缺口应力集中对40Cr钢高周疲劳性能的影响

用超声疲劳试验方法测定了40Cr钢光滑试样和缺口试样在105-1010周次范围内的疲劳寿命(S-N)曲线,研究了缺口应力集中对疲劳性能的影响。结果表明:两种试样的S-N曲线呈现连续下降特征;缺口应力集中对材料疲劳性能的影响表现出阶段性特征,存在一个临界疲劳断裂循环数Nc,当疲劳断裂周次NfNc,Kf随Nf的增加而减小。在107以上超高周范围内,Kf和疲劳缺口敏感系数g与Nf具有线性关系。     

40车轴钢的缺口疲劳性能

在保证车轴钢冶金质量和力学性能符合标准要求的基础上，对40车轴钢进行光滑试样与缺口试样疲劳性能试验，缺口试样按照EN 13261—2020标准加工，并借助扫描电子显微镜表征了钢的疲劳断裂形貌。结果表明：40车轴钢光滑试样与缺口试样比EN 13261标准中EA1N钢（相当于35钢）疲劳极限标准下限分别提高43%、18%，缺口试样提高的疲劳强度比光滑试样降低32%；光滑试样与缺口试样均为表面裂纹萌生机制，光滑试样为单源开裂，而缺口试样为多源开裂。     

机械零件疲劳强度中间插值二次回归计算法

基于不同理论应力集中系数Ｋ１下材料疲劳极限试验数据的统计分析，建立了机械零件疲劳极限σ－１ｃ的计算模型。当材料光谱试样的疲劳极限σ－１和机械零件的理论应力集中系数Ｋｔ已知时，利用此方法可比较方便、快速地求出机械零件的疲劳极σ１－ｃ的中间插值，且具有满足工程需要的足够精度。     

LZ50钢车轴的旋转弯曲疲劳性能

在LZ50钢车轴上制取了光滑试样和环形缺口试样,按照AAR M-101-2009新标准测试了其旋转弯曲疲劳性能,并观察了断口形貌。结果表明:光滑试样的旋转弯曲疲劳裂纹萌生于试样表面,而环形缺口试样的裂纹萌生于缺口前缘,并呈多源开裂形式,两种试样均形成与轴向垂直的平断口;当车轴的拉伸强度提高5%后,其光滑试样旋转弯曲疲劳性能也得到明显提高,环形缺口试样虽中值疲劳强度比光滑试样的降低18%,但其旋转弯曲疲劳极限缺口敏感系数仍与国外碳素钢车轴保持在同一水平上。     

有限厚度双材料板界面缺口附近三维弹性应力场分析

通过三维有限元计算来研究界面缺口附近的应力场，分析两种材料结合形成的有限厚度板缺口附近界面上下材料中应力集中系数和离面约束系数的影响因素。板的中面缺口根部附近界面上下两种材料中的相对应力集中系数是B／ρ、E2／E1和v2/v1的函数，随着E2／E1值和v2/v1的变化，界面缺口上下材料中的相对应力集中系数（K1)m-p/(Kt)p-б的变化相反。沿厚度方向界面缺口根部附近应力集中系数Kt均小于中面的（Kt)m-p，缺口根部界面上下材料应力集中系数Kt是2z/B、E2／E1和v2/v1的函数。沿厚度方向界面缺口根部附近界面上下材料的离面约束系数Tz也是2z/B、E2／E1和v2/v1的函数。缺口根部附近三维影响区域，当E2/E1＝1、v2/v1＝1时最小，随着E2/E1和v2/v1的增大而增大。     

缺口对ML20MnTiB钢拉伸强度的影响研究

对ML20MnTiB钢光滑试样和不同半径U形缺口试样进行拉伸试验,根据光滑试样所测得的数据,获得光滑试验真应力对数应变曲线。对各缺口试样的拉伸断裂过程进行仿真,获得各缺口试样拉伸过程中的应力、应变场,并对各缺口试样断裂面形态及断裂机理进行扫描电镜分析,由此研究缺口对ML20MnTiB钢拉伸强度的影响。研究结果表明,失效应力随缺口半径减小而增大,失效应变随缺口半径减小而减小,以光滑试样的本构关系及材料自身性能参数可以较为准确地预测缺口试样的拉伸加载过程。当应变增大时,试样产生缺口强化现象。各缺口试样的断裂机理均为韧窝型断裂,在应力三轴度及塑性应变的作用下,随着缺口半径的减小,断裂面的韧窝增大。     

机械加工残余应力对调质40CrNiMo钢疲劳特性的影响

机械加工在试样表面造成的残余压应力σｒ，若分布很浅（０．０１－０．０２ｍｍ），疲劳过程中σｒ极易衰减，从而对疲劳极无明显影响，疲劳仍限取决于裂纹萌生寿命。若σｒ分布较深（０．０５－０．０７ｍｍ），在疲劳过程中最外层应力衰减后，次外层残余应力与缺口应力综合作用，有可能在缺口根部产生非扩展裂纹，从而使疲劳极限取准地裂纹的止裂条件，提高了缺口试样疲劳极限。     

GZL铝合金疲劳特性研究

对一种新型高能耗铝合金ＧＺＬ光滑试样和缺口试样（应力集中系数ασ＝２）的疲劳性能进行了较详细的研究，使用了一种新方法处理成组法试验中不断的试验点，对铸态、热处理态的疲劳性能进行了讨论，给出关于微观组织对疲劳性能的影响机制，给出热处理态的缺口敏性系数。     

全层状TiAl基合金拉伸试验断裂过程及机理

通过对全层组织TiAl基合金不同缺口试样进行原位拉伸试验,结合数据分析以及断口形貌的观察,研究TiAl基合金拉伸的断裂过程和断裂机理.研究发现在原位拉伸试验中,对于直缺口试样,裂纹起裂于缺口根部,其断裂过程主要是主裂纹(沿层裂纹)首先起裂、扩展、连接,然后形成解理断裂起裂源.这一起裂源形成的同时,在刚好能满足Griffiths脆性解理断裂所需要的临界裂纹尺寸时,引起整个试件发生脆性解理断裂.裂纹首先起裂于层间,层间是薄弱环节,断裂方式是穿层断裂和沿层断裂的混合体,以穿层断裂为主.对于大圆弧和平板试样,这种材料的断裂过程是:首先在较高外加载荷下,在标距范围内比较薄弱的区域沿层开裂,随着外加载荷的进一步增加,这些微裂纹扩展,当微裂纹引起的应力集中和外加载荷共同作用足以引起微裂纹进一步贯穿的瞬间,即能量积累到一定程度时,试样整体发生解理断裂.很多试样的断裂是直接起裂的过程,一旦产生裂纹试样立即解理断裂,即这种材料的强度较高,材料很难产生裂纹,也不易损伤.     

一种高锰奥氏体低温钢的力学性能

测定了一种高锰奥氏体低温钢从室温到７７ Ｋ温度的拉伸和冲击性能以及７７Ｋ温度下的断裂韧度,并观察了其断口形貌特征。结果表明试验钢屈服强度与温度的关系为σ０．２＝３００＋１３９２．４ｅｘｐ（－０．０１０６Ｔ）;７７ Ｋ的σ０．２＝８８３Ｍｐａ,ＫＪ０．０５约为２３２ Ｍｐｓ·ｍ～１／２;低温沿晶断裂被完全抑制,７７ Ｋ的断口表现为以韧窝为主并混有少量准解理小刻面的特征。 Ｘ射线相分析表明该钢具有很高的组织稳定性,７７ Ｋ温度变形前后均为单相奥氏体组织。     

2Cr13钢离子渗氮处理的疲劳强度

２Ｃｒ１３钢离子渗氮处理提高光滑和缺口疲劳极限的幅度与疲劳薄弱环节有关。氮化光滑试样的薄弱环节在表下，疲劳强度主要取决于心部性能，较调质态提高仅为２５％，未能发挥氮化层性能潜力；缺口试验的薄弱环节的氮化层，疲劳强度主要取决于氮化性能，较调质态提高达９２％，使氮化层性能得到充分发展。用主要取决于心部性能的光滑疲劳极限与主要取决于氮化层性能的缺口疲劳极限计算的ｑ值具有非真实性。     

A metallurgical evaluation of tool wear and chip formation when machining pearlitic grey cast irons with dissimilar graphite morphologies

The need for superior in-service strength has meant that an increasing number of engineering components are now being made from pearlitic cast irons containing spheroidal graphite, rather than the more traditional cast irons containing flake graphite. Such changes of workpiece material have resulted in a rapid decline in tool life in many machining operations, particularly turning and facing.An investigation into the factors involved during chip formation which result in the observed patterns of tool wear is described in the work presented here. A series of turning tests were made on pearlitic grey cast irons containing flake (GA iron) and spheroidal (SG iron) graphite morphologies with cemented carbide (coated and uncoated) and ceramic tool materials. Built-up edge persisted to higher cutting speeds when cutting SG iron than GA iron, its periodic detachment causing attrition or fracture of the cutting edge. Smooth wear processes, probably caused by dissolution-diffusion and small strain discrete plastic deformation, were predominant on the rake and flank faces of the coated and ceramic tools when cutting both cast irons at high speed. Smooth wear was less rapid when cutting GA iron than SG iron because tool temperatures were reduced and “protective” nonmetallic layers, deposited from the chip-workpiece, interrupted dissolution-diffusion. When cutting SG iron, rapid wear of the uncoated cemented carbides was caused by attrition, while the relatively slower smooth wear, when cutting GA iron, was caused by dissolution-diffusion.     

Experimental verification of RV-MTS model for fracture in soda-lime glass weakened by a V-notch

Fracture phenomenon was experimentally investigated in a soda-lime glass weakened by a V-notch under tensile-shear loading. Fracture tests were conducted using a new test sample called the V-notched Brazilian disc (V-BD) specimen. The fracture resistance and fracture initiation angle were obtained initially from the test results. Afterward, a fracture model was utilized to estimate the experimental results. Very good correlation was found between the experimental and theoretical results both for the fracture resistance and the fracture initiation angle in notches having different notch angles and various notch tip radii. Experimental results revealed that for a constant notch tip radius, the failure load under pure tensile loading conditions decreases as the notch angle increases. For a constant notch angle, as the notch tip radius increases, the fracture load in the soda-lime glass V-BD specimens enhances in the whole domain from pure tensile to pure shear loading. Moreover, for a constant notch tip radius, the notch angle has almost no effect on the fracture initiation angle when the specimen is predominantly under tensile loading conditions.     

Observation of notch effect in Al6061-T6 specimens under tensile loading using digital image correlation and finite element method

The fracture of ductile materials such as Al6061-T6 usually starts from void nucleation. In this study, we investigated strengthening effect and stress triaxiality behavior in notched Al alloy specimens. Various specimens, one un-notched and two with different U-types of notch, one with large diameter (L-notched) and another with small diameter (S-notched), having the same width were selected. The distribution of strain in the full-field was examined using the digital image correlation (DIC) method. Under the tensile test, S-notched specimens showed a more pronounced strengthening effect than the L-notched specimens. The results of DIC confirmed that the notch tip of the S-notched specimen experienced high strain, while the L-notched specimen experienced high strain at the center. The maximum stress triaxiality moved rapidly from the notch tip to the center with the increase in notch radius over 10 mm. The results of the stress triaxiality analysis by finite element method were in good agreement with the above DIC measurement results and predicted the void generation and fracture sites in L-notched and S-notched specimens.     

轴向压缩载荷下陶瓷切口件的断裂准则

根据对切口试件的应力分析和脆性断裂理论，给出了压缩载荷下陶瓷切口件的断裂准则，并进行了验证。在压缩载荷下，陶瓷材料切口件的断裂强度与陶瓷材料的断裂强度之间存在定量关系。可根据陶瓷材料断裂强度的概率分布参数，求得压缩载荷下陶瓷材料切口件的带存活率的断裂准则，并进行了验证。     

Effect of heat treatment on the hydrogen delayed fracture of high strength spring steel

The purpose of this study is to investigate the effect of the changes in the tempering temperature on the hydrogen-delayed fracture resistance of high strength spring steel. The specimens with various microstructures are produced by differentiating the tempering temperature range, their mechanical characteristics are evaluated, and an impact test is carried out. The correlation with the change in mechanical properties is studied through the analysis of the microstructures with a scanning electron microscope. Hydrogen is forcefully injected inside the notched specimen electrochemically, and the hydrogen-delayed fracture resistance is evaluated using a constant loading test. The fractography of the fractured specimen is observed, and through a thermal desorption spectrometer analysis, the main trapping site of the diffusible hydrogen is investigated. The diffusible hydrogen in high strength spring steel is primarily trapped in dislocation, and the hydrogen-delayed fracture resistance increased depending on the increase in the tempering temperature.     

钢的回火与强度塑性的配合

淬火后回火是最常用的热处理工艺之一。然而,钢的强度与塑性常常随回火温度——时间参量的发言改变而向相反的方向变化。必须指出,这些强度与塑性值通常是由光滑试样单向拉力试验得来的,一个机械零件的实际承载能力或寿命往往既不单纯取决于材料的强度,也不单纯取决于塑性,而是根据服役条件决定于两者的合理配合。本文提供了几种结构钢淬火成为金属马氏体后回火对某些机械性能参量影响的研究结果。这些参量更接近于表征一些典型钢种的实际承载能力或服役寿命。它们包括：一次加载时的失效抗力：带有不同应力集中直至一个实际裂纹的极限拉伸强度,缺口敏感度,断裂韧性和冲击韧性。循环加载下的失效抗力：光滑与缺口试样的疲劳极限,过载持久寿命,重复冲击持久寿命,裂纹产生的应力循环次数及裂纹扩展速率。试验结果表明,几乎所有这些参量均在一定的回火温度下出现高峰、低谷或饱和拐点,它们反映了给定材料的强度与塑性之间的一种适当的配合关系,当服役条件改变时这些转折回火温度则向一定的方向转移,要求新的配合。从而为选择回火温度提供一些依据。通过回火转变的高压电镜薄膜透射研究,显示了不同回火温度下所发生的组织变化,为这些表征参量的变化提供了部分内在的联系数。     

Effect of notch geometry on the fatigue strength and critical distance of TC4 titanium alloy

Studying the effect of different geometric features of machined notch on the fatigue strength and critical distance has an important guiding role to understand the critical distance size effect and to predict the HCF strength of turbine engine fan blades after FOD. Systematically experimental investigations of geometrical characteristic effects on the 10⁶ cycle fatigue strength and critical distance for TC4 machined notched plates at stress ratios of R = 0.1 have been conducted. 123 specimens, including unnotched plates and three different types of notched plates (V-notches, U-notches and C-notches) with various notch root radii, depths and angles have been considered. The results indicate that the notch with small radius can significantly lead to high stress concentration and greatly reduce the HCF strength, while the notch angle and notch depth can affect the HCF strength to a certain extent. The K _t related model does not apply to describe the critical distance size effect perfectly. The critical distance has linear relationship with the notch root radius but no significant correction with the notch depth or notch angle. The findings of this study are beneficial for the size effect modeling and later fatigue strength evaluating of TC4 notched components.