R. Chona

Stereo-DIC Measurements of Thermal Gradient Effects on the Vibratory Response of Metals

Thermomechanical problems have been much less studied than their room temperature counterparts as challenges arise both with metrology and with interpretation of results. This effort aims to shrink this knowledge gap by investigating the influence of thermal effects on the high frequency vibratory response of metals. The present study concentrates on how an inhomogeneous temperature field (max. 600 °C) affects the vibratory response, and specifically mode shapes and resonant frequencies, of a vibrating plate. A plate made of a nickel-based superalloy, Hastelloy X, was heated by induction heating and the temperature distribution was estimated by measuring the out-of-plane curvature resulting form heating. Harmonic vibratory loading at frequencies exceeding 1 kHz was applied using a programmable shaker. Stereo-vision digital image correlation (stereo-DIC) was used to obtain a full-field representation of the vibrating plate. An image decomposition analysis technique based on Tchebichef polynomials was used to compare room and high temperature mode shapes. Results indicate that there is a small influence of temperature on resonant frequencies, even though mode shapes remain similar between room and high temperature vibration.

Effect of Two‐Dimensional Grading on the Thermomechanical Response of the Panel

Some of the advantages of functionally graded materials (FGM) are related to their ability to provide a better thermal protection and reduce delamination tendencies present in layered composites. In particular, in ceramic‐metal systems these goals can be achieved by increasing the concentration of ceramic particles in the region adjacent to the heated surface using a heterogeneous single layered structure. The unfortunate by‐products of such design are asymmetry about the middle surface of the structure and bending‐stretching coupling. As a result, displacements and stresses increase as compared to the symmetric counterpart, while the buckling loads and natural frequencies decrease. One of the possible solutions to the problem compensating for a reduced stiffness of FGM structures is based on the replacement of one‐dimensional grading with a two‐dimensional grading, including the regions with enhanced stiffness. The paper illustrates the formulation of the problem and peculiarities introduced in the solut...

Relating Fatigue Strain Accumulation to Microstructure using Digital Image Correlation

While fatigue has been studied extensively, fatigue life predictions are still phenomenological and relatively simple (e.g. Miner’s rule, Paris relationship, etc.). Many improvements have been suggested to such models to incorporate newly discovered phenomena, but fatigue life predictions still have limited accuracy and scope. A quantitative understanding of fatigue at the grain level would lead to models with better predictive capability and/or broader applicability. In this work, fatigue crack growth in Hastelloy X, a high-temperature nickel based alloy, was examined using an ex-situ digital image correlation technique. Electron backscatter diffraction (EBSD) was performed on a region of interest in front of the crack tip in a single edge notched tension specimen to obtain microstructural characteristics. The specimen was then fatigue loaded to advance the crack. At regular intervals of crack growth, the specimen was removed from the load frame and the region of interest was imaged with an optical microscope. By performing digital image correlation on these images, a full-field measure of the accumulated plastic strain was obtained as the crack approached and passed through the region of interest. Strain fields were compared to EBSD results to elucidate the relationship between microstructure and fatigue crack growth. The presence of strain concentrations at grain and (annealing) twin boundaries was seen to be instrumental in the evolution of plastic strain accumulation during fatigue.

Understanding Mechanisms of Cyclic Plastic Strain Accumulation under High Temperature Loading Conditions

In-depth analysis of deformation mechanisms, such as slip deformation and slip/twin interaction, on the micro-level can greatly enhance the understanding of the physics behind thermomechanical fatigue damage accumulation in ductile metals. This improved understanding is essential for the development of new fatigue models that are not phenomenological, and have better predictive capability than the current ones. Hastelloy X, a nickel based superalloy has been experimentally studied using multiscale experimentation under different cyclic plasticity and thermomechanical fatigue loading conditions. High resolution ex-situ Digital Image Correlation (DIC) was used to measure plastic strain accumulation as a function of load and temperature. By performing DIC experiments with sub-grain resolution we can relate the measured strain fields to the underlying microstructure through comparison with EBSD scans of the same region of interest. The results reveal a highly heterogeneous material response at the grain level and even at the sub-grain level with high strain concentrations at regions such as twin boundaries and triple points, and with strain variations even within particular grains. By making such measurements at regular intervals of load, the evolution of plastic strain during loading was observed. As loading increased heterogeneity also increased (i.e. regions with high strain became more strained and regions with low strain remained relatively unstrained).

Studying Thermomechanical Fatigue of Hastelloy X using Digital Image Correlation

Hastelloy X, a nickel based superalloy, has been extensively used in the past for high temperature applications. However, less is known about its thermomechanical fatigue response compared to other structural metals. Generally, the relation between microscale and macroscale is vital in order to identify specific phenomena that contribute to the cracked component fatigue lifetime. In this work, Hastelloy X notched samples were used to investigate fatigue crack nucleation near the notch and subsequent fatigue crack growth. Isothermal experiments at varying temperatures (up to 1,000°C) were performed while the specimen was fatigue loaded to advance the crack. Macroscale Digital Image Correlation (DIC) was performed on images taken at various stages of crack growth. In addition, using images obtained directly behind the crack tip, microscale DIC was used to quantify the effects of crack closure. The interaction of crack closure and far-field loading was investigated as a function of temperature. The combination of full-field macroscale and near-tip microscale measurements will aid in the development and understanding of a multiscale fatigue crack growth model.