---
term: "Proof Stress"
category: "material-testing"
shortDefinition: "Stress at a specified plastic strain offset on the engineering stress–strain curve when a distinct yield point is absent; Rp0.2 at 0.2% offset is the most common metals reporting convention."
formula: "Rp0.2 at εp = 0.2%"
relatedStandards: ["ISO 6892-1","ASTM E8"]
url: "https://vectorbtc.com.tr/resources/glossary/proof-stress/"
---

## What it measures

**Proof stress** reports the onset of **plastic deformation** when the stress–strain curve has no sharp yield knee. Laboratories specify an offset strain (typically **0.2% plastic strain**) and read the intersecting stress as **Rp0.2**.

## How it is tested

A tensile specimen is loaded in a **universal testing machine** with an **extensometer** on the gauge length. Software or manual analysis draws the offset line parallel to the elastic slope and records proof stress at the intersection.

## Standards and reporting

ISO 6892-1 and ASTM E8 define offset proof strengths (Rp0.1, Rp0.2, and others). Report the offset used, strain rate, and specimen geometry because proof stress is rate- and geometry-sensitive.

## Common errors

Machine compliance at low force, grip slippage, and misaligned extensometers inflate apparent proof stress. Using the wrong offset for spring or aerospace alloys can misstate elastic design limits.