Reliability-based Fracture Analysis for Shallow Shell Structure with the Dual Boundary Element Method

Reliability analysis has gained prominence as a stochastic approach to incorporate uncertainties in structural analysis. This study presents a novel methodology for evaluating the sensitivity of the crack tip stress intensity factor in a shallow shell structure. The work focuses on the reliability analysis of a shallow shell structure containing a crack originating from the corner of a fuselage window. The analysis incorporates uncertainties in both geometrical and loading parameters. The sensitivity of the crack tip stress intensity factors with respect to the considered uncertainties is determined. The Implicit Differentiation Method (IDM)-based First-Order Reliability Method (FORM) is utilized, and the results are compared to the results obtained from Monte Carlo Simulation (MCS), with a maximum difference of 2.99%. The reliability analysis aids in determining an appropriate inspection crack size that satisfies safety requirements, consequently facilitating the selection of the corresponding inspection technique

Statistical Inference of Equivalent Initial Flaw Size Distribution for Fatigue Analysis of an Anisotropic Material

A novel methodology for the fatigue life uncertainty quantification of anisotropic structures is presented in this work. The concept of the equivalent initial flaw size distribution (EIFSD) is employed to overcome the difficulties in small cracks detection and fatigue prediction. This EIFSD concept is combined with the dual boundary element method (DBEM) to provide an efficient methodology for modelling the fatigue crack growth. Bayesian inference is used to infer the EIFSD based on inspection data from the routine maintenance of the structure, simulated with the DBEM. A large amount of DBEM simulations were required for the Bayesian inference. Therefore, surrogate models are used as part of the inference to further improve computational efficiency. A numerical example featuring an anisotropic plate is investigated for demonstrating the proposed methodology. When considering a low level of uncertainty in the crack propagation parameters, an error of 0.12% was found between the estimated fatigue life obtained using the proposed method compared to actual fatigue life, and only 0.35% error when considering high level of uncertainty. The application of the estimated fatigue life can be used to determine an appropriate inspection interval for aircraft maintenance

Implicit differentiation-based reliability analysis for shallow shell structures with the Boundary Element Method

A novel methodology for evaluating the response sensitivities of shallow shell structures using the Boundary Element Method (BEM) is presented in this work. The implicit derivatives of the BEM formulations for shallow shell structures, with respect to the geometrical variables, such as curvature and thickness, have been derived for the first time and incorporated into an Implicit Differentiation Method (IDM). The IDM is employed in conjunction with the First Order Reliability Method (FORM) to evaluate the reliability of shallow shell structures. The accuracy of the IDM formulation is first validated against an analytical solution, with results showing a maximum difference of only 2.61%. The IDM was later validated against the Finite Difference Method (FDM), with results showing a maximum difference of only 0.11%. The IDM was also found to be significantly more efficient than the FDM, requiring 35% less CPU time when calculating sensitivities. This is further compounded by the fact that, unlike the FDM, the IDM does not require a step size. A numerical example featuring a circular shallow shell is used to demonstrate the application of the IDM-based FORM for assessing structural reliability. The uncertainty in curvature is set as a variable for the purpose of investigating its impact on reliability. The results of the reliability index obtained from the IDM-FORM are compared to the results obtained from FDM-FORM and were found to be very similar. An analysis of sensitivity is conducted to identify the most significant variables affecting reliability. It is found that uncertainties in curvature, thickness, and applied pressure distribution parameters have the largest impact on structural reliability. To demonstrate how the IDM could be used in practice, it was employed as gradient-based optimisation procedure featuring shallow-shell structures. The IDM was found to be a very efficient and accurate alternative to existing methods for calculating structural response sensitivities

Optimizing Sensor Paths for Enhanced Damage Detection in Large Composite Stiffened Panels - A Multi-Objective Approach

This work proposes a novel methodology for the automatic multi-objective optimisation of sensor paths in Structural Health Monitoring (SHM) sensor networks using Archived Multi-Objective Simulated Annealing (AMOSA). Using all of the sensor paths within a sensor network may not always be beneficial and could impair damage detection accuracy. Knowing which paths to include, and which to exclude, can require significant prior expert knowledge, which may not always be available, and may not result in optimal path selection. Therefore, this work proposes a novel automatic procedure for optimising sensor paths to maximise coverage level and damage detection accuracy, and minimise overall signal noise. This procedure was tested on a real-world large composite stiffened panel with many frames and stiffeners. Compared to using all of the available sensor paths, the optimized network exhibits superior performance in terms of detection accuracy and overall noise. It was also found to provide 35% higher damage detection accuracy compared to a network designed based on prior expert knowledge. As a result, this novel procedure has the capability to design high-performing SHM sensor path networks for structures with complex geometries, but without the need for prior expert knowledge, making SHM more accessible to the engineering community

A multi-fidelity modelling approach to the statistical inference of the equivalent initial flaw size distribution for multiple-site damage

A new methodology for the statistical inference of the Equivalent Initial Flaw Size distribution (EIFSD) using the Dual Boundary Element Method (DBEM) is proposed. As part of the inference, Bayesian updating is used to calibrate the EIFS based on data obtained from simulated routine inspections of a structural component from a fleet of aircraft. An incremental crack growth procedure making use of the DBEM is employed for the modelling of the simultaneous growth of cracks in the structure due to fatigue. Multi-fidelity modelling, in the form of Co-Kriging, is used to create surrogate models that act in place of the DBEM model for the expensive Monte Carlo sampling procedure required for the statistical inference of the EIFSD. The proposed methodology is applied to a numerical example featuring a long fuselage lap joint splice in presence of multiple site damage (MSD). Results show that the EIFSD can be accurately estimated with data from 50 inspections. The employed Co-Kriging models proved to be effective substitutes for the DBEM model, providing significant reductions in the computational cost associated with the implementation of the proposed statistical inference methodology

Preserving the impossible: conservation of soft-sediment hominin footprint sites and strategies for three-dimensional digital data capture.

Human footprints provide some of the most publically emotive and tangible evidence of our ancestors. To the scientific community they provide evidence of stature, presence, behaviour and in the case of early hominins potential evidence with respect to the evolution of gait. While rare in the geological record the number of footprint sites has increased in recent years along with the analytical tools available for their study. Many of these sites are at risk from rapid erosion, including the Ileret footprints in northern Kenya which are second only in age to those at Laetoli (Tanzania). Unlithified, soft-sediment footprint sites such these pose a significant geoconservation challenge. In the first part of this paper conservation and preservation options are explored leading to the conclusion that to 'record and digitally rescue' provides the only viable approach. Key to such strategies is the increasing availability of three-dimensional data capture either via optical laser scanning and/or digital photogrammetry. Within the discipline there is a developing schism between those that favour one approach over the other and a requirement from geoconservationists and the scientific community for some form of objective appraisal of these alternatives is necessary. Consequently in the second part of this paper we evaluate these alternative approaches and the role they can play in a 'record and digitally rescue' conservation strategy. Using modern footprint data, digital models created via optical laser scanning are compared to those generated by state-of-the-art photogrammetry. Both methods give comparable although subtly different results. This data is evaluated alongside a review of field deployment issues to provide guidance to the community with respect to the factors which need to be considered in digital conservation of human/hominin footprints

Prevalence and determinants of the use of self-tests by members of the public: a mixed methods study

Background Self-tests can be used by members of the public to diagnose conditions without involving a doctor, nurse or other health professional. As technologies to design and manufacture diagnostic tests have developed, a range of self-tests have become available to the public to buy over-the-counter and via the Internet. This study aims to describe how many people have used self-tests and identify factors associated with their use. Methods A postal questionnaire will elicit basic information, including sociodemographic characteristics, and whether the person has used or would use specified self-tests. Consent will be sought to recontact people who want to participate further in the study, and interviews and focus groups will be used to develop hypotheses about factors associated with self-test use. These hypotheses will be tested in a case-control study. An in-depth questionnaire will be developed incorporating the identified factors. This will be sent to: people who have used a self-test (cases); people who have not used a self-test but would use one in the future (controls); and people who have not used and would not use a self-test (controls). Logistic regression analysis will be used to establish which factors are associated with self-test use. Discussion Self-tests do have potential benefits, for example privacy and convenience, but also potential harms, for example delay seeking treatment after a true negative result when the symptoms are actually due to another condition. It is anticipated that the outcomes from this study will include recommendations about how to improve the appropriate use of self-tests and existing health services, as well as information to prepare health professionals for patients who have used self-tests

Annual variation in the levels of transcripts of sex-specific genes in the mantle of the common mussel, Mytilus edulis

Mytilus species are used as sentinels for the assessment of environmental health but sex or stage in the reproduction cycle is rarely considered even though both parameters are likely to influence responses to pollution. We have validated the use of a qPCR assay for sex identification and related the levels of transcripts to the reproductive cycle. A temporal study of mantle of Mytilus edulis found transcripts of male-specific vitelline coat lysin (VCL) and female-specific vitelline envelope receptor for lysin (VERL) could identify sex over a complete year. The levels of VCL/VERL were proportional to the numbers of sperm/ova and are indicative of the stage of the reproductive cycle. Maximal levels of VCL and VERL were found in February 2009 declining to minima between July - August before increasing and re-attaining a peak in February 2010. Water temperature may influence these transitions since they coincide with minimal water temperature in February and maximal temperature in August. An identical pattern of variation was found for a cryptic female-specific transcript (H5) but a very different pattern was observed for oestrogen receptor 2 (ER2). ER2 varied in a sex-specific way with male > female for most of the cycle, with a female maxima in July and a male maxima in December. Using artificially spawned animals, the transcripts for VCL, VERL and H5 were shown to be present in gametes and thus their disappearance from mantle is indicative of spawning. VCL and VERL are present at equivalent levels in February and July-August but during gametogenesis (August to January) and spawning (March to June) VCL is present at lower relative amounts than VERL. This may indicate sex-specific control mechanisms for these processes and highlight a potential pressure point leading to reduced reproductive output if environmental factors cause asynchrony to gamete maturation or release

Effect of simvastatin on bone markers in osteopenic women: a placebo-controlled, dose-ranging trial [ISRCTN85429598]

BACKGROUND: Hydroxymethylglutaryl coenzyme A reductase inhibitors increase new bone formation in vitro and in rodents. Results of epidemiologic analyses evaluating the association between use of these cholesterol-lowering drugs, bone mineral density and fracture have been mixed. METHODS: Women (n = 24) with osteopenia, assessed by broad band ultrasound attenuation, were randomized to simvastatin 20 mg, 40 mg or identical-appearing placebo for 12 weeks. Fasting lipid profiles and biochemical markers of bone formation (bone-specific alkaline phosphatase) and resorption (N-telopeptides and C-terminal propeptide of type 1 collagen) were measured at baseline, 6 and 12 weeks. RESULTS: Plasma low density lipoprotein-cholesterol concentration fell 7%, 39% (p < 0.01 vs baseline) and 47% (p < 0.01 vs baseline) after 12 weeks of treatment with placebo, simvastatin 20 mg and 40 mg, respectively. At baseline, bone marker concentrations were similar in the three treatment groups. At 6 and 12 weeks, bone marker concentrations were not different from baseline, and no significant differences in bone marker concentrations were observed between treatment groups at either 6 or 12 weeks. CONCLUSION: Among osteopenic women, treatment with simvastatin for 12 weeks did not affect markers of bone formation or resorption

Patient Uncertainty Questionnaire-Rheumatology (PUQ-R): development and validation of a new patient-reported outcome instrument for systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) in a mixed methods study

Background An in-depth qualitative exploration of uncertainty in systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) led to the development of a five-domain conceptual framework of patient uncertainty in these two conditions. The purpose of this study was to develop and evaluate a new patient-reported outcome (PRO) instrument for patient uncertainty in SLE and RA on the basis of this empirically developed conceptual framework. Methods Cognitive debriefing interviews were conducted to pre-test the initial items generated on the basis of the preliminary qualitative exploration of patient uncertainty in SLE and RA. Two separate field tests were conducted in five hospital sites to evaluate the measurement properties of the new instrument; the first to identify and form scales, and the second to assess measurement properties of the final version in an independent sample. Psychometric evaluation was conducted in line with the Rasch Measurement Theory (RMT), examining the extent to which sample to scale targeting was satisfactory, measurement scales were constructed effectively and the sample was measured successfully. Traditional psychometric techniques were also used to provide complementary analyses best understood by clinicians. Results Pre-testing supported the relevance, acceptability and comprehensibility of the initial items. Findings indicated that the Patient Uncertainty Questionnaire for Rheumatology PUQ-R instrument fulfilled the expectations of RMT to a large extent (including person separation index 0.73 – 0.91). The PUQ-R comprises 49 items across five scales; symptoms and flares (14 items), medication (11 items), trust in doctor (8 items), self-management (6 items) and impact (10 items) which further displayed excellent measurement properties as assessed against the traditional psychometric criteria (including Cronbach’s alpha 0.82 – 0.93). Conclusion The PUQ-R has been developed and evaluated specifically for patients with SLE and RA. By quantifying uncertainty, the PUQ-R has the potential to support evidence-based management programmes and research