THE EFFECTS OF SUB-CRITICAL CRACK GROWTH ON THE FRACTURE BEHAVIOUR OF CRACKED FERRITIC STEELS AFTER WARM PRESTRESSING

The effects of sub-critical crack extension on the fracture properties of ferritic steels after they have been subjected to warm prestressing are investigated. The crack growth is assumed to occur at the temperature of the final loading, after the warm prestressing and the subsequent unloading. Predictions of the fracture behaviour are made using continuum mechanics and a fracture criterion based on a modified J-integral. The results of these calculations are consistent with those of a micromechanistic model of cleavage fracture from a sharp crack. Both the theories predict that the beneficial elevation in fracture load produced by warm prestressing is maintained after sub-critical crack growth provided the latter is not greater than the compressive yielded zone formed on unloading.     

STATISTICAL MODELLING OF INTERGRANULAR BRITTLE FRACTURE IN A LOW ALLOY STEEL

The intergranular brittle fracture (IBF) behaviour of a low alloy steel 16MND5 (A508 Cl. 3) was investigated. A temper embrittlement heat treatment was applied to the material to simulate the effect of local brittle zones (ghost lines) which can be found in the as-received material condition. An increase in the Charpy V toughness transition temperature and a significant decrease in the fracture toughness measured on CT-type specimens were observed in the embrittled material, as compared to the reference material which was submitted to the same austenitizing and tempering heat treatment, but which was not subjected to the temper embrittlement treatment. Tensile tests on notched specimens were carried out to measure the Weibull stress and scatter in the results. A statistical model, the Beremin model, originally proposed for brittle cleavage fracture was applied to IBF. It is shown that this model is not able to fully account for the results, in particular for the existence of two slopes in a Weibull plot. Systematic fractographic observations showed that the low slope regime in this representation was associated with the existence of MnS inclusions initiating brittle fracture, while the larger slope was related to microstructural defects. Initiation of IBF from MnS inclusions can occur when the material is still elastically deformed while the second population of microstructural defects is active in the plastic regime. A modified statistical model based on the Beremin model and taking into account these specific aspects is proposed in the framework of the weakest link theory. The parameters of this model are identified from test results on notched specimens. It is shown that this model is able to predict the temperature dependence of fracture toughness and the scatter in the experimental results.     

A MICRO-MECHANICS MODEL OF CORROSION-FATIGUE CRACK GROWTH IN STEELS

Principles of Microstructural Fracture Mechanics (MFM) are used to develop a model for the characterization of environment-assisted short fatigue crack growth. Fatigue cracks are invariably initiated at corrosion pits formed at inclusions, hence the analysis includes stress concentration effects at pits that lead to the propagation of fatigue cracks the rates of which are considered to be proportional to the crack tip plastic displacement. This plasticity is constrained by microstructural barriers which are overcome in a non-aggressive environment at critical crack lengths only when the applied stress is higher than the fatigue limit. However, the superposition of an aggressive environment assists fatigue damage via crack tip dissolution, enhancement of crack tip plastic deformation, the introduction of stress concentrations at pits and a reduction of the strength of the microstructural barrier. These environment effects are manifested in a drastic reduction of the fatigue limit and higher crack propagation rates. The model is compared with fatigue crack propagation data of a BS251A58 steel tested in reversed torsion when submerged in a 0.6M NaCl solution.     

柴油发动机连杆断裂原因分析

应用金相、扫描电镜、能谱分析等手段，分析了柴油机连杆断裂原因。认为连杆的断裂是由于材料中存在的显微孔洞和夹杂物导致产生锻造裂纹，在随后调质过程中裂纹进一步扩展，致使使用时发生脆性断裂。     

EFFECT OF MIXED-MODE OVERLOADS ON ENVIRONMENTAL-HYDROGEN-INDUCED FRACTURE

Abstract— The effect of short periods of mixed-mode overloading on the environmental hydrogen induced fracture life of 0.42%C, 0.87%Cr, 0.21%Mo steel has been studied. Tests were performed in 0.5 mol/L H₂SO₄ solution under continuous hydrogen charging conditions using a weight loading system. Experimental results show that the application of mixed-mode overloads can cause more severe crack growth retardation than those of mode I. Possible mechanisms responsible for the retardation of subsequent crack growth, such as crack deflection, plasticity-induced residual compression stresses, dislocation shielding and overload damage, are examined.     

DETERMINATION OF THE ENERGETIC PARAMETERS CONTROLLING CLEAVAGE FRACTURE INITIATION IN STEELS

Abstract— The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies γ_pm and γ_mm, which define the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the γ_pm value remains constant around 7 J/m², it has been observed that γ_mm is dependent on temperature. At −196°C the value is lower than 50 J/m² and at room temperature it is higher than 200 J/m². This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.     

合金元素对烧结Nd—Fe—B永磁材料断裂强度影响的研究

研究了含Ｃｏ和不含Ｃｏ的烧结Ｎｄ－Ｆｅ－Ｂ永磁材料的抗弯强度和断裂韧性，并采用微观分析方法对它们的断裂行为进行了研究。结果表明，烧结Ｎｄ－Ｆｅ－Ｂ永磁材料的静变断裂主要为沿晶断裂。     

AN INVESTIGATION OF THE WARM PRESTRESSING (WPS) EFFECT IN A533B WELD METAL

Abstract— The WPS effect has been shown to be beneficial in increasing the lower shelf fracture toughness of A533B weld metal for both the load-unload-cool-fracture (LUCF) and the load-cool-fracture (LCF) cycles. The LCF cycle gives a greater apparent increase in fracture toughness than the LUCF cycle but only a small increment of load in the lower shelf region is needed to propagate failure compared with a much increased fracture load which is required in the LUCF case. Models predicting the increase in apparent fracture toughness by superposition of the stress and strain distributions from each step in the prestress cycle are shown to be non-conservative for failures occurring in the lower shelf brittle regime. There is evidence for the operation of a "microcrack blunting" mechanism, but the extent to which it contributes to the WPS effect is small. Stress-relief heat-treatments after prestressing remove nearly all the beneficial effects of WPS. Residual stresses clearly play an important role in the WPS phenomenon, and the models successfully predict experimentally observed trends. Prestressing "deactivates" failure initiation from the larger inclusions in the weld metal. This however does not affect the intrinsic fracture toughness of the material. Fracture in this weld metal, where initiation sites (inclusions) abound, appears to be a co-operative process, and fracture is controlled by inclusions of the mean (and hence most plentiful) diameter rather than the maximum diameter.     

我耐度钢耐延迟断裂性能的评价方法

鉴于目前还没有统一的延迟断裂实验方法，对不同研究者提出的各种延迟断裂实验方法进行了分析比较，重点介绍最近提出的耐延尺断裂性能的定量化评价方法。     

对接焊法兰盘断裂分析

用光学显微镜、扫描电镜和能谱仪分析了1Cr18Ni9Ti对接焊法兰盘断裂原因，结果表明，法兰盘首先产生应力腐蚀，加之材料中碳含量偏高，钛含量低且偏聚，焊接热影响区Cr23C5沿晶析出，使工件又产生晶闸腐蚀，降低了力学性能，导致产品在打压试验中断裂。     

铁素体钢断裂韧度尺寸效应的小冲孔试验

对六种不同厚度的试样进行了小冲孔试验,并进行了有限元模拟,此外还通过扫描电镜观察并分析了断裂面的微观结构。试验结果显示,试样的SP断裂变形能和断裂韧度随试样厚度的增大而增大。提出了裂纹起始遵循断裂应变准则及裂纹扩展遵循断裂能密度准则的观点。有限元模拟结果与试验结果基本吻合。断裂面呈典型的韧性断裂特征,试样变形后的半球状外表面布满微小的褶皱状突起和微裂纹。     

动态断裂过程的惯性效应

动态断裂过程存在着惯性效应，本文介绍了惯性效应的产生机制，数值模拟及其修正公式。     

THE FRACTURE MECHANISM IN 475°C EMBRITTLED FERRITIC STAINLESS STEELS

The mechanism of “475°C embrittlement” in age-hardened ferritic stainless steel, E-Brite and A129-4, is investigated. Experimental results for smooth tensile and notched bending fracture tests are interpreted using a finite element simulation of the stresses at fracture. Yield is characterised by profuse slip band formation. Transgranular fracture initiation is observed at slip band intersections with grain boundaries. Deformation twinning occurs during brittle fracture. Slip bands and deformation twins are identified using lattice rotations measured with electron back-scatter diffraction patterns. Mechanisms for the ductile-to-brittle transition are discussed.     

COMPRESSION FATIGUE CRACK GROWTH BEHAVIOUR OF AN ALUMINIUM ALLOY: EFFECT OF OVERLOADING

Abstract— The behaviour of fatigue cracks in an Al-alloy under cyclic compression, either with or without overloads, was studied. For constant-amplitude compressive cycling, a non-catastrophic (saturation) character of the fatigue crack behaviour was confirmed, with the final depth of a crack depending on the applied load level. Single (tensile or compressive) intermittent overloads were shown to re-activate a previously arrested crack while reversed (tensile—compressive or compressive—tensile) ones were also shown to maintain continual fatigue crack extension under otherwise fully compressive cycling.     

INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART I—EXPERIMENTAL

Abstract— The role of residual stresses in the warm prestress (WPS) effect has been investigated. Three types of specimen have been tested in this investigation: smooth uniaxial tensile specimens, blunt notched single edge notched bend (SENB) specimens and sharply precracked SENB specimens. Room temperature prestraining of uniaxial tensile specimens leads to a dramatic decrease in the measured nominal fracture stress at — 196°C. Such an embrittling effect may be expected to reduce the beneficial increase in subsequent fracture toughness commonly observed in WPS sequences. The blunt-notched specimens were prestressed in tension and compression. Compressive prestressing was found to lead to a decrease in subsequent fracture load whereas tensile prestressing leads to an increase. The load decrease following a compressive WPS was greater than the load increase following a tensile WPS. Various sequences of loading, unloading and cooling have been investigated and the differences in the subsequent fracture behaviour of specimens have been explained qualitatively by superposition arguments. The theories of Chell and Curry have been supported by the general trend of results.     

钢解理断裂应力的测量和物理意义

用二维平面应变有限元计算了不同尺寸四点弯曲（4PB）缺口试样缺口前的应力分布。通过测量解理起裂源的位置，精确测定了一种钢的细观解理断裂应力和σ1和宏观理解断裂应力σF。结果表明，σF的测量比σf更简单和方便，σf的值可以用σF估计。随试样尺寸的增加，σF和σf的值基本不变，σF的分散性不于σf。稳定的σF和σf的下限值可以用比Griffit-Owen试样尺寸大的试样测得。σf可精确用于细观解理断裂机理的分析，并表征了钢的本质韧性。建议σF作为一个潜在的工程参数，用于钢的理解断裂韧性评价和结构安全设计。     

INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART II—ANALYSIS

Abstract— Curry's model of the WPS effect has been applied to the results of a previous paper, and is extended to treat warm prestressing in blunt notched test-pieces. The effect of more complex prestress cycles is also predicted by an extrapolation of the model. The effects of the load-cool-fracture, LCF, cycle can be reasonably predicted for both sharply precracked and blunt notched specimens. For the sharply precracked specimens the effects of the load-unload-cool-fracture, LUCF, cycle at — 196°C are consistently overpredicted and this may be due to a decrease in the cleavage fracture stress at — 196°C of the material at the crack tip which has been subjected to repeated plastic straining by the combination of loading cycles. Modifications to the model are suggested which reduce the overproduction but a wide degree of scatter is observed in the experimental observations. Blunt notched specimens show a reasonable correlation between prediction and theory for the tensile LUCF cycle. Problems have been found in predicting the effect of various prestress cycles in different specimens due to the inherent variability in baseline fracture behaviour of the weld metal. It is concluded that the general trend of results is adequately explained by superposition models but that a greater understanding of local flow properties at a crack tip is required to achieve reasonable predictive success for weld metals such as A533BW.     

PMMA断口形态及其破坏机理

本文通过对PMMA蠕变试件断口的SEM分析,把材料蠕变破坏过程中主裂纹的发展划分为三个阶段,提出了决定材料承载寿命的因素。     

The anisotropic R-criterion for crack initiation

The anisotropic nature of mixed modes I-II crack tip plastic core region and crack initiation is investigated in this study using an angled crack plate problem under various loading conditions. Hill’s anisotropic yield criterion along with singular elastic stress field at the crack tip is employed to obtain the non-dimensional variable-radius crack tip plastic core region. In addition, the R-criterion for crack initiation proposed by the authors for isotropic materials is also extended to include anisotropy. The effect of Hill’s anisotropic constants on the shape and size of the crack tip plastic core region and crack initiation angle is presented for both plane stress and plane strain conditions at the crack tip. The study shows a significant effect of anisotropy on the crack tip core region and crack initiation angle and calls for further development of anisotropic crack initiation theory.     

储气塔阀门紧固螺栓的断裂分析

储气塔运行半年即发生了阀门紧固螺栓的断裂事故。采用化学成分分析、硬度测定、金相检验以及断口的宏、微分析等方法,对螺栓断裂原因进行了分析。结果表明,由于塔内腐蚀性介质的微量渗出,导致了阀门紧固螺栓的应力腐蚀断裂。