55 research outputs found
Failure Processes in Embedded Monolayer Graphene under Axial Compression
Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial compression. By monitoring the shifts of the 2D Raman phonons of rectangular flakes of various sizes under load, the critical strain to failure was determined. Prior to loading care was taken for the examined area of the flake to be free of residual stresses. The critical strain values for first failure were found to be independent of flake size at a mean value of –0.60% corresponding to a yield stress up to -6 GPa. By combining Euler mechanics with a Winkler approach, we show that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1–2 nm. These results were compared with DFT computations performed on analogue coronene/PMMA oligomers and a reasonable agreement was obtained
Compression Behavior of Single-layer Graphene
Central to most applications involving monolayer graphene is its mechanical
response under various stress states. To date most of the work reported is of
theoretical nature and refers to tension and compression loading of model
graphene. Most of the experimental work is indeed limited to bending of single
flakes in air and the stretching of flakes up to typically ~1% using plastic
substrates. Recently we have shown that by employing a cantilever beam we can
subject single graphene into various degrees of axial compression. Here we
extend this work much further by measuring in detail both stress uptake and
compression buckling strain in single flakes of different geometries. In all
cases the mechanical response is monitored by simultaneous Raman measurements
through the shift of either the G or 2D phonons of graphene. In spite of the
infinitely small thickness of the monolayers, the results show that graphene
embedded in plastic beams exhibit remarkable compression buckling strains. For
large length (l)-to-width (w) ratios (> 0.2) the buckling strain is of the
order of -0.5% to -0.6%. However, for l/w <0.2 no failure is observed for
strains even higher than -1%. Calculations based on classical Euler analysis
show that the buckling strain enhancement provided by the polymer lateral
support is more than six orders of magnitude compared to suspended graphene in
air
Strain-engineered graphene grown on hexagonal boron nitride by molecular beam epitaxy
Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene
Assessing micromechanical behaviour of PET cords in rubber matrix composites by laser Raman microscopy
The mechanical behaviour of PET cord-rubber composites has been investigated by adopting a multi-scale approach by combining standard tensile testing and laser Raman microscopy (LRM). Tensile tests were performed on cord-rubber composite and on its constituents to gain information on the mechanical response at the macro-scale. The behaviour at smaller scales was assessed by means of LRM, which has already been established as a technique that can yield values of stress or strain of reinforcement at the micro-scale. The effects of cord content, composite configuration and sample length have been examined. In particular, the efficiency of stress/ strain transfer to the embedded cord has been evaluated and correlated to the micromechanical behaviour through the ‘finite fibre length effect’ observed at the macro-scale
Development of a universal stress sensor for graphene and carbon fibres
Carbon fibres (CF) represent a significant volume fraction of modern
structural airframes. Embedded into polymer matrices, they provide significant
strength and stiffness gains over unit weight as compared to other competing
structural materials. Nevertheless, no conclusive structural model yet exists
to account for their extraordinary properties. In particular, polyacrynonitrile
(PAN) derived CF are known to be fully turbostratic: the graphene layers are
slipped sideways relative to each other, which leads to an inter-graphene
distance much greater than graphite. Here, we demonstrate that CF derive their
mechanical properties from those of graphene itself. By monitoring the Raman G
peak shift with strain for both CF and graphene, we develop a universal master
plot relating the G peak strain sensitivity of all types of CF to graphene over
a wide range of tensile moduli. A universal value of - average- shift rate with
axial stress of ~ -5{\omega}0^-1 (cm^-1 MPa^-1)is calculated for both graphene
and all CF exhibiting annular ("onion-skin") morphology
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