Yo-Han Yoo

Analysis of ceramic/metal armour systems

A combined numerical and experimental study for the analysis of ceramic/metal composite armour system against 40.7 g steel projectiles has been performed. The ballistic performance of the add-on lightweight armours was examined by varying the thickness of tiles, while maintaining equal areal density of the system. A numerical study using smoothed particle hydrodynamics scheme is promising since the major distinguishing features of composite armour systems such as, projectile erosion, crack propagation, ceramic conoid formation and failure of backing plate, are successfully captured. Simulation results for ballistic limits appear to match fairly well with the test values and reveal an optimum value of the front plate to back plate thickness ratio.

Enhanced strain of InAs quantum dots by an InGaAs ternary layer in a GaAs matrix

The present work demonstrates via numerical analysis that the presence of a thin InGaAs ternary layer around InAs quantum dots (QDs) reinforces the in-plane (err) and vertical (ezz) strain components of InAs quantum dots as compared to the QDs embedded directly in GaAs matrix, contrary to the general belief of strain relief. It has been further shown that such reinforced err and ezz states yields a decreased band-gap energy, i.e., the experimentally observed redshift in the literature.

Comparison of strain fields in truncated and un-truncated quantum dots in stacked InAs/GaAs nanostructures with varying stacking periods

Strain fields in truncated and un-truncated InAs quantum dots with the same height and base length have been compared numerically when the dots are vertically stacked in a GaAs matrix at various stacking periods. The compressive hydrostatic strain in truncated dots decreases slightly as compared with the un-truncated dots without regard to the stacking period studied. However, the reduction in tensile biaxial strain, compressive radial strain and tensile axial strain was salient in the truncated dot and the reduction increased with decreasing stacking period. From such changes in strain, changes in the band gap and related properties are anticipated.

Effect of work hardening on the critical indentation limit in spherical nano-indentation of thin film/substrate systems

Abstract A finite element analysis has been carried out to investigate the effect of work hardening of materials on the critical indentation limit (the ratio of indentation depth to the film thickness) to ensure the determination of the ‘film-only’ properties in spherical nano-indentation of thin film/substrate layered systems. The work hardening of the film decreased the critical limit whilst that of substrate did not show any appreciable influence. The film hardening effect was more apparent when a smaller indenter was used. For the case when the film is harder than substrate, the influence of film hardening was more apparent especially when the film/substrate yield stress ratio was low. However, for soft film/hard substrate system, the influence of work hardening was more or less similar regardless of the yield stress ratio. Thus, it is suggested that care has to be taken to include the influence of work hardening when drawing out a critical indentation limit.

Effect of the stacking period on the strain field in InAs/GaAs quantum dots

A finite-element analysis was carried out to study the effect of the vertical stacking period on the strain field in stacked InAs quantum dots embedded in a GaAs matrix. A comparison between stacked and unstacked structures was established. The hydrostatic strain was reinforced slightly as stacking occurred and was roughly constant regardless of the studied stacking period. On the other hand, the biaxial, radial and vertical strain components were relieved with stacking and the relief was significantly enhanced with a decrease in the stacking period. The drastic changes in the strain field with the stacking period due to strain field interaction are associated with the modification of electronic states and photoluminescence properties.

Spherical nano-indentation of a hard thin film/soft substrate layered system: II. Evolution of stress and strain fields

The evolution of stress/strain fields during spherical nano-indentation of a Si hard film/Al soft substrate layered system was investigated numerically. Observation of the stress and strain fields for a small indenter radius suggested that critical indentation depths could be correlated with elastic stress field interaction with the substrate (for 2% load-error-tolerable limit) or plastic deformation in the substrate (for 10% load-error-tolerable limit). The stress field interaction with the substrate or plastic deformation of the substrate provided an earlier indication of error than the load–displacement curves when the indenter radius was small. However, when the indenter radius was large, the latter was more sensitive to errors during the indentation process. Evolution of strain fields beyond the critical indentation limit was also investigated and is discussed.

Protection capability of dual flying plates against obliquely impacting long-rod penetrators

The protection capability of dual steel flying plates against obliquely impacting tungsten heavy alloy long-rod penetrators was simulated using NET3D code. The major variables considered in this work include thickness ratios of front to rear plate (5/0, 3.3/1.7, 2.5/2.5, 1.7/3.3, and 0/5) and plate velocity (0.2 and 0.5 km/s) at obliquity of 60°. Based on the residual kinetic energy of the penetrator after perforating the plates system, the protection capability of the system increased with an increase in the ratio of rear plate thickness. Such a trend was confirmed at a normal ordnance velocity (1.5 km/s) as well as a hypervelocity regime (2.5 km/s) and was shown to be best exploited when the relative velocity between the penetrator and rear plate was appropriate. A design guideline for an effective flying plate protection structure was proposed.

Spherical nano-indentation of a hard thin film/soft substrate layered system: I. Critical indentation depth

The effect of the radius of the spherical indenter on the critical nano-indentation depth to ensure reliable measurement of thin film properties was investigated numerically for a Si hard thin film/Al soft substrate layered system. It was predicted that the error in measuring load–displacement relations for a Si thin film increases with indentation depth as well as the radius of the indenter. It was also predicted that the critical indentation depth decreases as the indenter radius normalized to film thickness increases. It is therefore suggested that care needs to be taken as to the indentation depth depending on the ratio of indenter radius to film thickness, as quantified in this paper.

Effect of the Velocity of a Single Flying Plate on the Protection Capability Against Obliquely Impacting Long‐Rod Penetrators

The protection capability of a flying steel plate against obliquely impacting tungsten heavy alloy long‐rod penetrators was simulated using the NET3D code as a function of plate velocity ranging from −0.5 to 0.5 km/sec at an obliquity of 60°. The negativity in plate velocity was assigned if the plate had a velocity component in the direction of penetrator progress. Based on the residual kinetic energy of the penetrator after perforating the plate, the protection capability of the plate increased as the plate velocity decreased to a negative value at both a normal ordnance velocity (1.5 km/sec) and a hypervelocity (2.5 km/sec) impact. The defeat capability of the oblique plate increased as the impact velocity increased in the plate velocity range studied in this work. The interaction mechanisms between the penetrator and steel plate, responsible for these results, were investigated. The physical meaning of the results obtained in this work were discussed in the light of sensor‐activated and reactive armours.

A new concept for the determination of tensile properties of nanofilms and materials via nanoindentation

Here we present a new concept to determine tensile properties of nanofilms and materials via nanoindentation. The proposed methodology utilizes a conical or truncated conical indenter and requires the fabrication of an upwardly obtruded well shaped tube from the substrate. The downward stroke of the indenter along the centreline of the tube mainly produces tensile hoop stress in the upper region of the tube where the nanofilm exists. In the present work, the feasibility of the proposed method has been demonstrated through finite element analysis, as the first stage of a longer project on the topic. It has been demonstrated that Youngs modulus and the yield strength of the nanofilm can be suitably determined in tensile mode from the load–stroke relation.