Major Shear Band Research Articles

Fracture surface morphology of Mg-based bulk metallic glass and composite during quasi-static and dynamic compressive deformation

The different fracture features developed during quasi-static and dynamic compressive deformation of Mg-based bulk metallic glass and composite reinforced by SiC particulate were investigated. Mg-based amorphous glass fractures on one major plane and shows a planar and smooth fracture surface upon quasi-static loading. The fracture surface of composite is rough, which is composed of a great deal of small mirror planes in quasi-static tests. Rugged fracture surfaces were observed both in Mg-based amorphous glass and composite during dynamic deformation. A large area of molten liquid spreads over the whole fracture surface, indicating more severe temperature rise occurs during dynamic deformation. When encountering SiC particulates during propagation, cracks cut through (sometimes being stopped inside the particulate) or bypass the particle. The fact that cracks cut through or bypass particles reflects the strong bonding at the particle–matrix interfaces and the effective load transfer from matrix to the reinforcing particles. The block function to crack propagation due to the presence of particles is expected for shear bands before cracking initiates inside the shear bands, avoiding the formation of major shear band and the catastrophic failure likely.

Failure of Zr50Ti16.5Cu15Ni18.5 amorphous metallic ribbon

The deformation and fracture behavior of Zr50Ti16.5Cu15Ni18.5 bulk amorphous metal in the form of a thin ribbon have been determined in tensile test at room temperature. The fracture is localized in a major shear band and the fracture angle between the tensile stress axis and the fracture plane is close to 45°. Fractographic observations have revealed that the fracture surface of the amorphous metallic glass consists mainly of a vein-like pattern morphology. We present a scheme of three zones of fracture surface morphology: progressive smooth sliding region (A), dominating vein like pattern (B), and river-like ripples (C).

Dynamic compressive deformation and failure behavior of Zr-based metallic glass reinforced porous tungsten composite

The dynamic compressive deformation and fracture behavior of the Zr-based metallic glass reinforced porous tungsten composite were investigated at room temperature by means of the Split Hopkinson Pressure Bar (SHPB). Both fracture stress and fracture strain increased significantly compared to the pure metallic glass phase. The deformation behavior of the composite was found to be dominated by the ductile W phase and the 3D net structure of the W phase. It was found that the composite appeared to exhibit some work hardening during the dynamic compressive deformation. The failure mode of the specimen is a mixture of one major shear band and axial splitting, and the shear plane inclined ∼56° with respect to the loading axis. Scanning election microscope (SEM) was used to evaluate damage initiation and propagation. It was found that the increase of fracture stress and fracture strain is due to the interaction between localized shear banding and axial splitting, promoting additional fracture surface area, and large volume fraction of ductile W phase. The dynamic compressive deformation and fracture behavior of the composite are discussed by taking the effect of the complex stress state within the composite into account.

Deformation and evolution of shear bands under compressive loading in bulk metallic glasses

Shear band formation and failure mechanism in monolithic glasses and glass matrix composite exhibiting a wide range of plasticity were investigated by interrupted compression experiments. The major shear bands in monolithic glasses appear rapidly after a small deformation, and their numbers remain almost same in the later stages of deformation. The path of the crack growth does not coincide strictly with the shear band. The larger plastic strain in glasses exhibiting higher ductility is mainly accommodated in the primary shear band by forming larger shear offset prior to failure. The failure mechanism under compression is not pure shear, but mixed mode with evidence of a tensile component, which leads to the formation of microvoids (microcracks). Bridging of the microvoids leads to final fracture. In the case of glass composite containing particles, the microcracks at the interface arrest the propagation of the existing shear bands and form additional shear bands, improving plasticity.

Mechanical Behavior and Physical properties of Zr-Ti-Cu-Ni-Be Amorphous and Partially Crystallized Alloy extracted from a commercial golf club head

The deformation behavior of a bulk amorphous and crystallized amorphous <TEX>$Zr_{22.5}Ti_{14}Cu_{12.5}Ni_{10}Be_{22.5}$</TEX> alloy extracted from a commercial golf club head was characterized at room temperature ana <TEX>$300^{\circ}C$</TEX>. At room temperature, amorphous specimens revealed higher yield stress and ductility than partially crystallized alloy specimens. Amorphous alloy displayed some plasticity before fracture, which resulted from strain hardening and repeated crack initiation and propagation. The fracture is mainly localized on one major shear band, and the compressive fracture angle of the amorphous specimen between the stress axis and the fracture plane was about <TEX>$40^{\circ}$</TEX> Scanning electron microscope observations revealed mainly a vein-like structure in the amorphous alloy But the fracture surface of partially crystallized amorphous alloy consisted of vein-like and featureless fracture structure. The partially crystallized alloy extracted from the thick part of the club fractured in the elastic region, at a much lower stress level than the amorphous, suggesting that relatively coarse crystal particles formed during cooling cause the brittle fracture.

Ultra-high strength above 5000 MPa and soft magnetic properties of Co–Fe–Ta–B bulk glassy alloys

Glassy Co 43Fe 20Ta 5.5B 31.5 alloy rods with glass transition temperature of 910 K, followed by a large supercooled liquid region of 72 K were produced in the diameter range up to 2 mm by copper mold casting. The glassy alloy rods exhibit ultra-high fracture strength of 5185 MPa, high Young’s modulus of 268 GPa, high specific strength of 6.0 × 10 5 N m kg −1 and high specific Young’s modulus of 31 × 10 6 N m kg −1. The fracture strength, specific strength and specific Young’s modulus obtained by compression test are higher than all previous values reported for any crystalline and glassy alloys. The ultra-high strength of over 5000 MPa persists in the temperature range up to 698 K. The fracture occurred on one major shear band which was declined by 44° to the stress axis implying the slight deviation from the maximum shear stress plane. Excellent formability is manifested by large tensile elongation of 1400% and large reduction ratio in thickness above 90% in the supercooled liquid region. The ultra-high strength alloy exhibits simultaneously good soft magnetic properties: low coercivity of 0.25 A/m and an extremely high permeability of 550,000. The novel alloy is promising as a new ultra-high strength material with good deformability and soft magnetic properties.

Difference in compressive and tensile fracture mechanisms of Zr 59Cu 20Al 10Ni 8Ti 3 bulk metallic glass

The compressive and tensile deformation, as well as the fracture behavior of a Zr 59Cu 20Al 10Ni 8Ti 3 bulk metallic glass were investigated. It was found that under compressive loading, the metallic glass displays some plasticity before fracture. The fracture is mainly localized on one major shear band and the compressive fracture angle, θ C, between the stress axis and the fracture plane is 43°. Under tensile loading, the material always displays brittle fracture without yielding. The tensile fracture stress, σ F T, is about 1.58 GPa, which is lower than the compressive fracture stress, σ F C(=1.69 GPa). The tensile fracture angle, θ T, between the stress axis and the fracture plane is equal to 54°. Therefore, both θ C and θ T deviate from the maximum shear stress plane (45°), indicating that the fracture behavior of the metallic glass under compressive and tensile load does not follow the von Mises criterion. Scanning electron microscope observations reveal that the compressive fracture surfaces of the metallic glass mainly consist of a vein-like structure. A combined feature of veins and some radiate cores was observed on the tensile fracture surfaces. Based on these results, the fracture mechanisms of metallic glass are discussed by taking the effect of normal stress on the fracture process into account. It is proposed that tensile fracture first originates from the radiate cores induced by the normal stress, then propagates mainly driven by shear stress, leading to the formation of the combined fracture feature. In contrast, the compressive fracture of metallic glass is mainly controlled by the shear stress. It is suggested that the deviation of θ C and θ T from 45° can be attributed to a combined effect of the normal and shear stresses on the fracture plane.

Finite element modelling of the competition between shear bands in the early stages of thrusting: Strain localization analysis and constitutive law influence

Abstract Finite element simulation of a sandbox model of thrusting is performed using large strain analysis and two different non-associated elastoplastic constitutive laws (namely Drücker-Prager and Van Eekelen criteria). The analysis of strain localization using the Rice bifurcation criterion coupled with a kinematic indicator shows that, in the early stages of imbricated thrusting, the development of major shear bands can be influenced by some competition with second order bands. The influence of the vertical/horizontal stress ratio is checked, as well as the influence of Lode angle in the constitutive law. Significant differences are found between a classical plasticity criterion (Drücker-Prager) and a more realistic one (Van Eekelen) regarding the resistance of the model and the stress paths. These differences may result in erroneous fault type prediction.

The stages in a process of severe metallic wear : M. Kerridge and J. K. Lancaster (Research Laboratory, Associated Electrical Industries, Ltd., Aldermaston, Berks.) — Proc. Roy. Soc., A 236 (1956) 250–264; (11 fig., 12 ref.)

Metallic glasses (MGs) usually exhibit synchronously enhanced plasticity and strength with decreasing the testing temperature. Although great efforts have been made, why MGs show better plasticity at low temperature remains unclear. In this work, the shear band cracking and fracture mechanism of a relatively brittle TiZr-based MG was investigated through methods of low temperature compression and 3D X-ray tomography. Pronounced apparent softening and profuse internal shear-band cracks were observed along with the improved plasticity, enhanced yield strength, decreased average softening rate, and reduced area of vein pattern as decreasing the testing temperature. Moreover, the fracture features can be correlated well with the compressive properties; and the true rupture stress that is carried by the still-bonded part of major shear band was found to be very close to the yield strength, demonstrating that the apparent softening should mainly originate from the cracking rather than dilation of shear band. The decreased softening rate can be fitted by a diffusion model, implying reduced atomic mobility and increased cracking resistance. Consequently, the improved plasticity of TiZr-MG at low temperature was attributed to the suppression of instant fracture and the enhanced resistance to shear band cracking, rather than the change of shear band density.