Wrapping an adhesive sphere with an elastic sheet   /   Capillary buckling of a thin film adhering to a sphere
Peeling an orange or trying to map Earth show that making a portion of sphere flat is a hard task. The same kind of difficulty can lead architects to design buildings whose shapes are developpable surfaces, i.e., surfaces that can be fold from a planar surface without distortions. In fact, Gauss in his Theorema Egregium summarizes all these observations by demonstrating that it is impossible to transform a portion of sphere into a plane without stretching.
Here, we consider the adhesion of an elastic film on a sphere. The film need to be bent, but also stretched to be in contact with the sphere, as continents are stretched on a flat map of Earth. Two dimensionless parameters allow us to classify the different morphologies observed experimentally. The adhesion-induced instability leading to the undulatory morphology of the branches is also described in details.
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Plants that live in aquatic environments at the air-water interface face the challenge of survival when the water level increases. Being rooted to the underlying soil, some of them developped mechanisms to protect their genetic material: their petals bend into a cup-like structure, preventing the water from flooding. Nymphoides flowers may fully close, trapping an air bubble.
We investigate the conditions under which such flowers close in response to hydrostatic pressure by using a model experiment. We then show that inverting the problem leads to a means of grabbing a liquid with a passive device.
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Random blisters on stickers: metrology through defects
Blisters are commonly observed when a sticker is placed on a flat surface, as mimatching adhesion fronts are brought together. Similar blisters are also seen in various technological applications when a thin film delaminate from a substrate because of differential thermal expansion or swelling. Although undesired in many cases, the observation of such blisters can allow to measure mecahnical properties of thin films and adhesion energy.
We study the characteristics of elongated delamination blisters on a model experiment. Far from the tip, measuring the local curvature of the crest gives the ratio of the bending stiffness of the film by the adhesion energy. Looking at the curvature of the blister's tip gives also information on the film's thickness.
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Evaluation of transient loadings following warm-prestressing in RPV steels
The toughness of a material depends not only on its mechanical characteristics but also on its history of loading. For example, when a crack is loaded at a temperature Ta at a stress intensity factor Ka , no fracture will occur when decreasing the temperature while holdind constant the loading, even if the tenacity of the virgin material is exceeded: this effect is known as the warm prestress (WPS) effect.
We study the WPS effect on a 16MND5 steel that originates from a shell of a pressurized water reactor. Experiments confirm the WPS effect and show to which extent the load can be increased during the cooling without fracture. Numerical simulations allow to estimate local mechanical quantities at cleavage triggering and to compare the predictions of the Beremin and Bordet models for cleavage fracture.
Stamping and wrinkling of elastic plates
Stamping of metal plates between spherical dies is a common industrial process and can lead to the apparition of plastic wrinkles as a result from Gauss' Theorema Egregium. In the elastic case, a pattern of hierarchical reversible wrinkles can also be observed as the spherical molds are progressively closed down.
We study the characteristics of the wrinkles in the limit of high confinement, i.e., when the gap between the two molds is on the order of the thickness of the plate. In this limit, the number of wrinkles saturates and the stamping force reaches a maximum independent of the thickness of the plate. A model describing the mechanical behavior of the wrinkled plate is proposed.
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