L.H. Dai

Adiabatic Shear Banding Instability in Bulk Metallic Glasses

In this letter, a linear instability analysis was performed to highlight the mechanism of formation of adiabatic shear banding instabilities in bulk metallic glasses (BMGs). It is found that this instability is determined by the free volume coalescence-diffusion Deborah number. The most important findings are that both free volume coalescence softening and adiabatic heating softening exert an influence on the formation of adiabatic shear banding instability in BMGs, and higher strain rate promotes the growth of instability. These results are of particular significance in understanding the mechanism of formation of shear bands in BMGs.

Surface rippling on bulk metallic glass under nanosecond pulse laser ablation

We report an interesting surface ripple patterns in the irradiated area of a Zr-based bulk metallic glass by nanosecond pulse laser with single shot. Such surface rippling phenomenon can be ascribed to the Kelvin-Helmholtz instability at the interface between the molten layer and the expanding plasma plume. The analytical instability criterion is obtained via a perturbation analysis. Furthermore, the model demonstrates that the characteristic spacing of the ripples is dominated by the density, transverse velocity of the plasma wind, and the surface tension of the molten layer. The predicted spacing agrees well with the experimentally observed values. The results are fundamentally useful for laser-processing bulk metallic glasses (BMGs) and even for understanding the nature of flow in BMGs.

Dynamic fracture instability of tough bulk metallic glass

We report the observations of a clear fractographic evolution from vein pattern, dimple structure, and then to periodic corrugation structure, followed by microbranching pattern, along the crack propagation direction in the dynamic fracture of a tough Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit.1) bulk metallic glass (BMGs) under high-velocity plate impact. A model based on fracture surface energy dissipation and void growth is proposed to characterize this fracture pattern transition. We find that once the dynamic crack propagation velocity reaches a critical fraction of Rayleigh wave speed, the crack instability occurs; hence, crack microbranching goes ahead. Furthermore, the correlation between the critical velocity of amorphous materials and their intrinsic strength such as Youngs modulus is uncovered. The results may shed new insight into dynamic fracture instability for BMGs

Memory emission of printed carbon nanotube cathodes

The memory emission (ME) effect of printed carbon nanotube cathodes (PCNTCs) was reported. If the surface of PCNTCs is marked in a pattern by some methods, the emission image of this cathode will be the figure of the same pattern, just like the PCNTCsremember what happened in the past. We named this phenomenon as “memory emission.” According to the finding of field emission scanning electron microcopy, we suggested that the connecting CNT bundles protruding to the substrate and the electron conductive probability increasing from the substrate to the topside CNT emitter, which result in the emission enhancement of the marked place, were the reasons of the ME effect of PCNTCs. With the help of the ME effect of PCNTCs, the well-patterned emission image could easily be obtained.

Universal structural softening in metallic glasses indicated by boson heat capacity peak

Low-temperature heat capacity is systematically investigated in various glassy and crystalline polymorphs of a wide range of metallic glasses. We reveal that the boson heat capacity peak beyond the Debye level arises from both excess phonon scattering and background electronic excitation, and the two contributions are strongly coupled and also material-dependent. It is interesting to observe that the boson heat capacity peaks obey an inversely linear correlation between their heights and characteristic positions, which is mainly dominated by phonic anomalies. This indicates a universal structural softening among the studied glasses when the boson peak occurs. We further suggest a possibility that the linear evolution of the fast boson peaks can probe into the slow structural softening across the glass transition, and the two dynamic processes are controlled by the short-time shear modulus associated with local soft regions in fragile glasses.

Failure Assessment for the High-Strength Pipelines with Constant-Depth Circumferential Surface Cracks

In the oil and gas transportation system over long distance, application of high-strength pipeline steels can efficiently reduce construction and operation cost by increasing operational pressure and reducing the pipe wall thickness. Failure assessment is an important issue in the design, construction, and maintenance of the pipelines. The small circumferential surface cracks with constant depth in the welded pipelines are of practical interest. This work provides an engineering estimation procedure based upon the GE/EPRI method to determine the J-integral for the thin-walled pipelines with small constant-depth circumferential surface cracks subject to tension and bending loads. The values of elastic influence functions for stress intensity factor and plastic influence functions for fully plastic J-integral estimation are derived in tabulated forms through a series of three-dimensional finite element calculations for different crack geometries and material properties. To check confidence of the J-estimation solution in practical application, J-integral values obtained from detailed finite element (FE) analyses are compared with those estimated from the new influence functions. Excellent agreement of FE results with the proposed J-estimation solutions for both tension and bending loads indicates that the new solutions can be applied for accurate structural integrity assessment of high-strength pipelines with constant-depth circumferential surface cracks.