硅砖裂纹产生的机制

硅砖生产过程中由于砖体受环境温度的急剧变化,内部产生较强热应力,当强度达到制品的强度极限值时,即产生裂纹甚至开裂。硅砖裂纹主要产生在成型和烧成阶段,但是粉料配料、泥料混炼以及干燥、装车等工序控制不当也会产生裂纹废品。因此,要降低废品率,控制好成型和烧成工序至关重要,但其它工序的操作也不容忽视。     

大功率柴油机机体上止口裂纹的检查和修复

1概述190系列柴油机广泛用于钻井、发电、冶金。调车机车、船舶等方面，也是石油钻探用主要动力设备。机体是柴油机的主要部件，工作时，上止口承受约50吨的爆发力。由于设计、制造及其它方面的原因，上止口容易产生裂纹，受力后逐步扩展导致漏水，严重时需整机返修甚至报废。2190柴油机机体裂纹的检查由于机体尺寸很大，结构复杂，长期使用后表面划压伤痕很多，这给裂纹的检查带来极大不便。常用的磁粉探伤、X射线探伤、着色探伤、超声波探伤很难满足要求，须采用水洗型荧光渗透探伤剂探伤。2．1水洗型荧光渗透探伤简介水洗型荧光探伤剂由…     

在用蒸压釜停车检验与裂纹分析

针对某企业蒸压釜停车检验时,蒸压釜母材、焊缝、釜齿等部位均发现存在裂纹的现象,根据裂纹产生的部位和裂纹形貌特征,分析判断应力腐蚀裂纹、冷裂纹、疲劳裂纹及产生原因。     

墙体裂纹的解决方法

分析了墙体产生裂纹的原因,阐述了腻子在解决温变裂纹中的作用和原理,并指出裂纹防治方案实施中应注意的问题.     

厚度63.5mm的A572钢板探伤缺陷分析

63.5mm厚的A572Gr50钢板在进行超声探伤时检测到有疑似分层缺陷,对钢板进行了低倍、探伤、力学性能及扫描电镜的检验。结果标明:A572Gr50钢板材料化学成分符合相关标准规定要求;中部超声波探伤时检测到的缺陷不是分层缺陷,而是由密集不连续微裂纹(毫米尺度)、夹杂物和马氏体组织分布带组成的缺陷,不连续微裂纹是在残余应力作用下沿夹杂物聚集区和硬度较高的马氏体组织区形成的微裂纹,属于一种应力裂纹。     

裂纹形态对丁腈橡胶力学性能和耐疲劳性能的影响

丁腈橡胶为石化行业应用最为广泛的一类橡胶材料。在受到周期应力的作用下，丁腈橡胶内已存在的裂纹会不断扩展而最终导致橡胶断裂。本文考察了预制缺陷的种类及大小对丁腈橡胶疲劳过程中裂纹扩展情况的影响，并对断面形貌进行了观察。结果表明，在圆形、纵向和横向三种缺陷中，纵向缺陷对丁腈橡胶的力学性能和耐疲劳性能影响最小；对圆形缺陷来讲，随孔直径的增大，丁腈橡胶的耐疲劳性能和力学性能逐渐下降。     

16MnR钢表面裂纹的疲劳扩展

报告了在 P_x∶P_y=0,0.5和1三种双轴载荷作用下,对倾斜角等于0°、30°和45°的表面裂纹所作的疲劳扩展研究。讨论了表面裂纹扩展速率的计算方法,指出必须考虑不同的裂纹倾斜角和不同的双轴载荷比造成的裂纹扩展驱动力的变化对斜表面裂纹扩展速率的影响。因而采用裂纹投影法来处理斜表面裂纹的疲劳扩展问题并提出了修正的 Paris 方程。按照修正的 Paris 方程,根据不同的双轴载荷比和裂纹倾斜角分组整理了试验数据,并作了回归分析、假设检验,给出了含表面裂纹的16MnR 板材在三种不同的双轴载荷比情况下的裂纹扩展速率方程。     

烧结MgAlON复合刚玉材料微裂纹的制备与表征

以刚玉(ω(Al2O3)98%,粒度分为3-1 mm、≤1 mm、≤85um和≤3um四级)和粒度≤2 um的MgAlON为原料,聚乙烯醇溶液作结合剂,设计了3种含骨料的配方和4种不合骨料的配方,在液压机上于150MPa下保压5 s压制成型,于N2气氛下1 500℃保温2 h烧成,制成φ20mm×10 mm和3 mm×4 mm×36 mm的MgAlON复合刚玉试样,分别采用热震(1 100℃空冷10次)的方法和在维氏显微硬度计上施加不同压力(29.4、49和98 N)保压25 s的方法预制裂纹,然后在N2气氛保护下分别于1 100℃和1 550℃保温6 h进行裂纹愈合处理.研究了裂纹预制方法、试样的粒度组成以及获得维氏压痕时的载荷大小和保压时间对裂纹形状和尺寸的影响,愈合温度对压痕愈合情况及试样抗折强度的影响,并计算了维氏压痕的愈合速率.结果表明:(1)烧结MgAlON复合刚玉试样的热震裂纹形状不规则,长度不易准确测定.(2)维氏压痕近似菱形,虽没有出现由压痕顶角向4个方向发射的微细狭长裂纹,但用颗粒大小为数微米的粉料制备试样时容易控制维氏压痕的大小、形状和位置;在98 N载荷下保压25 s获得的压痕深度适中,形貌规则,能够愈合,便于下一步的愈合机理研究.(3)经1 100℃和1 550℃愈合处理后,压痕逐渐变小、变浅乃至消失,其抗折强度则逐渐恢复到原试样的水平.(4)压痕的愈合速率可以表示为: .     

不同泊松比复合材料的裂纹尖端应力场

具有平面负泊松比的碳纤维/环氧树脂非平衡复合材料层板具有很高的断裂韧性和缺口断裂强度。本文用复变函数一变分方法计算了负泊比材料的裂纹尖端应力场，并与常规的平衡复合材料进行了比较。本文重点研究了主应力方向与纤维夹角的关系。结果表明：负泊松比材料独特的裂纹尖端应力场有利于提高缺口断裂强度。     

A New Method for Precracking Beam for Fracture Toughness Experiments

A simple and versatile precracking method using a triangular notch as a crack starter in limited bending was developed, which is suitable for both brittle ceramics and quasi-plastic materials that are difficult to precrack by the conventional bridge-indentation technique. Slow growth of large crack in brittle or quasi-brittle ceramics was controlled and observed in situ in this way. The precracking tests performed on various ceramics exhibited high reliability and feasibility. The precracked specimens were subsequently used to measure the fracture toughness, and the resultant data showed that the fracture toughness determined by using the precracked specimens reflected the minimum value of the toughness measured in single edge-notched beam (SENB) tests.     

陶瓷材料断裂韧性测试新技术：静态爆破剂预制裂纹的研究

准确测试陶瓷材料断裂韧性Ｋ１Ｃ的关键是预制具有原生裂纹的试样。本文以单边切口梁法测试Ｋ１Ｃ为基础，提出了一种简便而有效的陶瓷材料原生裂纹的预制方法－静态膨胀法。研究结果表明：静态膨胀法中裂纹的扩展是稳态的；通过调节影响微裂纹产生与扩展的多种因素，可以准确地控制裂纹扩展的深度，预制出合适的原生裂纹，为准确评价结构陶瓷的断裂韧性提供了一种有效可行的方法。     

Fracture Toughness Evaluation of Small Thin Ceramic Specimens

A test method to evaluate the fracture toughness, K_IC, of thin, small, precracked ceramic specimens is described. The method is applicable for thin plates, wafers, self-supported layers, etc., especially when a large amount of material is not available for testing. The method consists of bonding a small, thin single-edge notched beam on one side of a metallic beam. A stress-free precrack with a square root singularity is achieved when the assembly is deformed in three-point bending.
The fracture toughness of a thin, alumina single-edge precracked beam was evaluated experimentally using this method, and compared with that obtained for similar specimens having a 0.3-mm-wide machined notch. Comparison with previously reported fracture toughness values suggests that even a very sharp machined notch overestimates the evaluated fracture toughness.     

Experimental Study of the Fracture Toughness of a Ceramic/Ceramic-Matrix Composite Sandwich Structure

A hybrid experimental–numerical approach has been used to measure the fracture resistance of a sandwich structure consisting of a 304 stainless steel/partially stabilized zirconia ceramic-matrix composite crack-arresting layer embedded in a partially stabilized zirconia ceramic specimen. The mode I fracture toughness increases significantly when the crack propagates from the ceramic into the ceramic-matrix composite region. The increased toughening due to the stainless steel particles is explained reasonably well by a toughening model based on processing-induced thermal residual stresses. In addition, several experimental modifications were made to the chevron-notch wedge-loaded double cantilever beam specimen to overcome numerous problems encountered in generating a precrack in the small, brittle specimens used in this study.     

Modified Specimen Preparation for Applied-Moment Double-Cantilever-Beam Testing

A method is discussed for preparing applied-moment double-cantileverbeam specimens used in fracture toughness (K_Ic) testing. This procedure eliminates the need to precrack the specimens, which often leads to specimen failure before testing. In addition to yielding toughness values which are in agreement with literature values, this method allows the experimenter greater control in selecting the location of failure initiation.     

Effect of Precrack "Halos" on Fracture Toughness Determined by the Surface Crack in Flexure Method

The surface crack in flexure method, which is used to determine the fracture toughness of dense ceramics, necessitates the measurement of precrack sizes by fractographic examination. Stable crack extension may occur from flaws under ambient, room-temperature conditions, even in the relatively short time under load during fast fracture strength or fracture toughness testing. In this article, fractographic techniques are used to characterize evidence of stable crack extension, a "halo," around Knoop indentation surface cracks. Optical examination of the fracture surfaces of a high-purity Al₂O₃, an AlN, a glass-ceramic, and a MgF₂ reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF₂ is due to indentation-induced residual stresses initiating crack growth. However, for the Al₂O₃ and the glass-ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.     

硅液相浸渍反应制备TiC/SiC和TiN/SiC复相陶瓷

以滤纸、酚醛树脂和氧化钛为原料,经过模压成型、固化、碳化及不同条件下渗硅制备了TiC/SiC和TiN/SiC复相陶瓷。通过X射线衍射和扫描电子显微镜研究了TiC/SiC和TiN/SiC复相陶瓷的微观结构和物相组成,测量了复相陶瓷的弯曲强度和断裂韧性。结果表明:真空条件下液态渗硅获得的TiC/SiC复相陶瓷具有多孔的微观结构,其弯曲强度和断裂韧性较小。氮气气氛下液态渗硅制备的TiN/SiC复相陶瓷结构致密,有较高的弯曲强度和断裂韧性。不同反应生成的TiC,TiN陶瓷颗粒对液态硅的润湿性不同,使得生成的复相陶瓷具有不同的微观结构。TiN/SiC复相陶瓷中TiN颗粒的引入,在基体与第二相颗粒间的界面上产生拉应力和压应力,使达到这一区域的裂纹偏转,从而获得增韧效果。     

Mechanical behaviour of a silicon nitride particulate ceramic composite

Mechanical behaviour of a particulate ceramic composite (Si₃N₄ + SiC) was investigated. Its strength and fracture toughness on heating up to 1300 °C were determined as well as stress–strain curves plotted for this temperature range were analyzed. It is emphasized that this material is not only heterogeneous but also inelastic, and its deformation and fracture behaviour differ considerably from those of conventional ceramics. It was established that SENB fracture toughness measurements on notched specimens in flexure were quite reliable. Thus, there is no need in employing sophisticated standard test methods for this purpose. Fracture resistance estimates by the edge fracture (EF) method demonstrated that this material exhibited a lower barrier to the onset of fracture and a nonlinearly rising R-line, i.e., it displayed the ability to resist crack propagation (R-curve effect). The fracture resistance F_R and initial fracture toughness K_Ii were also determined. This information is rather useful for analysis of its actual performance under mechanical loading. The model of a nozzle vane of the gas turbine was employed to illustrate that the EF method was appropriate for evaluating the uniformity of ceramic items by their fracture resistance.     

Double Torsion testing of thin porous zirconia supports for energy applications: Toughness and slow crack growth assessment

Thin, porous zirconia-based ceramic components are of high interest in energy application devices where they are used as structural ceramics. Mechanical reliability of such devices is not only dependent on the fracture toughness of the ceramic components, but also on their sensitivity to slow crack growth (SCG). In this work, the fracture toughness and SCG behavior of porous (4.5–45.5%) and thin (∼ 0.25 mm) 3Y-TZP ceramics are investigated using the Double Torsion method. The analysis of the double torsion data, previously developed for dense materials, was here assessed and adapted. The compliance of the samples was observed to change linearly with crack length and the measured stress intensity factor was dependent on crack length, as for dense materials. This dependency decreased by increasing the sample porosity. For all materials, the ratio of the SCG threshold to fracture toughness was of 0.56 ± 0.06.     

Determining the Toughness of Ceramics from Vickers Indentations Using the Crack-Opening Displacements: An Experimental Study

Recently, a method for evaluating the fracture toughness of ceramics has been proposed by Fett based on the computed crack-opening displacements of cracks emanating from Vickers hardness indentations. To verify this method, experiments have been conducted to determine the toughness of a commercial silicon carbide ceramic, Hexoloy SA, by measuring the crack-opening profiles of such Vickers indentation cracks. Although the obtained toughness value of K _o= 2.3 MPa·m^1/2 is within 10% of that measured using conventional fracture toughness testing, the computed crack-opening profiles corresponding to this toughness display poor agreement with those measured experimentally, raising concerns about the suitability of this method for determining the toughness of ceramics. The effects of subsurface cracking and cracking during loading are considered as possible causes of such discrepancies, with the former based on direct observations of lateral subsurface cracks below the indents.