49 research outputs found
Effects of constraint curvature on structural instability: tensile buckling and multiple bifurcations
Bifurcation of an elastic structure crucially depends on the curvature of the
constraints against which the ends of the structure are prescribed to move, an
effect which deserves more attention than it has received so far. In fact, we
show theoretically and we provide definitive experimental verification that an
appropriate curvature of the constraint over which the end of a structure has
to slide strongly affects buckling loads and can induce: (i.) tensile buckling;
(ii.) decreasing- (softening), increasing- (hardening), or constant-load (null
stiffness) postcritical behaviour; (iii.) multiple bifurcations, determining
for instance two bifurcation loads (one tensile and one compressive) in a
single-degree-of-freedom elastic system. We show how to design a constraint
profile to obtain a desired postcritical behaviour and we provide the solution
for the elastica constrained to slide along a circle on one end, representing
the first example of an inflexional elastica developed from a buckling in
tension. These results have important practical implications in the design of
compliant mechanisms and may find applications in devices operating in
quasi-static or dynamic conditions
Development of configurational forces during the injection of an elastic rod
When an inextensible elastic rod is 'injected' through a sliding sleeve
against a fixed constraint, configurational forces are developed, deeply
influencing the mechanical response. This effect, which is a consequence of the
change in length of the portion of the rod included between the sliding sleeve
and the fixed constraint, is theoretically demonstrated (via integration of the
elastica) and experimentally validated on a proof-of-concept structure
(displaying an interesting force reversal in the load/deflection diagram), to
provide conclusive evidence to mechanical phenomena relevant in several
technologies, including guide wire for artery catheterization, or wellbore
insertion of a steel pipe.Comment: 10 pages, 4 figures, Extreme Mechanics Letters (2015
Plastically-driven variation of elastic stiffness in green bodies during powder compaction. Part II: Micromechanical modelling
A micromechanical approach is set-up to analyse the increase in elastic
stiffness related to development of plastic deformation (the elastoplastic
coupling concept) occurring during the compaction of a ceramic powder.
Numerical simulations on cubic (square for 2D) and hexagonal packings of
elastoplastic cylinders and spheres validate both the variation of the elastic
modulus with the forming pressure and the linear dependence of it on the
relative density as experimentally found in Part~I of this study, while the
dependence of the Poisson's ratio on the green's density is only qualitatively
explained
Air-encapsulating elastic mechanism of submerged Taraxacum blowballs
In this article, we report the observation of an air-encapsulating elastic mechanism of Dandelion spherical seed heads, namely blowballs, when submerged underwater. This peculiarity seems to be fortuitous since Taraxacum is living outside water; nevertheless, it could become beneficial for a better survival under critical conditions, e.g. of temporary flooding. The scaling of the volume of the air entrapped suggests its fractal nature with a dimension of 2.782 and a fractal air volume fraction of 4.82 × 10−2 m0.218, resulting in nominal air volume fractions in the range of 14–23%. This aspect is essential for the optimal design of bioinspired materials made up of Dandelion-like components. The miniaturization of such components leads to an increase in the efficiency of the air encapsulation up to the threshold (efficiency = 1) achieved for an optimal critical size. Thus, the optimal design is accomplished using small elements, with the optimal size, rather than using larger elements in a lower number. The described phenomenon, interesting per se, also brings bioinspired insights toward new related technological solutions for underwater air-trapping and air-bubbles transportation, e.g. the body surface of a man could allow an apnea (air consumption of 5–10 l/min) of about 10 min if it is covered by a material made up of a periodic repetition of Dandelion components of diameter 18 μm and having a total thickness of about 3–6 cm