Physics · Mechanical Properties Of Solids · NEET
No. Elastic after-effect happens in a SINGLE loading and unloading: after you remove the force, the body takes a short time to return fully to its original shape, instead of snapping back at once. Elastic fatigue is about MANY cycles: after being stressed and released repeatedly for a long time, the material loses strength and can break even below its normal breaking stress. For NEET, remember: after-effect = time delay (one event); fatigue = weakening (repeated events).
This is elastic fatigue. Each stretch and release is one stress cycle. After thousands of cycles the internal structure is disturbed and the band cannot recover fully, so it stays loose and its elastic limit falls. This is why old rubber bands and springs become weak. NEET may phrase it as 'a wire subjected to repeated stress can break at a stress lower than its breaking stress' - that statement describes elastic fatigue.
For an ideal perfectly elastic body it would be instant. In real materials there is always a small delay. For quartz fibre and phosphor bronze the delay is extremely small (almost zero), which is why they are used in sensitive instruments like galvanometers and moving-coil meters. For glass fibre and rubber the delay is large. NEET loves the fact that quartz has a negligible elastic after-effect.
Elastic after-effect is a TIME delay in returning to the original state after the load is removed. Elastic hysteresis is about ENERGY: when you load and unload a material (like rubber), the loading and unloading stress-strain curves do not overlap, and the area between them is energy lost as heat. After-effect is about timing; hysteresis is about lost energy per cycle. See the linked page on elastic hysteresis for the loop.
Yes, because of elastic fatigue. A single load below the breaking stress will not break a fresh wire. But if that same sub-breaking stress is applied and removed repeatedly for a very long time, the wire develops internal weakness and can finally break at that lower stress. This is why machine parts, aircraft wings and bridges are designed with a factor of safety and are checked for fatigue failure.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the short delay a stretched or bent body takes to return fully to its original shape after the force is removed, instead of snapping back at once.
It is the loss of elastic strength in a material after it is stressed and released many times over a long period, so it can fail at a lower stress than a fresh sample.
Quartz (and phosphor bronze) have a negligible elastic after-effect, so they are used as suspension fibres in sensitive instruments like galvanometers.
Machine parts, springs, bridges and aircraft undergo repeated stress cycles. Fatigue can cause failure below the breaking stress, so a factor of safety and rest periods are used to prevent sudden failure.
Yes, it appears in Mechanical Properties of Solids as a short concept. NEET usually tests it as a statement or a difference from elastic fatigue and elastic hysteresis, not as a numerical.
