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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. 

Traditional polymer processing breaks polymer chains. The resulting networks of short chains have a low fatigue threshold. This paper shows that a low-intensity process preserves long chains, leading to a network of an increased fatigue threshold.

 

Poly(ethyleneterephthalate)(PET)is ather moplastic of high-volu me applications, andisiden- tified as Nu mber 1inthe ResinIdentification Code onsingle-use packages. The ester bondsinthe poly mer chains are prone to hydrolysis, but the rate of hydrolysis is extre mely lo w at roo m temperature. Here weshowthathydrolysiscausesPETtogrowcracksevenatroomtemperature and under lo w loads. The hydrolytic cracks greatly outrun erosion. When PET is sub merged in water andsubjectedto a fixedload,the crack velocityincreases with p H. At highloads,the crack gro ws rapidly, and hydrolysis is negligible, so that the crack gro ws with substantial plastic defor mation andthefracturesurfaceisrough. Atlo wloads,the crack gro wsslo wly and hydrolysis isfastenough,sothatthecrackgrows withnegligibleplasticdeformationandthefracturesurface is s mooth. These observations sho w that hydrolysis e mbrittles PET. Under develop ment for sus- tainability and healthcare are biodegradable and bio mass-derived poly mers, many of which have hydrolysablegroupsinthe mainchainsorcrosslinks.Theyareallpotentiallysusceptibletohy- drolyticcrackgrowthandembrittlement.Itishopedthatthisstudy willaidthedevelopmentand applications ofthese poly mers. 

We introduce a class of ultra-light and ultra-stiff sandwich panels designed for use in photophoretic levitation applications and investigate their mechanical behavior using both computational analyses and micro-mechanical testing. The sandwich panels consist of two face sheets connected with a core that consists of hollow cylindrical ligaments arranged in a honeycomb-based hexagonal pattern. Computational modeling shows that the panels have superior bending stiffness and buckling resistance compared to similar panels with a basketweave core, and that their behavior is well described by Uflyand-Mindlin plate theory. By optimizing the ratio of the face sheet thickness to the ligament wall thickness, panels maybe obtained that have a bending stiffness that is more than five orders of magnitude larger than that of a solid plate with the same area density. Using a scalable microfabrication process, we demonstrate that panels as large as 3 × 3 cm2 with a volumetric density of 20 kg/m3 and corresponding area density of 2 g/m2 can be made in a few hours. Micro-mechanical testing of the panels is performed by deflecting microfabricated cantilevered panels using a nanoindenter. The experimentally measured bending stiffness of the cantilevered panels is in very good agreement with the computational results, demonstrating exquisite control over the dimensions, form, and properties of the microfabricated panels. 

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.