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1. Role of deformation twinning in fatigue of CrCoNi medium-entropy alloy at room temperature

   https://doi.org/10.1016/j.scriptamat.2021.113985  

   Heczko, Milan; Mazánová, Veronika; Slone, Connor E.; Shih, Mulaine; George, Easo P.; Ghazisaeidi, Maryam; Polák, Jaroslav; Mills, Michael J. (September 2021, Scripta materialia)

   Cylindrical specimens of CrCoNi alloy with electropolished surfaces were subjected to constant total strain amplitude low cycle fatigue. The alloy exhibited an initial period of cyclic hardening followed by cyclic softening until failure occurred. At the end of hardening stage at the peak of cyclic stress, well-developed persistent slip markings (PSMs) consisting of extrusions and intrusions were associated with thin deformation twins. A sophisticated experimental workflow was designed to extract information from the surface and the bulk of tested material. A combination of SEM, EBSD, ECCI, FIB and HR-STEM was used to study the internal structure and the surface profiles around the deformation twins, which were produced during the initial period of cyclic loading. Furthermore, localized cyclic plastic strain and stress concentrations near deformation twins led not only to early, well-developed PSMs, but also to the activation of TWIP and TRIP plasticity even at low macroscopic stress amplitudes. 

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2. Multiscale mechanics of polydimethylsiloxane: A comparison of meso- and micro-cyclic deformation behavior

   https://doi.org/10.1002/app.55546  

   Lou, Lihua; Bacca, Nicole; Ma, Marshall S.; Nautiyal, Pranjal; Bifano, Thomas G.; Agarwal, Arvind (April 2024, Journal of Applied Polymer Science)

   Despite plenty of static and dynamic mechanical measurements and modeling for bulk polydimethylsiloxane (PDMS) specimens, a notable gap exists in comprehensively understanding the dynamic mechanics under large cycle, low strain conditions, especially for microscale samples. This study integrates tensile testing and nanoindentation techniques to compare dynamic mechanical response for bulk PDMS samples and μ-pillars. The results from cyclic tensile testing, which involved up to 10,000 cycles at a strain range of 10%–20%, indicate a stabilization of energy dissipation rate after the initial 25 cycles. This attributes to stress relaxation and strain hardening, validating by rapid dual-phase exponential decay in maximum stress, coupled with an incremental increase in elastic modulus. In comparison to tensile testing, μ-pillars exhibited a 0.82% reduction in stiffness, stabilizing ~600th cycle. Concurrently, there was an approximately twofold increase in approaching distance during the initial 120 cycles, and an approximately fourfold increase in dissipated energy over the first 80 cycles, before reaching a plateau. This lagging hysteresis effect attributes to the distribution of the resultant force, including top tension, bottom compression, and base tilt. Overall, this study illuminates temporal mechanical deformations in PDMS under two application scenarios, enhancing our understanding of PDMS mechanical behavior. 

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3. Assessment of an Alternative Implementation of the Dobry et al. Cyclic Strain Procedure for Evaluating Liquefaction Triggering

   Green, R.A. and (June 2019, Proc. 7th International Conference on Earthquake Geotechnical Engineering (7ICEGE))

   Despite its fundamental basis and many positive attributes, the cyclic strain approach has not been embraced by practice for evaluating liquefaction triggering. One reason for this may be the need to perform cyclic laboratory tests to develop a relationship among excess pore water pressure, cyclic strain amplitude, and number of applied strain cycles. Herein an alternative implementation of the strain-based procedure is proposed that circumvents this requirement. To assess the efficacy of this alternative implementation, Standard Penetration Test field liquefaction case histories are evaluated. The results are compared with both field observations and with predictions from a stress-based procedure. It was found that the strain-based approach yields overly conservative predictions. Also, a potentially fatal limitation of the strain-based procedure is that it ignores the decrease in soil stiffness due to excess pore pressure when representing the earthquake loading in terms of shear strain amplitude and number of equivalent cycles. 

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4. In-Situ Liquefaction Testing of a Medium Dense Sand Deposit and Comparison to Case History- and Laboratory-Based Cyclic Stress and Strain Evaluations

   https://doi.org/10.1007/978-3-031-11898-2_32  

   Stuedlein, A.W.; Jana, A. (September 2022, 4th International Conference on Performance Based Design in Earthquake Geotechnical Engineering)

   Wang, L.; Zhang, J.-M.; Wang, R. (Ed.)

   Observations of the dynamic loading and liquefaction response of a deep medium dense sand deposit to controlled blasting have allowed quantification of its large-volume dynamic behavior from the linear-elastic to nonlinear-inelastic regimes under in-situ conditions unaffected by the influence of sample disturbance or imposed laboratory boundary conditions. The dynamic response of the sand was shown to be governed by the S-waves resulting from blast-induced ground motions, the frequencies of which lie within the range of earthquake ground motions. The experimentally derived dataset allowed ready interpretation of the in-situ γ-ue responses under the cyclic strain approach. However, practitioners have more commonly interpreted cyclic behavior using the cyclic stress-based approach; thus this paper also presents the methodology implemented to interpret the equivalent number of stress cycles, Neq, and deduce the cyclic stress ratios, CSRs, generated during blast-induced shearing to provide a comprehensive comparison of the cyclic resistance of the in-situ and constant-volume, stress- and strain-controlled cyclic direct simple shear (DSS) behavior of reconstituted sand specimens consolidated to the in-situ vertical effective stress, relative density, and Vs. The multi-directional cyclic resistance of the in-situ deposit was observed to be larger than that derived from the results of the cyclic strain and stress interpretations of the uniaxial DSS test data, indicating the substantial contributions of natural soil fabric and partial drainage to liquefaction resistance during shaking. The cyclic resistance ratios, CRRs, computed using case history-based liquefaction triggering procedures based on the SPT, CPT, and Vs are compared to that determined from in-situ CRR-Neq relationships considering justified, assumed slopes of the CRR-N curve, indicating variable degrees of accuracy relative to the in-situ CRR, all of which were smaller than that associated with the in-situ cyclic resistance. 

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5. Nonlinear continuum description of the E. coli cell wall under high turgor pressure

   https://doi.org/10.1103/PhysRevE.111.064405  

   Albarrán, Octavio; Garcés, Renata; Po, Giacomo; Schmidt, Christoph F; Eldredge, Jeff D (June 2025, Physical Review E)

   The principal stress-bearing component of bacterial cell walls is the tough, covalently linked peptidoglycan network. This active material maintains the shape and integrity of bacterial cells in the presence of high turgor pressures while constantly growing. It is not well understood how its extreme material properties derive from its molecular structure. The large stress in the cell wall demands a description going beyond small-strain approximations, an approach that has not been taken in the biomaterials community.We here derive a continuum elastic model of E. coli peptidoglycan (PG) in two steps: We first reduce the complex microscopic structure of the PG layer to the spring elements of a 2D hyperelastic triangular network, and then interpret those results in the context of finite-strain continuum theory. We present a self-contained formalism to study stress-strain relationships for large deformations and arbitrary anisotropies. We obtain the minimal model compatible with the available data on E. coli morphology and mechanics. We show that the strain hardening of PG leads to a stiffness matrix that is linearly proportional to the turgor pressure of the cell. 

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