ANALYSIS OF MATERIAL ELASTICITY: STUDY OF SEVERAL MACROSCOPIC TO NANO CASES

• Main Author: Amalia, Nadya
• Format: Dissertations
• Language: Indonesia
• Subjects: Fisika
• Online Access: https://digilib.itb.ac.id/gdl/view/42127
• Institution: Institut Teknologi Bandung

An object or material has a rest shape that can undergo changes, whether in shape or size, due to loading by external forces or changes in temperature. Changes in shape or size are known as deformations. Basically, when deformation occurs, internal intermolecular forces in the material appear to counteract the load received. If the applied load is not too large, the force is able to completely resist the applied load and allow the material to reach a new equilibrium state and return to its rest shape when the load is removed, known as elastic deformation. Larger load may cause permanent deformation of the material, known as plastic deformation, or even structural failure. Elasticity as the material's ability to achieve new equilibrium and return to its original state when loading is removed is very important to understand. This is related to the importance of the design stage to avoid failure, for example in civil, mechanical, aerospace, and materials engineerings, as well as various other applications related to the mechanical behavior of material. Different materials can exhibit different elastic behavior and almost all engineering planning requires knowledge of the material elasticity. The resistance of material to elastic deformation is measured by the elastic modulus. The smaller the deformation due to loading, the more rigid the material, the greater the elastic modulus. As technology develops, there have been various methods used to determine the elastic modulus, such as tensile testing, compression testing, micro/nanoindentation, ultrasonic, resonance, and so on. However, each method has its own limitations, including tests that have to directly contact or even damage the sample, samples that must have a certain size and shape, the need for sophisticated equipment, and generally lacking accuracy. Therefore, until now an alternative method for determining the elastic modulus continues to be developed. The equilibrium relationship in beam theory, for material with cantilever structure, explains that changes in beam curvature depend on the dimensions and elasticity. Cantilever itself is a structure with one end position fixed and the other end free. Elastic modulus can be obtained from this relationship. For a slender beam, the flexibility is several orders greater than the extensibility, so axial stretching can be ignored. In addition, the contribution of shear force to beam bending is generally two times less than the contribution of the bending moment. Therefore, the effect of shear forces can also be ignored. An alternative non-destructive and non-contact method for estimating elastic modulus of material at a macroscopic (continuum) level through image processing of materials with cantilevered structures, which are subjected to bending due to their own weight, is one of the subjects and developed in this dissertation. The main issue in the measurement method is accuracy. Then, how is the accuracy of this method? To check whether this method is able to determine the elastic modulus of material accurately, a number of samples were tested. The estimation results were then compared with the elastic moduli of the samples measured directly using a tensile test device and the developed method was able to estimate the elastic moduli of sample materials with an error to the direct measurement results of less than 16%. This shows that the alternative method developed can considerably estimate the elastic modulus. Furthermore, because the bending shape of sample can be monitored continuously, the modulus of elasticity for long-term temperature increment can be determined. Amorphous polymer samples showed a sudden change in the bending shapes on certain temperature ranges, which were then identified as the glass transition temperatures. A model was proposed to explain the elastic moduli around the critical temperatures and the glass transition temperatures of the amorphous polymer samples have been successfully estimated. Interestingly, the glass transition temperature estimation results showed conformity with data obtained from direct measurements using the differential thermal analysis (DTA) device. The error was less than 6.7%. Since the elastic modulus changes abruptly around the glass transition temperature, the proposed method may be more efficient than measurement using DTA which is shown by a weak peak on the curve of the analysis. This method is the first to estimate the glass transition temperature of an amorphous polymer based on bending of the cantilever structure as well as the simplest and most potential for the development of new tools to determine the glass transition temperature of a material. Then, this dissertation also investigate a case in civil engineering that is still related to elastic deformation, namely high pile rebound (HPR) during the process of pile driving in fine-grained or cohesive (clay) soils. Excessive hammer blow on teh pile to overcome HPR produces high tensile and compression stresses that can damage the pile. This of course can result in loss of time, money, and energy. Based on case studies from a number of scientific reports it is known that HPR is a response caused by pore water incompressibility. Meanwhile, the characteristics of a soil affect the movement of pore water. In clay soils, water molecules are held in soil pores by physical and chemical/electrical forces due to their dipole-like behavior. In addition, surface-related forces are inversely related to particle size where small particles have a greater specific surface area than large particles. In addition to affecting surface area, smaller particles arrange themselves such that the soil's pores are smaller. Therefore, pile surface engineering by coating the pile with a material that likes water (hydrophilic) is hypothetically able to attract pore water molecules to the surface of the pile so that the incompressibility of pore water in clay can be anticipated. In this dissertation, titanium dioxide (TiO2) as a superhydrophilic material was used to engineer the surface properties of the pile. The pile driving test at the laboratory scale on the pile model showed that the coating was able to make the pile reach the same depth as the pile without coating with less blow counts. As for the pile driving test in granular soil, there was no difference between the pile with coating and the pile without coating. This is understandable because based on the characteristics of granular soils the pore water can diffuse much faster than in clay soils. These results indicate that pile coating with TiO2 can be considered as an alternative HPR solution and further study is needed. An empirical model was also derived and parameters distinguishing the engineering properties of clay and granular soil test were obtained. For macroscopic structures, modulus of elasticity is considered as an inherent material property which is independent of size. However, at the nanoscale, researchers have observed that the elastic behavior of materials cannot be explained using the concept of continuum mechanics and the constant value of elastic modulus. In fact, it not only differs from the bulk value, but also depends on the size in many cases. Two dominant physical mechanisms that cause dependence on elasticity behavior on size at the nanoscale are surface energy effects and non-local interactions. The second mechanism arises because of the discrete material structure and the fading interatomic force fluctuations in the phenomenological elastic modulus. If the surface energy effect has been discussed in depth in much of the literature, the fundamental explanation for the effects of non-locality based on the formalism of quantum mechanics is still meager. The third discussion in this dissertation is the study of the alleged dependence of elastic modulus on size based on its correlation with the distance between atoms, which also depends on the size due to interatomic force fluctuations, based on the formalism of quantum mechanics. A model was developed to explain this and the results of the comparison with experimental data from a number of scientific publications showed conformity.