Application of fracture mechanics concepts to the failure of steel constructions

The failure behaviour of wide plates with different defect geometry was investigated experimentally and quantitatively described by FE-calculations based on material properties, which were determined with small-scale specimens. It is shown, that in the temperature range where brittle, elastic-plastic and fully plastic failure of the wide plates occurs, the fracture behaviour can be predicted sufficiently precisely based on fracture mechanics parameters in terms of the J-Integral. These predictions require an exact evaluation of the material properties, such as stress-strain-curve and critical fracture mechanics values and, on the other side, of the loading conditions calculated in terms of J_l.     

Application of fracture mechanics to high-temperature creep rupture

Abstract

This paper reviews the crack opening displacement approach to fracture testing of materials at ambient temperature and considers extension of the work to higher temperatures, as one phase of an approach aimed at bringing various cracking behaviours within the same over-all fracture mechanics treatment.

Testing procedure uses BS 15 mild steel as l-inch plate at 350°C, with loading times up to 1000 hours. A rectangular specimen about 18 × 6 inches is slit for three quarters of its length along the centre of width, and the notched region is immersed at high temperature. The split ends protruding from the furnace are strained apart, and a transducer is projected into the furnace to provide a continuous measurement of crack opening displacement. CaDs produced are in reasonable agreement with values predicted by the general yielding theory. Intergranular crack extensions occurred at CODs of 14 to 17 × 10^?3 inches, were of the ‘pop-in’ type.

One specimen was cooled to 0°C after test so that the crack could be extended by cleavage. The COD at initiation was 5 × 10^?3 inches, indicating that the material had been substantially embrittled by hot straining.

Résumé

Après avoir revu l'ouverture et le déplacement des fissures pour des essais de rupture à température ambiante, les auteurs considèrent l'élargissement de ces conclusions a haute température comme approche pour examiner divers comportements des fissures dans un mécanisme global de la rupture.

Pour les essais, une tôle d'un pouce d'épaisseur d'acier doux BS?15 a été utilisée a 350°C, le temps maximal de mise en charge étant de 1000 heures. L'éprouvette rectangulaire déenviron 18 po. × 6 po. est entaillée en son centre sur les trois quarts de sa longueur; la région entaillée est maintenue à haute température. Les extrémités fendues de la tôle sont écartées l'une de l'autre hors du four et un palpeur dans le four donne en continu la propagation de la fissure. La propagation des fissures est en bon accord avec les valeurs données par la théorie de l'écoulement. L'extension de fissure intergranulaire se produit pour des valeurs de la propagation des fissures de 14 à 17 × 10^?3 pouces et sont réentrantes. Une éprouvette fut refroidie à 0°C aprés l'essai de fa?on à pouvoir faire avancer la fissure par clivage. La propagation de la fis sure était de 5 × 10^?3 pouces indiquant une fragilisation du métal par deformation à chaud.     

钢材断裂的原因分析

钢材在生产和使用过程中,有时发生断裂失效现象,引起这种现象的原因很多,比如：夹杂物型、组织型等。就影响钢材断裂的因素展开讨论和分析。     

钛微合金化冷弯结构空心型钢的开发研究

根据冷弯结构空心型钢产品组织性能的要求,结合薄板坯连铸连轧生产线（FTSR）的工艺特点,确定C350钢种的钛微合金化成分设计工艺;通过测定此成分下钢的连续冷却转变曲线,制定合理的控轧控冷工艺;对BOF→LF→FTSC工艺的系统进行优化,实现对钢中硅质量分数的有效控制。成品理化性能检测结果显示,最终成品拥有理想的组织性能、表面质量及冷成型性能,为钛微合金化钢生产工艺的进一步推广进行了有益尝试。     

A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel

A new test specimen configuration, designated the T-notch double cantilever beam (TNDCB), was developed, calibrated and employed for a fracture mechanics study of stress corrosion cracking (SCC) of cold worked Type-316 austenitic stainless steel exposed to hot aqueous solutions of 44.7 wt pct MgCl₂. The effects of stress intensity (K _I ), temperature (T) and electrochemical potential (E) upon the crack velocity (v) and fractography were investigated. The stress intensity (K _ISCC ) below whichv became immeasurably small was ∼12 MN·m^−3/2. Above this value, three regions of behavior were observed. Region I exhibitedK _I dependent cracking followed by Region II which exhibitedK _I independent cracking and an apparent activation energy of 63 to 67 kJ/mol, followed by Region III where cracking again became dependent uponK _I . The relative proportions of intergranular and transgranular crack paths were markedly dependent upon bothK _I andE, and less sensitive toT. Crack velocity was insensitive to small changes inE with respect to the free corrosion potentials (E _corr), but could be terminated by an applied active potential of ∼−0.35 V_SCE. The pH within the propagating crack was estimated to be <1.0 atE _corr, rising to ∼4.5 at −0.35 V_SCE. The mechanism of SCC was discussed with respect to film rupture events caused by crack tip plastic deformation, adsorption controlled processes on the metal surface, and hydrogen diffusion in the metal lattice. Alan J. RUSSELL, formerly Research Student, University of British Columbia     

提高冷弯结构空心型钢SS490B的性能稳定性

冷弯结构空心型钢SS490B是一种硼钛微合金化出口材,由热轧板卷剪裁后进行制管,对板卷的质量要求非常严格。如果板卷性能不稳定,在后续加工使用时会出现加工平台缺陷,影响用户的使用。在通钢FTSR生产线上,对影响冷弯结构空心型钢SS490B性能稳定性的因素进行分析,并根据分析的结果,通过稳定化学成分、提高铸坯质量、优化轧制温度制度、优化冷却工艺、稳定层流冷却、加装风吹装置以及制定相应监察制度等措施,能够稳定SS490B的力学性能,满足用户的使用要求。     

Application of a micromechanical damage model to the analysis of ductile fracture of a thermomechanically treated steel with different microstructures

A possibility to characterize the different stages of the process of ductile fracture from a more microscopic point of view are micromechanical damage models. On a steel 27MnSiVS6 microalloyed additionally with titanium, which belongs to the group of the microalloyed precipitation hardening ferritic-pearlitic steels, different states of microstructure (ferrite-pearlite, tempered martensite, bainite) were produced using thermomechanical treatment. On such specimens fracture mechanics tests were done with side-grooved 1/2 CT-specimens and the results were compared with those obtained by a numerical simulation based on the modified Gurson model. A good agreement between calculated and measured J_R-curves also for a thermomechanical treated steel with separations was found. The critical distance I_c has a great influence on the results of this numerical model. A qualitative relation between this parameter and the average inclusion spacing, which depends on the microstructure, was found.     

硼钛微合金化结构钢的物理化学相分析

研究了硼钛微合金化结构钢析出相的电解萃取方法;通过测定电解残渣的溶解曲线,确定了碳硼化物与氮化硼相定量分离的条件;测定了不同试验钢中各析出相的结构和含量,以及含硼结构钢中的固溶体、碳硼化物和氮化硼中的硼含量。结果表明:文献中所推荐的用氯化钾和柠檬酸水溶液作电解液的电解条件并不适合本合金,确定了以四甲基氯化铵和乙酰丙酮的甲醇溶液为电解液的新的电解条件,在此条件下可以将碳硼化物和氮化硼相定量分离;不同样品的碳硼化物的溶解时间不同,可以通过电解残渣的溶解曲线来确定。本文所确定的电解萃取方法已用于硼钛微合金化结构     

On the application of linear elastic fracture mechanics to hard steels

The fracture toughness of a high speed steel (M3-2PM) and a tool steel (AISI 01) was assessed by the application of LEFM in a nonconventional manner. The point of fracture initiation in bend test specimens was identified in a scanning electron microscope and the shape of the inclusion sites found was approximated by ellipses to allow a critical stress intensity factor to be calculated. In this way an assessment of the fracture initiating inclusion size as well as the matrix toughness was attained. The toughness results obtained for the M3-2PM steel were on average 18.0 MNJm^3/2 and for the AISI 01 steel 13.5 MNJm^3/2 both of which somewhat deviate from previously reported values in the hardness range 65 and 61 HRC respectively. The deviation falls within 30 pct in both cases.     

钢梁结构井架在竖井改造中的应用

主要阐述了钢梁结构井架的优缺点 ,介绍了钢梁结构井架在夹皮沟金矿二道沟分矿竖井改造中的应用及其材料消耗和施工工艺。     

Fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel

A fracture mechanics approach to hydrogen-assisted microdamage in eutectoid steel is presented. Fractographic analysis revealed micromechanical effects of hydrogen in the form of tearing topography surface (TTS). The progress of this microdamage is modeled as a macroscopic crack that extends the original fatigue precrack and involves linear elastic fracture mechanics principles. In this case, the change from hydrogen-assisted microdamage (TTS) to cleavagelike topography takes place when a critical stress intensity factor (K _H) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue precrack plus the TTS area). It is shown that the value K _H depends on experimental variables—mainly on the fatigue precracking regime—and its value may be associated with a characteristic level of stress intensity factor in the crack growth kinetics curve.     

Linear elastic fracture mechanics of duralumin

Abstract

One desirable improvement of high-strength aluminum alloys often showing considerable anisotropy is the increase of their notch toughness in certain grain orientations. Although we know that material properties such as yield stress, fracture toughness, fatigue or stress corrosion behaviour generally have a strong directional dependence, the characteristics of yielding and particularly of fracture, are not well documented. These two aspects are specially studied in this paper.

The basis forms the presentation of various methods for the testing of the fracture toughness parameter K _1C . The most suitable technique is then adopted and the toughness evaluated, covering a range of temperatures which are of particular interest in typical engineering applications. From a three-point bend test performed on a Charpy-type specimen provided with a fatigue crack and sidenotched, a fracture toughness parameter can be derived. Influence of the strain rate on fracture and its implications arising for certain technical problems are investigated in an impact test. This is believed to be the first application of an instrumented Charpy specimen in the toughness investigations of a high-strength Al alloy and in the determination of crack movements through the section. Effects of the orientation are then evaluated and a connection with some metallurgical factors indicated. Evidence is presen ted showing the fracture mechanics approach applied in the analysis of a part-through crack in a thin-walled cylinder.

Résumé

Les alliages d’aluminium à haute résistance présentant généralement une grande anosotropie et une amélioration de leurs propriétés consiste à augmenter leur résilience pour certaines orientations des grains. Même si nous savons que les propriétés des matériaux, comme la limite élastique, la résistance à la rupture, leur comportement en fatigue ou leur corrosion sont fortement anisotropes, les caractéristiques de l’écoulement plastique et de la rupture sont, de ce point de vue, mal connues. Cet article se penchera particulierement sur ces deux aspects.

En premier lieu sont présentés différentes méthodes pour l’évaluation du facteur de concentration des contraintes K _1C . La technique la plus appropriée est alors adoptée et la résistance mesurée dans un domaine de températures d’intérêt particulier pour les ingénieurs. Le facteur K _1C peut être obtenu à partir d’un essai de flexion fait sur une éprouvette de type Charpy ayant une fissure de fatigue et entaillée sur ses faces latérales. L’influence du taux de déformation sur la rupture et ses implications dans certaines applications techniques sont recherchées dans l’essai de résilience. Nous croyons que ce soit là la premiére application d’une telle éprouvette pour l’étude des duralumins et pour connaître le déplacement des fissures dans la section. L’effet de la texture est alors évalué et l’on indique des corrélations avec d’autres facteurs métallurgiques. On montre aussi une approche de la mécanique de rupture appliquée à l’analyse d’une fissure dans un cylindre à paroi mince.     

硼铬微合金钢的脆性断裂失效分析

为了研究硼铬微合金钢的脆性断裂失效机理,以工厂履带拖拉机支重轮的中心轴脆性断裂为研究背景,通过对中心轴断口的观察以及不同位置的组织和晶粒度的对比,得出了硼铬微合金钢的断裂失效机理。利用FactSage和Thermo- Calc软件对TiN夹杂物的析出机理和控制进行理论计算。结果表明,钛合金的不完全溶解和钛、氮元素质量分数的不合理控制,导致大量的大尺寸硬质TiN夹杂物在凝固过程中析出,使得钛元素的加入没有起到很好地细化晶粒的效果,中心区域的铁素体＋奥氏体组织的晶粒度过于粗大。大尺寸TiN夹杂物作为裂纹源和心部粗大的晶粒导致了硼铬微合金钢的脆性断裂失效。热力学计算表明,TiN夹杂物在凝固过程中形成,低钛低氮和钛、氮质量分数的合理搭配可以有效推迟TiN夹杂物的析出,降低其尺寸。较小的夹杂物尺寸和细小的晶粒均可以有效增强材料抵抗裂纹扩展的能力。     

Effects of heat treatment and testing temperature on fracture mechanics behavior of low-Si CA-15 stainless steel

This research studied the effects of heat treatment and testing temperature on fracture mechanics behavior of Si-modified CA-15 martensitic stainless steel (MSS), which is similar to AISI 403 grade stainless steel, which has been widely used in wall and blanket structures and in the pipe of nuclear power plant reactors, turbine blades, and nozzles. The results indicated that fracture toughness of low-Si CA-15 MSS is better than that of AISI 403. The specimens of the low-Si CA-15 MSS after austenitization at 1010 °C and then tempering at 300 °C have higher plane-strain fracture toughness (K _IC ) values for both 25 °C and −150 °C testing temperatures. However, the specimens tested at 150 °C cannot satisfy the plane-strain fracture toughness criteria. The fatigue crack growth rate is the slowest after austenitization at 1010 °C for 2 hours and tempering at 400 °C. Observing the crack propagation paths using a metallographic test, it was found that the cracking paths preferred orientation and branched along ferrite phase, owing to martensite-phase strengthening and grain-boundary-carbide retarding after 300 °C to 400 °C tempering. Also, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.     

The effect of rolling at different temperatures on the fracture toughness anisotropy of a C-Mn structural steel

Abstract

The toughness of rolled plates of a plain C-Mn steel was evaluated for three sections, at different temperatures, by means of small-scale Wedge Open Loading (WOL) specimens. These were machined to orient the fracture plane parallel to the rolling plane, the transverse plane or the longitudinal plane which, according to the ASTM Standards code (12) are termed the S-L, the L-T and the T-L orientations, respectively. In addition, small tensile specimens having the tensile axis perpendicular to the rolling plane were tested at different temperatures.

The plates had been soaked at various temperatures between 1200°C and 680°C (finishing temperatures ～980°C to ～680°C) and then continuously rolled to the final thickness of 3/4-in. (10). The observed toughness depended on finishing temperature and also on specimen orientation; the S-L orientation being the least and the L-T orientation the most tough in any given plate. Brittle fractures were observed on the rolling-plane (S-L orientation) of WOL specimens made from. plates that had been soaked at temperatures ≤800°C (finishing temperatures ≤760°C) and these were attributed to the microstructure and texture. Scanning electron micrographs disclosed that elongated MnS inclusions were principally responsible for the fracture toughness anisotropy for higher soaking and finishing temperatures.

Résumé

La résistance de plaques laminées d'un acier au carbone et manganèse fut évaluée à differentes températures, selon les trois orientations possibles du plan de rupture par rapport au plan de laminage. Les éprouvettes ont été usinées conformément à une homothétie: reduite de la géométrie WOL (wedge open loading) et de sorte que le plan de rupture soit parallèle au plan de laminage, au plan transversal ou au plan longitudinal Ces orientations sont appelées S-L, L-T, et T-L respectivement, suivant les normes de l'ASTM (12). De plus, de petites éprouvettes de traction dont les axes de traction sont perpendiculaires au plan de laminage, furent expérimentées a differentes températures.

Les plaques ont été maintenues à differentes températures entre 1200°C et 680°C (températures finales de 980°C a 680°C), puis laminées en continue jusqu'à une épaisseur finale de 3/4 po. (10). On trouve que la resilience observee depend de la température finale et de 1'orientation de l'eprouvette. L'orientation S-L a été la moins résiliente tandis que 1'orientation L-T a été la plus résiliente de toutes les plaques expérimentées. Des cas de rupture fragile ont été observés sur le plan de laminage (orientation S-L) d' éprouvettes WOL obtenues de plaques ayant été maintenues à des températures ≤ 800°C (températures finales ≤760°C), et ceux-ci sont attribués à la microstructure et la tex ture du rnateriau. Des micrographics obtenues du microscope électronique a balayage révélèrent que des inclusions allongées de MnS étaient en grande partie responsables de l'anisotropie de la résistance a la rupture pour de plus hautes températures de maintien et températures finales.