Wechselbiegeapparatur für die in situ-Untersuchung von Zerrüttungsvorgängen im Rasterelektronenmikroskop

An SEM stage (Fig. 1 and 2) is described which allows bending fatigue of metallic specimens in the vacuum of the specimen chamber. With the aid of inductive displacement pick-ups the angle of bending can be measured (Fig. 3) as well as the momentum of bending (Fig. 4). Thus the mechanical hysteresis can be registrated and the fatigue of the specimen can be controlled. Fig. 5 shows the geometry of the fatigue specimen, which has an electrolytically polished surface area of about 6 × 3 mm² on which the development of deformation topography can be studied during the fatigue experiment. The main advantages of the new fatigue apparatus are:

• 1 No interruption of cyclic straining is necessary for the investigation of the specimen surface.
• 2 The specimen is in vacuum during the complete experiment so that environmental influences are reduced.
• 3 The development of surface topography can be monitored continuously.
• 4 Differences of the topography (e. g. fatigue cracks) in the phase of tension and compression can be detected.

In the case of fatigue of polycrystalline metals, fatigue cracks often start at the surface of the specimen in slipbands produced by the cyclic straining. Fig. 6 shows typical fatigue slipbands with extrusions and intrusions for pure Al. A slipband crack in a later stage of fatigue is shown in Fig. 7. For the investigation of the distribution of fatigue cracks at the surface the possibility of watching the surface in different phases of bending is a great help. The cyclic crack opening – which depends on the crack depth – can be seen by eye on the TV-screen and can be recorded on video tape when the frequency of bending is about 1 cycle/s or can be photographed from the record display at very low frequencies of bending. Fig. 8 gives an example of the difference in crack opening in the phase of tension and compression of the specimen surface respectively.     

Acrylamide-occurrence in mixed concentrate feed for dairy cows and carry-over into milk

During the preparation of cooked foods acrylamide is formed from asparagine and reducing sugars at high temperatures. By-products of oil, starch and sugar production, which may be found in animal feed, partially result from processing steps using heat treatment that are similarly likely to form acrylamide. Possibly, pelletizing during the processing of mixed concentrates may also be involved in acrylamide formation. Thus the occurrence of acrylamide in animal feed and the potential for carry-over into animal products should be tested. Independently of the feed matrix, 1.5 g per day of acrylamide was fed to a cow for ten days resulting in a mean concentration of 175 µg/kg of acrylamide in the milk. From the data obtained the mean carry-over was found to be 0.24%, and a mean half-life time of 2.8 h was estimated. This means acrylamide was rapidly transformed in the cow. The acrylamide concentrations in three commercial mixed concentrates were respectively 180, 145 and 140 µg/kg feed. To test the possible effect of pelletizing, the peripheral zones were separately analysed. No difference in concentration was observed. Based on the carry-over rate estimated in this study, a maximum concentration of approx. 0.2 µg/kg of acrylamide would be expected in milk from cows fed with such feeds.     

Tribological characterization of thin hard coatings by reciprocating sliding tests

Thin hard coatings on metal or ceramic surfaces offer a large spectrum of improvements of the friction and/or wear behaviour of tribosystems. The development of coatings and the tailoring of their properties require test methods providing information about their friction and wear behaviour. A new wear test standard (ASTM) is under development for the evaluation of friction and wear quantities for sliding motions using the reciprocating sliding mode. The applicability of this test method to coated specimens was checked by testing uncoated and coated steel specimens in contact with alumina balls, whereby lower loads were used than in the ASTM proposal for bulk materials. Additionally, the influence of the relative humidity of the surrounding air at room temperature on friction and wear results was examined.     

Influence of grain refinement on the high temperature fretting behaviour of IN 738 LC

The wear and friction behaviour of Inconel 738 LC in contact with SiSiC was studied for the case of oscillating sliding motion. The test temperature was varied in the range from room temperature up to 700 °C. A large-grain IN 738 LC and a grain-refined modification were compared. In both cases, the wear dropped drastically at temperatures above 400–500 °C and remained low for the grain-refined modification, but increased again for the large-grain modification at temperatures above 600 °C. The high wear/low wear transition was accompanied by a transfer of metal on the ceramic surface.     

Microstructural Characterization of Wear Particles Formed During Tribological Stressing of TiC and Ti(C,N) Coatings

An attempt was undertaken to obtain a better understanding of the tribological properties of two wear-resistant coatings on tool steel by structural and microchemical analysis of wear particles using a transmission electron microscope. Coatings were deposited by physical vapor deposition and plasma-assisted chemical vapor deposition techniques and tribological properties were derived from reciprocating sliding tests of the coatings against alumina balls. Three types of wear particles were identified by electron diffraction and energy dispersive X-ray spectroscopy: nanocrystalline rutile (TiO₂), nanocrystalline graphite and microcrystalline graphite. Low coefficients of friction, of the order of 0.2, were attributed to the formation of solid lubricant films of sub-stoichiometric TiO_2-x Magnéli phases and/or graphite.     

Influence of Temperature and Relative Humidity on the Friction and Wear of Unlubricated Reciprocating Sliding Steel/ Steel Couples

Many tribosystems are subjected to different conditions with respect to temperature and humidity. Reciprocating sliding tests with steel/steel couples were performed using a laboratory test rig in air with varying relative humidity and temperature. During each test the friction force, the total linear wear, the electrical contact resistance, and the acoustic emission were recorded. Tests with self‐mated couples of bearing steel (100r6) and of stainless steel (X10CrNiMoNb18‐10/X5CrNi18‐9) in a ball‐on‐disc arrangement revealed small effects of temperature and humidity on friction, but a strong effect of the water vapour content on the wear rate of the system. Attempts were made to correlate changes of wear behaviour with different wear mechanisms.     

Variable-sample-rate delayless frequency-adaptive digital filterfor synchronized signal acquisition and sampling

In this paper, a hardware-based variable-sample-rate delayless frequency-adaptive cascaded digital filter is proposed. The open-loop frequency-adaptive and delayless property of a cascaded filter consisting of a median filter of length 3, a Blackman bandpass filter of length 30 and a predictive filter of length 25 is experimentally verified. Simulation results illustrating the filter's output response to input disturbances, in a synchronization signal acquisition application, are also presented     

Tribological behaviour of SiC?TiC?TiB2 composites under unlubricated sliding conditions

The friction and wear behaviour of eight different SiC TiC TiB₂ composite materials, with a practically constant SiC:TiC ratio of 1 and an increasing amount of TiB₂, was determined comparatively in oscillating sliding tests at room temperature under unlubricated conditions. The influence of the relative humidity (RH) of the surrounding air was investigated in tests in dry, normal, and moist air. All tests were performed against SiC balls and Al₂O₃ balls as counterbodies. The friction was affected by RH but barely at all by the composition of the composites. The wear resistance of the composites was found to be improved considerably by addition of TiB₂ in the range 20–60%. The highest wear resistance of the system was found when Al₂O₃ was used as the counterbody material.     

Laboratory fretting tests with thin wire specimens

Wire ropes, due to their construction and application, are prone to fretting damages. In order to know the wear behaviour of individual wires under fretting conditions, laboratory tests are required. The present work describes the preliminary fretting tests accomplished with thin steel wires to optimise the testing procedure. The tests were carried out with ‘crossed‐cylinders’ configuration varying the stroke and normal load. Afterwards, the fretted surfaces were characterised by means of an optical and scanning electron microscope, and a diamond stylus. No significant influence of selected parameters was detected and a good correlation was proved for on‐line measured parameters and off‐line obtained values. Copyright © 2007 John Wiley & Sons, Ltd.