Question 7
A stress–strain diagram is shown.
Identify the key feature represented by point X on the diagram.
That is it. Step through the hints below to see why. In 2024, 66.9% of Queensland students chose (A).
Not quite. X is where plastic deformation begins, and that point is the yield point. “Plastic limit” appeared in the 2019 syllabus list, but it is not the name of this point, and the 2025 syllabus no longer lists it. In 2024, 10.5% of Queensland students chose (B).
Not quite. The proportional limit is the end of the straight line, and QCAA marked that with W. Between W and X the curve has already started to bend. In 2024, 18.0% of Queensland students chose (C).
Not quite. The ultimate tensile strength is the highest point of the whole curve, and that is Y. X is only the top of the first, smaller rise. In 2024, 4.1% of Queensland students chose (D).
Start at the origin. The curve rises in a straight line, where stress is proportional to strain, and QCAA put W at the end of that line. W is the proportional limit, so (C) belongs to W.
$$\sigma\propto\varepsilon\quad\text{from }0\text{ to W}$$
Past W the line bends and climbs a little further to X, then the curve dips. That first peak is where the material gives way and starts to deform plastically. It is the yield point, and the stress there is the yield stress.
The dip after X is typical of low-carbon steel, which yields at an upper point and then carries on at a slightly lower stress.
Y is the highest point of the whole curve, the ultimate tensile strength, so (D) belongs to Y. Z is where the curve ends, at fracture.
“Plastic limit” appeared in the 2019 syllabus list, but it is not the name of any point QCAA marked here, and the 2025 syllabus no longer lists it. That rules out (B).
X sits just past the end of the straight line, at the top of the first rise before the curve dips. That is the yield point, so the stress at X is the yield stress. W is the proportional limit, Y the ultimate tensile strength and Z fracture.
The most common wrong answer was (C), the proportional limit. It sits just below the yield point, and on many textbook diagrams the two are drawn at the same spot. QCAA drew them apart here, with the proportional limit at W. Stress-strain diagrams are reduced content in the 2025 syllabus, yet one appeared in every paper from 2020 to 2025.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2024 QCAA Engineering external assessment, © State of Queensland (QCAA) 2024, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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