A fracture mechanics approach to high cycle fretting fatigue based on the worst case fret concept – I. Model development

An integrated analytical and experimental approach was taken to develop a fracture mechanics-based methodology for predicting the limiting threshold stress of high-cycle fretting fatigue in structural alloys. The contact stress field for two flat surfaces under fretting was analyzed via an integral equation technique. The local fretting stress field of the uncracked body was then utilized to obtain the stress intensity factor of an arbitrarily oriented fatigue crack using a continuum dislocation formulation. The limiting threshold stress ranges for the nonpropagation of fretting fatigue cracks were predicted on the basis that the fretting fatigue cracks are small cracks that exhibit a size-dependent growth threshold and propagate at stress intensity ranges below the large-crack threshold. In part I, the development of the worst-case fret (WCF) model is described. The influence of the limiting high-cycle fatigue (HCF) threshold stress on a variety of fretting fatigue parameters such as bearing pressure, pad geometry, shear stress, mode mixity, and coefficient of friction are elucidated by parametric calculations. In part II, the WCF model is applied to treating HCF of Ti-6Al-4V where model predictions are compared against critical experiments performed on a kilohertz fretting-fatigue rig.     

A SIMPLE RELIABILITY MODEL FOR THE FATIGUE FAILURE OF REPAIRABLE OFFSHORE STRUCTURES

A simple reliability model for fatigue failure of tubular welded joints used in the construction of offshore oil and gas platforms is proposed. The stress-life data obtained from large-scale fatigue tests conducted on tubular joints is used as the starting point in the analysis and is combined with a fracture mechanics model to estimate the distribution of initial defect sizes. This initial defect distribution is hypothetical but it agrees well with other initial defect distributions quoted in the literature and when used with the fracture mechanics model results in failure probabilities identical to those obtained from the stress-life data. This calculated distribution of initial defect sizes is modified as a result of crack growth under cyclic loading and the probability of failure as a function of fatigue cycles is calculated. The failure probability is modified by inspection, and repair and the results illustrate the trade-off between inspection sensitivity and inspection interval for any desired reliability.     

Hydrogen enhanced crack growth in 18 Ni maraging steels

The kinetics of sustained-load subcritical crack growth for 18 Ni maraging steels in high purity hydrogen are examined using crack-tip stress intensity,K, as a measure of crack driving force. Crack growth rate as a function of stress intensity exhibited a clearly definedK-independent stage (Stage II). Crack growth rates in an 18 Ni (250) maraging steel are examined for temperatures from -60°C to 100°C. A critical temperature was observed above which crack growth rates became diminishingly small. At lower temperatures the activation energy for Stage II crack growth was found to be 16.7 ± 3.3 kJ/mole. Temperature and hydrogen partial pressure are shown to interact in a complex manner to determine the apparentK _th and the crack growth behavior. Comparison of results on ‘250’ and ‘300’ grades of 18 Ni maraging steel indicate a significant influence of alloy composition and/or strength level on the crack growth behavior. These phenomenological observations are discussed in terms of possible underlying controlling processes. Formerly a Graduate Student and Research Assistant. Based on a thesis submitted in partial fulfillment of requirements for the M.S. degree in Metallurgy and Materials Science, Lehigh University, Bethlehem, PA.     

A new list compaction method

List compaction, or so-called ‘cdr-coding’, can greatly reduce the storage needs of list processing languages. However, existing methods do not perform well when several lists are being constructed simultaneously from the same heap, since the non-contiguous nature of the cells being allocated eliminates the opportunity for compaction. This situation arises not only in true parallel systems sharing a common memory, and sequential systems supporting multiple processes, but also quite often in purely sequential systems, where it is not uncommon to build several different lists simultaneously within a single loop. In this paper, a new list compaction method is presented that performs well during both sequential and ‘parallel’ list generation. The method is essentially a generalization of cdr-coding, in which lists are represented explicitly as linked vectors rather than implicitly as compacted memory. In addition, an encoding scheme is used that is as simple or simpler than all known encodings, and destructive operations are supported with no greater overhead than existing schemes. Performance figures in a simulated environment suggest that the strategy consistently performs better than conventional cdr-coding, with essentially the same complexity.     

An algebraic model for divide-and-conquer and its parallelism

A formal algebraic model for divide-and-conquer algorithms is presented. The model reveals the internal structure of divide-and-conquer functions, leads to high-level and functional-styled algorithms specification, and simplifies complexity analysis. Algorithms developed under the model contain vast amounts of parallelism and can be mapped fairly easily to parallel computers.This research was supported in part by DOE grant DOE FG02-86ER25012.     

Cathodic Debonding of Neoprene from Steel

X-ray photoelectron spectroscopy (XPS) has been used to determine the failure mechanisms in neoprene/steel adhesive bonds during cathodic delamination or during immersion in aqueous solutions of NaOH to simulate the effects of cathodic delamination at the bondline. Delamination of the rubber from substrates that had been pretreated by acid-etching began at the edges of the bonds and proceeded slowly inward. The centers of the bonds were always intact except for every long times when complete delamination of the rubber was observed. Failure was near the primer/oxide interface but with islands of rubber remaining on the substrate and islands of oxide remaining on the rubber. The failure mechanism was associated with degradation of the phenolic primer and with dehydrohalogenation of chlorinated rubber in the adhesive and primer. Dehydrohalogenation resulted in the formation of inorganic salts which could dissolve during exposure to water at high pH values, leading to large osmotic pressures at the interface. Debonding of rubber from polished substrates was much faster, leaving little rubber on the substrate failure surfaces and little oxide on the rubber failure surfaces, and indicating that mechanical interlocking of the rubber with the substrate has an important effect on the environmental stability of rubber-to-metal bonds.     

Code optimizations for lazy evaluation

Implementations of lazy evaluation for nonstrict functional languages usually involve the notion of a delayed representation of the value of an expression, which we call athunk. We present several techniques for implementing thunks and formalize a class of optimizations that reduce both the space and time overhead of these techniques. The optimizations depend on a compile-time inferencing strategy calledpath analysis, a generalization of strictness analysis that uncovers order-of-evaluation information. Although the techniques in this paper are focused on the compilation of a nonstrict functional language for a conventional architecture, they are directly applicable to most of the virtual machines commonly used for implementing such languages. The same techniques also apply to other forms of delayed evaluation such asfutures andpromises.This research was supported in part by the National Science Foundation under Grant DCR-8451415.     

Production of Landsat ETM+ reference imagery of burned areas within Southern African savannahs: comparison of methods and application to MODIS

Accurate production of regional burned area maps are necessary to reduce uncertainty in emission estimates from African savannah fires. Numerous methods have been developed that map burned and unburned surfaces. These methods are typically applied to coarse spatial resolution (1 km) data to produce regional estimates of the area burned, while higher spatial resolution (<30 m) data are used to assess their accuracy with little regard to the accuracy of the higher spatial resolution reference data. In this study we aimed to investigate whether Landsat Enhanced Thematic Mapper (ETM+)‐derived reference imagery can be more accurately produced using such spectrally informed methods. The efficacy of several spectral index methods to discriminate between burned and unburned surfaces over a series of spatial scales (ground, IKONOS, Landsat ETM+ and data from the MOderate Resolution Imaging Spectrometer, MODIS) were evaluated. The optimal Landsat ETM+ reference image of burned area was achieved using a charcoal fraction map derived by linear spectral unmixing (k = 1.00, a = 99.5%), where pixels were defined as burnt if the charcoal fraction per pixel exceeded 50%. Comparison of coincident Landsat ETM+ and IKONOS burned area maps of a neighbouring region in Mongu (Zambia) indicated that the charcoal fraction map method overestimated the area burned by 1.6%. This method was, however, unstable, with the optimal fixed threshold occurring at >65% at the MODIS scale, presumably because of the decrease in signal‐to‐noise ratio as compared to the Landsat scale. At the MODIS scale the Mid‐Infrared Bispectral Index (MIRBI) using a fixed threshold of >1.75 was determined to be the optimal regional burned area mapping index (slope = 0.99, r ² = 0.95, SE = 61.40, y = Landsat burned area, x = MODIS burned area). Application of MIRBI to the entire MODIS temporal series measured the burned area as 10 267 km² during the 2001 fire season. The char fraction map and the MIRBI methodologies, which both produced reasonable burned area maps within southern African savannah environments, should also be evaluated in woodland and forested environments.     

Recovery and fractionation of phosphorus retained by lightweight expanded shale and masonry sand used as media in subsurface flow treatment wetlands

Most subsurface flow treatment wetlands, also known as reed bed or root zone systems, use sand or gravel substrates to reduce organics, solids, and nutrients in septic tank effluents. Phosphorus (P) retention in these systems is highly variable and few studies have identified the fate of retained P. In this study, two substrates, expanded shale and masonry sand, were used as filter media in five subsurface flow pilot-scale wetlands (2.7 m3). After 1 year of operation, we estimated the annual rate of P sorption by taking the difference between total P (TP) of substrate in the pilot cells and TP of substrate not exposed to wastewater (control). Means and standard deviations of TP retained by expanded shale were 349 +/- 171 mg kg(-1), respectively. For a substrate depth of 0.9 m, aerial P retention by shale was 201 +/- 98.6 g of P m(-2) year(-1), respectively. Masonry sand retained an insignificant quantity of wastewater P (11.9 +/- 21.8 mg kg(-1)) and on occasion exported P. Substrate samples were also sequentially fractionated into labile P, microbial P, (Fe + Al) P, humic P, (Ca + Mg) P, and residual P. In expanded shale samples, the greatest increase in P was in the relatively permanent form of (Fe + Al) P (108 mg kg(-1)), followed by labile P (46.7 mg kg(-1)) and humic P (39.8 mg kg(-1)). In masonry sand, there was an increase in labile P (9.71 mg kg(-1)). Results suggest that sand is a poor candidate for long-term P storage, but its efficiency is similar to that reported for many sand, gravel, and rock systems. By contrast, expanded shale and similar products with high hydraulic conductivity and P sorption capacity could greatly improve performance of P retention in constructed wetlands.     

Guidelines for Two-Parameter Weibull Analysis for Flaw-Containing Materials

Two-parameter Weibull statistics used in the analysis of mechanical data from flaw-containing materials, such as ceramics and castings, are reviewed. Guidelines to estimate Weibull parameters with the linear regression technique are provided. Moreover goodness-of-fit tests for Weibull fits and calculating confidence intervals for the estimated Weibull modulus are discussed. Methods of estimating lower bounds as well as a new hypothesis test for comparing two estimated Weibull moduli are explained. The use of these guidelines is demonstrated by using data from the literature.