Monday 7 July 2014

Mufflers


Silencer is also termed as muffler. It is a device used for reducing the noise emitted by exhaust of an internal combustion engine, which is a major source of noise pollution. The IC engines are used for various purposes such as, in power plants, automobiles, locomotives, and in various manufacturing machinery. The main drawback of these engines are the sound produced by engines. It becomes a more vital concern when used in residential areas or areas where noise creates hazard. Generally, noise level of more than 80 dB is injurious for human being. The main sources of noise in an engine are divided in two parts first is the exhaust noise and second is the noise produced due to friction of various parts of the engine. The engine exhaust noise is the most dominant.
Generally are of two types:
 Absorptive Muffler

Reactive Muffler


The mixed (absorptive and reactive) muffler is shown:

Mixed Muffler

Thursday 10 October 2013

Fracture Mechanics

Introduction

Fracture mechanics deals with the measurement and analysis of crack. Fracture toughness is used as a term for measuring the resistance of a material to extension of a crack. Fracture toughness play an important role in application of fracture mechanics. This is used in the life prediction for a structure that contains the crack, damage tolerance design, oil and gas pipe line industries, and residual strength analysis for different engineering components and structures. The fracture toughness values may also use as in material properties against crack propagation, performance evaluation, and quality assurance for typical engineering structures, pressure vessels and petrochemical vessels and tanks and automotive, ship and aircraft structures and also in the railways. Therefore, fracture toughness testing and evaluation has been a very important subject in development of fracture mechanics method and its engineering applications. The terms K, J and CTOD are used as the evaluation for fracture toughness of materials.

Brief History of Fracture Mechanics

            According to Anderson, 2005 the causes for failure of structure is the ignorance during the design, manufacturing and the new design methods and new materials for replacement for old materials. The application of new design method and materials cause the catastrophic failure because the designer ignores some problems that are with them. So, testing and evaluation is become more important for new materials and design. Some of the historical failure which needs the fracture mechanics is given.
During the Second World War in 1943, the liberty ships are made which are cheaper and less time consuming because they uses welding in ships instead of welding. Nearly about 2700 ships are made and one half were damaged. The main causes of failure are the lack of toughness in steels, improper welds which contains cracks and the local stress concentration on the deck. After this some researchers at Naval Research Laboratory studied the fracture in detail and the field of fracture mechanics was born.
In 1983 polyethylene is used for gas pipe line in U.S. There are for maintenance the pinch clamping process was used. After the 6 years the flaw will grow that is generated from this process and gas will leak from this spread near the residential area and there were severe damage of houses were take place.
In 2003 the failure of the Space Shuttle Columbia was takes place. The reason was that foam insulation from external tank was strike with the left wing and the temperature was reached about 1450 oC.
There are many incidents which are in the form of catastrophic failure and many accidents were prevented from the use of fracture mechanics in design.
According to Zhu and Joyce, 2012 a long history of more than 50 years in the development of fracture mechanics theory. Different fracture toughness parameters have been proposed, and various fracture toughness test methods and experimental technologies have been developed and revised incrementally over the years. In the 1950s and 1960s, fracture mechanics focused on linear elastic materials testing using G and K. In the 1970s and 1980s, it was concentrated on the J-integral and on the CTOD testing for Elastic–plastic materials. In the 1990s, the ductile to brittle transition of ferritic steels and In the 2000s, the attention was on the crack tip opening angle (CTOA) testing for stable crack extension of thin-walled materials in low-constraint conditions.

Fracture Toughness Testing

A fracture toughness test measures the resistance of a material to crack extension. Such a test may yield either a single value of fracture toughness or a resistance curve, where a toughness parameter such as K, J, or CTOD is plotted against crack extension.
Linear elastic fracture mechanics (LEFM) is used for linear elastic materials. The KIC based testing is comes under LEFM. But it is applicable to materials which having lower toughness values and also because the materials have a sufficient amount of plasticity near the crack tip. So, an alternative fracture mechanics model is Elastic-plastic fracture mechanics (EPFM) is used to materials which having high fracture toughness and behaves like non-linear (i.e., plastic deformation). Crack-tip-opening displacement (CTOD) and the J contour integral that is associated with EPFM. Both parameters describe crack-tip conditions for elastic-plastic materials. Critical values of CTOD or J give nearly size-independent measures of fracture toughness, even for relatively large amounts of crack-tip plasticity.