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Eingeladener Vortrag
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The stress vs. strain curve of materials is affected the rate of imposed straining. Among the methods for dynamic testing the technique known as 'split Hopkinson pressure bar' (SHPB) has evolved into the most widely used one to exert high-speed straining. The theory behind it comprises simple equations to compute stress and strain. The reliability of the strain analysis can be assessed by digital image correlation (DIC). The present results indicate that the visually observed strain is smaller than predicted by theory.
The stress vs. strain curve of materials is affected the rate of imposed straining. Among the methods for dynamic testing the technique known as 'split Hopkinson pressure bar' (SHPB) has evolved into the most widely used one to exert high-speed straining. The theory behind it comprises simple equations to compute stress and strain. The reliability of the strain analysis can be assessed by digital image correlation (DIC). The present results indicate that the visually observed strain is smaller than predicted by theory.
Embedded conventional fibre Bragg grating (FBG) sensors with acrylate coating have been chosen for monitoring novel textile-reinforced composite materials with a complicated wavy structure as a result of reinforcing textures. Problems of monitoring this type of material occur due to the harsh fabrication technology of the composite and its complicated structure. Embedded fibre sensors are exposed to the influence of transverse and point-wise deformations. To what extent will these effects influence the measurement signal of a sensor? Temperature cycles, tension and vibration tests were carried out to answer this question. Transverse and point-wise deformation and micromechanical indentation tests on non-embedded FBG sensors were carried out to clarify the reliability of recorded signals.
The Split Hopkinson Pressure Bar (SHPB) is a method for material characterization which works at strain rates from 102 - 104 1/s. This method can be used for example in the field of military engineering, in the field of deformation technology, automobile indurstry, aerospace industry or in other industrial fields. Controlled tests with the SHPB device enable Information about the dynamic material deformaion behaviour in the named strain rate area. The setup of the SHPB is not standarized and the measurement principle is dependent on simplifications. Due to this amongst others the problem with the dispersion of the measurement signal occurs. Furthermore it is possible that measurement uncertainties appear caused by the misalignment of the SHPB setup. In this work the influence of the alignment and a dispersion correction by pulse shaping is presented. On the one side for the alignment calibration tests are done. And on the other side for the dispersion influence correction with pulse shaping tests are done with TiA16V4. This material was choosen due to the fact that it can be compared with existing literature data.
The Split Hopkinson Pressure Bar (SHPB) or
Kolsky Bar is known as a method for analysing
mechanical properties of a material under dynamic
load. Referring to the name it is a splitted
Hopkinson Pressure Bar. The Hopkinson Bar is
related to its inventor Bertram Hopkinson (1914).
He used it to analyze stone samples under dynamic
load. Later his construction was improved by
Davies (1948) [1] and Kolsky (1949) [2], who had
the idea to split the bar and put a sample in
between.
The technique is used to measure a lot of
different mechanical properties of a material e.g.
the dynamic Youngs modulus, deformation
behaviour or to chart the dynamic stress-strain
diagram. Further applications are the measurement
of the elastic wave and the analysis of the
propagation of the wave [2]. In the last years there
was still an interest in measuring dynamic
properties with a SHPB. Nevertheless no
comprehensive validation of the system exists.
Another problem is that the stress and the strain in
the sample are only calculated with simplified
estimations.
The aim of this paper is to give a first
impression of important points validating the
system. On account of different setups of the
SHPB a validation of the used system is necessary.
Hence it is important to look at the theory. Further
in this paper the measurement results will be
shown in a strain over time diagram.
The absolute detection of a gap distance of loose and ftxed extrinsic Fabry-Perot interferometer (EFPI) sensors with conventional measurement Systems is not possible. Due to this EFPI-sensors were deemed to be unfit for distance determination in comparision with FBG sensors. With the conventional measurement method a continous measurement of the EFPIsensors was necessary. Otherwise data about the distance change was lost. Additional it is necessary to detect the tumaround points in the interference signal with 100%. This made it worse for the analysis and the use of the sensor. At BAM Federal Institute for Materials Research and Testing a measurement System called “EFPI-Scan” is developed which can measure the absolute gap distance of four connected EFPI-sensors synchronous. Every measurement channel is temperature independetn and long term stable. Test measurements showed a deviation of maximum ±1 gm per hour. Furthermore it is possible to measure with different measurement modes (abolute, relative and a combination of both). Special emphasis should be placed to the absolute measurement accuracy of ± 5 pm at a gap distance determination ftom 40 up to 4000 pm.
Fiber optic sensors have gained increasing importance in recent years and are well established in many areas of industrial applications. In this paper, we introduce a concept of a self-diagnostic fiber optic sensor. The presented sensor is to resolve the problems of embedded fiber optic sensors in complex structures and to enable the validation under operational conditions. For this purpose, different magnetostrictive coated fiber optic sensors were developed and various experiments were performed to verify their mode of Operation and to determine the respective reproducibility. The measuring principle is illustrated by obtained experimental results, which showed a change in wavelength from 1 pm at a magnetic field strength change of 0.25 mT.
In addition, the temperature characteristics of the implemented magnetostrictive sensor were analyzed and an experimental factor of 1.5 compared to a reference fiber optic sensor was determined.
Fiber optic sensors have gained increasing importance in recent years and are well established in many areas of industrial applications. In this paper, we introduce a concept of a self-diagnostic fiber optic sensor. The presented sensor is to resolve the problems of embedded fiber optic sensors in complex structures and to enable the validation under operational conditions. For this purpose, different magnetostrictive coated fiber optic sensors were developed and various experiments were performed to verify their mode of operation and to determine the respective reproducibility. The measuring principle is illustrated by obtained experimental results, which showed a change in wavelength from 1 pm at a magnetic field strength change of 0.25 mT. In addition, the temperature characteristics of the implemented magnetostrictive sensor were analyzed and an experimental factor of 1.5 compared to a reference fiber optic sensor was determined.
In this work an air-based sensor network for the localization of persons at extensive areas is presented. The developed network consists of a localization device which the person is wearing (BodyGuard-System), a mobile relay station in the air, and a base station. All three parts communicate with the same radio chip. The BodyGuard-System is an inertial navigation system which was developed for localization in difficult environments with high accuracy and low measurement uncertainty. To increase the range of the system, a multi-hop network was built up. The measured data of the BodyGuard-System and the mobile relay station is visualized on a PC in the base station. This multi-hop network is necessary for example for fire department missions.
Structural health monitoring by distributed fiber optic sensors embedded into technical textiles
(2012)
Technical textiles with embedded distributed fiber optic sensors have been developed for the purposes of structural health monitoring in geotechnical and civil engineering. The distributed fiber optic sensors are based on Brillouin scattering in silica optical fibers and OTDR in polymer optical fibers. Such 'smart' technical textiles are used for reinforcement of geotechnical and masonry structures. The embedded fiber optic sensors provide online information about the condition of the structure and about the occurrence and location of any damage or degradation.--------------------------------------------------------------------------------------------------------------------------------------------------------
Technische Textilien mit integrierten faseroptischen Sensoren eröffnen neue Möglichkeiten der Zustandsüberwachung (structural health monitoring) in Geotechnik und Ingenieurbau. Die verteilt messenden Sensoren basieren auf der Brillouin-Streuung in Glasfasern und auf der OTDR in polymeroptischen Fasern. Derartige 'intelligente' technische Textilien werden in erster Line zur Verstärkung von geotechnischen Bauwerken und von Gebäuden genutzt. Die integrierten Sensoren liefern eine zeitnahe Information über den bestimmungsgemäßen Zustand des Bauwerks sowie über die Entstehung und den Ort von lokalen Bauwerksschäden.
The Split Hopkinson Pressure Bar (SHPB) is a materials testing apparatus primarily designed for the analysis of viscoplastic material behavior at very high strain rates.
Because of its complexity it is common practice to simplify the underlying exact mechanical theory by making strong assumptions. On the one hand this has to be done to achieve any results at all on the other hand one commits systematic errors. Therefore we utilize different numerical and Simulation methods to analyze and minimize these errors.
High quality tubular products are essential to the oil and gas industry. Quality control during their production focuses on the non-destructive detection of surface defects. The structured light technique is a candidate for the challenge to detect, monitor and evaluate such defects in real-time.
In the present study the automatic processing of structured light measurements is performed and validated. The algorithm for the automatic Analysis of inspection data has an advantage over current data evaluation methods based on individual assessments of operators.
Measurement and data recording systems are important parts of a holistic Structural Health Monitoring (SHM) system.
New sensor technologies such as fiber-optic sensors are often used; however, standards (or at least guidelines) are not yet available or internationally approved. This lack in standardization makes the acceptance of FOS technologies in complex SHM systems substantially difficult. A standard family for different FOS technologies is therefore being developed that should help to design SHM systems in an optimal way. International standardization activities take place in several standardization bodies such as IEC and ASTM, and within SHM societies such as ISHMII.
The paper reports on activities in standardization of fiber-optic sensors, on results already achieved, and on newly started projects. Combined activities of fiber sensor experts and SHM experts from Civil Engineering are presented. These contributions should help owners of structures as well as developers of sensors and monitoring systems to select effective and validated sensing technologies. Using these standards, both parties find recommendations how to proceed in development of SHM systems to evaluate the structural behavior based on e.g. standardized fiber optic sensors, and to derive necessary measures, e.g. the optimal maintenance strategy.
Standardization in fibre-optic sensing for structural safety - activities in the ISHMII and IEC
(2015)
Fiber-optic sensors are increasingly established in the sensor market. Their advantages have unquestionably been verified by numerous demonstrations to enhance the operational performance of aged structures or to monitor the structural behavior of safety-relevant structures or their components. However, there are some barriers in use due to a lack of extensive standardization of fiber-optic sensors. This leads very often to restraints in the users community. The paper shows the status in international standardization of fiber-optic sensors as well as current activities in leading institutions such as IEC and ISHMII and others with the purpose of providing relevant standards for a broader use of selected fiber-optic sensor technologies.