Mechinopedia For Mechanical Engineers

 Magnetic Crack Detection apparatus is designed to detect normally invisible cracks on or extending near to the surfaces of articles made of magnetic materials. The design of magnetic crack - detectors is based on the principle that, if a crack or flaw is present in a magnetic material through which a magnetic field is passing, the lines of force will be distorted near the fault owing to the local change of permeability associated with it. In the case of a piece without any flaw, lines of force will be uniform an straight. Cracks and flaws can also be detected by magnetizing a ferrous metallic part , then sprinkling very fine iron dust over it. The magnetic poles formed at the crack gather a line of dust and outline the crack. The special equipment for this magnetic testing for cracks is known as magnaflux. 

 These tests reveal discontinuities that are open to the surface and may use dyes or fluorescent materials. They are specially useful for non ferrous metals and non - metallic substances. The simple test involves dipping the component into kerosene, wiping it dry, and then coating it thinly with whiting. cracks will be revealed by a discolored 'line" appearing in the "whiting", this being produced when kerosene trapped in the cracks seeps out slowly. Dyes may be used instead of kerosene , but fluorescent materials are probably the most widely used.  In the fluorescent test, the sample is immersed for sometime in a hot bath of a strongly fluorescent compound such as anthracene, used as a penetrating agent. The solution enters the cracks, if any, and remains there. The metal is then dried and examined under a quartz tube mercury vapor lamp. Any penetrated solution will be detected by the fluorescence caused by the ultraviolet radiation of the light. 

 It relies upon transmission and reflection of ultrasonic beams or waves of frequencies between 100 kHz and 25MHz. The ultrasonic waves are usually produced by the the piezoelectric effect within the crystal probe which is placed on the surface of the specimen. Discontinuities below the surface cause reflection of the ultrasonic waves which appear as peaks upon the cathode - ray oscilloscope receiver. The size of the peak seen on the receiving tube is some indication of the size of the defect. The crystal probe thus becomes the receiver as well as transmitter. Ultrasonic techniques are useful for detecting cracks , voids, and defects far below the surface as well as near the surface. While radiation and ultrasonic tests are very sophisticated and yields excellent results under almost all circumstances, the older hammer test is still employed for the detection of internal defects. If a "sound" object , that is one free from large internal flaws, is struck sharply with a suitab

 These tests are based on absorption and dispersion of X-rays or Gamma Rays passing through the material. By means of  a luminescent screen or photographic plate , points of varying radiation intensity that occur at faults can be detected.  As radiation sources, X-rays produced by Betatron devices ( electron centrifuge ) or Gamma rays produces by radioactive decay processes are used. In these methods the radiation is passed through the metal being examined and is then allowed to impinge upon sensitive film. Thus, if an object has an internal defect , more radiation will be passed through the defective area than through the sound region and the defect will show up as dark area on the film. Unfortunately , scattering tends to obscure the defect, so that small defects may not be defected . In general , X rays are preferred for laboratory testing since they offer greater control over intensity and they produce sharper pictures.  X-ray equipment is generally large and bulky , whereas gamma

 This is always the first step in the inspection of an component and may be done with unaided eye or by using hand lens or a microscope. Although a seemingly elementary procedure, it should be carried out carefully and systematically on every component. 

NON - DESTRUCTIVE TESTING :  The Mechanical Tests carried out on metals as described before involve cutting or destroying of  the metal. But when a structure or metal part is not to be spoiled while testing, certain tests called " Non - destructive tests " are carried out to detect flaw and crack in the metal.  The methods of  Non - Destructive Testing are , * Visual Examination, * Radiographic Tests, * Ultrasonic Test, * Liquid Penetrant Tests  * Magnetic Particle Test. 

Destructive Test on metals is classified as ,  * Tensile Test * Compression Test  * Hardness Test  * Impact Test  * Fatigue Test * Creep and Stress - Rupture Test  

 As metals are exposed to temperatures within 40 percent of their absolute melting point, they begin to elongate continuously at low load; but at a temperature higher than this it becomes increasingly important. It is for this reason that the creep test is thought of a high - temperature test. A creep curve is a plot of the elongation of tensile specimen versus time, at a given temperature , under either a constant load or under a constant true stress.   Tests may run for a period of days to many years. A Typical creep curve shows four stages of elongation,  a) Instantaneous elongation following the application of the load.  b) Transient or primary creep, c) Secondary creep , d) Tertiary Creep. During the primary creep the rate of work hardening decreases because the recovery process or slow, but during secondary creep both rates are equal. In the fourth stage of creep, grain boundary cracks or necking may occur which reduces the cross - sectional area of the test specimen.  STRESS - R

 Although yield strength is suitable for criterion for designing components which are subjected to be static loads, for cyclic loading the behavior of a material must be evaluated under dynamic conditions. The fatigue strength or endurance limit of a material is therefore used in the design of parts subjected to repeated alternating stresses over an extended period of time. The fatigue test determines the stresses which a sample of a material of standard dimensions can safely endure for a given number of cycles. This is accomplished using a specimen having a round cross section, loaded at two points as a rotating simple beam, and supported as its ends. The top surface of such a specimen is always in compression and the bottom surface is always in tension. The maximum stress always occurs at surface , halfway along the length of the specimen , where the cross - section is minimum. For each complete rotation of the specimen, a point in the surface originally at the top center goes altern

 Metals sometimes fail when subjected to suddenly applied load or stress and in order to assess their capacity to stand sudden impacts , the impact test is employed. The impact test measures the energy necessary to fracture a standard notched bar by an impulse load and as such is an indication of the notch toughness of  the material under shock loading. Two major tests for determining impact toughness are Izod test and the Charpy test . Both of these methods use the same type of machine and both yield a quantitative value of the energy required to fracture. The two most common kinds of impact test use notched specimens loaded as beams. The beams may be simply loaded ( Charpy test ) or loaded as cantilevers (Izod test). The notch is usually a V-notch cut to specifications with a special milling cutter. The function of the notch is to ensure that the specimen will break as a result of  Impact load to which it is subjected. Without the notch, many alloys would simply bend without breaking

 At the present time micro hardness tests are widely used to determine the hardness of exceedingly thin layers, very small specimens and even separate structural components of alloys. Two types of tests , indentation and scratch, are made to determine micro hardness. The indentation test is based on combining a hardness tester, using a diamond pyramid indenter at low loads, with a metallurgical microscope. Loads from 1 to 200 g are applied. The microhardness number is determined from the formula, H = 1.8544 (P / d^2 ). in which P is the load in g, and d is the length of the diagonal in microns. Microhardness determination based on scratch test involves scratching the surface being tested by diamond under the action of a definite load. The width of the scratch is measured with a special microscope. The Hardness value is defined either as the width of the scratch obtained at a certain constant load or as the load at which the scratch of definite width is obtained. 

 The shore scleroscope consists of a small diamond pointed hammer of very small weight    ( about 2.4 g) which is allowed to fall freely from a height of 250 mm down a glass tube graduated into 140 equal parts . The height of the rebound is taken as the index of the hardness. This test is used for testing rolls and other machine components, and the instrument can be carried about in workshop. Shore value multiplied by 6 gives roughly the BHN on steels.  The scleroscope is very useful for testing the hardness of case - hardened surface which provides a very small hardness of the skin of the surface of the material. Micro hardness test by Vickers Test Method 

 The test is similar to that of the Brinell in that the hardness number is derived from the relationship between the applied load and the surface area of the indentation . The test consists in forcing a square based diamond pyramid ( with an angle 136* between opposite faces ) into the ground or even polished surface to be tested. The applied load is 5, 10, 30, 50, 100 or 120 kg . The Load is selected according to the thickness and hardness of the specimen.  The Vickers hardness number or diamond Pyramid Hardness (DPH), determined in such tests is the load per unit area of  the impression and is found from the formula : DPH = 1.8544 (P / d^2). where P = Load applied to the pyramid in kg.            d = diagonal of the impression in mm. As a rule , Vickers Hardness Number is determined from special tables in accordance with measured value of d ( length of the diagonal). Vickers and Brinell hardness numbers are expressed in same units and coincide for hardness up to about 400. At higher

The Rockwell Hardness Test is based on the indentation of the hard tip, or indenter, into the test piece under the action of two consecutively applied loads - minor (initial) and major (final). In order to eliminate zero error and possible surface effects due to roughness or scale, the initial or minor load is first applied and produces an initial indentation. A conical shaped diamond (called a brale) with 120* apex angle and 0.2 mm radius is used as the indenter or penetrator in the Rockwell test for hard materials. For softer materials, a hardened steel ball 1.5 mm in diameter is generally used. A number of different scales are used, each scale being suitable for certain classes of materials. It should be understood that each scale is entirely arbitary, the hardness number obtained having relevance to that particular scale only.  Rock well Hardness Scale , Scale - A   Diamond cone Indenter, Major load 60 kg , Dialed number - black , For materials - Cemented Carbides, thin steel , ca

  The Brinell Hardness Test is the most commonly adopted test for hardness of iron and steel. In this test, a Hardened ball is impressed on a flat polished surface of the sample under a load. The load is maintained and diameter of the impression made on the test piece is subsequently measured by means of microscope the order of  accuracy being +_ 0.1 mm. The Brinell Hardness number is obtained from the equation, BHN =  Load on Ball / area of indentation .          = 2P / 3.14* D (D-(D^2 - d^2)^(1/2) )  where BHN = Brinell Hardness Number,                  P = Load applied in kg,                  D = Diameter of ball in mm.                  d = Diameter of ball impression in mm.  The harder the metal, the higher its Brinell number will be. In practice, hardness number corresponding to a particular indentation diameter is read off from a table in which load, indenter size, indentation sizes, and hardness numbers are correlated. The Brinell test, irrespective of some limitations, does gi

 Hardness measurements for determining the properties of a machine component have found extensive application in the quality control of metals and metal products in all branches of industry due to the rapidity and simplicity of the tests and their non destructive character. The three most important forms of hardness test are  (1) Brinell Test , (2) Rockwell Test , (3) Diamond Pyramid .  (4) Shore Scleroscope . 

Compression test may be regarded as opposite to the tensile test in so far as the un axial load applied is compressive rather than tensile. The compression is rarely used as an acceptance test for structural materials. Since Brittle materials are unsuitable for the tension test these are usually used in compression to evaluate the strength properties of such materials. Brittle materials such as cast iron , concrete , mortar, brick, ceramics are commonly tested in compression. Specimens are usually plain right cylinders or prisms whose ends are made as nearly flat and parallel as possible to avoid eccentric loading. Occasionally specimens have enlarged ends to add to their lateral stability. A Universal Testing machine , fitted with compression plates, is usually used to apply the load, and the specimen should be measured in accordance with standard procedure before it is placed in the machine. After the specimen is properly placed and aligned, a small initial load is usually applied to

Testing is for the purpose of providing an engineer with the necessary data for his design calculations and determining whether a material, either in the raw or fabricated form., meets specifications. Its necessary that metals shall be tested so that their mechanical properties and especially their strength properties can be assessed and compared. Testing for mechanical properties will not always reveal the inherent unsoundness of a material. This is particularly true when a large latitude is provided for in the specimen. This necessitates both destructive and non- destructive testing. Destructive Testing uses various types of mechanical tests such as  Tensile,    Compression ,   Hardness ,    Impact ,   fatigue and   creep testing.    Non- destructive Testing ,on the other hand, includes    visual examination,    radiographic ,    ultrasonic ,   liquid penetrant and   magnetic particle testing.

A test almost universally employed to express mechanical properties and to supply the most useful fundamental information regarding the behavior of materials is the tensile test. Probably the testing machine most commonly used is called universal testing machine. The name "universal" is given in the sense that the machine may be adapted to carry out tension, compression ,direct shearing and bending test.

Universal testing Machine is used in This method . In Tensile test , the test piece is first prepared by turning the same piece to the standard shapes specified. This may be either round or flat.  Before commencing the test two gauge marks are made on the specimen longitudinally usually 50mm to 200mm apart according to the size of the test piece. The ends of the test piece are then gripped in the tensile testing machine and gradually increasing load is applied until failure is approached. The amount of elongation in the test piece caused by the load is measured accurately by a mechanical , electrical, or optical device called extensometer. As the loading of the test piece progresses, load and deformation readings are recorded simultaneously. The stress is calculated from the loads and the original dimension of the piece and this stress is plotted graphically with strain to show behavior of metal at different states.    The parameters which are used to describe the stress-strain curve