Identifying fabrication defects of metal packaged fibre Bragg grating sensors for smart pre-stressing strands

Metal packaged fibre Bragg grating sensors have the potential to provide reliable measurements of temperature and strain in high stress environments for the purpose of structural health monitoring. However due to the induction brazing fabrication process a small percentage of sensors are found to have defective brazed joints. We demonstrate a defect identification procedure derived from the batch temperature calibration of fibre Bragg grating sensors for temperature and strain measurement, allowing defective sensors to be identified before installation. The procedure was demonstrated on a sample of twelve temperature sensors fabricated for a small-scale smart pre-stressing strand validation test

In-situ temperature calibration procedure for temperature and strain fibre Bragg grating sensors for monitoring pre-stressing strands

In this work, we demonstrate active and passive methods for in-situ temperature calibration of fibre Bragg grating strain and temperature sensors. The method is suitable for characterizing sensors which are already attached to the steel reinforcements of civil structures. The proposed method, which involves the use of active induction heating or passive room temperature fluctuations, can be implemented using portable equipment, is time efficient, and can be used to calibrate attached sensors on-site, rather than in lab conditions. Preliminary results of the induction heating calibration show good agreement with pre-calibrated temperature sensors. In-situ calibration of fibre strain sensors, attached to a prestressing strand is also successfully carried out

Metal-Packaged fibre Bragg grating strain sensors for surface mounting onto spalled concrete wind turbine foundations

In this work, we demonstrate preliminary results for a hermetically sealed, metal-packaged fibre Bragg grating strain sensor for monitoring existing concrete wind turbine foundations. As the sensor is bolted to the sub-surface of the concrete, it is suitable for mounting onto uneven, wet and degraded surfaces, which may be found in buried foundations. The sensor was able to provide reliable measurements of concrete beam strain during cyclic three- And four- point bend tests. The strain sensitivity of the prototype sensor is currently 10 % of that of commercial, epoxied fibre strain sensors

Comparison of epoxy and braze-welded attachment methods for FBG strain gauges

This paper presents experimental results from fatigue and static loading tests performed on both epoxy and braze-welded FBG strain sensors. Most FBG attachment methods are relatively understudied, with epoxy the most commonly used. Long curing times and humidity sensitivity during curing render epoxy inappropriate for certain implementations. This work shows that a bespoke braze-welded attachment design is able to achieve a higher static failure limit of 22kN when compared to strain gauge epoxies, which fail at 20kN. Both methods demonstrate high fatigue life, with no significant deterioration after two million cycles. Epoxy swelling was observed when the sensors were held at a relative humidity of 96%, applying ~0.6 mϵ of tension to the FBG, whereas a braze-weld attachment was unaffected by humidity

Predicting freeze-thaw damage using tipping point analysis of strain data

This paper demonstrates how Tipping Point Analysis can be used to predict the onset of structural strain, induced by ice formation. In civil structures, water-ice transitions present many potential issues that can lead to structural damage or plant shutdowns. Examples include freeze-thaw damage in concrete and masonry, and ice build-up on moving parts such as wind turbine blades. Early indication of ice formation could prevent irreparable damage if this information could be used to actuate de-icing procedures. The transition considered in this work is the strain induced in a polypropylene container by the volume change of water as it freezes, measured using surface-mounted fibre-optic strain sensors. This first order phase transition can be detected early on using degenerate fingerprinting, which identifies “slowing down” of the noise prior to the critical point of the transition. Water was supercooled which, at freezing, causes a rapid increase in temperature, presenting an identifiable specific transition point for reference. The analysis was able to consistently predict freezing around 5-10 minutes prior to the transition. A linear relationship was found between mass calculated from the calorimetric equation and mass from experimental measurements. Strain could not be estimated from this mass, since the random process of freezing in an open top container causes an irregular distribution of force. These tests will allow the method and the model to be continually developed towards a more practical application

Amine-free CO2-switchable hydrophilicity solvents and their application in extractions and polymer recycling

Volatile organic solvents are used in almost every industry currently in operation. Their key advantage, being easily removed from products, is overshadowed by the issues associated with their vapour emissions, inhalation risks, smog formation, and flammability. There is, therefore, a dire need for replacement solvents that are much less volatile and yet are as easily removed from products. Switchable-hydrophilicity solvents (SHS) meet this need. SHS are solvents that alternate between water-miscible and water-immiscible states upon the application of a trigger. Carbon dioxide functions as the trigger, as it acidifies the solution and causes protonation of the switchable solvent, which in the past has almost always been a liquid amine. Anionic SHS (ASHS) systems, a recently discovered variation on the idea of an SHS, consist of a hydrophobic carboxylic acid paired with a water-soluble base. In these systems, the low miscibility carboxylic acid (in its neutral state) is combined with a base dissolved in water. A very small number of ASHS have been reported, but they can only function in the presence of an amine and an unacceptably large excess of water. We sought an SHS that has as little toxicity as possible, and for that reason, we preferred one that contains no amine at all. We now report that the combination of branched carboxylic acids and a substoichiometric quantity of either NaHCO3 or NaOH forms an SHS with very little toxicity and does not require a large excess of water. Two applications of these new switchable solvents are described

The role of angiogenic and antiangiogenic factors in the second trimester in the prediction of preeclampsia in pregnant women with type 1 diabetes

OBJECTIVE: To assess the association between circulating angiogenic and antiangiogenic factors in the second trimester and risk of preeclampsia in women with type 1 diabetes. RESEARCH DESIGN AND METHODS: Maternal plasma concentrations of placental growth factor (PlGF), soluble fms-like tyrosine kinase 1 (sFlt-1), and soluble endoglin (sEng) were available at 26 weeks of gestation in 540 women with type 1 diabetes enrolled in the Diabetes and Preeclampsia Intervention Trial. RESULTS: Preeclampsia developed in 17% of pregnancies (n = 94). At 26 weeks of gestation, women in whom preeclampsia developed later had significantly lower PlGF (median [interquartile range]: 231 pg/mL [120–423] vs. 365 pg/mL [237–582]; P < 0.001), higher sFlt-1 (1,522 pg/mL [1,108–3,393] vs. 1,193 pg/mL [844–1,630] P < 0.001), and higher sEng (6.2 ng/mL [4.9–7.9] vs. 5.1 ng/mL[(4.3–6.2]; P < 0.001) compared with women who did not have preeclampsia. In addition, the ratio of PlGF to sEng was significantly lower (40 [17–71] vs. 71 [44–114]; P < 0.001) and the ratio of sFlt-1 to PlGF was significantly higher (6.3 [3.4–15.7] vs. 3.1 [1.8–5.8]; P < 0.001) in women who later developed preeclampsia. The addition of the ratio of PlGF to sEng or the ratio of sFlt-1 to PlGF to a logistic model containing established risk factors (area under the curve [AUC], 0.813) significantly improved the predictive value (AUC, 0.850 and 0.846, respectively; P < 0.01) and significantly improved reclassification according to the integrated discrimination improvement index (IDI) (IDI scores 0.086 and 0.065, respectively; P < 0.001). CONCLUSIONS: These data suggest that angiogenic and antiangiogenic factors measured during the second trimester are predictive of preeclampsia in women with type 1 diabetes. The addition of the ratio of PlGF to sEng or the ratio of sFlt-1 to PlGF to established clinical risk factors significantly improves the prediction of preeclampsia in women with type 1 diabetes