强度等级对混凝土双K断裂参数的影响

将线弹性断裂理论与虚拟裂缝模型相结合,利用三点弯曲梁试件在试验中测得了最大荷载、裂缝口张开位移等参数,并据此计算了尺寸为1 100mm×300mm×300mm的3组不同强度等级混凝土试件的起裂荷载、临界有效裂缝长度、起裂断裂韧度、失稳断裂韧度(即双K断裂参数)。结果表明,起裂荷载、最大荷载、起裂断裂韧度、失稳断裂韧度均随试件强度等级的增大而增大,且起裂荷载占失稳荷载的比重亦随试件强度等级的增大而增大,试件从起裂到破坏持续时间随强度等级的增大而变短。起裂韧度与失稳韧度的比值小于起裂荷载与最大荷载的比值。试件设计强度等级越大,临界有效裂缝长度值越小,延续越差,脆性越大,由起裂到失稳的时间亦越短。     

非标准混凝土楔入劈拉试件断裂参数试验研究与尺寸效应分析

采用跨高比和初始缝高比恒定的非标准混凝土楔入劈拉试件,按照改进的试验加载装置进行试验可抵消竖向力的作用。以双K断裂理论为基础,采用紧凑拉伸试件的断裂韧度公式计算得到非标准楔入劈拉试件的断裂参数,并利用通用有限元软件ANSYS对试验计算结果进行验证。结果表明,采用紧凑拉伸试件计算公式和有限元方法均能准确计算非标准楔入劈拉试件的断裂韧度,两种方法误差仅为3%;在截面高度300~600mm范围内,起裂断裂韧度基本不随试件截面高度的变化而变化,失稳韧度在截面高度大于500mm时不具有明显尺寸效应。     

基于XFEM的混凝土楔入劈拉断裂过程模拟分析

为研究混凝土的楔入劈拉断裂特性,采用扩展有限元法数值模拟了不同缝高比、不同截面高度的混凝土楔入劈拉试件断裂过程,研究了混凝土楔入劈拉试件裂缝扩展特性和尺寸效应。结果表明,起裂韧度受初始缝高比和试件截面高度的影响较小,但不具有明显的尺寸效应;当缝高比大于0.5或截面高度大于500 mm时,失稳韧度才不具有尺寸效应。结果可为混凝土断裂特性研究提供参考。     

不同配筋率对钢筋混凝土三点弯曲梁断裂韧度的影响

为深入了解配筋率对钢筋混凝土断裂韧度的影响,制作了3组12根带预制裂缝的钢筋混凝土三点弯曲梁并做了断裂试验,根据试验导出了钢筋混凝土三点弯曲梁试件断裂参数解析计算公式,分析了不同配筋率对钢筋混凝土三点弯曲梁断裂韧度的影响程度。结果表明,钢筋混凝土三点弯曲梁试件的开裂荷载主要由混凝土主体结构所决定,最大荷载主要由混凝土内部配置的钢筋情况所决定;裂缝亚临界扩展相对值随着配筋率的增加呈线性增加;配筋率的变化对失稳断裂韧度有一定影响,但对起裂断裂韧度影响不大;配筋率的增加,可有效提高试件的韧性和抵抗失稳破坏的能力。     

Ⅰ型断裂尺寸效应及裂纹扩展阻力曲线的有限元分析

针对三点弯曲法在测定混凝土等材料断裂能时存在不同程度的尺寸效应现象,基于有限元软件ABAQUS,建立5种不同尺寸、几何相似的混凝土梁,采用线性内聚力模型模拟裂纹尖端微小内聚力区域的力学特性,分析单边带预制裂缝的三点弯曲梁的Ⅰ型断裂开裂过程,针对其尺寸效应和裂纹扩展阻力曲线开展研究。结果表明,随着试件尺寸的增大,试件断裂过程区的相对长度和名义强度减小,应力强度因子和峰值荷载及对应挠度值增大;小尺寸结构破坏模式的延性特性较显著,而大尺寸结构表现出更加显著的脆性特性;尺寸效应定律的线性形式能较好地反映名义强度和试件尺寸之间的关系;通过材料的裂纹扩展阻力曲线获得裂纹失稳扩展临界点所对应的宏观断裂能,其值与输入的数值模拟参数基本一致,采用裂纹扩展阻力曲线获取材料断裂能是可行的。     

混凝土Ⅱ型断裂破坏试验及其断裂韧度的确定

为了解混凝土Ⅱ型断裂破坏过程中应力应变变化规律并确定混凝土剪切断裂韧度,制备了4组32件单边切口半边对称加载试件,利用特殊的加载方式在5 000kN压力机上对试件做了混凝土Ⅱ型断裂破坏试验,并描述了各个试件破坏时的变化情况。试验结果表明,混凝土Ⅱ型断裂破坏外加荷载—时间关系曲线、外加荷载—应变关系曲线、荷载—裂缝尖端滑移位移曲线、应变—时间关系曲线各有特点,所得单边切口半边对称加载试件能够实现混凝土的剪切断裂,且断裂韧度与初始缝长大小无关。     

不同缝高比大坝混凝土断裂性能及声发射特性

为研究缝高比对大坝混凝土断裂性能的影响,对不同缝高比(0.3、0.4、0.5)的带缺口立方体大坝混凝土试件(边长为300、450 mm)分别开展楔入劈拉试验,同时采用声发射技术监测大坝混凝土的声发射特性.根据试验结果获得了荷载—裂缝口张开位移(P-CMOD)曲线、断裂韧度、断裂能及声发射特征参数.研究结果表明,300、...     

混凝土损伤参数的截面积效应试验研究

针对直接测量损伤目前难度较大的问题,提出采用加引气剂模拟损伤的方法,利用所加不同比例引气剂模拟不同程度的损伤,并对不同尺寸混凝土试件做了单轴压缩试脸,测定了相应的弹性模量,并计算出混凝土的初始损伤D0和损伤阈值Df,得到了损伤参数随试件截面积的变化规律,即损伤参数值随试件截面积的增大而增大,但增幅逐渐减小.     

基于断裂能的混凝土弹塑性损伤本构模型和尺寸效应研究

为研究混凝土断裂特性与尺寸效应,引入断裂能和等效应变表示混凝土的损伤演化规律,结合混凝土塑性变形特征,提出基于断裂能的混凝土弹塑性损伤本构模型,并建立相应的数值计算模型。为验证数值计算结果的合理性,引入改进的能量尺寸效应公式进行对比分析。通过对三点弯曲梁的有限元数值模拟和断裂参数敏感性分析发现,尺寸较小的梁失效时,断裂过程区未扩展至特征长度,限制了能量耗散,从而提高了梁的抗断裂能力,增强了梁的名义强度;随着尺寸增大,失效时的断裂过程区逐渐接近特征长度,梁的名义强度逐渐减小;当断裂过程区达到特征长度后,结构名义强度不再随梁尺寸的增大而减小。     

基于离散元的岩石断裂尺寸效应及断裂过程区演化特征分析

为了在离散元中研究岩石的非线性断裂过程,基于幂律型位移软化接触模型,针对一系列相似边切槽半圆盘花岗岩的三点弯曲试验进行数值模拟,并分析了其尺寸效应和断裂过程区演化规律。荷载位移曲线的峰后特征表明,随着半圆盘半径的增大,延性破坏逐渐过渡为脆性破坏,而测试结构的强度模拟结果与Baant尺寸效应理论吻合较好。在加载过程中,断裂过程区长度先增大后减小,并在荷载位移曲线的峰后达到最大值,该最大值与结构尺寸密切相关,当试件尺寸足够大,断裂过程区不受边界的约束而充分扩展,可达到饱和长度,而小尺寸试件的断裂过程区扩展会受到边界条件的限制。     

Evaluation of the effect of metallurgical variables on materials behaviour and reference curves

An integral part of the safety assessment of nuclear pressure vessels and piping is the quantitative estimation of defect growth in both a stable and an unstable manner during service. This estimation is essential for determining whether any defect detected during inspection should be repaired or whether the size of the defect even after its expected growth is small enough to leave the integrity of the vessel unaffected.

The most important stable defect growth mechanism is that of environmentally assisted cyclic crack growth. Recent results indicate that it is markedly affected by sulphur content and/or manganese sulphide morphology and distribution. This implies that an essential improvement in component safety has been gained by currently applied steelmaking practices, which result in extra low sulphur content, generally below 0·010 wt.%, and in the round shape and small size of inclusions through, e.g. calcium treatment, hence considerably reducing the effect of the environment on crack growth rate. This further implies that the ASME Section XI reference curves for environmentally accelerated cyclic crack growth are conservative for steels produced by current steelmaking practices.

The ASME Section XI applies predominantly linear elastic fracture mechanics to assess the effects of cracks on the integrity of nuclear power plant components. Unstable linear elastic fracture often propagates by a cleavage mechanism. The cleavage fracture process has recently been shown to be of a statistical nature in both ferritic and bainitic steels. The carbide size distribution plays a dominant role in controlling the fracture toughness of these steels. A cleavage fracture model has been developed, by which both the expectance value and the probability limits of the fracture toughness, K_IC, can be predicted. The probability limits given by the model are shown to be consistent with the experimental observations. The application of the model to the data on which the ASME Section XI reference fracture toughness curve is based indicates that the reference curve is slightly unconservative.     

Effects of geometry and flow properties on the fracture toughness of a C–Mn reactor pressure vessel steel in the lower shelf region

To estimate the fracture toughness of thick section nuclear reactor pressure vessel (RPV) steel in the irradiated condition, it is necessary to apply a size effect correction to the test results obtained on small-scale surveillance specimens. This correction is usually derived using toughness data obtained on different sizes of fracture mechanics testpieces made of non-irradiated material, for which the flow properties are quite different from those of material in the irradiated state. This paper describes the results of a fracture toughness test programme carried out on a C–Mn steel plate for two different specimen geometries (10 mm thickness precracked Charpy and 25 mm thickness compact tension) in the lower shelf region of the temperature/fracture–toughness curve. A comparison of the fracture behaviour and failure micromechanisms has been made for the material in the ‘start of life’ condition and after the application of cold prestraining, which was used to simulate the effects of neutron irradiation on flow properties during service. Although the Master Curve methodology predicts no size effects on the lower shelf, size effects were observed.     

Prediction of cleavage fracture from non-sharp defects using the Weibull stress based toughness scaling model

The structural integrity assessment of engineering plant is based on the principles of fracture mechanics that assume defects to be sharp cracks. Whilst conservative, this assumption may be overly conservative in some situations, e.g. for defects that are non-sharp. This paper describes the prediction of cleavage fracture initiation from blunt notches of varying root radii using the Weibull stress based toughness scaling model. Failure predictions are compared with the results of experiments performed on single edge notch bend SE(B) specimens containing both cracks and notches of varying notch root radii. Cleavage initiation sites were located close to the peak tensile stress ahead of the notch, implying that a tensile stress criterion is the main controlling factor for cleavage fracture. The cleavage fracture predictions from the toughness scaling model correlate well with the experimental data; but care needs to be taken to ensure that calibration of the Weibull parameters references fracture toughness data from constraint levels that span that of the defect of interest. This ensures the model interpolates between the constraint states used for calibration, rather than extrapolating outside the range of applicability.     

Fracture toughness of spun cast SG iron pipe

The fracture toughness properties of two spun cast, ferritic SG iron pipe samples have been investigated and the results compared with previous work. It was found that the COD at fibrous crack initiation ($\text{δ}{}_{\mathrm{<mtext>i</mtext>}}$) was independent of: (a) specimen size; (b) machined or fatigue precracking and (c) graphite nodule number and spacing. This is discussed with respect to the fracture initiation mechanism which was found to be independent of graphite nodules. The maximum load COD values and COD transition were similar for fatigue cracked and machined slit specimens. The COD values and COD transition temperature obtained from spun cast pipe are lower than those obtained in previous work on SG iron and it has been shown that the maximum load COD decreases with decrease in graphite nodule spacing. The effect of graphite nodule spacing on the ductile fracture propagation energy is discussed.     

局部钢纤维混凝土力学性能研究

针对不同掺量的局部钢纤维增强混凝土（PSFRC）构建了细观数值模型,采用统计损伤方法模拟了PSFRC从微裂纹萌生、扩展直至贯通的全过程,通过对破坏机理和力学性能的分析,得出PSFRC初裂强度、极限强度、弯曲韧性和变形能力比素混凝土、普通钢纤维混凝土有明显改善,并随钢纤维掺量的增加其改善程度增大,性价比提高,施工难度也大大降低。     

An assessment of failure probability in elastic-plastic conditions where stable tearing has occurred

An analysis is presented for calculating the failure probability of a structure subject to general elastic-plastic loading and where the fracture mode is ductile. The main statistical variables considered are fracture toughness, flow stress and defect size.The analysis is based on a development of the R6 methodology of defect assessment. The concept of a maximum load locus is developed from the failure assessment line. The maximum load locus is used to predict those combinations of materials properties and defect size that might combine to predict structural failure after some stable crack growth. The method of determining the maximum load locus is described and some examples presented.The maximum load locus is used to estimate the failure probability on the first loading of a structure. It is also shown that the maximum load locus can be used to estimate the failure probability of a proof loaded structure where, in general, there may be a change in the dominant failure mode between proof and fault loadings. Detailed examples are presented to illustrate the analysis.     

Measurement of fracture toughness transition behaviour of Cr–Ni–Mo–V pressure vessel steel using pre-cracked Charpy specimens

The potential application of small pre-cracked Charpy specimens for the prediction of the fracture toughness of the 1T-thickness specimens and the reference temperature T₀ has been examined. Transition fracture behaviour of plane sided, side-grooved and 1T SENB specimens, respectively, was investigated over a wide temperature range. The fracture toughness regions with various fracture initiation mechanisms were defined and ductile to brittle transition temperatures denoted. The fracture toughness transition region of small pre-cracked specimens was shifted to lower temperatures as compared with that of 1T SENB specimens. The fracture toughness data of small pre-cracked specimens have been size corrected (weakest link) to 1T thickness and used to establish the reference temperature T₀ and K_Jc(mean) fracture toughness vs. temperature curve. The calculated temperature T₀ has been in consistence with that of the 1T SENB specimen. However, some corrected fracture toughness data lay outside the scatter band of 1T thickness specimens and the shape of the K_Jc(mean) curve has been quite different from the K_Jc(med)(1T) curve. It was found out that the original measured fracture toughness results of corrected data points lying outside the scatter band violated the validity condition b₀R_p0.2/J_c≥30. Bearing in mind the work of Koppenhoefer and Dodds Jr. (Engng Fract Mechanics (1997);58:249–270), and the most recent analysis of Ruggieri et al. (Engng Fract Mechanics (1998);60:19–36), the fracture data of small pre-cracked specimens having the validity parameter lower than 50 have been first constraint adjusted using the cleavage fracture toughness scaling model of Dodds and coworkers (J Testing Evaluation (1991);19:123–134; Int J Fracture (1995); 74:131–161; Engng Fract Mechanics (1997);58:249–270), and only then size corrected. The K_Jc(mean) curve of such treated data was identical with K_Jc(med)(1T).     

Effect of temperature gradient on crack initiation

Compact tension (CT) specimens of pressure vessel steel A533B have been subjected to linear distributions of temperature along the crack line. The temperature dependence of fracture toughness for the A533B steel creates a fracture toughness gradient in such a specimen. The specimens simulated an irradiated pressure vessel wall and a functionally gradient material. The CT specimens subjected to linear distributions of temperature were tested to evaluate the fracture toughness for crack initiation. In all tests, the temperature at the crack tip was kept at −10°C or −55°C. If the value of the temperature gradient ahead of the crack tip exceeded a critical value, the fracture toughness deviated from the toughness obtained under a uniform temperature of −10°C or −55°C.