超高韧性水泥基复合材料延性断裂性能评价准则的试验研究

该文研究了超高韧性水泥基复合材料的断裂性能评价准则。对纤维体积掺量为1。5%、2。0%与2。5%的超高韧性水泥基复合材料试件进行了弯曲断裂试验,基于非线性断裂力学,应用双J参数评价其断裂性能,并分析了其J_R阻力曲线。结果表明,该材料的弯曲断裂表现出延性特征。应用双J参数评价该材料的断裂性能,当流入裂缝塑性区的能量大于起裂断裂能J_IC时,裂缝出现;随后,裂缝稳定扩展;当流入塑性区能量大于失效断裂能J_IF时,局部裂缝出现,材料进入失稳发展阶段。该材料的J_R阻力曲线存在三阶段线性关系,分别以宏观裂缝出现点与主裂缝局部化点为分界点。在UHTCC的变形稳态发展阶段,相同面积的裂纹发展量消耗的能量相同。     

全级配混凝土I型裂缝扩展全过程数值模拟

该文将起裂断裂韧度作为裂缝扩展的判定依据,应用ANSYS软件,对全级配混凝土I型裂缝扩展过程进行数值模拟,分别计算了混凝土楔入劈拉试件的荷载-裂缝口张开位移曲线、临界裂缝长度和双K 断裂韧度,并与溪洛渡大坝的断裂试验结果进行比较,吻合良好。同时,结合试验数据,将该文计算结果与《水工混凝土断裂试验规程》规定的标准尺寸试件断裂参数计算结果进行对比。结果表明:规程规定的方法也适用于大尺寸非标准试件双K 断裂参数的计算,其误差率在5%以内。此外,对于全级配混凝土,只要通过试验测得其弹性模量、抗拉强度、抗压强度和起裂荷载,即可用该文提出的方法计算混凝土的双K 断裂韧度和裂缝扩展全过程。应用该方法还可以得到全级配混凝土的K_R阻力曲线。     

桥梁高性能钢HPS485W疲劳裂纹扩展速率试验研究

以国产桥梁用高性能钢HPS485W为研究对象,对7.5mm、12.5mm和19.5mm的HPS485W紧凑拉伸试样分别在应力比R=0.1、R=0.5和R=0.8的疲劳荷载下进行疲劳裂纹扩展速率(da/dN)试验,采用七点递增多项式的方法进行局部拟合求得试样的疲劳裂纹扩展速率。与传统桥梁用钢14MnNb相比,该文试验测得高性能钢HPS485W具有更优越的忍受疲劳裂纹扩展能力。试验结果表明:试样厚度是影响疲劳裂纹扩展速率的关键因素;对同一厚度的试样,疲劳裂纹扩展速率随着应力比R的增大而增大。此外,对19.5mm试样在荷载比R=0.1的情况下,进行工程门槛值的试验测定和理论门槛值的数值求解,分析求得19.5mm的HPS485W的理论门槛值为7.22MPa·m1/2。该文试验得到的HPS485W疲劳裂纹扩展曲线,可用于高性能钢桥的抗疲劳、防断裂设计与寿命预测。     

基于连续损伤力学的UHTCC疲劳损伤扩展模型研究

为研究超高韧性水泥基复合材料的损伤扩展规律,该文对该材料的预制单边切口试件进行了三点弯曲疲劳试验。基于连续损伤力学,建立两种疲劳损伤扩展模型:一是以J积分作为自变量的双对数线性模型,二是以疲劳应力水平作为自变量的单对数线性模型。结果表明,两种模型计算结果与试验结果吻合良好。但J积分取值很大程度上依赖于试件的几何特征,导致模型1受试件形状影响较大;实际应用中,由于疲劳应力水平的获得相对直观方便,故模型2更加实用。     

疲劳荷载对橡胶混凝土损伤和断裂性能的影响

虽然橡胶混凝土塑性和疲劳性能较好,但由于掺入橡胶,其在疲劳荷载下离散性增大,损伤过程及最终的断裂机制均不明确。为研究橡胶混凝土在疲劳荷载下的损伤和断裂性能,基于声发射开展了不同橡胶掺量的混凝土在疲劳荷载下的三点弯曲疲劳断裂试验。计算有效裂缝长度,分析疲劳荷载下不同橡胶掺量的混凝土裂缝长度a的变化规律,并利用裂缝长度a和声发射累积能量E_AE分别定义了损伤变量D_a和D_AE;分析疲劳荷载下不同橡胶掺量的混凝土断裂能G_F的变化规律;利用声发射中的信号持续时间分析疲劳荷载下橡胶混凝土中裂缝出现和扩展的规律。结果表明,混凝土的断裂能随橡胶掺量的增加呈线性增加;在疲劳荷载下,裂缝长度a和D_a均呈倒S型规律变化,而D_AE呈正S型规律变化;声发射信号持续时间表明,在疲劳荷载下,橡胶混凝土中的裂缝总在荷载较小时出现或发生扩展。      

混凝土I型裂缝扩展准则比较研究

针对混凝土I型裂缝扩展问题,分别采用以起裂韧度为参数的裂缝扩展准则、最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则,数值模拟了强度等级C20、C40、C60、C80和C100的混凝土三点弯曲梁裂缝扩展全过程,获取了试件的荷载-裂缝口张开位移(P-CMOD)曲线并与试验结果进行了比较。结果表明,三种准则中以起裂韧度为参数的裂缝扩展准则计算得到的峰值荷载及P-CMOD全曲线与试验结果差别最小。随着混凝土强度等级的提高,最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则计算出的P-CMOD曲线与试验结果相比均有较为明显的偏离,但以起裂韧度为参数的裂缝扩展准则计算结果与试验曲线更为吻合。试验与计算结果表明,以起裂韧度为参数的裂缝扩展准则更适用于不同强度混凝土材料的断裂分析。     

采用峰值荷载法确定全级配水工混凝土断裂参数

该文采用峰值荷载法研究了全级配水工混凝土试件的断裂参数。通过有限元方法,得到了适用于全级配水工混凝土的非标准楔入劈拉试件的应力强度因子K、裂缝嘴张开口位移CMOD计算公式以及裂缝张开位移COD与CMOD比值COD/CMOD的相关曲线。基于相关断裂试验,通过该文所提公式得到了全级配水工混凝土的等效断裂韧度与临界裂缝尖端张开口位移CTOD_c等断裂参数。研究结果表明:峰值荷载法可应用于截面尺寸不同而缝高比相同的全级配水工混凝土试件,峰值荷载法只需实测各试件的峰值荷载,就可方便确定全级配水工混凝土的断裂参数。     

沥青路面Top-Down裂缝的数值分析

沥青路面Top—Down裂缝是在沥青路面表面出现的一种典型的损坏形式。在荷载的重复作用下，这种裂缝会由上向下疲劳扩展，严重影响路面的使用功能甚至导致路面结构的破坏。以Paris公式为基础，采用三维有限元方法分析了荷载作用下TopDown裂缝尖端的应力强度因子，并裂缝的疲劳扩展规律与疲劳寿命。     

刚柔复合式路面沥青层温度疲劳损伤及开裂研究

运用损伤力学、传热学、断裂力学理论和有限元法,分析了沥青层的温度疲劳损伤特性、疲劳寿命、裂缝扩展等。研究结果表明:温度疲劳损伤主要集中在沥青层表面,随着温变次数的增加,沥青层表面的损伤度线性增加;随着沥青层模量与温变幅值的增加,沥青层表面的损伤度不断增加;随着温变次数的增加,沥青层表面的水平拉应力线性减小;随温变幅值的增大,沥青层的疲劳寿命不断减小;基于高温效应的沥青层最小临界厚度为4cm;增加沥青层厚度和减小沥青层模量可以起到延缓沥青层温度裂缝扩展的作用。研究结果对刚柔复合式路面的合理设计和推广应用有重大意义。     

C形环小试样疲劳裂纹扩展试验方法与应用

航空航天、核反应堆工程等重要工程结构的安全控制对材料断裂性能有重要需求,小尺寸试样断裂韧性测试需求日益凸显。该文提出了一套用于小尺寸构件疲劳裂纹扩展速率测试的C形环小试样试验方法,设计了试样构形和试验夹具,并基于有限元分析和柔度法理论,建立了C形环小试样裂纹长度预测公式和应力强度因子K的表达式。对于低压转子材料26NiCrMoV11-5钢,采用C形环小试样和标准CT试样完成了疲劳裂纹扩展试验,获得了材料的疲劳裂纹扩展特性,并讨论了新方法的有效性。     

Effect of load ratio on the deformation behaviour of fatigue crack tips

To investigate the change in the manner of plastic deformation at a fatigue crack tip under different loading conditions, the fresh surface area which is produced in one fatigue cycle at the crack tip was measured by an electrochemical method under various load ratios. The fresh surface area produced in each fatigue cycle is in proportion to the crack propagation rate in the Paris regime. This indicates that the crack tip geometry at K_max maintains a similar profile through the Paris regime. Furthermore, the fresh surface area is independent of load ratio at a given da/dN in the Paris regime. This suggests that the shape of the fatigue crack tip at K_max should be independent of load ratio at a given da/dN, and that the shape at K_min is also independent of load ratio, whereas the size of the deformed region at the crack tip is in proportion to da/dN.     

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。     

Static mode fatigue crack propagation and generalized stress intensity correlation for fatigue–brittle polymers

Stable fatigue crack propagation is predominantly described by the Paris power law correlation of the crack growth rate with the amplitude cyclic stress intensity. The Paris relationship works well for most ductile materials but does not capture the response for fatigue–brittle materials lacking a cyclic damage mechanism, including ceramics and many polymers. Instead, crack growth rate of fatigue–brittle materials correlates to the peak cyclic stress intensity factor, \(\hbox {K}_{\mathrm{max}}\). This work shows that \(\hbox {K}_{\mathrm{max}}\) correlation of fatigue crack growth is derived directly from static mode crack tip behavior with constant correlation coefficients, and that \(\Delta \hbox {K}\) correlations are not generally applicable for static mode crack propagation in fatigue–brittle polymers. This derivation predicts load ratio, frequency, and waveform effects, which are included in a general static mode fatigue crack propagation law. Fatigue crack propagation data of a known fatigue–brittle polymer are presented to demonstrate static mode crack propagation behavior correlation with \(\hbox {K}_{\mathrm{max}}\) with constant parameters.     

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.     

Influence of woven ply degradation on fatigue crack growth in thin notched composites under tensile loading

This paper deals with the fatigue of the through-the-thickness crack propagation in thin notched composite laminates made of two glass woven plies. It highlights the different crack growths between warp and weft directions of the woven ply. Experimental results show a decrease of the crack growth rate per cycle with the increase of the crack initiation time. Moreover, it has been shown that it is necessary to take into account the fatigue damage of the woven plies in term of loss of rigidity in the initiation phase. The fatigue crack growth rates are then quantified using Paris law type equations and linear elastic fracture mechanics (LEFM).     

Fatigue crack growth analysis of different adhesive systems: Effects of mode mixity and load level

The aim of this paper is to investigate the fatigue fracture behavior of three different adhesive systems (epoxy‐based, acrylic, and a rubber‐like adhesive). To achieve this, double cantilever beam specimens were manufactured with different adhesives and tested under several mode mixities and different load levels. Fatigue crack growth rate was evaluated through a Paris law equation. For postprocessing, the compliance‐based beam method was used. Results showed that the variation of the threshold energy with load level is more pronounced for the epoxy‐based adhesive. The crack propagation life is higher for the acrylic adhesive. Although, for pure mode I conditions, the normalized threshold of the rubber‐like adhesive is lower, for pure mode II, it was higher than the epoxy‐based adhesive. Due to the normalization by the static fracture energy, the slope of the Paris law was approximately constant for all the adhesive systems.     

An accumulation damage model for fatigue fracture of ferroelectric ceramics

The accumulated plastic displacement criterion for crack propagation in traditional materials is extended to develop equations to predict the fatigue crack growth of ferroelectric ceramics subjected to combined electromechanical loads. The crack-line is perpendicular to the poling direction of the medium. An electric saturation zone and a stress saturation zone are assumed to develop at the crack tips when the medium is subjected to external electromechanical loads. This assumption makes it possible to obtain the accumulated plastic deformation in closed form. A fatigue crack growth law, which is a fourth-power function of the effective stress intensity factor, similar to the well-known Paris law, is derived. Graphical results for the effect of electric load on the effective crack tip stress intensity factor and crack growth rate are provided.     

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。     

Q345R钢多个过载作用下疲劳裂纹扩展行为研究

单一过载使得疲劳裂纹扩展速率减缓,可以提高疲劳寿命,但对于多个过载作用下疲劳裂纹扩展仍未明确,有待于进一步研究。针对Q345R标准紧凑拉伸试样,在常幅循环加载条件下引入多个拉伸过载,研究多个过载作用对疲劳裂纹扩展行为的影响。研究结果表明:在保持应力比、过载位置和过载比不变的情况下,随着过载间距增加,迟滞效应先增加后减小;过载间距循环数是单一过载迟滞循环数的一半时,da/dN_min达到最小,迟滞效应最明显。采用含有过载交互作用参数Ø_I的修正Wheeler模型对不同的过载间距疲劳裂纹扩展行为试验结果进行预测,预测的结果与试验结果能够很好的吻合。