The effect of nozzle design on laser micro-machining of M2 tool steels

This paper shows how computational techniques have been used to develop axi-symmetric, straight, sonic-line, minimum length micro nozzles that are suitable for laser micro-machining applications. Gas jets are used during laser micro-machining processing applications to shield the interaction zone between laser and workpiece material, and they determine the machining efficiency of such applications. The paper discusses the nature of laser-material interactions and the importance of using computational fluid dynamics to model pressure distributions in short nozzles that are used to deliver gas to the laser-material interaction zone. Experimental results are presented that highlight unique problems associated with laser micro machining using gas jets.

On the selective removal of steel by laser-assisted vortex machining

Of all laser-based processes, laser machining has received little attention compared with others such as cutting, welding, heat treatment and cleaning. The reasons for this are unclear, although much can be gained from the development of an effcient laser machining process capable of processing diffcult materials such as high-performance steels and aerospace alloys. Existing laser machining processes selectively remove material by melt shearing and evaporation. Removing material by melting and evaporation leads to very low wall plug effciencies, and the process has difficulty competing with conventional mechanical removal methods. Adopting a laser machining solution for some materials offers the best prospects of effcient manufacturing operations. This paper presents a new laser machining process that relies on melt shear removal provided by a vertical high-speed gas vortex. Experimental and theoretical studies of a simple machining geometry have identifed a stable vortex regime that can be used to remove laser-generated melt effectively. The resultant combination of laser and vortex is employed in machining trials on 43A carbon steel. Results have shown that laser slot machining can be performed in a stable regime at speeds up to 150mm/min with slot depths of 4mm at an incident CO2 laser power level of 600 W. Slot forming mechanisms and process variables are discussed for the case of steel. Methods of bulk machining through multislot machining strategies are also presented.

The use of fibre optic sensors to monitor pipeline response to tunnelling

This paper describes the use of fibre optic sensing with Brillouin Optical Time-Domain Reflectometry (BOTDR) for near-continuous (distributed) strain monitoring of a large diameter pipeline, buried in predominantly granular material, subjected to a pipe jack tunnelling operation in London Clay. The pipeline, buried at shallow depth, comprises 4.6 m long sections connected with standard bell and spigot type joints, which connect to a continuous steel pipeline. In this paper the suitability of fibre optic sensing with BOTDR for monitoring pipeline behaviour is illustrated. The ability of the fibre optic sensor to detect local strain changes at joints and the subsequent impact on the overall strain profile is shown. The BOTDR strain profile was also used to infer pipe settlement through a process of double-integration and was compared to pipe settlement measurements. The close approximation of the measured pipe settlement provides further confidence in fibre optic strain sensing with BOTDR to investigate the intricacies of pipeline behaviour, pipe-soil interaction and interaction between pipe sections when subjected to ground movement. Copyright ASCE 2006.

On the percussion drilling of SI using a 20W MOPA based YB fiber laser

A study on the nanosecond fiber laser interaction with silicon was performed experimentally for the generation of percussion drilled holes. Single pulse ablation experiments were carried out on mono crystalline 650μm thick Si wafers. Changes of the mass removal mechanism were investigated by varying laser fluence up to 68 J/cm2 and pulse duration from 50 ns to 200 ns. Hole width and depth were measured and surface morphology were studied using scanning electron microscopy (SEM) and optical interferometric profilometry (Veeco NT3300). High speed photography was also used to examine laser generated plasma expansion rates. The material removal rate was found to be influenced by the pulse energy, full pulse duration and pulse peak power. Single pulse ablation depth of 4.42 μm was achieved using a 200 ns pulse of 13.3 J/cm 2, giving a maximum machining efficiency of 31.86 μm per mJ. Holes drilled with an increased fluence but fixed pulse length were deeper, exhibited low recast, but were less efficient than those produced at a lower fluence. The increased peak power in this case led to high levels of plasma and vapour production. The expansion of which, results in a strong driving recoil force, an increase in the rate and volume of melt ejection, and cleaner hole formation. The experimental findings show that for efficient drilling at a given energy, a longer, lower peak power pulse is more desirable than a high peak power short pulse.

A comparison of two models for the vibration response of piled foundations to inertial and underground-railway-induced loadings

The vibration response of piled foundations due to ground-borne vibration produced by an underground railway is a largely-neglected area in the field of structural dynamics. However, this continues to be an important aspect of research as it is expected that the presence of piled foundations can have a significant influence on the propagation and transmission of the wavefield produced by the underground railway. This paper presents a comparison of two methods that can be employed in calculating the vibration response of a piled foundation: an efficient semi-analytical model, and a Boundary Element model. The semi-analytical model uses a column or an Euler beam to model the pile, and the soil is modelled as a linear, elastic continuum that has the geometry of a thick-walled cylinder with an infinite outer radius and an inner radius equal to the radius of the pile. The boundary element model uses a constant-element BEM formulation for the halfspace, and a rectangular discretisation of the circular pile-soil interface. The piles are modelled as Timoshenko beams. Pile-soil-pile interactions are inherently accounted for in the BEM equations, whereas in the semi-analytical model these are quantified using the superposition of interaction factors. Both models use the method of joining subsystems to incorporate the incident wavefield generated by the underground railway into the pile model. Results are computed for a single pile subject to an inertial loading, pile-soil-pile interactions, and a pile group subjected to excitation from an underground railway. The two models are compared in terms of accuracy, computation time, versatility and applicability, and guidelines for future vibration prediction models involving piled foundations are proposed.

Theoretical study of line narrowing in a sub-terahertz monolithic dual laser source with an integrated reflector

A monolithic design is proposed for low-noise sub-THz signal generation by integrating a reflector onto a dual laser source. The reflectivity and the position of such a reflector can be adjusted to obtain constructive feedback from the reflector to both lasers, thus causing a Vernier feedback effect. As a result, 10-fold line narrowing, the narrowing being limited by the resolution of the simulation, is predicted using a transmission line model. Finally, a simple control scheme using an electrical feedback loop to adjust laser biases is proposed to maintain the line narrowing performance. This line narrowing technique, comprising a passive integrated reflector, could allow the development of a low-cost, compact and energy-efficient solution for high-purity sub-THz signal generation. © The Institution of Engineering and Technology 2014.

Coupled analysis of building damage due to tunnelling

Excavation works in urban areas require a preliminary risk damage assessment. In historical cities, the prediction of building response to settlements is necessary to reduce the risk of damage of the architectural heritage. The current method used to predict the building damage due to ground deformations is the Limiting Tensile Strain Method (LTSM). This method is based on an uncoupled soil-structure analysis, in which the building is modelled as an elastic beam subject to imposed greenfield settlements and the induced tensile strains are compared with a limit value for the material. This approach neglects many factors which play an important rule in the response of the structure to tunneling induced settlements. In this paper, the possibility to apply a settlement risk assessment derived from the seismic vulnerability approach is considered. The parameters that influence the structural response to settlements can be defined through numerical coupled analyses which take into account the nonlinear behaviour of masonry and the soil-structure interaction.

BISTABLE PERFORMANCE OF CLOSELY COUPLED TWIN STRIPE LASERS.

Results are given for bistable effects in closely coupled twin stripe lasers. These devices use controlled adjustment of asymmetric transverse optical gain to obtain bistability. Various bistable effects have been observed. Initially the authors reported a large light/current hysteresis loop obtained as the drive current to the laser was raised and lowered. Information concerning the bistable mechanisms was then obtained by applying small current pulses into each stripe. It was thus found that bistability was involved with the switching from one stable laser waveguiding mechanism to another. More recently the experimental measurement system has been much improved. Through the use of computer control of motorised micromovements and computer controlled data management, time resolved near and far field, and charge carrier concentration distribution measurements have been more accurately carried out. The paper will outline briefly this system, and report on how it has helped to reveal new mechanisms of bistability in twin stripe lasers.

A CMOS analogue predistortion circuit for wideband optical fibre links

The laser is a major source of nonlinearity for optical fibre communication systems. In this paper, we propose a CMOS analogue predistortion circuit to reduce laser nonlinearity for wideband optical fibre links. The circuit uses a nonlinearity having the inverse transfer characteristic of the directly modulated vertical cavity surface emitting laser (VCSEL). It is shown by post-layout simulation that the predistortion circuit shows 12dBm improvement in the optical fibre system. The optical fibre transmitter front-end with predistortion lineariser is being fabricated using the austriamicrosystems (AMS) 0.3 5μm CMOS technology.

Security considerations in the design and peering of RFID discovery services

There is growing interest in Discovery Services for locating RFID and supply chain data between companies globally, to obtain product lifecycle information for individual objects. Discovery Services are heralded as a means to find serial-level data from previously unknown parties, however more realistically they provide a means to reduce the communications load on the information services, the network and the requesting client application. Attempts to design a standardised Discovery Service will not succeed unless security is considered in every aspect of the design. In this paper we clearly show that security cannot be bolted-on in the form of access control, although this is also required. The basic communication model of the Discovery Service critically affects who shares what data with whom, and what level of trust is required between the interacting parties. © 2009 IEEE.