A general method for estimation of fracture surface roughness: Part I. Theoretical aspects

An assumption-free general method is developed for quantitative estimation of fracture surface roughness from the measurements performed on the fracture profiles generated by sectioning planes which are normal to the average topographic plane of the fracture surface. The input data are the profile roughness parameter,R _L, and angular orientation distribution of line elements on the fracture profile,f(α). It is shown that

A general method for estimation of fracture surface roughness: Part II. Practical considerations

Surface roughness is an important attribute of fracture surfaces. An assumption-free method for estimation of surface roughness presented in Part I^[1] is analyzed further here. It is shown that three vertical sectioning plane orientations mutually at an angle of 120 deg contain sufficient information for a reliable estimation of surface roughness; in most of the cases, the sampling error due to measurements on a limited number (three) of vertical section orientations should be less than ±6 pct with a confidence limit of 95 pct. A simplified procedure is presented for calculation of a profile structure factor from the measurements of the profile frequency function. A practical example of application of the present analysis involving measurement of the fracture surface roughness of a metal-matrix composite is discussed.     

A general method for estimation of fracture surface roughness: Part II. Practical considerations

Surface roughness is an important attribute of fracture surfaces. An assumption-free method for estimation of surface roughness presented in Part I^[1] is analyzed further here. It is shown that three vertical sectioning plane orientations mutually at an angle of 120 deg contain sufficient information for a reliable estimation of surface roughness; in most of the cases, the sampling error due to measurements on a limited number (three) of vertical section orientations should be less than ±6 pct with a confidence limit of 95 pct. A simplified procedure is presented for calculation of a profile structure factor from the measurements of the profile frequency function. A practical example of application of the present analysis involving measurement of the fracture surface roughness of a metal-matrix composite is discussed.     

A geometric model for fatigue crack closure induced by fracture surface roughness

Mechanisms for fatigue crack closure under plane strain conditions have recently been identified at very low (near-threshold) stress intensities in terms of effects of excess corrosion deposits or fracture surface roughness in promoting premature closure of the crack. In the present paper, a geometric model is presented for crack closure induced by fracture surface roughness. This model specifically addresses the contribution from both Mode I and Mode II crack tip displacements in addition to considering the nature of the fracture surface morphology. The implications of this model are briefly discussed in light of the roles of grain size, yield strength, microstructure, and crack size in influencing near-threshold fatigue behavior in engineering alloys.     

A general procedure for the estimation of interdependence.

Dependencies among observations are of interest to psychologists for both methodological and substantive reasons. A general procedure is developed that can be used to estimate dependencies among observations, given a set of a priori expectations about the structure of those dependencies. The utility of this general procedure is illustrated for specific cases: the estimation of serial dependency in time-series data, of dyadic and group interdependence in social psychology, and of reliability. The utility of the approach derives from its generality and from its ability to handle randomly missing data. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The skeleton after spinal cord injury. Part 1. Theoretical aspects

Sublesional osteoporosis occurs after acute spinal cord injury (SCI), preferentially weakening the skeleton below the level of the neurological lesion. Although its pathogenesis is unclear, it resembles post-menopausal, high turnover osteoporosis. Physical and pharmacologic therapies are currently being tested for their ability to prevent early loss and restore lost bone. Although treatment strategies hold promise, preservation of skeletal strength after injury may ultimately rest on lifestyle decisions made early in life. If skeletal strength is to be maintained after SCI, ways must be found to optimize skeletal strength prior to injury, arrest early losses, and stabilize, if not restore, lost bone over time.     

Mitochondrial diseases. Part I -- general review]

Inborn mitochondrial diseases (MD) may result from molecular defects involving the mitochondrial or the nuclear genome, so they may be transmitted maternally or as Mendelian traits; some cases occur sporadically. Numerous secondary causes of mitochondrial disorders are also known. Impairment of mitochondrial oxidation leads to the defective energy production, and further, to cellular damage. MD should be suspected when progressive signs occur, especially involving nervous system and muscles; other organs as heart, liver and kidney may also be affected.     

Experimental study of fracture surface roughness on rocks with crack velocity

In experimental animals, administration of 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) leads to extensive, but incomplete, loss of 5-hydroxytryptamine (5-HT) innervation in the brain. Here, we report the effects of MDMA on 5-HT neuronal function measured in the rat in vivo using electrophysiological and microdialysis techniques. Two weeks after administration of an established neurotoxic regimen of MDMA (20 mg/kg s.c., twice daily for 4 days) we found; 1) no change in either the density or the firing activity of 5-HT neurons in the dorsal raphé nucleus; 2) no change in basal extracellular 5-HT in either the frontal cortex or the hippocampus, although extracellular 5-hydroxyindoleacetic acid was reduced by about 50% in both regions; and 3) no change in the amount of 5-HT released in the hippocampus in response to electrical stimulation (5 Hz) of either the dorsal or medial raphé nucleus, but a marked reduction in the amount of 5-HT released in the frontal cortex after electrical stimulation of the dorsal raphé nucleus. In summary, although MDMA causes marked 5-HT neurotoxicity, our data suggest that 5-HT cell firing is unchanged and, furthermore, that 5-HT release is maintained in some (but not all) forebrain regions even in response to physiological levels of stimulation.     

Thermal stress fracture of refractory lining components: Part I. Thermoelastic analysis

This paper presents a mathematical model and corresponding stress analysis suitable for the development of theoretical criteria useful for the design and selection of refractory components of linings of high-temperature furnaces and metallurgical process vessels. Refractory lining components, for which the maximum principal tensile stress fracture criterion is assumed valid, are simulated using a two-dimensional constant heating rate thermoelastic model. Dimensional analysis and the finite element numerical method are utilized to develop a general solution for the maximum principal tensile stress in a rectangular shape of arbitrary length and width in terms of thermal and mechanical properties and heating and cooling rate. Implications of the general stress solution with regard to fracture behavior and design recommendations are discussed with reference to results previously reported in the literature.     

Hydrogen-Assisted ductile fracture in spheroidized 1520 Steel: Part I. axisymmetric tension

The effects of hydrogen on ductile fracture were investigated in a spheroidized steel similar to AISI 1520, which contained negligible amounts of P and S. Cylindrical tensile data indicated that hydrogen does not significantly affect void initiation and the early stage of void growth. Instead the effects of hydrogen were observed in the final fracture stages. It was inferred that hydrogen could assist the later stage of void growth by an internal pressure and affect the center crack growth in tension by the enhanced strain localization caused by hydrogen. The fracto-graphic response to hydrogen in spheroidized steels was rationalized by the combination of these two effects, that is, internal pressurization and enhanced strain localization. The critical role of manganese sulfide was manifested by comparison with previous work. Formerly with Carnegie Mellon University.     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments

Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA) composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response. The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450°C as-rolled condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged (PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)—based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments

Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA) composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response. The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450 °C as-rolled condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged (PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)-based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

The surface tension of liquid silver alloys: Part I. Silver-gold alloys

The sessile drop technique has been used to determine the surface tension of silver-gold alloys. The techniques of data reduction are discussed and an improved one is presented. The measurements are in good agreement with previous data reported in the literature for pure silver and gold. The surface tension of alloys at 1108‡C was found to behave according to the prediction of the perfect solution model. Formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University     

A general mechanism of martensitic nucleation: Part I. General concepts and the FCC → HCP transformation

Consideration of the martensitic nucleation process as a sequence of steps which take the particle from maximum to minimum coherency leads to the hypothesis that the first step in martensitic nucleation is faulting on planes of closest packing. It is further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur in such a way as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the stacking fault energy is shown to consist of both volume energy and surface energy contributions. When the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that the fault energy associated with the simultaneous dissociation of an appropriate group of dislocations (e.g. a finite tilt boundary segment) can be zero or negative. This condition leads to the spontaneous formation of a martensitic embryo. For the specific case of the fcc → hcp martensitic transformation in Fe-Cr-Ni alloys, the defect necessary to account for spontaneous embryo formation at the observedM _s temperatures may consist of four or five properly spaced lattice dislocations. Such defects are considered to be consistent with the known sparseness of initial martensitic nucleation sites. This paper is Part I of a three-part series based on a thesis submitted by G. B. Olson for the degree of Sc.D. in Metallurgy at the Massachusetts Institute of Technology in June 1974.     

The mechanism of brittle fracture in a microalloyed steel: Part I. Inclusion-induced cleavage

The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-to-brittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notch-tip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusions in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents.     

Bioelectrical impedance techniques in medicine. Part I: Bioimpedance measurement. First section: general concepts

After a brief historical overview, the concept of electrical impedance is introduced as a principle of transduction calling attention to the possible mechanisms by which a physiological event may change impedance, i.e., by geometric, resistivity, and/or permittivity changes. Thereafter, since impedance measurements usually require the injection of current, its biological effects are discussed in order to establish the safety criteria. Finally, the elements found in an impedancimetric circuit and their respective nature are presented and described. The particular behavior of the biological impedance and the electrode/electrolyte interface appear immediately as strikingly important. The section ends with a bird's-eye view of the basic circuitry to measure impedance. Each subsection is closed by partial conclusions to underline the relevant concepts.     

Acquired scalp alopecia. Part I: A review

In this two-part series we review the acquired scalp alopecias. A broad spectrum of diseases result in alopecia. In this first part we provide a framework for the assessment and diagnosis of scalp hair loss, and begin covering the individual conditions. The non-scarring alopecias covered include effluvium, androgenetic alopecia, alopecia areata, trichotillomania, and loose anagen syndrome. The scarring alopecias cause permanent pilosebaceous follicle loss; the lymphocyte-associated scarring alopecia described encompasses lichen planopilaris, discoid lupus erythematosus, pseudopelade, and follicular mucinosis. Part II will cover the neutrophil-associated and infiltrative processes causing scarring alopecia followed by the medical management of alopecia. There is particular reference to newly described conditions and progress in the understanding of older conditions. More recently characterized conditions include the loose anagen syndrome, chronic telogen effluvium, and the frontal fibrosing variant of lichen planopilaris.     

Relationship between fracture toughness, fracture path, and microstructure of 7050 aluminum alloy: Part I. Quantitative characterization

The fracture toughness of Al-Zn-Mg-Cu-based 7XXX aluminum alloys decreases with an increase in the extent of recrystallization. In this contribution, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the 7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging condition. The fracture path is quantitatively correlated to fracture toughness, and the bulk microstructural attributes estimated via stereological analysis. In the companion article, these quantitative data are used to develop and verify a multiple-fracture micromechanism-based model that relates the fracture toughness to a number of microstructural parameters of the partially recrystallized alloy plate.     

Interface morphology development during stress corrosion cracking: Part I. Via surface diffusion

The initiation of a crack in a specimen under tensile or compressive stresses is treated from the point of view of perturbation analysis. A surface distortion is Fourier analyzed into a series of waves and the amplitude response of a single component of varying frequency is theoretically investigated. The response of the individual components yields a Griffith-type criterion for wave amplitude growth. The model is applied to alloy systems undergoing stress corrosion cracking via surface diffusion.