Damage in elastomers: healing of internally nucleated cavities and micro-cracks.

Following on the work of Poulain et al. (Damage in elastomers: Nucleation and growth of cavities, micro-cracks, and macro-cracks, Int. J. Fract., 2017, 205, 1-21), this paper presents an investigation of the response of cavities/cracks internally nucleated within a transparent PDMS elastomer that is confined between two firmly embedded stiff beads and subjected to quasistatic cyclic loading-unloading. Specifically, it is observed that cracks that nucleate and propagate to reach tens of microns in length during the loading can heal completely upon unloading. They do so autonomously within a time scale of seconds. Furthermore, the regions of the elastomer that experience healing appear to acquire higher strength or toughness.

Mechanical probing of icelike water monolayers.

Direct measurements of the normal force interactions between a mica-tungsten contact pair at various humidity levels reveal the presence of repulsive forces at about 0.5 nm before intimate contact. Such repulsive interactions begin to appear above 20% RH and are fully developed in the range of 38-45% RH. Using the DMT model of contact, a reduced elastic modulus of approximately 6.7 GPa is extracted from these repulsive interactions and attributed to the presence of icelike water on mica at room temperature. The collapse of such structures was also inferred from the measurements.

On scale dependence in friction: transition from intimate to monolayer-lubricated contact.

Scale dependence in friction is studied in the present paper using the newly developed mesoscale friction tester (MFT). A transition in frictional shear strength from several hundreds of MPa to several tens of MPa was observed over a very limited range of contact radii (20-30 nm) in both ambient and dry environments. Thus, a single apparatus has been able to establish these two limits which are consistent with the values previously obtained from friction experiments using atomic force microscopy (AFM) and the surface force apparatus (SFA), respectively. Consequently, it is hypothesized here that a shear strength in the hundreds of MPa results from intimate contact (solid-solid) and a shear strength in the tens of MPa results from a monolayer-lubricated contact. Furthermore, both the probe size and the normal pressure govern the interfacial conditions in the contact zone and it is these conditions, rather than the nominal environment, which in turn determine the resulting shear strengths. A continuum analysis based on the Lifshitz theory for van der Waals interactions is used to explain the quantized shear strengths which were obtained from our experiments and previous AFM and SFA friction experiments. This quantized friction behavior [J.N. Israelachvili, P.M. McGuiggan, A.M. Homola, Science 240 (1988) 189] results from the discrete separation due to the different interfacial conditions that can arise between two sliding surfaces. The consistency between the analysis and the experimental results shows that this analysis is applicable for nonwear friction with single asperity contact.

On the modified Tabor parameter for the JKR-DMT transition in the presence of a liquid meniscus.

The JKR, DMT, Maugis models and Tabor parameter for contact under normal loading have been developed based mainly on solid-solid (van der Waals) interactions. In this case, the characteristic length scale for the adhesive forces in the Tabor parameter is the equilibrium interatomic spacing. However, for contact in humid environments, where a liquid meniscus may be present, capillary forces with a longer force range related to the Kelvin radius dominate. Fogden and White [J. Colloid Interface Sci. 138 (1990) 414] introduced a parameter that includes the Kelvin radius for the JKR-DMT transition. This topic was also addressed by Maugis and Gauthier-Manuel [J. Adhes. Sci. Technol. 8 (1994) 1311] who included capillary effects within the frame work that Maugis had previously established. The parameters introduced by Fogden and White and Maugis and Gauthier-Manuel can be viewed as a modified Tabor parameter for the JKR-DMT transition. In the present work, the Kelvin equation linking the Kelvin radius and the relative humidity was explicitly included in the modified Tabor parameter. This provided a quantitative description of the JKR-DMT transition in terms of the relative humidity. This parameter was examined via load and contact radius measurements, where the latter were obtained from Bowden and Tabor's assumption that the friction force f=tauA. The friction experiments were conducted at two different humidity levels using a newly-developed mesoscale friction tester (MFT), which provides a very wide range of contact radii. The modified Tabor parameter was used to reexamine data from pull-off experiments in water and cyclohexane vapor environments [L.R. Fisher, J.N. Israelachvili, Colloids Surf. 3 (1981) 303 and H.K. Christenson, J. Colloid Interface Sci. 121 (1988) 170]. Finally, guidelines are presented for the appropriate choice of contact mechanics models to be used in interpreting data from SFA and AFM experiments in humid environments.

Mesoscale scanning probe tips with subnanometer rms roughness.

Surface smoothness of probe tips is critical for applications, such as measuring surface tension of various liquids, oscillatory hydration forces, and interfacial shear strengths from friction experiments. In this study we establish conditions for fabricating tips with smooth surfaces by controlling the electrochemical polishing process throughout the tip evolution rather than following the current practice of producing tips by the drop-off method. Polishing is conducted under a constant voltage, with the wire immersed below the nominal air/electrolyte interface by no more than one-half of the wire diameter and stopping the etching at different current levels. This process provides a tip radius range of approximately 100 nm to 5 microm for a tungsten wire with a 0.2 mm diameter. Alternatively, the wire can be placed above the nominal air/electrolyte interface but within the meniscus until the current drops to zero. In this case, the tip radii range from 5 to 50 microm. In both cases, atomic force microscopy scans of these tips show that the surface rms roughness is about 0.3 nm.

Mathematical Model analysis of Wallstent and Aneurx: dynamic responses of bare-metal endoprosthesis compared with those of stent-graft.

We performed this study in order to analyze the mechanical properties of bare-metal Wallstent endoprostheses and of AneuRx stent-grafts and to compare their responses to hemodynamic forces. Mathematical modeling, numerical simulations, and experimental measurements were used to study the 2 structurally different types of endoprostheses. Our findings revealed that a single bare-metal Wallstent endoprosthesis is 10 times more flexible (elastic) than is the wall of the aneurysmal abdominal aorta. Graphs showing the changes in the diameter and length of the stent when exposed to a range of internal and external pressures were obtained. If the aorta is axially stiff and resists length change, a force as large as 1 kg can act in the axial direction on the aortic wall. If the stent is not firmly anchored, it will migrate. In contrast, a fabric-covered, fully supported, stent-graft such as the AneuRx is significantly less compliant than the aorta or the bare-metal stent. During each cardiac cycle, the stent frame tends to move due to its higher elasticity, while the fabric resists movement, which might break the sutures that join the fabric to the frame. Elevated local transmural pressure, detected along the prosthesis graft, can contribute to material fatigue.

3D surface imaging of the human female torso in upright to supine positions.

Three-dimensional (3D) surface imaging of breasts is usually done with the patient in an upright position, which does not permit comparison of changes in breast morphology with changes in position of the torso. In theory, these limitations may be eliminated if the 3D camera system could remain fixed relative to the woman's torso as she is tilted from 0 to 90°. We mounted a 3dMDtorso imaging system onto a bariatric tilt table to image breasts at different tilt angles. The images were validated using a rigid plastic mannequin and the metrics compared to breast metrics obtained from five subjects with diverse morphology. The differences between distances between the same fiducial marks differed between the supine and upright positions by less than 1% for the mannequin, whereas the differences for distances between the same fiducial marks on the breasts of the five subjects differed significantly and could be correlated with body mass index and brassiere cup size for each position change. We show that a tilt table-3D imaging system can be used to determine quantitative changes in the morphology of ptotic breasts when the subject is tilted to various angles.

On the Evaluation of the Elastic Modulus of Soft Materials Using Beams with Unknown Initial Curvature.

A nonlinear optimization procedure is established to determine the elastic modulus of slender, soft materials using beams with unknown initial curvature in the presence of large rotations. Specifically, the deflection of clamped-free beams under self-weight - measured at different orientations with respect to gravity - is used to determine the modulus of elasticity and the intrinsic curvature in the unloaded state. The approach is validated with experiments on a number of different materials - steel, polyetherimide, rubber and pig skin. Since the loading is limited to self-weight, the strain levels attained in these tests are small enough to assume a linear elastic material behavior. This nondestructive methodology is also applicable to engineered tissues and extremely delicate materials in order to obtain a quick estimate of the material's elastic modulus.

Toughening effect of strain-induced crystallites in natural rubber.

We study fracture propagation in stretched natural rubber sheets. Experimental results in specimens stretched less than 3.8 times show a monotonic increase in the crack speed with stretch and can be explained by a numerical model based on neo-Hookean theory and Kelvin dissipation. In specimens stretched more than 3.8 times, strain-induced crystallites act as reinforcing and toughening fillers and significantly increase fracture resistance, like nanostructures in other polymeric or biological materials. Consequently, as we increase the amount of stretch, fractures travel slower and slower, and eventually halt altogether.

Interaction of shear waves and propagating cracks.

Shear waves generated from an ultrasonic transducer are used to twist dynamically growing crack fronts; the response of crack front to such external perturbations is examined in order to investigate the primary cause of surface roughening in brittle materials. The response of the crack front is found to be linear in amplitude and frequency of the perturbing wave and without persistence. The response to random perturbations, e.g., by localized material inhomogeneities at the free surface, is also discussed.