Graham John McShane

The Underwater Blast Resistance of Metallic Sandwich Beams With Prismatic Lattice Cores

The finite element method is used to evaluate the underwater blast resistance of monolithic beams and sandwich beams containing prismatic lattice cores (Y-frame and corrugated core) and an ideal foam core. Calculations are performed on both free-standing and end-clamped beams, and fluid-structure interaction effects are accounted for. It is found that the degree of core compression in the free-standing sandwich beam is sensitive to core strength, yet the transmitted impulse is only mildly sensitive to the type of sandwich core. Clamped sandwich beams significantly outperform clamped monolithic beams of equal mass, particularly for stubby beams. The Fleck and Deshpande analytical model for the blast response of sandwich beams is critically assessed by determining the significance of cross-coupling between the three stages of response: in stage I the front face is accelerated by the fluid up to the point of first cavitation, stage II involves compression of the core until the front and back faces have an equal velocity, and in stage III the sandwich beam arrests by a combination of beam bending and stretching. The sensitivity of the response to the relative magnitude of these time scales is assessed by appropriately chosen numerical simulations. Coupling between stages I and II increases the level of transmitted impulse by the fluid by 20‐30% for a wide range of core strengths, for both the free-standing and clamped beams. Consequently, the back face deflection of the clamped sandwich beam exceeds that of the fully decoupled model. For stubby beams with a Y-frame and corrugated core, strong coupling exists between the core compression phase (stage II) and the beam bending/stretching phase (stage III); this coupling is beneficial as it results in a reduced deflection of the back (distal) face. In contrast, the phases of core compression (stage II) and beam bending/stretching (stage III) are decoupled for slender beams. The significance of the relative time scales for the three stages of response of the clamped beams are summarized on a performance map that takes as axes the ratios of the time scales. DOI: 10.1115/1.2198549

Dynamic Compressive Response of Stainless-Steel Square Honeycombs

The dynamic out-of-plane compressive response of stainless-steel square honeycombs has been investigated for impact velocities ranging from quasi-static values to 300 ms -1 . Square-honeycomb specimens of relative density 0.10 were manufactured using a slotting technique, and the stresses on the front and back faces of the dynamically compressed square honeycombs were measured using a direct impact Kolsky bar. Three-dimensional finite element simulations of the experiments were performed to model the response and to help interpret the experimental results. The study has identified three distinct factors governing the dynamic response of the square honeycombs: material rate sensitivity, inertial stabilization of the webs against buckling, and plastic wave propagation. Material rate sensitivity and inertial stabilization of the webs against buckling cause the front and back face stresses to increase by about a factor of two over their quasi-static value when the impact speed is Increased from 0 to 50 ms -1 . At higher impact velocities, plastic wave effects cause the front face stress to increase linearly with velocity whereas the back face stress is almost independent of velocity. The finite element predictions are in reasonable agreement with the measurements.

Young's modulus measurement of thin-film materials using micro-cantilevers

The need for a simple and effective characterization technique for thin-film materials which are widely used in MEMS (micro-electro-mechanical systems), using commonly available equipment, has prompted consideration of cantilever beam-based methods. The advantages of this class of techniques which employ a scanning surface profiler to deform micro-cantilevers are simplicity, speed, cost and wide applicability. A technique for extracting Youngs modulus from static deflection data is developed in this paper and validated in experiments on thin-film specimens of silicon nitride deposited on a silicon substrate under different conditions. Finite element analysis is used to assess the influence of factors affecting the bending of thin films, and thus guide the analysis of micro-cantilever deflection data for reliable characterization of the material.

Thermal Modeling of Al-Al and Al-Steel Friction Stir Spot Welding

This paper presents a finite element thermal model for similar and dissimilar alloy friction stir spot welding (FSSW). The model is calibrated and validated using instrumented lap joints in Al-Al and Al-Fe automotive sheet alloys. The model successfully predicts the thermal histories for a range of process conditions. The resulting temperature histories are used to predict the growth of intermetallic phases at the interface in Al-Fe welds. Temperature predictions were used to study the evolution of hardness of a precipitation-hardened aluminum alloy during post-weld aging after FSSW.

Force measurements on U-shaped electrothermal microactuators: Applications to packaging

The current paper critically reviews the prospects for the electrothermal actuation of elastic fixtures used as packaging elements for opto-electronic components. A convenient design methodology is presented together with a practical scheme for both prototyping out-of-plane bimorph actuators and measuring the vertical forces that they can deliver. A test bench has been assembled capable of measuring both the displacement and the restoring force delivered by such actuators which are patterned using laser micromachining of a bilayer consisting of 500 nm titanium tungsten (Ti-W) and 3 μm silicon nitride (SiN) thin films on a silicon substrate. An analytical model is derived to predict the dependence of the restoring force on the input electrical power and topology of the actuator. Experimental results are presented for bilayer actuators made of Ti-W/SiN in which attainable forces are of the order of 25 μN for input powers of 70 mW. An approximate theoretical model correlates well on the measured results of restoring force for different actuator geometries and supply currents. A packaging prototype was successfully tested using 550 μm long U-shape actuators with a gap width of 200 μm. These were able to move macroscopic components with rotations of up to 3°.

Process Modelling of Friction Stir Welding for Aerospace Aluminium Alloys

A process model for the prediction of post weld hardness has been applied to Friction Stir Welding (FSW) of 2xxx, 6xxx and 7xxx aerospace aluminium alloys. The model has been used to predict hardness maps for welds in both thin sheet and thick plate material. The different weld configurations and tool geometries have been simulated using a proven thermal model to predict the temperature profile during welding. The thermal data have then been used as inputs to the hardness prediction model. This model is calibrated using isothermal softening data for each alloy and postweld natural ageing is also accounted for. A direct comparison of the model performance with measured hardness has been performed, and the predicted and measured profiles agree well, despite the fact that the model ignores the effect of deformation during FSW on hardness. The model is used to predict the effect of tool geometry (in particular the shoulder/pin ratio) on the hardness throughout the weld zone.

Static and Dynamic Properties of Semi-Crystalline Polyethylene

Properties of extruded polymers are strongly affected by molecular structure. For two different semi-crystalline polymers, low-density polyethylene (LDPE) and ultra-high molecular weight polyethylene (UHMWPE), this investigation measures the elastic modulus, plastic flow stress and strain-rate dependence of yield stress. Also, it examines the effect of molecular structure on post-necking tensile fracture. The static and dynamic material tests reveal that extruded UHMWPE has a somewhat larger yield stress and much larger strain to failure than LDPE. For both types of polyethylene, the strain at tensile failure decreases with increasing strain-rate. For strain-rates 0.001–3400 s−1, the yield stress variation is accurately represented by the Cowper–Symonds equation. These results indicate that, at high strain rates, UHMWPE is more energy absorbent than LDPE as a result of its long chain molecular structure with few branches.

Micro-cantilevers for thin films: Young's modulus

A simple and effective characterisation technique based on micro-cantilever beams for thin film materials using commonly available equipment — scanning surface profiler — is described. The advantages of this class of techniques are simplicity, speed, cost and a wide applicability. A technique for extracting the Young’s modulus from static deflection data is developed and validated in experiments on thin film specimens of silicon nitride deposited on a silicon substrate under different conditions. Finite element analysis is used to assess the influence of factors affecting the bending of thin films, and thus guide the analysis of micro-cantilever deflection data for reliable characterisation of the material.