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Residual Stress, Thermomechanics & Infrared Imaging, Hybrid Techniques and Inverse Problems, Volume 8 of the Proceedings of the 2017 SEM Annual Conference & Exposition on Experimental and Applied Mechanics, the eighth volume of nine from the Conference, brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on a wide range of areas, including:

Residual Stress Measurements Stress Analysis from Thermal Measurements Damage & Defect Analysis Using Infrared Techniques Inverse Methods in Plasticity Inverse Problem Methodologies in Experimental Mechanics



Chapter 1. Residual Stresses in Bovine Femurs

The slitting method has become well-established for determining residual stresses in engineering materials. This study develops and applies a version of that method using a small slot to find residual stresses vs. depth in layers near the surface of bovine femurs. Results are obtained for the central region (diaphysis) of hydrated femurs from both mature and young cows. The magnitude of residual stresses was found to be greatest in thin layers near the surface, typically 100–200 μm deep. Residual stresses in those layers were compressive in mature femurs at the circumferential location tested, but tensile in hydrated young femurs.

Yongbo Zhang, Drew Nelson

Chapter 2. Experimental Stress Analysis of Unsymmetrical, Irregularly-Shaped Structure Containing an Arbitrarily-Shaped Hole

This paper describes the ability to process load induced temperature information with an Airy stress function in real polar coordinates and some local known boundary conditions to determine the stresses experimentally in an isotropic linear elastic finite arbitrarily-shaped structure containing an irregularly-shaped hole. The proposed method simultaneously smooths the measured data, separates the stress components, and evaluates the individual stress components full-field, including at the boundary of the hole (location of highest tensile stress).

B. Kalayciogli, A. Alshaya, R. Rowlands

Chapter 3. Quantitative Calorimetry and TSA in Case of Low Thermal Signal and Strong Spatial Gradients: Application to Glass Materials

In the present paper, the thermo-mechanical characterization of a holed glass sample under cyclic loading is carried out. Due to the low thermoelastic response obtained for such a material, the thermal movie has been preliminary filtered. The experimental stress field obtained from the Thermoelastic Stress Analysis (TSA) is well correlated to the finite element model. It validates both the use of this experimental technique to study the thermoelastic response of brittle materials and the filtering methodology. Finally, the corresponding calorimetric response has been determined by using a simplified formulation of the heat diffusion equation. This permits to quantify heat sources and to carry out energy balances.

Guillaume Corvec, Eric Robin, Jean-Benoît Le Cam, Pierre Lucas, Jean-Christophe Sangleboeuf, Frédéric Canevet

Chapter 4. A New Denoising Methodology to Keep the Spatial Resolution of IR Images Equal to 1 Pixel

This paper proposes a noise suppression methodology to improve the spatio-temporal resolution of infrared images. The methodology is divided in two steps. The first one consists in removing the noise from the temporal signal at each pixel. In the second step, the residual offset is identified by considering thermal images for which no mechanical loading is applied. In this case, the temperature variation field is homogeneous and the value of temperature variation at each pixel is theoretically equal to zero. The method is first tested on numerical images. The results demonstrate that this approach permits to keep the spatial resolution of infrared images equal to 1 pixel. The methodology is then applied to characterize thermal activity of a defect at the surface of inorganic glass submitted to cyclic mechanical loading.

Guillaume Corvec, Eric Robin, Jean-Benoît Le Cam, Jean-Christophe Sangleboeuf, Pierre Lucas

Chapter 5. Calorific Signature of PLC Bands Under Biaxial Loading Conditions in Al-Mg Alloys

This paper investigates the thermomechanical behavior of Al-Mg alloys submitted to biaxial loading until fracture. The study aims to characterize calorimetric signature accompanying the formation and propagation of Portevin-Le Chatelier (PLC) bands induced by such a loading condition. Full kinematic and thermal fields on the specimen surface were characterized by using Digital Image Correlation (DIC) and infrared thermography (IRT). Heat source field was reconstructed from the temperature field and the heat diffusion equation. The heat source map enables us to visualize spatio-temporal gradients in the calorimetric response of the material and to investigate the kinematics of PLC bands induced by equibiaxial tensile loading. Under certain conditions, heat source maps can be seen as mechanical dissipation maps. At the specimen centre, the heat source exhibits jumps that fit with jumps of temperature and equivalent deformation rate.

Jean-Benoît Le Cam, Eric Robin, Lionel Leotoing, Dominique Guines

Chapter 6. How Does Cristallizable Rubber Use Mechanical Energy to Deform?

Strain-induced crystallization (SIC) is responsible for the hysteresis loop observed in the mechanical response of Natural Rubber (NR). The present paper aims at determining the physical origin of such mechanical energy dissipation. For that purpose, temperature variations are measured by using infrared thermography during cyclic uniaxial tensile tests at ambient temperature. Heat sources (heat power densities) produced or absorbed by the material due to deformation processes are deduced from temperature fields by using the heat diffusion equation. Energy balance performed for each deformation cycle shows that crystallization/melting process does not produce intrinsic dissipation. The crystallization/melting process dissipates mechanical energy without converting it into heat. Hence, the whole dissipated mechanical energy corresponds to energy used by the material to change its microstructure. The demonstration that NR is able to dissipate mechanical energy without converting it into heat explains its ability to resist the crack growth and the fatigue loading.

Jean-Benoît Le Cam

Chapter 7. Use of Bulge Test Geometry for Material Property Identification

The bulge test geometry, sometimes called blister or burst test, has a long history of use for material property identification. Paper materials are thin with relatively low stiffness; in a bulge test paper materials will exhibit a combination of membrane and plate behavior. We have developed a VFM examination to identify the in-plane stiffnesses of this type of material by incorporating both membrane and plate internal work.

John M. Considine, X. Tang

Chapter 8. Crystal Plasticity Parameter Identification by Integrated DIC on Microscopic Topographies

The present study unravels details of the micromechanical behavior of a micro-specimen made of IF-steel. A triangular prism is machined via focused ion beam (FIB) and contains two ferritic grains. Four experimental tools are integrated to identify the material’s crystal parameters: (i) an optical confocal microscope captures height profile images, (ii) an in-situ tensile stage prescribes the loading history to the macro-specimen, (iii) a global Digital Image Correlation (DIC) algorithm measures the 3D surface displacement fields, and iv) an extension of Integrated-DIC for 3D displacement fields is implemented to assess the micromechanical behavior. It is demonstrated that with this methodology the identification of the boundary conditions and crystal plasticity parameters is successfully achieved.

J. P. M. Hoefnagels, M. Bertin, C. Du, F. Hild

Chapter 9. Comparison of Residual Stress Characterization Techniques Using an Interference Fit Sample

Residual stress in an engineering component induced from processing is pervasive and can impact the component’s performance significantly. There are numerous destructive and non-destructive techniques that are available to determine the residual stresses in a component. In this work, an interference fit sample was manufactured from a titanium alloy. The sample was equipped with a set of strain gauges to measure the strains induced by the interference process used for sample assembly. Energy dispersive diffraction experiment using synchrotron radiation was conducted to measure the lattice strains in the interference fit sample. Hole drilling measurements were also conducted on the sample. The non-destructive X-ray result is compared with strain gauge measurements, and found to be in good agreement when appropriately averaged.

Jun-Sang Park, John Okasinski, Jonathan Almer, Paul Shade, T. J. Turner

Chapter 10. Influence of Thermographic Image Filtering on Hybrid TSA

Hybrid thermoelastic stress analysis (Hybrid-TSA) is an experimental thermographic method that has been successfully applied to plenty of structures and loading situations. The main objective is the separation of the TSA stresses, namely the sum of the two principal in-plane stresses. While Hybrid-TSA yields a general better understanding of a component’s elastic response, it is mostly useful for structural integrity analysis in terms of stress variation due to fatigue or cracking for instance. In practice, TSA stress separation is accomplished using an Airy stress function, which is derived from compatibility and equilibrium conditions, and is frequently represented in the form of a finite series of coefficients. To date, the analyses mostly focused on determining the appropriate number of coefficients for a given noisy TSA image, excluding the filtering of the TSA raw data. The present study deals with the influence of filtering operations on the results of the method. Synthetic TSA stress fields corrupted by added noise are considered. Gaussian filters are then applied to reduce the difference between theoretical and reconstructed TSA stresses using a reduced number of points on the structure. The influence of the noise level is discussed. The study provides information for a better separation of stresses at an optimal computational cost.

W. A. Samad, X. Balandraud

Chapter 11. Optical Analysis of Residual Stress with Minimum Invasion

Electronic Speckle-Pattern Interferometry (ESPI) has been applied to analyze welding-induced residual stresses. A tensile load is applied to a butt-welded aluminum alloy specimen (tungsten arc bead-on-plate welding on a 100 × 20 mm plate of 5 mm thick) at loading levels up to 20 % of the yield stress, and the acceleration field is evaluated from subtraction of the displacement fields taken in two consecutive time steps. Based on our previously proposed algorithm, the residual stress is estimated from the acceleration field. The resultant residual stress data are compared with experimental results for the same welded specimen based on acoustoelasticity and X-ray diffractometry. Numerical analysis based on Finite Element Modeling has also been conducted. The residual stress data resulting from all the four methods agree with one another, commonly exhibiting the generally accepted concepts that thermal expansion causes tensile residual stress along the weld line and that the effect is greater at the ending side of butt-welding than starting side. The maximum tensile residual stress along the weld line is estimated to be 76.9 ± 12.8 MPa.

Sanichiro Yoshida, Fumiya Miura, Tomohiro Sasaki, Daniel Didie, Shahab Rouhi

Chapter 12. Determination of Constitutive Properties in Inverse Problem Using Airy Stress Function

A new inverse problem formulation is developed using the Airy stress function. Inverse methods are used to determine the constitutive properties of a graphite/epoxy laminated composite loaded vertically by processing measured values of v-displacement component with an Airy stress function in complex variables. Displacements are recorded using digital image correlation. The traction-free conditions on the symmetrically located sided notches are satisfied analytically using conformal mappings and analytic continuation. The traction-free on the vertical free edge and a symmetrical condition on horizontal line of symmetry are imposed discretely. The primary advantage of this new formulation is the direct use of displacement data, eliminating the need for numerical differentiation when strain data is required. The inverse method algorithm determined the constitutive properties with errors range from 2% to 10%. Selection of Airy coefficients, test geometry configuration and comparison with other inverse methods will be addressed.

A. Alshaya, John M. Considine, R. Rowlands

Chapter 13. High-Speed Infrared Imaging for Material Characterization in Experimental Mechanic Experiments

Heat transfers are involved in many phenomena such as friction, tensile stress, shear stress and material rupture. Among the challenges encountered during the characterization of such thermal patterns is the need for both high spatial and temporal resolution. Infrared imaging provides information about surface temperature that can be attributed to the stress response of the material and breaking of chemical bounds. In order to illustrate this concept, high-speed infrared sequences were recorded during tensile and shear testing experiments carried out on steel and aluminum samples. Results from split-Hopkinson experiments carried out on a polymer material at high strain-rate are also presented. The results illustrate how high-speed and high-definition infrared imaging in the midwave infrared (MWIR, 3–5 μm) spectral range can provide detailed information about the thermal properties of materials undergoing mechanical testing.

Marc-André Gagnon, Frédérick Marcotte, Philippe Lagueux, Vince Morton

Chapter 14. A Spatio-Temporal Approach for iDIC-Residual Stress Measurement

Recently, the Integrated-Digital Image Correlation (iDIC) has been proposed as a simple and effective approach for residual stress measurement. iDIC is a variant of the classical Digital Image Correlation where the “standard” displacement functions are replaced by problem-specific ones. By this simple modification, stress components become the unknowns of the problem, thus allowing a single-pass analysis. However, implementation of the Integral Method for estimation of depth-dependent Residual Stress components is difficult. In particular, the Least Squares approach is not possible.This work suggests a two-pass approach: in the former the direct solution of the triangular linear system is solved. In the latter, the previous estimates are used as starting point for a global minimization involving all the acquired images.

Antonio Baldi

Chapter 15. Detection of Early Stage Material Damage Using Thermophysical Properties

This paper presents a preliminary study on the effects of fatigue induced microstructural damage on the thermophysical properties of carbon fiber polymer composites commonly used for aerospace applications. The goal is to identify thermal properties that may serve as viable indicators of early stage material damage during structural health monitoring (SHM). A series of fatigue tests were conducted on multilayer composite specimens with peak stress σmax = 0.55σu (the static strength) and stress ratio R = 0.1. The cyclic load was paused periodically at predefined cycle intervals starting from 100 cycles through end of test at 150 k cycles. At each pause, the front side of the specimen is instantaneously heated with a high intensity flash followed by temperature measurements by two thermocouples attached at the front and back sides the rectangular specimen. Simultaneously, IR images are recorded using high speed camera. Changes in thermophysical properties including heat transfer rate (Q), thermal conductivity (k), heat capacity (c) were computed as functions of fatigue cycles. Preliminary data shows that the time-temperature (T-t) evolution is correlated to the number of fatigue cycles or consumed fatigue life of the specimens.

Mulugeta A. Haile, Natasha C. Bradly, Michael D. Coatney, Asha J. Hall

Chapter 16. Repeatability of Contour Method Residual Stress Measurements for a Range of Material, Process, and Geometry

This paper examines precision of the contour method using five residual stress measurement repeatability studies. The test specimens evaluated include: an aluminum T-section, a stainless steel plate with a dissimilar metal slot-filled weld, a stainless steel forging, a titanium plate with an electron beam slot-filled weld, and a nickel disk forging. These specimens were selected to encompass a range of typical materials and residual stress distributions. Each repeatability study included contour method measurements on five to ten similar specimens. Following completion of the residual stress measurements an analysis was performed to determine the repeatability standard deviation of each population. In general, the results of the various repeatability studies are similar. The repeatability standard deviation tends to be relatively small throughout the part interior and there are localized regions of higher repeatability along the part perimeter. The repeatability standard deviations over most of the cross-section range from 5 MPa, for the aluminum T-section, to 35 MPa, for the stainless steel forging. These results provide expected precision data for the contour method over a broad range of specimen geometries, materials, and stress profiles.

Mitchell D. Olson, Adrian T. DeWald, Michael R. Hill

Chapter 17. System Identification of Structures with Modal Interference

Previous studies show that when a system has repeated modes in frequency domain, the multiple-input and multiple-output (MIMO) concept can be applied to effectively identify the modal parameters. In this study, the MIMO concept is extended to the time domain for system identification of systems with modal interference. When performing a modal analysis of a structural system with close modes or high damping, the results of identification may be poor due to modal interference, and the estimation of system order is important to the effectiveness of modal identification. By introducing the concept of the singular value decomposition (SVD) in Eigensystem Realization Algorithm (ERA), and simplifying the identification process of ERA, the system order will be estimated effectively even for a system having close (even repeated) modes. In addition, the correlation matrix is also used to perform SVD, and then directly identify modal parameters, i.e., omitting the additional process of constructing the generalized Hankel matrix, which provides a more efficient method of identification of modal parameter. Also, in this paper, the SVD algorithm is introduced to the identification process of Ibrahim Time-Domain Method. The order of system matrix is efficiently determined, and the modal identification of a system with close modes through the ITD method can then be well implemented.

Chang-Sheng Lin

Chapter 18. Influence of Printing Constraints on Residual Stresses of FDM Parts

The Fused Deposition Modelling (FDM) is nowadays one of the most widespread and employed processes to build complex 3D prototypes directly from a STL model. In this technique, the part is built as a layer-by-layer deposition of a feedstock wire. This typology of deposition has many advantages but produces rapid heating and cooling cycles of the feedstock material that introduces residual stresses in the part during the build-up. Consequently, warping, de-layering and distortion of the part during the print process are common issues in FDM parts and are related to residual stresses. The common techniques employed to obtain parts of correct shape and dimensions, such as depositing glue on the bed, have the aim to constrain the object on the printing bed, though this increases the residual stresses in the parts. The aim of the present work is to measure the residual stresses in several points of printed parts, both on top and bottom, in order to verify if the constrain conditions used during the printing produce substantial variation from a point to another. The residual stresses have been measured in ABS parts employing the hole-drilling method. In order to avoid the local reinforcement of the strain gage, an optical technique, i.e. ESPI (electronic speckle pattern interferometry), is employed to measure the displacement of the surface due to the stress relaxation and, consequently, calculate the residual stresses.

C. Casavola, A. Cazzato, V. Moramarco, G. Pappalettera
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