Q.State the symptoms and sources of stress.

Stress, Symptoms, and Sources — A First Look

Imagine you are holding a rubber band between your fingers. When you pull it gently, it stretches a little and snaps back. Pull harder, and it stretches more — but you can feel it resisting. Pull too hard, and it either stays stretched out (permanent damage) or snaps.

That "pull" you apply is stress. The stretching you see is the symptom (strain). And the reason the rubber band resists — its material, thickness, and internal bonds — is the source of that stress response.

In engineering, stress is not a feeling — it is a physical quantity. It is the internal force that particles of a material exert on each other when an external load tries to deform them. The symptom is the deformation (strain) you can measure. The source is whatever is causing the internal forces to arise: an applied load, a temperature change, or even the material's own weight.

The Precise Statement

Stress is the internal resistance per unit area that develops inside a body when an external force is applied.

Symptom (strain) is the resulting deformation — the change in shape or size relative to the original.

Source is the external agent (force, pressure, thermal expansion, etc.) that triggers the internal stress.

Mathematically, for a simple bar under axial load:

where $\sigma$ is the stress (Pa or N/m²), $F$ is the external force (N), and $A$ is the cross-sectional area (m²). The symptom — strain $\epsilon$ — is:

where $\Delta L$ is the change in length and $L_0$ is the original length.

The source is the force $F$ itself — but also the way it is applied. A force that is too large, applied suddenly, or repeated many times can produce different stress responses even in the same material.

Why This Distinction Matters

A student new to this often mixes up stress and its source. The source is external (the load you apply). Stress is internal (the material's reaction). The symptom is what you observe — the stretching, bending, or twisting.

For example, if you stand on a wooden plank:

• Source: your weight (a force of about 600 N for a 60 kg person)
• Stress: the internal forces per unit area inside the plank, trying to hold you up
• Symptom: the plank bends slightly downward (strain)

If the source is too large, the symptom becomes permanent — the plank cracks or breaks. That is failure.

Three Common Sources of Stress

Each source produces a different kind of internal stress, and each material responds with its own symptom (strain) depending on its stiffness (Young's modulus).

The Big Picture

Stress is not visible — you cannot see it. But you can calculate it from the source (the load) and the geometry (the area). The symptom (strain) is what you can measure. Together, they tell you whether a structure is safe, whether it will deform too much, or whether it will break.

Start with the rubber band. Then move to a steel rod. Then to a bridge. The idea is the same: source → internal stress → observable symptom.