Photoelastic force measurements in granular materials

Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local of stress. In this review paper, we summarize past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus

Determination of the characteristic directions of lossless linear optical elements

We show that the problem of finding the primary and secondary characteristic directions of a linear lossless optical element can be reformulated in terms of an eigenvalue problem related to the unimodular factor of the transfer matrix of the optical device. This formulation makes any actual computation of the characteristic directions amenable to pre-implemented numerical routines, thereby facilitating the decomposition of the transfer matrix into equivalent linear retarders and rotators according to the related Poincare equivalence theorem. The method is expected to be useful whenever the inverse problem of reconstruction of the internal state of a transparent medium from optical data obtained by tomographical methods is an issue.Comment: Replaced with extended version as published in JM

Calibration and evaluation of optical systems for full-field strain measurement

The design and testing of a reference material for the calibration of optical systems for strain measurement is described, together with the design and testing of a standardized test material that allows the evaluation and assessment of fitness for purpose of the most sophisticated optical system for strain measurement. A classification system for the steps in the measurement process is also proposed and allows the development of a unified approach to diagnostic testing of components or sub-systems in an optical system for strain measurement based on any optical technique. The results described arise from a European study known as SPOTS whose objectives were to begin to fill the gap caused by a lack of standards

Application of Sharafutdinov's Ray Transform in Integrated Photoelasticity

We explain the main concepts centered around Sharafutdinov's ray transform, its kernel, and the extent to which it can be inverted. It is shown how the ray transform emerges naturally in any attempt to reconstruct optical and stress tensors within a photoelastic medium from measurements on the state of polarization of light beams passing through the strained medium. The problem of reconstruction of stress tensors is crucially related to the fact that the ray transform has a nontrivial kernel; the latter is described by a theorem for which we provide a new proof which is simpler and shorter as in Sharafutdinov's original work, as we limit our scope to tensors which are relevant to Photoelasticity. We explain how the kernel of the ray transform is related to the decomposition of tensor fields into longitudinal and transverse components. The merits of the ray transform as a tool for tensor reconstruction are studied by walking through an explicit example of reconstructing the $\sigma_{33}$-component of the stress tensor in a cylindrical photoelastic specimen. In order to make the paper self-contained we provide a derivation of the basic equations of Integrated Photoelasticity which describe how the presence of stress within a photoelastic medium influences the passage of polarized light through the material

Shaking table testing of an existing masonry building: assessment and improvement of the seismic performance

This paper aims to assess and improve the seismic performance of an existing masonry building with flexible floors, representative of a Portuguese building typology—‘gaioleiro’ buildings. The study involved seismic tests and dynamic identification tests of two models (nonstrengthened and strengthened) in the shaking table. Each model was subjected to several seismic tests with increasing amplitude. . Before the first test and after each seismic test, the dynamic identification of the model was carried out, aiming at obtaining their seismic vulnerability curves based on a damage indicator obtained from the decrease of the frequencies of the modes. In the strengthened model, steel elements were used to improve the connection between walls and floors, together with ties in the upper stories. The results show that adopted strengthening technique is effective for reducing the seismic vulnerability of ‘gaioleiro’ buildings, namely for improving the out-of-plane behavior of the facades.The first author acknowledges the financial support from the Portuguese Foundation for Science and Technology (FCT) through grant SFRH/BD/32190/2006. The work was also supported by the project 'New Integrated Knowledge based approaches to the protection of cultural heritage form Earthquake-induced Risk' (NIKER FP7-ENV-2009-1-224123) from the European Commission

Experimental behavior of masonry wall-to-timber elements connections strengthened with injection anchors

Out-of-plane failure mechanisms observed in stone masonry buildings subjected to seismic action are often a direct result of poor connections between structural elements. During a seismic event these weak connections become incapable of assuring proper load transmission. Therefore, the need to prevent these phenomena is of critical importance in understanding the behavior of unstrengthened masonry buildings along with the necessity of developing effective strengthening solutions. This paper presents injection anchors as a viable option to improve anchorage between masonry and timber elements on historical buildings, as for example wall-to-timber framed wall or wall-to-timber diaphragm connections. The experimental campaign consisted of quasi-static monotonic and cyclic pullout tests performed on real scale specimens, representative of wall-to-timber framed wall connections found in late 19th century buildings of downtown Lisbon, Portugal. Combined cone-bond failure was obtained in all 7 tests. Boundary conditions of the specimens greatly affected the results in terms of maximum pullout force, dissipated energy, and strength degradation. Displacement ductility of the strengthened connections is high. The force-displacement curves clearly pointed out the influence on the results of the wall's compressive stress state and the contribution of friction in the grout/masonry interface.This work was partially funded by project FP7-ENV-2009-1-244123-NIKER of the 7th Framework Program of the European Commission, which is gratefully acknowledged. Authors would like to thank the technical staff of the Structures Lab. of University of Minho for the assistance provided preparing and carrying out the tests and also, Monumenta, Ltd. for the construction of the masonry walls and Cintec (R) for the installation of the injection anchors

Local stress measurement using the thermoelastic effect

A technique for measuring local stresses in metallic specimens is proposed and tested. The technique depends on the experimental measurement of temperature changes in stressed members due to adiabatic elastic deformation. At a free boundary in a body under plane stress, these temperature changes are directly related to the value of the tangential principal stress. The technique is suited for measurement of stress-concentration effects, since the temperature changes can be measured with thermocouples featuring extremely small junctions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43937/1/11340_2006_Article_BF02326342.pd