2.25Cr1Mo和2.25Cr1MoV钢高温疲劳裂纹扩展规律研究

对2.25C r1M o和2.25C r1M oV钢在常温到500℃的温度区间内的多个温度上进行疲劳裂纹扩展速率测试和分析,对疲劳断口形貌进行观察,得到了两种材料的高温疲劳裂纹扩展规律。结果表明:两种材料高温下疲劳裂纹扩展规律基本一致;裂纹扩展速率随着温度的升高而增大;疲劳裂纹扩展方式由穿晶型为主向沿晶型为主转变。     

电工陶瓷材料裂纹扩展规律的研究

本文应用双扭法研究了四种电瓷材料静态疲劳裂纹扩展规律。试验结果表明:在空气介质中,所有试验电瓷材料均存在明显的裂纹亚临界扩展现象,且随裂纹尖端应力强度因子(K_1)增大呈现三个阶段关系。在不同介质中,裂纹扩展的敏感性按下列顺序变小:水、空气和煤油。试验还表明电瓷材料静态疲劳裂纹扩展规律与材料的配方、环境和受力等因素有关。     

直流电场对LiNbO3晶体亚临界裂纹扩展的影响

本工作采用双扭曲试验法(DT法),着重研究了极化条件、直流电场的强度和取向对晶体学取向不同的LiNbO_3晶体中裂纹亚临界扩展行为的影响,并对晶体由裂纹临界扩展导致断裂的形貌作了分析。实验结果表明:LiNbO_3晶体试样的裂纹亚临界扩展方程指数n随晶体学取向的不同而不同,这与裂纹扩展所沿晶面的断裂能有关。未极化的LiNbO_3晶体受直流电场作用时,其亚临界裂纹扩展的加快,系压电晶体电致伸缩效应所致;极化的LiNbO_3晶体试样在直流电场作用下,其亚临界裂纹扩展主要是受晶体的逆压电效应的影响。     

压力容器不同服役周期下初始裂纹临界尺寸研究

测量了压力容器用钢在3.5%Na Cl溶液中的应力腐蚀门槛值KISCC和腐蚀疲劳裂纹扩展速率da/d N。通过逆推法计算了不同服役周期下初始裂纹或缺陷的临界尺寸,发现在a/c1的情况下裂纹主要沿着深度a方向扩展,而在长度c方向基本没有扩展,裂纹的形状逐渐由椭圆形向圆形转变。     

有机玻璃YB-DM-11的疲劳裂纹扩展特性研究

研究了试样类型、应力比对有机玻璃YB-DM-11疲劳裂纹扩展速率、疲劳裂纹扩展平均寿命的影响。结果表明：试样类型对有机玻璃疲劳裂纹扩展速率影响很小,但紧凑拉伸试样的寿命大于中心裂纹拉伸试样;对于中心裂纹拉伸试样,当应力比为正时,应力比增大,疲劳裂纹扩展速率增加,疲劳裂纹扩展平均寿命减小;应力比为负时,裂纹闭合效应导致裂纹扩展加速;负应力比时的裂纹扩展速率大于正应力比,但疲劳裂纹扩展平均寿命明显小于正应力比。     

16MnR钢高温疲劳裂纹扩展行为试验研究

对16MnR钢在常温、150℃、300℃和425℃下的疲劳裂纹扩展速率进行了测试和分析,得到了四种温度下的疲劳裂纹扩展规律。结果表明：该材料在150℃和300℃的疲劳裂纹扩展速率比常温和425℃时低,425℃时疲劳裂纹扩展速率最高。     

模拟压力容器接管的新型十字试板的角裂纹疲劳扩展性能

用新型十字试板进行了双轴载荷下的角裂纹疲劳扩展性能的研究,以模拟压力容器接管处角裂纹的疲劳扩展行为。试板角裂纹尖端的应力强度因子采用三维边界元法计算,裂纹疲劳扩展的形貌变化采用降载勾线法记录。试验结果表明,裂纹疲劳扩展规律与双向载荷比及裂纹初始形状有关,裂纹沿长度方向的扩展速率可以用 Paris 公式表示,但在深度方向则不然。     

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     

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。     

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。     

Mixed-Mode Fracture in Soda-Lime Glass Using Indentation Flaws

Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.     

2.25Cr-1Mo复杂应力状态下低周疲劳寿命预测

通过对无预裂纹圆柱形缺口试件的常温、高温低周疲劳总寿命试验以及对带有预裂纹圆柱形缺口试件的常温、高温裂纹扩展寿命试验,并利用ＮＨＲＤＳ有限元程序进行了缺口附近轴对称问题的循环应力和应变计算,研究了非均匀分布复杂应力状态下低周疲劳寿命。结果表明,２ ．２５Ｃｒ １Ｍｏ 材料复杂应力状态下低周疲劳总寿命和裂纹扩展寿命可采用当量形式的Ｍａｎｓｏｎ Ｃｏｆｆｉｎ 公式进行表征。     

Crack Initiation by Indentation Fatigue in Lead Alkali and Soda–Lime Glass

An indentation technique using a conventional Vickers microhardness tester was used to evaluate fatigue properties of lead-alkali and soda-lime silica glasses. The specimens were indented repeatedly at the same point with subcritical loads until radial cracks were initiated. The number of cycles to initiate the cracks at different subcritical loads demonstrated typical fatigue curves for both glasses. The uniqueness of the experiment was that the diagonal lengths of the deformed cavity were observed to increase with the number of cycles. This increase of the deformed cavity for a certain number of cycles prior to the visibility of crack initiation was analyzed, correlating the elastic-plastic phenomenon and the accumulation of the residual stress in each cycle.     

Fracture and Subcritical Crack Growth in Soda-Lime Glass under Combined Modes I and II Loading

Fracture and subcritical crack growth characteristics under combined modes I and II loading were studied using the compact tension shear specimens of soda–lime glass. The maximum normal stress criterion gives a good agreement with the experimental mode I–mode II fracture toughness envelope for initially straight cracks and kinked cracks. Subcritical crack growth characteristics were determined under sustained modes I and II loading in water. The values of K _I and K _II were calculated approximately by replacing the subcritical kinked crack with an assumed straight crack ā, and the K _Imax value based on the maximum normal stress criterion was used to describe this subcritical kinked crack growth. The experimental results show that subcritical crack growth under pure mode I, pure mode II, and various combined modes I and II loading can be well described by the K _Imax value based on the approximate maximum normal stress criterion.     

Crack Propagation Behavior of Y-TZP Ceramics

Y-TZP ceramics present the twofold advantage of a very fine grain size and a phase transformation leading to some reinforcement, making them useful in hip joint head prostheses. The subcritical crack growth of such a material that mainly drives the lifetime of the components has been evaluated for wide crack velocity ranges down to 10^-11 m/s by using both double torsion and static fatigue tests. It appeared that subcritical crack growth was activated by H₂O. It is shown that two types of flaws should be considered: surface flaws caused by machining located in the compressive layer present at the surface, and so-called volume flaws. The subcritical crack growth and the toughness of both types of defects are different. The relevance of subcritical crack growth analysis for such surface cracks is discussed. The compressive layer at the surface leads to a threshold value below which no crack growth occurs.     

Influence of Composition on Fatigue Behavior and Threshold Stress Intensity Factor of Borosilicate Glasses

Fatigue behavior of borosilicate glasses was studied using the analysis of subcritical propagation of Vickers indentation cracks. Glasses containing various amounts of glass-network modifiers, mainly soda, were considered. Cone and median/radial crack systems were observed, depending on glass composition, indenter geometry, and test environment. Indentation tests were performed in water, holding the maximum load for durations ranging from 15 s to 6 d. The analysis of the crack length as a function of dwell time allowed evaluation of the subcritical growth parameters and fatigue limit at crack arrest. The influence of composition on fatigue parameters and fatigue limit was discussed in terms of fourfold-coordinated boron atoms fraction compared with the content of glass-network modifier ions.     

Characterisation of subcritical crack growth in ceramics using indentation cracks

Abstract

The introduction of indentation cracks into brittle materials has proved to be a useful tool in characterising subcritical crack growth, notably in efficiently measuring the kinetic growth parameters and in defining whether the material exhibits a fatigue threshold. The accuracy of the subcritical crack growth parameters obtained using indentation mechanics can be excellent, provided the stress intensity factor associated with the indentation cracks is well characterised. Indentation cracks can also be used to measure crack velocity as a function of the applied stress intensity factor by direct observation. In such testing, it is critical that the changes in crack shape as the crack extends are known or accurately predicted. Numerical simulations suggest that the shape changes can be influenced not only by the testing geometry but also the growth kinetics. Finally, it is shown that the fatigue threshold can be determined by allowing median cracks to extend subcritically during indentation.     

Determination of Subcritical Crack Growth Parameters by In Situ Observation of Indentation Cracks

A technique to determine stress intensity factor—crack velocity (K— v ) relationships for subcritical crack growth from in situ observation of indentation cracks is described. To minimize the effect of residual contact stresses and lateral crack interaction, measurements were made only on cracks that had undergone significant subcritical crack growth. Crack shapes were determined fractographically from crack-arrest markings, produced by temporary unloading during the crack extension process. The subcritical crack growth parameters obtained by this technique were in excellent agreement with those determined from dynamic fatigue and previous studies.     

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。     

Fracture Behavior of the Space Shuttle Thermal Protection System

Stable crack-growth and fracture-toughness experiments were conducted using precracked specimens machined from LI-900 reusable surface insulation (RSI) tiles of the space shuttle thermal protection system (TPS) at room temperature. Similar fracture experiments were conducted on fracture specimens with preexisting cracks at the interface of the tile and the strain isolation pad (SIP). Stable crack growth was not observed in the LI-900 tile fracture specimens which had a fracture toughness of 12.0 kPaμm. The intermittent subcritical crack growth at the tile-pad interface of the fracture specimens was attributed to successive local pull-outs due to tensile overload in the LI-900 tile and cannot be characterized by linear elastic fracture mechanics. No subcritical interfacial crack growth was observed in the fracture specimens with densified LI-900 tiles where brittle fracture initiated at an interior point away from the densification.