Detailed Surface Measurements Research Articles

Observations of modal coupling due to bolted joints in an experimental benchmark structure

The goal of this paper is to present a set of measurements from a benchmark structure containing two bolted joints to support future efforts to predict the damping due to the joints and to model nonlinear coupling between the first two elastic modes. Bolted joints introduce nonlinearities in structures, typically causing a softening in the natural frequency and an increase in damping because of frictional slip between the contact interfaces within the joint. These nonlinearities pose significant challenges when characterizing the response of the structure under a large range of load amplitudes, especially when the modal responses become coupled, causing the effective damping and natural frequency to not only depend on the excitation amplitude of the targeted mode, but also the relative amplitudes of other modes. In this work, two nominally identical benchmark structures, known in some prior works as the S4 beam, are tested to characterize their nonlinear properties for the first two elastic modes. Detailed surface measurements are presented and validated through finite element analysis and reveal distinct contact interactions between the two sets of beams. The free-free test structures are excited with an impact hammer and the transient response is analyzed to extract the damping and frequency backbone curves. A range of impact amplitudes and drive points are used to isolate a single mode or to excite both modes simultaneously. Differences in the nonlinear response correlate with the relative strength of the modes that are excited, allowing one to characterize mode coupling. Each of the beams shows different nonlinear properties for each mode, which is attributed to the different contact pressure distributions between the parts, although the mode coupling relationship is found to be consistent between the two. The test data key finding are presented in this paper and the supporting data is available on a public repository for interested researchers.

Spalling of concrete cover induced by reinforcement

This paper investigates the phenomenon of cover spalling in reinforced concrete induced by bond or by the action of an inner pressure. This research is based on an experimental programme comprising a series of bond tests and a series of tests with inner-pressure on cylindrical openings. The inner-pressure test series (aimed at representing the conditions occurring for instance due to corrosion of reinforcement) was performed with hydraulic inflator devices embedded within concrete openings near to the free concrete surface. By means of detailed surface measurements performed with Digital Image Correlation, the mechanisms triggering spalling failures and the associated resistance are discussed and analysed thoroughly. This series investigates in addition a number of phenomena relevant to spalling failures, such as the influence of the casting position, group and size effects. The observed response, analysed by means of a mechanical analogy, is later used to investigate a series of structural tests performed on pull-out specimens. This analysis highlights the analogies and differences between the two types of tests (subjected to an imposed pressure or to bond stresses). On that basis, a comprehensive approach for treatment of bond-related cases failing by cover spalling is proposed, showing consistent agreement to the experimental evidence.

MULTI-SENSOR DOCUMENTATION OF METRIC AND QUALITATIVE INFORMATION OF HISTORIC STONE STRUCTURES

Abstract. This paper focuses on the integration of multi-sensor techniques regarding the acquisition, processing, visualisation and management of data regarding historic stone structures. The interdisciplinary methodology that is carried out here comprises of two parts. In the first part, the acquisition of qualitative and quantitative data concerning the geometry, the materials and the degradation of the tangible heritage asset each time, is discussed. The second part, refers to the analysis, management and visualization of the interrelated data by using spatial information technologies. Through the paradigm of the surveying of the ancient temple of Pythian Apollo at Acropolis of Rhodes, Rhodes Island, Greece, it is aimed to highlight the issues deriving from the separate application of documentation procedures and how the fusion of these methods can contribute effectively to ensure the completeness of the measurements for complex structures. The surveying results are further processed to be compatible and integrated with GIS. Also, the geometric documentation derivatives are combined with environmental data and the results of the application of non-destructive testing and evaluation techniques in situ and analytical techniques in lab after sampling. GIS operations are utilized to document the building materials but also to model and to analyse the decay extent and patterns. Detailed surface measurements and geo-processing analysis are executed. This integrated approach, helps the assessment of past interventions on the monument, identify main causes of damage and decay, and finally assist the decision making on the most compatible materials and techniques for protection and restoration works.

Large and primarily updip afterslip following the 2012 Mw 7.6 Nicoya, Costa Rica, earthquake

AbstractWe present detailed surface measurements of the first 3.5 years of postseismic deformation following the 5 September 2012 moment magnitude (Mw) 7.6 Nicoya, Costa Rica, earthquake. The dominant signal in the first 2.5 years is uniform horizontal trenchward motion totaling 7–26 cm across 40 stations. Trenchward velocity is strongly diminished by mid‐2014 and appears by 2016 to have begun reversing. We invert the first 2.5 years to determine the corresponding afterslip on a detailed 3‐D interface. Results show significant afterslip both updip and downdip of the main coseismic rupture zone, with as much as 1.7 m of offset in two patches at 15–20 km depth and immediately updip of the maximum coseismic slip. This updip slip represents an important mechanism to address unrelieved interseismic locking, although sufficient strain energy remains to generate up to a Mw 7.1 event near the coastline. The afterslip patches are anticorrelated with strongly clustered aftershocks at the same depth, which is indicative of varying frictional behavior along strike. An additional patch of slip is colocated with reoccurring slow‐slip events beneath the Gulf of Nicoya. The magnitude of the observed slip, however, cannot be sufficiently explained by the known slow‐slip events. Ongoing measurements will be crucial to understanding the relocking process in Nicoya.

A new reservoir model to support environmental monitoring of the Orakeikorako geothermal system

Orakeikorako is one of several protected geothermal systems in the Taupo Volcanic Zone (TVZ) in the centre of the north island of New Zealand. Its beautiful surface features are highly valued for both their cultural significance and for tourism. Many other geothermal systems in the TVZ have been developed for energy utilisation including Ngatamariki which borders Orakeikorako. The interaction of geothermal systems is poorly understood and is an area of active research in New Zealand. A new reservoir model has been developed to support environmental monitoring of the Orakeikorako system and to assist future investigations of its interaction with neighbouring geothermal systems. It also demonstrates that accurate reservoir models can be developed by using detailed surface measurements for calibration when down-hole data are not available. This is important both for modelling protected geothermal systems and also for assessment of systems in the early stages of development.

Radiation exposure in the moon environment

During a stay on the moon humans are exposed to elevated radiation levels due to the lack of substantial atmospheric and magnetic shielding compared to the Earth's surface. The absence of magnetic and atmospheric shielding allows cosmic rays of all energies to impinge on the lunar surface. Beside the continuous exposure to galactic cosmic rays (GCR), which increases the risk of cancer mortality, exposure through particles emitted in sudden nonpredictable solar particle events (SPE) may occur. SPEs show an enormous variability in particle flux and energy spectra and have the potential to expose space crew to life threatening doses. On Earth, the contribution to the annual terrestrial dose of natural ionizing radiation of 2.4mSv by cosmic radiation is about 1/6, whereas the annual exposure caused by GCR on the lunar surface is roughly 380mSv (solar minimum) and 110mSv (solar maximum). The analysis of worst case scenarios has indicated that SPE may lead to an exposure of about 1Sv. The only efficient measure to reduce radiation exposure is the provision of radiation shelters.Measurements on the lunar surface performed during the Apollo missions cover only a small energy band for thermal neutrons and are not sufficient to estimate the exposure. Very recently some data were added by the Radiation Dose Monitoring (RADOM) instrument operated during the Indian Chandrayaan Mission and the Cosmic Ray Telescope (CRaTER) instrument of the NASA LRO (Lunar Reconnaisance Orbiter) mission. These measurements need to be complemented by surface measurements.Models and simulations that exist describe the approximate radiation exposure in space and on the lunar surface. The knowledge on the radiation exposure at the lunar surface is exclusively based on calculations applying radiation transport codes in combination with environmental models.Own calculations are presented using Monte-Carlo simulations to calculate the radiation environment on the moon and organ doses on the surface of the moon for an astronaut in an EVA suit and are compared with measurements. Since it is necessary to verify/validate such calculations with measurement on the lunar surface, a description is given of a radiation detector for future detailed surface measurements. This device is proposed for the ESA Lunar Lander Mission and is capable to characterize the radiation field concerning particle fluencies, dose rates and energy transfer spectra for ionizing particles and to measure the dose contribution of secondary neutrons.

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor—Part II: Heat Transfer Results

Measurements using a novel heat flux sensor were performed in an internal ribbed channel representing the internal cooling passages of a gas turbine blade. These measurements allowed for the characterization of heat transfer turbulence levels and unsteadiness not previously available for internal cooling channels. In the study of heat transfer, often the fluctuations can be equally as important as the mean values for understanding the heat loads in a system. In this study, comparisons are made between the time-averaged values obtained using this sensor and detailed surface measurements using the transient thermal liquid crystal technique. The time-averaged heat flux sensor and transient TLC results showed very good agreement, validating both methods. Time-resolved measurements were also corroborated with hot film measurements at the wall at the location of the sensor to better clarify the influence of unsteadiness in the velocity field at the wall on fluctuations in the heat flux. These measurements resulted in turbulence intensities of the velocity and heat flux of 20%. The velocity and heat flux integral length scales were about 60% and 35% of the channel width, respectively, resulting in a turbulent Prandtl number of 1.7 at the wall.

Geophysical investigations in sweden for the characterization of a site for radioactive waste disposal — An overview

The geophysical measurements carried out for the investigations of prospective sites for radioactive waste disposal in Sweden are described. The investigations comprise detailed surface measurements with magnetic, electromagnetic, electric and seismic techniques. Measurements are made with a line spacing of 40 m and a station spacing of 20 m in an area of 4–6 km 2. At each site 7–10 deep boreholes are drilled. A number of geophysical logs are run in the boreholes, including several electric logs, providing data on temperature, salinity, and gamma ray. These geophysical data are evaluated together with the results from geological and hydrological investigations to make a three-dimensional geological and tectonical model of the site. This model defines the geometry used in the numerical modelling of the groundwater flow. In the case of thin, resistive overburden the surface investigations proved to be effective in the detection of dipping fracture zones. A line spacing as low as 40 m is needed to get adequate mapping of the fracture zones. The methods used cannot be considered efficient for mapping sub-horizontal fracture zones. The borehole logs provide data for the identification of fracture zones as well as different lithological units and give qualitative information of the hydraulic conditions. The temperature and salinity logs have provided the best hydraulic information, whereas, after testing, the induction log (borehole slingram), the differential resistance log and the VLF-log were found to be not effective for our investigations. The borehole techniques used, normally give information only about a relatively small volume of rocks around the boreholes. To obtain a detailed description of the bedrock between the boreholes, development of cross-hole geophysical, high-resolution techniques of considerable range, is needed.

Investigation of Three-Dimensional Shock/Boundary-Layer Interactions at Swept Compression Corners

This paper presents the first phase of an experimental study of a three-dimensional shock wave/turbulent boundary-layer interaction at a swept compression corner. Compression corners of 16 and 24 deg angles, whkh cause near-incipient and well-separated two-dimensional flows at Much 3, are systematically swept through angles up to 50 deg while maintaining a constant streamwise corner angle. The resulting three-dimensional flows are studied by way of detailed surface measurements and a few exploratory flowfield surveys. Tests of four Reynolds numbers reveal that a Reynolds number influence on the interaction length remains in effect across the available range of sweep. Cylindrical symmetry is obtained along the compression corner at many of the conditions tested. The corner flow differs little from the two-dimensional case for sweep angles less than about 10 deg, above which the interaction length begins to increase dramatically. This increase agrees qualitatively with the assumption of two-dimension al flow normal to the swept corner. Flowfield surveys show that the boundary layer breaks away from the surface to form a free layer at sweep angles up to at least 40 deg. Further, measured yaw angles within the swept interaction deviate only a few degrees from the streamwise direction except near the model surface.