Use Of Experimental Tests Research Articles

Key role of boundary conditions for the 2D modeling of crack propagation in linear elastic Compact Tension tests

In fracture mechanics, the use of experimental tests are fundamental to characterize the material properties in terms of crack initiation and propagation behavior. When modeled in boundary value problems, simplifications need to be made. Notably, the loading has to be reduced to a set of boundary conditions and the choice between plane stress and plane strain has to be done in the 2D case. Here we focus on the Compact Tension (CT) test which is a fracture setup commonly used to measure the fracture toughness at crack propagation onset and we question the possibility to use it to study crack propagation. For this, the tests are monitored by digital image correlation and compared to finite element method simulations. Three ways to guide the choice between plane stress and plane strain hypotheses are proposed. They lead to the same conclusion that the plane stress conditions are the most relevant for the geometry of the samples used here. The key role of boundary conditions is highlighted by testing several models, with imposed force or displacement boundary conditions, against the experimental data. Imposed force boundary conditions on the pin are shown to be able to reproduce the experiments before crack propagation and to be insensitive to the way this force is applied, in line with Saint Venant principle. The results with imposed displacement are in contrary very sensitive to their distribution along the pin. While the stage before propagation is accurately predicted by imposed forces, we show that for the propagation phase, Saint Venant is put in default and accurate results can only be obtained by imposing the displacement fields issued from the digital image correlation. These results can be extended to other fracture experiments, involving pin loading, like the Compact Tension Shear (CTS) or the (Tappered) Double Cantilever Beam ((T)DCB) tests.

Assessment of stress transfer in laminated structural power composites produced with mechanically-connected electric double-layer capacitors

Enabling energy-storing functions in structural composites is an appealing strategy to reduce the weight of systems in electric transport. Structural power composites (SPC) can be made by introducing energy-storing layers in traditional composite laminates, either as embedded devices or as thin integrated interleaves simultaneously performing load-bearing and electrochemical energy storage functions. However, the solid electrolyte in these layered architectures is typically a soft polymer, thus making the SPC very prone to delamination. Mechanical interconnection using structural matrix bridges between plies, analogous to metal connectors, provides a solution to alleviate delamination risks while maintaining the structural integrity of the SPC even under the most unfavourable combinations of loads. This paper reports on the use of experimental tests and FEM simulations to study the micromechanics of interlaminar polymer connectors in SPCs. It explores the effect of the connector area on the shear strength and energy density. The results provide a validated methodology to design laminated structural composites with a tailored balance of multi-functional properties of wide application to SPC of different constituents, with electrochemical energy storage either as capacitors or batteries.

Consumer inertia in energy markets: Insights from behavioral economics

The transition towards deregulated energy markets requires a dynamic participation by consumers. Yet, in many European countries, a high degree of consumer inertia is observed: the rate of switching to new tariffs and providers is far from being satisfactory. Neoclassical consumer choice models cannot explain this phenomenon, unless assuming that perceived transactions costs are disproportionately high. This paper discusses how more realistic assumptions about consumer behavior can help interpret low switching rates. In particular, it examines psychological aspects (e.g., loss aversion, present bias, ambiguity aversion) and cognitive biases (e.g., choice overload, overconfidence) that can explain consumer stickiness in energy markets. Different behavioral traits point at different policy interventions. Therefore, such analysis illustrates how crucial it is that policy-makers aiming at reducing consumer inertia take these behavioral aspects into account and make use of experimental testing when laying out interventions.

Integrity assessment and determination of residual fatigue life of vital parts of bucket-wheel excavator operating under dynamic loads

In this paper are presented results of tests and analyses of complex dynamic loads carried out on the bucket-wheel excavator SchRs 650/5 × 24 Krupp, as well as assessment of service life of vital welded structures of a bucket-wheel excavator boom subjected to cyclic loading with a variable amplitude through the use of experimental tests carried out in order to determine operational strength and growth of a fatigue crack for one structural part. Bucket-wheel excavator was built by “Thyssen Krupp” company, Germany. Outer loads, or in other words digging forces for the overburden and coal have been calculated on the basis of measured values of actual current intensity of the bucket-wheel drive and recorded output values of changeable loads. Correlations between the power of the bucket-wheel drive system and adequate hourly production, depending on the overall digging resistance which affects the stress condition of the bucket-wheel, were also determined. Results of the theoretical and experimental analysis of natural and forced oscillations of the support structure for various exploitation conditions are presented in first part of the paper. Deformations εi determined by tensometric measurements on the rotating shaft of the bucket-wheel were converted into tangential stresses through the introduction of the modulus of elasticity and Poisson's ratio, which, along with the polar moment of inertia of the cross-section, define the moment of rotation on the bucket-wheel shaft. Through the use of the load - strength comparison method (maximization of the ratio of load and strength indicators) the application factor of the gear with the largest number of turns KA has been determined.In the second part of the paper methodological approach for the assessment of service life of vital welded structures of a bucket-wheel excavator boom was presented. Assessment was done in order to determine operational strength and growth of a fatigue crack, through the use of experimental tests. Realized researches and results presented in this paper offer great possibilities for the analyses of behaviour of vital welded structures of the bucket-wheel boom. By the application of the measurement device with 8 channels for registration and processing of electric signals HBM Spider 8 and measurement tapes HBM 6/350 × XY31 deformations were measured at vital welded structures of the boom in the area of the bucket-wheel. The objective of the test is to determine if there is a possibility of occurrence of plastic deformations or initial cracks due to fatigue at vital welded structures and to obtain data which define crack growth.

Numerical Framework to Determine Instability Modes for Static Vehicles under Partial Submergence

Studies on instabilites of static (parked) vehicles that are partially submerged during flood events have captured worldwide attention in the previous years. The response of varying hydrodynamics effects to vehicles have been the interests reported. Indeed, the vehicle shapes have been found to significantly affect the response towards its hydrodynamics. These works, however, were experimental in nature. As the development of complex exterior designs of vehicles progressively made with the expansion of technologies, repeating the tedious experimental testing on another set of vehicle designs may not be practical. Herein this paper attempts to provide guidance on the use of a simple numerical framework to compensate the use of experimental testing. The proposed framework has been validated with a series of experimental testing conducted on a 1:24 scaled model that was controlled to be partially submerged under sub-critical conditions. Keeping close to the assumptions that the submergence levels of the vehicles are significantly influencing the buoyancy and drag forces, the numerical approach had given a simpler mechanism on the calculations of the submerged area (AD) and volume (V) of the vehicle, when this could not be computed easily during experiments. The parameters AD and V coupled with a graphical presentation of instability threshold values, would provide quick computations on the corresponding forces acting on the any similar characteristics static vehicle accordingly.

Determination of Residual Fatigue Life of Welded Structures at Bucket-Wheel Excavators through the Use of Fracture Mechanics

This paper presents a methodological approach for the assessment of service life of vital welded structures of a bucket-wheel excavator Sch Rs 650/5x24 (’Thyssen Krupp’, Germany) boom, subjected to cyclic loading with a variable amplitude through the use of experimental tests carried out in order to determine operational strength and growth of a fatigue crack. Realized researches and results presented in this paper offer great possibilities for the analyses of behaviour of vital welded structures of the bucket-wheel boom. By the application of the measurement device with 8 channels for registration and processing of electric signals HBM Spider 8 and measurement tapes HBM 6/350xXY31 deformations were measured at vital welded structures of the boom in the area of the bucket-wheel, made of steels St 37.2 and St 52.3 in accordance with standard DIN 17100, or steels S235J2G3 and S355J2G3 in accordance with standard EN 10025-2. The objective of the test is to determine if there is a possibility of occurrence of plastic deformations or initial cracks due to fatigue at vital welded structures. Tests that refer to the growth of the fatigue crack located at the welded joint have been carried out by bending at three points with asymmetric load R = 0.5 (R = σmin / σmax) at the specimen with a single edge notch. Tests were performed through the use of controlled force, ranging between Fmax and Fmin at the high-frequency pulsator ‘Cracktronic’, while obtainment of data regarding the crack growth was carried out through the use of measurement gauge ARM A-10.

Effects of Surface Roughness on the Kinetic Friction of SiC Nanowires on SiN Substrates

The effects of surface roughness on the kinetic friction between SiC nanowires and SiN substrates were investigated by use of experimental testing and numerical modelling. The experimental measurements showed that the shear stress, or the frictional force per unit projected contact area, was significantly affected by the substrate roughness, decreased from 0.38 to 0.02 MPa for the increase in roughness from 0.5 to 23 nm. A power-law relationship between frictional stress and surface roughness was found. The numerical modelling based on the lowest energy principle and the Monte Carlo method revealed that the substrate effect was through the variation in the number of contact asperities between a nanowire and a substrate, which was much fewer on a rougher surface. The real contact area also exhibited a power-law dependence on the substrate roughness. The frictional forces normalized using the real contact areas obtained from the simulation were reasonably consistent, varying from 127 to 166 MPa for the five substrates of different roughnesses.

A method of reproducing complex, multi-object anatomical structures

PurposeThis paper aims to present a method of reproducing multi-object structures from materials of diverse physical properties with the use of models fabricated by means of rapid prototyping (RP) techniques.Design/methodology/approachA process of modelling complex anatomical structures of soft tissues and bones using mandible models as examples was described. The study is based on data acquired through standard computed tomography. Physical models of examined objects were fabricated with RP technology from a 3D-CAD virtual model.FindingsIn the analysis of complex medical issues, beside numerical methods, one can simultaneously make use of experimental tests to verify obtained results. In the case of experimental tests, it is necessary to fabricate physical models with appropriate material properties. RP techniques used in the method ensure accurate reproduction of the external shape of the fabricated model, whereas consecutive stages allow us to construct moulds and create internal structures within a finished model by wax cast models.Practical implicationsThe application of a physical RP model makes the identification of medical problem more efficient and the reconstruction of pathological alterations for experimental tests clearer. It prevents the simplification of assumptions to experimental analysis. The approach may reduce costs of fabricating models for experimental studies and offers the possibility of using materials of desired properties.Originality/valueThe approach developed by the authors and presented in this paper was submitted for patent protection as “A Method of Reconstructing Medical Models with Internal Structure and the Use of Materials of Diverse properties” – patent application no. P.398644.

Characterisation of blast loading in complex, confined geometries using quarter symmetry experimental methods

Explosions in confined spaces lead to complicated patterns of shock wave reflection and interactions which are best investigated by use of experimental tests or numerical simulations. This paper describes the design and outcome of a series of experiments using a test cell to measure the pressures experienced when structures were placed inside to alter the propagation of shock waves, utilising quarter symmetry to reduce the size of the required test cell and charge. An 80 g charge of PE4 (a conventional RDX-based plastic explosive) was placed at half height in one corner of the test cell, which represents the centre of a rectangular enclosure when symmetry is taken into consideration. Steel cylinders and rectangular baffles were placed within the test cell at various locations. Good reproducibility was found between repeated tests in three different arrangements, in terms of both the recorded pressure data and the calculated cumulative impulse. The presence of baffles within the test cell made a small difference to the pressures and cumulative impulse experienced compared to tests with no baffles present; however, the number and spacing of baffles was seen to make minimal difference to the experienced pressures and no noticeable difference to the cumulative impulse history. The paper presents useful experimental data that may be used for three-dimensional code validation.

Effect of DIC Spatial Resolution, Noise and Interpolation Error on Identification Results with the VFM

AbstractThe use of experimental tests that involve full‐field measurements to characterize mechanical material properties is becoming more widespread within the engineering community. In particular digital image correlation (DIC) on white light speckles is one of the most used tools, thanks to the relatively low cost of the equipment and the availability of dedicated software. Nonetheless the impact of measurement errors on the identified parameters is still not completely understood. To this purpose, in this paper, a simulator able to numerically simulate an experimental test, which involves DIC is presented. The chosen test is the Unnotched Iosipescu test used to identify the orthotropic elastic parameters of composites. Synthetic images are generated and then analysed by DIC. Eventually the obtained strain maps are used to identify the elastic parameters with the Virtual Fields Method (VFM). The numerical errors propagating through the simulation procedure are carefully characterized. Besides, the simulator is used to compare the performances of DIC and the grid method in the identification process with the VFM. Finally, the influence of DIC settings on the identification error is studied as a function of the camera digital noise level, in order to find the best testing configuration.

Effect of filler material on local and global behaviour of buckling‐restrained braces

AbstractThe nonlinear response of buildings has attracted a tremendous amount of attention in recent years. Braces, as lateral force‐resisting elements of a structure, are designed to not only react in the elastic region, but also to exhibit nonlinear response beyond the elastic limit. However, buckling in compression drastically degrades the performance of braces under earthquake loading. Buckling restrained braces (BRBs) have been evolved into very effective systems for severe seismic applications. They prevent buckling in compression through the encasing of core steel into a steel tube and confining infill concrete.The effect of infill material is investigated in this research through the use of experimental tests. Filler material may be concrete, grout or mortar, as well as granular material such as compacted aggregate. Moreover, lightweight concrete or lean concrete may be utilized as filler to reduce the overall structural weight. Furthermore, the need for unbonding material may not arise when sand and gravel mixture is used. Nevertheless, the strength of the aggregate should be such that no buckling or strength deterioration is observed. Parametric studies on BRB characteristics are carried out in this research. Results of cyclic loading tests are then provided for individual cases to characterize the effect of response parameters of BRB assemblages. Copyright © 2009 John Wiley & Sons, Ltd.

Providing Ride-Through Capability to a Doubly Fed Induction Generator Under Unbalanced Voltage Dips

This paper analyzes the effect of unbalanced voltage over doubly fed induction generators (DFIGs) and presents a novel control strategy, named dynamic programming power control plus (DPPC+), based on dynamic programming control. The high penetration of wind energy in the electrical grids demands for new requirements for the operation of wind energy conversion systems (WECSs). DFIG is the most employed WECS, and the DPPC+ guarantees their operation under unbalance conditions achieving the required objectives. Although the technique can be implemented to control both rotor and grid converters, we hereby expound the former, which regulates stator active and reactive power. The validation of the results obtained with DPPC+ has been performed through the use of experimental tests on a 20-kW test bench, consisting of a DFIG and induction motor drive. The obtained results show that the DPPC+ is suitable for achieving a good dynamic response while controlling current distortion and power and/or torque oscillations for both steady-state conditions and unbalanced voltage dips, showing the low-voltage ride-through capability.

Dynamic Programming Power Control for Doubly Fed Induction Generators

This paper presents a novel control strategy, dynamic programming power control (DPPC), to be applied to doubly fed induction generators most commonly used in wind energy applications. Although the technique can be implemented to control both rotor and grid converters, it will hereby be expounded the former, which regulates stator active and reactive powers. The results obtained are compared with those from other techniques, such as direct torque/power control (DTC/DPC) through the use of experimental tests, and indicate that DPPC achieves gains in dynamic response and considerable improvements in terms of ripple reduction and frequency spectrum as a result of constant switching frequency operation. The validation of results has been performed through experimental tests on a 6-kW generator.

An examination of Stone Age/Bronze Age adzes and axes of red deer (Cervus elaphus L.) antler from the Leine Valley, near Hannover

Red deer antler was, together with stone, one of the most important raw materials for tool manufacture in prehistoric times. Both the physical characteristics of this material and the manufacturing technology of antler tools are examined. The use of experimental tests and analytical studies of the recovered antler finds allow us to propose, with a great degree of confidence, that the main use of antler axes during the Stone Age/Bronze Age was in woodworking.