SHARP VS BLUNT CRACK HYPOTHESES IN THE STRENGTH OF GLASS - A CRITICAL-STUDY USING INDENTATION FLAWS
SHARP VS BLUNT CRACK HYPOTHESES IN THE STRENGTH OF GLASS - A CRITICAL-STUDY USING INDENTATION FLAWS
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DOI：
10.1111/j.1151-2916.1985.tb15246.x
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发表时间：
1985-01-01
影响因子：
3.9
通讯作者：
GONZALEZ, AC
中科院分区：
文献类型：
--
作者：
LAWN, BR;JAKUS, K;GONZALEZ, AC
脆性裂纹的尖端结构是原子级的尖锐还是具有圆形轮廓的基本问题与当前的强度控制缺陷描述有关。这两种对立观点之间的区别在于强度公式中的控制缺陷尺寸;第一种是裂纹长度，第二种是尖端半径。关于这一问题的有力证据是从钠钙玻璃的老化测试中获得的，使用压痕作为受控缺陷。随着新产生的缺陷在应力失效之前暴露于潮湿环境的增加，观察到惰性强度的增加。这种强度增加在大约1天后饱和，具体取决于环境物种。这一趋势反映了模具在早期磨损缺陷老化研究中的报告。然而，尽管Mould得出结论认为，强化一定是由于尖端变圆，但本试验显示，压痕诱导的裂纹实际上在老化期间延伸。断裂力学分析表明，这种延伸使与中心接触变形区相关的残余裂纹张开应力松弛。因此，它的结论是，在整个压痕后的演变过程中，裂纹保持尖锐的外部条件对强度的影响，表现为只有通过这些裂纹上的驱动力。已退火的缺陷（即，其残余驱动力已被去除）没有显示出这种老化效应。断裂力学分析还表明，在饱和老化区域，压痕裂纹似乎停止增长，断裂驱动力在Michalske宏观裂纹增长研究中描述的零速度阈值区域。与以前的解释相反，这表明裂纹在这个区域不会变钝，否则强化会变陡而不是变平。这一结果的影响有关的基本裂纹扩展规律的不可侵犯的性质进行了讨论。
The fundamental question as to whether the tip structure of brittle cracks is atomically sharp or has a rounded contour is examined in relation to current descriptions of strength‐controlling flaws. The distinction between the two opposing viewpoints lies in the controlling flaw dimensions in the strength formulation; crack length in the first and tip radius in the second. Definitive evidence on the issue is obtained from aging tests on soda‐lime glass, using indentations as controlled flaws. An increase in the inert strength is observed with increased exposure of the newly created flaws to moist environments prior to stressing to failure. This strength increase saturates after approximately 1 day, depending on the environmental species. The trend mirrors that reported by Mould in an earlier aging study on abrasion flaws. However, whereas Mould concluded that the strengthening must be due to tip rounding, the present tests reveal that the indentation‐induced cracks actually extend during the aging period. A fracture mechanics analysis shows that such extension relaxes residual crack‐opening stresses associated with the central contact deformation zone. It is accordingly concluded that the cracks remain sharp throughout their postindentation evolution; the influence of extraneous conditions on the strength is manifested only through the driving forces on these cracks. Flaws which have been annealed (i.e., which have had their residual driving forces removed) show no such aging effects. The fracture mechanics analysis also shows that in the region of saturated aging, where the indentation cracks appear to stop growing, the fracture driving force is in the region of the zero‐velocity threshold described in the macroscopic crack growth studies by Michalske. Contrary to previous interpretation, the indication is that the cracks do not blunt out in this region, for otherwise the strengthening would steepen rather than level out. Implications of this result concerning the inviolate nature of basic crack growth laws are discussed.