Measurement and analysis of fatigue crack deformation on the macro- and micro-scale

The paper describes an experiment which performs in-situ loading of a small compact tensionspecimen in a scanning electron microscope. Images are collected throughout a number of successive loadincgcycles. These are then analysed using digital image correlation (DIC) in order to produce crack flankdisplacements as a function of load. This data is then compared with a simple elastic approach, and it is concludedthat elastic-plastic analysis is required in order to accurately capture the displacements close to the crack tip. Asimple approach due to Pommier and Hamam is therefore employed. This gives a better representation of thedata, but predicts a variation of crack tip displacement, ?, which is difficult to explain from a physical perspective.The need for a more sophisticated analysis of the data is therefore highlighted

An analytical geometrical model for secondary dendrite arm detachment

A simple geometrical model of a ripened secondary dendrite arm is used to investigate the curvature at the neck of the arm where it joins the primary trunk. It is found that the negative K-(1) component of the curvature does not fully balance the peak in the positive K-(2) component

Measurement and analysis of fatigue crack deformation at the micro-scale

This paper introduces the use of digital image correlation for the measurement of surface displacements in the neighbourhood of a crack tip, both at the macro- and micro- scale. Various methods of interpreting the measured data and producing a crack driving force are then discussed, including the use of the full CJP model. A reduced set of parameters are then proposed, corresponding to the three principal interaction forces between the plastic enclave and the surrounding elastic material. Our own results, and those of Vasco Olmo, previously reported in the literature are then reanalysed using this new framework, and excellent agreement between two independent experiments is obtained. Implications for the analysis of further data sets are then discussed

Rapid solidification morphologies in Ni3Ge: Spherulites, dendrites and dense-branched fractal structures

Single-phase β-Ni3Ge has been rapidly solidified via drop-tube processing. At low cooling rates (850–300 μm diameter particles, 700–2800 K s−1) the dominant solidification morphology, revealed after etching, is that of isolated spherulites in an otherwise featureless matrix. At higher cooling rates (300–75 μm diameter particles, 2800–25,000 K s−1) the dominant solidification morphology is that of dendrites, again imbedded within a featureless matrix. As the cooling rate increases towards the higher end of this range the dendrites display non-orthogonal side-branching and tip splitting. At the highest cooling rates studied (25,000 K s−1), dense-branched fractal structures are observed. Selected area diffraction analysis in the TEM reveals the spherulites and dendrites are a disordered variant of β-Ni3Ge, whilst the featureless matrix is the ordered variant of the same compound. We postulate that the spherulites and dendrites are the rapid solidification morphology and that the ordered, featureless matrix grew more slowly post-recalescence. Spherulites are most likely the result of kinetically limited growth, switching to thermal dendrites as the growth velocity increases. It is extremely uncommon to observe such a wide range of morphologies as a function of cooling rate in a single material

Development and optimisation of micromechanical testing techniques to study the properties of meniscal tissue

In this paper we present the results from a recent micromechanical investigation aimed at developing methodologies for testing and understanding the fundamental behaviour of meniscal tissue. To achieve this, we employed two distinctly different, but equally relevant mechanical testing platforms – uniaxial tensile testing and Dynamic Mechanical Analysis. The results from the tensile tests revealed that the studied material exhibits non-linear stress-strain behaviour and that its viscoelastic properties are timedependent. Furthermore, by using DMA it was possible to perform walking and running simulations, which provided furtherinformation of the strain=time response of the meniscal samples. The importance of accurate specimen preparation and actual method development are also presented and discussed in detail

Advanced microscopy analysis of the micro-nanoscale architecture of human menisci

The complex inhomogeneous architecture of the human meniscal tissue at the micro and nano scale in the absence of artefacts introduced by sample treatments has not yet been fully revealed. The knowledge of the internal structure organization is essential to understand the mechanical functionality of the meniscus and its relationship with the tissue’s complex structure. In this work, we investigated human meniscal tissue structure using up-to-date non-invasive imaging techniques, based on multiphoton fluorescence and quantitative second harmonic generation microscopy complemented with Environmental Scanning Electron Microscopy measurements. Observations on 50 meniscal samples extracted from 6 human menisci (3 lateral and 3 medial) revealed fundamental features of structural morphology and allowed us to quantitatively describe the 3D organisation of elastin and collagen fibres bundles. 3D regular waves of collagen bundles are arranged in “honeycomb-like” cells that are comprised of pores surrounded by the collagen and elastin network at the micro-scale. This type of arrangement propagates from macro to the nanoscale

Mechanical behaviour of rapidly solidified copper: effects of undercooling and strain rate

In this paper we present the results, from what we believe is the first ever attempt to study the mechanical behaviour of pure highly undercooled Cu specimens. The data revealed that the strength of the studied system increases not only with an increase of the level of undercooling, but also with the rate of testing. Microstructural analysis demonstrated that at undercoolings above 200 K the specimens underwent a transition from dendritic to a grain refined structure, accompanied with a break in the stress-undercooling relationship. It is suggested that on this occasion the transition was the result of two competing mechanisms: dendrite fragmentation and recrystallization. Finally, the relationship between the resultant grain sizes and measured stresses is compared against the Hall-Petch Law

The functionally grading elastic and viscoelastic properties of the body region of the knee meniscus

The knee meniscus is a highly porous structure which exhibits a grading architecture through the depth of the tissue. The superficial layers on both femoral and tibial sides are constituted by a fine mesh of randomly distributed collagen fibers while the internal layer is constituted by a network of collagen channels of a mean size of 22.14 μm aligned at a 30∘ inclination with respect to the vertical. Horizontal dog-bone samples extracted from different depths of the tissue were mechanically tested in uniaxial tension to examine the variation of elastic and viscoelastic properties across the meniscus. The tests show that a random alignment of the collagen fibers in the superficial layers leads to stiffer mechanical responses (E = 105 and 189 MPa) in comparison to the internal regions (E = 34 MPa). All regions exhibit two modes of relaxation at a constant strain (τ1=6.4 to 7.7 s, τ2 = 49.9 to 59.7 s)

Evidence for dendritic fragmentation in as-solidified samples of deeply undercooled melts

The congruently melting, single phase intermetallic β-Ni3Ge has been subject to rapid solidification via drop-tube processing. We establish that the rapidly solidified material growing during the recalescence phase of solidification can be distinguished from the post-recalescence material in the as-solidified sample by the degree of chemical ordering displayed. This can in turn be used to visualize the material from the recalescence phase of solidification. At intermediate cooling rates this recalescence material consists of fragmented dendrites. The occurrence of fragmentation is compared against established theoretical models based on the growth of Rayleigh instabilities with excellent agreement being found. EBSD mapping is used to establish the relationship between these dendritic fragments and the final grain size distribution. The dendritic fragments are found to be poor nuclei for new grains and the fragmented dendrites do not consistently give rise to classical grain refined structures