In Situ Generated Shear Bands in Metallic Glass Investigated by Atomic Force and Analytical Transmission Electron Microscopy

Plastic deformation of metallic glasses performed at temperatures well below the glass transition proceeds via the formation of shear bands. In this contribution, we investigated shear bands originating from in situ tensile tests of Al88Y7Fe5 melt-spun ribbons performed under a transmission electron microscope. The observed contrasts of the shear bands were found to be related to a thickness reduction rather than to density changes. This result should alert the community of the possibility of thickness changes occurring during in situ shear band formation that may affect interpretation of shear band properties such as the local density. The observation of a spearhead-like shear front suggests a propagation front mechanism for shear band initiation here.

A mesoscopic analysis of a localized shear band propagation effect on the deformation and fracture of coated materials

The numerical simulations of the deformation and fracture in an iron boride coating – steel substrate composition are presented. The dynamic boundary-value problem is solved numerically by the finite-difference method. A complex geometry of the borided coating – steel substrate interface is taken into account explicitly. To simulate the mechanical behavior of the steel substrate, use is made of an isotropic strain hardening model including a relation for shear band propagation. Local regions of bulk tension are shown to arise near the interface even under simple uniaxial compression of the composition and in so doing they determine the mesoscale mechanisms of fracture. The interrelation between plastic deformation in the steel substrate and cracking of the borided coating is studied. Stages of shear band front propagation attributable to the interface complex geometry have been revealed. The coating cracking pattern, location of the fracture onset regions and the total crack length are found to depend on the front velocity in the steel substrate.

Discontinuous yielding transition of amorphous materials with low bulk modulus

The yielding transition of amorphous materials is studied with a two-dimensional Hamiltonian model that allows both shear and volume deformations. The model is investigated as a function of the relative value of the bulk modulus B with respect to the shear modulus μ. When the ratio B/μ is small enough, the yielding transition becomes discontinuous, yet reversible. If the system is driven at constant strain rate in the coexistence region, a spatially localized shear band is observed while the rest of the system remains blocked. The crucial role of volume fluctuations in the origin of this behavior is clarified in a mean field version of the model.

A Technique for High-Speed Microscopic Imaging of Dynamic Failure Events and Its Application to Shear Band Initiation in Polycarbonate

An experimental technique is reported, which can image the deformation fields associated with dynamic failure events at high spatial and temporal resolutions simultaneously. The technique is demonstrated at a spatial resolution of ~1 μm and a temporal resolution of 250 ns, while maintaining a relatively large field of view (≈ 1.11 mm × 0.63 mm). As a demonstration, the technique is used to image the deformation field near a notch tip during initiation of a shear instability in polycarbonate. An ordered array of 10 μm diameter speckles with 20 μm pitch, and deposited on the specimen surface near the notch tip helps track evolution of the deformation field. Experimental results show that the width of the shear band in polycarbonate is approximately 75 μm near the notch-tip within resolution limits of the experiments. The measurements also reveal formation of two incipient localization bands near the crack tip, one of which subsequently becomes the dominant band while the other is suppressed. Computational simulation of the experiment was conducted using a thermo-mechanically coupled rate-dependent constitutive model of polycarbonate to gain further insight into the experimental observations enabled by the combination of high spatial and temporal resolutions. The simulation results show reasonable agreement with the experimentally observed kinematic field and features near the notch-tip, while also pointing to the need for further refinement of constitutive models that are calibrated at high strain rates (~105/s) and also account for damage evolution.

Kinematics of the Central Alpine Fault Mylonite Zone, Tatare Stream, South Island, New Zealand

<p>The hanging wall of the Alpine Fault (AF) near Franz Josef Glacier has been exhumed during the past ~3 m. y. providing a sample of the ductilely deformed middle crust via obliquereverse slip on the AF. The former middle crust of the Pacific Plate occurs as an eastward-tilted slab that has been upramped from depths of ~25–35 km. A mylonitic high strain zone abuts the eastern edge of the AF in Tatare Stream. This ductile shear zone is locally ~2 km thick. The Tatare Stream locality is remarkable along the AF in the Central Southern Alps for the apparent lack of near surface segmentation of the fault there; instead its mylonitic shear zone appears uniformly inclined by ~63° to the SE. I infer this foliation is parallel to the shear zone boundary (SZB). In the distal part of the mylonite zone in extensional C' shear bands cross-cut the older non-mylonitic Alpine foliation (S3), and deflect that pre-existing fabric in a dextral-reverse sense. Based on the attitude of these shears the ductile shearing direction in the Alpine mylonite zone (AMZ) during extensional shear band activity is inferred to have trended 090 ± 6° (2σ), which is ~20° clockwise of sea floor spreading based estimates for the azimuth of the Pacific Plate motion. This indicates that slip on this central part of the AF is not fully “unpartitioned”. Measurements of the mean spacing, per-shear offset, C’ orientation, and per-shear thickness on >1000 extensional C’ shears provides perhaps the largest field-based data set of extensional shear band geometrical parameters so far compiled for a natural shear zone. The mean spacing between C’ shears decreases towards the AF from ~6 cm to ~0.2 cm. The per-shear offsets (8.2 ± 5 mm 1σ) and thickness (128 ± 20 1σ) of the extensional shears remains consistent despite a finite shear strain gradient. Using shear offset data I calculate a bulk finite shear strain accommodated by slip on C’ shears of 0.4 ± 0.3 (1σ), and a mean intra-shear band (C’ local) finite shear strain of 12.6 ± 5.4 (1σ). Consistency in the intra-shear band finite shear strain throughout the mylonite zone, together with increased C’ density implies that the quartzose rocks have behaved with a strain hardening rheology as the shears evolved. The dominant C’ (synthetic) extensional shears are disposed at a mean dihedral angle of 30° ± 2.2 (2σ), whereas the C’’ (antithetic) shears are 135 ± 3° (2σ) to the foliation (SZB). The C’ and C’’ shears appear to lie approximately parallel to planes of maximum instantaneous shear strain rate from which I obtain an estimate for Wk of 0.5 for the AMZ. I have measured the geometrical orientation of Mesozoic Alpine Schist garnet inclusion trails and tracked these pre-mylonitic age porphyroblastic garnets through the distal and main mylonite zones to determine their rotational response to late Cenozoic shearing. Electron microprobe analysis indicates that all the garnets examined in Tatare Stream are prograde from the regional (M2) Barrovian metamorphism. The mean inclusion trail orientations in the distal mylonite zone have been forward rotated by 35° relative to their equivalent orientation in the adjacent, less deformed non-mylonitic Alpine Schist. This rotation is synthetic to the dextralreverse shear of the AF zone. The rotation of approximately spherical shaped garnet porphyroblasts in the distal mylonite implies a finite shear strain of 1.2 in that zone. In the main part of the mylonite zone an additional forward rotation of 46° implies a finite shear strain there of 2.8. The inclusion trail rotational axis measured trends approximately perpendicular to the shear direction and parallel to the inferred late Cenozoic vorticity vector of ductile shearing. Using GhoshFlow, a program for simulating rotation of rigid passive objects in plane strain general shear a new kinematic vorticity number (Wn) estimate of 0.5 – 0.7 is established for the AMZ. The transition zone between the distal mylonite and the main mylonite zone, though little described in the literature, is well exposed in Tatare Stream. A distinct quartz rodding lineation, inherited from the non-mylonitic schist as an object into the mylonite zone, is distorted in the plane of the foliation across the transition from SW plunges to NE plunges. Because the foliation plane is here parallel to the SZB and by special reference to strongly curved lineation traces I have been able to isolate the pure shear component of deformation considering a simple 2D deformation on that slip plane; by modeling the distortional reorientation of inherited lineations in that plane. The direction of maximum finite elongation that I calculate in this plane trends 89 ± 3.8° (2σ). I believe this records the finite strain related to the co-axial component only. The parallelism of the previously calculated mylonitic ductile shearing direction to this stretching direction (also trending 090) indicates that the late Cenozoic ductile flow path in the central AMZ has been approximately monoclinic. I estimate a Wn of 0.8 ± 0.06 (2σ) based on the observed finite shearing in the mylonite zone (garnet rotation) and on the co-axial strain observed deforming the inherited lineations.</p>