Question 1

How is true stress different from engineering stress?

Accepted Answer

Engineering stress divides load by the original undeformed cross-sectional area, while true stress divides load by the instantaneous cross-sectional area during deformation. As load increases, the cross-sectional area decreases, making true stress more accurate, especially during necking and with large strains in ductile materials.

Question 2

Why does true stress continue increasing during the necking phase?

Accepted Answer

True stress remains proportional to the applied load but inversely proportional to the cross-sectional area. During necking, although load may decrease, the area decreases faster, causing true stress to keep increasing until the specimen ruptures. This behavior is accurately captured in true stress-strain diagrams.

Question 3

What is true strain and how is it calculated?

Accepted Answer

True strain sums successive elementary strain increments, where each increment equals the distance change between gauge marks divided by the corresponding length. This cumulative approach reflects material behavior more accurately than engineering strain, providing consistent results across tensile and compressive tests.

Question 4

When is engineering stress-strain data less accurate than true stress-strain data?

Accepted Answer

Engineering stress-strain diagrams become less accurate with large strains, particularly post-yield in ductile materials and during necking. True stress-strain diagrams maintain consistency during necking and across different test types, making them more reliable for analyzing material deformation behavior under various loading conditions.

Question 5

How does cross-sectional area change affect true stress calculations?

Accepted Answer

True stress is calculated by dividing the instantaneous load by the current cross-sectional area. As deformation progresses, the area decreases, which directly increases true stress values. This inverse relationship ensures true stress accurately reflects the material's actual stress state throughout loading.

Question 6

Why are true stress-strain diagrams preferred for material analysis?

Accepted Answer

True stress-strain diagrams provide accurate depiction across all deformation phases, including necking, and remain consistent for both tensile and compressive tests. Unlike engineering stress-strain diagrams used for convenience, true diagrams reflect actual material behavior, making them essential for precise engineering analysis.

Question 7

What role does the gauge length play in calculating true strain?

Accepted Answer

Gauge length determines the reference points for measuring successive length changes. Each increment of distance between gauge marks, divided by the corresponding length at that instant, yields elementary strain. Accumulating these increments produces true strain, accurately capturing the material's deformation history.

Vrai stress et vraie tension

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