The finite element analysis of composite laminates and shell structures subjected to low velocity impact

In this investigation, the composite laminate and shell structures subjected to low velocity impact are studied by the ANSYS/LS-DYNA finite element software. The contact force is calculated by the modified Hertz contact law in conjunction with the loading and unloading processes. In the case of composite laminate, the impact-induced damage including matrix cracking and delamination are predicted by the appropriated failure criteria and the damaged area are plotted. Two types of shell structure, cylindrical and spherical shells, are considered in this paper. The effects of various parameters, such as shell curvature, clamped or simple supported boundary conditions and impactor velocity are examined through the parametric study. Numerical results show that structures with greater stiffness, such as smaller curvature and clamped boundary condition, result to a larger contact force and a smaller deflection. The impact response of the structure is proportional to the impactor velocity.     

Porosity and mould filling of titanium castings

A vacuum/Ar pressure casting machine was used to study porosity and mould filling of cast bridges of Ti. Two series of experiments were carried out: (1) The melting and the mould chamber were separated by a thin Ti foil until penetrated by the Ti melt. The mould chamber including the mould cavity was evacuated. (2) No separating Ti foil was applied. The pressure in the mould chamber was then controlled by diffusion of Ar through the investment. The moulds were made of two investments with different gas permeability. A standardized wax pattern for a five-unit bridge was used. Mould filling was evaluated by visual inspection and by measuring the missing lengths of the margins of the crowns. The porosity was studied by x-ray radiography and density measurements. In the first series mould filling was satisfactory in all cases, while the porosity was substantial for high Ar pressures. Without a Ti foil (second series) all castings contained little porosity, but the mould filling was adequate only for the high permeability investment. The experiments show that porosity can be avoided by maintaining small pressure differences between the melting chamber and the mould cavity. Adequate mould filling is promoted by minimal back pressures from trapped Ar gas in the mould.     

Approximating a Robot Inverse Kinematics Solution Using Fuzzy Logic Tuned by Genetic Algorithms

A new scheme based on recursive fuzzy logic is presented in this paper for solving the point-to-point inverse kinematics problem of serial robots. To improve the convergence problem in the whole workspace, the membership functions of the fuzzy logic are searched for, tuned, and optimised using a simple genetic algorithm. A dominant joint, which brings the end-effect closer to the desired target, has to be selected before the implementation of the fuzzy logic in order to reduce the number of fuzzy logic iterations. The inverse kinematics solution of robots is usually obtained by direct inversion of the kimenatics equations, but this technique often leads to a singular Jacobian matrix during the calculations. The work presented in this paper provides a direct approach to the calculation of the kinematics inverse problem which bypasses the kinematic singularities. Computer simulations of the proposed scheme confirm the findings of the theoretical developments.     

Semi-natural Forested Landscape under a Bark Beetle Outbreak: a case study of the Bohemian Forest (Czech Republic)

Long-term and lasting human interference in forest ecosystems in the Bohemian Forest has resulted in the loss of some equilibrium mechanisms and hence increased susceptibility to ecological disturbances. Two methods for estimating bark beetle outbreaks in spruce forests are reported: an assessment of the susceptibility of individual trees to attack according to crown structure characteristics; and the use of remotely sensed data and image processing techniques to quantify changes in forest cover. The results document a significant increase in the area of highly damaged and dead forest classes, from 1% in 1987 to 16% in 1998, and a decrease in the area of healthy and moderately damaged spruce classes from 71% in 1987 to 52% in 1998. A possible approach to managing mountain Norway spruce ecosystems invaded by bark beetle in the Bohemian Forest is proposed.     

Magneto-Absorption in the α Phase of Solid Oxygen at Megagauss Magnetic Fields

Optical absorption spectra of solid oxygen α phase and liquid oxygen were measured in high magnetic fields over 100 T using a single turn coil method. The absorption originates from the bimolecular transition was observed. The spectra strongly reflect the local magnetic interaction between two oxygen molecules. The shape of the absorption spectrum of the α oxygen changed dramatically with increasing magnetic fields, even though the absorption intensity of liquid oxygen simply declined. This difference cannot be explained by only the local magnetic interaction, and we speculate that the field induced lattice distortion plays an important role in the solid oxygen α phase.     

Chemiluminescence‐Generating Nanoreactor Formulation for Near‐Infrared Imaging of Hydrogen Peroxide and Glucose Level in vivo

Water‐dispersed all‐in‐one nanoprobes composed of densely integrated peroxyoxalate fuel and a cyanine dye are formulated to optimize the nanoscopic chemiluminescence reaction. It is demonstrated that the chemiluminescent nanoformulation can generate bright near‐infrared signal in response to external hydrogen peroxide that is biologically implicated with cell signaling and diseases. Successful imaging of endogenously overproduces hydrogen peroxide and indirect determination of glucose level in vivo with the chemiluminescent nanoprobes offers an opportunity for early diagnosis of diseases.     

A novel design for a flow field configuration, of a direct methanol fuel cell

We proposed and tested a new and novel arrangement for a direct methanol fuel cell consisting of one inlet for a methanol solution and four outlets for oxidant gas (air), in both the anode and cathode flow fields. It utilizes different operating temperatures of 40 °C and 60 °C, and different methanol solution flow rates of 5 ml min⁻¹, 10 ml min⁻¹, and 20 ml min⁻¹. Test results indicate a significant reduction in produced CO₂ gas in the anode flow channels and product water in the cathode flow channels; consequently, cell performance can be greatly improved. Furthermore, methanol crossover can also be avoided and reduced.     

Resistance of European and hybrid aspen wood against two brown-rot fungi

The decay resistance of European (Populus tremula L.) and hybrid (P. tremula x tremuloides) aspen wood against brown-rot fungi was investigated after three different treatments, i.e. conventional drying, press drying and heat treatment. For both aspen species, the mass loss after exposure to Gloeophyllum trabeum was higher than after exposure to Coniophora puteana, regardless of the wood treatment. Conventionally and press dried aspen wood had similar mass losses. However, heat treatment appears to be an effective method to improve the decay resistance of aspen wood, reducing the mass loss by about 30% compared with conventional and press drying. As a function of mass loss, wood exposed to G. trabeum had lower moisture content than wood exposed to C. puteana. This is thought to be due to differences in the degradation pattern between the two brown-rot fungi. On the other hand, European aspen appears to be slightly more resistant to decay than hybrid aspen.     

Stress analysis of adhesively-bonded lap joints

The adhesively-bonded joints considered in this investigation include single-lap joint and double-lap joint. A simplified one-dimensional model based on the classical elasticity theory is presented. The shear deformation in the adhesive is assumed constant across the adhesive thickness. The analytical solutions of shear stress in the adhesive and longitudinal stress in the adherend are obtained and compared with the numerical solutions determined by the two-dimensional finite element method.