桥梁高性能钢HPS485W断裂韧性试验研究

为了研究桥梁高性能钢的断裂韧性,对中国舞阳钢厂生产的高性能钢HPS 485W进行了一系列的断裂韧性试验研究。通过夏比V形缺口冲击试验得到HPS 485W在不同温度下的冲击韧性,并应用Boltzmann函数求解韧脆转变温度曲线,试验结果表明:与传统桥梁用钢相比,HPS 485W冲击韧性更高且韧-脆转变温度更低。由HPS 485W延性断裂韧度(J_IC)试验,测得板厚18mm和28mm的HPS 485W试样的J积分值,试验结果表明HPS 485W具有优良的断裂韧性。对8mm和14mm的HPS 485W进行裂纹尖端张开位移(CTOD)试验,试验结果表明HPS 485W有良好的塑性和韧性。基于HPS 485W拉伸试验获得的应力-应变曲线结果,运用失效评定曲线方法对CTOD试验值进行评定,HPS 485W的母材断裂韧性(CTOD)落在评定曲线的合格范围内。该文的研究为高性能钢桥的安全评定和防断裂设计提供了试验依据。     

低温环境下桥梁钢Q345qD疲劳裂纹扩展行为研究

为了明确在寒冷地区服役桥梁钢的疲劳裂纹扩展行为,以16 mm厚桥梁钢Q345qD为研究对象,完成了室温和低温下的夏比冲击韧性试验、疲劳裂纹扩展速率试验和疲劳裂纹扩展门槛值试验。结果表明,夏比冲击功和试样断口剪切断面率随温度的降低而减少;在应力比0.1、0.2和0.5条件下,疲劳裂纹扩展速率均随温度降低而变缓,该桥梁钢的疲劳韧-脆转变温度点在-60℃以下;在室温~-60℃,其裂纹扩展速率均对应力比的变化不敏感;应力比0.1条件下的疲劳裂纹扩展门槛值随温度的降低有略微增大的趋势。该批次桥梁钢表现出了良好的抵抗低温疲劳裂纹扩展性能,防止低温脆性破坏成为疲劳设计的重点;试验数据能为钢结构桥梁的进一步抗低温疲劳和防低温冷脆断裂设计提供参考。     

14MnNbq焊接桥梁钢的疲劳裂纹扩展行为研究

该文系统地研究了14MnNbq桥梁钢焊接热影响区的疲劳裂纹扩展行为。首先,由中心穿透裂纹(MT)试样疲劳裂纹扩展试验,获得了不同应力比R下的疲劳裂纹扩展速率和门槛值;然后考察了应力比R的影响,给出了适于不同应力比的疲劳裂纹扩展速率和门槛值的一般表达式;最后提出了一种由疲劳裂纹扩展门槛值▽Kth确定闭合参数U的新方法,将控制疲劳裂纹扩展的有效应力强度因子幅度写为▽Keff=▽K-▽Kth,由此讨论闭合参数U的确定方法。研究结果表明:对于14MnNbq焊接桥梁钢,该文给出的疲劳裂纹扩展速率表达式与试验结果符合得相当好。     

WNQ570桥梁钢及其对接焊缝疲劳裂纹扩展性能试验研究

对WNQ570桥梁钢及其对接焊缝进行了疲劳裂纹扩展速率和疲劳裂纹扩展门槛值测定试验,采用两种不同数据拟合方法分别得到具有95%保证率的疲劳裂纹扩展参数。结果表明:本批次的WNQ570钢材具有良好的疲劳裂纹扩展性能,其中对接焊缝疲劳裂纹扩展速率高于母材;在应力强度因子幅值处于10 MPa·m1/2～70 MPa·m1/2的常规区间时,基于单试件数据点的处理结果对应的裂纹扩展速率明显高于基于成组数据点的处理结果;WNQ570的疲劳裂纹扩展速率随应力比增加而增加,疲劳裂纹扩展门槛值随应力比增加而降低。     

Ti-6Al-4V钛合金的疲劳裂纹扩展规律

针对熔模铸造Ti-6Al-4V钛合金的等幅疲劳裂纹扩展速率和疲劳裂纹扩展门槛值进行了研究。结果表明：该钛合金CT试样的疲劳裂纹扩展门槛值高于CCT试样的疲劳裂纹扩展门槛值,同一类试样的疲劳裂纹扩展门槛值随着应力比的增加呈下降趋势;疲劳裂纹扩展速率随着平均应力的增加以及应力水平的增加而增大;根据疲劳裂纹扩展试验数据拟合了Ti-6Al-4V钛合金Paris方程和Walker方程中的相关材料参数,以为材料的使用寿命评估及损伤容限设计提供参考。     

高强度结构钢系列的疲劳裂纹扩展速率试验研究

为研究高强度结构钢的疲劳性能,对Q460C、Q550D、Q690D和Q960D四种国产高强钢母材进行了疲劳裂纹扩展速率试验。每种钢材均取3个10 mm厚的C(T)紧凑拉伸试样,在应力比R=0.1的条件下施加恒幅疲劳荷载,利用显微镜目测记录每个试样的裂纹扩展长度a和对应的循环次数N。对每个试样的试验结果,分别采用七点递增多项式法、Smith法和割线法进行处理,计算裂纹扩展速率和应力强度因子幅值数据组,得到对应于Paris公式中的参数,比较不同方法拟合结果的优劣性并分析原因。再将同种钢材三个试样的数据合成一组,同样采用三种方法计算,并与单试样数据的结果作对比。最后给出了四种钢材Pairs公式中的疲劳裂纹扩展速率参数,并与其他文献中各种结构钢疲劳裂纹扩展速率的参数值进行对比评价。研究表明该文高强度结构钢的疲劳裂纹扩展速率会随着强度的提高而降低,并且这四种高强钢的疲劳裂纹扩展速率都比BS7910中针对普通强度结构钢推荐的疲劳裂纹扩展速率要低。     

拉-压循环加载下铝合金疲劳裂纹扩展的压载荷效应研究

应用弹塑性有限元方法与增量塑性损伤理论指出疲劳裂纹扩展的压载荷效应是裂纹尖端塑性损伤的结果, 建立了在拉-压循环加载下铝合金疲劳裂纹扩展速率的双参数预报模型, 对LY12-M 高强铝合金MT 试件在应力比R=0、-0.5、-1、-2 进行了疲劳裂纹扩展实验。结果表明:当最大应力强度因子K_max相同时, 恒幅拉压加载(应力比R<0)的疲劳裂纹扩展速率明显高于恒幅拉拉加载(应力比R=0)的情况, 拉-压循环载荷的压载荷部分对疲劳裂纹扩展速率具有促进作用。该文得出的LY12-M 铝合金在拉-压循环加载下的疲劳裂纹扩展速率预报模型与实验结果符合较好。     

温度对TC4-DT损伤容限型钛合金疲劳裂纹扩展行为的影响

对TC4-DT损伤容限型钛合金在150℃,25℃下的疲劳裂纹扩展速率da/dN进行了测试,给出了扩展速率和应力强度因子幅值△K之间的关系曲线.用SEM对2种温度下断口形貌进行了观测,实验结果表明,150℃的疲劳裂纹扩展速率试样具有较低的疲劳裂纹扩展速率,25℃的疲劳裂纹扩展速率试样具有较低的门槛值;稳态扩展区解理断裂和条带循环机制共存,150℃的da/dN试样中的疲劳辉间距比25℃试样细;快速扩展区的断口形貌呈韧窝型静载断裂特征,150℃的da/dN试样中的韧窝比25℃试样深.     

加载载荷对ADB610钢疲劳裂纹扩展性能的影响研究

针对ADB610新型低碳贝氏体钢,采取紧凑拉伸试样、恒幅加载方式,在应力比为0.1时进行3种不同加载载荷下多试样的疲劳裂纹扩展试验,并测出裂纹扩展长度a和循环寿命N的a-N试验数据。然后采取七点递增多项式法对a-N试验数据拟合计算疲劳裂纹扩展速率。结果表明,加载载荷较小时,ADB610钢循环寿命较长;在疲劳裂纹扩展的初始阶段,加载载荷小的疲劳裂纹扩展速率相对较慢,但随着疲劳裂纹逐渐扩展,加载载荷小的疲劳裂纹扩展速率反而更大一些。     

30Cr1Mo1V钢汽轮机转子高应力段的疲劳裂纹扩展速率

在某已服役了16 a的30Cr1Mo1V钢汽轮机转子的高应力段取样制作成紧凑拉伸试样,用MTS 810.50试验机进行室温和538℃下的疲劳裂纹扩展速率试验。结果表明：该钢疲劳裂纹稳定扩展阶段的疲劳裂纹扩展速率适用于Paris公式,室温下的疲劳裂纹扩展速率方程为da/dN=2.2101×10-8（ΔK）2.9163,538℃下的疲劳裂纹扩展速率方程为da/dN=9.8794×10-8（ΔK）2.6844;对于30Cr1Mo1V转子钢,温度升高,疲劳裂纹扩展速率加快;30Cr1Mo1V转子钢在疲劳裂纹稳定扩展阶段存在转折点,将该阶段又细分为两段,经过转折点后疲劳裂纹扩展速率的增速减慢;与原始材料相比,已服役16 a的30Cr1Mo1V钢汽轮机转子高应力段材料的疲劳裂纹扩展速率增大。     

Improved test method for very low fatigue‐crack‐growth‐rate data

Currently, in North America, the threshold crack‐growth regime is experimentally defined by using ASTM Standard E647, which has been shown in many cases to exhibit anomalies due to the load‐reduction (LR) test method. The test method has been shown to induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold. In this paper, the fatigue‐crack growth rate properties in the threshold and near‐threshold regimes for a titanium alloy, Ti‐6Al‐4V (STOA), are determined by using the LR test method and an improved test method. The improved method uses ‘compression–compression’ precracking, as developed by Pippan, Topper and others, to provide fatigue‐crack‐growth rate data under constant‐amplitude loading in the near‐threshold regime, without load‐history effects. Tests were conducted over a wide range in stress ratios (R = 0.1–0.7) on compact C(T) specimens for three different widths (25, 51 and 76 mm). The slitting method was used on 51 mm C(T) specimens to confirm that the material did not contain significant levels of residual stresses from forming and/or machining. A crack‐mouth‐opening‐displacement gage was used to monitor crack growth. Data from the ASTM LR method gave near‐threshold values that were found to be dependent upon the specimen width. However, data from the compression precracking constant amplitude (CPCA) loading method gave near‐threshold data independent of specimen width. A crack‐closure analysis was performed for both the LR and CPCA data, to correlate data at the various stress ratios. The CPCA data correlated well with the effective stress‐intensity factor range against rate relation, whereas the LR data exhibited significant threshold fanning with both stress ratio and specimen width.     

Fatigue life prediction of a structural steel under service loading

Fatigue life prediction techniques for variable amplitude load histories are reviewed. The fatigue crack growth rate and crack closure responses of BS4360 50B steel are determined for a service load history experienced by a gas storage vessel. Crack propagation rates are found to be independent of specimen thickness. Crack growth is successfully predicted by linear summation using the Paris law; no significant improvement is achieved by incorporating crack closure into the analysis. The particular choice of cycle counting technique is also found to have an insignificant effect on the predicted fatigue life. The load-interaction model proposed by Willenborg et al correctly indicates the absence of retarded growth, whilst the Wheeler and Führing models erroneously predict retarded crack growth.     

带中心孔的钛合金扩散连接层合板裂纹扩展特性

为了研究含孔TC4钛合金扩散连接层合板的裂纹扩展行为,进行了含ϕ6中心孔的8mm厚单层板材、三层(3+2+3)扩散连接层合板和三层(3+2+3)含ϕ12止焊区扩散连接层合板的疲劳对比试验,试验中施加标识载荷,通过断口判读裂纹形态和尺寸,得到裂纹扩展(a,N)数据,建立了裂纹扩展da/dN-a曲线,对单层板、层合板和止焊层合板的裂纹扩展行为和规律进行了对比。结果表明:单层板出现规则的半椭圆形孔壁裂纹;层合板由于存在层合界面的影响,出现不规则的半椭圆孔壁裂纹;而止焊层合板以角裂纹为主,裂纹扩展过程分为三阶段;三类试件的da/dN-a曲线可用双对数线性关系描述;三层板的裂纹扩展特性不弱于单层板;止焊层合板的裂纹扩展性能有明显改善,提高了含孔层板的损伤容限特性。     

超高韧性水泥基复合材料疲劳裂缝扩展公式的理论与试验研究

该文基于Paris公式,对具有应变硬化和多缝开裂特征的超高韧性水泥基复合材料(UHTCC)的疲劳裂缝扩展理论进行了理论研究,并通过试验进行验证。研究表明,Paris公式适用于UHTCC。其中:裂缝扩展参数由裂缝面积扩展A表示;与之相应,使用复合断裂能增长量#x00394;J代替应力强度因子变化量#x00394;K。基于试验,该文求出UHTCC的疲劳裂缝扩展门槛值,即当疲劳过程中的断裂能幅值#x00394;J小于某一临界值#x00394;J_th时,疲劳裂缝不扩展。在疲劳过程中,UHTCC的裂缝覆盖面积随疲劳过程呈三阶段线性发展,与疲劳变形的发展趋势一致。UHTCC的疲劳裂缝扩展随疲劳最大荷载与荷载幅值的增大而加速发展。     

Development of disc bending fatigue test technique for equi-biaxial loading

The disc bending fatigue test technique was developed to investigate the fatigue life under an equi-biaxial loading condition. In this test, a uniform thickness disc specimen was subjected to a bending load by applying air pressure on the specimen surface. Eleven specimens made of Type 316 stainless steel were tested in a room temperature ambient environment. The crack initiation and growth behaviors during the test were observed through a transparent window. The fatigue life was defined when the peak pressure measured near the specimen surface was reduced to 95% of the supplied air pressure. The fatigue life obtained by the disc bending fatigue test was shorter than that obtained by the uniaxial and plate bending fatigue tests for the same principal strain range. It was confirmed that the equi-biaxial loading condition reduced the fatigue life. The finite element analysis together with test results revealed that the crack was initiated at the edge of the specimen when the specimen thickness was less than 1.0 mm. The specimen thickness should be 1.2 mm in order to maximize the strain range at the specimen center. It was concluded that the disc bending fatigue test can derive the fatigue life under an equi-biaxial loading condition, for which strain range is measured at the specimen center.     

FATIGUE CRACK GROWTH OF SILICON NITRIDE WITH CRACK WEDGING BY FINE FRAGMENTS

Abstract— Fatigue crack growth tests of silicon nitride Si₃N₄ were carried out under four-point bending using square bar specimens at room temperature. A pre-crack was introduced by a bridge indentation method. Decreasing K -type tests at stress ratios of R = 0.1 and 0.6 and also under static load were first carried out, and after observing the cessation of crack growth, K -increasing tests were performed except for the case of a static load. Crack closure was observed on most specimens by the elastic compliance method. Furthermore, SEM observations of the crack paths were made to see what was happening during crack growth. The threshold and the region of steady crack growth were observed more clearly under cyclic loading, and an effect of load cycling certainly existed which became more evident when the maximum stress intensity factor K _max approached the threshold. A wedge effect, caused by fine fragments on the crack surface, played an important role in crack closure behaviour of each specimen, and it is suggested that the crack growth rate is controlled by both the effective stress intensity range Δ K _eff and the effective mean stress intensity factor K _m,eff at least as a first approximation.