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1. Stress-corrosion cracking of polypropylene in harsh oxidizing environments

   https://doi.org/10.1016/j.eml.2023.102080  

   Kim, Jong-hyoung; Song, Won-Seok; Jiao, Quan; Vlassak, Joost J. (November 2023, Extreme Mechanics Letters)

   Thermoplastic pipes are widely used in the semiconductor industry, where they are used to drain highly corrosive liquid waste. When exposed to oxidizing environments, thermoplastic pipes can undergo stress-corrosion cracking (SCC), potentially causing them to fail prematurely in the absence of appropriate design and maintenance guidelines. Here, the stress-corrosion cracking behavior of polypropylene, commonly used in waste drainage pipes for dilute sulfuric acid/hydrogen peroxide mixtures (Piranha solutions), is investigated as a function of applied energy release rate. Sub-critical crack growth experiments are performed with compact tension specimens in sulfuric acid/hydrogen peroxide mixtures using a custom constant-force loading system to evaluate the effects of temperature and chemical composition on SCC crack growth. The activation energy for the SCC process is 99.7 ± 15.3 kJ/mol, and the crack growth rate depends sensitively on the concentrations of sulfuric acid and hydrogen peroxide in the mixture. We propose a practical guideline to calculate the service life of polypropylene pipes in Piranha solutions using crack velocity curves and show that accidental exposure to a concentrated Piranha solution can significantly reduce service life. 

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2. Osmotic instability in soft materials under well-controlled triaxial stress

   https://doi.org/10.1016/j.jmps.2022.105195  

   Wang, Zhengjin; Liu, Junjie; Chen, Peijian; Suo, Zhigang (March 2023, Journal of the Mechanics and Physics of Solids)

   Living tissues and some engineering materials contain water. When a wet material loses water, high triaxial tensile stress may build up and cause instability. The mechanism of instability under triaxial tension has attracted great attention, but quantitative study remains an ongoing chal- lenge. Here we develop an experimental method to apply well-controlled triaxial tensile stress and observe osmotic instability in situ. We synthesize a hydrogel in an elastomer tube with strong adhesion between them. The elastomer dissolves minute amount of water, but allows water to diffuse out and places the hydrogel under homogeneous, equal-triaxial, tensile stress. We develop a method to determine the stress as a function of time. The transparent setup enables observation of various types of osmotic instabilities, including cavity nucleation, crack propagation, and surface undulation. Notably, our method enables the measurement of crack speed from ~10−5 m/ s to a limit comparable to the Rayleigh wave speed ~1 m/s. We observe a large jump in crack speed at a critical energy release rate. This work opens opportunities to study the physics of soft materials under high triaxial tension. 

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3. Why does an elastomer layer confined between two rigid blocks grow numerous cavities?

   https://doi.org/10.1016/j.jmps.2023.105223  

   Hao, Sida; Suo, Zhigang; Huang, Rui (April 2023, Journal of the Mechanics and Physics of Solids)

   For a thin layer of elastomer sandwiched between two rigid blocks, when the blocks are pulled, numerous cavities grow in the elastomer like cracks. Why does the elastomer grow numerous small cracks instead of a single large crack? Here we answer this question by analyzing an idealized model, in which the elastomer is an incompressible neoHookean material and contains a penny-shaped crack. To simulate one representative crack among many, the model is axisymmetric with zero radial displacement at the edge. When the rigid blocks are pulled by a pair of forces, a hydrostatic tension develops in the elastomer. At a critical hydrostatic tension, a small crack deforms substantially, as predicted by an elastic instability, resulting in an unbounded energy release rate. Consequently, the small crack initiates its growth, regardless of the toughness of the elastomer. As the crack grows, the energy release rate decreases, so that the crack arrests. Meanwhile, the rigid blocks constrain deformation of the elastomer far away from the crack, where hydrostatic tension remains high, allowing other cracks to grow. For an elastomer of thickness H, shear modulus , and toughness , the crack radius and spacing decrease as the normalized toughness increases. Therefore, a tough elastomer of small modulus and thickness will grow numerous small cracks when confined by two rigid blocks and pulled beyond a critical force. 

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4. In Situ Near Crack Tip pH Measurements to Confirm Alkaline Crack Conditions Causing Crack Arrest in Cathodically Polarized AA5456-H116

   https://doi.org/10.5006/4591  

   Montiel, Gabriella C; Esmaeely, Saba Navabzadeh; Marino, Gabriella; Pope, Jackson; Free, Brandon; Catledge, Katrina E; McAllister, Donald; Warner_Locke, Jenifer S (August 2024, Corrosion)

   In situ crack tip pH measurements for corrosion fatigue (CF) cracks in sensitized AA5456-H116 loaded under low loading frequencies show that cathodic polarization can arrest actively growing stress corrosion cracking (SCC) and CF cracks and produce a local alkaline crack tip pH. A method for measuring crack tip pH in situ was developed. For AA5456-H116 under a single level of high sensitization, CF experiments while loading in the Paris regime at a loading frequency of 0.1 Hz were conducted under freely corroding conditions and a cathodic polarization of −1.3 VSCE. Results show that under freely corroding conditions the crack actively grows, and the crack tip pH is slightly acidic, while at −1.3 VSCE an alkaline crack tip develops with a pH of 10 to 12. The findings of this study support the earlier published hypothesis that crack arrest of SCC and low loading frequency CF cracks is due to corrosion-induced blunting after the development of highly alkaline conditions that cause corrosion of the crack tip region blunting and halting the crack. 

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5. Composites retard hydrolytic crack growth

   https://doi.org/10.1016/j.eml.2021.101433  

   Jiao, Quan; Shi, Meixuanzi; Yin, Tenghao; Suo, Zhigang; Vlassak, Joost J. (October 2021, Extreme Mechanics Letters)

   Degradable polymers are under intense development for sustainability and healthcare. Evidence has accumulated that the chemical reaction that decomposes a polymer an also grow a crack. Even under a small load, the crack speed can be orders of magnitude higher than the overall rate of degradation, leading to premature failure. Here, we demonstrate that a crack slows down markedly in a composite of two degradable materials. In a homogeneous degradable material, the stress concentrates at the crack tip, so that a relatively small applied stretch induces a high stress and a high rate of reaction. The fracture behavior of a composite that consists of two degradable materials, a stiff material for the fibers and a compliant material for the matrix, with strong adhesion between both, is different: The soft matrix blunts the crack and distributes the stresses at the crack tip over a long length of the fibers. The same rate of reaction requires a larger applied stretch. This stress de-concentration retards crack growth in the composite. We demonstrate this concept using a composite made of stiff polydimethylsiloxane (PDMS) fibers in a soft PDMS matrix. In the presence of water molecules in the environment, siloxane bonds in the PDMS hydrolyze, causing hydrolytic crack growth. We show that a hydrolytic crack grows much more slowly in a PDMS composite than in homogeneous PDMS, and may even arrest in the composite. It is hoped that this concept will contribute to the development of degradable materials that resist premature failure. 

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