What is the Yield Point of a Metal

The yield point is the stress level where a metal starts to permanently deform. Below this stress, the metal behaves like a spring – it bends but bounces back to its original shape when you release the force. Above the yield point, the metal stays bent even after you remove the load.

Engineers use the yield point as a critical safety threshold. They design structures so that normal operating stresses stay well below this point, ensuring buildings don’t permanently sag, car parts don’t deform, and bridges maintain their shape over decades of use.

Upper and Lower Yield Points

Some metals, especially mild steel, show two distinct yield points instead of just one. The upper yield point is a brief stress peak where the metal first starts to yield. Immediately after, the stress drops to a lower yield point where plastic deformation continues at a constant, lower stress.

This happens because of how atoms interact inside the metal. Carbon and nitrogen atoms in steel act like tiny anchors that pin the metal’s internal structure in place. Breaking these anchors requires extra force – that’s the upper yield point.

Once these anchors break free, the metal flows more easily at the lower yield point. You can actually see this happening as visible bands called Lüders bands that spread across the metal surface during testing.

Engineers typically use the lower yield point for design calculations since it represents the sustained stress level during plastic flow. Materials like aluminum alloys don’t show this two-step behavior – they yield gradually instead.

How Is the Yield Point Measured

The yield point is measured using a tensile test where a metal sample is pulled apart at a controlled rate. A machine records both the applied force and how much the sample stretches, creating a stress-strain curve that shows exactly where yielding begins.

For metals with a clear yield point like mild steel, you’ll see a sudden drop or “knee” in the curve. But many metals transition gradually from elastic to plastic behavior, making the exact yield point harder to spot.

When there’s no sharp yield point, engineers use the 0.2% offset method. They draw a line parallel to the elastic portion of the curve, offset by 0.2% strain. Where this line intersects the actual curve defines the yield strength – representing the stress that causes a tiny but permanent 0.2% deformation.

Factors Affecting the Yield Point

Several key factors determine a metal’s yield strength:

• Material Composition and Microstructure: Adding elements like carbon, chromium, or manganese to steel increases yield strength by making it harder for the metal’s internal structure to slip and deform. Smaller grain size also increases yield strength – think of grains as puzzle pieces where the boundaries between pieces act as barriers to deformation.
• Heat Treatment: Quenching and tempering steel can double or triple its yield strength by creating a harder internal structure. Annealing has the opposite effect – it softens the metal and reduces yield strength by relieving internal stresses and allowing grains to grow larger.
• Cold Work (Strain Hardening): Cold-rolling or drawing metal increases its yield strength by creating a tangle of internal defects that resist further deformation. Cold-rolled steel can have 50% higher yield strength than the same steel that’s been hot-rolled.
• Strain Rate: Metals appear stronger when deformed quickly. A car bumper in a crash experiences higher effective yield strength than the same material loaded slowly. The effect is usually modest but becomes important in impact situations.
• Temperature: Heat makes metals weaker – aluminum at 300°C might have half the yield strength it has at room temperature. Cold generally increases yield strength but can make metals brittle. Engineers must account for service temperature when selecting materials, especially for applications like jet engines or Arctic pipelines.

Yield Strength of Common Metals

FAQs

What’s the difference between yield strength and tensile strength?

Yield strength marks when permanent deformation begins, while tensile strength is the maximum stress before the material breaks. A metal yields first, continues to deform while getting stronger (work hardening), then eventually fractures at its tensile strength.

Can yield strength change over time?

Yes, factors like fatigue loading, corrosion, and temperature exposure can alter yield strength. Cold work during service can increase it, while high temperatures or stress corrosion can decrease it significantly.