Pentti Kettunen

Carbon-carbon composite

Abstract A problem in the production of carbon-carbon composites from carbon fiber reinforced thermoset composites through pyrolization is the cracking of the matrix due to internal stresses caused by the large shrinkage of the pyrolizing matrix and the simultaneous expansion of the fibers. Subsequent infiltration by carbon or silicon is limited to cracks open to the surface and will not reach the closed cracks in the interior. The present paper illustrates improvements as applied to the case of unidirectional fiber reinforcement capable of producing carbon-carbon composites without cracking the matrix and furthermore getting the matrix into amorphous state. As a result of this, at room temperature the composite shows the strength of HSLA steel together with reasonable pseudo ductility. At 1500 °C, the strength reaches the values of 500–1000 N mm −2 . As the density is only 1.25–1.35 g cm −3 , the specific strength of the composite is very high.

Densification of plasma-sprayed titanium and tantalum coatings

Thermal spraying of corrosion- resistant coatings of titanium and tantalum is difficult; dense coatings are not produced, and oxidation of these metals increases coating porosity. In this study, oxidation during plasma spraying was reduced with a shrouding system. Porosity and oxide content also were minimized by optimizing the spraying parameters. After optimization, the coatings still had open porosity and thus were incapable of protecting the substrate material against corrosion in water solutions containing 3 % NaCl. Therefore, posttreatments for improvement of corrosion resistance were studied. Electron beam fusion produced corrosion resistance equal to or better than that of bulk commercial samples of titanium and tantalum.

Reversible component ΔBr of the stress-induced change in magnetization as a function of magnetic field strength and stress amplitude

Abstract The reversible component Δ B r of the stress-induced change in the magnetic flux density in polycrystalline iron and nickel was studied during cyclic stressing as a function of the magnetic field strength H and the stress amplitude σ. The frequency of the stressing was 25 Hz. Test specimens were magnetized with a magnetic field parallel to the stress axis. In iron the reversible change Δ B r has three different stages A, B and C as a function of the applied magnetic field strength H . In demagnetized state in the beginning of stage A the change Δ B r in the magnetic flux density is zero and, as the magnetic field strength H increases, Δ B r increases and reaches a maximum at a magnetic field strength H 1 corresponding to the knee point of the initial hysteresis curve of the test material. In stage B the reversible component Δ B r decreases and reaches a minimum value at a magnetic field strength H 2 corresponding to the onset of magnetic saturation. At the magnetic saturation the change δ B r in the magnetic flux density remains low. In nickel Δ B r has only two stages A and B. In stage A the reversible component Δ B r increases and in stage B the component Δ B r decreases as a function of H . The differences in Δ B r in iron and in nickel are explained on the basis of the different magnetocrystalline anisotropy energies in these two materials. The observed nonlinear behaviour of the change in magnetization as a function of stress amplitude is explained on the basis of plastic deformation.

A method for measuring particle velocity in thermal spraying

Abstract A mechanical method was established to determine the particle velocity in plasma spraying processes based on rotating slotted double cylinders or discs. The particle velocity was found to be a function of the mass of particles penetrating through the slots at different rotating speeds. The results obtained revealed a close agreement with those measured by the laser Doppler anemometry method and scanning confocal Fabry—Perot interferometer. The method provides a simple and accurate way of detecting the particle velocity in thermal spray processes. In addition, the method appears to be a low cost way of establishing measurement unit and high endurance in thermal spraying under such adverse conditions. This may lead to a wide application of the method on an industrial scale.

Effect of cyclic straining of the harmonic amplitude spectrum of magnetoelastically generated voltage uB

Abstract The amplitude spectrum of the magnetoelastically induced voltage u B was studied during cyclic stressing of polycrystalline iron and nickel. Test specimens were loaded with a sinusoidically varying stress with a frequency of 25 Hz. The amplitude spectrum of voltage u B is a typical harmonic spectrum consisting of numerous harmonic components. The fundamental frequency of the spectrum was 25 Hz, the same as the loading frequency. The amplitude of the high frequency components of the spectrum increased in the beginning of stress cycling and decreased towards the fracture of the test specimen. This indicates that the cyclic plastic deformation has an effect on the micromagnetization of the material. An increase in the amplitude of the harmonics were observed even at stress amplitudes where no plastic deformation can be measured with an extensometer. The present results show that the magnetic measurements can be used to study the changes in the material microstructure and fatigue caused by cyclic stressing.

Small-angle neutron scattering from fatigued copper single crystals

Abstract Copper single crystals oriented for single and multiple slip were studied by small-angle neutron scattering (SANS) after fatigue tests at a constant plastic shear strain amplitude y γ pl = 0.15% . For the SANS measurements, a special sample holder was used in order to obtain more reliable information about the small differences in the neutron scattering from individual samples. The size of the voids in the samples oriented for single slip increased as a function of the number of fatigue cycles, varying from 0.74 nm to 0.91 nm, but the volume fraction remained more or less constant. The dislocation density observed by SANS was found to increase from 6 × 107 mm−2 at τmax/2 to 4 × 108 mm−2 at τmax, but remained constant in the saturation stage of cyclic strain hardening. The SANS results obtained are in good agreement with positron annihilation data and transmission electron microscopy observations from the same specimens.

Dependence of the high-frequency components of the magnetoelastic voltage on plastic deformation and stress amplitude during stepwise cyclic stressing of ferromagnetic material

Abstract The high-frequency components F H of the magnetoelastic voltage u B induced during cyclic stressing of ferromagnetic material has been investigated. The true root mean square value B M of the high-frequency components was measured as a function of stress amplitude σ. The effect of strain ageing on the behaviour of B M was also studied. It was found that B M depends on the stress amplitude, plastic deformation, the dislocation arrangements and the mobility of individual dislocations. During successive stepwise increases in the stress amplitude it was found that the B M value of F H increases in the elastic and microplastic deformation range. B M reaches a maximum at a stress amplitude corresponding to the onset of the macroscopic plastic deformation Δepl. In technically pure iron the onset of the macroscopic deformation range Δepl was measured to be 0.5 × 10−4. The B M value of the high-frequency components F H decreases when the stress amplitude exceeds the value σB corresponding to the onset of ...

Softwood degradation after being several thousand years in shore front mud

Two man-made wooden objects, called here a “boat” and an “outrigger”, were found above one another in 2004 and 2006 in Kuusamo, Finland. A layer of shore front mud and turf covered them entirely. With partial excavations, samples were taken for age determination and structural studies from both of the items. As measured by the radiocarbon method with a probability of 95.4 %, the items appear to be from periods 5480 – 5300 CalBC and 5025 – 4770 CalBC for the boat and the outrigger, respectively.

Background to the topics concerned in the present papers

The present background description concentrates on the structure of wood xylem, on the mechanical strength properties under monotonic external loading, on the degradation of the structure of wood and somewhat also on the influence of thermal treatment (heat treatment) on the structure and properties of wood. The presentation is largely based on a recent publication of the author /1/. In this manner, the description tries to illustrate the status of the present general knowledge in the fields, into which the present papers bring new results and ideas for a better understanding of the structure and behavior of wood.

Monotonic Strength Properties of Siberian Yellow Pine

Strength values of the sapwood of Siberian yellow pine were measured in a system with orthogonal coordinates along the axial, radial, and tangential directions of the cell structure. Highest strength was the axial normal strength and lowest the tangential normal strength. The difference between these two values was 87-fold. Shear strength values remained between the two normal strength values. The highest shear strength appeared in tangential direction across the reinforcing fibers, i.e., on the plane perpendicular to the axial direction. Lowest shear strength appeared in tangential direction on the plane perpendicular to radial direction. The variations are due to orientation of cells and of fiber reinforcement in the cell wall laminas, especially in the middle layer of the secondary cell wall.