Determine the onset of plastic deformation on a tensile specimen or locally on the real component.
Testing service · Strength properties
Test yield strength and proof stress
ReH, ReL and Rp0.2 describe when a material leaves the elastic range. Testawell identifies the suitable route: a standard tensile test for global reference values or local i3D® indentation for component zones.
Both properties mark the transition from predominantly elastic to permanent plastic deformation. Below this range, the unloaded material approximately returns to its original shape. Above it, plastic strain remains. This transition is crucial for design, material selection, process release and quality control.
If the curve shows a distinct yield phenomenon, upper yield strength ReH and lower yield strength ReL are evaluated. For a continuous transition, a defined permanent strain is used, commonly the 0.2% proof stress Rp0.2.
Stress-strain curves reveal the elastic range, yielding, hardening and fracture behaviour.
ReH, ReL or Rp0.2: which property is required?
Property
Meaning
Typical curve
ReH
Highest stress before the first distinct yielding.
Material with a pronounced yield point.
ReL
Lowest stress in the subsequent yield range, evaluated according to the standard.
Pronounced yield or Lüders behaviour.
Rp0.2
Stress at which 0.2% permanent strain is reached by the defined evaluation.
Continuous transition without a clear yield point.
Rm
Ultimate tensile strength: the highest engineering stress.
Must not be confused with yield or proof stress.
Stress is based on the original cross-section S0 and is normally reported in MPa. Rp0.2 is determined from the stress-strain curve with the 0.2% offset method.
How are yield strength and proof stress tested in tension?
For metallic materials, ISO 6892-1 defines tensile testing at room temperature. A defined specimen is loaded axially while force and strain are recorded. A suitable extensometer is important because crosshead travel also contains compliance from the machine, grips and setup.
Define specimen and cross-section: geometry, orientation and S0 must match material form and standard.
Align and grip: suitable tooling limits bending and slip.
Measure force and strain: strain rate and instrumentation are matched to the requirement.
Evaluate the curve: determine ReH/ReL or Rp0.2 and further properties.
Tensile testing remains the reference route when required by a standard, when global values are needed, or when fracture elongation, reduction in area and behaviour up to fracture must be assessed.
Local alternative
Assess proof stress with i3D® indentation
If a suitable tensile specimen is unavailable or a small component zone must be resolved, indentation plastometry can provide a local comparison value. A defined indent is measured in 3D, including plastic pile-up, and compared with an inverse finite-element model.
The outputs include a plastic stress-strain curve and indentation-derived comparison values RIp0.2 and RIm. The superscript identifies them as indentation-derived, rather than unqualified tensile-test values.
Tensile test or i3D indentation: select the appropriate route
Criterion
Tensile test
i3D indentation
Result character
Global standard reference for a defined specimen
Local indentation-derived comparison value
Proof stress
Rp0.2, ReH or ReL
RIp0.2
Specimen
Standard geometry, often machined
Prepared local surface on specimen or component
Spatial resolution
Average over the gauge length
Profiles, grids and maps of local zones
Additional outputs
Elongation, reduction of area, behaviour to fracture
Plastic flow curve, local RIm, heatmaps
Typical use
Qualification and normative reference
Components, welds, gradients and screening
When does local indentation not replace tensile testing?
Use a tensile test when a contract or standard explicitly requires it, when behaviour to fracture and ductility are central, or when anisotropy and global response must be quantified. Local indentation is complementary: it resolves zones and reduces specimen extraction, but requires a suitable material response, surface and validation context.
Measured topography and numerical response are combined to identify the local material model.
Information required for test planning
material, heat-treatment state and expected strength range
component or specimen geometry and accessible test zone
applicable standard, drawing or customer specification
surface condition and available preparation route
number and distribution of points, profiles or grids
required reference, validation and reporting format
Related test routes and applications
Global reference
Tensile testing
Standard properties from a defined tensile specimen.
Local method
i3D indentation
Flow curves and comparison values from small component zones.
Comparison
Validation
Relate local indentation data to tensile references.
Spatial resolution
Gradients & transitions
Map proof-stress changes across local zones.
Frequently asked questions about yield strength and proof stress
What is the difference between yield strength and proof stress?
Materials with a distinct onset of yielding are described by upper and lower yield strength ReH and ReL. If no distinct transition exists, a specified plastic strain is used, commonly the 0.2% proof stress Rp0.2.
How is the 0.2% proof stress Rp0.2 determined?
A line parallel to the elastic slope is offset by 0.2% strain. Its intersection with the stress-strain curve defines Rp0.2. Accurate strain measurement and a standard-compliant tensile test are essential.
Can proof stress be assessed without a tensile specimen?
For local tasks, i3D indentation plastometry can derive an indentation-based comparative proof stress from a low-destructive indent, its 3D topography and an inverse FE model. It does not automatically replace every tensile test required by a standard.
When is the i3D indentation method useful?
It is especially useful on real components, small material volumes, welds, heat-affected zones, additive structures, gradients and campaigns with many local points.
Can indentation also provide tensile strength?
Depending on suitability and validation context, the method can derive a plastic flow curve and indentation-based comparison values for proof stress and tensile strength.
What information is needed to plan a test?
Provide material, component or specimen geometry, relevant zone, expected strength range, standards or customer requirements, number of points, surface condition and whether a global reference or local map is needed.
