Thixotropic yield stress materials show a shear-induced solid-liquid transition at the yielding point, characterized by yield stress and yield strain. It is well known in the literature that the elastic modulus and the yield stress of thixotropic materials increase with aging time. In the current work, we propose a discussion on the brittleness of a suspension of swollen bentonite in water, focusing mainly on the role of aging times on the yield strain and on the critical strain at the linear to nonlinear viscoelastic transition of the material. The yield strain was measured in creep and constant shear rate start-up experiments, whereas the linear to nonlinear viscoelastic transition was evaluated from Fourier transforms on transient data in oscillatory shear stress amplitude sweeps. We show that aging increases material brittleness since the yield strain decreases with the resting time. On the other hand, the linear to nonlinear viscoelastic transition strain is surprisingly unaffected by the aging process. Other thixotropic systems were also investigated: 8 and 10 wt. % suspensions of bentonite in water and a 2 wt. % suspension of Laponite^® in tap water. These lead to similar observations, showing constant linear to nonlinear viscoelastic strains and decreasing yield strains over increasing aging times. These findings bring relevant information to the intricate open-discussion issue on how to describe the behavior of thixotropic materials below the yield stress.

中文翻译：

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老化对膨润土和锂皂石悬浮液屈服和预屈服行为影响的实验证据

触变性屈服应力材料在屈服点显示出剪切诱导的固液转变，以屈服应力和屈服应变为特征。文献中众所周知，触变材料的弹性模量和屈服应力随着老化时间的增加而增加。在目前的工作中，我们提议讨论溶胀膨润土在水中的悬浮液的脆性，主要集中在老化时间对屈服应变和材料从线性到非线性粘弹性转变的临界应变的作用上。屈服应变是在蠕变和恒定剪切速率启动实验中测量的，而线性到非线性粘弹性转变是根据振荡剪切应力振幅扫描中瞬态数据的傅立叶变换来评估的。我们表明时效会增加材料的脆性，因为屈服应变随着静置时间的延长而降低。另一方面，线性到非线性粘弹性转变应变令人惊讶地不受老化过程的影响。还研究了其他触变系统：8 和 10 重量。% 膨润土在水中的悬浮液和 2 wt. Laponite 的悬浮液百分比^®在自来水中。这些导致类似的观察结果，显示恒定的线性到非线性粘弹性应变和随着老化时间的增加而降低屈服应变。这些发现为关于如何描述屈服应力下触变材料行为的复杂公开讨论问题提供了相关信息。