Fracture toughness properties of high-strength martensitic steel within a wide hardness range

Fracture toughness tests were carried out on six grades of high-strength martensitic steel within the hardness range from 270 to 475 HB. Four types of tests were performed: (a) Charpy V-notch (CVN) impact over the temperature range −120 to 60 °C, (b) plane strain fracture toughness, K _IC , near the onset of crack growth, (c) fracture toughness, J _IC , near the initiation of slow crack growth, and (d) fracture toughness, J _iC , and crack tip opening displacement (CTOD _iC ) at the onset of slow crack growth using direct current potential drop (DCPD) technique. Further, true plane strain fracture toughness, K _o , at the onset of crack initiation was determined. Fracture toughness behavior including the measured and determined values of CVN, K _IC , K _o , J _IC , K _iC , and CTOD _iC have been interrelated over the entire hardness range using the various analytical and empirical correlations reported in the literature. The results indicate that the steel acquires the optimum fracture toughness properties at a hardness of 305 HB, corresponding to a tempering temperature of 630 °C. Further, the steel exhibits a slight 300 °C temper embrittlement phenomenon.     

Enhanced fracture toughness in carbon-nanotube-reinforced amorphous silicon nitride nanocomposite coatings

Multiwalled-carbon-nanotube (MWCNT)-reinforced silicon nitride coatings were grown to evaluate the toughness contribution of nanotubes in a ceramic coating. An MWCNT array was first grown using catalytic chemical vapor deposition of acetylene on a silicon substrate. This aligned MWCNT preform was then infiltrated with an amorphous silicon nitride matrix by low-pressure chemical vapor deposition of dichlorosilane (DCS) and ammonia (NH₃). The fracture toughness of this material was determined by generating cracks using nanoindentation and then employing finite-element analysis to estimate the bridging toughness contribution of the MWCNTs. The MWCNT bridging toughness of the composites is determined to be ～5.6 MPa m^1/2, which is seven times higher than that of the matrix. The interfacial frictional stress is also estimated and ranges from 7 to 20 MPa.     

Mechanical properties of compacted graphite cast iron with different microstructures

Tensile strength, fracture toughness and impact properties were evaluated in compacted graphite (CG) cast iron with ferritic, pearlitic and ausferritic microstructures. Ultimate tensile strengths for the ferritic and pearlitic samples were 337 and 632 MPa respectively. The austempered samples showed a significant increment in the strength and recording values between 675 and 943 MPa. The fracture toughness test revealed that the stress–intensity factor K_IC was 34·0 MPa m^1/2 for the ferritic CG iron, 39·7 MPa m^1/2 for the pearlitic and between 51·0 and 58·0 MPa m^1/2 for the austempered irons. On the other hand, CG iron with ferritic matrix exhibited the best impact properties with absorbed energy of 33·3 J. The absorbed energy of the pearlitic CG iron was the lowest, 14·3 J, while the austempered samples showed values between 17·2 and 28·4 J. Complementing these results, the critical crack size was also analysed.     

Failure modes and materials performance of railway wheels

In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, K _ID , is obviously lower than static fracture toughness, K _IC , and has the same trend as K _IC . The ratio of K _ID /σ _YD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.     

Al2O3/WB2 composite ceramic tool material reinforced with graphene oxide self-assembly coated silicon nitride

Graphene oxide coated silicon nitride coated powder was prepared by electrostatic self-assembly technique. Microstructure analysis shows that GO is uniformly coated on the surface of silicon nitride and evenly dispersed in the matrix material. The microstructure analysis and performance tests were carried out on ceramic tools with different volume contents of Si₃N₄@GO coated powder. The results show that when the content of Si₃N₄@GO is 7. 94 vol%, the comprehensive mechanical properties of Al₂O₃/WB₂/(Si₃N₄@GO) ceramic tool materials are the best, with the Vickers hardness, flexural strength and fracture toughness 18.7GPa, 740 MPa and 8 .2MPa·m^1/2, respectively. Compared with the Al₂O₃/WB₂ ceramic tool material, the ceramic tool with Si₃N₄@GO coated powder increased the flexural strength and fracture toughness by 30.3% and 36.7%.     

脆性材料断裂韧性测试及表现断裂韧性的估算

用２种测试技术测试了脆性材料的断裂韧韧性,一种是利用楔入法在悬臂梁试件预裂出自然裂纹;另一种是用研磨法在三点弯曲试件上作出微米级的尖缺口裂纹。分别测试了３种材料的断裂韧性,并用提出的模型分别评估了多晶和非晶脆性材料的断裂韧性。     

Tensile and fracture toughness properties of SiC<Subscript>p</Subscript> reinforced Al alloys: Effects of particle size,particle volume fraction,and matrix strength

The goal of this work was to evaluate the effects of particle size, particle volume fraction, and matrix strength on the monotonic fracture properties of two different Al alloys, namely T1-Al2124 and T1-Al6061, reinforced with silicon carbide particles (SiC_p). From the tensile tests, an increase in particle volume fraction and/or matrix strength increased strength and decreased ductility. On the other hand, an increase in particle size reduced strength and increased the composite ductility. In fracture toughness tests, an increase in particle volume fraction reduced the toughness of the composites. An increase in matrix strength reduced both K _crit and δ_crit values. However, in terms of K _Q (5%) values, the Al6061 composite showed a value similar to the corresponding Al2124 composite. This was mainly attributed to premature yielding caused by the high ductility/low strength of the Al6061 matrix and the testpiece dimensions. The effect of particle size on the fracture toughness depends on the type of matrix and toughness parameter used. In general, an increase in particle size decreased the K _Q (5%) value, but simultaneously increased the amount of plastic strain that the matrix is capable of accommodating, increasing both δ_crit and K _crit values.     

Fracture of ECAP-deformed iron and the role of extrinsic toughening mechanisms

The fracture behaviour of pure iron deformed by equal-channel angular pressing via route A was examined. The fracture toughness was determined for different specimen orientations and measured in terms of the critical plane strain fracture toughness, K_IC, the critical J integral, J_IC, and the crack opening displacement for crack initiation, COD_i. The results demonstrate that the crack plane orientation has a pronounced effect on the fracture toughness. Different crack plane orientations lead to either crack deflection or delamination, resulting in increased fracture resistance in comparison to one remarkably weak specimen orientation. The relation between the microstructure typical for the applied deformation route and the enormous differences in the fracture toughness depending on the crack plane orientation will be analyzed in this paper.     

Fracture toughness of CA6NM alloy,quenched and tempered,and of its welded joint without PWHT

CA6NM quenched and tempered steel is used in hydraulic turbine rotors, pumps and compressors. The objective of this research is to determine the fracture toughness of tempered and quenched CA6NM alloy, and of its welded joints without post-welded heat treatment (PWHT). To this end, compact tension (CT) test pieces are milled from pieces of CA6NM steel for evaluation of the toughness of the alloy used in a hydraulic turbine. Due to the elasto-plastic condition of the material, the test pieces are tested by means of the J integral concept, setting out the resistance curve J–R and the crack initiation J _IC. In welded joints produced from ingots, without PWHT, the fragility they show does not allow the J–R curve for the CT test pieces to be drawn up, and the toughness is characterized by means of the K _IC concept. The welding procedure looks at the probable conditions for repair of cavitation wear to the turbine, where PWHT cannot be carried out. The results confirmed the higher toughness for the CA6NM steel, with values approximately three times higher than those obtained in the welded joints without PWHT. In terms of the fracture, the CA6NM steel shows ductile behaviour while the welded joint without PWHT shows fragile behaviour.     

Processing and properties of sintered reaction-bonded silicon nitride with Y2O3–MgSiN2: Effects of Si powder and Li2O addition

The effects of Si powder and Li₂O addition on the processing, thermal conductivity and mechanical properties of sintered reaction-bonded silicon nitride (SRBSN) with Y₂O₃–MgSiN₂ sintering aids were studied. Addition of Li₂O provides a less-viscous liquid phase that results in a more uniform and finer pore structure in RBSN with the coarser Si powders, but the pore structure plays a less important role in the densification of RBSN. The thermal conductivity of SRBSN without porosity decreases with increased Al impurity content and also decreases with the Li₂O addition regardless of the Si purity. The impurest coarse Si powder produces the lowest thermal conductivity (93 W m⁻¹ K⁻¹) but the highest four-point bending strength (∼700 MPa) and a higher fracture toughness (∼10 MPa m^1/2). However, the purer fine Si powder produces the highest thermal conductivity (119 W m⁻¹ K⁻¹) and highest toughness (∼11 MPa m^1/2) but the lowest strength (∼500 MPa).     

Tantalum oxide coatings as candidate environmental barriers

Tantalum (Ta) oxide, due to its high-temperature capabilities and thermal expansion coefficient similar to silicon nitride, is a promising candidate for environmental barriers for silicon (Si) nitride-based ceramics. This paper focuses on the development of plasma-sprayed Ta oxide as an environmental barrier coating for silicon nitride. Using a D-optimal design of experiments, plasma-spray processing variables were optimized to maximize coating density. The effect of processing variables on coating thickness was also determined. X-ray diffraction (XRD) was use to ascertain that the as-sprayed coatings were comprised of α- and β-Ta₂O₅, but were fully converted to β-Ta₂O₅ after a 1200 °C heat treatment. Grain growth of the Ta₂O₅ followed a time dependence of t ^0.2 at 1200 °C.     

Fatigue crack growth resistance of SiC<Subscript>p</Subscript> reinforced Al alloys: Effects of particle size,particle volume fraction,and matrix strength

The main aim of this work was to study the effects of particle size, particle volume fraction, and matrix strength on the long fatigue crack growth resistance of two different grades of Al alloys (Al2124-T1 and Al6061-T1) reinforced with SiC particles. Basically, it was found that an increase in particle volume fraction and particle size increases the fatigue crack growth resistance at near threshold and Paris regimen, with matrix strength having a smaller effect. Near final failure, the stronger and more brittle composites are affected more by static modes of failure as the applied maximum stress intensity factor (K _max) approaches mode I plane strain fracture toughness (K _IC).     

Toughness and fractography of austenitic type 304 stainless steel with sensitization treatments at 973 K

It is shown that the fracture toughness in type 304 austenitic stainless steel can be obtained from convenient measurements of the dimple size at the bottom of the cup portion in the fracture surface of round tension samples, employing SEM techniques and quantitative fractography. K_IC measurements were performed for sensitized samples after heating for 1, 12, 24, and 48 h at 973 K. Fracture toughness is shown to decrease monotonically with increasing time of heat treatment. Two models based on axial stress conditions, Krafft's model and a model proposed by Schwalbe and Backfisch, were tested and compared with Barsom and Pellegrino's model, which reflects closely the necessary plane strain conditions for K_IC measurements. The crack growth increment was considered equal to the dimple size, which was measured by means of the mean intercept length of three dimensional bodies or mean chord in space, L₃.     

Failure analysis of beta-C titanium alloy high-pressure vessels

An attempt has been made to apply the linear elastic fracture mechanics concept to Beta-C titanium alloy pressure vessels that exhibited brittle fractures during hydrotesting. Based on the results of stress analysis on the real structures and fracture surface examinations, a stress-intensity factor,K _IC, was estimated. TheK _IC value of the material in the cracking direction was measured by a surface semi-elliptical crack method. It was found that theK _IC value of the material is very close to the estimated stress-intensity factorK _I during failure, which places the pressure vessels in a critical condition in that a small variation in flaw size may cause a catastrophic failure. A compromise must be made betweenK _IC and the required yield strength. In this restricted case, the yield strength of the material should be controlled in the range of 1150 to 1200 MPa to avoid brittle fracture and the possible occurrence of yield during hydrotesting. Control of microstructure and other mechanical properties is also discussed in this investigation.     

Microstructure origin of strength and toughness of a premium rail steel

The mechanical properties, fracture toughness, fracture surface morphology, and failure mechanisms of different layers in a premium railhead were studied. Correlation between the mechanical properties and the failure mechanisms for each of the layers was made. It has been found that the microstructure and mechanical properties of the top layer are different from those of the inner layers, while the middle layer and the layer near the web demonstrated similar mechanical properties, microstructure, and fracture toughness. The top layer displayed 15% higher tensile strength than the other two layers. However, the strain to failure of the top layer, 11%, is only about 60% of that of the inner layers, 17.5%. The top layer has a fracture toughness, K _Ic , of 75 MPa m^1/2. This value for the inner layers is about 95 MPa m^1/2. Thus, the heat treatment decreases the ductility and fracture toughness of the top layer of the railhead. Transition from the brittlelike fracture mechanism of the top layer into a more ductile mechanism of the inner layers was also found.     

Fracture behaviour and fracture toughness of ductile closed-cell metallic foams

Standard fracture mechanics tests were carried out on two different types of aluminium foam, ALPORAS^® foams and ALULIGHT^® foams, with a variety of densities. Standard fracture toughness tests on compact tension (CT) specimens with widths from 50 mm to 300 mm and in situ tests in the scanning electron microscope were performed. Fracture toughness values in terms of the critical stress intensity factor, K_IC, the critical J-integral, J_IC, and the critical crack-tip opening displacement, COD_5,i, were determined. To identify the fracture process, local deformation measurements were performed on the foam surfaces with a digital image processing system.From the deformation measurements it is evident that the deformation is strongly localised on different length scales. A relatively large fracture process zone, 6–8 cells in height, is developed, where only few of them are heavily deformed. On the cell wall level the deformation is again strongly localised to the thinnest parts of the cell wall, where cracks initiate and propagate. The crack propagates through the foam, building many secondary cracks and crack bridges. The comparison of K vs. Δa (crack extension), J vs. Δa and COD vs. Δa with the current fracture processes at the crack tip and the load–displacement response reveals that COD gives the most reliable measured values to characterise the fracture toughness.     

Gradients in elastic modulus for improved contact-damage resistance. Part I: The silicon nitride–oxynitride glass system

Silicon nitride (Si₃N₄)-based graded materials were fabricated with controlled, unidirectional gradients in elastic modulus from the surface to the interior. This was accomplished by infiltrating a low modulus silicon oxynitride glass into a dense, higher modulus, Si₃N₄ ceramic. Elastic Hertzian indentation (spherical indenter) experiments were performed on both the graded and the monolithic Si₃N₄. While Hertzian indentation of the monolithic ceramic resulted in classical cone cracks, such cracks were completely suppressed in the graded materials at comparable load levels, despite the lower strength and lower toughness of the surface layer comprising glass. Finite element analysis (FEA) of the stresses associated with the indentation was also performed to gain insight into the mechanism for the enhanced contact damage resistance in the graded materials. The computational analysis revealed that the maximum tensile stresses outside the Hertzian contact circle, which drive the cone-cracks, are reduced by approximately 30% relative to those present in the monolithic silicon nitride. This reduction in the tensile stresses more than compensates for the lower toughness at the graded material surfaces, relative to the monolithic Si₃N₄. The FEA also allowed us to develop some strategies for elastic–modulus-gradients that would lead to further improvements in the cone-crack suppression characteristics of graded materials in general.     

Simulation design for the composition of zirconia composite ceramic tool

The relationship between the fracture toughness increment (ΔK _IC) resulting from toughening mechanisms, such as phase transition, residual stress, geometry effect, and grain bridging, and the volume fraction of zirconia was established to simulate and design the composition of a zirconia-matrix composite tool, thereby avoiding “trial-and-error” experiments. The composition of the ZrO₂/Al₂O₃ ceramic tool was simulated in accordance with the requirement for fracture toughness. It was shown that the simulated result was in agreement with experiment and that the established simulation model was to some extent valid in predicting the composition of the zirconia-matrix composite ceramic tool with dispersed α-Al₂O₃. Thus, a new type of ceramic tool material, a ZrO₂/Al₂O₃ composite, was developed by adding α-Al₂O₃ to ZrO₂ on the basis of the results of the computer simulation.     

Mechanical properties of polycrystalline Ti3SiC2 at ambient and elevated temperatures

Dense polycrystalline Ti₃SiC₂ samples were fabricated by reactively hot-isostatic pressing (HIPing) a mixture of elemental Ti, Si and C powders. The mechanical properties, including load–strain response, bending strength, fracture toughness and crack propagation, were investigated from ambient temperature to 1573 K. Non-linear stress–strain responses were observed in the polycrystalline Ti₃SiC₂ materials at ambient temperature. It is conceivable that the inelastic deformation is attributable to micro-deformations that consist of slip between micro-lamellae within individual grains and the formation of microcracks between grains. The polycrystalline Ti₃SiC₂ exhibited a brittle-to-ductile transition at about 1473 K; above this the Ti₃SiC₂ samples deformed plastically and exhibited high strains (>1.5%), whereas below 1373 K only limited inelastic deformation was observed prior to fracture. The mode I fracture toughness, K_IC, was measured by the single-edge notched beam (SENB) method to be 4.52 MPa m^1/2 at ambient temperature. Both fracture strength and fracture toughness decrease only slightly with increasing temperature up to 1273 K, above which they decrease more rapidly and reach half of their room-temperature values by 1473 K.     

Tensile Test Specimens with a Circumferential Precrack for Evaluation of Interfacial Toughness of Thermal-Sprayed Coatings

A new testing procedure to evaluate the interfacial toughness of thermal-sprayed coatings has been developed. The newly designed test specimen is a modification of the pin test with an artificially introduced weak interface, which is expected to open up easily under tensile loading and act as a circumferential precrack along the interface between a coating and the substrate. This configuration makes it possible to calculate the stress intensity factor K _Int at the tip of the precrack, which can be expressed as , where σ₀ is the apparent average stress, a the crack length, R the specimen radius, and F _I the geometrical correction function. Finite-element analysis was carried out to calculate the correction function F _I for various values of a/R. In the experiments, the flat surface of a pin was grit-blasted and a ring-shaped area from the periphery was covered with carbon using a pencil and set into a mating dice. SUS316L stainless steel was plasma-sprayed onto the flat surface of the pin and the dice. Then, tensile load was applied to the pin to break the weak interface containing the carbon and finally the unmodified coating-substrate interface. The load required to pull out the pin was measured for various specimen parameters such as a and R. The results indicate that the adhesion of the tested coatings can be represented by interface toughness of 1.9 ± 0.1 MPa m^1/2. As a consequence, this testing procedure can be considered as a viable method to evaluate adhesion of a thermal-sprayed coating on a substrate.