Ultrasonic measurement of core material temperature, phase 1

High temperature ultrasonic temperature measurements in nuclear rocket engine to determine feasibility of rhenium sensor as high temperature senso

Ultrasonic measurement of core material temperature, phase 2

Sheaths for pulse-echo ultrasonic temperature senso

Critical assessment of failure criteria for adhesively bonded composite repair design

Due to the high stress concentration and non-linear deformation in bonded joints, accurate strength prediction remains challenge. The aim of this paper is to evaluate the accuracies of different failure criteria and computational methodologies for bonded composite joints and their suitability as an engineering design tool. A total of four analytical and four numerical predictive models were evaluated against experimental results obtained from single lap and skin-doubler joints. Experimental observations reveal two main failure modes: cohesive and first-ply fracture. Strain-based models based on cohesive properties were found to be applicable only to joints exhibiting cohesive failure. Fracture mechanics-based models, on the other hand, can predict both cohesive and composite ply failure

Load ratio effects in the fatigue crack propagation of composite laminates and bonded joints

Fatigue cracks, if left unchecked, can lead to the catastrophic failure of the structure. Hence fatigue is an important design consideration. The development of a validated fatigue design methodology for bonded composite repairs is impeded by a lack of design criterion that is independent of load ratios, geometry and mode mixity. Consequently design methodology is largely empirical and involves large safety factors despite the recent research effort in this area. A critical review of the literature reveals that existing parameters correlating to the crack growths rates are proposed as a means of curve fitting and do not correctly account for the effects of mean loads. Therefore the objective of this Thesis is to develop a mechanistic fatigue model for bonded joints and composite laminates that is capable of quantifying load ratio effects. Firstly, by considering the theoretical similitude requirements in fatigue, a correlating parameter ΔGeq has been proposed. By maintaining similitude condition, this scaling parameter will be independent of specimen size. However the effects of load ratio can still be observed in the Mode I fatigue behaviour for both bonded joints and composite laminates. Therefore a new correlating parameter is needed to quantify these effects. In order to identify the mechanisms responsible for this behaviour, experimental tests and numerical simulations have been carried on two crack propagation mechanisms; cohesive debonding of bonded structures and delamination crack of composite laminates, under two crack propagation modes, Mode I and II. Plasticity induced crack closure is identified as the primary cause of load ratio effects in the Mode I fatigue behaviour of bonded joints. Elastic-plastic analysis is performed on the bonded double cantilever beam model. The bonded joints are shown to experience significant level of crack closure and this phenomenon is quantified. A new correlating parameter, based on the crack closure model, is proposed and successfully eliminates the influence of mean loads. Load ratio also has a strong influence on the delamination growth along 0°//0° interface. Experimental evidence showed extensive fibre bridging in the wake of the crack. Hence this fatigue behaviour is hypothesised to be attributed to plasticity induced crack closure and/or fibre bridging. The Hill’s plastic potential is used to simulate the anisotropic yielding of the lamina. The numerical results eliminate plasticity induced crack closure to be the cause of the observed load ratio effects in the experimental data. However it is also found that crack closure can occur in delamination along 45º//0º and 90º//0º interfaces. Therefore this leaves fibre bridging as the only possible source for the strong influence of load ratio. Rather than proposing new empirical relation to describe the observed fatigue behaviour, this Thesis develops a fundamental understanding of the effects of mean load and its mechanisms in bonded joints and composites. A scaling parameter is proposed which describes the crack tip conditions by considering the physical process. This will provide engineers a fatigue criterion which can be used to design a bonded composite repair for an arbitrary load ratio and mixed mode ratio

Synthesis and evaluation of new high polymers for coating applications annual summary report no. 1, 26 jun. 1964 - 25 jun. 1965

Synthesis of organic and semi-organic polymers useful as high temperature coating materia

Scaling parameter for fatigue delamination growth in composites under varying load ratios

Fatigue delamination growth in composite laminates is strongly influenced by mean loads or load ratios. Description of this behaviour currently relies on empirical curve fitting, which renders it difficult to predict fatigue lives of composite structures subjected to variable amplitude fatigue loading. This paper presents a new scaling parameter that is consistent with the similitude concept and incorporates the crack-tip shielding effects of fibre bridging under fatigue loading. Static and fatigue experiments were carried out on IM7/977-3 composite laminates under mode I and mode II. Large-scale fibre bridging was observed as a major toughening mechanism under both static and fatigue loading. To correctly account for the effect of fibre bridging, an inverse method was developed to determine the traction stresses acting in the crack wake. The new scaling parameter, accounting for the effect of bridging by cross-over fibres, is shown to unify the fatigue growth rates under different load ratios obtained in this study

Synthesis and evaluation of new high temperature polymers for coating applications Technical summary report, 26 Jun. 1964 - 25 Sep. 1966

Synthesis, characterization, and evaluation of organic and semiorganic heat resistant polymer coating

Plasticity induced crack closure in adhesively bonded joints under fatigue loading

The mean load of a cyclic loading has a large effect on fatigue crack growth rates in metallic materials and bonded joints. In metallic structures, this effect has been attributed to plasticity-induced crack closure, but little is known about the mechanism responsible for this mean load effect on fatigue crack growth in adhesively bonded joints. This paper presents a computational investigation of the plasticity-induced crack closure mechanism affecting disbond growth in adhesively bonded joints under fatigue loading. The results show that the ratios of crack-opening and crack-closure are approximately independent of the level of plastic constraint, indicated by the ratio between the plastic zone size and the adhesive thickness. An effective strain-energy release rate parameter, which accounts for the crack closure behaviour, has been developed as a new correlating parameter for disbond growth. Comparisons with the experimental results pertinent to four different adhesive bonded joints reveal that this new correlating parameter is capable of unifying the fatigue growth rates by eliminating the effect of mean loads

Implications of the cosmic microwave background power asymmetry for the early universe

Observations of the microwave background fluctuations suggest a scale-dependent amplitude asymmetry of roughly 2.5 sigma significance. Inflationary explanations for this 'anomaly' require non-Gaussian fluctuations which couple observable modes to those on much larger scales. In this Letter we describe an analysis of such scenarios which significantly extends previous treatments. We identify the non-Gaussian 'response function' which characterizes the asymmetry, and show that it is non-trivial to construct a model which yields a sufficient amplitude: many independent fine tunings are required, often making such models appear less likely than the anomaly they seek to explain. We present an explicit model satisfying observational constraints and determine for the first time how large its bispectrum would appear to a Planck-like experiment. Although this model is merely illustrative, we expect it is a good proxy for the bispectrum in a sizeable class of models which generate a scale-dependent response using a large eta parameter.Comment: 5 pages. v2: Minor changes to match version published in Phys. Rev.

Discriminating among Earth composition models using geo-antineutrinos

It has been estimated that the entire Earth generates heat corresponding to about 40 TW (equivalent to 10,000 nuclear power plants) which is considered to originate mainly from the radioactive decay of elements like U, Th and K, deposited in the crust and mantle of the Earth. Radioactivity of these elements produce not only heat but also antineutrinos (called geo-antineutrinos) which can be observed by terrestrial detectors. We investigate the possibility of discriminating among Earth composition models predicting different total radiogenic heat generation, by observing such geo-antineutrinos at Kamioka and Gran Sasso, assuming KamLAND and Borexino (type) detectors, respectively, at these places. By simulating the future geo-antineutrino data as well as reactor antineutrino background contributions, we try to establish to which extent we can discriminate among Earth composition models for given exposures (in units of kt$\cdot$ yr) at these two sites on our planet. We use also information on neutrino mixing parameters coming from solar neutrino data as well as KamLAND reactor antineutrino data, in order to estimate the number of geo-antineutrino induced events.Comment: 24 pages, 10 figures, final version to appear in JHE