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The prosthetic knee joint shown opposite uses a high-performance polyethylene bearing between the titanium metal supports - Leaving Cert Engineering - Question 2 - 2016
Question 2
The prosthetic knee joint shown opposite uses a high-performance polyethylene bearing between the titanium metal supports. (a) (i) Describe two critical properties ... show full transcript
Worked Solution & Example Answer:The prosthetic knee joint shown opposite uses a high-performance polyethylene bearing between the titanium metal supports - Leaving Cert Engineering - Question 2 - 2016
Step 1
Describe two critical properties to be considered in the selection of polyethylene as a bearing material in this prosthetic device.
Answer
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Biocompatibility: The material must be compatible with biological tissues to minimize rejection or adverse reactions. Polyethylene is known for its low friction and excellent wear resistance, making it suitable for prolonged contact with human tissues without causing harm.
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Durability: The bearing must withstand repetitive mechanical stress and wear over time. Polyethylene provides high mechanical strength, contributing to its longevity in a dynamic environment like joint movement.
Step 2
Describe, with the aid of a diagram(s), a mechanical test suitable for measuring the hardness of titanium.
Answer
A suitable test for measuring hardness is the Vickers hardness test. In this test, a diamond-indented square-based pyramid is pressed into the titanium surface under a specific load to induce an indentation. The size (diagonal length) of the indentation is then measured under a microscope. The hardness value is calculated using the formula:
where F is the applied load, and d is the average diagonal length of the indentation.
Step 3
Using the graph paper supplied, plot the stress-strain diagram for metal A and the stress-strain diagram for metal B using the same graph axes, as shown opposite.
Answer
Plot the provided values on a graph:
- Plot for Metal A: Plot points (0, 45), (1, 90), (1.5, 135), (2, 200), (2.75, 275), (3.35, 335), (3.45, 345), (3.4, 340)
- Plot for Metal B: Plot points (0, 16), (0.28, 28), (0.4, 40), (0.63, 63), (0.75, 75), (0.79, 79), (0.78, 78), (0.75, 75)
Ensure that the axes are labeled with Strain (x1000) on the horizontal and Stress (N/mm²) on the vertical.
Step 4
Step 5
Compare metal A and metal B under the following headings: Ductility, Tensile strength.
Answer
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Ductility: Metal A exhibits good ductility, indicating its capability to deform under tensile stress without fracturing. Metal B, however, displays little to no elastic properties and is more ductile, making it easier to stretch but also weaker overall.
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Tensile Strength: Metal A has a high tensile strength of 345 N/mm², making it suitable for load-bearing applications. In contrast, Metal B has a significantly lower tensile strength of 79 N/mm², indicating it is less robust under similar stress conditions.
Step 6
Describe, with the aid of a diagram(s), the principle of eddy current testing.
Answer
Eddy current testing utilizes electromagnetic induction to detect imperfections. An alternating current flows through a coil, generating a magnetic field. When the coil is brought near a conductive material, eddy currents are induced within the material. Any discontinuities in the material will disturb these currents. A probe measures these variations, indicating potential defects.
Step 7
Outline two reasons why non-destructive tests are used during the manufacture of an engine crankshaft.
Answer
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Material Integrity: Non-destructive testing (NDT) techniques allow manufacturers to identify flaws or defects within the crankshaft without damaging it. This is crucial for ensuring the component's safety and performance.
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Cost Efficiency: By identifying potential issues early in the manufacturing process, NDT saves costs related to scrap material and rework, leading to improved overall production efficiency.