Effect of aging treatment on fracture toughness in ZM61 magnesium alloy

The effect of aging treatment on fracture toughness in Mg–6Zn–1Mn (wt-%) was investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, uniaxial tensile and fracture toughness tests, respectively. The results showed that the fracture toughness of Mg–6Zn–1Mn alloy can be enhanced by aging treatment. The fracture toughness and strength showed a reverse trend in single aged and double aged alloy. Synergetic effect of fine grains and precipitates improved the fracture toughness more sharply than aging treatment. The precipitate free zones and grain boundary precipitates made the largest contribution to the reduction of toughness. Under as extruded and aged conditions, the main origins of cracks were elastic incompatibility and plastic deformation.     

Fracture toughness of Ti-15Al-8Nb alloy under mixed mode I/III loading

The effect of mixed mode I/III loading on fracture toughness of Ti-15 at.% Al-8 at.% Nb alloy, which undergoes stress-induced martensitic transformation, was investigated for four different grain sizes. The fracture toughness under mixed mode I/III loading was found to be significantly higher than that under mode I loading in all cases. The results were explained on the basis of the stress and strain fields ahead of a mixed mode crack and its influence on the martensitic transformation zone.     

Microstructural evolution and its influence on the impact toughness of GH984G alloy during long-term thermal exposure

The microstructure evolution and its effect on the impact toughness of a new Ni-Fe based alloy GH984 G,used in 700℃ ultra-super critical coal-fired power plant,were investigated during thermal exposure at 650℃-750℃ for up to 10,000 h.The results show that the impact toughness at room tempe rature drops rapidly at the early stage during thermal exposure at 700℃ and then has no significant change even if after exposure for 10,000 h.The significant decline of the impact toughness is attributed to the coarsening of M₂₃C₆ carbides at grain boundaries,which weakens the grain boundary strength and leads to the aging-induced grain boundary embrittlement.The M₂₃C₆ carbides have almost no change with further thermal exposure and the impact toughness also remains stable.Additionally,the impact toughness rises with the increase of thermal exposure temperature.The size of γ' after thermal exposure at 750℃ for10,000 h is much bigger than that at 650℃ and 700℃ for 10,000 h.There fore,the intragranular strength decreases significantly due to the transformation of the interaction between γ' and dislocation from stro ngly coupled dislocation shearing to Orowan bowing.More plastic deformation occurs within grains after thermal exposure at 750℃ for 10,000 h,which increases the impact toughness.     

Influence of the filler metal on the mechanical properties of Ti-6Al-4V electron beam weldments

The influence of various filler metals on the mechanical properties of 17 mm thick Ti-6Al-4V electron beam welded joints has been analysed. Autogeneous welded joints exhibit higher toughness when compared to the parent plate but this improvement was less marked than that observed in plasma arc welded joints. To achieve better toughness, without suffering unacceptable losses of strength, different morphologies of commercially pure titanium filler metals have been employed. Using 0.50 mm thick sheet as filler metal leaded to maximum toughness but as counterpart a significant decrease in strength was observed. To obtain high toughness while maintaining a high strength level 0.25 mm sheet and 1 mm diameter filler metals are recommended. Fractographic examination of the failed specimens helped to explain the fracture behaviour of the different welded joints.     

Computational evaluation of flexural toughness of FRC and fracture properties of plain concrete

In this paper, a computational tool concerning the computation of flexural and fracture toughness of cement based composites is presented. Firstly, RILEM’s (Réunion Internationale des Laboratoires d’Essais de Matériaux) recommendations related to the analysis of FRC in three-point bend tests are discussed in their relevant aspects regarding the computational implementations. The determination of other mechanical properties such as the Young modulus has been added to the program. Taking this into account, a new formulation based on displacements is used. In the second part of the paper, the determination of fracture properties of concrete, such as the fracture energy, G _F , and the fracture toughness, K _IC ^S , is discussed regarding the computational strategies used in the implementations. Several features whereby anterior data can be reanalyzed, obtained from other standards and recommendations, have been incorporated into the program, therefore allowing comparative studies and back analysis activities.     

Effect of fracture toughness on abrasive wear resistance of steels

Various abrasive wear mechanisms were reviewed and an abrasive wear modeling experiment is assessed. Abrasive wear resistance of non-heat treated and heat treated steels has been determined by using a pin-abrasion machine with five abrasive papers, which grinds on a small pin of test materials. The mass loss of test material during abrasive wear was determined gravimetrically. A correlation between abrasive wear resistance and Mode II fracture toughness of materials was established. The effect of fracture toughness on abrasive wear resistance of steels was outlined.     

铝镁合金平面应力断裂韧性测试研究

为测试铝镁合金平面应力的断裂韧性,根据HB 5261-1983《金属板村KR曲线试验方法》要求,在MTS-810试验机上,对铝镁合金中心裂纹拉伸试样(CCT)进行裂纹扩展阻力曲线KR与平面应力断裂韧性KC的测试;分析屈曲、裂纹长度的修正以及KR曲线有效数据点等因素的影响.测试结果表明:铝镁合金材料的平面应力断裂韧性KC较平面应变断裂韧性KIC高40％.     

Improvement of room temperature fracture toughness in Cr2Nb-based alloys by heat treatment

The microstructure and room temperature fracture toughness of binary Cr/Cr₂Nb alloys annealed at 1653 K for 30 h were investigated at both the hypo- and hypereutectic compositions. The experimental results indicate that the high temperature heat treatment has a beneficial effect on the room temperature fracture toughness of the Cr₂Nb/Cr alloys. After the heat treatment, the room temperature fracture toughness of the hypo- and hypereutectic alloys are increased by about 212 and 203%, which are 15 and 8 times higher than that of as-cast Cr₂Nb Laves phase (1·2 MPa m^1/2). The fractographic analysis indicates that in the annealed condition, the strengthen of lamellar eutectic cohesive strength can provide significant toughening of the matrix by crack deflection, crack blunting and crack bridging mechanisms.     

Effect of pre-strain on fracture toughness of ductile structural steels under static and dynamic loading

The ductile fracture process consists of void nucleation, growth and coalescence. The whole ductile process can be divided into two successive steps: (I) the initial state to void nucleation, followed by (II) void growth up to void coalescence. Based on this suggestion, resistance to ductile fracture could be divided into the resistance to stage I and stage II, and accordingly the whole fracture toughness could be regarded to be due to contributions from stages I and II. The fracture toughness contributed from the two steps is, respectively, denoted as void nucleation-contributed fracture toughness and void growth-contributed fracture toughness. The effect of plastic pre-strain on the fracture toughness of ductile structural steels under static and dynamic loading (4.9 m/s) within the ductile fracture range was evaluated by summing contributions due to void nucleation-contributed and void growth-contributed fracture toughness. The effect of strain rate on fracture toughness was also investigated by the same means. The results show that both plastic pre-strain and high-speed loading decrease the void nucleation-contributed fracture toughness while their effects on the void growth-contributed fracture toughness depend on the variations in strength and ductility. Moreover, fracture toughness of structural steels generally decreases with increasing strain rate.     

时效制度对7B04铝合金断裂韧性的影响

为研究不同时效制度对7B04铝合金断裂韧性的影响,测定了15种时效工艺的常规力学性能和断裂韧度,并利用扫描电镜对拉伸断口进行分析.通过比较不同时效工艺制度的性能指标和断口形貌,总结出时效制度对7B04铝合金断裂韧度的影响规律.结果表明:随着第二级时效温度的提高和时效时间的延长,7B04铝合金强度下降,断裂韧性提高,其中,温度对性能的影响更大;采用115℃×7 h 185℃×13 h双级时效工艺时,对应的强度和断裂韧度指标均比T73和T76工艺对应的指标高.     

Effect of strain rate and temperature on the deformation behavior in a Ti-23.1Nb-2.0Zr-1.0O titanium alloy

The compression behavior of a Ti-23.1 Nb-2.0Zr-1.0O (at.％) alloy was investigated at strain rates from 0.1 s-1 to 1000 s-1 and temperatures from 100 ℃ to 200 ℃ on a Gleeble 3800 system and Split Hopkinson Pressure Bar (SHPB) compressive tester.Optical microscopy,electron backscatter diffraction (EBSD),X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to characterize the microstructure evolution during the deformation.Numerous deformation phenomena,including dislocation slip,twinning of both {332}<113> and {112}<111> modes,stress-induced α" martensite (SIMα") and stress-induced ω (SIω) transformations,were observed.The preferred activation of twinning and SIω transformations was observed in the sample compressed at lower temperatures and/or higher strain rates.The underlying mechanism is that twinning and stress induced phase transformations are attribute to higher stress concentrations at β grain boundaries and additional energy supplied by a higher strain rate,as well as high stacking fault energy because of higher temperature.     

Influence of temperature on impact fracture behavior of an alloy steel

In this paper, the influence of temperature on impact toughness and fracture behavior of alloy steel (AISI Classification 8320) is presented and discussed. Impact toughness decreased with a decrease in test temperature. The extrinsic influence of temperature on impact toughness–fracture resistance relationships is rationalized in light of the conjoint and mutually interactive influences of intrinsic microstructural features, local stress states and macroscopic fracture behavior.     

Experimental research on the compressive fracture toughness of wing fracture of frozen soil

It is commonly found that not only bending fracture but also compressive fracture occur frequently in compression, furthermore, in some specific conditions, compressive fracture sometimes has dominant effect on frozen soil. Therefore, it is extremely necessary to study the mechanical characteristics of the compressive fracture of frozen soil and to investigate the damage and fracture mechanism of frozen soil based on the previous research on frozen soil damage in compression. This study draws on the ideas and methods used in compression fracture research on ice that is very similar to frozen soil, and specific clay in Shenyang region was adopted as the experimental material, to make compressive specimens containing tilted wing crack of different angles, and uniaxial unconfined compression fracture experiments were conducted at different temperatures and loading rates. The fracture toughness K_IC and K_IIC of the main crack tip of the specimens are calculated with obtained experimental results and the law of K_IC and K_IIC changing with tilted angles, temperatures and loading rate is obtained to gain an insight to damage mechanism of frozen soil in compression. This paper presents a meaningful attempt for the research on compressive fracture of frozen soil, so as to better solve practical engineering problems.     

On scattering of measured values of fracture toughness parameters

The values of fracture toughness K _1C of C-Mn steels and weld metal were calculated from values of COD measured in standard specimens tested at low temperatures. The analysis of scattering of 42 values of K _1C measured in normalized C-Mn steel showed that the highest value of K _1C could be as large as 360 percent of the lowest. The factors affecting the scattering were investigated in detail and it was found that the scattering was mainly caused by the variation of locations of cleavage initiation. The local fracture stress σ was found to be the most stable parameter and combined with the minimum cleavage distance min which is determined by the triaxiality of stress reaching a critical value, it could be used to characterize the lower boundary of fracture toughness of steels. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of mixed mode I/III loading on fracture toughness of mild steel at various strain rates

Abstract

The effect of loading angle &phis; on the fracture toughness of mild steel at various strain rates has been studied. The fracture toughness was found to decrease with increasing loading angle (or increasing mode III component) at strain rates 10^-5 to 10⁰ s^-1 where ductile fracture was observed. Under impact conditions (strain rate 10² s^-1), fracture was by cleavage and the fracture toughness was found to increase with increasing loading angle. The results showed that the mixed mode fracture behaviour of mild steel changed from Class C in the strain rate range 10^-5 to 10⁰ s^-1 to a combination of Class A and B under impact conditions. In the strain rate range 10^-5 to 10^-2 s^-1, the fracture toughness behaviour with increasing strain rate was found to be similar for the three loading angles studied, namely &phis;= 0, &phis;= 30 and &phis;= 45&deg;. At the strain rates 10^-2 to 10² s^-1, fracture toughness at &phis;= 0&deg; decreased sharply, while for loading angles &phis;= 30&deg; and &phis;= 45&deg;, the fracture toughness increased with strain rate. The increase in mixed mode fracture toughness with strain rate in this strain rate regime has been attributed to the inertial effects which are known to reduce the T stress ahead of the crack.     

On the effects of an electric field on the fracture toughness of poled piezoelectric ceramics

Central crack specimens have been used to study the effects of an applied electric field on the fracture toughness of poled soft lead zirconate titanate ceramics (PZT-5). The ultrasonic lapping technique was used to machine the central crack of the specimens used. The present experimental study illustrated that changing the field from negative to positive reduced the fracture toughness of a specimen subjected to an applied electric field. These experimental results were in agreement with the observations made by Park and Sun (J. Am. Ceram. Soc. 78 (1995) 1475) and Heyer et al. (Acta Mater. 46 (1998) 6615). Finite element results were employed to compare with the experimental data. Four fracture criteria, i.e. total energy release rate, mechanical energy release rate, local energy release rate and strain energy release rate, were compared with the experimental results. It was found the local energy release rate, which was defined based on a strip electric saturation model, was in broad agreement with the experimental observations. Moreover, it was found that the strain energy density criterion was unable to describe the effect of electric field on fracture toughness when a large electric field was applied.     

Interpreting the stress ratio effect on delamination growth in composite laminates using the concept of fatigue fracture toughness

This paper provides a study on fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be interface configuration independent but significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface roughness is observed to be similar in different interfaces at the same stress ratio. But it is obviously more rough for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.     

Effect of long-term thermo-oxidative aging on weight and fracture toughness of polycyanate neat resin

This study focuses on the effect of long-term thermo-oxidative aging on weight and fracture toughness (K_IC) of polycyanate ester (PCy) neat resin (FSD-M-08178). Thermo-oxidative aging was conducted at 180 °C up to 8000 h under atmospheric pressure in order to investigate susceptibility to thermo-oxidative environment in the PCy. Weight and K_IC changes were investigated with chemical structure changes due to thermo-oxidative degradation. The chemical structure changes were analyzed by means of Fourier transformed infrared spectroscopy with attenuated total reflection system. To investigate degree of degradation, carbonyl index (CI) was analyzed with triazine ring peak which is characteristic peak of the cured polycyanate resin. Single-edge-notch bending test was conducted for each aged (non-aged, 100, 197, 500, 1000, 2000, 4000, and 8000 h) condition to investigate K_IC change and evaluated with CI. The result indicated that the weight increased first 500 h. Then, the weight decreased. The result also indicated that the K_IC sharply dropped first 100 h. After 100 h, the K_IC was almost same up to 8000 h. The result suggested the K_IC depend on the CI which was chemical structure changes of the specimen surface due to thermo-oxidation degradation.     

Measuring the fracture toughness of ultra-thin films with application to AlTa coatings

An experimental technique is presented for measuring the fracture toughness of brittle thin films. In this technique, long rectangular membranes are fabricated from the film of interest using standard silicon micromachining techniques. A focused ion beam is then used to introduce pre-cracks of different lengths along the centerline of the membranes and the membranes are pressurized until rupture. The fracture stress of the membrane is measured as a function of pre-crack length and the fracture toughness of the film is determined from a simple fracture mechanics analysis. The technique is applicable to a wide range of materials and is especially suited for ultra-thin films. We have demonstrated the experimental procedure for a 150 nm AlTa intermetallic film and obtained a room-temperature fracture toughness of K _1c = 4.44 ± 0.21 MPam^1/2.     

Investigation of fracture mechanism of 6063 aluminum alloy under different stress states

Fracture mechanisms in a 6063 aluminum alloy were investigated and analyzed carefully by in-situ tensile tests in SEM with a vacuum chamber. Specimens used were designed to produce different stress states. Studies indicated that with stress triaxiality (σ _m/σ _e) decreasing, the fracture modes changed from normal fracture to shear fracture and the fracture surfaces changed from the dimples and intragranular dominated fracture mode to the shear dominated fracture mode. The grain boundaries of the 6063 aluminum alloy were the weakest positions. In the case of high stress triaxiality, the grain boundary cracks were produced by normal stress or by the incompatibility of deformation between neighboring grains, and the normal stress dominated the crack propagation. In the case of low stress triaxiality, the boundary cracks were produced by the relative slipping of grains against neighboring grains, and the shear stress dominated the crack propagation. The final fracture of the specimens occurred by connections of cracks through transgranular cracking of the ligaments among these cracks.