δ_5间接测量法测试管线钢止裂韧性CTOA试验研究

为解决多数实验室不具备δ5专用夹式引伸计而无法实现δ5间接测量法对裂纹尖端张开角CTOA的测试问题,该文另辟蹊径借鉴直接法测试CTOA中的数值图像相关方法,通过工业摄像机采集试样撕裂过程图像,然后在图像上直接测试δ5和裂纹扩展量Δa,进而求得裂纹尖端张开角CTOA。采用该方法重点测试3件X80管线钢试样的稳态裂纹扩展阶段临界CTOA值(CTOA)c,并依据试验断裂形式从原理上分析试验结果的有效性,确认被测X80管线钢的(CTOA)c为10.32°。     

CTOA测试中的约束分析

针对实验研究约束不易操作、难以测量的状况,采用有限元Gruson孔洞模型建立SENB试件、CT试件和MCT试件的有限元模型,研究试件类型、厚度对裂尖轨迹的影响,以及试件厚度、裂纹扩展量和约束表征参数对裂尖约束的影响。研究结果表明:试件类型对于裂尖轨迹影响较小,试件厚度对裂尖轨迹影响显著;约束沿厚度方向和裂纹扩展方向变化明显,全局约束因子αg和应力三轴度Am等效,该结论对CTOA测试及理论分析有较大的理论价值和指导作用。     

X70管道钢裂纹尖端张开角的试验研究

高压富气输送管道要求发展管道钢的裂纹尖端张开角(CTOA)的测试方法。采用准静态的撕裂过程和长裂纹扩展长度试件,进行了X70管道钢的裂纹扩展试验,用摄像机跟踪拍摄裂纹扩展全过程,从获得的照片上直接测量出裂纹扩展全过程的CTOA值,分析了裂纹扩展中的断裂力学行为。试验结果表明:试件的韧带厚度越小,越容易获得稳定的扩展过程,且稳态裂纹扩展的CTOA值随试件韧带厚度的增加而增大,太厚的试件不易得到稳态扩展过程。韧带厚度为4mm、8mm试件的稳态的裂纹扩展阶段分别为7.6°、11.3°,裂纹扩展长度与韧带厚度比在4―25、4―10之间。     

单晶γ-TiAl合金中裂纹沿[111]晶向扩展的分子动力学研究

为了从微观角度探索γ-TiAl合金中特定晶向的裂纹扩展机理,研究了γ-TiAl合金中[111]晶向微裂纹扩展的过程及其断裂机理。首先在单晶γ-TiAl合金中预置[111]晶向的微裂纹,然后通过分子动力学方法模拟该裂纹的扩展过程,最终分析了裂尖原子组态变化、微裂纹扩展路径以及应力-应变情况。研究表明,该晶向的微裂纹不是沿直线扩展,而是启裂时裂尖发生偏转,表现出明显的取向效应;微裂纹以裂尖发射滑移位错以及裂尖上形成孪晶的方式进行扩展;受边界的影响,微裂纹扩展到一定阶段会在边界位错堆积处萌生子裂纹,且扩展机制与主裂纹类似;在两个裂纹尖端发射滑移位错的相互作用下,在主裂尖前端再次萌生子裂纹,最终主、子裂纹相连导致断裂;微裂纹扩展过程中的应力分布主要集中于裂尖和扩展过程中形成的孪晶面上,并且随着微裂纹的扩展,裂尖应力值随时间的增大而减小。     

温度对单晶γ-TiAl合金裂纹扩展的影响的分子动力学模拟

用分子动力学方法从原子尺度对单晶γ-TiAl合金中心裂纹的扩展机理进行了研究,模拟了不同温度下预制中心裂纹的扩展过程。结果表明:随着温度升高,裂纹的启裂时间变长,启裂应力值分别为5.64GPa、4.58GPa和4.27GPa;裂尖和边界发射的位错数目随温度的升高而增多;温度为300K时,裂纹先脆性扩展,出现分枝后,裂纹通过裂尖发射位错向前扩展,扩展过程为塑性扩展;温度达750K时裂纹出现分枝,扩展过程为塑性扩展,此时的裂纹扩展速率慢于300K时的裂纹扩展速率;950K时裂纹没有出现分枝,扩展过程为塑性扩展且扩展速率最快;三种温度下裂纹扩展过程均出现裂尖钝化与偏折现象。     

基于数字图像相关技术的裂纹扩展测量

常用的管道钢断裂韧度测试方法包括夏比冲击试验(CVN)、落锤撕裂试验(DWTT)、裂纹尖端张开位移(CTOD)试验、裂纹尖端张开角(CTOA)试验、双试件法等。双试件法对低韧性材料的测试结果比高韧性材料的要准确,对于表征高韧性的钢管材料的断裂韧度来说,CVN和DWTT方法会受与韧性断裂扩展没有关系的能量耗散的影响,CTOA被很多国内外的学者认为测量结果准确度高,简便易行,是比较方便好用的止裂判据。在对金属裂纹扩展各种测试方法进行系统分析的基础上,研究了基于数字图像相关技术的稳定裂纹扩展阻力测量技术,开发了一种高精度快速测量硬件和软件处理系统,实际应用证明了该系统的实用性和可靠性。     

基于声发射技术的30CrMnSi钢断裂机理研究

采用三点弯曲试样加载过程声发射监测实验,结合断口SEM观察及X射线能谱分析,考察了30CrMnSi合金钢延性断裂的整个物理过程.结果表明:30CrMnSi钢的断裂过程从宏观上可以分为裂尖塑性变形、裂纹亚临界扩展和裂纹失稳扩展三个阶段.其机理如下:裂尖塑性变形过程是位错的滑移、堆积而引起的孔洞形核和长大;裂纹亚临界扩展过...     

A537钢在盐水中疲劳裂纹尖端塑性区尺寸减小与声发射

研究了Ａ５３７低强度结构钢在空气和３．５％ＮａＣｌ中性水溶液中疲劳裂纹扩展及其相应的声发射规律。结果表明腐蚀疲劳裂纹扩展过程中声发射活动下降。由于裂纹尖端塑性区内的位错运动是疲劳与腐蚀疲劳裂纹扩展的主要声发射源，所以认为声发射活动性下降是氢使尖塑性尺寸减小所致。用激光散斑干涉法裂纹尖端应变分布，证明氢的存在使裂尖塑性区尺寸减小，氢的存在同提高裂尖局部材料的流变应力。     

内嵌弹性区和结合力模型在薄膜/基底界面断裂中的应用

薄膜/基底结构是微电子学和材料科学中广泛应用的典型结构。由于加工工艺中材料力学、热学性能失配等原因导致的薄膜中出现的残余应力,是界面裂纹的萌生和扩展的重要原因。采用三参数(Γ₀, /σ_y,t)的修正的断裂过程区结合力模型,讨论了在塑性氛围下裂尖解理断裂的过程,裂尖应力分布,裂尖形貌和表征裂纹尖端断裂过程区特征参数对断裂过程的影响,并应用到均质金属薄膜/陶瓷基底结构中残余应力导致界面裂纹起裂和扩展的全过程分析中。     

Fe—3%Si 单晶体裂纹尖端位错行为及氢效应的 TEM 动态观察~*

在透射电镜中对 Fe-3%Si 单晶体薄膜进行拉伸试验,观察了在受力状态下裂纹尖端的位错行为及固溶氢的影响。结果表明,在受力时裂尖会自动发射位错。位错一旦从裂尖发射,便很快离开裂纹尖端以反塞积群形式存在于塑性区中。在裂纹尖端与塑性区之间存在一无位错区(DFZ)。裂纹扩展包括裂尖发射位错的塑性过程以及 DFZ 解理的脆性过程。固溶的氢改变了裂纹尖端的位错分布,促进了位错从裂尖发射,致使 DFZ 的长度减小甚至消失。     

CTOA of a stable crack in a thin aluminum fracture specimen

A crack tip opening angle (CTOA) resistance curve was generated from the moiré interferometry data of thin single edge notched (SEN) and central notched (CN), 2024-T3 aluminum fracture specimens. This CTOA resistance curve, which has a steady state value of 6°, was then used to propagate the cracks in elastic–plastic finite element models of the CN specimen and a CN specimen with a simulated multiple site damage. The CTOA of curved crack growth in a biaxial fracture specimen scattered between 4° and 8° but the resultant crack tip opening displacement, which is the vector sum of the mode-I and the mode-II crack tip sliding displacement, remained a constant 0.18 mm. The CTOA of a rapidly propagating crack in 1.6 mm thick, 7075-T6 SEN specimens increased from 4.5° at a low-crack velocity to a constant 7° at the terminal crack velocity.     

Crack tip opening angle measurement through a girth weld in an X100 steel pipeline section*

Crack tip opening angle (CTOA) is becoming one of the most accepted methods for characterizing fully plastic fracture. It provides a measure of the resistance to fracture for a material in cases where there is a large degree of stable‐tearing crack extension during the fracture process. Our current pipeline research uses the CTOA test as an alternative, or addition, to the CTOD (crack tip opening displacement) and the fracture energy characterization provided by the J‐integral approach. A test technique was developed for measurement of CTOA that uses a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software were used to record the progression of the crack tip and to estimate the CTOA. In this article, CTOA data on crack growth orientations perpendicular to pipeline girth welds are presented. The CTOA for X100 high strength bainitic gas pipeline steel is reported. Two different specimen gauge sections, 3 mm and 8 mm, were used and the effect of the specimen thickness on the CTOA is discussed. The results show a change in the CTOA as the crack grows into the heat affected zone (HAZ). A slight improvement in the fracture resistance is measured, and through the weld, a slight decrease in fracture resistance is observed.     

Comparison of the measured and predicted crack propagation behaviour of Zr–2.5Nb pressure tube material

The crack propagation behaviour of Zr–2.5Nb pressure tube material was studied through comparison of the measured and predicted behaviour. Three-dimensional finite element simulations of compact tension and burst specimens were performed using the crack tip opening angle (CTOA) as a fracture criterion to allow for crack propagation. To obtain reasonable agreement with the measured force, crack extension, displacement, and crack tunnelling behaviour from compact tension simulations, a non-constant CTOA profile was required. This CTOA profile was then used as the crack propagation criterion in simulations of burst specimens where it was again found that there was reasonable agreement obtained with experiment.     

Constraint-corrected fracture failure criterion based on CTOD/CTOA

In engineering design, a difficulty has always existed in those standard laboratory tests that cannot accurately predict the behavior of large structures like pipelines due to the different constraint levels. At present, extensive work has been done to characterize the constraint effects on fracture toughness by introducing a second parameter, while the systematic research on constrained transformation is inadequate. To address this issue, the ductile fracture process of X65 SENB specimen is simulated through the finite-element method coupled with the Gurson–Tvergaard–Needelman model. The parameters crack tip opening displacement (CTOD) and crack tip opening angle (CTOA) are chosen to characterize the fracture behaviors. The effects of specimen thickness on fracture toughness based on CTOD/CTOA and constraints ahead of crack tips in SENB specimen are studied. The results indicate that the critical values of CTOD/CTOA decrease with the increase of specimen thickness, but the constraint parameters are opposite. Furthermore, it finds that there is a near linear relationship between critical values of CTOD/CTOA and the stress constraint ahead of the crack tip. Thus, a constraint-corrected fracture failure criterion based on CTOD/CTOA is proposed, which can be used for the prediction and simulation of stable tearing crack growth in specimens and structures, made of this steel with any thickness value.     

A new data analysis technique to examine crack tip opening angle values tested in API X65 pipe steel

This paper presents a new data analysis technique to rapidly identify the region of stable crack growth in crack tip opening angle (CTOA) testing of a modified double cantilever beam. The technique basically used the load–displacement data from CTOA testing on API X65 steel, which demonstrated a region of constant slope after the peak load. In total, 22 data points (out of 90) fell in this region, for which the variation of CTOA versus crack length remained almost constant. The CTOA measurement was conducted from crack edges and from a fine reference grid, using photographs from two cameras in front and rear sides of the CTOA specimen. As the visual analysis of the individual photographs is tedious and rather lengthy, the presented technique can be easily used for rapid and precise identification of the mean CTOA from those data in the constant slope region of the load–displacement plot.     

Quantifying crack tip displacement fields with DIC

Crack paths under both fatigue and fracture conditions are governed by the crack tip displacement field and the material deformation characteristics, including those influenced by metallurgical anisotropy. Experimental techniques such as thermoelasticity and photoelasticity have been successfully used to characterise the elastic stress fields around cracks but they do not take into account either plasticity or anisotropy. Considerable work has been carried out to characterise crack tip stress fields from displacement measurements. The current method of choice for obtaining displacement field data is digital image correlation (DIC) which has undergone significant advances in the recent years. The ease of use and capabilities of the technique for full field displacements has led to improved methods for characterising crack tip displacement fields based on data obtained from DIC. This paper gives an overview of some of the applications of DIC for crack tip characterisation such as K, T-stress and crack tip opening angle (CTOA) measurements as well as data obtained from 3D measurements of a propagating crack.     

Microtopography for ductile fracture process characterization: Part 1: Theory and methodology

The mechanics of ductile fracture is receiving increased focus as the importance of integrity of structures constructed from ductile materials is increasing. The non-linear, irreversible mechanical response of ductile materials makes generalized models of ductile cracking very difficult to develop. Therefore, research and testing of ductile fracture have taken a path leading to deformation-based parameters such as crack tip opening displacement (CTOD) and crack tip opening angle (CTOA). Constrained by conventional test techniques and instrumentation, physical values (e.g. crack mouth opening displacement and CTOA angles) are measured on the test specimen exterior and a single through-thickness “average” interior value is inferred. Because of three-dimensional issues such as crack curvature, constraint variation, and material inhomogeneity, inference of average parameter values may introduce errors. The microtopography methodology described here measures and maps three-dimensional fracture surfaces. The analyses of these data provide direct extraction of the parameters of interest at any location within the specimen interior, and at any desired increment of crack opening or extension. A single test specimen can provide all necessary information for the analysis of a particular material and geometry combination.     

AN ENGINEERING FRACTURE PARAMETER FOR NON-J-CONTROLLED CRACK GROWTH

Abstract— An engineering fracture parameter (CTOA) is studied under non- J -controlled growth. Based on the asymptotic solution of the crack tip field and the concept of the ₁ integral, an analytical relation linking the crack tip opening angle (CTOA) and the ₁ integral is established. A numerical investigation of the process of crack growth is carried out by the finite element method. The whole field solution, fracture parameters and indeterminate parameters are determined by FE analysis. An experimental investigation on a ductile structural steel has also been conducted and the crack resistance curve (CTOA)_R is obtained. The numerical and experimental results validate the theoretical analysis and show that they are consistent. It can be concluded that CTOA is always constant during steady crack propagation.     

Crack Tip Opening Angle Measurement Methods and Crack Tunnelling in 2024‐T351 Aluminium Alloy

Abstract: The crack tip opening angle (CTOA) fracture criterion is one of the most promising fracture criterion used to characterise the stable tearing process in metallic materials. Traditional methods used for the experimental characterisation of the CTOA involve accurate identification of the crack tip at each tearing event. Recently alternative methods have been proposed that reduce the necessity of accurately defining the current crack and rely more on the shape of the crack flanks to define the CTOA. In addition, these methods define an ‘apparent crack tip’, which may be different from the actual surface crack tip and may provide insight into the amount of crack‐front tunnelling that is occurring. In the current research, compact tension specimens fabricated from 6.35 mm thick 2024‐T351 aluminium alloy plate were evaluated to investigate different CTOA measurement methods and their potential for estimating crack‐front tunnelling. In addition to characterizing the CTOA, fatigue marker bands were employed to map the evolution of crack‐front tunnelling. The experimental critical CTOA values obtained from the alternative methods were noticeably lower than that obtained from the traditional approach and showed noticeably more scatter. When compared to the experimentally obtained marker bands, the alternative methods indicated limited potential for predicting crack‐front tunnelling.     

Microtopography for ductile fracture process characterization Part 2: application for CTOA analysis

The crack tip opening angle (CTOA) is seeing increased use to characterize fracture in so-called “low constraint” geometries, such as thin sheet aerospace structures and thin-walled pipes. With this increase in application comes a need to more fully understand and measure actual CTOA behavior. CTOA is a measure of the material response during ductile fracture, a “crack tip response function”. In some range of crack extension following growth initiation, a constant value of CTOA is often assumed. However, many questions concerning the use of CTOA as a material response-characterizing parameter remain. For example, when is CTOA truly constant? What three-dimensional effects may be involved (even in thin sheet material)? What are the effects of crack tunneling on general CTOA behavior? How do laboratory specimen measurements of CTOA compare to actual structural behavior?Measurements of CTOA on the outer surface of test specimens reveal little about three-dimensional effects in the specimen interior, and the actual measurements themselves are frequently difficult. The Idaho National Engineering and Environmental Laboratory (INEEL) use their microtopography system to collect data from the actual fracture surfaces following a test. Analyses of these data provide full three-dimensional CTOA distributions, at any amount of crack extension. The analysis is accomplished using only a single specimen and is performed entirely after the completion of a test. The resultant CTOA distributions allow development of full and effective understanding of CTOA behaviors. This paper presents underlying principles, various sources of measurement error and their corrections, and experimental and analytical verification of CTOA analysis with the microtopography method.