Specific aspects on crack advance during J-test method for structural materials at cryogenic temperatures

Cryogenic elastic plastic, J‐integral investigations on metallic materials often show negative crack extension values with respect to resistance curve J‐R. According to the present ASTM standard, the use of unloading compliance technique relies on the estimation procedure of the crack lengths during the unloading sequences of the test. The current standard, however, does not give any specific procedure for treating such negative data. To date, the applied procedure uses the shifting of the negative crack extension values either to the onset of the blunting line or to the offset of the resistance curve. The present paper represents a solution of the negative crack length problem on the basis of a mechanical evaluation procedure of the unloading slopes. The achieved progress using this evaluation technique is demonstrated on different materials such as cryogenic high toughness stainless steels, low carbon ferritic steel and aluminum alloys from the series of 7000 and 5000. In addition, this work deals with the crack tunnelling phenomenon, observed for high toughness materials, and shows the reduction of this crack extension appearance by using electro discharge machining (EDM) side groove technique. The differences between EDM processed side grooves and standard V‐notch machining have been investigated within these test series.     

HIGH CYCLE FATIGUE AND THRESHOLD BEHAVIOUR OF POWDER METALLURGICAL Mo AND Mo-ALLOYS

Abstract A detailed characterization of the room temperature fatigue properties of powder metallurgical Mo, Mo–W and Ti–Zr–Mo (TZM) alloys is presented. In particular the factors affecting fatigue crack nucleation and growth behaviour are described. Fatigue tests were carried out by conventional rotating-bending and compared with results from a time-saving 20 kHz resonance push-pull test method. Fatigue strength data were determined by a statistical evaluation of test results from a sufficiently large number of specimens. The results show an increase in fatigue strength with alloying additions. Fatigue cracks were observed nucleating at highly localized slip bands at the specimen surface with the fatigue crack zones comprising only a small fraction of the total specimen cross-section. Fatigue crack growth rates at low stress intensities and threshold stress intensity values for crack growth were determined for a stress ratio of R =– 1 using a 20 kHz resonance test method. These latter values were found to be sensitively dependent on microstructure, composition and processing history.     

FINITE ELEMENTS FOR MAGNETOSTATIC FIELDS WITH DISTRIBUTED CURRENT DENSITIES

ABSTRACT

This paper presents a general finite element method for the solution of fields due to general three-dimensional vector-valued sources in terms of the magnetic scalar potential, between boundaries of axisymmetric shape. In the space occupied by the sources, and there only, a correction field defined by a vector quantity must be added. This correction field is obtained by using a vector potential ā, subject to the Coulomb convention. The latter convention may be relaxed to allow ā to be divergenceless in the mean. In many magnetic field problems, the sources (current-carrying coils), occupy only a small part of the problem region. Since the correction field is non-zero only in the current-carrying space, it is computationally relatively cheap to find. Use of the procedure is illustrated by the end winding model of an electric machine.     

Saturation Factors for Calcium Hydroxide and Calcium Sulfates in Fresh Portland Cement Pastes

Aqueous extracts from five cement pastes at w/c = 0.5 were analyzed for Ca²⁺, SO^2–₄, Na⁺, and K⁺. Degrees of saturation with respect to portlandite, gypsum, and syngenite were determined at ages from 6 min to 3 h, taking into account ionic association effects. Portlandite saturation is usually attained within a few minutes of mixing, but rates of saturation vary. The maximum degree of saturation is 2 to 3 times the equilibrium value and is attained within 2 h. Initial super-saturation with respect to gypsum, and in some cases syngenite, occurs at very early ages and declines to the saturation level within 12 min. Results are interpreted by use of a solution-precipitation model for cement hydration. The calculated critical size of portlandite nuclei at early ages is ∼0.1 μm. Slow growth of portlandite nuclei suggests a surface poisoning phenomenon.     

Dual Dynamical Systems and their Representation by System Functions†

The system function concept is an extension of the usual application of the transform method to linear systems. In this paper it is shown that there are two system functions which can be naturally associated with a given system and that each one can be interpreted in two ways. These alternate interpretations are used to show that the same system function provides a means for calculating the time-domain output of a given system and the transform-domain output of the dual system (defined as the adjoint of the given system with reversal of the time scale ordering).     

Proteolytic activity among psychrotrophic bacteria isolated from refrigerated raw milk

Psychrotrophic bacteria were isolated from refrigerated raw milk from a processing plant in Southern Brazil. Psychrotrophic counts were between 4.9 and 7.8 log cfu/mL, and 5.3 to 7.2 log cfu/mL, for samples collected at the truck and the milk storage silo, respectively. Among the bacterial isolates, 90% were Gram-negative. Most strains presented low proteolytic activity, but strains of Burkholderia cepacia, Klebsiella oxytoca and Aeromonas sp. showed higher than 20 U/mL on azocasein as substrate. Crude proteases from selected strains were resistant to conventional heat treatments and caused coagulation of UHT milk after 5 days storage at room temperature.