Crack initiation stress and strain of jointed rock containing multi-cracks under uniaxial compressive loading: A particle flow code approach

The ratio of crack initiation stress to the uniaxial compressive strength(SCI,B/SUC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength(B,UCB,CI,A,A/SSSS) were studied by performing numerical stress analysis on blocks having multi flaws at close spacing's under uniaxial loading using PFC3 D. The following findings are obtained: SCI,B/SUC,B has an average value of about 0.5 with a variability of ± 0.1. This range agrees quite well with the values obtained by former research. For joint inclination angle, β=90°,B,UCB,CI,A,A/SSSS is found to be around 0.48 irrespective of the value of joint continuity factor, k. No particular relation is found betweenB,UCB,CI,A,A/SSSS and β; however, the average B,UCB,CI,A,A/SSSS seems to slightly decrease with increasing k. The variability ofB,UCB,CI,A,A/SSSS is found to increase with k.Based on the cases studied in this work,B,UCB,CI,A,A/SSSS ranges between 0.3 and 0.5. This range is quite close to the range of 0.4to 0.6 obtained for SCI,B/SUC,B. The highest variability of ± 0.12 forB,UCB,CI,A,A/SSSS is obtained for k=0.8. For the remaining k values the variability ofB,UCB,CI,A,A/SSSS can be expressed within ± 0.05. This finding is very similar to the finding obtained for the variability of SCI,B/SUC,B.

Modal and Fatigue Life Analysis on Beam with Multiple Cracks Subjected to Axial Force

Based on the transfer matrix method and Forman equation,a new method is proposed to conduct the modal and fatigue life analysis of a beam with multiple transverse cracks.In the modal analysis,the damping loss factor is introduced by the complex elastic modulus,bending springs without mass are used to replace the transverse cracks,and the characteristic transfer matrix of the whole cracked beam can be derived.In the fatigue life analysis,considering the interaction of the beam vibration and fatigue cracks growth,the fatigue life of the cracked beam is predicted by the timing analysis method.Numerical calculation shows that cracks have a significant influence on the modal and fatigue life of the beam.

Identifying crack initiation stress threshold in brittle rocks using axial strain stiffness characteristics

As an estimate for the in-situ spalling strength around massive underground excavations to moderately jointed brittle rocks, crack initiation stress marks the initiation of rock micro fracturing. It is crucial to accurately identify crack initiation stress level by proper method. In this study, confined compression tests of sandstone samples are used to examine the validity/applicability of proposed axial strain stiffness method. The results show that by highlighting the minuscule changes in stress-strain curve, the axial strain stiffness curve provided further insight into rock failure process and revealed five stages:(a) irregular fluctuation,(b) nearly horizontal regular fluctuation,(c) irregular fluctuation gradually decreasing to zero,(d) extreme fluctuation, and(e) near zero, which mainly correspond to five stages of stress–strain curve. The ratio of crack-initiation stress to peak strength determined using this approach is 0.44–0.51, similar to the ranges previously reported by other researchers. In this method, the key is to accurately detect the end point of the stage(b), "nearly horizontal regular fluctuation" characterized by a sudden change in axial strain stiffness curve, and the sudden change signifies crack initiation in rock sample. Finally, the research indicates that the axial strain stiffness curve can provide a mean to identify the crack-initiation stress thresholds in brittle rocks.

THE CONSTITUTIVE RELATION OF CRACK-WEAKENED ROCK MASSES UNDER AXIAL-DIMENSIONAL UNLOADING

An accurate and efficient numerical method for solving the crack-crack interaction problem is presented. The method is mainly by means of the dislocation model, stress superposition principle and Chebyshev polynomial expansion of the pseudo-traction. This method can be applied to compute the stress intensity factors of multiple kinked cracks and multiple rows of periodic cracks as well as the overall strains of rock masses containing multiple kinked cracks under complex loads. Many complex computational examples are given. The dependence of the crack-crack interaction on the crack configuration, the geometrical and physical parameters, and loads pattern, is investigated. By comparison with numerical results under confining pressure unloading, it is shown that the crack-crack interaction under axial-dimensional unloading is weaker than those under confining pressure unloading. Numerical results for single faults and crossed faults show that the single faults are more unstable than the crossed faults. It is found from numerical results for different crack lengths and different crack spacing that the interaction among kinked cracks decreases with an increase in length of the kinked cracks and the crack spacing under axial-dimensional unloading.

联合Self-attention与Axial-attention的机场跑道裂缝分割

机场跑道裂缝形态多样、方向各异、长短不一且粗细不均,通常不具有统计规律。现有的各类裂缝分割算法难以在此类复杂场景中落地。针对上述问题,提出了联合self-attention与axial-attention的机场跑道裂缝分割网络(CSA-net),通过引入自注意力模块、轴向注意力模块、可变形卷积模块,提取裂缝的局部特征和全局语义特征。通过transformer decoder还原特征图的原始尺寸,融合了不同尺度间的分割结果,保留尽可能多的细节信息,使得CSA-net有更好的分割精度。在机场跑道实拍的数据集上进行的测试表明,针对裂缝的像素级分割指标F1-score达到了78.91%,高于目前各类裂缝分割算法。

轴流压缩机动叶片断裂失效分析及对策

某公司催化裂化轴流压缩机组在服役过程中因动叶片发生断裂导致装置异常停工。将机组解体后,通过采用宏观形貌、化学成分、微观组织、扫描电镜及能谱等检测手段对叶片的断口进行了分析。经分析发现,叶片断裂的主要原因是在交变应力作用下,叶片外弧面的腐蚀坑处存在应力集中,使疲劳裂纹发生扩展导致叶片断裂。文章结合以往国内外轴流压缩机叶片断裂案例中的经验和教训,针对叶片断裂失效问题,从运行管理、检修质量、涂层等3方面提出了应对措施和建议。

ON THE PLANE PIEZOELECTRIC PROBLEM OF A LOADED CRACK TERMINATING AT A MATERIAL INTERFACE

The plane problem of a crack terminating at the interface of a bimaterial piezoelectric, and loaded on its faces, is treated. The emphasis is placed on how to transform this problem into a non-homogeneous Hilbert problem. To make the derivation tractable, the concept of the axial conjugate is introduced and related to the complex conjugate. The angle between the crack line and the interface may be arbitrary. Numerical results are given to illustrate the stress singularity at crack tip.

Mechanical Behavior of Polyurethane Polymer Materials under Triaxial Cyclic Loading:A Particle Flow Code Approach

Polyurethane polymer grouting materials were studied with conventional triaxial tests via the particle flow code in two dimensions（PFC^（2D）） method, and the simulation results agreed with the experimental data. The particle flow code method can simulate the mechanical properties of the polymer. The triaxial cyclic loading tests of the polymer material under different confining pressures were carried out via PFC^（2D） to analyze its mechanical performance. The PFC^（2D） simulation results show that the value of the elastic modulus of the polymer decreases slowly at first and fluctuated within a narrow range near the value of the peak strength; the cumulative plastic strain increases slowly at first and then increases rapidly; the peak strength and elastic modulus of polymer increase with the confining pressure; the PFC^（2D） method can be used to quantitatively evaluate the damage behavior of the polymer material and estimate the fatigue life of the materials under fatigue load based on the number and the location of micro-cracks. Thus, the PFC^（2D） method is an effective tool to study polymers.

SELF-SIMILAR SOLUTIONS OF FRACTURE DYNAMICS PROBLEMS ON AXIALLY SYMMETRY

By the theory of complex functions, a penny-shaped crack on axially symmetric propagating problems for composite materials, was studied. The general representations of the analytical solutions with arbitrary index of self-similarity, were presented for fracture elastodynamics problems on axially symmetry by the ways of self-similarity under the ladder-shaped loads. The problems dealt with can be transformed into Riemann-Hilbert problems and their closed analytical solutions are obtained rather simple by this method. After those analytical solutions are utilized by using the method of rotational superposition theorem in conjunction with that of Smirnov-Sobolev, the solutions of arbitrary complicated problems can be obtained.

Crack Detection in Pipes with Different Bend Angles Based on Ultrasonic Guided Wave

Pipeline plays an indispensable role in process industries,because the progressing crack-like defects of in it may result in serious accidents and significant economic losses.Therefore,it is essential to detect the cracks occurred in pipelines.The axial crack-like defects in elbows with different angle are inspected by using the T(0,1)mode guided waves,in which different configurations including 45°,90°,135°and 180°(straight pipe)are considered respectively.Firstly,the detection sensitivity for different defect location is experimentally investigated.After that,finite element simulation is used to explore the propagation behaviors of T(0,1)mode in different bend structures.Simulation and experiment results show that the crack in different areas of the elbow can affect the detection sensitivity.It can be found that the detection sensitivity of crack in the middle area of the elbow is higher compared to the extrados and intrados of the elbow.Finally,the mode conversion is also investigated when the T(0,1)crosses the bend,and the results show that bend is a key factor to the mode conversion phenomenon which presents between the T(0,1)mode and F(1,2)mode.

纤维掺量对超高性能混凝土纤维桥接应力的影响效应研究

为确保采用超高性能混凝土UHPC的结构在进行抗裂计算时合理考虑纤维桥接应力的作用,设计了纤维掺量分别为1%、2%和3%的带缺口狗骨形试件,开展了UHPC材料轴拉试验,确定了不同受拉性能等级UHPC的纤维桥接应力发展规律以及纤维掺量对UHPC纤维桥接应力的影响效应。结果表明:1)在1%~3%的纤维掺量范围内,UHPC材料的宏观开裂强度、抗拉峰值强度、纤维桥接应力和裂缝控制能力均随纤维掺量的增加而增大;2)若采用相同纤维特征的UHPC材料,则能桥接的最大裂缝宽度基本相同;3)应变软化和低应变硬化UHPC开裂后均有应力陡降现象,而高应变硬化UHPC开裂后没有应力陡降现象,说明采用高应变硬化UHPC无需配筋即可实现构件的弥散开裂,若采用应变软化和低应变硬化UHPC则需要配筋才能实现弥散开裂。

考虑残余应力影响的桥式起重机多轴疲劳寿命评估

桥式起重机桥架结构是典型的焊接结构,在日常服役时主梁焊缝区域经常出现疲劳裂纹。为保证起重机的服役安全,基于多尺度有限元分析方法建立危险裂纹区域的多级子模型;在二级子模型上建立热-结构耦合焊接有限元模型并模拟分析了焊接的全过程,通过模拟焊后热处理分析对残余应力的影响;结合多轴临界面法构建了以应变幅值作为多轴损伤参量的疲劳分析方法,评估了危险裂纹区域的疲劳性能。结果表明:所提方法将结构整体疲劳问题转化成局部焊接位置的疲劳问题,不仅增加了计算效率,还能考虑疲劳细节处更多的影响因素,为起重机焊缝部位的疲劳寿命评估和延寿提供了一定的参考。

2198-T8铝锂合金多轴疲劳性能及裂纹扩展行为

对2198-T8铝锂合金薄壁管状试样在不同加载条件下进行了拉扭复合疲劳裂纹扩展试验。结果明,随着裂纹扩展阶段的等效应力幅降低,试样的多轴疲劳寿命升高。当应力幅比升高时,多轴疲劳寿命降低,轴向应变变大,扭向应变变小,扭向应变集中滞后于轴向应变集中。结合数字图像相关技术,拟合疲劳裂纹扩展速率模型,并采用两种临界平面模型对裂纹扩展方向进行预测,其中最大正应力平面对裂纹扩展方向的预测效果最好。随着应力幅比的升高,裂纹扩展区微观形貌由磨损形貌变为磨损与疲劳条带共存的形貌,裂纹扩展模式由II型变为I+II型,最终变为以I型为主。

大数据背景下基于轴裂纹的贝叶斯网络故障诊断模型研究

本研究专注于利用贝叶斯网络的建模方法进行故障诊断,特别关注轴裂纹故障。研究目标是通过比较和应用4种不同的贝叶斯网络模型来确定在轴裂纹故障诊断中最为精确的模型。实验过程中,首先从故障模拟器中收集和整理了轴裂纹故障的相关数据,这些数据包括可能导致轴裂纹的各种参数,如应力水平、材料属性和运行时间等。数据集被划分为训练集和测试集,以便进行模型训练和精度验证。在稳定的实验环境、精准的数据采集设备以及高效的计算机处理系统的支持下,运用4种贝叶斯网络建模方法对训练集进行了模型构建。随后,通过对测试集的验证,对比这4种模型的诊断精度得到最优的诊断模型。这一发现揭示了该方法在故障诊断领域的优势,并为提升工业设备的运行安全性和维护效率提供了实用工具。本研究的创新之处在于将4种不同的贝叶斯网络建模方法应用于轴裂纹故障诊断,并通过实证对比分析确定了最优模型。这一研究成果不仅丰富了故障诊断领域的理论方法,也为实际工程中的轴裂纹故障诊断提供了高效、精准的解决方案。

单轴应力状态下石灰岩体静态破碎裂纹演化规律及应用

为了研究静态破碎过程中裂纹演化规律,采用真三轴岩石力学试验机对200 mm×200 mm×200 mm的立方体石灰岩试样进行了单轴应力状态下静态破碎试验,并利用声发射空间定位技术对静态破碎过程中裂纹起裂与扩展进行监测。结果表明:单轴应力下裂纹起裂位置为钻孔中下部,并先后形成两组裂纹,每组裂纹沿孔壁对称分布并大致成一直线;两组裂纹的扩展速率均先快后慢,分为快速增长和快速下降两个阶段,并且下降阶段的裂纹扩展速率小于增长阶段扩展速率;先生裂纹的扩展速度大于后生裂纹;每条裂纹的扩展速度具有方向的差异性,即平行孔轴向方向大于与垂直孔轴向方向;基于弹性力学、断裂力学理论,建立了静态破碎剂作用力与裂纹扩展半径的关系,并通过试验结果验证了该模型的合理性。将研究结果应用于某高瓦斯矿实际地质条件下的强制放顶工程中,确定了静态破碎强制放顶的基本参数,获得了良好的工业效果。

基于外穿式交流电磁场检测的钻杆轴向裂纹在役检测技术研究

钻杆是钻井过程中易受破坏的关键部件,在其服役过程中进行检测,对于减少钻具失效事故具有重要意义。针对钻杆在役检测的特点,提出一种新型外穿式交流电磁场检测方法,建立针对钻杆外表面轴向裂纹的外穿式交流电磁场检测有限元仿真模型,分析钻杆表面感应电磁场分布以及裂纹引起的电磁场畸变信号与裂纹尺寸关系,在此基础上提取裂纹识别特征信号,基于仿真模型分析激励线圈提离高度影响,设计开发井口钻杆外表面轴向裂纹在役检测系统,并进行试验测试,仿真和试验结果表明:外穿式交流电磁场检测探头所提取的钻杆轴向裂纹特征信号Bx和Bz分别反映裂纹深度和长度,具有定量识别能力;激励线圈提离高度可满足在役检测需要;外穿式交流电磁场检测系统利用周向阵列检测线圈可有效检测钻杆表面轴向裂纹。

轴对称断裂动力学问题的自相似解

通过复变函数论的方法 ,对复合材料的圆盘状裂纹的轴对称性扩展问题进行研究· 采用自相似方法 ,并根据任意的自相似指数的断裂动力学问题 ,进行自相似求解 ,导出解析解的一般表示· 应用该法可以迅速地将所论问题转化为Riemann_Hilbert问题 ,并可以相当简单地得到问题的闭合解· 利用这些解并采用叠加原理 ,就可以求得任意复杂问题的解·

最佳自增强下厚壁圆筒表面轴向裂纹研究

基于有限元软件ANSYS分析了厚壁圆筒的最佳自增强压力。计算了在相应工作压力下自增强圆筒内表面轴向裂纹Ι型应力强度因子。考虑了裂纹的几何形状、深度等影响因素,对圆筒内外两侧同时存在裂纹的情况进行了研究。结果表明:最佳自增强压力随工作压力而变化;自增强后应力强度因子的减少量随裂纹加深而降低;外部裂纹可使内裂纹应力强度因子降低1%左右。

静态破碎剂的轴向膨胀力学性能实验研究

针对静态破碎剂轴向膨胀力学行为进行了深入研究,提出了测量静态破碎剂膨胀压的轴向输出法,设计了测试系统及核心钢筒组件.分别采用轴向输出法和外管法对静态破碎剂膨胀压进行了测试.通过对比在不同孔径条件下两种方法测得的膨胀压,分析了孔径大小对静态破碎剂轴向膨胀压及其输出率的影响.结果表明:轴向输出法能够可靠准确测得静态破碎剂膨胀压;在固定的高径比条件下,静态破碎剂轴向膨胀压随孔径尺寸增大而增加;孔径小于50 mm时,自封孔效应导致静态破碎剂输出轴向膨胀压的能力减弱;孔径大于150 mm时,轴向膨胀压与径向膨胀压趋于一致,轴压输出率达100%.

管道轴向裂纹检测脉冲远场涡流传感器设计与仿真分析

脉冲远场涡流检测方法结合了脉冲检测频率丰富以及远场方法适于铁磁性管道检测的优势,因此文中采用脉冲远场技术对管道轴向裂纹进行了检测。首先分析了脉冲远场涡流的检测原理,通过提取感应电压信号的负峰值和过零时间作为特征量可以分析管道的检测信息,在此基础上设计了4种不同结构的传感器模型,比较了4种模型过渡区的远近、对轴向裂纹检测灵敏度的高低以及对不同壁厚管道检测的结果。仿真结果表明:与其它模型相比,连通激励磁路的传感器模型具有更好的检测效果。