The mechanics of solid-state nanofoaming.

Solid-state nanofoaming experiments are conducted on two polymethyl methacrylate (PMMA) grades of markedly different molecular weight using CO2 as the blowing agent. The sensitivity of porosity to foaming time and foaming temperature is measured. Also, the microstructure of the PMMA nanofoams is characterized in terms of cell size and cell nucleation density. A one-dimensional numerical model is developed to predict the growth of spherical, gas-filled voids during the solid-state foaming process. Diffusion of CO2 within the PMMA matrix is sufficiently rapid for the concentration of CO2 to remain almost uniform spatially. The foaming model makes use of experimentally calibrated constitutive laws for the uniaxial stress versus strain response of the PMMA grades as a function of strain rate and temperature, and the effect of dissolved CO2 is accounted for by a shift in the glass transition temperature of the PMMA. The maximum achievable porosity is interpreted in terms of cell wall tearing and comparisons are made between the predictions of the model and nanofoaming measurements; it is deduced that the failure strain of the cell walls is sensitive to cell wall thickness.

Notch sensitivity of orthotropic solids: interaction of tensile and shear damage zones.

The macroscopic tensile strength of a panel containing a centre-crack or a centre-hole is predicted, assuming the simultaneous activation of multiple cohesive zones. The panel is made from an orthotropic elastic solid, and the stress raiser has both a tensile cohesive zone ahead of its tip, and shear cohesive zones in an orthogonal direction in order to represent two simultaneous damage mechanisms. These cohesive zones allow for two modes of fracture: (i) crack extension by penetration, and (ii) splitting in an orthogonal direction. The sensitivity of macroscopic tensile strength and failure mode to the degree of orthotropy is explored. The role of notch acuity and notch size are assessed by comparing the response of the pre-crack to that of the circular hole. This study reveals the role of the relative strength and relative toughness of competing damage modes in dictating the macroscopic strength of a notched panel made from an orthotropic elastic solid. Universal failure mechanism maps are constructed for the pre-crack and hole for a wide range of material orthotropies. The maps are useful for predicting whether failure is by penetration or kinking. Case studies are developed to compare the predictions with observations taken from the literature for selected orthotropic solids. It is found that synergistic strengthening occurs: when failure is by crack penetration ahead of the stress raiser, the presence of shear plastic zones leads to an enhancement of macroscopic strength. In contrast, when failure is by crack kinking, the presence of a tensile plastic zone ahead of the stress raiser has only a mild effect upon the macroscopic strength.

Crack kinking at the tip of a mode I crack in an orthotropic solid.

The competition between crack penetration and crack kinking is addressed for a mode I macroscopic crack in an orthotropic elastic solid. Cohesive zones of finite peak strength and finite toughness are placed directly ahead of and orthogonal to the plane of the parent crack. The cohesive zone ahead of the crack tip is tensile in nature and leads to crack penetration, whereas the inclined zones slide without opening under a combined shear and normal traction, and give crack kinking. Thereby, the competition between continued crack growth by penetration ahead of the crack tip versus kinking is determined as a function of the relative strength and relative toughness of the cohesive zones. This competition is plotted in the form of a failure mechanism map, with the role of material orthotropy emphasized. Synergistic toughening is observed, whereby the parent crack tip is shielded by the activation of both the tensile and shear (kinking) cohesive zones, and the macroscopic toughness is elevated. The study is used to assess the degree to which various classes of composite have the tendency to undergo kinking.

Hierarchical macroscopic fibrillar adhesives: in situ study of buckling and adhesion mechanisms on wavy substrates.

Nature uses hierarchical fibrillar structures to mediate temporary adhesion to arbitrary substrates. Such structures provide high compliance such that the flat fibril tips can be better positioned with respect to asperities of a wavy rough substrate. We investigated the buckling and adhesion of hierarchically structured adhesives in contact with flat smooth, flat rough and wavy rough substrates. A macroscopic model for the structural adhesive was fabricated by molding polydimethylsiloxane into pillars of diameter in the range of 0.3-4.8 mm, with up to three different hierarchy levels. Both flat-ended and mushroom-shaped hierarchical samples buckled at preloads one quarter that of the single level structures. We explain this behavior by a change in the buckling mode; buckling leads to a loss of contact and diminishes adhesion. Our results indicate that hierarchical structures can have a strong influence on the degree of adhesion on both flat and wavy substrates. Strategies are discussed that achieve highly compliant substrates which adhere to rough substrates.

The toughness of adipose tissue: measurements and physical basis.

The measured toughness J(C) of adipose and dermal porcine tissues are 4.1 and 17 kJ m(-2), respectively, via a trouser tear test. An assessment is made of the contribution to overall toughness from the microstructural elements. The analysis suggests that the toughness of adipose tissue is determined by the collagen network that surrounds the adipocytes. The volume fraction of the interlobular septa is sufficiently low for it to make a negligible contribution to the macroscopic toughness.

The penetration of a soft solid by a liquid jet, with application to the administration of a needle-free injection.

Liquid jet injections have been performed on human skin in vivo and silicone rubber using Intraject needle-free injectors. The discharge characteristics of the liquid jet were measured using a custom-built test instrument. The experiments reveal that a high-speed liquid jet penetrates a soft solid by the formation and opening of a planar crack. The fluid stagnation pressure required for skin penetration decreases with increasing diameter of the liquid jet. These findings are consistent with the slow-speed penetration of a soft solid by a sharp-tipped punch. It is demonstrated that the Shergold-Fleck sharp-tipped punch penetration model [Shergold, O.A., Fleck, N.A., 2004. Mechanisms of deep penetration of soft solids. Proc. Roy. Soc. Lond. A 460, 3037-3058.] gives adequate predictions for the pressure required to penetrate a soft solid by a high-speed liquid jet.

Experimental investigation into the deep penetration of soft solids by sharp and blunt punches, with application to the piercing of skin.

An experimental study has been conducted on the penetration of silicone rubbers and human skin in vivo by sharp-tipped and flat-bottomed cylindrical punches. A penetrometer was developed to measure the penetration of human skin in vivo, while a conventional screw-driven testing machine was used to penetrate the silicone rubbers. The experiments reveal that the penetration mechanism of a soft solid depends upon the punch tip geometry: a sharp tipped punch penetrates by the formation and wedging open of a mode I planar crack, while a flat-bottomed punch penetrates by the growth of a mode II ring crack. The planar crack advances with the punch, and friction along the flanks of the punch leads to a rising load versus displacement response. In contrast, the flat-bottomed punch penetrates by jerky crack advance and the load on the punch is unsteady. The average penetration pressure on the shank cross section of a flat-bottomed punch exceeds that for a sharp-tipped punch of the same diameter In addition, the penetration pressure decreases as the diameter of the sharp-tipped punch increases. These findings are in broad agreement with the predictions of Shergold and Fleck [Proc. R. Soc. London, Ser. A (in press)] who proposed models for the penetration of a soft solid by a sharp-tipped and flat-bottomed punch.