Nondestructive Evaluation of Fatigue in Titanium Alloys
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H. ROSNER*, N. MEYENDORF*, S. SATHISH**, T.E. MATIKAS*** *Fraunhofer Institute Nondestructive Testing, UniversitAt 37, 66123 Saarbrilcken, Germany "**Centerfor Materials Diagnostics, University of Dayton, Dayton OH 45469-0121 ***Former Director of the Center for Materials Diagnostics, University of Dayton
ABSTRACT Dissipated heat has been measured by thermographic technique during fatigue experiments on Ti-6AI-4V. Surface temperature of the specimen was found sensitive to the amount of fatigue damage accumulated in the material. An increased heat dissipation due to fatigue can be related to continuous change in the microstructure (increased dislocation density, stacking faults etc.) of the material. A method based on passive thermography can be proposed to monitor damage accumulation in Ti-6AI-4V due to cyclic loading. INTRODUCTION Cyclic loading of materials continuously degrades the microstructure and eventually leads to catastrophic failure. The methodologies used for predicting the fatigue and fracture life of materials assumes a pre-existence of a crack and examines the behavior of its growth under cyclic loading. These approaches have provided excellent design criteria for materials as well as components [1]. Although presence of a crack and its growth is detrimental for the life of the material, the processes that lead to the formation of cracks is very important in understanding the degradation of material and to develop experimental techniques to detect the onset of degradation and failure. At present, no reliable technique is available, neither to predict nor to detect the onset of failure in materials. Recently a nondestructive method based on nonlinear
acoustics has been used to follow continuously fatigue damage accumulation in titanium alloys [2]. It has also been observed that heat generation caused by internal friction during fatigue loading in steel samples can be utilized to study damage accumulation [3,4]. The aim of the present research is to investigate the applicability of infirared (IR) thermographic method of measuring heat generation during cyclic loading of titanium alloys to continuously monitor the progression of fatigue damage. In order to detect heat generated in a material by passive IR thermography, an effective method is required to excite heat in the specimen. In the present work mechanical loading has been applied to generate heat. It is also possible to use ultrasonic waves to load the sample to induce heat [5,6]. Under a load, the temperature of the material changes, and the characteristics of the changes in temperature is detected with IR-thermography. The change in temperature is caused
by two separate mechanisms. One of the mechanisms is thermoelasticity of the material, while the other, which is more important for fatigue characterization, is the heat dissipation caused by internal friction [7]. An analysis of the dissipated heat as the material is fatigued is used for the investigation of damage accumulation. IR thermography has local resolution and takes a short time (o
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