Proof Complexity of Modal Resolution

Journal of Automated Reasoning - We investigate the proof complexity of modal resolution systems developed by Nalon and Dixon (J Algorithms 62(3–4):117–134, 2007) and Nalon et al. (in:...     

The Mechanical Properties of Dry, Electrospun Fibrinogen Fibers

Due to their low immunogenicity, biodegradability and native cell-binding domains, fibrinogen fibers may be good candidates for tissue engineering scaffolds, drug delivery vehicles and other medical devices. We used a combined atomic force microscope (AFM)/optical microscope technique to study the mechanical properties of individual, electrospun fibrinogen fibers in dry, ambient conditions. The AFM was used to stretch individual fibers suspended over 13.5 μm wide grooves in a transparent substrate. The optical microscope, located below the sample, was used to monitor the stretching process. Electrospun fibrinogen fibers (diameter, 30-200 nm) can stretch to 74% beyond their original length before rupturing at a stress of 2.1 GPa. They can stretch elastically up to 15% beyond their original length. Using incremental stress-strain curves the viscoelastic behavior of these fibers was determined. The total stretch modulus was 4.2 GPa while the relaxed elastic modulus was 3.7 GPa. When held at constant strain, fibrinogen fibers display stress relaxation with a fast and slow relaxation time of 1.2 s and 11 s.In comparison to native and electrospun collagen fibers, dry electrospun fibrinogen fibers are significantly more extensible and elastic. In comparison to wet electrospun fibrinogen fibers, dry fibers are about 1000 times stiffer.     

Fate of infected and eroded hemodialysis grafts and autogenous fistulas

Between 1970 and 1991, we placed 1,090 grafts (bovine and polytetrafluoroethylene) in 1,041 patients and created 1,034 autogenous fistulas in 856 patients for hemodialysis. Subsequent revisions for complications resulted in a total of 3,944 operations performed in patients with grafts and 1,633 operations in patients with autogenous fistulas. A total of 255 infections developed in 158 of the patients with grafts, whereas 8 infections developed in 7 patients with autogenous fistulas. The puncture infection rate was 5%/yr (12%/yr for a second puncture infection). The clean wound infection rate was 3% for grafts and 0.4% for autogenous fistulas. We made an attempt to salvage the graft, usually with a segmental bypass, in 75% of patients with a graft infection. Grafts were salvaged in 80% of patients in whom salvage was attempted (60% of all patients with an infection). The results in the few patients with infected autogenous fistulas were relatively poor.     

Tensile failure of pultruded glass-polyester composites under superimposed hydrostatic pressure

The tensile properties of five types of pultruded 0·52 V_f glass-fibre-polyester rods were investigated by extending waisted round specimens at atmospheric and superimposed hydrostatic pressures, −H, to 300 MPa. The maximum principal stress at fracture, −700 MPa, decreased, with the superimposition of −H, approximately by its magnitude. As −H increased the failure surfaces became flatter, the amount of fibre pull-out decreased and transverse cracks became shorter or were eliminated. Glass fibres in the failure surfaces were resin free, and failure of the glass fibre bundles appeared to control the fracture process in the entire pressure range for all materials. The decrease in maximum principal tensile stress with increasing −H indicates that the glass fibre failure process is not controlled by a critical tensile stress. Failure criteria are discussed, and in the tension-compression-compression octant of stress space the relevant criteria appear to be strain energy and deviatoric tensile stress, strain and strain energy for these GRPs and glass itself.     

Three-parameter yield criterion for a brittle polyester resin

The tensile and compressive strengths of three polyester resins were measured under superposed hydrostatic pressure extending to 300 MPa, in an attempt to establish yield criteria. The polyesters were brittle in uniaxial tension at all pressures, and accordingly, a third testing geometry, diametral compression of a disc, was employed to complete the two or three necessary parameters to establish the yield surface in stress space. From the biaxial (disc) and axial compressive test data, the atmospheric tensile yield strength (higher than the fracture strength) was computed to be ～67 MPa in comparison with the compressive strength of ～120 MPa, their ratio 0.56 being significantly less than the more common 0.75 found for thermoplastics and epoxides. The data for compressive yield strength under superposed pressure were compared with the predictions of the two-parameter pyramidal, conical and paraboloidal criteria and the fit, though reasonable for the latter, could be significantly improved if a further independent material parameter was employed to give a three-parameter pyramidal criterion (the principal stresses σ₁, σ₂ and σ₃ being measured in MPa) of the form 0.0150σ₁?0.0039σ₂?0.0083σ₃=1