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.     

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.     

Fracture properties of interfacially doped Nb-Al2O3 bicrystals: II,relation of interfacial bonding,chemistry and local plasticity

As described in part I, Nb and α-Al₂O₃ single crystals were diffusion bonded in UHV for several different orientations of the two constituents. These were doped to give defined levels of interface coverage of Ag, Ti, Y or S. The influence of these dopants on the fracture behaviour of the several metal–ceramic interfaces was studied. Submonolayer doping causes large changes in fracture energy J_c: a strong reduction with Ag, and especially S; possibly, a small decrease with Y; and, in contrast, enhancement with Ti. Auger electron spectroscopy data indicated that the interfaces have excess oxygen and provided information on the bonding states and the local crack path.The influences of plasticity of the metal part and of interface chemistry on the bicrystal bond strength are evaluated. The change in the work of attraction, W_at, is derived by means of interface thermodynamics using measured surface and interface coverages of dopant atoms. An empirical relationship is then developed between the interfacial J_c and W_at, which directly demonstrates increasing toughness and associated crack-tip plasticity with increasing interface bonding. The toughness spans two orders of magnitude while W_at varies by 2.5 times. At a critical level of W_at, a transition causes a virtual jump in interfacial J_c; this is interpreted as an incipient blunting transition that is frustrated by the rate sensitive plasticity in the Nb.     

Crack tip process zone domain switching in a soft lead zirconate titanate ceramic

Non-180° domain switching leads to fracture toughness enhancement in ferroelastic materials. Using a high-energy synchrotron X-ray source and a two-dimensional detector in transmission geometry, non-180° domain switching and crystallographic lattice strains were measured in situ around a crack tip in a soft tetragonal lead zirconate titanate ceramic. At K_I = 0.71 MPa m^1/2 and below the initiation toughness, the process zone size, spatial distribution of preferred domain orientations, and lattice strains near the crack tip are a strong function of direction within the plane of the compact tension specimen. Deviatoric stresses and strains calculated using a finite element model and projected to the same directions measured in diffraction correlate with the measured spatial distributions and directional dependencies. Some preferred orientations remain in the crack wake after the crack has propagated; within the crack wake, the tetragonal 0 0 1 axis has a preferred orientation both perpendicular to the crack face and toward the crack front.     

High temperature fracture toughness of TZM alloys with different kinds of grain boundary particles

The elastic-plastic fracture toughness J_IC of two titanium-zirconium-molybdenum (TZM) alloys with different kinds of grain boundary particles was estimated at elevated temperatures using the convenient J_IC test method with a suitable depth of side-groove for determining the J_IC at the maximum load point. It was found that the convenient J_IC test method can be successfully applied to evaluate high temperature fracture toughness at least up to 1200 °C. The J_IC values at temperatures ranged from 800 °C to 1200 °C were almost constant regardless of temperature, while the J_IC values of the TZM with carbide particles were higher than those of the TZM with oxide particles. The TZM with different forging rates showed similar J_IC values, which suggested the effect of forging rate would be not significant at high temperatures.     

Toughness of aluminium alloy foams

The fracture behaviour of closed cell aluminium-based foams (trade-name “Alulight”) is characterized for the compositions Al–Mg1–Si0.6 and Al–Mg1–Si10 (wt%), and for a relative density in the range 0.1–0.4. The toughness testing procedures are critically analysed, and the origins of the observed R-curve behaviour for metal foams are explored. A major contribution to the observed increasing crack growth resistance with crack advance is in the development of a crack bridging zone behind the crack tip. The crack bridging response is quantified in terms of a crack traction vs extra displacement curve by performing independent tests on deep notch specimens. The area under the bridging traction vs extra displacement curve from the deep notch tests is approximately equal to the measured initiation toughness J_IC, in support of the crack bridging concept. A line spring model is then used to interpret the fracture response. The effect of material composition and relative density upon the initiation toughness is measured, and the accuracy of an existing micromechanical model for the fracture toughness of a brittle foam is assessed. Finally, the reduction in tensile and compressive strengths due to the presence of an open hole is determined; it is found that the Alulight foams are notch-insensitive, with the net section strength equal to the unnotched strength.     

An experimental investigation of constraint effects on mixed mode fracture initiation in a ductile aluminium alloy

The effect of constraint on ductile fracture initiation from a notch tip under mode I and mixed mode (involving modes I and II) loading is investigated. To this end, mixed mode fracture experiments are performed with Compact Tension Shear (or CTS) specimen of a ductile 2014-O aluminium alloy. The constraint effects are investigated by considering specimens with two crack length to width ratios. The effect of crack tip constraint on the relationship between the critical value of the J-integral at fracture initiation (J_c) and M_p is examined. Further, the micromechanics of mixed mode ductile fracture initiation is investigated by performing fractographic studies and metallographic examination of the mid-plane region of the specimen near the notch tip.     

Acoustic Emission Methodology to Evaluate the Fracture Toughness in Heat Treated AISI D2 Tool Steel

In this article, fracture toughness behavior of tool steel was investigated using Acoustic Emission (AE) monitoring. Fracture toughness (K _IC) values of a specific tool steel was determined by applying various approaches based on conventional AE parameters, such as Acoustic Emission Cumulative Count (AECC), Acoustic Emission Energy Rate (AEER), and the combination of mechanical characteristics and AE information called sentry function. The critical fracture toughness values during crack propagation were achieved by means of relationship between the integral of the sentry function and cumulative fracture toughness (KICUM). Specimens were selected from AISI D2 cold-work tool steel and were heat treated at four different tempering conditions (300, 450, 525, and 575?°C). The results achieved through AE approaches were then compared with a methodology proposed by compact specimen testing according to ASTM standard E399. It was concluded that AE information was an efficient method to investigate fracture characteristics.     

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.     

D406A钢焊接接头断裂韧度测试

根据英国标准BS7448断裂韧度试验标准,采用多试样法,测试了D406A超高强度钢焊接接头的J-R曲线.取尺寸为B(板厚)×2B、缺口方向为板厚方向、带预制疲劳裂纹的标准试样进行三点弯曲试验.测试试件焊缝和热影响区的载荷-施力点位移曲线,从而得到J积分.然后对数据点进行拟合得到J-R曲线,计算出J积分临界值,从而解决了由于板厚不足而无法直接测试焊接接头临界应力强度因子的问题,为固体火箭发动机壳体设计提供依据.     

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.     

P92钢高温断裂韧性的试验

文中以P92耐热钢为研究对象，研究了不同尺寸的开侧槽和不开侧槽的紧凑拉伸试样在630 °C下的断裂韧性，得到了相应的的阻力曲线及断裂韧度J_Q. P92钢在高温下为典型的韧性断裂机制. 基于三维有限元计算对侧槽的拘束效应进行表征，结果表明，侧槽可明显提高试样的拘束水平，试样尺寸越小，J阻力曲线差异越明显. 随载荷增大，非侧槽试样的拘束变化更明显，开侧槽将导致试样阻力曲线不同. 试样尺寸及结构的改变，对韧性材料的阻力曲线影响较大，而对断裂韧度值影响较小. 试样开侧槽之后裂纹扩展更平齐，可优化断裂韧性试验过程.     

Resistance-curve and fracture behavior of Ti–Al3Ti metallic–intermetallic laminate (MIL) composites

The R-curve and fracture toughness behavior of single-edge notch beams of Ti–Al₃Ti metallic–intermetallic laminate (MIL) composites has been investigated. Composites with 14, 20, and 35% volume fraction Ti, with a corresponding intermetallic layer thickness of ~540, ~440, and ~300 microns, respectively, were tested in crack arrester and crack divider orientations. In the arrester orientation, the R-curve could not be determined for the two highest Ti volume fraction compositions as the main crack could not be grown through the test samples. In the divider orientation, R-curves were determined for all three Ti volume fractions tested. The laminate composites were found to exhibit more than an order of magnitude improvement in fracture toughness over monolithic Al₃Ti. Crack bridging and crack deflection by the Ti layers were primarily responsible for the large-scale bridging conditions leading to the R-curve behavior and enhanced fracture toughness. Estimates of steady-state toughness under small-scale bridging conditions were in close agreement with experimental results.     

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。     

Effect of tempering on the toughness of a Cr- Mo bainitic steel

The effect of tempering on impact and fracture toughness properties of a Cr-Mo bainitic steel was studied in the quenched and stress relieved (Q & SR) condition. The lowest tempering parameter used resulted in considerable improvement in impact properties. Further tempering increased the upper shelf energy, had a minor effect on the transition temperature, and increased both the initiation fracture toughness (J_IC) and the tearing modulus(T). However, the effect on J_IC andT was much greater than the effect on the impact upper shelf energy. The results were discussed in light of the changes in microstructure and flow properties due to tempering.     

Effect of notch-root radius on the fracture toughness of composite Si3N4 ceramics

In-situ silicon nitride and a whisker-reinforced silicon nitride-silicon nitride composite, densified via gas pressure sintering and hot pressing, respectively, were evaluated using the single-edge V-notched beam (SEVNB) fracture toughness technique. The mean value ofK _IC for each material was 5.7 and 7.9 MPa·m^1/2, respectively, and the toughness was influenced by the presence of the elongated Si₃N₄ grains in the microstructure. The notch radius was observed to have the same effect as a sharp crack when notch-root radius was smaller than 10 μm, which was considered to be a realK _IC for these materials.     

Corrosion behaviour of zinc deposits obtained under pulse current electrodeposition: Effects of coumarin as additive

The corrosion behaviour of zinc deposits obtained under pulsed current electrodeposition from an acidic chloride bath in the presence and absence of coumarin has been investigated. The effects of pulse peak current density (J_p) on the morphology of zinc deposits were studied by scanning electron microscopy. An increase in J_p from 40 to 280 A dm⁻² yields deposits with a finer grain size. The refinement of the grain size was more considerable in the presence of coumarin (J_p = 280 A dm⁻²). The preferred orientation of zinc deposits was studied by X-ray diffraction. At J_p = 40 A dm⁻², the preferred orientation of zinc deposits was (1 0 3) and changed to (0 0 2) at J_p = 80 A dm⁻². An increase in J_p to 280 A dm⁻² did not change the preferred crystallographic orientations except for an increase in the peak intensity of the (0 0 2) plane. In the presence of coumarin, the preferred crystallographic orientations changed at J_p = 280 A dm⁻² from the (0 0 2) plane to the (1 0 3) plane. The corrosion behaviour was investigated in an aerated 3.5% NaCl solution; the anodic polarization and electrochemical impedance spectroscopy curves were performed. The corrosion resistance of zinc deposits was improved by increasing the pulse peak current density (J_p); whereas, the presence of coumarin did not improve the corrosion resistance.     

Fracture properties of interfacially doped Nb-A12O3 bicrystals: I,fracture characteristics

The influence of orientation and impurities on the fracture behavior of Nb–sapphire interfaces was studied using notched bending tests. Single crystals were diffusion bonded in UHV for different interface orientations. The bicrystals were doped to produce prescribed fractional interfacial coverages of Ag. The interfacial impurity content was measured after fracture with Auger spectroscopy.The tougher bicrystals exhibit significant nonlinearity in loading. A J-integral analysis was used to account for the large plastic zones. For undoped bicrystals bonded at 1400°C, the interfacial fracture energy ranged from J_c of 77 to 2100 J/m² depending on the interface planes of the Nb and sapphire. Greater toughnesses were derived from bonding at 1300°C, owing to less oxygen contamination of the Nb. Interfacial doping by Ag atoms leads to a strong reduction of J_c at coverages of only 0.2 to 0.5 of a monolayer. Higher fracture energy is caused by greater plastic deformation in the Nb as observed by slip lines on the metal fracture surface. Evaluation of the loading and fracture characteristics revealed that sharp precursor cracks developed initially in the ceramic. Extensive crack blunting also occurs, especially for the tougher bicrystals, but is often followed by erratic or unstable extension during which far less plasticity occurs, apparently owing to the rate sensitivity for Nb deformation.     

Fracture of severely plastically deformed Ta and Nb

A comparative study focusing on the fracture characteristics of severely plastically deformed (SPD) Nb and Ta is presented. Since many SPD-processes lead to typically elongated microstructures, orientation dependent fracture toughness measurements with three different crack orientations were performed. Among the various testing orientations, two exhibit a considerably high fracture toughness in both materials. The high fracture resistance in these orientations will be discussed on the basis of the significance of crack deflection and delamination processes. In both materials the lowest fracture toughness was found in the third testing orientation parallel to the grain elongation. However, between Nb and Ta the fracture toughness differs in this orientation by a factor of three, rendering Nb even parallel to the grain alignment to a quite fracture resistant material.     

有害杂质及Ce对8090合金薄板不同加载方向断裂特性的影响

测定了含不同浓度有害杂质及Ｃｅ的８０９０合金薄板不同方向力学性能及断裂性能．结果表明：Ｆｅ，Ｓｉ使各方向的断裂韧性下降，并使４５°方向上的裂纹启裂阻力下降，Ｎａ，Ｋ除对Ｔ－Ｌ方向的启裂阻力不造成明显损害外，能够损害各方向的断裂性能．在合金中添加０．０９％Ｃｅ即可显示有益的合金化作用；如果添加０．２８％Ｃｅ，可明显改善含一定量Ｆｅ，Ｓｉ的合金薄板各方向的断裂性能，其主要原因在于Ｃｅ可通过使再结晶组织均匀，细化晶粒，减少晶界析出相及改善断裂过程等机制抑制杂质对断裂性能的危害．