Question 2. Materials Testing. Introduction. Special test have been developed to determine: Various strenghts Ductility Hardness Toughness Elasticity Fatigue Creep. Introduction Cont.

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Question 2 Materials Testing Mr. Breslin/Mr Harvey

Introduction Special test have been developed to determine: • Various strenghts • Ductility • Hardness • Toughness • Elasticity • Fatigue • Creep. Mr. Breslin/Mr Harvey

Introduction Cont. • These tests are used by engineers to find out how materials are going to perform when they are put into use. • Slight variations in some of the test for the testing of polymers. Mr. Breslin/Mr Harvey

Types of Testing 2 Types of Testing. • Destructive. • Non-destructive testing. Mr. Breslin/Mr Harvey

Destructive Testing • Tensile testing. • Hardness testing. • Birnell, Rockwell and Vickers. • Impact testing. • Izod and Charpy. • Fatigue testing. Mr. Breslin/Mr Harvey

Non-Destructive Testing(N.D.T.) • Macroscopic and Microscopic. • Liquid Dye Penetrate Testing • Eddy currency testing. • Ultra sonic testing. • X-ray testing. • Magnetic particle testing. Mr. Breslin/Mr Harvey

Tensile Testing • Tensile testing is about stretching a piece of material and seeing how it behaves. • Used to determine ductility. • Material is placed in a extensometer. • Materials being tested must have a specified cross-sectional area, two dot punches a specified distance apart. Mr. Breslin/Mr Harvey

Information Obtained from Tensile Test • Youngs Modulus; • proof or yield stress; • tensile strength; • percentage elongation; • percentage reduction in area; • properties such as ductility and shear strength can be measured. Mr. Breslin/Mr Harvey

Tensile Test Cont. • After the test has been completed the distance between the dots will be measured. • The material will try to return to its original length, this is known as elasticity. • The tests will continue with increasing loads which are graphed. Mr. Breslin/Mr Harvey

Graph of Tensile Test • The extension is proportional to the load. • Stages in graph. • Elasticity extension. (limit of proportionality). the materials does not return to its original limit. • Plastic extension. Large extension of specimen. Ends at yield point. • Necking range. Specimen breaks (cup and cone fracture). Mr. Breslin/Mr Harvey

The initial straight line (0P)of the curve characterizes proportional relationship between the stress and the deformation (strain). • The stress value at the point P is called the limit of proportionality - Hook’s Law • Where E is a constant, known as Young’s Modulus or Modulus of Elasticity • The value of Young’s Modulus is determined by the nature of the material and is nearly insensitive to the heat treatment and composition. Mr. Breslin/Mr Harvey

The value of Young’s Modulus is determined mainly by the nature of the material and is nearly insensitive to the heat treatment and composition. • Modulus of elasticity determines stiffness - resistance of a body to elastic deformation caused by an applied force. Mr. Breslin/Mr Harvey

The line 0E in the Stress-Strain curve indicates the range of elastic deformation – removal of the load at any point of this part of the curve results in return of the specimen length to its original value. • The elastic behavior is characterized by the elasticity limit (stress value at the point E): • A point where the stress causes sudden deformation without any increase in the force is called yield limit (yield stress, yield strength): Mr. Breslin/Mr Harvey

Tensile Strength • As the load increases, the specimen continues to undergo plastic deformation and at a certain stress value its cross-section decreases due to “necking” (point S in the Stress-Strain Diagram). • At this point the stress reaches the maximum value, which is called ultimate tensile strength (tensile strength): Mr. Breslin/Mr Harvey

Continuation of the deformation results in breaking the specimen - the point B in the diagram. • The actual Stress-Strain curve is obtained by taking into account the true specimen cross-section instead of the original value. • Other important characteristic of metals is ductility - ability of a material to deform under tension without rupture. Mr. Breslin/Mr Harvey

PROOF STRESS • Hard steels and non-ferrous metals do not have defined yield limit, therefore a stress, corresponding to a definite deformation (0.1% or 0.2%) is commonly used instead of yield limit. • This stress is called proof stress or offset yield limit (offset yield strength): • The method of obtaining the proof stress is shown in the NEXT SLIDE Mr. Breslin/Mr Harvey

Ductile Fracture • Ductile materials behave elastically when a gradually increasing force is applied up to a certain point. • Beyond this point plastic deformation occurs. • As the force is increased, necking occurs which reduces the components cross sectional area until the component fractures. • The fracturing occurs in a cup and cone formation. Mr. Breslin/Mr Harvey

Brittle Fracture • In brittle fracture failure occurs before any significant plastic deformation has occurred. • The surface of the fractured material appears bright and granular. Mr. Breslin/Mr Harvey

Force Extension Graph • High ductility. Bottom line C on graph. Example: Soft copper. • Good ductility. Middle line B on graph. Example: Aluminium. • Low ductility. Top line A on the graph. Example: Softened brass. Mr. Breslin/Mr Harvey

FRACTURES A.) = BRITTLE FRACTURE B.) = DUCTILE FRACTURE C.) = COMPLETELY DUCTILE FRACTURE Mr. Breslin/Mr Harvey

Elastic Limit • The point up to which the extension is proportional to the load applied and beyond which a material stays stretched and therefore an increase in load produces a larger extension in the material. • This signifies the end of the elastic range for the material being tested. Mr. Breslin/Mr Harvey

Example • A tensile test on a specimen material gave the following results Stress (N/mm2) 44 110 220 264 300 330 340 352 Strain (x 1000) 0.50 1.25 2.50 3.00 3.75 5.00 5.75 7.50 Plot the stress-strain graph and determine: Mr. Breslin/Mr Harvey

EXAMPLE QUESTION Q.) The following data were obtained from a tensile test on a specimen of an aluminium alloy. Using the graph paper supplied, plot the stress-strain diagram and then determine: (i) The 0.1% proof stress; (ii) Young’s Modulus of Elasticity for the specimen. Mr. Breslin/Mr Harvey

Plot the Stress-Strain Graph. • From the graph, the 0.1% proof stress is 325N/mm2 • Youngs Modulus: = Stress = 125 = 90kn/mm2 Strain 4 Mr. Breslin/Mr Harvey

EXAMPLE QUESTION Q.) The following data was obtained from a tensile test on a specimen of 10mm diameter and gauge length 60mm Using the graph paper supplied, plot the load-extension diagram and determine: (i) The tensile strength; (ii) Young’s Modulus for the specimen. Mr. Breslin/Mr Harvey

Stress • It is the amount of load carried by a unit area. • STRESS = load of force sectional area Mr. Breslin/Mr Harvey

Strain • Formula = Extension Original length. Mr. Breslin/Mr Harvey

Young's Modulus • Formula = stress strain kN/ mm² For example shown earlier Mr. Breslin/Mr Harvey

Tensile Strength • It is the maximum force applied to the specimen before it breaks divided by its cross sectional area. • Tensile Strength = MAX LOAD CSA Mr. Breslin/Mr Harvey

(i) Tensile strength Mx Load = 142KN = 142KN = 1.808KN/MM2 CSA ΠR X R 78.55 (ii) Youngs Modulus • Select point on elastic region of diagram eg. (32,0.4) • Diameter = 10mm, Gauge length = 60mm. Mr. Breslin/Mr Harvey

Factor of Safety • If a particular part is to withstand a load of 600kN, the engineer might allow for a force of 1800kN. Factor of safety = 3. • The part may have to withstand shock loads such as wind. Mr. Breslin/Mr Harvey

Hardness of Materials • Hardness is materials to indention (scratching) on the surface. • Hardness in some materials can be changed with heat treatments. Mr. Breslin/Mr Harvey

Vickers Hardness Test • Indenter: square pyramid angle 136° • Width of hole is measured. • More accurate than brinell. • Can be used on hard materials. Mr. Breslin/Mr Harvey

Brinell Hardness Test • Indenter: Hardened steel ball. Diameter 1 to 10mm. • Width of hole is measured. • Cannot be used on very thin material. • Ball may deform with very hard material. Mr. Breslin/Mr Harvey

Rockwell Hardness Test • Indenter: Ball and cone. • Depth of hole is measured. • Ball used on soft material. • Diamond cone used on hard material. Mr. Breslin/Mr Harvey