Papers by Author: Viorel Goanta

Abstract: The paper proposes to present the results of the evaluation of glass fiber reinforced plastics (GFRP) used in the construction of wind turbine blades. In a wind turbine, the blades are the most exposed to damages and the defects which appear are various and are connected with the type of manufacture, simple/complex loading, environmental conditions etc. In order to increase the lifetime span and to analyze the degradation phenomena during the materials functioning, destructive evaluation tests are performed to determine the mechanical property, by testing pure shear on specimens Iosipescu, from GFRP with woven reinforcement at [± 45°] and [0°/90°], with the shear fixture, endowment of Technical University Gh.Asachi Iasi.

Abstract: Experimental investigation methods and techniques are used to obtain accurate information on the tension and deformation of materials. Results underlying the limit stress state that aply to materials at this moment come from experiments on plain specimens and specimens showing stress concentrations. The study of stress concentrations is necessary to determine how it influences the limit stress state, distribution of stresses and yielding section. The article presents a series of specimens with stress concentrations and finite element analysis for thorough research.

Abstract: Testing cruciform specimens subjected to biaxial tension is one of the most widely used experimental techniques and more accurate at this time to determine the mechanical properties of materials and to verify the failure theories. This type of experiment allows the continuous monitoring of behavior of materials from the beginning of deformation until fracture under different ratios of forces and directions of the deformation, which transforms it into a very versatile testing method. We have varied the number of parameters and their values in order to achieve a uniform distribution of biaxial state of stresses and strains in the area tested. In theory, any material can be tested by stretching a biaxial cruciform specimen, but must be investigated in what way the shape of the specimen influence the data obtained. In this paper are presented the requirements that must be fulfilled by the samples used for tensile / compression biaxial tests and the design of cruciform specimens through FEA that meet these demands.

Abstract: When loaded over their yield strength, materials suffer significant plastic deformations. In such zones, the degree of plastic deformation depends on the characteristics of the material and also on the nature of the loading. As known, the degree of plastic deformation may be correlated with the value of hardness, determined in the plastically-deformed zone. The present paper discusses the three types of testings applied, which led to the fracture of some specimens and, consequently, to plastic deformations in the immediate vicinity of the fractured surfaces. The testings, performed on Compact Tension Specimens, aimed at determining the following characteristics developed by the Mechanics of fracture: the crack growth rate, da/dN, fracture toughness for materials with brittle behavior, K_Ic, and fracture toughness for the materials with a preponderantly ductile character, J_Ic. Along the surfaces fractured during these testings, plastic deformations occur, differentiated exactly by the load differences. Under such conditions, testings were made upon the 1C45 (1.053) steel, with the characteristics established by the manufacturer, as well as upon the same steel, yet subjected to an annealing thermal treatment, and the variations of Vickers hardness were recorded both along the fractured surfaces and from one specimen to another.

Abstract: In this paper we present the experimental results obtained after determining hardness on samples previously subjected to fatigue. Firstly, 6 identical samples have been subjected to stress fatigue in the elastic range a number of 10⁵, 5∙10⁵, 10⁶, 2∙10⁶, 3∙10⁶ and 4∙10⁶ cycles. For all samples we used the same form of stress cycle, respectively, sinusoidal, and the same values of maximum, minimum force and the amplitude of the cycle (50 kN, 30 kN and 10 kN). It is noted that the maximum value of the load was less than that at which samples fall within the plastic deformation. Therefore, the original loading of samples was performed in the elastic range. For each of the six samples determinations of levels of hardness were performed, with the value of indentation load of 10 kgf. Indentations were made on the samples, along the length of the calibration, at a distance of 10 mm one within the other. As it will be seen below, in the areas with the highest hardness were performed several indentations, in order to determine the highest hardness area, which, in our view, also presents the largest degree of plastic deformation.

Abstract: In order to determine the fracture toughness of the materials presenting high hardness values in the superficial layers, the Vickers micro-indentation was imposed as a reliable procedure. That method became attractive because of the relative simplicity of the experimental technique and because of its low cost. There are several calculus relationships that could be applied using the data provided by that method, in order to determine the material fracture toughness. The determination of fracture toughness using the Vickers indentation method is based on the analysis of radial cracks propagation, from the corners of the indentation trace. The length of these cracks is connected with the material fracture toughness, on the basis of some semi-empirical calculus relations that are taking into account the indentation load and some physical characteristics of the test material, as Young’s modulus and Poisson’s coefficient. In the present paper, fracture toughness was determined on a series of ceramic samples, made of the same material, but with different geometrical shapes and obtained by applying different technological procedures. The influence of some technological parameters on the fracture toughness was evaluated. The material fracture toughness was determined, into the vicinity of the propagated cracks (in a sample that could be a final product), on an area with a specified geometric contour. As a preliminary stage, a step by step FEM analysis was made, into the Vickers indentation material region, for different values of indentation load. In this manner, it was proved that the maximum stress value, on the perpendicular direction, as related to the crack diagonal plane, is always located at the peak of the indentation trace, and that is the effective start-point of cracking, for this type of indentation.