Simon Lawes

The role of micro-computed tomography in forensic investigations☆

The use of micro-CT within forensic practice remains an emerging technology, principally due to its current limited availability to forensic practitioners. This review provides those with little or no previous experience of the potential roles of micro-CT in forensic practice with an illustrated overview of the technology, and the areas of practice in which micro-CT can potentially be applied to enhance forensic investigations.

Evaluation of the tribological properties of DLC for engine applications

Diamond-like carbon (DLC) coatings are used in automotive engines for decreasing friction and increasing durability. There are many variants of DLC films which provide a wide range of mechanical, physical and tribological properties. The films can be extremely hard (>90 GPa), give low coefficients of friction against a number of counterfaces and exhibit low wear coefficients. The films are often considered to be chemically inert. The properties of DLC films depend to a large degree on the relative proportions of graphitically- (sp2) and diamond-like (sp3)-bonded carbon but the inclusion of elements such as hydrogen, nitrogen, silicon, tungsten, titanium, fluorine and sulphur can dramatically change their tribological response. Two different types of DLC, a WC/C amorphous hydrogenated DLC (WC/C a-C : H) coating and an amorphous hydrogenated DLC (a-C : H) have been investigated. The mechanical and tribological properties have been evaluated by nanoindentation, scratch and wear testing and friction testing in an instrumented cam–tappet testing rig. The deformation mechanisms and wear processes have been evaluated by scanning electron and atomic force microscopy. The results show that the harder a-C : H film was more wear resistant than the softer WC/C a-C : H film and performed better in the cam–tappet testing rig.

Performance Assessment of a New Variable Stiffness Probing System for Micro-CMMs.

When designing micro-scale tactile probes, a design trade-off must be made between the stiffness and flexibility of the probing element. The probe must be flexible enough to ensure sensitive parts are not damaged during contact, but it must be stiff enough to overcome attractive surface forces, ensure it is not excessively fragile, easily damaged or sensitive to inertial loads. To address the need for a probing element that is both flexible and stiff, a novel micro-scale tactile probe has been designed and tested that makes use of an active suspension structure. The suspension structure is used to modulate the probe stiffness as required to ensure optimal stiffness conditions for each phase of the measurement process. In this paper, a novel control system is presented that monitors and controls stiffness, allowing two probe stiffness values (“stiff” and “flexible”) to be defined and switched between. During switching, the stylus tip undergoes a displacement of approximately 18 µm, however, the control system is able ensure a consistent flexible mode tip deflection to within 12 nm in the vertical axis. The overall uncertainty for three-dimensional displacement measurements using the probing system is estimated to be 58 nm, which demonstrates the potential of this innovative variable stiffness micro-scale probe system.

Focussed arc tungsten inert gas brazing of zinc-coated steels:

The brazeability of automotive zinc-coated steels depends on several factors. These include the morphology of the joint and the welding parameters selected. However, more fundamental material factors such as the composition of the coating, method of coating and coating thickness also have a significant effect. In this study, five commercially available and widely used automotive zinc-coated steels are investigated to assess brazeability. Surface zinc content and the coating type are shown to have a marked effect on the quality of the resulting joint. This is shown by surface analysis of the joint to determine evenness and bridging capability of the filler material and a cross-sectional analysis of the joints. Differences in wettability and contact length of the filler material and zinc-coated steel substrate are observed. It was found that electro-galvanised steel exhibited the best brazeability of the materials investigated here. Wettability of spreading angles as low as 17.3°, most uniform contact length and best bridging capability due to the filler material forming a metallic bond with the substrate were observed. However, pores were present in cross-sections. Galvannealed steel also showed good wetting with no embedded defects. Other steels used (galvanised and magnesium–aluminium zinc steels) presented problems with uniformity, high spreading angles of the filler material and poor bridging characteristics.

Effect of Ultrasonic Treatment on the Microstructure of A201 Aluminium Alloy for Thixoforming

It is known that the introduction of high intensity ultrasonic waves into liquid and solidifying metals leads to a non‐dendritic and fine grain structure which is the requirement for semi‐solid feedstock production. The effect of vibration time on the semi‐solid microstructure of the A201 aluminium alloy billets fabricated with the ultrasonic treatment in the liquid state was studied in this paper. It was observed that the application of ultrasound technology can break up and distribute the dendrites which are present in the as‐cast alloy. A suitable thixotropic microstructure with relatively rounded and fine globules could be obtained by ultrasonically treating liquid metal at 690 °C for a treatment time of 1 minute, cooling to room temperature and then reheating to the semi‐solid state. This shows the ultrasonic treatment could be an economic and alternative route to produce A201 semi‐solid feedstock for thixoforming.

Optimization of surface measurement for metal additive manufacturing using coherence scanning interferometry

Abstract. Surface topography measurement for metal additive manufacturing (AM) is a challenging task for contact and noncontact methods. We present an experimental investigation of the use of coherence scanning interferometry (CSI) for measuring AM surfaces. Our approach takes advantage of recent technical enhancements in CSI, including high dynamic range for light level and adjustable data acquisition rates for noise reduction. The investigation covers several typical metal AM surfaces made from different materials and AM processes. Recommendations for measurement optimization balance three aspects: data coverage, measurement area, and measurement time. This study also presents insight into areas of interest for future rigorous examination, such as measuring noise and further development of guidelines for measuring metal AM surfaces.

Development of CO2 snow cleaning for in situ cleaning of µCMM stylus tips

Contamination adhered to the surface of a µCMM stylus tip compromises the measurement accuracy of the µCMM system, potentially causing dimensional errors that are over ten times larger than the uncertainty of a modern µCMM. In prior work by the authors, the use of a high pressure CO2 gas stream was demonstrated to achieve significant cleaning rate for a range of contaminant without damage to the stylus tip surface. This paper explores the practical challenges of achieving effective stylus tip cleaning in situ on µCMM systems. Two types of snow cleaning approaches were evaluated for their coverage of cleaning, thermal impact and gas flow forces. This work then presents a novel multi-nozzle prototype system using pulsed snow streams to achieve cleaning coverage over the entire stylus tip, and balances forces from the snow streams reducing drag force imparted by the gas stream to levels comparable to the probing force of µCMMs, as well as allowing automated cleaning procedure integrated into a µCMM system.

Evaluation of the Capabilities and Damage Risk of Cleaning Methods for Micro-CMM Stylus Tips

The dimensional accuracy of a micro-CMM is significantly affected by contamination adhered to the stylus tip during use. Contaminant particles can cause dimensional errors that are orders of magnitude greater than those reported in the literature. To reduce such errors, this study evaluates the suitability of three cleaning methods (brushing, laser cleaning and snow cleaning) for removing surface contamination on a micro-CMM stylus tip. The cleaning capability of each method is experimentally investigated. Due to the fragile nature of the styli, possible damage (mechanical and thermal) to the tip is assessed. Overall, snow cleaning was found to possess higher cleaning capability and lower risk of damage than the other two methods.

In situ low-cost and adaptable braze tool evaluation system with vision analysis

In this study, a low-cost, flexible process monitoring system for focussed arc tungsten inert gas welder tool electrodes is presented. The system is capable of acquiring and processing geometric data that are used to define acceptance criteria. This system is used to characterise tool wear phenomena in repetitive brazing runs ensuring that the tool is operating within its process window. It was found, using this apparatus, that a build-up of zinc oxide, originating from the zinc-coated steel, on the braze tool prohibits arcing and delivery of a good quality joint. The vision system can identify build-up of this layer before failure of ignition. Measurements showed that after 990 s of continuous arcing, the zinc oxide build-up would cause system malfunction. The vision system gave a warning of zinc build-up at 577 s.

Compressive Behavior of Aluminium Foams Prepared by Powder Metallurgy Method

The compression behavior of closed cell aluminum foams with different density has been examined. Stress-strain curves were obtained. The microstructure evaluation of cell walls was investigated using X-ray computer tomography. A series of images of internal microstructures were given, the structural deformation evolution was analyzed. The images of cell wall evolution shown that crack appeared in cell wall after a small deformation of elastic deformation. As the compression proceeded, further cracks occur and some of the cell walls fracture. The fracture area become expanded, a part of dislocation on the sample side is formed. As the fracture area spreads, some regions become dense. The stress-strain curves showed brittle characteristics. As the relative density increases, yield strength and elastic modulus of aluminum foams increased. The best fit lines are obtained, where, is 0.97 and 0.2 respectively.