Change In Fracture Mechanism Research Articles

Tiny yet tough: Maximizing the toughness of fiber-reinforced soft composites in the absence of a fiber-fracture mechanism

Tiny yet tough: Maximizing the toughness of fiber-reinforced soft composites in the absence of a fiber-fracture mechanism

Effect of Welding Heat Input on the Microstructure and Impact Toughness of HAZ in 420 MPa-Grade Offshore Engineering Steel

In the present work, the effect of the welding heat input on the microstructure, martensite–austenite (M–A) constituents, and impact toughness of the coarse-grained heat-affected zone (CGHAZ) in offshore engineering steel with Ca deoxidation is studied. With the heat input increased from 50 to 100 kJ/cm, the HAZ toughness decreased rapidly, while the measured microhardness decreases steadily. The grain sizes are increased from 52 to 132 μm, and the width of bainite lath increased from 0.4 to 2 μm. The area fraction of lath bainite (LB) decreased, while the area fraction of granular bainite (GB) increased. The average width of M–A constituents grows from 0.3 to 0.6 μm, and the average length grows from from 0.5 to 0.9 μm. Its area fraction is increased from 5.3 to 8.6% and then decreased to 6.1%, and its number density decreased from 0.7 to 0.2 μm−2. The morphologies of M–A constituents change from dot-like to slender and blocky, which are deleterious to impact toughness. The fracture mechanism changes from ductile to quasicleavage and cleavage as the heat input is increased. As the M–A constituents are always found as the cleavage initiation, they should be responsible for the decrease in HAZ toughness when the heat input is above 100 kJ/cm.

Evolution of Primary and Eutectic Si Phase and Mechanical Properties of Al2O3/Al-20Si Composites under High Pressure

To further improve the mechanical properties of Al-Si alloys. The phase, microstructure and mechanical properties of Al2O3/Al-20Si composites under different pressures were studied. The results show that the phase of Al2O3/Al-20Si composites are composed of α-Al phase, β-Si phase and Al2O3. Under the condition of hot-pressing sintering (0.02 GPa), a large number of Si phases with irregular shape and sharp angle are distributed on the α-Al matrix. Under high pressure solidification, the growth of primary Si phase is inhibited and the eutectic Si is spheroidized obviously. The microhardness of Al2O3/Al-20Si composite increases from 102.5 HV0.05 at 0.02 GPa to 156.4 HV0.05 at 4 GPa, which increases by 52.6%. The compressive strength increased from 381.5 MPa at 0.02 GPa to 469.1 MPa at 4 GPa, increasing by 23%. With the increase of solidification pressure, the fracture mechanism changes from cleavage fracture to quasi cleavage fracture.

Investigating the stress level impact on the creep rupture behaviour of 2195-T84 Al-Li alloy: Experimental and constitutive modelling

In the present study, the creep rupture behaviour and microstructural evolution of 2195-T84 Al-Li alloy are investigated at different tensile stresses. It is found that as the applied stress during the creep rupture process increases, the corresponding creep strain and creep rate significantly increase. Moreover, the evolution of microstructures, including precipitates, dislocation density and creep cavities at different stages is characterized using a transmission electron microscope (TEM), X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The main strengthening precipitates are identified as T1 (Al2CuLi) and θ′ (Al2Cu) phases. Obtained results show that as the applied stress increases, T1 and θ′ phases are gradually coarsened. This coarsening is more pronounced for θ′ phase. Furthermore, the creep cavities are mainly distributed at the interface between the insoluble second phase particles and the matrix, and their average sizes gradually increase as the applied stress increases. Meanwhile, the density and size of dimples on the fracture surface gradually decrease as the applied stress increases. Moreover, the main fracture mechanism changes from transgranular dimple fracture to quasi-cleavage fracture. Based on the microstructural evolution, a novel set of unified creep rupture constitutive model is proposed. The established model incorporates the evolution of microstructures, including the dislocation density, average length of T1 and θ′ precipitates and creep cavitation, and correlates microstructural variables with creep rate. The results calculated by the constitutive model are in good agreement with the experimental data, which validates the proposed model.

Fatigue crack growth behavior of friction stir processed super duplex stainless steels (SAF- 2507)

The effects of friction stir processing (FSP) on the fatigue crack growth (FCG) behavior of super duplex stainless steel (SDSS) SAF 2507 was investigated. Both base metal (BM) and friction stir processed (FSPed) compact tension (CT) samples were fatigue tested under constant amplitude load; at a stress ratio (R) of 0.1 and frequency of 10 Hz. Crack length was monitored using a video camera. The crack growth behavior for both BM and FSPed CT specimens was characterized using stress intensity factor and the J-integral computed using elastic-plastic finite element analysis (FEA). Optical and scanning electron microscopes (SEM) were used to analyze the fracture surfaces. The results of this study show that FSPed specimens have higher fatigue crack growth (FCG) resistance and fatigue life as compared to base material. In addition, the crack propagation under the above-mentioned conditions has revealed changes in fracture mechanisms, due to FSP. The microscopic analyses of the fracture surface of FSPed specimens revealed numerous crack initiations sites and complicated crack propagation paths which contributed to the retardation of the crack propagation.

Effects of heat treatment on the microstructure, texture and mechanical property anisotropy of extruded 2196 Al-Cu-Li alloy

The mechanical properties and microstructures of the extruded 2196 Al-Cu-Li alloy plates under different heat treatment parameters are tested and characterized along different directions. It is found that the mechanical property anisotropy of 2196 Al-Li alloy is significantly affected by heat treatment. The anisotropy is mainly caused by the differences of grain morphology, microtexture, recrystallization degree and secondary particles characteristics in different directions. The specimens in 0° and 90°directions have high strength, while the specimen in 45° direction has the best ductility. The main microtexture of alloy are {111}<112>oriented A*1 and A*2 shear textures, and aging treatment has little effect on the grain morphology and microtexture. The decrease of mechanical property anisotropy after aging treatment is mainly due to the increase of number density and distribution uniformity of T1 phase. The elimination of PFZ and the increase of grain boundary strength are also conducive to the decrease of anisotropy. The dislocations introduced by pre-stretching is beneficial to the uniform precipitation of the fine phases, resulting in the optimal mechanical property homogeneity of the alloy. The ductility in different directions is also anisotropic, and the fracture mechanism changes from ductile to intergranular and cleavage fracture modes after heat treatment.

Long-Term Stability of a RAFT-Modified Bulk-Fill Resin-Composite under Clinically Relevant Versus ISO-Curing Conditions.

The addition of RAFT (reversible addition-fragmentation chain transfer) agents to the matrix formulation of a bulk-fill resin composite can significantly decrease the required curing time down to a minimum of 3 s. Evaluating the long term-stability of this resin composite in relation to varied curing conditions in an in-vitro environment was this study’s goal. Specimens were produced according to either an ISO or one of two clinical curing protocols and underwent a maximum of three successive aging procedures. After each one of the aging procedures, 30 specimens for each curing condition were extracted for a three-point bending test. Fragments were then stereo-microscopically characterized according to their fracture mechanism. Weibull analysis was used to quantify the reliability of each aging and curing combination. Selected fragments (n = 12) underwent further testing via depth-sensing indentation. Mechanical values for either standardized or clinical curing were mostly comparable. However, changes in fracture mechanism and Weibull modulus were observed after each aging procedure. The final procedure exposed significant differences in the mechanical values due to curing conditions. Curing conditions with increased radiant exposure seemingly result in a higher crosslink in the polymer-matrix, thus increasing resistance to aging. Yet, the clinical curing conditions still resulted in acceptable mechanical values, proving the effectiveness of RAFT-polymerization.

Fracture mechanism and toughness of a rolled magnesium alloy under dynamic loading

Static and dynamic fracture experiments are performed using fatigue pre-cracked three-point bend specimens of a rolled AZ31 Mg alloy on a servo-hydraulic universal testing machine and a Hopkinson bar setup, respectively. The results are interpreted using in-situ optical imaging along with digital image correlation analysis. Microstructural analysis reveals that the fracture mechanism changes from twin-induced quasi-brittle cracking for static loading to micro-void growth and coalescence under dynamic loading accompanied by decrease in tensile twinning near the tip with loading rate. By contrast, the density of twins near the far-edge of the ligament and associated texture change enhance strongly with loading rate. The fracture toughness increases dramatically at high loading rates which is attributed to enhanced work of separation and dissipation in the background plastic zone.

Microstructural and Fractographic Analysis of CuAlNi Shape Memory Alloy before and after Heat Treatment

The paper presents comparison of microstructure and fracture surface morphology of the CuAlNi shape memory alloy (SMA) after different heat treatment procedures. The investigation was performed on samples in as-cast state and heat treated states (solution annealing at temperatures of 850 °C / 60’ / H2O and 920 °C / 60’ / H2O along with tempering at two different temperature 150 °C / 60’ / H2O and 300 °C / 60’ / H2O). The microstructure of the samples was examined by optical (OM) and scanning electron microscope (SEM) equipped with device for EDS analysis. The obtained fracture surfaces were examined by SEM. Optical and scanning electron microscopy showed martensitic microstructure in all investigated samples. However, the fractographic analysis of samples after tensile testing reveals significant changes in fracture mechanism. In both solution annealed states the results shows transgranular type of fracture, but after tempering at two different temperatures the difference is obvious. After tempering at 150 °C, along with transgranular type of fracture appear some areas with intergranular type of fracture. After tempering at 300 °C, fracture surface reveals completely intergranular type of fracture.

Effects of ethylene‐octene copolymer (POE) on the brittle to ductile transition of high‐density polyethylene/POE blends

AbstractThree kinds of ethylene‐octene copolymers (POE) were melt‐blended with high‐density polyethylene (PE‐HD) in different proportions. Detailed characterizations were conducted to analyze their structural differences of POE and its effects in toughening PE‐HD. The higher molecular weight POE can improve the toughness of PE‐HD. 60:40 PE‐HD/POE is elongated to break up to 700% while impact strength is 84.7 kJ/m2 at −30°C, which is 21‐fold of PE‐HD. In the brittle to ductile transition (BDT) during impact, the fracture mechanism changes from the crazing mode to the shear yield‐plastic deformation mode. The BDT temperature decreases as the POE molecular weight and its content increase. The interface strength in tension is estimated to access their effects. The Boltzmann‐type models were successfully extended to describe the typical S‐shaped curves in BDT of notched impact strength vs POE content or temperature. The supplementary decay model is suggested for the attenuation in toughening. Transition map in impact is proposed to select the use range of composition (c) and temperature (T) for high toughness. The curves are converted into 3D graph of T‐c‐impact strength for illustrating their coupling‐separate effects, and further into the contour map of impact strength in T‐c space for finding their partial equivalence.

Fracture mechanism and model of 18Ni maraging steel

ABSTRACT18Ni maraging steel is normally used to make key components by machining processes. It is necessary to understand the fracture mechanism and behaviour of 18Ni maraging steel for the study and further optimisation of machining processes. In this paper, the Split Hopkinson Tensile Bar tests accompanied by quasi-static mechanical tests are carried out to relate the stress triaxiality, deformation temperature and strain rate with the fracture strain. During the building up of the fracture model, it is noticed that the change of fracture mechanism from 600°C to 900°C brings about a non-linear relationship of fracture strain with the deformation temperature. This results in a large error in fitting the Johnson–Cook (J-C) fracture model, so a modified fracture model is put forward.

Effect of Shock and Vibration Loading on the Fracture Mechanisms of a VT23 Titanium Alloy

The structure nonuniformity a VT23 titanium alloy after shock and vibration loading is established to lead to larger plasticity of the material and changes in fracture mechanisms. Localization of deformation processes influences the sizes and quantitative distribution of tear dimples on the fracture surface. The approach was advanced to detect tear dimples on the fracture surface of the alloy, in particular after shock and vibration loading. The fractographic inspection was used to compute the dimple parameters, among them area, number, equivalent diameter, and visual depth. The above parameters for the conglomerates of detected dimples became the basis for statistical analysis and establishment of additional fracture mechanisms of the alloy at the microlevel. It provides the grounds the grounds for preliminary comparative evaluation of the cond of titanium alloys, modified by shock and vibration loading, using fractographic examination data.

Evolution of Microstructure and Properties of ZYK530 Magnesium Alloy during Extrusion-Forging compound Forming

The evolution of microsturcture and mechanical properties of ZYK530 Magnesium alloy during extrusion and die forging compound forming process were studied and analyzed. The results show that the as-cast ZYK530 alloy mainly consists of a-Mg matrix and I-Mg3Zn6Y and a small amount of W-Mg3Y2Zn3 and Mg2Zn3 phases. During extrusion to die forging forming, the grain size of as-cast ZYK530 Magnesium alloy is refined and uniform under the action of three-dimensional compressive stress and high temperature deformation. The I-Mg3Zn6Y phase of ZYK530 magnesium alloy decrease relatively, while the distribution of W-Mg3Y2Zn3 is more obvious, a small amount of Z-Mg12ZnY and Mg-Zn phases and undetermined phases can be observed in the matrix. Tensile test results show that the tensile strength, yield strength and elongation of extruded preformed specimens are increase by 32.6%, 18% and 18% respectively compared with as-cast specimens. The mechanical properties of final forged specimens are improved by 9%, 38.6% and 23% respectively compared with those of extruded preformed specimens. The fracture mechanism changes form as-cast cleavage fracture to mixed fracture mechanism of toughness and brittleness.

Влияние механизма дисперсионного твердения на закономерности пластической деформации и разрушения ванадийсодержащей высокоазотистой аустенитной стали

Nitrogen alloying of austenitic steels increases their corrosion resistance and improves mechanical properties. During heat treatment, high-nitrogen austenitic steels tend to the precipitation hardening and the increase of strength characteristics. In the current paper, the authors studied the effect of the duration of age-hardening at the temperatures of 700 °С and 800 °С on the structure, phase composition, plastic flow behavior, and fracture mechanisms of V-alloyed high nitrogen chrome-manganese austenitic Fe-19Cr-22Mn-1.5V-0.3C-0.86N (mass %) steel. The study revealed that after water-quenching at 1200 °С, the specimens possess the high strength properties, ductility and contain large (300-500 nm) (V,Cr)(N,C) particles. Aging at temperatures of 700 °С and 800 °С facilitates complex reactions of austenite discontinuous decomposition with the Cr 2 N-plate formation in grains and continuous decomposition with the formation of vanadium nitride-based particles in austenite. During the long-term aging (50 h at 700 °C and 10 h at 800 °C), the intermetallic σ-phase appears in specimens. At age-hardening, the observed phase transformations cause the changes in macro- and micro-mechanism of fracture in the specimens of steel under the study. In the initial state, the specimens show mainly the ductile transgranular fracture. After age-hardening, the fracture mechanism changes into the mixed mechanism with the elements of brittle intergranular and ductile transgranular fractures. When increasing the duration of aging and implementation of complex reactions of decomposition of solid solution, the specimens are fractured by the quasi-cleavage mechanism. The specimens aged at temperatures of 700 °С and 800 °С have quite similar precipitation hardening mechanisms, though the increase in aging temperature leads to the rising of the decomposition rate of solid solution. The sequence of transformations described above and the corresponding sequence of changes in the mechanisms of steel fracture are implemented faster when increasing the aging temperature.

Experimental Evaluation of the Influences of Water on the Fracture Toughness of Mudstones with Bedding

The effect of water on fracture toughness in various modes of mudstones was investigated using semicircular bend (SCB) specimens exposed to three-point bendings. Natural mudstone specimens are obtained using a special coring method and are then classified into three types (A, B, and C) corresponding to three directions/configurations of bedding planes (divider, arrester, and transverse). The results show that the Type A (divider configuration) specimens possess the largest fracture toughness value for all tested modes and same soaking time, whereas the Type C (transverse configuration) specimens have the smallest one. By increasing soaking time, the fracture toughness in all three modes decreases and the fracture mechanism changes from brittle failure to ductile failure. Among them, the Type C specimens have the highest degree of degradation for each soaking time period. Regarding the fracture modes, the degradation degree ofKIcis higher than that ofKIIcfor all three types of mudstone specimens. In addition,KIIc/KIcratio increases when soaking time is extended. Furthermore, in the initial and short soaking time stages, the experimentalKIIc/KIcresults are consistent with theoretical findings from modified MTS criterion. However, after being soaked 300 minutes for three types of specimens, the test curves deviate from the theoretical curves. Analogously, the mixed-mode I/II ratio ofKefftoKIcis consistent with the theoretical values in the initial stage when the degree of damage is low. With soaking time increasing, the experimental curve is gradually deviated from the theoretical curve. When soaking time reaches 300 minutes, the deviation is substantial. And the test data for the Type-A specimens are observed to provide better agreements with theoretical predictions by modified MTS criterion than those for the other two types of specimens.

Microstructure and mechanical properties of laser metal deposited AlSi10Mg alloys

ABSTRACTThin-walled specimens with more than 150 layers were deposited by laser metal deposition without cracks and concaving deformation. The microstructure in each layer could be divided into three zones according to the morphology. The homogeneity deteriorated with the rising of the input. The tensile strength dropped 29.7% when the porosity increased from 0.53% to 1.88%. Controlling the oxygen under 0.5% and optimising the heat input, the as-built tensile strength reached 360 MPa. The fracture elongation was enhanced from 3.9% to 12.7% when the heat input was increased from 480 to 1200 W. The decrease of the secondary dendrite arm spacing and the change of fracture mechanism is the main reason leading to the strengthening of the mechanical properties.

Influence of Thermal Deformation Parameters on Mechanical Properties and Microstructure Evolution of AA7075 Aluminum Alloy during Hot Stamping-Quenching Process

Hot tensile tests have been performed on AA7075 aluminum alloy to investigate the effects of temperature and strain rate on its deformation behavior during the hot stamping-quenching process. The corresponding mechanical properties and associated microstructures have also been studied. The results indicate that the flow behavior curves exhibit work hardening, a dynamic equilibrium, then a dramatic decrease, ultimately leading to fracture. Ductility of over 30% can be obtained at relatively low deformation temperature and strain rate. The fracture mechanism changes from ductile transgranular fracture to ductile intergranular fracture owing to the weakening of the grain-boundary strength at high temperatures. The strength of the tested samples after artificial aging treatment increases with the deformation temperature after solid-solution heat treatment. It is more appropriate to choose a temperature of 400°C and a strain rate of 0.1–1 s−1 for AA7075 aluminum alloy parts to simultaneously obtain the desired ductility and final strength in this process.

Effect of Thermal Cycle on Microstructure Evolution and Mechanical Properties of Selective Laser Melted Low-Alloy Steel.

Low-alloy steel samples were successfully fabricated by selective laser melting (SLM). The evolution of the microstructure and the mechanical properties were investigated with different values of the energy area density (EAD). The results revealed that the initial solidification microstructures of the single tracks with different EADs were all martensite. However, the microstructures of bulk samples under different EADs were not martensite and differed significantly even from one another. When EAD increased from 47 to 142 J/mm2, the mixed lower bainite and martensite austenite microstructure changed to granular bainite; further, the morphology of bainite ferrite gradually changed from lath to multilateral. Moreover, with the increase of EAD, the grain size was remarkably reduced because of the increasing austenitizing periods and temperature during thermal cycling. The average grain size was 1.56 µm, 3.98 µm, and 6.31 µm with EADs of 142 J/mm2, 71 J/mm2, and 47 J/mm2, respectively. Yield strength and tensile strength of the SLM low-alloy steel increased with the increase in EAD; these values were significantly more than those of the alloys prepared by traditional methods. The microstructure of the SLM low-alloy steel samples is not uniform, and the inhomogeneity becomes more significant as EAD decreases. Simultaneously, when EAD decreases, the fracture mechanism changes from ductile to a mixture of ductile and brittle fracture; this is in contrast to the samples prepared by traditional methods. This study also found a stress concentration mechanism around large pores during plastic deformation that resulted in a brittle fracture. This indicates that large-sized pores significantly degrade the mechanical properties of the specimens.

Hot Ductility and Fracture Phenomena of Low‐Carbon V–N–Cr Microalloyed Steels

The hot ductility of low‐carbon V–N–Cr microalloyed steel is studied using a thermomechanical simulator. The steel is solution treated at 1350 °C and cooled at 15 °C s−1 to the test temperature in the range of 650–1200 °C. Next, they are strained to failure at a strain rate of 4 × 10−3 or 4 × 10−2 s−1, and ductility troughs are obtained. The results reveal a ductility trough in V–N–Cr steels in the range of 650–900 °C with a minimum reduction in area (RA) value of 13.5 % at 800 °C and strain rate of 4 × 10−3 s−1. The occurrence of dynamic recrystallization from 950 to 1200 °C has a substantial contribution to high ductility and on the formation of film‐like ferrite at prior austenite grain boundaries. The fracture mechanism changes from intergranular to highly ductile microvoid coalescence in the temperature range of 800–1200 °C and at a higher strain rate. Ductility loss in the V–N–Cr steel is caused by the presence of a high density of VN and/or V(C,N) precipitates, which are linearly present within the matrix, and a small number of random precipitates are present at the boundary.

Improved microstructure and mechanical properties for SnBi solder alloy by addition of Cr powders

In this work, Cr powders with several weight fractions (0, 0.1, 0.2 and 0.3 wt%) were doped into SnBi solder. The effects of Cr on the microstructure and mechanical properties of composite solders were investigated. The test results show that the microstructure of SnBi-0.2Cr solder is distinctly refined compared to the pure SnBi solder. The elongation of solder slab and the UTS of solder joint are respectively enhanced by 56% and 10% with 0.2 wt% Cr in tensile tests. Moreover, the creep depth decreases by 34% when the content of Cr is 0.2 wt%. The morphologies of Cu6Sn5 IMC layers transforms from the elongated scallop-like to the continuous rough scallop-like and the thicknesses of Cu6Sn5 IMC layers are decreased in composite solder joints. It is obvious that the fracture mechanism changes from brittle fracture to ductile fracture in both solder joint and slab.