悬臂弯曲加载10CrNiMo钢的S-N曲线试验研究

采用有限元方法,计算了在悬臂弯曲加载方式下,不同载荷作用时板状光滑试样施力点处的挠度;采用成组试验法,通过载荷控制模式,进行了应力比为-1时10CrNiMo钢的S-N曲线试验,并对试样断口进行了观察和分析。结果表明:在试验载荷范围内,所设计试样施力点处的挠度变化,在试验机作动器的有效行程内,可满足S-N曲线试验,通过试验获得了两种常用置信度下10CrNiMo钢的S-N曲线;在大载荷往复作用时,表面裂纹从试样上、下两个表面萌生并扩展所形成的面积基本相当,但在小载荷循环作用下,表面裂纹通常先从试样的一个表面萌生并扩展,当裂纹扩展至一定程度时,试样另一个表面才开始萌生疲劳裂纹,并协同前一表面的裂纹共同扩展至试样断裂。产生这种现象的原因,和不同大小的载荷开动试样上下两个表面材料内部滑移系的数量、材料内部组织的不均匀性,以及裂纹在扩展过程中前缘应力状态的变化有关。     

表面裂纹在不同控制模式下的扩展特性

在应变、载荷和位移三种控制模式下,采用Gross板状试样,研究了10CrNi5Mo高强钢在悬臂弯曲加载方式下低周疲劳表面裂纹扩展特性,研究结果表明:表面裂纹扩展速率d(2c)/dN在位移和载荷控制模式下差别不大,且都高于应变控制模式;三种控制模式都可以用裂纹前缘名义总应变范围ΔεT作为统一参量来描述表面裂纹疲劳扩展速率,且表达式与Paris公式类似,均满足幂函数规律;试验中通过不断调整载荷或位移的大小来保证应变恒定,会增加试验的操作难度和工作强度。     

疲劳载荷下工业纯铁表面与内部裂纹长度的研究初探

疲劳裂纹扩展试验中,采用表面裂纹长度作为疲劳裂纹长度进行疲劳裂纹扩展计算和疲劳裂纹扩展模型建立的过程中,会导致计算的最终结果和模型产生一定的误差。对工业纯铁板材试样进行疲劳裂纹扩展试验,分析疲劳裂纹扩展过程中裂纹长度与裂纹扩展速率的关系,以及对疲劳裂纹扩展断口形貌特征进行观察。结果表明,上述2种方法均可来确定表面裂纹长度和内部裂纹长度之间的关系。     

裂纹扩展片技术在弧形面上半椭圆表面裂纹长度测试中的应用

简述了裂纹扩展片技术的研究现状和测试原理,标定出应用在单臂悬梁试样弧形面上的单片和串联双片10mm裂纹扩展片的裂纹长度增量与电压增量关系方程。通过扩展片与试样裂纹扩展同步性研究,证明了裂纹扩展片技术应用于弧形面上半椭圆表面裂纹长度测试的可行性。     

一种悬臂弯曲加载表面裂纹扩展试样及其应用

介绍了一种带有上下对称圆弧的板状试样,试样上圆弧几何中心预制有半椭圆表面裂纹,这种设计能够较好地模拟压力容器具有表面裂纹时的特点.利用这种试样,可以完成悬臂弯曲加载恒总应变控制下的低周疲劳表面裂纹扩展速率试验,获得表面裂纹扩展长度和深度的关系,最终建立表面和深度裂纹扩展速率与总应变的函数关系.据此,可为含有表面裂纹体的压力容器低周疲劳剩余使用寿命评估提供依据.     

几种潜艇耐压壳体用钢的低周疲劳性能研究

本文叙述了作者及其合作者在潜艇耐压壳体钢低周疲劳性能研究工作中取得的进展,重点介绍了光滑板状弯曲试样的低周疲劳性能和板状弯曲加载条件下的表面裂纹扩展行为,讨论了表面裂纹扩展速率与总应变范围的关系和与塑性应变范围的关系。通过对弯曲低周疲劳表面裂纹扩展特征的研究,确立了低周疲劳表面裂纹扩展试验程序。并且通过对三种潜艇耐压壳体钢进行的表面裂纹扩展速率测定,获得了d(2c)/dN=A(ΔεT)~n的定量关系(ΔεT为总应变范围)。文中示出的低周疲劳曲线和d(2c)/dN-ΔεT曲线,可供材料研究、潜艇结构设计工作者使用。     

悬臂弯曲加载金属材料疲劳损伤的测量方法研究

基于损伤力学理论,提出了一种金属材料疲劳损伤测量的新方法。即采用光滑板状试样,通过悬臂弯曲加载方式,以10CrNiMo钢为对象,研究金属材料疲劳损伤与其宏观参量——施力点位移特征量的相关性。研究结果表明,金属材料的疲劳损伤与其施力点位移特征量峰值满足特定的函数关系。据此,可以通过监测施力点位移特征量峰值的变化,来间接测量金属材料的疲劳损伤,并和相同加载条件下该材料的疲劳损伤临界值进行比较,从而为金属材料的安全服役和剩余寿命评估提供依据。     

全片层与双态组织高铌TiAl合金高温原位拉伸研究EI北大核心CSCD

利用原位观察的方法研究了全片层(Fully Lamellar,FL)组织与双态(Duplex,DP)组织高铌TiAl合金在750℃下的拉伸力学行为。结果表明:DP组织具有较好的抗拉强度和塑性,其抗拉强度为958MPa,断面收缩率为45.1%,FL组织抗拉强度较低且几乎没有塑性;随着拉伸应力的增加,DP组织合金试样表面存在一个大量裂纹萌生和扩展的过程,这些裂纹均匀分布在试样表面,主要以萌生为主,几乎难以扩展,而在FL组织中只能观察到少量的裂纹萌生现象;拉伸断裂后的DP组织合金的裂纹是由多个孔洞合并而成,呈蠕虫状且尺寸很小,FL组织中与拉伸方向垂直或呈一较大夹角的片层团界及片层界较易萌生裂纹,在临近断口区域,裂纹主要以穿层扩展为主。     

小试样高通量蠕变及蠕变裂纹扩展试验装置设计

对于某些取样受限的结构,如在役构件、薄壁件、焊接接头、功能性梯度结构,无法采用传统试样测试获得高温蠕变及裂纹扩展性能,小试样测试方法使得此类构件的高温力学性能的获取成为可能。但现有小试样蠕变试验装置用途单一且存在试样氧化的问题,无法满足试验要求。本文设计一种基于小试样的材料蠕变及蠕变裂纹扩展性能测试装置,装置配备专用夹具和真空系统,可满足不同种类小试样真空环境下的高温试验,避免试样氧化,并可同时完成6个同类或不同类型小试样的蠕变和蠕变裂纹扩展试验。装置采用马弗炉对试样加热,最高试验温度可达1 200℃。采用光栅位移传感器测量小试样变形量,直流电位法测量裂纹长度,提高了变形测量精度。试验结果表明,该装置可以精确测量小试样位移和裂纹长度,用以研究材料蠕变及裂纹扩展性能。     

10Ni5CrMoV钢双轴弯曲应力下表面裂纹疲劳扩展特性研究

以10Ni5CrMoV钢为研究对象,在加载条件与构件使用环境相近条件下,在原板厚大尺寸试样表面引入人工缺口模拟实际构件中存在的类表面裂纹缺陷,采用双轴弯曲疲劳试验方法,开展原板厚大尺寸试样双轴弯曲应力下的表面裂纹疲劳扩展特性试验研究。结果表明,裂纹半长c和裂纹深度a都随循环次数N单调递增,且曲线a-N满足一定的线性关系。建立表面裂纹半长c与深度a之间的定量关系式预测模型,从而定量解决裂纹深度尺寸不好直接测量的难题。     

考虑应变梯度效应的三点弯梁模型解析研究--第二部分:局部化带的位移及转角

基于梯度塑性理论,分析了应变软化及真实裂纹扩展阶段的局部化带的张拉位移和转角。在弹性阶段,可以由弹性理论来确定二者的关系。真实裂纹出现后,利用平衡条件、几何条件及梯度以来的应变软化本构关系,得到了真实裂纹长度与局部化带长度的关系。当真实裂纹刚出现时,局部化带长度达到最大值。在任何阶段,局部化带到中性轴的距离单调降低,局部化带的张拉位移和转角受梁深、带宽、弹模及下降模量等的影响。弹模及下降模量越大,带宽越小,则局部化带的张拉位移和转角都增加。而且,在前两个阶段,张拉位移都线性增加,但在后两个阶段,转角都非线性增加。     

光面爆破的岩石裂纹扩展规律研究

光面爆破是一种控制爆破技术,为了实现更为精确的控制爆破效果,需要对光面爆破的岩石裂纹扩展规律进行研究。在光面爆破的初始裂纹产生阶段,利用数值模拟的方法,对光面爆破的弹性力学简化模型进行分析,得到了初始裂纹的起裂点位置以及初始裂纹长度。在爆轰气体驱动下,初始裂纹二次扩展。从裂纹扩展的能量角度,分析了裂纹扩展速度与裂纹长度的定量关系。基于断裂力学对裂纹二次扩展条件和二次扩展范围进行理论分析,定量描述了裂纹二次扩展长度与孔壁压力的关系。最后,在对光面爆破致裂机理认知的基础上,提出光面爆破中装药不耦合系数、炮孔间距、光面层厚度等参数的计算方法。     

光面爆破的岩石裂纹扩展规律研究

光面爆破是一种控制爆破技术,为了实现更为精确的控制爆破效果,需要对光面爆破的岩石裂纹扩展规律进行研究。在光面爆破的初始裂纹产生阶段,利用数值模拟的方法,对光面爆破的弹性力学简化模型进行分析,得到了初始裂纹的起裂点位置以及初始裂纹长度。在爆轰气体驱动下,初始裂纹二次扩展。从裂纹扩展的能量角度,分析了裂纹扩展速度与裂纹长度的定量关系。基于断裂力学对裂纹二次扩展条件和二次扩展范围进行理论分析,定量描述了裂纹二次扩展长度与孔壁压力的关系。最后,在对光面爆破致裂机理认知的基础上,提出光面爆破中装药不耦合系数、炮孔间距、光面层厚度等参数的计算方法。     

On the stress wave induced curving of fast running cracks — a numerical study by a time-domain boundary element method

Summary Fast crack propagation in dynamically loaded plane structures is investigated. The major point of interest is the evolution of the crack trajectory under the influence of stress waves which are generated and repeatedly reflected at the specimen boundaries. Since these waves may lead to arbitrary mixed-mode and time-dependent loading of the crack tip, both the direction and speed of crack advance are determined from a fracture criterion.Starting point is a system of time-domain boundary integral equations which describes the initial boundary value problem of a linear elastic body containing an arbitrarily growing crack. The unknown displacements and/or tractions on the exterior boundary and the displacement jumps across the crack are computed numerically by a collocation method in conjunction with a time-stepping scheme. Crack growth is modelled by adding new boundary elements of constant length at the running crack tip.The method proves to be of sufficient accuracy when applied to problems treated with other numerical techniques. Moreover, the simulation of dynamic crack propagation under various geometry and loading conditions enables the reproduction and analysis of complex phenomena observed experimentally.     

A New Procedure for Mode I Fracture Characterization of Cement‐Based Materials

Fracture characterization under mode I loading of a cement‐based material using the single‐edge‐notched beam loaded in tree‐point‐bending was performed. A new method based on beam theory and crack equivalent concept is proposed to evaluate the Resistance‐curve, which is essential to determine fracture toughness with accuracy. The method considers the existence of a stress relief region in the vicinity of the crack, dispensing crack length monitoring during experiments. A numerical validation was performed by finite element analysis considering a bilinear cohesive damage model. Experimental tests were performed in order to validate the numerical procedure. Digital image correlation technique was used to measure the specimen displacement with accuracy and without interference. Excellent agreement between numerical and experimental load–displacement curves was obtained, which validates the procedure.     

Simulation of stage I-crack growth using a hybrid boundary element technique

Short fatigue crack propagation often determines the service life of cyclically loaded components and is highly influenced by microstructural features such as grain boundaries. A two-dimensional model to simulate the growth of these stage I-cracks is presented. Cracks are discretised by displacement discontinuity boundary elements and the direct boundary element method is used to mesh the grain boundaries. A superposition procedure couples these different boundary element methods to employ them in one model. Varying elastic properties of the grains are considered and their influence on short crack propagation is studied. A change in crack tip slide displacement determining short crack propagation is observed as well as an influence on the crack path.     

A predictive fatigue life model for anodized 7050 aluminium alloy

The objective of this study is to predict fatigue life of anodized 7050 aluminum alloy specimens. In the case of anodized 7050-T7451 alloy, fractographic observations of fatigue tested specimens showed that pickling pits were the predominant sites for crack nucleation and subsequent failure. It has been shown that fatigue failure was favored by the presence of multiple cracks. From these experimental results, a fatigue life predictive model has been developed including multi-site crack consideration, coalescence between neighboring cracks, a short crack growth stage and a long crack propagation stage. In this model, all pickling pits are considered as potential initial flaws from which short cracks could nucleate if stress conditions allow. This model is built from experimental topography measurements of pickled surfaces which allowed to detect the pits and to characterize their sizes (depth, length, width). From depth crack propagation point of view, the pickling pits are considered as stress concentrator during the only short crack growth stage. From surface crack propagation point of view, machining roughness is equally considered as stress concentrator and its influence is taken into account during the all propagation stage. The predictive model results have been compared to experimental fatigue data obtained for anodized 7050-T7451 specimens. Predictions and experimental results are in good agreement.     

The effect of applied displacement rate on the observation of crack instability

This paper presents an analysis of the effect of displacement rate on the load-displacement behavior of strain-rate insensitive materials upon crack propagation. It is shown that if a crack is being loaded continuously during its propagation the point of crack instability does not correspond to a sudden load-drop and indeed the load may continue to increase. This phenomenon results in a higher absorbed energy to fracture during rapid loading.     

J-INTEGRAL APPROACH TO MODE III FATIGUE CRACK PROPAGATION IN STEEL UNDER TORSIONAL LOADING

Mode III fatigue crack propagation tests were conducted on circumferentially cracked bars of a medium carbon steel under a constant value of the J-integral range. The ΔJ value was evaluated from the loading part of the hysteresis loop of the applied torque and the angle of twist. The fracture surface was macroscopically flat for all cases examined in the present study. The crack propagation rate decreased with crack extension, because of the shear contact of the crack faces. The crack propagation rate, without contact shielding, obtained by extrapolating the relation between the crack propagation rate and the crack length to the pre-crack length, was a power function of ΔJ irrespective of the initial notch depth.     

Application of incubation time approach to simulate dynamic crack propagation

The incubation time criterion for dynamic fracture is applied to simulate dynamic crack propagation. Being incorporated into ANSYS finite element package, this criterion is used to simulate the classical dynamic fracture experiments of Ravi-Chandar and Knauss on dynamic crack propagation in Homalite-100. In these experiments a plate with a cut simulating the crack was loaded by an intense pressure pulse applied on the faces of the cut. The load consisted of two consequent trapezoidal pulses. This, in the experimental conditions used by Ravi-Chandar and Knauss, resulted in a crack initiation, propagation, arrest and reinitiation. Dependence of the crack length on time was measured in those experiments. The results for crack propagation obtained by FEM modelling are in agreement with experimental measurements of crack length histories. This result shows the applicability of the incubation time approach to describe the initiation, propagation and arrest of dynamically loaded cracks.