A stress-strain curve is the material's behaviour written as a graph: pull the test piece, record how much load it takes and how far it stretches, and plot one against the other. For a stamped or drawn part the interesting part of that graph is not the last point before it breaks - it is where the curve leaves the straight line, and the shape of the curve after it does.
What does a stress-strain curve tell you?
A stress-strain curve shows elastic behaviour (the straight section, where the part returns to shape), the yield point where permanent deformation starts, the ultimate tensile strength where the load peaks, and the elongation at break. For forming, three further values matter more than the headline strength: the strain-hardening exponent n, the plastic strain ratio r, and the uniform elongation. High n and high r mean the material spreads thickness evenly and resists thinning, which is what allows a deeper draw. Low n and low r mean the material localises and tears, so the same part needs more drawing stages, larger radii and more annealing. The curve is measured to ISO 6892-1 or ASTM E8, and the mill certificate should carry the values you specified.
The shape of the curve, section by section
- Elastic region - a straight line. Its slope is Young's modulus, and it is the same for every grade of a given metal family, so it does not help you choose a steel.
- Yield point - where the line stops being straight. This is the stress the part sees when it springs back, so it is the number behind springback and the reason a formed part never comes out at the die angle.
- Strain hardening - the rising curve after yield. The exponent of that rise is n. A high n delays the onset of localised thinning, which is exactly the property a deep drawn wall depends on.
- Ultimate tensile strength - the peak of the curve. Buyers often treat UTS as the headline number, but for forming it is mostly a limit: stronger usually means harder to form.
- Necking and fracture - after the peak the deformation concentrates in one band and the part tears. Total elongation covers the whole test; uniform elongation stops at the peak, and it is the one that predicts formability.
Typical values for the materials we form
| Material | Yield Rp0.2 | Tensile Rm | Elongation A80 | n | r |
|---|---|---|---|---|---|
| DC04 deep drawing steel | 140–210 MPa | 270–350 MPa | ≥ 38% | ≈ 0.21–0.24 | ≈ 1.6–2.0 |
| DC01 general cold-rolled | 240–310 MPa | 370–450 MPa | ≥ 28% | ≈ 0.18–0.20 | ≈ 1.3–1.5 |
| 304 stainless | ≥ 210 MPa | 520–720 MPa | ≥ 40% | ≈ 0.45 (high, but very high work hardening) | ≈ 1.0 |
| 430 stainless | ≥ 250 MPa | 450–600 MPa | ≥ 18% | ≈ 0.20 | ≈ 1.0–1.1 |
| C11000 copper (soft) | 60–90 MPa | 220–260 MPa | ≥ 40% | ≈ 0.35 | ≈ 0.9–1.0 |
| Pure iron (DT4E equivalent) | ≈ 120–180 MPa | ≈ 270–350 MPa | ≥ 30% | ≈ 0.20–0.24 | ≈ 1.5–1.8 |
Ranges are the typical published positions for these families, not a specification. n and r depend on grade, thickness, rolling direction and test method, so a programme that depends on them should have them on the mill certificate rather than take them from a table.
Why n and r matter more than strength
A drawn cup fails when one band of the wall thins faster than the rest. Strain hardening is what resists that: if the thin band is also the strongest band, deformation spreads instead of concentrating. High n is that effect, and it is why a deep drawing grade of steel costs more than a general cold-rolled grade of the same thickness - the difference is not strength, it is how the curve behaves after yield. High r works on the other axis: it describes how much the sheet resists thinning through the thickness compared with flow in the plane, and a high r is directly linked to how deep a cup can be drawn in one stroke.
What to put on a drawing or an RFQ
- Grade and standard - DC04 to EN 10130, 304 to EN 10088-2, C11000 to ASTM B152.
- Thickness in millimetres with the tolerance and the standard it is held to.
- The properties you actually need: r and n for a deep drawn part, uniform elongation for a stretch-formed part, yield for a part that must stay flat after forming.
- Surface and coating, because a coating changes friction and therefore the forming behaviour as much as the curve does.
- Whether the part is drawn, stretched or bent - the same curve is read differently for each.
Key point
If a part tears, the fix is usually in the material or in the die, and the stress-strain curve tells you which. Tearing at the same place every stroke points at the die radius or the blank holder force; tearing that starts early and moves points at n and r, which is a material specification problem that no amount of tool adjustment will solve.
Frequently asked questions
What is the difference between engineering and true stress-strain curves?
Engineering stress divides load by the original area, so the curve falls after the peak. True stress divides by the instantaneous area, so it keeps rising. Forming simulation uses true stress-strain data, which is why a curve copied from a data sheet rarely matches a real forming trial.
What does a high r value mean?
A high plastic strain ratio means the sheet resists thinning through the thickness better than it resists flow in the plane of the sheet. In practice a high r allows a deeper draw in one stage, which is why drawing grades are specified on r as much as on elongation.
Why does 304 stainless crack when I try to draw it deeply?
304 has a very high strain-hardening exponent but a low r value, so it spreads work hardening quickly and thins in the wall. Deep 304 parts usually need more stages, more intermediate annealing and more generous radii than the same part in DC04.
Can you anneal between drawing stages?
Yes. Intermediate annealing is common on deep or multi-stage drawn parts, and batch annealing is an outsourced process for us - we manage it in the route and disclose it on the quotation.
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