Experimental and numerical analysis of the deformed process of cracks/cavities

In this paper, the three-point bending tests of beams with a small single-edge crack are conducted to demonstrate the reliability and accuracy of the laser diffraction technique in detecting its deformation and evolution. As a comparison, the quasi-static analysis of the beam subjected to 3-point bending is further proceeded using the finite element method and the related results have been in good agreement with those from the tests.

Polyethyleneterephthalate provides superior retention of endothelial cells during shear stress compared to polytetrafluoroethylene and pericardium

Background

Polyethyleneterephthalate (PET) and polytetrafluoroethylene (PTFE) are polymers successfully used as large diameter arterial grafts for peripheral vascular surgery. However, these prosthetic grafts are rarely used for coronary bypass surgery because of their low patency rates. Endothelialisation of the lumenal surface of these materials may improve their patency. This study aimed to compare the endothelialisation of PET, PTFE and pericardium by examining their seeding efficiency over time and the effect of various shear stresses on retention of endothelial cells.

Methods


Ovine endothelial cells at 4 × 105 cells/cm2 were seeded onto PET, PTFE and pericardium, and cultured for 1–168 hours. Cell coverage was determined via en face immunocytochemistry and cell retention was quantified after being subjected to shear stresses ranging from 0.018 to 0.037 N/m2 for 15, 30 and 60 minutes.

Results

Endothelial cells adhered to all of the materials one hour post-seeding. PET exhibited better cell retention rate, ranging from 66.9 ± 5.6% at 0.018 N/m2 for 15 min to 44.7 ± 1.9% at 0.037 N/m2 for 60 minutes, when compared to PTFE and pericardium (p < 0.0001, three-way ANOVA).

Conclusion

PET shows superior retention of endothelial cells during shear stress compare to PTFE and pericardium.

Shear-free perfect fluids with a solenoidal magnetic curvature

We investigate shear-free perfect fluid solutions of the Einstein field equations where the fluid pressure satisfies a barotropic equation of state and the spatial divergence of the magnetic part of the Weyl tensor is zero. We prove, with the exception of certain quite restricted special cases within the class of solutions in which there exists a Killing vector aligned with the vorticity and for which the magnitude of the vorticity ω is not a function of the matter density μ alone, that such a fluid is either non-rotating or non-expanding. In the restricted cases the equation of state must satisfy an over-determined differential system.

Adhesion performance of polymer nanofibres under shear loading

Geckos have extraordinary wall-climbing ability because of the millions of hairs with micro/nano fibrillar structures on their feet. Mimicking gecko's feet is of scientific and engineering importance for development of physical adhesion materials and devices. The design of gecko-inspired physical adhesives seems to be geometry dominated. In this study, Finite Element Method (FEM) has been used to analyse the vertical peel-off force of polyporpylene (PP) nanofibres having different fibre dimensions, inclining angels and contact areas on a flat glass substrate. It has been found that the main parameters affecting the frictional adhesion are fibre diameter and fibre aspect ratio, the inclining angle between the fibre and the substrate surface, and the intimate contact areas. Our analysis has shown that PP nanofibres with a diameter of less than 200nm can generate less peel-off force than fibres of larger diameters, indicating more stable adhesion with the glass substrate for thinner fibres. A bent fibre with more intimate contact area can bear more shear force than a straight fibre with less contact area. Also, under the same shear loading, fibres with an inclining angle of less than 30° provide a low peel off force.

Field test study on the shear behaviour of stressed skin diaphragm spanning several purlins

Although profiled steel sheets have the diaphragm effect in steel structure, since there is no specific code to follow in China, it is only used as the reservation of the structural stiffness.So it results in economic wastes.In order to investigate the contribution of stressed skin to the overall stiffness, field tests are carried out to measure the lateral stiffness of the frames with and without stressed skin diaphgram.Based on the superposition principle and using the curve fitting method, the test results are quantified.The shear rigidity of stressed skin diaphgram spanning several purlins is gained indirectly.It shows that the shear rigidity of the diaphragm between two frames is even close to the lateral stiffness of one frame.

Development of adiabatic shear band in cold-rolled titanium

The evolution of adiabatic shear localization in commercial titanium subjected to heavy cold rolling was investigated. The evolution of the morphology, microhardness, local shear strain, and local temperature increments were systematically studied and estimated. A shear band with about 25m in width was formed and fine nanograins with a range of dimensions varying from 20 to 160nm and had a mean size of about 70nm were observed inside the centre of shear band after 83% cold-rolling. Microhardness test shows that hardness within the shear band is markedly higher than that of the surrounding matrix. The calculated shear strain and maximum temperature increase within the shear band are much higher than that of the overall deformed sample. The initiation of shear localization may depend on geometric perturbation instead of thermal ones.

Interfacial energy states of moisture-exposed cracks in mica

Results of crack growth observations on mica in water-containing environments are described. The study focuses on equilibrium crack states for reversed loading cycles, i.e., for initial propagation through virgin solid and subsequent retraction-repropagation through healed or misoriented-healed interfaces. Departures from these equilibrium states are manifest as steady-state forward or backward crack velocities at specific applied loads. The equilibria are thereby interpreted as quiescent, threshold configurations G = W_E, with G the Griffith mechanical-energy-release rate and W_E the Dupré work of adhesion, on crack velocity (v-G) diagrams. Generally, W_E is found to decrease with concentration of water, in accordance with a Gibbs formalism. Hysteresis is observed in the forward-backward-forward crack propagation cycle, signifying a reduction in the adhesion energy on exposure of the open interface to environmental species prior to healing. This hysteresis is especially marked for those interfaces that are misoriented before healing, indicating that the structure of the underlying solid substrate as well as of the intervening fluid is an important consideration in the interface energetics. The equilibrium states for different environments can be represented on a simple energy-level diagram, as differences between thermodynamic end-point states: initial, closed-interface states refer to crystallographic bonding configurations ahead of the crack-tip adhesion zone; final, open interface states refer to configurations behind the crack-tip zone. The significance of this diagram in relation to the fundamental atomic structure of interfaces in fracture and other adhesion geometries, including implications concerning kinetics, is discussed.

Pressurized internal lenticular cracks at healed mica interfaces

The equilibrium states of internal penny cracks at interfaces in thin-sheet bodies are investigated. Consideration is given to cracks held open by a center-loading force from an entrapped particle in combination with a uniform pressure from a fixed mass of entrapped gas. A fracture mechanics analysis indicates that under these conditions the cracks are stable, but are amenable to growth from an enhancement in net pressure (increase in internal pressure or decrease in external pressure) or effective particle size. Essential details of the theory are confirmed by experiments on lenticular cracks at healed interfaces in muscovite mica. The results are pertinent to flaw responses in brittle ceramic systems where structural integrity is an issue.

Repulsive interaction between coplanar cracks in the double–cantilever geometry

Experiments on thin mica sheets are used to demonstrate that coplanar cracks in double-cantilever beam specimens do not universally attract each other, as conventionally portrayed, but, at long range, actually repel. An elasticity analysis explains the repulsion in terms of a compression zone, 0.35 times the beam half-thickness ahead of the crack tip, generated by bending moments from the cantilever arms on the remaining specimen section.