Lec 1-converted.pptx

Mechanics of Materials Lab Fall 2018

Lecture 1 (Material Testing Lab) By: Engr. Hilal Yousafzai [email protected]

Department of Mechanical Engineering, IIU Islamabad

Lab outline

1. Study of Material testing Lab. 2. Study of universal testing m/c 3. Tensile test on UTM for a mild steel specimen. 4. Compression test on cement mortar cube 5. Shear test on mild steel specimen 6. Torsion test on ductile steel, Cast iron & brass.

Book Recommended: 1. Mechanics of Materials by F.P Beer & ER Johnston 2. Mechanics of Engg Materials by FV Warnock P.P Benham & R.J Crawford.

Marks Distribution:

Assignments + Project + Quiz

(25%)

Attendence

(10%)

Viva Exam

(15%)

Final Exam

(50%)

Total

100%

Lets begin…..

mechanics of materials?

Strength of materials, also called mechanics of materials, deals with the behavior of solid objects subject to stresses and strains. In mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation.

Materials Testing • Mechanical properties are obtained by mechanical

testing. • Mechanical testing is used for developing design data, maintaining quality control and providing data in failure analysis.

cont,.

 Mechanical testing is usually destructive and requires test specimens of the material to be machined or cut to the specific shape required by the test method.

Mechanical Properties ∗ Describe material when a force is applied to it. ∗ Determined through testing, usually involving destruction of material. ∗ Extremely important to consider in design.

Types of mechanical properties a) b) c) d) e) f) g)

Ductility Toughness Brittleness Hardness Plasticity Elasticity Strength

Ductility

∗ Ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire.

Tensile test of an AlMgSi alloy The local necking and the cup and cone fracture surfaces are typical for ductile metals.

Schematic appearance of round metal bars after tensile testing. a) Brittle fracture b) Ductile fracture c) Completely ductile fracture

Toughness • Toughness is the ability of a material to absorb energy

and plastically deform without fracturing • Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing. • It is also defined as the resistance to fracture of a material when stressed. • Toughness requires a balance of strength and ductility.

Brittleness

∗ A material is brittle if, when subjected to stress, it breaks without significant deformation (strain). ∗ Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ∗ Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses (which do not deform plastically) and some polymers, such as PMMA and polystyrene. ∗ Many steels become brittle at low temperatures (see ductile-brittle transition temperature), depending on their composition and processing.

Brittle fracture in cast iron tensile test pieces

Graph comparing stress-strain curves for ductile and brittle materials.

Hardness

∗ Hardness is the degree of resistance to indentation, penetration, abrasion and wear. ∗ Indentation hardness measures the resistance of a sample to permanent plastic deformation due to a constant compression load from a sharp object; they are primarily used in engineering and metallurgy fields. ∗ Common indentation hardness scales are Rockwell, Vickers, Shore, and Brinell.

Plasticity

 Plasticity is the tendency of a material to undergo permanent deformation underload.

Elasticity

 Elasticity (or stretchiness) is the physical property of a material that returns to its original shape after the stress (e.g. external forces) that made it deform. The relative amount of deformation is called the strain.

Strength

 Strength is the ability of a material to withstand various loads to which it is subjected during a test or service.

Materials Testing

 2 types of materials testing: a) Destructive test -results in the part being destroyed during the quality control testingprogram. b) Non destructive test - is done in such a manner that the usefulness of the product or part is not damaged or destroyed.

Non Destructive Test

 Non destructive inspection techniques enable inspectors to check properties critical to the safe performance of metal parts without causing damage to the parts themselves.  This test is concerned with testing for cracks and flaws

Types of non destructive test

a) b) c) d)

Dye penentrant test Radiographic test Magnetic particle test Ultrasonic test

Dye Penentrant Test

 It is easy to use and economical. 1) The specimen is coated with a red liquid dye which soaks into the surface crack or flaw. 2) The liquid is then washed off and the part dried. 3) Flaws and cracks show up red against the white background of the developer.

Cracks in aweld

Radiographic test

 This test involves passing gamma rays (X-rays) through a part to detect flaws in the metal.  The developed film has an image of the internal structure of thepart.  A defect will show up on the film as a dark area.  X-rays are very sensitive and are capable of inspection any thickness of almost any kind of materials.

Magnetic particle test

 This test is used to detect flaws on or near the surface of iron-based metals. 1) The part is first magnetized. 2) It is then either dusted with fine iron powder or coated with a solution of iron particles. 3) Flaws in the workpiece cause the lines of magnetic force to become distorted and break through the surface. 4) There they attract concentrations of the iron particles, which reveal defects in the metal.

Concentration of the iron particle shows defect in themetal

Ultrasonic test

 This test use ultasonic sound waves to detect cracks and flaws in almost any material that can conduct sound.  Sound waves can also be used to measure the thickness from one side of the material.

Cont.

 The human ear can hear sound waves with frequencies ranging from 20 to 20,000 Hertz.  Sound waves that vibrate with a frequency greater than 20,000 Hz are inaudible and are called ultrasonic.

Cont.

 These high frequency sound waves are produced by a piezoelectric transducer.  The transducer is electrically pulsed and then vibrates at its own natural frequency. 1) In order to operate, the transducer must be joined to the piece being tested by a liquid coupling such as a film of oil, glycerin or water.

Cont.

2) The high frequency sound waves are transmitted through the material. 3) The flaws reflect the sound waves and are detected on an oscilloscope. There is no size limitation on work that can be tested by ultrasonic test.

Ultrasonic Test

DESTRUCTIVETEST

Destructive Testing

• Destructive testing is a costly and time consuming technique. A specimen is selected at random from a large number of pieces. • Two types of destructive test: a) Hardness test b) Impact test

Hardness Test

• Indentation hardness measures the resistance of a sample to permanent plastic deformation due to a constant compression load from a sharp object. • types of hardness test: a) Rockwell b) Brinell

Rockwell Hardness Test

 The Rockwell hardness tester functions according to the depth of penetration made in metal by a specific kind of penetrator point forced by a given load.

Rockwell Hardness Tester

Cont.

 The Rockwell hardness test uses two loads that are applied sequentially.  A minor load of 10 kg is applied that helps seat the indenter and remove the effect of surface irregularities.  A major load which varies from 60 kg to 150 kg is then applied.  The difference in depth of indentation between the major and minor loads provides the Rockwell hardness number.  This number is taken directly from the dial on the machine.

Brinell Hardness Tester

 Brinell hardness test use a machine to press a 10 mm diameter, hardened steel ball into the surface of the test specimen.  This machine applies a load for a specific period of time and causes an indentation that is used to calculate hardness.

Cont.

Cont.

 The load applied to the steel ball depends on the type of metal undertest.  The load is usually applied for 10 to 15seconds.  The diameter of the indentation is measured to ±0.5 mm using a low magnification portable microscope.