Universal Testing Machine (UTM)

A universal testing machine is used to test the tensile stress and compressive stress of materials. It is named after the fact that it can perform many standard tensile and compression tests on materials, components, and structures. It has the following units

1.        Loading unit:

           In this unit the specimen is clamped and the desired load is applied. It consist of the following components:

•         Table
•         Table cover
•         Column
•         Screw drive
•         Upper and lower cross head
•         Upper and lower jaws
•      Upper and lower clamping handles

   2.      Control unit:

             This unit controls the motion of the jaws and application of load. It consist of the following components:

·        Automatic control

·        Manual control (hydraulic pump control, jaws up & down motion)

·        Dial gauge (load range selector,zero adjustment, span for calibration)

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Yield and Ultimate strength

Definition of Yield strength:

            The yield strength or yield point of a material is defined as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied load is removed.

Importance:

Knowledge of yield point is vital when designing a component since it generally represents an upper limit of the load that can be applied.

Formula :

              Yield strength= yield load/area

Definition of Ultimate strength:

            Ultimate strength is the maximum stress that a material can withstand without failure or rapture. It is given by:

                               Ultimate strength = ultimate load/Area

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FIVE BASIC ELEMENTS OF MPI

SOURCE:-

Source produces the probing medium or investigating medium. In this MPI test the current is the source element.

MODIFICATION:

            The ability to change or vary the magnetic field if there is any crack or discontinuity in the specimen. Here in the MPI test the current is the modifying element.

DETECTION:   

  The material which detect that how much change occur in the magnetic field around the specimen. Here the iron magnetic particle is the detective element.

INDICATION:-

            The element which indicate the position of the crack in the specimen. The magnetic field pattern or signals is the indicating element.

INTERPRETATION:-

            The observer which sees the signals or pattern of magnetic field which give the results of the test is called interpreter.

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Magnetic Particle Testing Inspection

Magnetic particle testing is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles. The surface will produce magnetic poles and distort the magnetic field in such a way that the iron particles are attracted and concentrated making defects on the surface of the material visible.

How to perform the test:

First put the specimen into the A.C current carrying coil and then take the specimen into the beaker. And pour some kerosene oil into the beaker and put some iron powder in it. The magnetic field produce by A.C current around the specimen. If there is any crack present in the specimen then there will be greater concentration of iron particle s around the crack or defect present in the specimen. If there is no crack then the iron particles will arrange themselves throughout the specimen.

TYPES:-

            There are two types of magnetic particle inspection test.

•  CONTINUOUS:- wet method.
•   RESIDUAL:- dry method. 

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Non Destructive Test

Nondestructive testing (NDT) use test methods to examine an object, material or system without impairing its future usefulness. Non-destructive testing is often required to verify the quality of a product or a system. Various types of commonly performed NDT are

1.       Eddy Currents inspection

2.       Magnetic Particle Inspection (MPI or MT)

3.       Radiography

4.       Ultrasonics

5.       X-ray Technique

6.       Penetrant Test

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Evaporator ( In a refrigeration System)

Function of evaporator:

·         In a refrigeration system, an evaporator is a device which enables a volatile liquid to vaporize for the purpose of removing heat from a refrigerated space or product.

·         It is one of the main components in a refrigeration system.

·         A refrigerant before entering the evaporator is usually a mixture of liquid and vapor due to its expansion through the expansion valve, and enter the evaporator at low temperature and pressure.

·         The liquid refrigerant vaporizes inside the evaporator to absorb heat from the object to be cooled.

·         In many cases the vapor in a superheated state is sucked into the compressor to prevent the liquid slugging.

Main Types of Evaporator:

•            Bare tube coil evaporator
•            Plate surface evaporator
•            Finned tube type evaporator

 Bare tube coil evaporator:

·         Bare tube refers the tube whose inner and outer surfaces are both smooth.

·         Bare-tube evaporators are also called prime-surface evaporators.

·         Bare-tube coils are available in a number of sizes, shapes and designs

·         Spiral bare-tube coils are often employed for liquid chilling.

·         Large ceiling-hung bare-pipe coils employing natural convection air circulation are sometimes used in frozen storage rooms and in storage coolers where the circulation of large quantities of low velocity air is desirable.

·         They are also used as either “dry” or “spray coils,” in conjunction with centrifugal blowers to provide high-velocity chilled air for blast-cooling or freezing operations.

·         The bare-tube evaporator is generally operated dry-expansion.

Bare tube coil evaporator

 

 Plate surface evaporator:

·         There are several types of plate-surface evaporators.

·         Some are construction of two flat sheets as metal so embossed and welded together as to provide a path for refrigerant flow between them.

·         This type of plate-surface evaporators has the advantages of easy cleaning and low cost in manufacturing.

·         It can be readily formed into the various shapes required to serve as structural components, for example, the walls of a household refrigerator or reach-in cooler, when it is constructed in a boxlike shape.

·         The plate construction offers some secondary heat transfer surface, but is also useful for cooling flat packaged products that contact the surface.

Plate surface evaporator

 

 Finned tube type evaporator:

·         As the name suggested, finned tube evaporators have the fins installed on the bare tube to enhance the heat transfer from the air to the refrigerant in the tube.

·         The fins here act as a secondary heat exchange surface to improve the evaporator efficiency from increasing overall evaporator surface area.

·         Finned evaporators are used extensively in residential and commercial refrigeration and air conditioning applications.

·         In order for them to be effective, there must be good thermal contact between the evaporator fins and the tubing surface.

·         This can be accomplished by several ways.

·         One method is to solder the fin directly to the tubing.

·         Another method is to slip the fin over the tubing and expand the tubing by pressure or some such means so that the fin locks onto the tubing surface.

·         A variation of the latter method is to flare the fin hole slightly to allow the fin to slip over the tube.

·         After the fin is installed, the flare is straightened and the fin is securely locked to the tube.

Finned tube type evaporator

 

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