The effect of periodic-random loading on fatigue crack growth

A theory of fatigue crack growth based on the concept of damage accumulation is presented which takes some account of the effect of periodic-random loading. The Dugdale model of plasticity is used to calculate the distribution of the energy dissipated during stress cycling in the plastic zones of a crack embedded in a material sample of infinite extent. It is shown how to calculate the damage accumulated by decomposing the random group of stress levels into significant complete stress cycles of various amplitudes. A simple short numerical algorithm is presented which performs this decomposition. A crack growth law is derived having a very simple form which automatically incorporates the condition for catastrophic failure.

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Résumé On présente une théorie sur la croissance des fissures de fatigue basée sur le concept du dommage cumulatif, et tenant compte de l'effet de mise en charges aléatoires. On recourt au modèle de plasticité de Dugdale pour le calcul de la distribution de l'énergie dissipée lors du cycle de contrainte dans les zônes plastiques d'une fissure noyée dans un matériau de dimension infinie.On montre comment calculer le dommage cumulatif en décomposant les blocs de contraintes de niveau aléatoire en collectifs de contraintes cycliques d'amplitudes variables. Un algorithme numérique simple est présenté pour effectuer cette décomposition. On en tire une loi de propagation de la fissure dont la forme est très simple et qui comporte automatiquement les conditions susceptibles de conduire à une rupture brutale.

     

Specific allograft reactivity in the sea star Dermasterias imbricata

Previous work in our laboratories and at other institutions has shown that fluoroscopic images recorded on a video disc can be used successfully for producing computerized-axial-tomograms. The work described in this paper gives a quantitative analysis of the capabilities of such imaging systems, in conjunction with a particular method of data processing, for detecting and imaging changes in object absorptivity. Relations between the degree of contrast or absorptivity and object size required by this type of system can be inferred from the data.     

The lipid composition of plasma membrane subfractions originating from the three major functional domains of the rat hepatocyte cell surface

1. The neutral and phospholipid compositions of three rat liver plasma membrane subfractions originating predominantly from the three major functional domains of the hepatocyte viz the blood sinusoidal, contiguous and bile canalicular fractions, were determined. 2. The sinusoidal and canalicular plasma membrane subfractions, both of which were vesicular, contained a higher lipid to protein weight ratio than the contiguous plasma membrane subfraction that consisted of membrane strips, junctional complexes and some larger vesicles. The three plasma membrane subfractions contained a similar neutral lipid to phospholipid ratio. The highest unesterified cholesterol content was associated with the canalicular plasma membrane subfraction. 3. The phospholipid profiles of the three subfractions were generally similar. However, the canalicular plasma membrane subfraction contained a higher proportion of sphingomyelin than the other subfractions. 4. Correlations between the neutral and phospholipid composition of the subfractions and membrane integrity and function are discussed, especially with respect to a possible role of lipids in governing the resilience of the canalicular plasma membrane to the action of bile salts.     

Quantitative phase-sensitive imaging in a transmission electron microscope

This paper presents a new technique for forming quantitative phase and amplitude electron images applicable to a conventional transmission electron microscope. With magnetised cobalt microstructures used as a test object, we use electron holography to obtain an independent measurement of the phase shift. After a suitable calibration of the microscope, we obtain quantitative agreement of the phase shift imposed on the 200 keV electrons passing through the sample.     

Within-child associations between family income and externalizing and internalizing problems.

Within-child associations between family income and child externalizing and internalizing problems were examined using longitudinal data from the NICHD Study of Early Child Care and Youth Development (2004a, 2004b; N = 1,132). Variations in income effects were estimated as a function of whether families were poor, whether mothers were partnered, and the number of hours mothers and their partners were employed. On average, children had fewer externalizing problems during times when their families' incomes were relatively high than during times when their families' incomes were relatively low; the estimated benefits of increased income were greatest for children who were chronically poor. For both externalizing and internalizing problems, income was most strongly associated with problems when chronically poor children's mothers were partnered and employed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Testing associations between young children's relationships with mothers and teachers.

The current study was conducted to examine associations between children's maternal attachments and the quality of their relationships with teachers at 3 time points. The quality of the teacher-child relationship at 54 months, kindergarten, and 1st grade was regressed on blocks of predictors that included maternal attachment at 15, 24, and 36 months. There were 4 main findings. First, insecure/other attachment was a negative predictor of the quality of the teacher-child relationship at all 3 time points. Second, avoidant attachment was a negative predictor of the quality of the teacher-child relationship at 54 months. Third, in kindergarten, avoidant children in more hours of school had better quality relationships with teachers than avoidant children in fewer hours. Fourth, there were associations among the quality of the teacher-child relationship at all 3 time points. Implications for practice are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Rheological investigation of shear-induced structure changes in multiblock copolymers

Styrene-butadiene multiblock copolymers were examined with both newly introduced and established rheological techniques and by transmission electron microscopy (TEM) to evaluate shear-induced structural changes in these polymers. Transient rheological tests (based on superposed flow principles) were developed which probed structural changes that occur in the copolymers during and at the cessation of steady shear. Data from these tests indicated that for the cylindrical morphology copolymer (SB1) there were structural changes occurring during steady shear that were recovered upon cessation of shear. The recovery process took place on time scales that could be significant in processing. The lamellar morphology material (SB2) did not exhibit this recovery behavior. Longer-term structure changes were investigated using established techniques and showed differences between the cylindrical and lamellar copolymers. When tested at 210°C, peaks in tan δ occurred at 30 rad/s for SB1 and at 0.5 rad/s for SB2 with saturation strain levels of 150 strain units (SU) for SB1 and 80 SU for SB2. TEM analysis of SB2 indicated that, although rheological changes are significant up to 80 SU, better alignment of the domain morphology can be achieved at moderately low strains of 7 SU. This indicates that the copolymers' rheological changes, which occur as a result of steady shearing, may not be due entirely to domain alignment, but may also be due to more local molecular rearrangements (e.g., chain disentanglement). © 1996 John Wiley & Sons, Inc.     

The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the γ/β microstructure of the feedstock powder, and the VPS coating consisted of a single (γ) phase. A 3-h, 1100 °C heat treatment in vacuum converted the single-phase VPS coating to a two-phase γ/β microstructure and coarsened the γ/β microstructure of the HVOF coating. Oxidation of free-standing as-sprayed and heat-treated coatings of each type was carried out in air at 1100 °C for a duration of 100 h. Parabolic rate constant(s), K _p, were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in K _p following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower K _p values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner α-Al₂O₃ layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a single-layer oxide, α-Al₂O₃. The formation of a thin α-Al₂O₃ layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.     

Microstructure and wear properties of aluminum/aluminum-silicon composite coatings prepared by cold spraying

Composite coatings containing aluminum and aluminum-11.6 wt.% silicon eutectic alloy phases of varying compositions were fabricated using cold spraying. Coating contained a uniform distribution of the two phases. The hardness of the coatings increased as the volume fraction of Al-Si in the coating increased. The length to width ratio of the splats was found to be larger for Al particles compared to Al-Si particles. Dry sliding ball-on-plate wear tests indicated that the wear volume loss was similar for the Al and Al/Al-Si composite coatings in spite of the increase in microhardness. This discrepancy is explained by the inter-splat delamination mechanism. The coefficient of friction of aluminum coating reduced on Al-Si addition.