Strain Wiley Online Library : Strainhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2F%28ISSN%291475-1305John Wiley & Sons, Incen© John Wiley & Sons Ltd0039-21031475-13052013-04-01T00:00:00-05:00April 201349295198Identifying the Magnitude and Location of a Load on a Slender Beam Using a Strain Gauge Based Force Transducerhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12035Identifying the Magnitude and Location of a Load on a Slender Beam Using a Strain Gauge Based Force TransducerE. T. Bednarz, W. D. Zhu, S. A. Smith2013-05-23T06:06:18.192554-05:00doi:10.1111/str.12035John Wiley & Sons, Inc.10.1111/str.12035http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12035Full Papern/an/aABSTRACT

A unique strain gauge based method is developed to identify the magnitude and location of a load on a slender beam with variable cross sections, and pinned, firm rest, soft rest, pinned-fixed, and fixed boundary conditions. Four uniaxial strain gauges are mounted to the bottom surface of the beam, and the bending moment diagram of the beam can be constructed using measured strains on the beam. By combining individually scaled strain gauge outputs, the magnitude and location of the load can be accurately identified. The strain gauge based force transducer methodology is experimentally validated on prismatic beams with firm rest, soft rest, firm rest-fixed, and fixed boundary conditions, and a continuously tapered beam with rest boundary conditions. The force transducer methodology is independent of the boundary conditions of the beam, and the error from strain gauge drift due to uniform thermal expansion on a prismatic beam can cancel out.

]]> A unique strain gauge based method is developed to identify the magnitude and location of a load on a slender beam with variable cross sections, and pinned, firm rest, soft rest, pinned-fixed, and fixed boundary conditions. Four uniaxial strain gauges are mounted to the bottom surface of the beam, and the bending moment diagram of the beam can be constructed using measured strains on the beam. By combining individually scaled strain gauge outputs, the magnitude and location of the load can be accurately identified. The strain gauge based force transducer methodology is experimentally validated on prismatic beams with firm rest, soft rest, firm rest-fixed, and fixed boundary conditions, and a continuously tapered beam with rest boundary conditions. The force transducer methodology is independent of the boundary conditions of the beam, and the error from strain gauge drift due to uniform thermal expansion on a prismatic beam can cancel out.Experimental Investigation of Strain Behaviour of Heated Cement Paste and Concretehttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12032Experimental Investigation of Strain Behaviour of Heated Cement Paste and ConcreteT. Ring, M. Zeiml, R. Lackner, J. Eberhardsteiner2013-05-23T05:41:10.824115-05:00doi:10.1111/str.12032John Wiley & Sons, Inc.10.1111/str.12032http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12032Full Papern/an/aABSTRACT

The material degradation of concrete subjected to fire events has a severe influence on the load-carrying capacity of support structures. Spalling of concrete layers, exposing the reinforcement bars and degradation of the material properties (Young's modulus, compressive strength) may lead to significant damage of the reduced cross-section and, therefore, cause failure of the structure. In order to understand the stress build-up at the heated surface caused by thermal expansion due to fire loading, finally leading to damage and spalling of concrete, the strain behaviour of cement paste and concrete exposed to combined thermo-mechanical loading is the focus of this work. Hereby, the evolution of thermal strains, Young's modulus and Poisson's ratio with increasing temperature are investigated experimentally. For this purpose, the specimens are loaded uniaxially while the temperature is increased up to 800 °C. The obtained results provide the proper basis for the development of realistic material models, allowing more sophisticated simulations of structures exposed to fire.

]]> The material degradation of concrete subjected to fire events has a severe influence on the load-carrying capacity of support structures. Spalling of concrete layers, exposing the reinforcement bars and degradation of the material properties (Young's modulus, compressive strength) may lead to significant damage of the reduced cross-section and, therefore, cause failure of the structure. In order to understand the stress build-up at the heated surface caused by thermal expansion due to fire loading, finally leading to damage and spalling of concrete, the strain behaviour of cement paste and concrete exposed to combined thermo-mechanical loading is the focus of this work. Hereby, the evolution of thermal strains, Young's modulus and Poisson's ratio with increasing temperature are investigated experimentally. For this purpose, the specimens are loaded uniaxially while the temperature is increased up to 800 °C. The obtained results provide the proper basis for the development of realistic material models, allowing more sophisticated simulations of structures exposed to fire.Digital Imaging Methodology for Measuring Early Shrinkage Cracking in Concretehttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12034Digital Imaging Methodology for Measuring Early Shrinkage Cracking in ConcreteL. Ruiz-Ripoll, B. E. Barragán, S. Moro, J. Turmo2013-05-13T23:32:13.441136-05:00doi:10.1111/str.12034John Wiley & Sons, Inc.10.1111/str.12034http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12034Full Papern/an/aABSTRACT

The measurement of early shrinkage cracking in concrete is important to prevent aesthetic issues and avoid surface cracking that could lead to reinforcement corrosion and reduce the durability, long-term service life and integrity of a structure. Moreover, the lack of standards and subjectivity of the very few methodologies proposed so far complicate its estimation.

This research presents a new imaging methodology for evaluating and quantifying early shrinkage cracking patterns. The methodology was developed testing highly restrained square concrete slabs subjected to severe conditions of restraint and moisture loss. Its quantification consisted of photographing, processing the pictures and highlighting the cracks. For the first time, early shrinkage cracking in concrete can be measured through an experimental technique and quantified by means of geometric figures. In this way, more precise and automatic results are achieved, as flat figures adapt to the shape of cracks and store their properties. Therefore, parameters such as the total cracked area, total crack length, maximum crack width or average crack width were easily calculated.

The results demonstrated the suitability of the wind tunnel test to produce significant cracking patterns, as well as the great capacity of the imaging methodology to identify and characterize the cracking pattern.

]]> The measurement of early shrinkage cracking in concrete is important to prevent aesthetic issues and avoid surface cracking that could lead to reinforcement corrosion and reduce the durability, long-term service life and integrity of a structure. Moreover, the lack of standards and subjectivity of the very few methodologies proposed so far complicate its estimation. This research presents a new imaging methodology for evaluating and quantifying early shrinkage cracking patterns. The methodology was developed testing highly restrained square concrete slabs subjected to severe conditions of restraint and moisture loss. Its quantification consisted of photographing, processing the pictures and highlighting the cracks. For the first time, early shrinkage cracking in concrete can be measured through an experimental technique and quantified by means of geometric figures. In this way, more precise and automatic results are achieved, as flat figures adapt to the shape of cracks and store their properties. Therefore, parameters such as the total cracked area, total crack length, maximum crack width or average crack width were easily calculated. The results demonstrated the suitability of the wind tunnel test to produce significant cracking patterns, as well as the great capacity of the imaging methodology to identify and characterize the cracking pattern.Image-based Continuous Displacement Measurements Using an Improved Spectral Approachhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12031Image-based Continuous Displacement Measurements Using an Improved Spectral ApproachF. Mortazavi, M. Lévesque, I. Villemure2013-05-13T23:11:33.604509-05:00doi:10.1111/str.12031John Wiley & Sons, Inc.10.1111/str.12031http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12031Full Papern/an/aABSTRACT

Digital Image Correlation algorithms capable of determining continuous displacement fields are receiving growing attention in the field of mechanical properties identification. In this paper, we develop an Improved Spectral Approach (ISA) to reconstruct continuous displacements based on their Fourier decomposition. This approach leads to a time and memory-efficient algorithm, thanks to the fast Fourier transform. Moreover, the Fourier-based decomposition enables accurate heterogeneous measurements. Improvements consist in increasing the accuracy and convergence rate as well as dealing with non-periodic displacements and images. Furthermore, a theoretical framework is presented to quantify the noise sensitivity of the ISA from which useful information is retrieved. The approach is evaluated using synthetic images deformed by heterogeneous displacement fields. Comparisons show that the introduced modifications lead to lower uncertainties by one order of magnitude in the case of non-periodic images and displacement field studied. Moreover, first-order (SO1) and second-order (SO2) subset-based Digital Image Correlation algorithms are compared with the ISA. The comparisons herein reveal that the uncertainties of the ISA are 6–9 times smaller than those of the SO1 due to insufficiency of the first-order shape function for the estimation of heterogeneous displacements, while being slightly smaller than those of the SO2. Moreover, as the image smoothness decreases, the uncertainties of the SO2 deviate from those of the ISA and the exact displacements. The presented approach shows great potentials for challenging applications such as strain measurements at microstructural levels.

]]> Digital Image Correlation algorithms capable of determining continuous displacement fields are receiving growing attention in the field of mechanical properties identification. In this paper, we develop an Improved Spectral Approach (ISA) to reconstruct continuous displacements based on their Fourier decomposition. This approach leads to a time and memory-efficient algorithm, thanks to the fast Fourier transform. Moreover, the Fourier-based decomposition enables accurate heterogeneous measurements. Improvements consist in increasing the accuracy and convergence rate as well as dealing with non-periodic displacements and images. Furthermore, a theoretical framework is presented to quantify the noise sensitivity of the ISA from which useful information is retrieved. The approach is evaluated using synthetic images deformed by heterogeneous displacement fields. Comparisons show that the introduced modifications lead to lower uncertainties by one order of magnitude in the case of non-periodic images and displacement field studied. Moreover, first-order (SO1) and second-order (SO2) subset-based Digital Image Correlation algorithms are compared with the ISA. The comparisons herein reveal that the uncertainties of the ISA are 6–9 times smaller than those of the SO1 due to insufficiency of the first-order shape function for the estimation of heterogeneous displacements, while being slightly smaller than those of the SO2. Moreover, as the image smoothness decreases, the uncertainties of the SO2 deviate from those of the ISA and the exact displacements. The presented approach shows great potentials for challenging applications such as strain measurements at microstructural levels.Consistent Identification of CFRP Viscoelastic Models from Creep to Dynamic Loadingshttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12033Consistent Identification of CFRP Viscoelastic Models from Creep to Dynamic LoadingsJ. Berthe, M. Brieu, E. Deletombe, G. Portemont, P. Lecomte-Grosbras, A. Deudon2013-04-08T05:57:46.341053-05:00doi:10.1111/str.12033John Wiley & Sons, Inc.10.1111/str.12033http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12033Full Papern/an/aABSTRACT

Rate-dependent models require creep or mechanical tests at various strain rates in order to be identified and validated. Different geometries coexist for creep and static tests (normative geometry) and for dynamic tests. Therefore, due to geometrical sample considerations, experimental results could be inconsistent for identification or validation procedures, inducing, for example, differences on the shear modulus only due to the change of geometry.

The objective of this work is to present an improved sample geometry that allows to obtain consistent mechanical tests results at various strain rates highlighting the rate dependencies of laminates. In particular, a complete mechanical validation of the sample geometry for dynamic tests is successfully performed in order to avoid inconsistency. Results of static and dynamic tests on the validated geometry are analysed, and the rate dependency of the elastic properties of the UD T700GC/M21 mesoscopic ply is highlighted on a wide strain rate range (10−3 to 102 s−1). Finally, the identification of a non-linear viscoelastic model is performed on dynamic and creep tests results in order to obtain a representative model for dynamic, static and creep loadings, and to demonstrate the importance of introducing the improved geometry for the dynamic tests.

]]> Rate-dependent models require creep or mechanical tests at various strain rates in order to be identified and validated. Different geometries coexist for creep and static tests (normative geometry) and for dynamic tests. Therefore, due to geometrical sample considerations, experimental results could be inconsistent for identification or validation procedures, inducing, for example, differences on the shear modulus only due to the change of geometry. The objective of this work is to present an improved sample geometry that allows to obtain consistent mechanical tests results at various strain rates highlighting the rate dependencies of laminates. In particular, a complete mechanical validation of the sample geometry for dynamic tests is successfully performed in order to avoid inconsistency. Results of static and dynamic tests on the validated geometry are analysed, and the rate dependency of the elastic properties of the UD T700GC/M21 mesoscopic ply is highlighted on a wide strain rate range (10−3 to 102 s−1). Finally, the identification of a non-linear viscoelastic model is performed on dynamic and creep tests results in order to obtain a representative model for dynamic, static and creep loadings, and to demonstrate the importance of introducing the improved geometry for the dynamic tests.Determination of Ductile Fracture Parameters of a Dual-Phase Steel by Optical Measurementshttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12030Determination of Ductile Fracture Parameters of a Dual-Phase Steel by Optical MeasurementsG. Gruben, D. Vysochinskiy, T. Coudert, A. Reyes, O.-G. Lademo2013-03-15T08:07:01.87661-05:00doi:10.1111/str.12030John Wiley & Sons, Inc.10.1111/str.12030http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12030Full Papern/an/aABSTRACT

Marciniak–Kuczynski and Nakajima tests of the dual-phase steel Docol 600DL (www.ssab.com/) have been carried out for a range of stress-states spanning from uniaxial tension to equi-biaxial tension. The deformation histories of the specimens have been recorded by digital images, and the displacement and strain fields have been determined by post-processing the images with digital image correlation software. The fracture characteristics of the material are presented by means of the stress triaxiality, the Lode parameter and the equivalent strain. These parameters are evaluated on the surface of the specimens based on the optical field measurements and assumptions regarding the mechanical behaviour of the material. Additionally the minor versus major principal strains up to fracture are presented. It is found that the material displays a significantly lower ductility in plane-strain tension than in uniaxial tension and equi-biaxial tension, and that it, in the tests exposed to local necking, undergoes large strains between the onset of necking and fracture. Fractographs of selected specimens reveal that fracture is due to growth and coalescence of voids that occur in localised areas governed by shear-band instability.

]]> Marciniak–Kuczynski and Nakajima tests of the dual-phase steel Docol 600DL (www.ssab.com/) have been carried out for a range of stress-states spanning from uniaxial tension to equi-biaxial tension. The deformation histories of the specimens have been recorded by digital images, and the displacement and strain fields have been determined by post-processing the images with digital image correlation software. The fracture characteristics of the material are presented by means of the stress triaxiality, the Lode parameter and the equivalent strain. These parameters are evaluated on the surface of the specimens based on the optical field measurements and assumptions regarding the mechanical behaviour of the material. Additionally the minor versus major principal strains up to fracture are presented. It is found that the material displays a significantly lower ductility in plane-strain tension than in uniaxial tension and equi-biaxial tension, and that it, in the tests exposed to local necking, undergoes large strains between the onset of necking and fracture. Fractographs of selected specimens reveal that fracture is due to growth and coalescence of voids that occur in localised areas governed by shear-band instability.Patient-Specific Finite Element Modelling and Validation of Porcine Femora in Torsionhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12029Patient-Specific Finite Element Modelling and Validation of Porcine Femora in TorsionNicholas J. Emerson, Amaka C. Offiah, Gwendolen C. Reilly, Matt J. Carré2013-03-12T06:10:53.788798-05:00doi:10.1111/str.12029John Wiley & Sons, Inc.10.1111/str.12029http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12029Full Papern/an/aABSTRACT

The accuracy of biomechanical simulation has been improved by using high-resolution computed tomography (CT) to define the geometry and material parameters. This technique has been used to assess numerous systems, including the mechanical properties of bone, fixation techniques post-fracture and the performance of bone microarchitecture. In this study, a semi-automated process for converting CT data into finite element (FE) models was used to model the mid-shaft (diaphysis) of porcine femoral samples under sub-maximal torsional and compressive load. Physical validation was undertaken to investigate if the all-important geometry and material property mapping functioned correctly. Porcine femoral specimens were imaged using contiguous helical CT, which was converted to FE models using ScanIP from Simpleware, Exeter, UK. The heterogeneous material properties were estimated using density–elasticity relationships proposed in literature for human bone samples. Laboratory testing performed favourably, with a linear strain response validating the use of the array of linear material models used in simulation. The simulation procedure also performed well. Linear regression and mean error calculation demonstrated accurate correlation between predicted (from simulation) and observed (measured within the laboratory) results that offered improvement over the accuracy within comparative testing for human samples. Using FE modelling on a patient-specific basis offers potential in a number of scenarios, including the determination of injury risk and design of protective equipment. The increased accessibility of animal samples allows large-scale fracture testing of complex loading mechanisms and the potential to consider younger animal samples (to investigate the behaviour of developing bone). Spiral fractures of long bones have been demonstrated to be an indicator of non-accidental injury in children. Combining the increased accuracy in torsional simulation in this study with younger sample testing may be employed to attempt to determine the causes of fracture from post fracture scans, aiding in the diagnosis of non-accidental injury.

]]> The accuracy of biomechanical simulation has been improved by using high-resolution computed tomography (CT) to define the geometry and material parameters. This technique has been used to assess numerous systems, including the mechanical properties of bone, fixation techniques post-fracture and the performance of bone microarchitecture. In this study, a semi-automated process for converting CT data into finite element (FE) models was used to model the mid-shaft (diaphysis) of porcine femoral samples under sub-maximal torsional and compressive load. Physical validation was undertaken to investigate if the all-important geometry and material property mapping functioned correctly. Porcine femoral specimens were imaged using contiguous helical CT, which was converted to FE models using ScanIP from Simpleware, Exeter, UK. The heterogeneous material properties were estimated using density–elasticity relationships proposed in literature for human bone samples. Laboratory testing performed favourably, with a linear strain response validating the use of the array of linear material models used in simulation. The simulation procedure also performed well. Linear regression and mean error calculation demonstrated accurate correlation between predicted (from simulation) and observed (measured within the laboratory) results that offered improvement over the accuracy within comparative testing for human samples. Using FE modelling on a patient-specific basis offers potential in a number of scenarios, including the determination of injury risk and design of protective equipment. The increased accessibility of animal samples allows large-scale fracture testing of complex loading mechanisms and the potential to consider younger animal samples (to investigate the behaviour of developing bone). Spiral fractures of long bones have been demonstrated to be an indicator of non-accidental injury in children. Combining the increased accuracy in torsional simulation in this study with younger sample testing may be employed to attempt to determine the causes of fracture from post fracture scans, aiding in the diagnosis of non-accidental injury.Experimental and Numerical Crashworthiness Investigation of the Empty and Foam-Filled Conical Tube with Shallow Spherical Capshttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12028Experimental and Numerical Crashworthiness Investigation of the Empty and Foam-Filled Conical Tube with Shallow Spherical CapsA. Ghamarian, H. R. Zarei, M. A. Farsi, N. Ariaeifar2013-03-06T08:00:30.293263-05:00doi:10.1111/str.12028John Wiley & Sons, Inc.10.1111/str.12028http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12028Full Papern/an/aABSTRACT

This paper experimentally investigates the crash responses of the empty and polyurethane foam-filled conical tubes with shallow spherical caps under quasi-static axial loading. To find more details about the energy absorption mechanism, finite element methods is used to simulate the crush process. In terms of finding more efficient and lighter crash absorbers particularly, the energy absorption and specific energy absorption and load ratio have been considered. The influences of the tube geometrical and material parameters such as radius of spherical region, wall thickness, length, semi-apical angle and foam density on the energy absorption mechanism have been investigated. This study provides practical information for the use of thin-walled tubes with shallow spherical caps as energy absorbers in aerospace applications to design reentry of sounding rocket based on foam-filled conical tube with shallow spherical caps. © 2013 Blackwell Publishing Ltd

]]> This paper experimentally investigates the crash responses of the empty and polyurethane foam-filled conical tubes with shallow spherical caps under quasi-static axial loading. To find more details about the energy absorption mechanism, finite element methods is used to simulate the crush process. In terms of finding more efficient and lighter crash absorbers particularly, the energy absorption and specific energy absorption and load ratio have been considered. The influences of the tube geometrical and material parameters such as radius of spherical region, wall thickness, length, semi-apical angle and foam density on the energy absorption mechanism have been investigated. This study provides practical information for the use of thin-walled tubes with shallow spherical caps as energy absorbers in aerospace applications to design reentry of sounding rocket based on foam-filled conical tube with shallow spherical caps. © 2013 Blackwell Publishing LtdIssue Informationhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12001Issue Information2013-03-24T23:28:00.32229-05:00doi:10.1111/str.12001John Wiley & Sons, Inc.10.1111/str.12001http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12001iiiIdentification and Avoidance of Systematic Measurement Errors in Lamb Wave Observation With One-Dimensional Scanning Laser Vibrometryhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12015Identification and Avoidance of Systematic Measurement Errors in Lamb Wave Observation With One-Dimensional Scanning Laser VibrometryM. N. Neumann, B. Hennings, R. Lammering2012-12-06T01:07:06.629883-05:00doi:10.1111/str.12015John Wiley & Sons, Inc.10.1111/str.12015http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.1201595101

Abstract:  Scanning laser vibrometry is a widely used tool to observe Lamb wave fields for structural health monitoring (SHM) purposes. Lamb waves propagate over long distances in thin-walled structures and interact with structural inhomogeneities, for example, damages, in spite of wavelengths several times of the damage size. In SHM of sheets and glass- or carbon-fibre-reinforced plastic plates, this effect is used for determining the position as well as the size of structural faults. With the often employed one-dimensional vibrometry, a geometrically induced, systematic error occurs when measuring oblique-angled motion. This error can be, in the specific case of Lamb waves, of a non-negligible quantity. The nature of this geometrical measurement error in general and concerning Lamb waves in special is discussed analytically for both amplitude and phase data. It is shown that this matter should be taken into account in some applications.

]]> Abstract:  Scanning laser vibrometry is a widely used tool to observe Lamb wave fields for structural health monitoring (SHM) purposes. Lamb waves propagate over long distances in thin-walled structures and interact with structural inhomogeneities, for example, damages, in spite of wavelengths several times of the damage size. In SHM of sheets and glass- or carbon-fibre-reinforced plastic plates, this effect is used for determining the position as well as the size of structural faults. With the often employed one-dimensional vibrometry, a geometrically induced, systematic error occurs when measuring oblique-angled motion. This error can be, in the specific case of Lamb waves, of a non-negligible quantity. The nature of this geometrical measurement error in general and concerning Lamb waves in special is discussed analytically for both amplitude and phase data. It is shown that this matter should be taken into account in some applications. Locating the Crack Tip Using Displacement Field Data: A Comparative Studyhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12017Locating the Crack Tip Using Displacement Field Data: A Comparative StudyM. Zanganeh, P. Lopez-Crespo, Y. H. Tai, J. R. Yates2012-12-06T01:06:43.331906-05:00doi:10.1111/str.12017John Wiley & Sons, Inc.10.1111/str.12017http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12017102115

Abstract:  The evaluation of stress intensity factors from experimentally determined crack-tip stress or displacement fields almost always requires that the location of the crack tip is identified beforehand. In this work, a study has been performed to compare how different methods to locate the crack-tip position from the displacement field around a crack tip obtained by digital image correlation influence the estimation of stress intensity factors. The methods used were two constrained Newton type methods: the trust-region reflective Newton method and quasi-Newton method; an unconstrained direct search method: the Nelder–Mead Simplex method; a constrained genetic algorithm; and a constrained Pattern Search (PS) method. It is shown that the Newton type methods are less accurate compared with the direct search methods studied. The PS method was found to be the most accurate. Furthermore, the PS method was found to be about twice as fast as the Simplex method and 10 times faster than a Genetic Algorithm for the same computing hardware and the same input data.

]]> Abstract:  The evaluation of stress intensity factors from experimentally determined crack-tip stress or displacement fields almost always requires that the location of the crack tip is identified beforehand. In this work, a study has been performed to compare how different methods to locate the crack-tip position from the displacement field around a crack tip obtained by digital image correlation influence the estimation of stress intensity factors. The methods used were two constrained Newton type methods: the trust-region reflective Newton method and quasi-Newton method; an unconstrained direct search method: the Nelder–Mead Simplex method; a constrained genetic algorithm; and a constrained Pattern Search (PS) method. It is shown that the Newton type methods are less accurate compared with the direct search methods studied. The PS method was found to be the most accurate. Furthermore, the PS method was found to be about twice as fast as the Simplex method and 10 times faster than a Genetic Algorithm for the same computing hardware and the same input data. Experimentally Determined Stresses Associated with Elliptical Holes Using Polar Coordinateshttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12018Experimentally Determined Stresses Associated with Elliptical Holes Using Polar CoordinatesA. A. Khaja, R. E. Rowlands2013-03-21T00:55:22.934841-05:00doi:10.1111/str.12018John Wiley & Sons, Inc.10.1111/str.12018http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12018116124

Abstract:  This paper determines the individual components of stress experimentally in a finite tensile plate containing an elliptical hole using a series representation of an Airy stress function in terms of polar coordinates. The Airy coefficients are evaluated from measured temperature information. As well as satisfying force equilibrium, the thermoelastic results are substantiated independently using strain gages and finite element analysis. Employing real, rather than the commonly used complex, variables can be easier for such problems, avoids inconveniences associated with conformal mapping and the concept is applicable to experimental analysis of arbitrarily shaped and/or loaded cut-outs.

]]> Abstract:  This paper determines the individual components of stress experimentally in a finite tensile plate containing an elliptical hole using a series representation of an Airy stress function in terms of polar coordinates. The Airy coefficients are evaluated from measured temperature information. As well as satisfying force equilibrium, the thermoelastic results are substantiated independently using strain gages and finite element analysis. Employing real, rather than the commonly used complex, variables can be easier for such problems, avoids inconveniences associated with conformal mapping and the concept is applicable to experimental analysis of arbitrarily shaped and/or loaded cut-outs. Experimental Technique for Measuring the Long-term Transfer Length in Prestressed Concretehttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12019Experimental Technique for Measuring the Long-term Transfer Length in Prestressed ConcreteJ. R. Martí-Vargas, L. A. Caro, P. Serna2012-12-06T01:06:59.669911-05:00doi:10.1111/str.12019John Wiley & Sons, Inc.10.1111/str.12019http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12019125134

Abstract:  This article presents a proposal of a test set-up and methodology for testing the transfer length evolution through time of prestressing reinforcement in pretensioned prestressed concrete members, aimed at providing a basis for standardization. The proposed test method is based on the instantaneous and time-related analysis of the prestressing reinforcement force profile at only one end of a pretensioned prestressed concrete member. The basis of the test method and the requirements of the prestressing frame and its components are presented, as well as the test procedure stages and the measurement devices. The interpretation of the test results and the criteria to determine both the initial and the long-term transfer lengths are explained. A test method application and the equipment for testing seven-wire prestressing strands have been designed. Some experimental results are provided to validate the test. A comparative analysis of test reliability with other existing experimental methods is also included.

]]> Abstract:  This article presents a proposal of a test set-up and methodology for testing the transfer length evolution through time of prestressing reinforcement in pretensioned prestressed concrete members, aimed at providing a basis for standardization. The proposed test method is based on the instantaneous and time-related analysis of the prestressing reinforcement force profile at only one end of a pretensioned prestressed concrete member. The basis of the test method and the requirements of the prestressing frame and its components are presented, as well as the test procedure stages and the measurement devices. The interpretation of the test results and the criteria to determine both the initial and the long-term transfer lengths are explained. A test method application and the equipment for testing seven-wire prestressing strands have been designed. Some experimental results are provided to validate the test. A comparative analysis of test reliability with other existing experimental methods is also included. Dissipative Behaviour Analysis of Reprocessed Polyethylene Terephthalate using Digital Image Correlation and Updated Finite Element Analysishttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12021Dissipative Behaviour Analysis of Reprocessed Polyethylene Terephthalate using Digital Image Correlation and Updated Finite Element AnalysisA. S. Caro-Bretelle, P. Ienny, L. K. Nait-Ali, A. Bergeret2013-01-29T05:09:17.795721-05:00doi:10.1111/str.12021John Wiley & Sons, Inc.10.1111/str.12021http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12021135146

Abstract:  Polyethylene terephthalate (PET) is a semi-crystalline polyester that is usually processed by conventional methods. However, the degradation of this polyester during the melting process impacts its macroscopic behaviour, which is particularly true for recycled PET that is subjected to multiple processes at the melting temperature by extrusion, injection-moulding or pelletizing. In this work, the impact of reprocessing on the mechanical properties of PET is studied. This approach requires exploring the influence of implementation on both its own mechanical performance factor and on the damage linked to the striction problem. An experimental protocol was developed to study large strain responses through a set of sequenced and instrumented uniaxial traction tests based on the standard ISO-5272-1-A and hourglass geometries. Updated finite element simulations were coupled with digital image correlation, thereby providing access to the parameters of a phenomenological model. This coupled approach enables researchers to understand both the complex shape of the stress–strain curve and the well-known dilatant mechanism that is observed during uniaxial deformation.

]]> Abstract:  Polyethylene terephthalate (PET) is a semi-crystalline polyester that is usually processed by conventional methods. However, the degradation of this polyester during the melting process impacts its macroscopic behaviour, which is particularly true for recycled PET that is subjected to multiple processes at the melting temperature by extrusion, injection-moulding or pelletizing. In this work, the impact of reprocessing on the mechanical properties of PET is studied. This approach requires exploring the influence of implementation on both its own mechanical performance factor and on the damage linked to the striction problem. An experimental protocol was developed to study large strain responses through a set of sequenced and instrumented uniaxial traction tests based on the standard ISO-5272-1-A and hourglass geometries. Updated finite element simulations were coupled with digital image correlation, thereby providing access to the parameters of a phenomenological model. This coupled approach enables researchers to understand both the complex shape of the stress–strain curve and the well-known dilatant mechanism that is observed during uniaxial deformation. Experimental Characterisation of Aluminium 6082 at Varying Temperature and Strain Ratehttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12022Experimental Characterisation of Aluminium 6082 at Varying Temperature and Strain RateS. L. Lemanski, N. Petrinic, G. N. Nurick2013-01-11T02:54:57.28638-05:00doi:10.1111/str.12022John Wiley & Sons, Inc.10.1111/str.12022http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12022147157

Abstract:  This article describes the experimental methodology used in overcoming the challenges of performing tests and recording results on specimens, which are suitable for such a wide range of test conditions. Uniaxial tensile tests were conducted on aluminium alloy 6082-T352 at varying temperatures and strain rates to validate testing techniques and to determine the effect of these parameters upon this material. The applied strain rate varied over several orders of magnitude – using a screw-driven tensometer for quasi-static loading (6.9 × 10−4 s−1), a hydraulic piston rig for moderate strain rate (4.0 × 101 s−1) and a tensile Hopkinson bar for high strain rate (1.5 × 103 s−1). Temperature was varied using a heat gun, and the air temperature was measured using a thermocouple in the hot air stream. Specimen temperature is determined by finite element modelling, and this correlates well with other work. Although it would have been possible to improve the design of individual tests for specific test conditions, an important objective was to conduct the entire series of tests in as consistent a manner as possible. The procedure for characterising the stress–strain behaviour for this material under these different loading conditions is also considered in some detail, as the real material behaviour deviates from simplified elasto-plastic material models. Results presented for Al 6082 samples show a slight increase in yield stress with increasing strain rate, and a decrease in yield stress with increasing temperature.

]]> Abstract:  This article describes the experimental methodology used in overcoming the challenges of performing tests and recording results on specimens, which are suitable for such a wide range of test conditions. Uniaxial tensile tests were conducted on aluminium alloy 6082-T352 at varying temperatures and strain rates to validate testing techniques and to determine the effect of these parameters upon this material. The applied strain rate varied over several orders of magnitude – using a screw-driven tensometer for quasi-static loading (6.9 × 10−4 s−1), a hydraulic piston rig for moderate strain rate (4.0 × 101 s−1) and a tensile Hopkinson bar for high strain rate (1.5 × 103 s−1). Temperature was varied using a heat gun, and the air temperature was measured using a thermocouple in the hot air stream. Specimen temperature is determined by finite element modelling, and this correlates well with other work. Although it would have been possible to improve the design of individual tests for specific test conditions, an important objective was to conduct the entire series of tests in as consistent a manner as possible. The procedure for characterising the stress–strain behaviour for this material under these different loading conditions is also considered in some detail, as the real material behaviour deviates from simplified elasto-plastic material models. Results presented for Al 6082 samples show a slight increase in yield stress with increasing strain rate, and a decrease in yield stress with increasing temperature. Reduction of Noise-Induced Bias in Displacement Estimation by Linear Off-Pixel Digital Image Correlationhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12023Reduction of Noise-Induced Bias in Displacement Estimation by Linear Off-Pixel Digital Image CorrelationW. Tong2013-01-11T02:55:12.25994-05:00doi:10.1111/str.12023John Wiley & Sons, Inc.10.1111/str.12023http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12023158166

Abstract:  A linear digital image correlation algorithm is proposed to eliminate noise-induced bias in one-dimensional translation estimation using noisy images. The algorithm uses linear interpolation for both initial and current images at off-pixel positions and solves directly the displacement parameter by minimizing a sum-of-squared-differences coefficient. Both analytical results and numerical simulations using synthetic image sets show that there is indeed no noise-induced bias in the displacement estimation using the proposed algorithm if the off-pixel positions in both images are chosen properly according to the relative displacement between two images. When the displacement is only known initially within a range of ±0.5 pixels from the actual displacement, an iterative procedure using the algorithm is able to obtain the displacement estimation with a residual bias that converges to the noiseless subpixel approximation bias. A further refinement of the off-pixel analysis algorithm will be needed so the remaining residual bias due to subpixel approximation can also be significantly reduced.

]]> Abstract:  A linear digital image correlation algorithm is proposed to eliminate noise-induced bias in one-dimensional translation estimation using noisy images. The algorithm uses linear interpolation for both initial and current images at off-pixel positions and solves directly the displacement parameter by minimizing a sum-of-squared-differences coefficient. Both analytical results and numerical simulations using synthetic image sets show that there is indeed no noise-induced bias in the displacement estimation using the proposed algorithm if the off-pixel positions in both images are chosen properly according to the relative displacement between two images. When the displacement is only known initially within a range of ±0.5 pixels from the actual displacement, an iterative procedure using the algorithm is able to obtain the displacement estimation with a residual bias that converges to the noiseless subpixel approximation bias. A further refinement of the off-pixel analysis algorithm will be needed so the remaining residual bias due to subpixel approximation can also be significantly reduced. Diffusing-Wave Spectroscopy Contribution to Strain Analysishttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12024Diffusing-Wave Spectroscopy Contribution to Strain AnalysisM. Erpelding, B. Dollet, A. Faisant, J. Crassous, A. Amon2013-02-04T05:51:35.19602-05:00doi:10.1111/str.12024John Wiley & Sons, Inc.10.1111/str.12024http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12024167174

Abstract:  We present a new full-field strain measurement method based on diffusing-wave spectroscopy. Our technique makes it possible to measure strains in the vicinity of the surface of highly light-scattering materials. Its main feature is an extreme sensitivity: the range of deformations measured is 10−5–10−3. To validate the measurements, experimental results from several plane stress configurations are compared with theoretical and numerical calculations. Furthermore, we propose an extension of the method for non-scattering materials.

]]> Abstract:  We present a new full-field strain measurement method based on diffusing-wave spectroscopy. Our technique makes it possible to measure strains in the vicinity of the surface of highly light-scattering materials. Its main feature is an extreme sensitivity: the range of deformations measured is 10−5–10−3. To validate the measurements, experimental results from several plane stress configurations are compared with theoretical and numerical calculations. Furthermore, we propose an extension of the method for non-scattering materials. Eco-Friendly Laminates: From the Indentation to Non-Destructive Evaluation by Optical and Infrared Monitoring Techniqueshttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12026Eco-Friendly Laminates: From the Indentation to Non-Destructive Evaluation by Optical and Infrared Monitoring TechniquesS. Sfarra, C. Ibarra-Castanedo, C. Santulli, F. Sarasini, D. Ambrosini, D. Paoletti, X. Maldague2013-02-18T06:01:50.817326-05:00doi:10.1111/str.12026John Wiley & Sons, Inc.10.1111/str.12026http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12026175189

Abstract:  In this work, the combined effect of indentation damage and of manufacturing defects of a hybrid laminate including jute hessian cloth (plain weave) and hemp fibres in an epoxy matrix has been investigated. With this aim, various non-destructive evaluation (NDE) techniques have been employed, such as near-infrared (NIR) reflectography, infrared thermography (IRT), holographic interferometry (HI) and digital speckle photography (DSP). In particular, two different methods of heating were applied during IRT data collection: pulse thermography and square pulse thermography (SPT). The first one using a mid-wave infrared (IR) camera, while the second one using a long-wave IR camera. In the same way, two different cameras working into the near- and short-wave IR spectra were used, to compare different results from ∼ 0.74 to 14 μm. Data were processed applying principal component thermography (PCT), correlation and the robust second-order blind identification (SOBI-RO) algorithms. The latter is used for the first time to our knowledge in this work. The defects found were enhanced by image subtraction between the reflectogram and the transmittogram, distance transform and image fusion. In particular, data fusion from IRT and DPS images allowed clearly defining the extension of the indentation damage.

]]> Abstract:  In this work, the combined effect of indentation damage and of manufacturing defects of a hybrid laminate including jute hessian cloth (plain weave) and hemp fibres in an epoxy matrix has been investigated. With this aim, various non-destructive evaluation (NDE) techniques have been employed, such as near-infrared (NIR) reflectography, infrared thermography (IRT), holographic interferometry (HI) and digital speckle photography (DSP). In particular, two different methods of heating were applied during IRT data collection: pulse thermography and square pulse thermography (SPT). The first one using a mid-wave infrared (IR) camera, while the second one using a long-wave IR camera. In the same way, two different cameras working into the near- and short-wave IR spectra were used, to compare different results from ∼ 0.74 to 14 μm. Data were processed applying principal component thermography (PCT), correlation and the robust second-order blind identification (SOBI-RO) algorithms. The latter is used for the first time to our knowledge in this work. The defects found were enhanced by image subtraction between the reflectogram and the transmittogram, distance transform and image fusion. In particular, data fusion from IRT and DPS images allowed clearly defining the extension of the indentation damage. Application of a Multi-Camera Stereo DIC Set-up to Assess Strain Fields in an Erichsen Test: Methodology and Validationhttp://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12027Application of a Multi-Camera Stereo DIC Set-up to Assess Strain Fields in an Erichsen Test: Methodology and ValidationY. Wang, P. Lava, S. Coppieters, P. V. Houtte, D. Debruyne2013-02-18T06:02:25.535752-05:00doi:10.1111/str.12027John Wiley & Sons, Inc.10.1111/str.12027http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111%2Fstr.12027190198

Abstract:  A multi-camera stereo digital image correlation (MC-DIC) set-up is presented to obtain full displacement and strain fields of a sheet-metal specimen subjected to an Erichsen test. The set-up is composed of several conventional stereo DIC systems (two camera set-up), each of which tracks the deformation of an aspect of the specimen. The individual measurements, including the geometries and the displacements, are then converted to the same reference frame to integrate into a global view. Afterwards, the strain is calculated based on the composed displacement field. It is found that the geometry and the displacement fields of the bulged specimen are ideally stitched, and smooth strain fields are obtained. The influences of the reference frame transformation and the stitching procedure on the MC-DIC measurement are investigated. A rigid motion test is performed to validate the displacement measurement. It is discussed that the global field is more reliable than the individual measurements for this test set-up.

]]> Abstract:  A multi-camera stereo digital image correlation (MC-DIC) set-up is presented to obtain full displacement and strain fields of a sheet-metal specimen subjected to an Erichsen test. The set-up is composed of several conventional stereo DIC systems (two camera set-up), each of which tracks the deformation of an aspect of the specimen. The individual measurements, including the geometries and the displacements, are then converted to the same reference frame to integrate into a global view. Afterwards, the strain is calculated based on the composed displacement field. It is found that the geometry and the displacement fields of the bulged specimen are ideally stitched, and smooth strain fields are obtained. The influences of the reference frame transformation and the stitching procedure on the MC-DIC measurement are investigated. A rigid motion test is performed to validate the displacement measurement. It is discussed that the global field is more reliable than the individual measurements for this test set-up.