Low Mechanical Load Research Articles

Structural basis for the fracture toughness of the shell of the conch Strombus gigas.

Natural composite materials are renowned for their mechanical strength and toughness: despite being highly mineralized, with the organic component constituting not more than a few per cent of the composite material, the fracture toughness exceeds that of single crystals of the pure mineral by two to three orders of magnitude. The judicious placement of the organic matrix, relative to the mineral phase, and the hierarchical structural architecture extending over several distinct length scales both play crucial roles in the mechanical response of natural composites to external loads. Here we use transmission electron microscopy studies and beam bending experiments to show that the resistance of the shell of the conch Strombus gigas to catastrophic fracture can be understood quantitatively by invoking two energy-dissipating mechanisms: multiple microcracking in the outer layers at low mechanical loads, and crack bridging in the shell's tougher middle layers at higher loads. Both mechanisms are intimately associated with the so-called crossed lamellar microarchitecture of the shell, which provides for 'channel' cracking in the outer layers and uncracked structural features that bridge crack surfaces, thereby significantly increasing the work of fracture, and hence the toughness, of the material. Despite a high mineral content of about 99% (by volume) of aragonite, the shell of Strombus gigas can thus be considered a 'ceramic plywood' and can guide the biomimetic design of tough, lightweight structures.

A simple and efficient thermal link assembly for cryocoolers

The thermal connection between a cryocooler cold tip and any object to be cooled (the thermal load) is very often a technical challenge. The allowed mechanical load on the cooler cold finger is generally strongly restricted. It is therefore difficult to design a mechanically anchored thermal link with low induced mechanical load and high thermal conductance. The proposed heat link concept is based on an evaporation/condensation process between the cooler cold tip and the thermal load.

Study of combustion characteristics of a compression ignition engine fuelled with dimethyl ether

This paper presents the combustion characteristics of a light-duty direct-injection diesel engine operating on dimethyl ether (DME). The indicated pressure diagrams and injector needle lifts are recorded and the combustion characteristics are demonstrated and compared with those of an engine operated on diesel fuel. The experimental and calculated results show that the DME engine has a longer delay of injection and duration of injection, a lower maximum cylinder pressure and rate of pressure rise, as well as a shorter ignition delay compared with those of a diesel engine. The DME engine has a low mechanical load and combustion noise, a fast rate of diffusion combustion and a shorter combustion duration than that of a diesel engine. It has the ideal pattern of compression ignition engine heat release.

High bone turnover in the elderly

Objectives: To document lifestyle-associated variations in biochemical markers of bone metabolism in advanced age. Study Design: Cross-sectional study. Setting: Department of Geriatrics, University Hospital, Basel, Switzerland. Subjects: Three hundred twelve ambulatory and 193 institutionalized men and women, aged 54 to 99yrs. Outcome Measurements: Biochemical markers of bone resorption (urinary deoxypyridinolin and N-telopeptides), serum vitamin D metabolites, and serum intact parathyroid hormone (iPTH) concentrations were assessed. Mean values of all parameters were correlated with a six-grade activity score. Physical activity score reflected the degree of weight bearing, ranging from walking independently to primarily bed-bound. Ambulatory subjects were measured in summertime and institutionalized subjects in wintertime to accentuate seasonality of vitamin D hormone levels. Results: Excretion of bone resorption markers was significantly higher in the institutionalized and physically inactive patients compared with those who were ambulatory and physically active ( p = .0001). Vitamin D deficiency (25-hydroxyvitamin D of <12ng/mL) was present in 86% of institutionalized and 15% of ambulatory subjects, and 1,25-dihydroxyvitamin D serum concentrations were lower in institutionalized than in ambulatory subjects ( p = .0001). Mean serum concentrations for iPTH were similar for both the institutionalized and ambulatory groups. When subjects were arranged according to activity score, mean excretion of urinary bone resorption markers increased with degree of inactivity. Conclusion: Despite the difference in vitamin D metabolites, iPTH serum concentrations did not differ significantly between the institutionalized and ambulatory groups. However, institutionalized and physically inactive participants had markedly elevated excretion of urinary bone resorption markers compared with ambulatory and physically active subjects. These results suggest that high bone turnover in the elderly may be caused by physical inactivity related to low mechanical loading rather than secondary hyperparathyroidism.

Evaluation of the plastic zone in an elastic-plastic dissimilar materials joint

In a dissimilar materials joint very high stresses develop near the free edge of the interface, even for low mechanical loading or after a homogeneous change of temperature, due to the different mechanical properties and different thermal expansion coefficients of the joined components. If one or both materials in the joint exhibit plastic material behaviour, after a small loading plastic deformation occurs. The aim of this paper is to find the relationship between the size and shape of the plastic zone, the loading, the material properties of the joined components and the geometry of the joint. For mechanical loading some empirical equations have been found to evaluate the size and shape of the plastic zone, based on the pure elastic stress analysis. The size and shape of the plastic zone calculated with the empirical equations are in good agreement with those from the Finite Element Method (FEM). The stress distributions in the elastic and elastic-plastic material are presented as well.

Fluctuation analysis of motor protein movement and single enzyme kinetics.

We studied fluctuations in the displacement of silica beads driven by single molecules of the motor protein kinesin, moving under low mechanical loads at saturating ATP concentrations. The variance in position was significantly smaller than expected for the case of stepwise movement along a regular lattice of positions with exponentially distributed intervals. The small variance suggests that two or more sequential processes with comparable reaction rates dominate the biochemical cycle. The low value is inconsistent with certain recently proposed thermal ratchet models for motor movement as well as with scenarios where the hydrolysis of a single ATP molecule leads to a cluster of several steps. Fluctuation analysis is a potential powerful tool for studying kinetic behavior whenever the output of a single enzyme can be monitored.

Mechanical behavior of a continuous fiber-reinforced aluminum matrix composite subjected to transverse and thermal loading

T he transverse properties of an aluminum alloy metal matrix composite reinforced by continuous alumina fibers have been investigated. The composite is subjected to both mechanical and cyclic thermal loading. The results of an experimental program indicate that the shakedown concept of structural mechanics provides a means of describing the material behavior. When the loading conditions are within the shakedown region the material finally responds in an elastic manner after initial plastic response, and for loading conditions outside the shakedown region the material exhibits a rapid incremental plastic strain accumulation. The failure strain varies by an order of magnitude according to the operating conditions. Hence for high mechanical and low thermal loading the failure strain is small, while for low mechanical and high thermal loading the failure strain is large.

The Development of a Direct Injection Diesel Combustion System for Low Noise, Emissions and Mechanical Loading

In recent years the whole climate of diesel engine development has changed with the advent of noise and emissions legislation combined with the demand for higher specific output. The conventional direct injection combustion chambers that have been used in the 1 litre/cylinder class of engine now present difficulties to the engineer in meeting these increasingly stringent demands. The development history of a novel combustion system, ‘Quadram’, is presented, leading up to its introduction into quantity production. Emphasis has been placed on achieving the legislative targets while retaining cost effective fuel injection equipment, in an intensely competitive market complicated by worldwide fuel quality variations. The ‘Quadram’ system is shown to display the key feature of achieving optimum performance with retarded injection timing, without any compromise to engine durability.

Failure Analysis of Brittle Fracture in Nonceramic Insulators

Time dependent transverse cracking of the glass reinforced polymer (GRP) core rod in nonceramic insulators (NCI) under low mechanical loads, commonly referred to as brittle fracture, has been investigated. Tension tests were performed on GRP core rods with E glass, ECR glass as well as epoxy and polyester matrices using nitric and oxalic acid environments in order to obtain the relationship between applied stress and the time to rupture. While both E glass and ECR glass remain prone to brittle fracture, ECR has a substantially superior resistance to stress corrosion cracking than E glass. Scanning electron microscopy of fracture surfaces has produced a quantitative relationship between the stress history and the fracture mirror size on the fibers. Results of an epoxy cone insulator end fitting are presented, and the mechanism of brittle fracture is discussed.

The Incremental Strain Growth of Elastic-Plastic Bodies Subjected to High Levels of Cyclic Thermal Loading

A linearized method of analysis proposed in an accompanying paper [1] is used to obtain the ratchet rate for two types of thermal loading problems where parts of the structure experience reversed plastic straining. For structures that can shakedown plasticially it is found that for a given increment of load beyond the plastic shakedown boundary, the rate of ratchet increases with increasing level of thermal loading. When a structure is unable to shakedown plastically it ratchets at low mechanical loading as the result of a localized mechanism that involves some reversed plasticity. It is shown that the ratchet rate in such situations can be substantial but its value is very dependent on the local curvature of the yield and not the accuracy of the yield surface itself.

Experimental theramal ratchetting data for a component with a stres concentration

The result from eleven tests of lead alloy model shouldered tubes, subjected to constant axial mechanical loads and cyclic, axisymmetric thermal shocks are described. Dwell periods between 0.5 h and 24 h allowed creep to occur between the thermal shocks. Electrical resistance strain gauges were used to measure ratchet and creep strains. At low mean axial stresses (&lt; 0.94σy), it was found that the ratchet strains increased with load and dwell period. It was also found that the ratchet strains in the fillet region were significantly larger than those in the shank and are induced at lower mechanical loads than those in the shank. For higher stresses, the ratchet strains were found to be relatively insensitive to both load and dwell period. Also, the ratchet strains in the fillet and shank were found to be of the same magnitude.