A new experimental technique has been devised to measure residual stresses in ductile materials with a combination of laser speckle pattern interferometry and spot heating. The speckle pattern interferometer measures in-plane deformations while the heating provides for very localized stress relief. The residual stresses are determined by the amount of strain that is measured subsequent to the heating and cool-down of the region being interrogated. A simple lumped parameter model is presented to provide a description of the method. This description is followed by presentations of the results of finite element analyses and experimental results with uniaxial test specimens. Excellent agreement between the experiments and the computer analyses were obtained.

The results presented in this paper clearly demonstrate that the dynamic range of this measurement technique can be improved substantially over the earlier experiments. It is just as clear that a more systematic study must be performed to quantify these improvements and to generate usable calibrations. These results are also encouraging in the sense that this technique may now be appropriate for other materials with high thermal diffusivities. Previous attempts to measure residual stresses by laser annealing and electronic speckle pattern interferometry have been successful for moderate to high stress levels. The method uses an infrared laser for relieving stress in a small spot. A dab on temperature indicating paint is applied to the spot and a specklegram of the spot and the surrounding area is captured. The paint is then heated with a laser until it melts. The heat is transferred from the paint into the metal resulting in a small amount of localized stress relief as the yield stress of the material drops below the stress levels surrounding the spot. Once the spot and area around it have cooled a second speckle-gram is captured and the images are processed to determine the in-plane strain. The amount of stress relief depends on the melting temperature of the paint since yield stress is a function of temperature. The measurement of local stress relief by heating is subject to limitations that result from thermal expansion competing with the reduction in yield stress of the spot at the elevated temperature. That is, as the spot is heated it tends to temporarily reduce the stress in the region surrounding the spot as it expands into this surrounding region. This limits the amount of stress relief that can occur. This can be overcome to some extent by using higher temperature paints, which in turn lowers the yield stress in the heated spot. At some point, however, the thermal expansion overtakes the surrounding stress field and can even drive it into compression. Furthermore, for tension levels on the order of eighty percent or less of the yield stress, the sub-micrometer deformations result in less than a single fringe. The strains indicated by such sub-fringes are comparable to noise levels that occur from air turbulence, environmental thermal variations and so forth. Thus, for both fundamental and practical reasons the technique was modified to increase the fringe count at lower stress levels. The authors have successfully performed two separate experiments to raise the fringe count. One method was simply to start observing the fringes (or strains) immediately after annealing. Not only can several fringes be obtained in this way but a clear relationship has been observed with the stress levels. The other approach was to cool an area surrounding the region of interest and then observe the net strain after thermal equilibrium is reestablished. Both methods have shown the ability to handle lower tension levels than were measurable by the earlier procedure.

Residual Stress, Thermomechanics& Infrared Imaging, Hybrid Techniques and Inverse Problems, Volume 9 of the Proceedings of the 2015SEM Annual Conference & Exposition on Experimental and Applied Mechanics, the ninth 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: Inverse Methods Inverse Methods in Plasticity Varying Length Scales Harsh Environments Opto-Acoustical Methods Hybrid Experimental Residual Stress Modelling and Advances in Measurements Thermomechanics General Material Response Infrared Imaging

The residual stress is a common phenomenon in composite materials. They can either add to or significantly reduce material strength. Because of the increasing demand for high-strength, lightweight materials such as composites and their wide range of applications; it is critical that the residual stresses of composite materials are understood and measured correctly.The first edition of this book consists of thirteen chapters divided into two parts. The first part reviews destructive and non-destructive testing (NDT) techniques for measuring residual stresses. There are also additional chapters on using mathematical (analytical and numerical) methods for the calculation of residual stresses in composite materials. These include the simulated hole drilling method, the slitting/crack compliance method, measuring residual stresses in homogeneous and composite glass materials using photoelastic techniques, and modeling residual stresses in composite materials. The second part of the book discusses measuring residual stresses in different types of composites including polymer and metal matrix composites. The addition of nanoparticles to the matrix of polymeric composites as a new technique for the reduction of residual stresses is also discussed.In the Second Edition of this book, each of the original chapters of the first edition has been fully updated, taking into account the latest research and new developments. There are also five new chapters on the theoretical and experimental studies of residual stresses in the composite integrated circuits; residual stresses in additive manufacturing of polymers and polymer matrix composites; residual stresses in metal matrix composites fabricated by additive manufacturing; the eigenstrain based method for the incremental hole-drilling technique; and the estimation of residual stresses in polymer matrix composites using the digital image correlation technique.Residual Stresses in Composite Materials, Second Edition, provides a unique and comprehensive overview of this important topic and is an invaluable reference text for both academics and professionals working in the mechanical engineering, civil engineering, aerospace, automotive, marine, and sporting industries. - Presents the latest developments on theoretical and experimental studies of residual stresses in composites - Reviews destructive and non-destructive testing (NDT) techniques for measuring residual stresses - Discusses residual stresses in the polymer matrix, metal matrix, and ceramic matrix composites - Considers the addition of nanoparticles to the matrix as a new technique for reduction of residual stresses in polymeric composites - Introduces the latest advancements of research on the residual stresses in additive-manufactured polymer and metal matrix composites

A new technique for the measurement of residual stresses is presented. The technique is based on strain measurements following thermal stress relaxation. The heat input is supplied by a low power infrared laser and the strain is measured with speckle pattern correlation interferometry. This paper presents a comprehensive overview of the technique and an example of how it has been applied in a practical situation.

Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Volume 3 of the Proceedings of the 2018 SEM Annual Conference & Exposition on Experimental and Applied Mechanics, the third volume of eight 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 optical methods ranging from traditional photoelasticity and interferometry to more recent DIC and DVC techniques, and includes papers in the following general technical research areas: New Developments in Optical Methods & Fringe Pattern Analysis; DIC Applications for Challenging Environments; Optical Methods in SEM: History & Perspective; Mechanical Characterization of Materials & Structures with Optical Methods; Bioengineering.