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Materials Testing

Leaving cert Engineering HL Q2

QuestionAnswer
Explain the term metal fatigue. Metal fatigue is failure due to on/off loading or cyclic stressing. Fatigue failure begins as a minute crack which grows under the action of fluctuating stress.
Describe the general details of an impact test Test piece is notched and held in the vice of the machine. A pendulum strikes the test piece. Some energy is absorbed in breaking the piece. The numerical value for the toughness of the material is given by how far the pendulum swings after the test piece
What is the difference between Izod and Charpy Izod = test piece held vertical, 167 Joules of force, Notch on same side that the pendulum strikes it. Charpy = Test piece held horizontal, 300 Joules of force, Notch on opposite side that the pendulum strikes
For a mild steel test specimen used in a tensile test, what changes does the test piece undergo during the test in the elastic region of the graph During the elastic extension stage, the specimen extends proportionately with applied load and will return to the original state if the load is removed.
For a mild steel test specimen used in a tensile test, what changes does the test piece undergo during the test in the plastic region of the graph The plastic stage gives an increase in specimen length with a relatively small increase in load. It will not return to its original length. After the point of ultimate tensile strength it will begin to neck and then fracture with a "cup and cone fracture"
Name two non-destructive tests to detect surface flaws Visual inspection Liquid penetrant test
Name two non-destructive tests to detect internal flaws Ultrasonic test X-ray test
Describe the operation of a tensile testing machine The test piece can be round or rectangular section. It is clamped in the tensiometer at both ends. It is stretched by the tensiometer with huge force until it breaks. The results are plotted on a graph.
Describe the general operation of a hardness testing machine The test piece is put on the machine table, an indenter is forced onto the surface of the test piece with an exact unit of force, the indenter is removed and the size of the indentation left is used to calculate a value for the hardness of the test piece
Describe the brinell hardness test A spherical or ball indenter is used. The force of the machine divided by the diameter of the indentation is used to calculated a value for hardness. The Brinell test is used for softer materials as the ball indenter can deform under large loads.
Describe the Vickers hardness test A square based pyramid indenter made from diamond is used. The diagonals of the indentation left is used to calculate the area of the indentation.Hardness is calculated by dividing by the area of indentation. The Vickers test is used for harder material.
Describe the Rockwell hardness test A rounded cone indenter made from diamond is used. The depth of the indentation made is used to calculate hardness. Used for harder materials as the diamond will not deform
Describe the principles of x-ray testing Radiation emitted from an x-ray tube passed through the wp & onto a film turning it black. Thicker test pieces absorb more radiation leaving a lighter shade on the film. Defects in the wp won't absorb as much radiation & show up as dark spots on the film.
Describe how copper, mild steel and brass test pieces would behave during a tensile test Copper = Very ductile. Large elongation under a small load Brass = Strong but brittle. Very little enlogation Mild steel = Very strong. not excessivly brittle or ductile.
For a stress strain graph, how do you find Young's modulus of elasticity Take the second pair of figures from the data table provided. Use these figures to calculate stress/strain.
For a stress strain graph, how do you find 0.1% proof stress Find 1.0 for strain at the bottom of the graph. Draw a line from this point parallel to the straight part of the graph. Find the intersection point of the new line with the graph, from here draw a horizontal line to the left to find a value for Stress
For a load extension graph, how do you calculate Stress Take the second pair of figures from the data table provided. Stress= load/CSA (cross sectional area) if it is a round test piece, CSA is found by using the formula for the area of a circle, pi multiplied by radius squared. stress is found in the table.
For a load extension graph, how do you calculate Strain Take the second pair of figures from the data table provided. Strain= extension/original length . Extension is found in the table, original length is the gauge length of the test piece given in the question.
For a load extension graph, how do you calculate Young's modulus First calculate stress (load/csa) and strain (extension/original length). Then calculate Young's modulus= stress/strain using the answers found earlier
What property do the Izod and Charpy tests give a value for Impact toughness. How well a material will be able to absorb an impact and not fail for example, a steel crash barier
What property do the brinell, vickers and rockwell tests give a value for Hardness. How well a material will be able to resist indentation of wearing for example a gear wheel in a car gearbox
What property do the tensile test give a value for Tensile strength of pulling strength for example the steel cables in the Golden gate bridge.
Describe the basic principles of Eddy current testing A coil with high frequency A/C is placed on the test piece producing eddy currents in it. A magnetic field is produced in the test specimen by the currents. A defect will distort this magnetic field and will be shown on a screen by a search coil.
What material is Eddy current suited for Non- ferrous metal materials
Name two non-destructive tests to detect flaws just below the surface Magnetic particles test Eddy current test
Outline three advantages of using non-destructive tests (NDT) in industrial engineering. - that parts can be tested for quality - that expensive components need not be destroyed during testing - tests can be carried out for flaws at surface level or internally.
Describe the basic principles of Ultrasonic testing The probe has an emitter and reciever. The screen shows how long it takes the signal to be reflected back. A flaw in the test piece will reflect the signal back sooner.
Explain the term metal creep Creep is the slow deformation of a material over time resulting from a constant force acting on the material. Creep is more likely to occur if materials are subjected to high temperatures.
Outline some reasons for the mechanical testing of metals to the point of destruction Can give specific values for properties such as hardness, toughness, ductility, strength, etc. Can determine suitability of materials before they are used in a product Method of quality control Determines the success of heat treatments on products.
Distinguish between Microscopic and Macroscopic examination of metals Microscopic allows for more detailed examination of grain size and some impurities. A microscope is used for this. Macroscopic is a visual inspection process. Physical manufacturing defects can be detected by eye or low powered magnifying glass.
Why are non-destructive tests used in the manufacture of engine parts - that these parts can be tested for quality; - that expensive components need not be destroyed during testing; - tests can be carried out for flaws at surface level or internally;
Identify some factors necessary to prevent early fatigue failure in a component • Make allowance for potential fatigue during component design. • Removal of sharp corners. • Reduce vibration in service. • Improve surface finish. • Prevent or minimise corrosion
Created by: todd.ciaran