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1. Twinning in rolled AZ31B magnesium alloy under free-end torsion

   https://doi.org/https://doi.org/10.1016/j.msea.2020.140405  

   Carneiro, L.; Culbertson, D.; Yu, Q.; Jiang, Y. (January 2021, Materials science engineering)

   null (Ed.)

   The mechanical response and microstructure evolution in a rolled AZ31B magnesium alloy were experimentally characterized using companion thin-walled tubular specimens under free-end monotonic torsion. The tubular specimens were made with their axes along the normal direction of the rolled magnesium plate. The shear stress-shear strain response shows a subtle sigmodal shape that is composed of four distinctive stages of strain hardening. Basal slips and tension twinning are operated throughout the shear deformation. Both tension twinning and compressing twinning are favored. Growth and interaction of tension twins with multiple variants lead to formation of twin-twin boundaries (TTBs). The collective hardening effects by twin boundary (TB) and TTB result in a unique rise of the strain hardening rate in Stage II and III. In addition to primary twins, tension-compression double twins and tension-compression-tension tertiary twins with detectable sizes are observed in the tension-twin favorable grains whereas compression-tension double twins are detected in the tension-twin unfavorable grains; all of which become more observable with the increasing shear strain. During Stage IV deformation where TTB formation exhausts, non-basal prismatic slips become more significant and are responsible for the progressive decrease in strain hardening rate in this stage. Swift effect, which is commonly observed in textured materials, is evidenced under free-end torsion. The origin of Swift effect is confirmed to be dislocation slips at a shear strain less than 5% but is predominantly due to tension twinning at a larger plastic strain. 

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2. Strain rate sensitivity of rotating-square auxetic metamaterials

   https://doi.org/10.1016/j.ijimpeng.2024.105128  

   Koohbor, Behrad; Uddin, Kazi Zahir; Heras, Matthew; Youssef, George; Miller, Dennis; Sockalingam, Subramani; Sutton, Michael A; Kiel, Thomas (January 2025, International Journal of Impact Engineering)

   This study provides an in-depth analysis of the mechanical behavior of rotating-square auxetic structures under various strain rates. The structures are fabricated using stereolithography additive manufacturing with a flexible resin. Mechanical tests performed on structures include quasi-static, intermediate, and high strain rate compression tests, supplemented by high-speed optical imaging and two-dimensional digital image correlation analyses. In quasi-static conditions (5 × 10–3 s-1), multiscale measurements reveal the correlation between local and global strains. It is shown that cell hinges play a significant role in structural deformation and load-bearing capacity. In drop tower impact conditions (intermediate strain rate of ca. 200 s-1), the auxetic structures display significant strain rate hardening compared to loading at quasi-static rates. The thin-hinge structures maintain a Poisson's ratio of approximately -0.8, showing higher auxeticity than slow-rate compression tests. High strain rate conditions (ca. 2000s-1) activate additional deformation mechanisms, including a delayed state of equilibrium exemplified by a heterogeneous distribution of lateral strains, possibly due to stress wave interactions and inertial stresses. The study further reveals nonlinear correlations between Poisson's ratio, strain, and strain rate, indicating reduced auxeticity at higher strain rates. These observations are discussed in terms of complex wave interactions and the strain rate hardening characteristics of the base polymer. 

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3. Twinning characteristics in rolled AZ31B magnesium alloy under three stress states

   https://doi.org/https://doi.org/10.1016/j.matchar.2021.111050  

   Carneiro, L.; Culbertson, D.; Zhu, X.; Yu, Q.; Jiang, Y. (May 2021, Materials characterization)

   null (Ed.)

   Stress-strain responses and twinning characteristics are studied for a rolled AZ31B magnesium alloy under three different stress states: tension along the normal direction (NDT), compression along the rolled direction (RDC), and torsion about the normal direction (NDTOR) using companion specimens interrupted at incremental strain levels. Tension twinning is extensively induced in twinning-favorable NDT and RDC. All the six variants of tension twin are activated under NDT, whereas a maximum of four variants is activated under RDC. Under NDTOR, both tension twins and compression twins are activated at relatively large strains and twinning occurs in a small fraction of favored grains rather than in the majority of grains. Secondary and tertiary twins are observed in the favorably-orientated grains at high strain levels. Deformation under each stress state shows three stages of strain hardening rate: fast decrease (Stage I), sequential increase (Stage II), and progressive decrease (Stage III). The increase in the hardening rate, which is more significant under NDT and RDC as compared to NDTOR, is attributed to the hardening effect of twin boundaries and twinning texture-induced slip activities. The hardening effect of twin boundaries include the dynamic Hall-Petch hardening induced by the multiplication of twin boundaries (TBs) and twin-twin boundaries (TTBs) as well as the hardening effect associated with the energetically unfavorable TTB formation. When the applied plastic strain is larger than 0.05 under NDT and RDC, the tension twin volume fraction is higher than 50%. The twinning-induced texture leads to the activation of non-basal slips mainly in the twinned volume, i.e. prismatic slips under NDT and pyramidal slips under RDC. The low work hardening under NDTOR is due to the prevailing basal slips with reduced twinning activities under NDTOR. 

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4. In Situ Study of Twin Boundary Stability in Nanotwinned Copper Pillars under Different Strain Rates

   https://doi.org/10.3390/nano13010190  

   Chang, Shou-Yi; Huang, Yi-Chung; Lin, Shao-Yi; Lu, Chia-Ling; Chen, Chih; Dao, Ming (January 2023, Nanomaterials)

   The nanoscopic deformation of ⟨111⟩ nanotwinned copper nanopillars under strain rates between 10^−5/s and 5×10^−4/s was studied by using in situ transmission electron microscopy. The correlation among dislocation activity, twin boundary instability due to incoherent twin boundary migration and corresponding mechanical responses was investigated. Dislocations piled up in the nanotwinned copper, giving rise to significant hardening at relatively high strain rates of 3–5×10^−4/s. Lower strain rates resulted in detwinning and reduced hardening, while corresponding deformation mechanisms are proposed based on experimental results. At low/ultralow strain rates below 6×10^−5/s, dislocation activity almost ceased operating, but the migration of twin boundaries via the 1/4 ⟨10-1⟩ kink-like motion of atoms is suggested as the detwinning mechanism. At medium strain rates of 1–2×10^−4/s, detwinning was decelerated likely due to the interfered kink-like motion of atoms by activated partial dislocations, while dislocation climb may alternatively dominate detwinning. These results indicate that, even for the same nanoscale twin boundary spacing, different nanomechanical deformation mechanisms can operate at different strain rates. 

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5. Experimental Characterization of High-Strain-Rate Viscoelastic and Damage Behavior in Anisotropic Soft Materials Using Laser-Induced Inertial Cavitation

   https://doi.org/10.1007/s11340-026-01315-0  

   Wang, S; Bao, J; Chen, J; Santacruz, S R; Estrada, JB; Fan, D Emma; Yang, J (May 2026, Experimental Mechanics)

   Abstract BackgroundCharacterizing soft materials at ultra-high strain rates ($$>10^{3}\ \mathrm {s^{-1}}$$ $>{10}^3{s}^{-1}$) remains a significant challenge due to their nonlinear, large-deformation, and rate-dependent mechanical behavior. Despite these challenges, understanding material response in this regime is essential for a wide range of engineering and biomedical applications, including laser eye surgery, lithotripsy, and high-rate energy deposition in soft tissues. Laser-induced cavitation (LIC) has recently emerged as a powerful experimental approach for subjecting soft materials to extreme, localized loading rates on the microscale, regimes that are difficult to access using conventional mechanical testing methods. However, most biological tissues are anisotropic, optically opaque, and prone to damage at high strain rates, and their orientation-dependent ultra-high-rate mechanical behavior remains poorly understood. ObjectiveThe objective of this study is to develop and validate an experimental method that is based on laser-induced inertial cavitation and enables quantitative measurement of anisotropic soft materials and biological tissues at ultra-high strain rates, while explicitly accounting for fiber-directional mechanics and damage evolution. MethodsLIC experiments were performed in synthetic anisotropic polyvinyl alcohol (PVA) hydrogels and fresh chicken breast tissue using nanosecond-duration pulsed lasers to generate ultra-high-rate deformation. Simultaneously, cavitation bubble dynamics were captured using ultra-high-speed videography at 1-2 million frames per second. Two nonlinear hyper-viscoelastic constitutive models (a Poynting-Thomson model and a generalized Maxwell model) were developed and integrated with the measured bubble dynamics to quantify fiber-directional material response and investigate rate-dependent damage mechanisms. In addition, complementary quasistatic and oscillatory shear rheometry tests were conducted to provide low-rate mechanical benchmarks and investigate the rate dependency of soft materials’ mechanical behavior. ResultsLIC experiments revealed pronounced anisotropic bubble dynamics, with preferential elongation along fiber directions in both PVA hydrogels and chicken breast tissue. Model-based fitting of major-axis bubble radius-time histories enabled the extraction of effective ultra-high-rate directional moduli and critical stretch thresholds for damage initiation. ConclusionsIn summary, this study introduces a new experimental methodology for characterizing anisotropic soft materials at ultra-high strain rates. The experimental data and analysis approaches obtained from this introduction of experimental and computational methods will benefit future studies on high-strain-rate material damage and tissue injury, laser and ultrasound-related medical procedures, and anisotropic tissue constitutive modeling. 

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