MICROSCOPIC MECHANISMS AND MECHANICS OF CRAZE GROWTH AND FRACTURE
MICROSCOPIC MECHANISMS AND MECHANICS OF CRAZE GROWTH AND FRACTURE
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DOI：
10.1080/01418617908239285
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发表时间：
1979-01-01
期刊：
影响因子：
--
通讯作者：
KRAMER, EJ
中科院分区：
文献类型：
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作者：
LAUTERWASSER, BD;KRAMER, EJ
孤立的空气银纹是在聚苯乙烯(PS)薄膜中产生的，这些薄膜粘合在铜栅上，通过拉伸这些薄膜来产生。银纹厚度分布r(X)是由沿着银纹拍摄的一系列透射电子显微照片直接确定的。用光学密度法测定了银纹沿线频繁间隔的纤丝体积分数v1和纤丝延伸率λ的局部数值。由这些参数计算出银纹表面的位移分布(X)、银纹表面的应力分布(X)和银纹纤维上的真实应力。λ(X)分布提供了确凿的证据，表明银纹是通过将新的聚合物从银纹表面吸引到纤维中而不是通过现有纤维的蠕变而随着长度的增长而增加的。轮廓在银纹尖端有一个适度的最大值，但在银纹尖端后面约15μm的距离上缓慢下降到比所施加的拉应力低约10%的值。拉伸出的银纹细丝的λ是拉伸时表面应力S的函数。在不断增长的银纹尖端后面拉伸的纤维形成了中脉，在银纹面中间的银纹层中形成了一层高λ。在中脉和银纹尖端观察到的最高λ值几乎是在玻璃中的分子纠缠之间完全伸展PS分子所需的值；进一步伸展必然导致链解缠和原纤维断裂。虽然中脉纤维中的链解缠可能是断裂的第一步，但现有银纹中裂纹的扩展导致裂纹前的S大幅增加，作为响应，两层极高λ的银纹纤维从银纹表面拉出。裂纹现在更倾向于在这些层中扩展，而不是在中脉，并从一层跳到另一层，在PS断口表面造成熟悉的鳕鱼或补丁图案。
Isolated air crazes have been produced in thin films of polystyrene (PS) bonded to copper grids by straining these in tension. The craze thickness profile,r(x), was directly determined from a series of transmission electron micrographs taken along the craze. Local values of the craze fibril volume fractionv1and fibril extension ratio λ were established at frequent intervals along the craze by optical densitometry of the micrographs. The craze surface displacement profilew(x), craze surface stress profileS(x) and the true stressatin the craze fibrils are computed from these parameters. The λ(x) profile provides conclusive evidence that the craze increases in thickness as it grows in length by drawing new polymer from the craze surface into the fibrils rather than by creep of the existing fibrils. TheSprofile exhibits a modest maximum at the craze tip but falls slowly over a distance of about 15 μmbehind the craze tip to a value about 10% below the applied tensile stress. The λ of the drawn craze fibrils is a function of the surface stressSat the time they are drawn. The fibrils drawing at highSjust behind the growing craze tip result in formation of the midrib, a layer of high λ in the craze midway between the craze surfaces. The highest values of λ observed, in the midrib and the craze tip, are nearly those required to completely extend PS molecules between molecular entanglements in the glass; further extension must result in chain disentanglement and fibril rupture. While chain disentanglement in the fibrils of the midrib is the probable first step in fracture, propagation of a crack in an existing craze results in a large increase in S ahead of the crack, and two layers of very-high-λ craze fibrils draw from the craze surfaces in response. The crack now prefers to propagate in these layers, rather than in the midrib, and jumps from one to the other causing the familiar mackerel or patch pattern on PS fracture surfaces.