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Review Article

Appl. Mech. Rev. 2017;69(5):050801-050801-24. doi:10.1115/1.4037966.

Instabilities in solids and structures are ubiquitous across all length and time scales, and engineering design principles have commonly aimed at preventing instability. However, over the past two decades, engineering mechanics has undergone a paradigm shift, away from avoiding instability and toward taking advantage thereof. At the core of all instabilities—both at the microstructural scale in materials and at the macroscopic, structural level—lies a nonconvex potential energy landscape which is responsible, e.g., for phase transitions and domain switching, localization, pattern formation, or structural buckling and snapping. Deliberately driving a system close to, into, and beyond the unstable regime has been exploited to create new materials systems with superior, interesting, or extreme physical properties. Here, we review the state-of-the-art in utilizing mechanical instabilities in solids and structures at the microstructural level in order to control macroscopic (meta)material performance. After a brief theoretical review, we discuss examples of utilizing material instabilities (from phase transitions and ferroelectric switching to extreme composites) as well as examples of exploiting structural instabilities in acoustic and mechanical metamaterials.

Commentary by Dr. Valentin Fuster
Appl. Mech. Rev. 2017;69(5):050802-050802-18. doi:10.1115/1.4038257.

Mechanics at the nanoscale is radically different from mechanics at the macroscale. Atomistic simulations have revealed this important fact, and experiments are being performed to support it. Specifically, in situ testing is being performed by researchers using different approaches with different material systems to interrogate the material at the nanoscale and prove or disprove many of the proposed models. This paper attempts to provide a fairly comprehensive review of the in situ testing that is being performed at the nanoscale, together with a brief description of the models that in situ testing are being used to verify. This review paper intends to primarily provide a broad snapshot of in situ testing of different nanocarbon-based polymeric nanocomposite materials.

Commentary by Dr. Valentin Fuster

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