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1. Ductile Fracture in Plane Stress

   https://doi.org/10.1115/1.4052106  

   Torki, Mohammad; Benzerga, Ahmed Amine (January 2022, Journal of Applied Mechanics)

   null (Ed.)

   Abstract A micromechanics-based ductile fracture initiation theory is developed and applied for high-throughput assessment of ductile failure in plane stress. A key concept is that of inhomogeneous yielding such that microscopic failure occurs in bands with the driving force being a combination of band-resolved normal and shear tractions. The new criterion is similar to the phenomenological Mohr–Coulomb model, but the sensitivity of fracture initiation to the third stress invariant constitutes an emergent outcome of the formulation. Salient features of a fracture locus in plane stress are parametrically analyzed. In particular, it is shown that a finite shear ductility cannot be rationalized based on an isotropic theory that proceeds from first principles. Thus, the isotropic formulation is supplemented with an anisotropic model accounting for void rotation and shape change to complete the prediction of a fracture locus and compare with experiments. A wide body of experimental data from the literature is explored, and a simple procedure for calibrating the theory is outlined. Comparisons with experiments are discussed in some detail. 

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2. In Situ Observations of Shear Localization and Fracture in Machining

   https://doi.org/10.1115/MSEC2024-124723  

   Yadav, Shwetabh; Chawla, Harshit; Sagapuram, Dinakar (June 2024, American Society of Mechanical Engineers)

   Using direct high-speed imaging, we study the transition between different chip formation modes, and the underlying mechanics, in machining of ductile metals. Three distinct chip formation modes — continuous chip, shear-localized chip, and fragmented chip — are effected in a same material system by varying the cutting speed. It is shown using direct observations that shear-localized chip formation is characterized by shear band nucleation at the tool tip and its propagation towards the free surface, which is then followed by plastic slip along the band without fracture. The transition from shear-localized chip to fragmented chip with increasing cutting speed is triggered by crack initiation at the free surface and propagation towards the tool tip. The extent to which crack travels towards the tool determines whether the chip is partially fragmented or fully fragmented (discontinuous). It is shown that shear localization precedes fracture and controls the crack path in fragmented chip formation. Dynamic strain and strain-rate fields underlying the each chip formation mode are quantified through image correlation analysis of high-speed images. Implications for using machining as an experimental tool for fundamental studies of localization and shear fracture in ductile metals are also discussed. 

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3. Experimental evidence for both progressive and simultaneous shear during quasistatic compression of a bulk metallic glass

   https://doi.org/10.1063/1.4942004  

   Wright, Wendelin_J; Liu, Yun; Gu, Xiaojun; Van_Ness, Katherine_D; Robare, Steven_L; Liu, Xin; Antonaglia, James; LeBlanc, Michael; Uhl, Jonathan_T; Hufnagel, Todd_C; et al (February 2016, Journal of Applied Physics)

   Two distinct types of slip events occur during serrated plastic flow of bulk metallic glasses. These events are distinguished not only by their size but also by distinct stress drop rate profiles. Small stress drop serrations have fluctuating stress drop rates (with maximum stress drop rates ranging from 0.3–1 GPa/s), indicating progressive or intermittent propagation of a shear band. The large stress drop serrations are characterized by sharply peaked stress drop rate profiles (with maximum stress drop rates of 1–100 GPa/s). The propagation of a large slip is preceded by a slowly rising stress drop rate that is presumably due to the percolation of slipping weak spots prior to the initiation of shear over the entire shear plane. The onset of the rapid shear event is accompanied by a burst of acoustic emission. These large slips correspond to simultaneous shear with uniform sliding as confirmed by direct high-speed imaging and image correlation. Both small and large slip events occur throughout plastic deformation. The significant differences between these two types require that they be carefully distinguished in both modeling and experimental efforts. 

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4. Tidal Modulation of Ice Streams: Effect of Periodic Sliding Velocity on Ice Friction and Healing

   https://doi.org/10.3389/feart.2022.719074  

   McCarthy, Christine; Skarbek, Rob M.; Savage, Heather M. (April 2022, Frontiers in Earth Science)

   Basal slip along glaciers and ice streams can be significantly modified by external time-dependent forcing, although it is not clear why some systems are more sensitive to tidal stresses. We have conducted a series of laboratory experiments to explore the effect of time varying load point velocity on ice-on-rock friction. Varying the load point velocity induces shear stress forcing, making this an analogous simulation of aspects of ice stream tidal modulation. Ambient pressure, double-direct shear experiments were conducted in a cryogenic servo-controlled biaxial deformation apparatus at temperatures between −2°C and −16°C. In addition to a background, median velocity (1 and 10 μm/s), a sinusoidal velocity was applied to the central sliding sample over a range of periods and amplitudes. Normal stress was held constant over each run (0.1, 0.5 or 1 MPa) and the shear stress was measured. Over the range of parameters studied, the full spectrum of slip behavior from creeping to slow-slip to stick-slip was observed, similar to the diversity of sliding styles observed in Antarctic and Greenland ice streams. Under conditions in which the amplitude of oscillation is equal to the median velocity, significant healing occurs as velocity approaches zero, causing a high-amplitude change in friction. The amplitude of the event increases with increasing period (i.e. hold time). At high normal stress, velocity oscillations force an otherwise stable system to behave unstably, with consistently-timed events during every cycle. Rate-state friction parameters determined from velocity steps show that the ice-rock interface is velocity strengthening. A companion paper describes a method of analyzing the oscillatory data directly. Forward modeling of a sinusoidally-driven slider block, using rate-and-state dependent friction formulation and experimentally derived parameters, successfully predicts the experimental output in all but a few cases. 

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5. A Cavity-Based Micromechanical Model for the Shear-Band Failure in Metallic Glasses Under Arbitrary Stress States

   https://doi.org/10.1115/1.4062724  

   Gao, Yanfei (December 2023, Journal of Applied Mechanics)

   Abstract Deformation and fracture of metallic glasses are often modeled by stress-based criteria which often incorporate some sorts of pressure dependence. However, detailed mechanisms that are responsible for the shear-band formation and the entire damage initiation and evolution process are complex and the origin of such a pressure dependence is obscure. Here, we argue that the shear-band formation results from the constitutive instability, so that the shear-band angle and arrangements can be easily related to the macroscopic constitutive parameters such as internal friction and dilatancy factor. This is one reason for the observed tension-compression asymmetry in metallic glasses. The free volume coalescence leads to precipitous formation of voids or cavities inside the shear bands, and the intrinsic “ductility” is therefore governed by the growth of these cavities. Based on a generalized Stokes–Hookean analogy, we can derive the critical shear-band failure strain with respect to the applied stress triaxiality, in which the cavity evolution scenarios are sharply different between tension-controlled and shear/compression-dominated conditions. This is another possible reason for the tension-compression asymmetry. It is noted that diffusive-controlled cavity growth could also be the rate-determining process, as suggested by the recent measurements of shear-band diffusivity and viscosity that turn out to satisfy the Stokes–Einstein relationship. This constitutes the third possible reason for the tension-compression asymmetry. 

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