预制裂纹304SS试样慢拉伸过程EN和SKP表征

应用电化学噪声技术(EN)研究了慢应变速率拉伸(SSRT)条件下预制疲劳裂纹的304SS试样在4mol/LNaCl+0.01 mol/LNa2S2O3溶液中的腐蚀行为,EN的时域与频域谱图显示实验后裂纹发生了扩展.扫描Kelvin探针(SKP)分析也证实了裂纹试样经慢拉伸实验后裂纹出现扩展.     

应变速率和电位对应力腐蚀裂纹扩展的影响

以滑移-溶解-再钝化模型为基础,推导出应力腐蚀裂纹扩展速率与裂尖应变速率和电位之间的理论公式.计算表明,在裂纹扩展速率与裂尖应变速率的关系曲线中有两个特征区域.裂纹扩展速率在区域I随裂尖应变速率增加而增大,而在区域II不随裂尖应变速率的改变而变化.用慢应变速率拉伸技术（SSRT）测量了304L不锈钢的裂纹增长速率.当电位控制在区域II的阳极区时,理论计算的裂纹扩展速率与实验得到的结果比较吻合.     

稀土元素对2090铝锂合金疲劳短裂纹门槛值的影响

测出２０９０铝锂合金的ΔＫｔｈ～Δａｔｈ，ΔＫｃｌ．ｔｈ～Δａｔｈ等曲线，并选用７０７５铝合金作对比试验，定量地评定了该合金疲劳短裂纹扩展抗力，探明了基短裂纹效应强的本质原因。同时，研究了稀土元素铈的影响。结果表明，在２０９０铝锂合金中添加微量铈，可较大提出其Δｋｉ，也有益于其ΔＫＣｌ．ｔｈ，因此，不仅消除了该合金疲劳短裂纹扩展抗力低的本质因素，还使其疲劳长裂纹扩展抗力进一步提高。     

PC及PC/ABS共混物的疲劳裂纹扩展研究

研究了聚碳酸酯(PC)和PC/ABS高分子材料的疲劳裂纹扩展规律,利用改进柔度法测量其裂纹扩展速率,采用扫描电子显微镜(SEM)观察其断口形貌,分析疲劳裂纹扩展机理.在较大裂纹扩展速率(10-6～10-3mm/cycle)范围内,PC/ABS的疲劳裂纹扩展速率可以用Paris公式da/dN=9·5587×10-5(ΔK)2·88381来描述.高分子材料PC的疲劳裂纹扩展速率约为高分子材料PC/ABS的3倍.高分子材料PC/ABS疲劳裂纹面上的特征以韧窝为主,较低裂纹扩展速率对应较小的韧窝,较高裂纹扩展速率对应较大的韧窝.高分子材料PC疲劳裂纹面有明显的不连续裂纹扩展带,其裂纹面相对较平.     

聚芳醚酮高温长时间下的断裂韧性

本文采用三点弯曲测平面应变断裂韧性ＪＩＣ的实验方法，研究了高温（２００℃）长时间放置过程中，聚芳醚酮的高温断裂韧性随时间的变化规律．结果表明，随放置时间的增长，断裂韧性ＪＩＣ和临界塑性屈服区尺寸△ＬＣ都逐渐减小；而屈服强度和抵抗裂纹继续扩展的阻力ｄＪ／ｄ（△ａ）皆增大，且在短时间内有较大增值．从分子运动的角度对上述各物理量的时间依赖性进行了讨论．     

电化学方法研究X120管线钢氢致裂纹行为

采用Davanathan-Stachursky双电池渗氢装置研究了X120试验管线钢中电化学氢渗透动力学行为,运用电化学充氢方法对X120管线钢进行预制微裂纹,采用SEM观察钢中夹杂物及其与氢致裂纹（HIC）形成的关系。 实验结果表明,随着充氢电流密度的增大,饱和阳极电流I∞、氢扩散通量J∞、氢有效扩散系数Deff均逐渐增大;HIC萌生与钢中夹杂物化学成分有关,X120管线钢中主要含有Al、Mn的硫化物、氧化物夹杂,氮化钛、铌的复合夹杂以及Ca-Al-O-S的混合物夹杂,HIC更易于在含Al、Mn的夹杂物处萌生及形成,且裂纹形核后一般沿轧制方向生长。     

H2S环境中UNS J91540不锈钢的电化学行为对SCC的影响

应用慢应变速率拉伸试验和电化学方法研究了湿硫化氢溶液中UNS J91450不锈钢的电化学行为对应力腐蚀开裂的影响.结果表明,本实验条件下UNS J91540不锈钢具有较高的SCC敏感性,并随着溶液pH的降低而明显增大.该不锈钢的电化学行为对它的腐蚀开裂(SCC)敏感性具有重要影响.开路电位下该不锈钢的SCC行为同时受氢脆作用和裂纹尖端阳极溶解作用的影响.在pH较低的介质中析氢电流较高而裂纹尖端阳极溶解作用略低,而在pH较高的介质中析氢电流较低而裂纹尖端的阳极溶解作用相对增强.氢脆作用对UNS J91540不锈钢SCC敏感性影响更显著.     

连续碳纤维增强的聚芳醚酮在Ⅰ型循环载荷下的层间破坏

用双悬臂梁(DCB)试件研究了连续碳纤维增强的聚芳醚酮复合材料(CF/PEK-C),在Ⅰ型循环载荷作用下的层间裂纹扩展行为.循环载荷采用载荷控制模式,最小载荷与最大载荷之比为0.5.在疲劳试验中,仍然发现有“阻力曲线”现象存在.层间裂纹扩展速率用指数定律与相应的应变能释放速率联系起来,并对结果进行了讨论.     

连续碳纤维增强的聚芳醚酮在Ⅰ型循环载荷下的层间破坏

 用双悬臂梁(DCB)试件研究了连续碳纤维增强的聚芳醚酮复合材料(CF/PEK-C),在Ⅰ型循环载荷作用下的层间裂纹扩展行为.循环载荷采用载荷控制模式,最小载荷与最大载荷之比为0.5.在疲劳试验中,仍然发现有“阻力曲线”现象存在.层间裂纹扩展速率用指数定律与相应的应变能释放速率联系起来,并对结果进行了讨论.     

60CrMnMo钢热疲劳裂纹生成与长大及稀土的作用

采用金相和扫描电镜等手段对６０ＣｒＭｎＭｏ钢热疲劳裂纹的萌生和长大规律进行了分析，并考查了稀土在钢中的作用。结果表明：热疲劳裂纹萌生于钢中夹杂物处，热疲劳裂纹的生长，不仅是裂纹自身发展的结果，更主要的是裂纹的相互联接。钢中加入稀土后，可以变质钢中夹杂物，从而抑制了热疲劳裂纹的生成与长大。     

稀土对9Cr2Mo钢抗淬裂性能的影响

利用电镜和金相等手段研究了稀土元素对 9Cr2Mo钢抗淬裂性能的影响。9Cr2Mo钢中加入稀土元素可以提高出现裂纹的淬火次数 ,延缓淬火裂纹的扩展速度 ,改变淬火裂纹的扩展途径 ,从而提高 9Cr2Mo钢的抗淬裂能力。     

The effects of annealing on fatigue crack propagation in polyethylene

Fatigue crack propagation tests on annealed and quenched medium-density polyethylene showed the annealed specimens to have much lower resistance to crack initiation and subsequent propagation. Although the same fracture mechanism, in which the brittle crack gradually becomes more ductile, prevailed in both cases, the voided and fibrillated crack tip root craze in the annealed material was much weaker that the nonfibrillated quenched root craze. Microstructural analyses indicate that the annealed material had separate crystallite populations, whereas the quenched material had a more homogeneous morphology. The highest melting fraction of the annealed material was composed of lamellae that were about 270 Å thick, and the quenched lamellae were estimated to be 160 Å thick. The reduced fatigue crack propagation resistance of the annealed material was suggested to be a result of a lower concentration of tie molecules and its reduced damping capability, compared to the quenched material. © 1995 John Wiley & Sons, Inc.     

Crack propagation in both porous compacts and pigmented films prepared from cellulose derivatives — a comparison of computer simulation with experimental observation

A model originally developed to simulate crack propagation in viscoplastic materials, where the micromechanism consists of void growth, has been evaluated for both pigmented films and porous compacts of cellulose derivatives. The program allows both visualization of crack growth and the calculation of crack velocity. The program is easy to use, enabling many simulations to be performed with minimum effort. The agreement with experimental observation both qualitatively and quantitatively is very good.     

Influence of Phase Miscibility on the Crack Propagation Kinetics of Nanostructured Binary S‐(S/B)‐S Triblock Copolymer Blends

Summary: The crack propagation kinetics of binary styrene‐(styrene/butadiene)‐styrene triblock copolymer blends based on one with symmetrical (LN4) and another with asymmetrical (LN3) molecular architecture is discussed with respect to post‐yield crack‐tip blunting and stable crack propagation behavior while highlighting the dynamic mechanical properties of the blends. The crack‐tip opening displacement (CTOD) rate is revealed to be sensitive to phase behavior, which is in agreement with a transition in phase miscibility in a critical composition range of 40–60 wt.‐% of LN3. Analyses of R‐curves from CTOD‐values reveal that kinetics of crack propagation is controlled by phase behavior, whereas the resistance to stable crack initiation is largely dependent on the composition. Our investigation offers new possibilities to tailor and optimize the crack resistance (crack propagation stability) of block copolymer blends through the control of phase miscibility and hence, fundamentally, adds a new dimension to the development of novel materials based on toughened nanostructured polymers.

Crack resistance curves for LN3 blends having different compositions.     

Fatigue crack propagation mechanisms in poly(methyl methacrylate) by in situ observation with a scanning laser microscope

Crack propagation tests were performed on an amorphous polymer, poly(methyl methacrylate), to investigate fatigue crack propagation mechanisms. A scanning laser microscope with a newly developed tensile testing machine was used to observe in situ crack propagation in compact‐type specimens. A crack usually propagated within the craze located at the crack tip under both static and cyclic loading conditions. When a crack stably propagated into the craze under static loading conditions, bright bands composed of the broken craze were observed at the edges along the crack wakes. However, there were successive ridges and valleys in place of bright bands along the crack wakes under cyclic loading conditions. When stable fatigue cracks were propagated at the loading half‐cycle in each cycle, new craze fragments appeared that were similar to the bright bands under static loading. However, the thickness of these fragments decreased in the following loading cycle, and a new valley was formed. This suggested that the valleys were formed by the contact between the fracture surfaces near the crack tip during unloading. Fatigue crack propagation is thought to be due to fibrils weakened by crack closure between fracture surfaces. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3103–3113, 2001     

Experimental and theoretical investigation of stress corrosion crack (SCC) growth of polyethylene pipes

Degradation of polymers is usually manifested in a reduction of molecular weight, increase of crystallinity in semicrystalline polymers, increase of material density, a subtle increase in yield strength, and a dramatic reduction in toughness. Stress corrosion cracking (SCC) results from strongly coupled thermo-mechano-chemical processes, and is sensitive to material composition and morphology. The individual crack propagation stage is critical in determining the lifetime of pipe. Based on author's previous works, crack layer (CL) theory model is adopted in this study to describe the individual stress corrosion (SC) crack propagation kinetics and the time interval from crack initiation to instability and break through. The effect of localized chemical degradation at the crack tip on SC crack growth kinetics is addressed. Typical SC crack growth is presented and discussed as a step-wise manner based on the proposed model. In addition, scanning electron microscopy (SEM) observation and Fourier transform Infrared spectroscopy (FTIR) analysis of failed samples obtained by accelerated SCC tests are applied to validate the proposed model. SEM is useful to identify the change of fracture mechanisms from chemically driven crack to mechanically driven crack by the formation of visible striations. FTIR analysis enables tracking of the accumulation of chemical degradation by detecting the amount of carbonyls on the crack surface. Carbonyl index is defined to compare the amount of chemical degradation quantitatively. The purpose of this paper is to continue to develop the technical theory and understanding behind SCC phenomena to facilitate all polymer pipe industries and in particular the polyethylene pipe industry to design better resins and piping systems.     

Influence of the β‐crystalline phase on the mechanical properties of unfilled and calcium carbonate‐filled polypropylene: Ductile cracking and impact behavior

The β‐crystalline form of isotactic poly(propylene) (PP) has been long recognized to have a greater mechanical absorption capacity than the α‐crystalline form. This is of major importance for improving impact properties and crack resistance of injection‐molding parts. Unfilled PP samples together with calcium carbonate‐filled PP samples having various β/α‐phase ratios, with nearly constant morphological parameters, have been investigated from the standpoint of ductile crack propagation and impact behavior. The presence of the β‐crystalline phase turned out to improve both properties. The β spherulites are notably more prone to craze initiation than α spherulites that display a propensity for cracking. Subsequent crack propagation appears to be faster in the latter ones. The plastic zone ahead from the crack tip broadens, and the specific plastic energy increases with increasing β‐phase content. The lower elastic limit of the β phase is likely to promote the early crazing. However, the suspected higher density of tie molecules in β spherulites provides more numerous and stiffer microfibrils. The impact strength of PP is also improved by the presence of β crystals as a result of greater energy‐absorption capabilities. However, filled samples turned out insensitive to the β phase. A discussion is made about the origins of the β‐phase‐induced improvement of the mechanical properties. The possible role of the β → α transition is also explained. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 31–42, 2002     

Decompression wave speed and crack velocity measurements during S4 test in water pressurized plastic pipes: Part 1. Experimental methods

In a Small Scale Steady State (S4) test apparatus of ISO 13477, instrumentations were designed and successfully adapted to determine decompression wave speed and crack velocity during rapid crack propagation event in water-filled plastic pipes. The basic design for decompression wave speed measurement involved the use of high-frequency dynamic pressure transmitters, located external to the water-filled pipe and connected to pressure measurement positions inside the pipe, by means of stainless steel tubes. For the crack velocity measurements, timing wire system with the required circuitry capable of giving the precise temporal indication of the propagating crack was designed and employed. In this paper, detailed design of instrumentations adapted to the S4 test apparatus and the assessment techniques used to obtain decompression wave speed and crack velocity are described. It was also demonstrated that the methods developed were viable for these measurements which are known to affect the rapid crack propagation behavior in water-filled plastic pipes.