拉弯复合应力下焊接头表面裂纹疲劳扩展规律的研究

通过对16MnR钢对焊的弓形试极疲劳裂纹扩展试验，研究了在拉弯复合应力下对焊接头表面裂纹疲劳扩展规律。采用Newman－Raju公式分析了有限宽板在拉弯复合应力下表面裂纹的应力强度因子的计算。试验研究表明在拉弯复合应力下当a／t≤0．8时表面裂纹疲劳扩展规律仍可用Paris公式来描述，并且c向和a向的Paris系数之间仍存在着C_c＝0．9~nC_a的关系。     

表面裂纹疲劳扩展形貌变化规律的研究

首先进行了１６Ｍｎ板材半椭圆表面裂纹疲劳扩展试验。试验结果表明，采用Ｎｅｗｍａｎ－ｒａｊｕ应力强度因子时，表面裂纹深度方向和表面方向的扩展速率均符合Ｐａｒｉｓ公式。在此基础上，从Ｎｅｗｍａｎ－Ｒａｊｕ应力强度因子计算式和Ｐａｒｉｓ疲劳裂纹扩展速率公式出发，通过理论和数值分析，建立了３类表面裂纹在受拉伸、弯曲、和拉弯组合加载下的疲劳扩展形貌统一表达式，并通过疲劳试验结果对其进行了考核。结果表明，采用统一表达式预测裂纹形貌的各项统计指标均优于其他两种表达式     

共聚聚丙烯疲劳裂纹扩展研究

参照金属材料疲劳裂纹扩展速率试验方法,采用材料疲劳试验机对无规共聚聚丙烯(PP–R)材料进行了Ⅰ型疲劳裂纹的扩展速度及裂纹扩展机理的研究,测试得到了疲劳裂纹扩展每0.5 mm长度下所对应的循环次数,进而拟合出da/d N(疲劳裂纹扩展长度与循环次数之比)与ΔK(应力强度因子)的疲劳裂纹扩展速率曲线图,发现PP–R疲劳裂纹扩展可分为裂纹扩展源阶段,裂纹扩展稳定阶段和裂纹快速扩展阶段3个阶段,其中扩展源阶段出现明显的增韧现象,稳定扩展阶段呈现出明显的线性特点;同时选取了4个典型的断面区域进行了扫描电子显微镜拍照,得到了4种差异很大的断面显微组织并进行了分析。     

07MnCrMoVR钢焊件疲劳裂纹扩展速率

为鉴定０７ＭｎＣｒＭｏＶＲ钢焊件的疲劳性能，通过试验得到了焊接接头以及焊接热影响区的疲劳裂纹扩展速率，同时给出了它们存活率为９７．７３％时的各自疲劳裂纹扩展速率，并对焊接接头与焊接热影响区疲劳裂纹扩展速率进行比较。最后还把结果与“在役含缺陷压力容器安全评定规程”［１］和ＢＳＩＰＤ６４９３［２］推荐结果进行了对比，认为该钢种疲劳裂纹扩展速率在其容许范围内慢于１６ＭｎＲ钢的疲劳裂纹扩展速率     

铸钢焊接接头裂纹扩展行为研究

简要介绍了某深水气田开发项目中出现的高强度铸钢类型，针对该材料的焊接，选用气体保护药芯焊丝开发焊接工艺。测定了焊接接头区域疲劳裂纹扩展速率及疲劳裂纹扩展门槛值，结果表明：整体焊接接头疲劳裂纹扩展性能较好，耐裂纹性能按高低顺序为焊缝>热影响区>铸钢母材。     

水介质中4340钢超高压换热器疲劳裂纹扩展速率

针对超高压换热管常用的ＡＩＳＩ４３４０钢，对表面裂纹的试板采用浸水腐蚀的方法，研究了４３４０钢在水介质中表面裂纹的疲劳扩展速率，并与空气中的扩展速率进行了比较。试验结果表明，腐蚀介质的存在加速厂裂纹扩展，并随裂纹的扩展，其加速作用越来越明显。水中存在的氧是导致腐蚀和加速裂纹扩展的主要原因。     

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。     

45^#钢中Ⅰ＋Ⅲ复合型疲劳裂纹的转型扩展及断口分析

以带有不同角度侧斜裂纹的紧凑拉伸试件研究了Ⅰ＋Ⅲ复合型裂纹在单向拉伸疲劳载荷作用下的转型情况,着重研究转型过程中应力强度因子（ΔK）和断口形态的变化.有限元计算表明,Ⅰ＋Ⅲ复合型裂纹是向Ⅰ型裂纹转化,侧斜角越大,Ⅲ型应力强度因子变化幅（ΔKⅢ）越大,裂纹转型越快;断口分析表明试件疲劳断口的疲劳源呈多源性,裂纹萌生于紧凑拉伸试件线切割缺口处的微裂纹;在转型过程中,带侧斜角试件断裂面较为粗糙,断面上存在较为粗大的撕裂棱和二次裂纹;疲劳辉纹更加不连续,不均匀;完全转型后断裂面较为平坦,撕裂棱和二次裂纹尺寸减小.即转型是通过在疲劳源形成与原裂纹面成一定角度的台阶,在裂纹的随后扩展中通过不同台阶与台阶的连接而转向,最后转到Ⅰ型扩展方向.     

纳米晶体材料疲劳裂纹扩展研究

纳米晶体材料独特的微观结构和变形机制,使得其裂纹的萌生和扩展有别于其它粗晶材料,本文对一型裂纹的生长建立了理论模型,该模型描述了裂纹的萌生于扩展并且讨论了晶粒尺寸和轴向偏差角对纳米晶体材料表面的疲劳裂纹扩展的影响。该模型结果表明,随着晶粒尺寸和轴向偏差角的增大,裂纹生长速度也伴随着增大,与实验结果相符。     

加载频率变化的腐蚀疲劳裂纹扩展速率数学模型

试验得出了 0 Cr1 8Ni9钢在质量分数为 0 .1 % Na Cl溶液中 4个不同加载频率下的腐蚀疲劳裂纹扩展速率。结合试验结果 ,并采用数学分析方法 ,给出了环境加速因子 C( f)的表达式 ,进而提出了一种腐蚀疲劳裂纹扩展速率的 dad N～ (ΔK ,f )数学计算模型。     

球形容器接管环带区球壳侧应变场在线性条件下的工程估算研究

本文用电测法对３台带接管的球形容器（两台带正交接管，一台带切向接管）接管环带区的应变进行了测试，对应变分布进行了分析。认为球形容器接管环带区的峰伍应变位于离焊缝较近的球壳侧，且峰值应变和压力具有线性关系。在线性条件下，对球壳侧应变的工程估算进行了探讨，得出的经验式可供压力容器安全评定时参考。     

加氢反应器接管焊缝疲劳裂纹的扩展研究

对 2 .2 5Cr1Mo钢在室温和 42 0℃下的疲劳裂纹扩展速率进行了测试和分析 ,得到了两种温度下的疲劳裂纹扩展规律 ,并根据测试结果对一台加氢反应器接管连接焊缝裂纹的疲劳扩展进行了计算和安全评估 ,为该反应器的安全使用提供了理论依据。     

Experimental investigation on fatigue property at room temperature of C/SiC composites machined by rotary ultrasonic milling

Rotary ultrasonic milling technology (RUM), as a surface strengthening machining method, was proposed to utilize in processing of C/SiC composites for enhancing anti-fatigue performance innovatively. Static tensile, intermittent fatigue and residual strength test were carried out. Due to constant impingement of high-frequency and low-amplitude vibration, surface residual compressive stress was formed near 2 GPa maximally. Axial thermal residual stress in fiber achieved -662.4 MPa proved by loading-unloading test. The peak value of fatigue damage parameter was reduced significantly. RUM surface restrains most of interface cracks because of residual compressive stress, and hinder the growth of fiber cracks for better machined surface quality. The damage accumulation, the first stress redistribution and fiber reinforcement stage were delayed. Average damage rate was decreased by 80.5 %. Residual tensile strength of RUM C/SiC was improved after fatigue, up to 95.8 % of tensile strength. The strengthening effect from RUM on fatigue property of C/SiC is significant and valuable.     

Origin of the Static Fatigue Limit in Oxide Glasses

Oxide glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. The log of crack velocity decreases linearly with decreasing stress intensity factor in Region I. For some glasses, at a lower stress intensity, K_o, log v asymptotically diminishes where there is no measurable crack growth. The same glasses exhibit static fatigue, or a decreasing strength for increasing static loading times, as cracks grow and stress intensity eventually reaches the fracture toughness. In this case, some glasses exhibit a low stress below which no fatigue/failure is observed. The absence of slow crack growth under a low stress intensity factor is called the fatigue limit. Currently, no satisfactory explanation exists for the origin of the fatigue limit. We show that the surface stress relaxation mechanism, which is promoted by molecular water diffusion near the glass surface, may be the origin of the fatigue limit. First, we hypothesize that the slowing down of slow crack growth takes place due to surface stress relaxation during slow crack growth near the static fatigue limit. The applied stress intensity becomes diminished by a shielding stress intensity due to relaxation of crack tip stresses, thus resulting in a reduced crack velocity. This diminishing stress intensity factor should result in a crack growth rate near the static fatigue limit that decreases in time. By performing Double Cantilever Beam crack growth measurements of a soda‐lime silicate glass, a decreasing crack growth rate was measured. These experimental observations indicate that surface stress relaxation is causing crack velocities to asymptotically become immeasurably small at the static fatigue limit. Since the surface stress relaxation was shown to take place for various oxide glasses, the mechanism for fatigue limit explained here should be applicable to various oxide glasses.     

10~#钢管锐缺口疲劳裂纹萌生寿命试验研究

为了确定存有大量轴向表面裂纹缺陷的氧气管道能否安全使用,进行了含真实表面缺陷和人工缺陷的实物钢管疲劳试验,并根据试验结果提出了锐缺口疲劳裂纹萌生寿命估算式和钢管的安全性分析。结果表明,钢管能够在现有工况下安全使用。     

自增强厚壁筒表面裂纹尖端应力强度因子的柔度标定法

采用柔度法对液压疲劳实验中的自增强厚壁筒表面裂纹尖端应力强度因子进行了标定,结果表明这是一种求得表面裂纹尖端应力强度因子的有效方法。     

The Mechanism of Crack Formation in Rubbers under the Effect of Ozone

Abstract

The formation and growth of cracks under an effect of ozone were investigated in different types of rubbers, such as NR, PBR, BMSR, SBR and in composite materials based on these rubbers. The study was carried out by the reflection-light optical microscopy method with using the MMP-4 microscope. In the presence of microinhomogeneities in a surface structure the crack formation is shown to start from those defects. In the absence of inhomogeneities the cracks are formed uniformly over the whole surface of a specimen. It is found that even insignificant quantities of low-modulus rubber in the PBR-based composite material may result in changing the shape of a crack during its growth. An attempt is undertaken to describe the shape of formed cracks quantitatively. The calculational and experimental data are presented.

The influence of deformation on the number of cracks per surface unit, on their shape, geometrical size, cracking degree is studied for example of natural rubber specimen.     

Effect of the blooming of chemical curatives on the cyclic fatigue life of natural rubber filled with a silanized silica nanofiller

When partly soluble chemical curatives are mixed with raw rubber, they migrate to the rubber surface, which can be detrimental to the rubber properties. Two rubber compounds with different amounts of curatives were prepared by mixing natural rubber with a high loading of precipitated amorphous white silica nanofiller. The silica surfaces were pretreated with bis(3‐triethoxysilylpropyl)‐tetrasulfide coupling agent to chemically adhere silica to the rubber. The chemical bonding between the filler and rubber was optimized via the tetrasulfane groups of bis(3‐triethoxysilylpropyl)‐tetrasulfide by adding accelerator and activator. The rubber compounds were cured and stored at ambient temperature for up to 65 days. One compound showed extensive blooming as a function of storage time. The cyclic fatigue life of the rubber vulcanizates was subsequently measured at a constant strain amplitude and test frequency at ambient temperature. The blooming of the chemical curatives reduced the cyclic fatigue life of the rubber vulcanizate by more than 100%. The migrated chemical curatives produced a thin layer approximately 15 μm in size beneath the rubber surface. When the rubber was stressed repeatedly in the fatigue test, cracks initiated in this layer and subsequently grew, causing the fatigue life of the vulcanizate to decrease. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011