Select any two from (a)(i), (ii) or (iii) below and explain the difference between the terms in each: (i) Brinell hardness test and Vickers hardness test; (ii) Yield strength and ultimate tensile strength; (iii) Izod impact testing and Charpy impact testing - Leaving Cert Engineering - Question 3 - 2021

Yield strength is defined as the amount of stress that a material can withstand without permanent deformation. In a tensile test curve, it corresponds to the point where the material begins to yield, which is indicated by a transition in the slope of the curve.

Conversely, ultimate tensile strength (UTS) reflects the maximum stress that the material can endure before failure. This point is found at the peak of the stress-strain curve, where the material’s ability to bear stress is at its highest before necking occurs.

1. Start by marking the stress values on the y-axis and strain values on the x-axis on the graph paper.
2. Plot points for each of the stress-strain pairs: (0.60, 0.00), (125, 1.40), (195, 2.30), (260, 3.00), (330, 5.00), (350, 7.00), and (352, 8.50).
3. Connect the plotted points with a smooth curve to represent the stress-strain relationship for the aluminium alloy.

The 0.1% proof stress can be determined from the stress-strain graph by drawing a line parallel to the initial linear portion of the curve, offset by 0.1% (or 0.001 in terms of strain). It intersects the curve at a point; the corresponding stress value at this intersection is the 0.1% proof stress. From the context given, the 0.1% proof stress was determined to be 325 N/mm².

Young’s Modulus (E) is calculated as the ratio of stress (σ) to strain (ε) in the material's linear elastic region.

Using the values from the tensile test, the modulus can be calculated as:

$E = \frac{\sigma}{\epsilon} = \frac{260 \text{ N/mm²}}{3.0 \times 10^{-3}} = 87 \text{ N/mm²}$

Liquid penetrant testing involves several stages:

1. Surface Preparation: The test surface is cleaned to remove any contaminants.
2. Application of Penetrant: A colored or fluorescent dye is applied to the surface, allowed to dwell to penetrate any defects.
3. Removal of Excess Penetrant: After dwell time, excess dye is wiped off the surface to prepare for the next stage.
4. Developer Application: A developer is applied to pull the penetrant out into visible defects, providing a clear visual indication for evaluation.
5. Inspection: After a minimum of 10 minutes, the areas are inspected under appropriate lighting, such as UV light for fluorescent penetrants.

X-ray radiography is commonly used to find internal defects, including cavities in welds. In this method, an X-ray source is placed on one side of the welded joint, and a film is positioned on the opposite side. As X-rays penetrate the joint, any flaws or cavities will affect the density of the exposure on the film. The developed film reveals areas of concern where the radiation was absorbed less due to the presence of internal voids. Safety measures must be taken during this process due to the radiation involved.