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With more than 180.000 engineers and scientists VDI Association of German Engineers and VDE Association for Electrical, Electronic & Information Technologies are belonging to the major technical and scientific associations in Europe. Both organizations promote the advancement of technology and among other things by developing recognized technical regulations as national and international standards. Since 2011 Technical Committee 2.15 Optical Measuring Methods for Structure Analysis and Monitoring as a common committee of both organizations is working among other things on the development of standards for optical measuring and testing methods. The committee consists of 27 members representing industrial users, system manufacturers, research institutes and universities. Currently the committee is drafting the new standard VDI/VDE 2626 Part 1 - Optical measuring procedures – Image correlation methods – Basics and equipment, which will be introduced in this presentation.
Draft VDI/VDE 2626 Part 1 describes practical acceptance and reverification procedures for the evaluation of the accuracy of optical measuring systems based on digital image correlation (DIC) methods. The document applies to DIC measurement systems used a) for full-field determination of two- or three-dimensional displacement of surfaces, and b) for
full-field determination of the elongation or strain of surfaces. The standard describes quality parameters as well as methods and reference objects for their determination. The quality parameters serve to specify DIC systems, and to compare different systems. They are equally suitable a) for acceptance tests as well as b) for reverification of DIC systems within the framework of a quality management system.
The acceptance test is used to determine whether the DIC system complies with the quality parameter limits specified by the manufacturer, or with contractually agreed limits by the manufacturer and user, respectively. For this test the quality parameters zero-displacement error, zero-strain error, and displacement error are defined and used. Zero-displacement error and zero-strain error describe minimum measurement uncertainties of the DIC system a user has to reckon with. They are influenced by different uncertainty contributions. The quality parameter displacement error is used to check the ability of the DIC system to determine absolute displacements within the measurement volume and to confirm the traceability of this measurement to the SI system.
Reverification of DIC systems serves to ensure long-term compliance with specified limits for the quality parameters. By comparing the results of successive reverification measurements of the quality parameters used for the acceptance test, it is possible to analyze trends with respect to changes in DIC system characteristics.
At present, Wind Turbine Generators (WTGs) operating in onshore and offshore wind farms are primary sources of renewable energy around the world. Cylindrical grouted sleeve connections are usually adopted in these WTG structures to connect the upper structure and foundation for ease of installation. These structures including grouted connections experience considerable adverse loading during their lifetimes. Settlements were reported inside similar connections used in energy structures especially oil and gas platforms, which were installed in last three decades. Thus, repair and rehabilitation of such connections in existing wind structures should also be planned ahead to keep them operating in the future. The nature of failure and crack generation in grouted connections are crucial prior to adopt a strengthening strategy. This pilot study is carried out to actualize the failure mechanism in the grouted connection, when subjected to axial loading. A novel reusable scaled cylindrical grouted connection with shear keys was designed and tested for its load bearing behaviour. The mechanical test was accompanied by classical measuring techniques (e.g. displacement transducer) as well as non-destructive measuring techniques (e.g. digital image correlation (DIC), acoustic emission analysis (AE)). The failure mechanism incorporating slippage of the shear keys and cracking of the grout was investigated. The capacity and applicability of such test mould were also discussed. The knowledge is expected to pave way towards repair of deteriorated grouted connections with similar geometry and failure pattern.
The paper presents a comparison between different numerical modelling approaches aiming to simulate the in-plain behaviour of three types of earthen materials, namely adobe masonry, rammed earth and cob. For this purpose, uniaxial and diagonal compression tests were carried out, which allowed determining important mechanical parameters, such as compressive strength, Young’s modulus, Poisson’s ratio, shear strength and shear modulus. Furthermore, the tests allowed assessing the level of non-linear behaviour of the respective stress strain relationships as well as the failure modes. The experimental results were then used for the calibration of numerical models (based on the finite element method) for simulating the non-linear behaviour of the earth materials under in-plane shear loading. Both macro- and micro-modelling approaches were considered for this purpose. The procedures adopted for model calibration established the reliability of various modelling strategies for the different loading conditions. The simplified Approach based on macromodelling shows a satisfactory accuracy and low computational costs. The results reproducing the uniaxial compression are in good correspondence with the post-elastic behaviour observed in the experimental campaign. The micro-modelling approach adopted to reproduce the shear behaviour, even with higher computational cost, represents a suitable tool to predict the adobe masonry and rammed earth collapse mechanisms.
The tensile behavior of strain-hardening cement-based composites (SHCC) is usually investigated on macroscopic scale by means of direct tension tests or bending tests. Additionally, the micromechanical properties of the composites are often described based on single fiber tension and pull-out tests. Such investigations, performed both on macroscopic and microscopic scales, are based on ‘classical’ force and displacement measuring techniques. Advanced test methods such as digital image correlation (DIC) and acoustic emission analysis (AE) may facilitate the identification and the analysis of the failure mechanisms in SHCC, which is important for both monitoring loaded SHCC elements and
further material development and optimization. In this study, these two techniques are combined to characterize the failure mechanisms of three different types of SHCC in direct tension tests. The results are related to data of stress and strain measurements. It is shown that DIC provides detailed spatially resolved and stress related strain measurements. Furthermore, it is demonstrated that AE allows for the localization of active cracks, quantification of the damage accumulation under increasing stresses, and characterization of the dominant crack bridging mechanisms and failure modes observed in the different types of SHCC.
The sustainable and resource-efficient production of wind energy plants requires the use of modern high-strength fine-grain structural steels. This applies to both foundation and erection structures. During the assembly of steel structures, unacceptable defects can occasionally be found in the weld area. In most cases, the economical solution would be local thermal gouging of the affected areas and re-welding. Due to the high shrinkage restraint of the joint groove in the overall structure, the superposition of design and welding-induced residual stress can lead to crack formation and ultimately to component failure, particularly in interaction with the degradation of the microstructure and mechanical properties high-strength steels during the repair process.
In this context, component-related investigations (FOSTA-P1311/IGF-Nr. 20162 N) focusing on welding residual stress evolution during local thermal gouging and rewelding of S500MLO and S960QL are carried out using in-situ digital image correlation (DIC) and ex-situ X-ray diffraction methods (XRD). Therefore, self-restrained specimens were systematically gouged and rewelded with defined welding heat control (heat input, working temperature) and parameters. By means of structural mechanics calculations, geometries of self-restrained specimens were identified, that represent defined rigidity conditions of repair welds of real components. It could be shown that with increasing restraint intensity significantly higher residual stresses occur in the weld metal and heat affected zone. Furthermore, it revealed that the transverse stresses along the weld seam decrease at the weld seam ends leading to different stress state during gouging and welding. XRD analysis of the local residual stresses after cooling to RT longitudinal and transverse to the weld direction showed a good comparability with global DIC analyses.
High entropy alloys (HEA) are a new class of materials that have been investigated since the early 2000s and offer great potential to replace conventional alloys. However, since they sometimes have significant contents of expensive alloying elements such as Co or Ni, their use is only conceivable in highly stressed areas of components. For this purpose, the weldability with conventional alloys such as high-alloy austenitic steels must be investigated. In addition to the resulting microstructure, the mechanical properties are also fundamental for the usability of HEAs in DMWs. For this purpose, TIG welds of CoCrFeMnNi HEA (cold rolled and recrystallized state) with AISI 304 austenitic steel are investigated. These mechanical properties are analyzed in this work by means of tensile tests and local hardness measurement. The local strain behavior of the welded joints is also characterized by means of Digital Image Correlation (DIC). The results of the local hardness measurement show a clear influence of the initial condition of the HEA on the HAZ. Thus, the HEA in the cold-rolled condition shows a clear softening because of recrystallization processes in the HAZ. On the other hand, there is no influence on the hardness of the weld metal, which is approx. 200 HV0.1 in both cases. The tensile tests show a consistent failure of the weld in the weld metal. However, regardless of the HEA condition, strengths in the range of the recrystallized HEA (RM ~ 550–600 MPa) are achieved, although with significantly reduced fracture elongations.
The essence of dynamic failure is closely linked to the formation of adiabatic shear bands (ASB), which result from the localization of shear strain under high deformation speeds accompanied by a rapid temperature increase. Understanding this phenomenon is crucial in view of safety issues when impacts of fast rotating machine components (i.e. aircraft turbine blades) may occur. Our contribution addresses both the experimental evidence and characterization of ASBs due to high-speed impact tests at the Split HOPKINSON pressure bar (SHPB) setup and the finite element analysis to determine the parameters of the underlying constitutive model, which is closely related to JOHNSON-COOK (JC) material model.
Experimental investigations were performed on notched shear specimens made of the fine -grained structural steel S690QL and the displacements in the regions affected by shear localization were measured with subset-based local Digital Image Correlation (DIC). The displacement fields, obtained in the SHPB tests, were considered as an objective to validate and to identify the constitutive parameters with. The JC model could reasonably reproduce the displacement distribution. In order to overcome the issues with mesh dependency we provide a nonlocal extension based on the implicit gradient model approach.
A safety or security related assessment of explosions, accidental and intentional scenarios alike, often necessitate performance of replication-tests. Such test results are necessary to clarify the causes within the scope of forensic investigations. To gain important insights into the behavior of structures and materials under such loading, field tests may also be performed in accordance with different test standards. To determine the resistance of building-structures after explosions, estimation of the residual load-bearing capacity in addition to the assessment of dynamic structural response and damage to the building components is important. In most cases an evaluation of structural integrity is based only on the visual damage, resulting in an overestimation of the residual capacity.
The Bundesanstalt für Materialforschung und -prüfung (BAM) operates the Test site for Technical Safety (TTS) on an area measuring about 12 km2 in the Federal State of Brandenburg for execution of true-to-scale explosion tests. At the TTS, building component testing was performed to assess the suitability of different non-destructive testing methods to characterize the dynamic structural response and damage resulting from the detonation of high explosives.
Different blast-loading scenarios were realized by varying the net explosive mass and the standoff distance with all scenarios representing a near-field detonation. The test object was a reinforced concrete wall 2 m high, 2.5 m wide and 20 cm thick, fixed at both vertical edges. The dynamic loading of the wall was characterized with 8 piezoelectric pressure sensors flush-mounted on the front surface, thus measuring the reflected pressures from the shock wave. The tests were conducted with the aim of characterizing the global behavior of the wall under dynamic shock loading and the resulting local damage pattern, respectively. High speed digital image correlation was implemented in combination with multiple acceleration sensors to observe the rear surface of the wall to chart the dynamic deflection during the loading and to determine the residual deformation after the loading had ceased. In addition, one test specimen was instrumented with fiber optic sensor cables, both fixed to the rebars and embedded in the concrete-matrix, respectively. Firstly, these sensors were interrogated during the blast test by a distributed acoustic sensing (DAS) device using a particularly high sampling rate to measure the shock-induced vibrations in the structure with high temporal resolution. This delivers information on dynamics of compression and tension cycles from within the structure. Secondly, the local damage-pattern emerging during the series of blasts was determined via distributed fiber optic strain sensing (DSS) by interrogating the embedded fiber optic sensors with a high spatial resolution DSS device after each blast. This enabled the characterization of non-visual damage to the structure, in particular with regard to the formation of localized cracks in the concrete matrix. The DSS was further complimented by a structure-scanner based on ultrasonic measurements.
Our contribution describes this new test approach in detail. Results of the three datasets, namely dynamic shock loading, global behavior of the test object and the local damage pattern will be presented. The suitability of the implemented measurement methods will be discussed in combination with the challenges in their application for technical safety evaluation of building components under explosive loading.
High entropy alloys (HEA) are a new class of materials that have been investigated since the early 2000s and offer great potential to replace conventional alloys. However, since they sometimes have significant contents of expensive alloying elements such as Co or Ni, their use is only conceivable in highly stressed areas of components. For this purpose, the weldability with conventional alloys such as high-alloy austenitic steels must be investigated. In addition to the resulting microstructure, the mechanical properties are also fundamental for the usability of HEAs in DMWs. For this purpose, TIG welds of CoCrFeMnNi HEA (cold rolled and recrystallized state) with AISI 304 austenitic steel are investigated. These mechanical properties are analyzed in this work by means of tensile tests and local hardness measurement. The local strain behavior of the welded joints is also characterized by means of Digital Image Correlation (DIC). The results of the local hardness measurement show a clear influence of the initial condition of the HEA on the HAZ. Thus, the HEA in the cold-rolled condition shows a clear softening because of recrystallization processes in the HAZ. On the other hand, there is no influence on the hardness of the weld metal, which is approx. 200 HV0.1 in both cases. The tensile tests show a consistent failure of the weld in the weld metal. However, regardless of the HEA condition, strengths in the range of the recrystallized HEA (RM ~ 550–600 MPa) are achieved, although with significantly reduced fracture elongations.