure evolution was investigated using SEM and optical microscope. The mechanical properties were assessed using tensile and hardness tests, finally the rapture surfaces of the used samples were studied to reveal the fracture mechanism in the presence of Be. The results revealed that the platelet β intermetallic phases were transformed into relatively harmless Chinese script Be–Fe phase and eutectic Si phases changed from flake-like particles into fine ones. The corresponding ultimate tensile strength (UTS) and elongation values increased from 270 MPa to 295 MPa and 3.7% to 4.7%, respectively. Additionally, the hardness of A380 alloy decreased continuously with increasing Be content. While the fracture surfaces of the unmodified A380 alloy tensile samples showed a clear brittle fracture nature, while finer dimple and fewer brittle cleavage surfaces were seen in the alloys with Be addition. Moreover, in the presence of Be, due to the refined phases, there has been a decrease in the values of hardness.

Effects of vibration frequency on microstructure, mechanical properties, and fracture behavior of A356 aluminum alloy obtained by expendable pattern shell casting

A simple, economic, and effective mechanical vibration method was introduced into the solidification process of A356 aluminum alloy during the expendable pattern shell casting process, and the effects of vibration frequency on microstructure, mechanical properties, and fracture behavior of the A356 alloy were investigated. Obtained results showed that the sizes and morphologies of α-Al primary phase and eutectic silicon particles were significantly improved by the mechanical vibration, and the mechanical properties and density of the A356 alloy greatly increased. With increasing vibration frequency, the grain size and secondary dendrite arm spacing (SDAS) continuously decreased, and the shape factor increased, and the mechanical properties and density of the A356 alloy gradually increased. With a vibration frequency of 100 Hz, the grain size and SDAS decreased by 32 and 19 %, respectively, and the shape factor increased by 262 %, and the average length, width, and aspect ratio of the silicon particles decreased by 45, 6, and 42 %, respectively, compared to that of the sample without vibration. Meanwhile, the tensile strength, yield strength, elongation, and hardness of the A356 alloy sample were, respectively, 35, 42, 57, and 28 % higher than those of the sample without vibration. In addition, the mechanical vibration changed the fractograph of the A356 alloy from a clear brittle fracture nature of the alloy without vibration to an obvious dimple fracture nature, and with the increase of vibration frequency, the dimples were very deep and well distributed with a high density.

Macroscopic Bifurcation and Fracture Mechanism of Polymethyl Methacrylate

Polymethyl methacrylate (PMMA) is one of important engineering materials and has been widely applied in various fields. Owing to the catastrophically brittle fracture nature, exploring the fracture mechanism of PMMA is of crucial significance to guide its engineering application. In this work, tensile tests with several kinds of sample geometries including unnotched samples and notch samples with different widths were designed to systematically investigate the macroscopic bifurcation of PMMA. Based on the sufficient experimental evidences, the traditional critical stress intensity criterion was found inappropriate for predicting the macroscopic bifurcation of PMMA. Finally, the fracture features were explicitly clarified based on the detailed experimental observations, and a new explanation to elucidate the fracture mechanism of PMMA was proposed.

Effects of rare earth elements addition on microstructures, tensile properties and fractography of A357 alloy

The effects of rare earth (RE) containing Ce and La elements addition on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy under as-cast and T6 conditions were systematically investigated in this study. Obtained results showed that the addition of RE obviously reduced the sizes of the α-Al primary phase and eutectic silicon particles as well as SDAS value and improved the morphology of eutectic silicon particles. The optimum level of added RE content were 0.2wt%, and the aspect ratio of eutectic silicon particles of the A357 modified alloy under as-cast and T6 conditions decreased 142% and 174%, respectively, compared with the unmodified alloy. In addition, the addition of RE greatly improved the tensile properties of A357 alloy as result of the significant improvement in microstructure, especially in elongation under T6 condition. The fracture surfaces of the A357 unmodified alloy tensile samples showed a clear brittle fracture nature, and its fracture path passed through the eutectic silicon particles and displayed a transgranular fracture mode, leading to poorer ductility. The fracture path of the A357 modified alloys passed through the eutectic phase along the grain boundaries of the α-Al primary phase, and the fracture generated by dimple rupture with cracked eutectic silicon particles, and it showed an intergranular fracture mode, resulting in superior ductility.

Multiscale Fracture Model for Quasi-Brittle Materials

Fracture of quasi-brittle heterogeneous materials is steered by processes at several different scale levels. These processes can progress independently or affect each other. In order to model fracture of such materials one should account for all rupture processes contributing to overall fracture process. This paper is presenting structural-temporal approach for analysis of multiscale nature of brittle fracture. Notion of spatial-temporal cell for different scale levels is introduced. Problem of experimental determination of a fixed scale level is discussed. Possible interconnections of this scale level with higher and lower scale levels are discussed. It is shown that this can give a possibility to predict fracture on a higher (real) scale level having experimental data obtained on a lower (laboratory) scale. This possibility is of extreme importance for many applications where the possibility to evaluate material strength properties on real structure scale level does not exist (ex. geological objects, big concrete structures, trunk pipelines, etc.).

Distribution of the Mechanical Strength of Solid Catalysts

The mechanical strength of solid catalysts is distributed over a wide range of values. The suitability of the normal, lognormal and Weibull distributions to model the catalyst strength variation was judged by three goodness-of-fit criteria: the coefficient of determination, Akaike information criterion and Kolmogorov–Smirnov test. It is concluded that the Weibull distribution is most universal to represent the catalyst strength data, though sometimes it may not be the optimal candidate. It is elucidated that the catalyst strength variation results from the brittle fracture nature of the mechanical failure of solid catalyst materials. The low-strength/probability part of the catalyst strength distribution is the key domain for the mechanical reliability, while the mean strength that has traditionally been taken as a measure of the catalyst strength is of less importance. It is also pointed out that a mechanical strength distributed in a narrow window, i.e., with a high Weibull modulus, is beneficial to industrial applications of solid catalysts.

Main Features of Designing With Brittle Materials

Brittle material behavior and mode of failure are contrasted with those characteristics of ductile materials. The stochastic nature of brittle fracture, which results from the random occurrence of fracture-initiating microstructural imperfections, necessitates a probabilistic fracture mechanics approach to design with brittle materials. It is also clearly shown which main properties of brittle materials have to be optimized to improve the reliability of mechanically loaded components made of brittle materials. Important features of designing with brittle materials are elucidated and illustrated by an exemplary design calculation of a ceramic disc spring. It is shown how even environmentally induced subcritical crack growth, characteristic of ceramic materials, can be adequately accounted for in the assessment of reliability.

Probabilistic approach for prevention of structural failure

A high strength low alloy steel was heat treated to produce two different microstructures. Multiple precracked specimens were tested at a low temperature, and a large scatter in the fracture toughness values was observed. The scatter is attributed to the probabilistic nature of brittle fracture. Based upon the Griffith equation and the 'weakest link' theory, it is derived analytically that the fracture toughness values follow a Weibull distribution, with a Weibull modulus of 4. The experimental data were used to examine the theoretical model, which was found to be applicable only when the fracture is truly controlled by the weakest link. The weakest link theory should be used with caution, although usually it errs on the safe side in a reliability assessment.