Accurate Evaluation of the Mechanical Properties of Grey Cast Iron

Abstract

Grey cast iron has been regarded, in some circles, as being brittle, low in reliability and one where the usual design equations cannot be applied. The reason is usually attributed to the presence of randomly distributed graphite flakes. This presentation discusses, both experimentally and theoretically, the notch strength, bending strength, and the scattering of tensile strength of grey cast iron, and suggests a new approach based on plasticity theory for the accurate evaluation of these characteristics. Experiments showed that grey iron has a very low notch sensitivity in circumferentially notched bars, but that strength decreases in edge-notched plates. At the temperature of liquid nitrogen, notch sensitivity is very high. In ductile iron, strength increases in notched specimens. These results have been interpreted by the stress distribution, taking into account non-elastic stress/strain behaviour, and using a fracture criteria with an over-stressed depth, δ· The δ is a necessary region for fracture and its minimum value is related to the graphite eutectic cell size. Nominal bending strength, which is about twice that of the tensile strength and which varies with the beam height and sectioned geometry, was also explained by these considerations. The scatter in strength, expressed by the coefficient of variation, was 2–9% in grey iron, a figure not as large as expected. The value was higher in the case of small test pieces, whilst the average strength was unchanged. These results were easily explained by the strength- hardness relationship and by a relationship derived from the strength theory of bundled threads, in which the fracture process is similar to that of cast iron.

In the analyses, the fracture behaviour of cast iron was assumed to be non-elastic, and ductile rather than elastic and brittle. It is shown that reasonably accurate strength evaluation of cast-iron structures is possible by considering these characteristics.     

额外纯Cu对Cu-Nb复合线材力学及导电性能的影响

通过拉伸试验和四点测电阻法,对中心含额外纯Cu的Cu-Nb复合线材的力学及导电性能进行测试,并利用SEM观察其微观结构和断口形貌。结果表明:从断口中心到边沿,断裂方式从正断逐渐转向剪切断裂。中间纯Cu有利于导电性和强度的优化。线材的强度随着Nb芯丝间距的减小呈指数上升趋势。当间距小于100 nm时,尺寸效应对强度有显著影响,且77 K下强度随间距减小而增大的速度大于室温下强度随间距减小的速度。而当间距大于300 nm时,尺寸效应对强度的贡献很小。     

A model of heating a flux-cored wire in arc metallizing and analysis of the structure of the coating

Summary

This paper describes HAZ‐notched CTOD tests of multipass welds in SMYS = 420–460 MPa class high‐strength steels for offshore structural applications. The weld metal strength overmatch causes different fracture behaviour depending on the actual CGHAZ toughness. When the CGHAZ is completely embrittled, the weld metal strength overmatch leads to the lower bound critical CTOD value. This is due to elevation of the local stress in the CGHAZ caused by the restraint effect of the overmatched weld metal. The fracture surface is generally flat, and brittle fracture originates from the CGHAZ sampled by the fatigue crack front. A larger fraction of the CGHAZ along the crack front gives a smaller critical CTOD value. When the CGHAZ has moderate toughness, however, the weld metal strength overmatch may produce a higher critical CTOD value at brittle fracture initiation. This is due to crack growth path deviation towards the base metal. Plastic deformation preferentially accumulates to a greater extent on the softer base metal side before the critical stress conditions for brittle fracture initiation occur in the CGHAZ. This asymmetrical plastic deformation promotes deviation of ductile crack growth from the crack tip CGHAZ. In this case, the critical CTOD value does not always reflect the CGHAZ toughness itself.

A notch location nearer the weld metal sometimes causes fracture initiation in the weld metal if the fatigue crack tip samples the CGHAZ. Such experimental data do not reflect the real CGHAZ toughness.

The significance of the critical CTOD value obtained in the tests must be determined in the fracture toughness evaluation of the weld CGHAZ. This paper presents a procedure for evaluation of CTOD test results obtained for HAZ‐notched welds that considers the strength mismatch effect.     

Effect of hook characteristics on the fracture behaviour of dissimilar friction stir welded aluminium alloy and mild steel sheets

ABSTRACT

Under tensile shear loading, fracture modes of dissimilar lap welds produced by friction stir scribe technology were studied. Three fracture modes were observed. For zone A fracture, the initial crack was restrained, and the joint ultimately fractured in the base mild steel. For zone B fracture, the initial crack progressed through the aluminium sheet just above the Al/steel interface. For zone C fracture, the initial crack proceeded along the steel hook to the aluminium sheet surface. Fracture mode and joint strength were greatly influenced by steel hook size, and the steel hook size was affected by welding parameters and tool scribe height. In this study, the experimental joint strength achieved the calculated joint load limit.     

On the transformation-induced work-hardening behavior of Zr47.5Co47.5Al5 ultrafine-grained alloy

The work-hardening behavior of Zr_47.5Co_47.5Al₅ ultrafine-grained alloy, obtained by water-cooled copper mold casting, has been investigated. The Cu- and Ni-free alloy shows high compressive strength combined with large plasticity. The work-hardening behavior and the remarkable enhancement of the ductility and strength origin from the B2 to B33 deformation-induced martensitic transformation associated with the high strength ultra-fine grains. The fracture mode of the alloy is both intergranular and transgranular fracture. Our current finding may offer useful guidelines to design materials with improved strength and ductility as practical engineering and biomedical applications.     

The influence of the reinforcing particle shape and interface strength on the fracture behavior of a metal matrix composite

A numerical analysis of the reinforcing particle shape and interface strength effects on the deformation and fracture behavior of an Al/Al₂O₃ composite is performed. Three-dimensional calculations are carried out for five elastic–brittle particles embedded into the elastic–plastic matrix, the reinforcing particle shape being varied from spherical to strongly irregular. It is shown that microstructural heterogeneity of the composite gives rise to a complex stress–strain state in the vicinity of particle boundaries and hence to near-interface areas undergoing tensile deformation both in tension and compression. Within the strain range under study, compressive strength is not achieved, either in compression or in tension, i.e., all cracks grow only under tensile stress. Particle fracture is found to occur by two mechanisms: interface debonding and particle cracking. Individual and combined effects of the particle shape, interface strength, and loading conditions on the fracture mechanisms are analyzed.     

高质量金刚石自支撑膜织构与断裂强度的关系研究

利用直流电弧等离子体喷射化学气相沉积在Mo基体上制备了不同织构的金刚石厚膜。用扫描电境(SEM)观察金刚石膜的形貌，用XRD表征晶体取向，用极图和取向分布函数法计算金刚石膜的不同织构，利用三点弯曲法测量金刚石膜的断裂强度。结果表明，金刚石膜的断裂强度随着衍射峰强度比值，I(111)／I(220)的增大而不断降低。{110}织构的金刚石膜具有最高的断裂强度。     

The Joint Strength and Fracture Mechanisms of TC4/TC4 and TA0/TA0 Brazed with Ti-25Cu-15Ni Braze Alloy

In this paper, Ti-25Cu-15Ni (mass ratio) braze alloys were prepared by vacuum arc melting. Additionally, the TA0 pure titanium and TC4 titanium alloy were brazed with the Ti-25Cu-15Ni braze alloy at 960, 980, 1000, 1020, and 1040 °C. The effects of the braze temperature on the tensile strength of the TA0 and TC4 joints and their fracture mechanisms were studied. The maximum tensile strength of the TA0 joints of 219.9 ± 0.1 MPa was achieved at a brazing temperature of 980 °C, and the maximum tensile strength of the TC4 joints of 832.9 ± 0.1 MPa was achieved at the same brazing temperature. These results indicate that their ideal joint strength is comparable. According to the fractography results of the TA0 joints, a mixed fracture morphology is indicated. The TA0 fracture surface is dominated by cleavage fracture with a small contribution from ductile fracture. The TC4 joint fracture arises from cleavage.     

Effect of Vacuum Annealing on the Characteristics of Plasma Sprayed Al2O3-13wt.%TiO2 Coatings

Adhesion strength is one of the critical properties for plasma-sprayed coating. In this study, the plasma-sprayed Al₂O₃-13wt.%TiO₂/NiCrAl coatings were annealed at 300-900?°C for 6?h in vacuum. The tensile bond strength and porosity of the coatings were investigated. The microstructure and the fracture were studied using optical microscopy, scanning electron spectroscopy, and x-ray diffraction. It was found that the tensile bond strength of coatings increased with the increase of annealing temperature until 500?°C, reaching the maximum value of 41.2?MPa, and then decreased as the annealing temperature continues to increase. All coatings presented a brittle fracture and the fracture occurred inside the ceramic coatings except for the coating annealed at 500?°C, which had a brittle-ductile mixed fracture and the fracture occurred at the interface of bond coating and the substrate.     

The fracture toughness of a cordierite square lattice

A single-edge-notch bend test was developed for the measurement of fracture toughness of a ±45° square lattice made from an elastic-brittle solid. The finite element method was used to obtain the K-calibration for an orthotropic elastic solid, with elastic constants equal to that of the ±45° square lattice. Finite element simulations were also used to predict the fracture strength of the cracked lattice, with failure dictated by the tensile strength of the cell-wall material. The conditions for K-dominance and for failure by a net-section strength criterion of the specimen were thereby determined. A set of fracture tests on single-edge-notch bend specimens made from the ceramic cordierite validate the predictions. The measured fracture toughness of the cordierite lattices scales linearly with the stockiness of the lattice.     

异种钢闪光对焊接头蠕变断裂力学行为

分别钢闪光对焊制备的高强匹配（ＢＷ－ＷＨ－ＢＬ）、低强匹配（ＢＨ－ＷＬ－ＢＭ）和中强匹配（ＢＬ－ＷＭ－ＨＢ）的;宏观力学不均匀异种钢接头进行了蠕变及蠕变断裂,试验,并用有限元方法分析了不同匹配接头蠕变断裂的力学因素。结果表明,高强匹配和低强匹配接头均沿头邻近焊缝界面的低强度母材或焊缝侧发生断裂,并怕特征;中强匹配接头蠕变断裂位置在远离焊缝界面的低强度母材区,呈延性断裂性质。应力三轴性是控制高强匹配     

扩散处理后的FeCrNi涂层结合强度与断口特征研究

对无底层和有NiCrAl底层的FeCrNi涂层进行扩散处理,之后使涂层在机械作用力下断裂,通过观察断面的SEM形貌,研究了涂层的断裂机制、涂层与基体间以及涂层内部颗粒间的结合状态,分析了扩散处理提高涂层结合强度的机理。涂层的断口基本上呈现出由暗色区域、白亮区域和未熔颗粒构成。暗色区域内断口面主要为层状剥离断裂;白亮区域内部含有滑移带和台阶面,断裂以脆性解理和塑性滑移混合方式进行。     

The influence of powder characteristics on mechanical properties of Al2O3–TiC–Co ceramic materials prepared by Co‐coated Al2O3/TiC powders

Ultrafine Al₂O₃–TiC–Co (ATC) ceramic is prepared in order to improve the bending strength and fracture toughness of ceramic materials. The ultrafine Co‐coated Al₂O₃ and TiC powders have been synthesized by electroless plating at room temperature, and the composite powders were sintered by hot‐pressing to compact ATC samples. The average bending strength, average hardness and average fracture toughness values of ATC ceramic with different particle sizes and Co contents were investigated. The toughening mechanism of the ultrafine ATC ceramic was studied by transmission electron microscopy (TEM) and ceramic performance testing methods. The results show that the relative density, bending strength and fracture toughness values increase remarkably with the increase of Co content. The ultrafine grain of original powders is beneficial to improve the relative density, strength and toughness values of ATC ceramic. The Co phase hinders the growth of ATC ceramic grains during sintering. The Co phase forms a three‐dimensional mesh skeleton structure during sintering, improving the fracture toughness and strength of the composite ceramic.     

圆柱螺旋弹簧的正断/切断型疲劳断裂模式与提高其疲劳断裂抗力的途径

结合承受扭转切应力和轴向正应力圆柱体的受力分析,讨论了圆柱螺旋弹簧发生的正应力断裂,纵向以及横向切应力断裂等3种疲劳断裂模式。结果指出,喷丸引入的残余正应力(即应力强化机制)只影响正应力而不影响纵/横向切应力断裂模式的疲劳强度(寿命)。但喷丸引起表面形变层内的组织结构改性(即组织结构强化机制)却能提高所有3种断裂模式的疲劳断裂抗力。     

Influence of Heat Treatment on the Microstructure and Property of Mg-Nd-Gd-Zn-Zr Alloy

研究了热处理工艺对Mg-Nd-Gd-Zn-Zr稀土镁合金组织和性能的影响。结果表明,随固溶温度提高,晶粒长大明显,材料的伸长率降低,但第二相数量减少,材料断裂方式由第二相开裂和穿晶断裂相结合的方式转为穿晶解理断裂;固溶时间延长,材料的抗拉强度变化不大,伸长率有所提高,但材料内部含氧量增加,氧化程度加剧;采用分级时效可以提高材料的伸长率,但抗拉强度有所下降。     

Effect of Heat Treatment, Pre-stress and Surface Hardening on Fracture Toughness of Micro-Alloyed Steel

Micro-alloyed steels are being increasingly accepted by industry in various fields of application and are available with a wide variety of microstructures. Extensive literature is available on their microstructure-property relationships. The superior mechanical properties of micro-alloyed steels are caused by fine-grained microstructures and precipitation of micro-alloying elements such as V, Ti and Nb that led to an improvement in yield strength, in the product of tensile strength and total elongation and in Charpy V-notch impact energy as well. The microstructural changes caused by heat treatment or residual stress state caused by surface hardening or mechanical means may influence the fracture toughness of these micro-alloyed steels. It is in this context that the present work begins with experimental determination of quasi-static initiation fracture toughness (J _1c) of low carbon (0.19%) micro-alloyed steel in as-rolled condition without any heat treatment. The study further explores the effect of normalizing, shot-peening and cyaniding followed by shot-peening on fracture toughness of as-rolled steel under study. The normalizing heat treatment, shot-peening and cyaniding followed by shot-peening—each indicates a positive influence on initiation fracture toughness. Results, when compared, show that cyaniding followed by shot-peening have led to a 2.7 times increase in J _1c. Cyaniding followed by shot-peening may therefore be considered as having the most positive influence on initiation fracture toughness in as-rolled condition for the type of micro-alloyed steel under study. Although initiation fracture toughness is in general known to decrease with increase in yield strength in LEFM arena, the micro-alloyed steel under study when normalized displayed simultaneous improvement in yield strength and J _1c. All these observed effects of normalizing, shot-peening and cyaniding on initiation fracture toughness (elastic-plastic fracture mechanics) were explained on the basis of microstructural study and stress depth profiles.     

超声辐照对碳纳米管增强环氧树脂黏度和力学性能的影响

采用超声辐照处理多壁碳纳米管增强环氧树脂复合材料,分析了超声辐照条件对复合材料的黏度、力学性能及拉伸断口显微形貌的影响。结果表明:温度一定时,多壁碳纳米管增强环氧树脂体系的黏度随超声辐照时间的延长而逐渐降低;超声辐照时间为5min时,环氧树脂的拉伸强度、弯曲强度、拉伸剪切强度达到最大值,较处理前分别提高了37%,167%和86%;经超声辐照后,拉伸断口形貌显示出更多的韧性断裂特征。     

Degradation of strength properties and its fracture behaviour of TiB2-TiC-based composite ceramic cutting tool materials at the high temperature

Strength retention is important for tool materials at high temperature because cutting temperature in machining is ranged from room temperature to 1000 °C. A study examining the strength properties and fracture behaviour of TiB₂-TiC-based composite ceramic cutting tool materials is presented at different temperatures. MoSi₂ and SiC additives are considered to investigate their effects on the density, microstructure, strength and failure mechanism of composites. It is found that the addition of SiC contributed more to the high-temperature strength of composites than MoSi₂, but it did not improve the room-temperature strength, despite grain refinement. The TBAVS8 composite has a flexural strength of 800 MPa at room temperature and can retain 75% at 900 °C. At room temperature, the fracture behaviour of composites was dominated by the strong bonding of the Ni binder phase. At high temperatures, the softer Ni binder phase was pinned, and its sliding was inhibited by SiC particles, which decelerated the strength degradation.     

Temperature dependence of rupture strength of the amorphous alloy Ni82.1Cr7.8Si4.6Fe3.1Mn0.3Al0.1Cu<0.1B2

Rupture strength of the amorphous alloy based on nickel (Ni_>80) has been measured in the temperature region 293–650 K under loading with a rate of 25 MPa/s. The samples in the form of 60–70 μm bands were prepared by spinning and were stabilized by temperature and force treatments, the amorphous state being preserved. The observed linear decrease in the rupture strength with increasing test temperature was analyzed in the framework of the kinetic thermal fluctuation theory of fracture. The initial barrier for elementary events of fracture of the alloy (activation energy), coefficients of local overstresses in the samples, and the theoretical strength of the alloy has been found.     

Determination of lower bound fracture toughness of a high strength low alloy steel welded joint

Abstract

The use of high strength low alloy steels for high performance structures (e.g. pressure vessels and pipelines) requires high strength consumables to produce an overmatched welded joint. This globally overmatched multipass welded joint contains two significantly different microstructures, as-welded and reheated. In this paper, the influence of weld metal microstructure on fracture behaviour is estimated in comparison with the fracture behaviour of composite microstructures (as-welded and reheated). The lower bound of fracture toughness for different microstructures was evaluated by using the modified Weibull distribution. The results, obtained using specimens with crack front through the thickness, indicated low fracture toughness, caused by strength mismatching interaction along the crack front. In the case of through thickness specimens, at least one local brittle microstructure is incorporated in the process zone at the vicinity of the crack tip. Hence, unstable fracture occurred with small, or without, stable crack propagation. Despite the fact that the differences between the impact toughness of a weld metal and the that of base metal are insignificant, the fracture toughness of a weld metal can be significantly lower.