In multipass welding, the intercritically reheated coarse grained heat affected zone (HAZ) demonstrates the worst toughness in the welded joint, since it contains a high carbon martensite w...

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Effect of morphology of martensite–austenite phase on fracture of weld heat affected zone in vanadium and niobium microalloyed steels

Mechanical devices are being introduced whose size scale is well below that of conventional mechanical test specimens. The smallest devices have sizes in the nanometer range, though ag ood proportion of structural devices are of the micrometer scale. Development of these products raises the question of how their mechanical behaviour and reliability may be predicted. Conventional macroscopic test data can be used, but these are obtained using specimens whose size is much larger than the devices themselves. There is a risk that performance predictions will be inaccurate, due to the existence of size effects. This paper covers small size scale testing in metallic specimens and devices, concentrating on free- standing specimens. To begin, some examples of micro-scale devices are given. Fabrication methods for small metallic devices are then briefly described. This is followed by a review of experimental observations of mechanical properties in various metallic materials at the micro-scale, highlighting the differences in results from different research groups and the gaps in our current knowledge. A section on computational and predictive modelling is included, in recognition of the role of modelling in device design and testing. Overall, the findings are that size effects are common, particularly in crystalline samples when the grain size is similar to one or more of the specimen dimensions. However, observations of size effects differ between studies and mechanical properties can vary widely, even for the same type of material. As a consequence, the relationships between specific device processing methods, specimen size and material properties must be adequately understood to ensure successful performance.

A review of deformation and fatigue of metals at small size scales

An attempt to establish a non-empirical relationship between the Charpy V-notch energy CVN and the fracture toughness K
Ic is presented. We focus our study on the lower shelf of fracture toughness and on the onset of the ductile-to-brittle transition of a A508 Cl.3 low alloy structural steel. The methodology employed is based on the `local approach'. Brittle cleavage fracture is modelled in terms of the Beremin (1983) model, whereas the ductile crack advance preceding cleavage in the transition region is accounted for with the GTN model (Gurson, 1977; Tvergaard, 1982; Tvergaard and Needleman, 1984. Mechanical testing at different strain rates and temperatures allowed the establishment of the constitutive equations of the material in a rate dependent formulation. Numerous fracture tests on different specimen geometries provided the large data set necessary for statistical evaluation. All specimen types were modelled with finite element analysis. Special consideration was taken in order to handle the dynamic effects in the Charpy impact test in an appropriate way. The fracture toughness could be predicted from Charpy impact test results, on the lower shelf, by applying the `local approach'. In the transition region the parameters of the Beremin model were found to deviate from those established on the lower shelf. Detailed fractographic investigations showed that the nature of `weak spots' inducing cleavage fracture changes with temperature. It is concluded that the Beremin model must be refined in order to be applicable in the ductile-to-brittle transition region.

Determination of the Fracture Toughness of a Low Alloy Steel by the Instrumented Charpy Impact Test

In this paper, we present a method for determining the initial contact point and nanoindentation load–indentation depth characteristics for soft materials. The method is applied to the prediction of the load–indentation depth characteristics of polydimethylsiloxane. It involves the combined use of Johnson–Kendall–Roberts and Maugis–Dugdale adhesion theories and nonlinear least squares fitting in the determination of the initial contact point, the transition parameter, and the contact radius at zero contact load. The elastic modulus and the work of adhesion are also extracted from the load–indentation depth curves.

Nanoindentation method for determining the initial contact and adhesion characteristics of soft polydimethylsiloxane

There has been a huge expansion in the laying of pipelines for the transmission of fossil fuels over large distances and in dire environments. One particular problem associated with the steels used to fabricate the pipes is that of the anisotropy of mechanica lp roperties, especially the toughness. This review represents an attempt to critically assess the steels and the orientation dependence of their mechanical properties, with the aim of establishing a basis for further progress.

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Mechanical Anisotropy in Steels for Pipelines

This paper presents results of an experimental study of fatigue in LIGA Ni micro-electro-mechanical systems (MEMS)/thin films produced by electroplating from a sulfamate bath at a current density of 50 mA/cm2. Following a brief description of microstructure and micro-tensile properties, the results of stress-life (S-N) experiments are presented for specimens with thicknesses of 70 and 270 μm. Specimens with the thicker cross-sections (270 μm thick) are shown to have comparable fatigue resistance to annealed bulk Ni in the as-plated condition. The thinner specimens (70 μm thick) have comparable fatigue resistance to hardened Ni, and better fatigue resistance than the thicker samples. The underlying fatigue fracture modes are elucidated via scanning electron microscopy. The implications of the results are then discussed for the failure analysis of LIGA Ni MEMS structures.

An investigation of fatigue in LIGA Ni MEMS thin films

The effect of non-metallic inclusions on cleavage in wrought steel has been studied using a resulphurized mild steel. The cleavage fracture stress was measured at –196°C and found to exhibit an orientation dependence which is attributed to the morphology of the inclusions. In the short transverse and transverse testing orientations, cleavage facets were seen to initiate at the edges of the elongated inclusions. It is proposed that the inclusions do not themselves act as cleavage crack nuclei, but are internal sites of stress concentration that facilitate carbide-initiated cleavage in the adjacent matrix. In the longitudinal testing orientation, inclusions are not associated with sites of cleavage nucleation. It is proposed that in this orientation decohesion of the inclusion/matrix interface causes a reduction in stress triaxiality which inhibits cleavage nucleation. Measurements of fracture toughness K1c at –196°C show no effect of testing orientation. This is attributed to the low probability o...

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Effect of non-metallic inclusions on cleavage fracture

Cleavage Fracture in a Eutectoid and Hypoeutectoid Steel

Research into Carbon Nanotubes and their applications is fast becoming an extremely popular topic, and any means to greatly improve the synthesis process has a huge marketability. While investigating the feasibility of continuous production of single-walled carbon nanotubes in a vertical Swirled Fluid Chemical Vapour Deposition (CVD) reactor, it was discovered that helical nanotubes were lifted from the reactor by the gas current while straight tubes remained behind. Investigation into the merits provided by the helical structure illustrated the greatly increased likeliness for helical tubes to be lifted from the reactor by the carrier gas giving rise to positive speculation of their possible use in vertical CVD reactors in the future.

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Comparison of Straight and Helical Nanotube Production in a Swirled Fluid CVD Reactor

The mechanical properties of sisal fibre, loofah matt, epoxy resin and their resulting composites were determined experimentally. The influence of volume fraction of the reinforcing fibres and matt on the mechanical properties of the composites was investigated. It was found that loofah matt reduces the mechanical properties of epoxy resin and as such loofah is not a suitable reinforcing material. Sisal fibre on the other hand does improve the mechanical properties of epoxy resin; the improvement being dependent on fibre volume fraction. This makes fibre reinforced composites useful practical materials. Based on the “effective reinforcement” and the advent of multiple matrix fracture, design limitations for the composites were set. Journal of Agriculture, Science and Technology Vol.3(1) 2001: 53-65

F. P. L. Kavishe

Papers

An investigation of fatigue in LIGA Ni MEMS thin films

Effect of non-metallic inclusions on cleavage fracture

Cleavage Fracture in a Eutectoid and Hypoeutectoid Steel

Comparison of Straight and Helical Nanotube Production in a Swirled Fluid CVD Reactor

An Investigation of the Mechanical Properties of Sisal Fibre, Loofah Matt And their Composites with Epoxy Resin