Structural and Vibration Analysis of a Machine Shaft using Finite Element Analysis

Copyright © 2019 by author(s) and International Journal of Trend in Scientific Research and Development Journal. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0) (http://creativecommons.org/licenses/ by/4.0) ABSTRACT The present study is a simulation of a machine shaft. The study will be done on FEM simulation software called Ansys 14.5, where a modal would be developed which will undergo a process of meshing. Meshing will divide the modal in extremely small units without changing the shape of actual geometry which will help the software to study the change at every small unit of the model. Then the of the modal would be defined in terms of inlet and outlet thereafter the boundary condition and design equation would be applied to get the desired result.


INTRODUCTION
Human consumption of the Earth's resources increases the need for a sustainable development as an important ecological, social, and economic theme. Reengineering of machine tools, in terms of design and failure analysis, is defined as steps performed on an obsolete machine to return it to a new machine with the warranty that matches the customer requirement. To understand the future fatigue behaviour of the used machine components, it is important to investigate the possible causes of machine parts failure through design, surface, and material inspections. Failure analysis is an indispensable tool that is used widely by industry sector to develop or improve the product design. The failures of machine elements are studied extensively by scientists to find methods in order to identify their causes and to prevent them from reoccurring. To determine the failure modes, analytical, experimental, and finite element analysis methods can be used.
A structure as a whole, and each individual member, must be designed with reference to the three 'Ss': strength, stiffness and stability: Strength is the ability to carry the applied loads without yielding or breaking. Examples of strength failures are a cable which snaps, a vehicle body which' crumples, and a glass panel being smashed.
Stiffness is the ability to carry the applied loads without too much distortion. A material can only sustain stress at the expense of some strain ( ). Sometimes the strain, even though very small, may be the limiting factor. For example, a machine tool must be stiff enough to produce the required accuracy in machining, and a camera tripod must be stiff enough to prevent camera shake.
Stability is the ability to carry compressive loads without collapsing or buckling out of shape. For example, a metal rod in compression longitudinally will suddenly bow out of shape under a compressive stress which is well below the compressive yield stress.

Fatigue Failure
Fatigue is caused by cyclical stresses, and the forces that cause fatigue failures are substantially less than those that would cause plastic deformation. Confusing the situation even further is the fact that corrosion will reduce the fatigue strength of a material. The amount of reduction is dependent on both the severity of the corrosion and the number of stress cycles.
Once they are visible to the naked eye, cracks always grow perpendicular to the plane of maximum stress. Figure 1.2 shows the fracture planes caused by four common fatigue forces. Because the section properties will change as the crack grows, it's crucial for the analyst to look carefully at the point where the failure starts to determine the direction of the forces. For example, while it is common for torsional fatigue forces to initiate a failure, the majority of the crack propagation could be in tension. That's because the shaft has been weakened and the torsional resonant frequency has changed.
Figure2. shows the fracture planes caused by four common fatigue force One of the more common causes of shaft failure is due to fatigue. Metal fatigue is caused by repeated cycling of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigues cracks initiate and propagate in regions where the strain is most sever. The concept of fatigue is very simple when a motion is repeated the object that is doing thework becomes weak. associated with cyclic loading. These limits are expressed by an S-N diagram, as shown in Fig. 1.4. For steel, these plots become horizontal after a certain number of cycles. In this case, a failure will not occur as long as the stressis below 27 klbf/in. No matter how many cycles are applied. However, at 10 cycles, the shaft will fail if the load is increased to 40bf/in. The horizontal line is known as the fatigue or endurance limit. For the types of steels commonly used for motors, good design practice dictates staying well below the limit. Problems arise when the applied load exceeds its limits or there is damage to the shaft that causes a stress raiser. Cracks initially propagate along the slip bands at around 45 degrees to the principal stress direction this is known as Stage I fatigue crack growth. When the cracks reach a length comparable to the materials grain size, they change direction and propagate perpendicular to the principal stress. This is known as Stage II fatigue crack growth.

Theories of Failure
Unfortunately, there is no universal theory of failure. Instead, over the years several hypotheses have been formulated and tested, leading to today's accepted practices. Being accepted, we will characterize these "practices" as theories as most designers do. Structural metal behaviour is typically classified as being ductile or brittle. Theories have been developed for the static failure of metals based upon the two classes of material failure: Ductile metals yield Brittle metals fracture The various theories are as follows: maximum principal stress theory also known as rankine's theory maximum shear stress theory or guest and tresca's theory maximum principal strain theory also known as st. venant's theory total strain energy theory or haigh's theory maximum distortion energy theory or vonmises and hencky's theory

Meshing
Meshing is a critical operation in FEM in this process the CAD geometry is divided into numbers of small pieces.

Conclusions
From the above results it can be concluded that the FEM analysis of shaft, vibration analysis and fatigue life analysis have been done.This shows the better results as compared to previous work. In this study the maximum principal stress, deformation, maximum von misses stress, fatigue life and corresponding to their stress act on the shaft and vibration analysis have been done. All these things are described in the previous chapter. According to the result stability frequency of design shaft is 0 Hz up to the 25000 Hz Damped frequency. The maximum alternative stress occurs on the shat after applying 10 Lac cycles of shaft is approximate 86 Mpa.

Scope of future work
Different material can also be trying for better performance and surface refinement can be done for better and accurate results. The results provided in this work can be experimentally verified.