Prepared weld cross-section for local mechanical characterisation of weld metal, the heat-affected zone and base material. Illustration.
Mechanical weld testing · Local material properties
Weld testing: local strength and stress-strain curves
How do strength and plastic deformation vary across a weld? Testawell uses indentation plastometry to determine local flow curves and tensile benchmark values in suitable regions of weld metal, the heat-affected zone and base material – including where a conventional tensile specimen cannot be extracted.
Why examine weld metal, the heat-affected zone and base material separately?
Welding produces regions with different thermal histories. The weld metal has melted and solidified. The adjacent heat-affected zone (HAZ) has not melted, but the heat can change its microstructure and properties. Unaffected base material lies further away.
These regions may start to deform plastically at different stresses and harden differently as deformation continues. A transverse tensile test describes the combined response of the joint. Describing individual regions requires additional data assigned to local positions. This is the purpose of our local weld characterisation.
Where are the regions with lower or higher strength than the base material?
How do local flow curves differ between the weld metal and HAZ?
What changes when welding parameters or heat treatment are modified?
Which material differences should be considered in a component calculation?
What does a local stress-strain curve tell you?
A flow curve describes the stress needed for increasing permanent, or plastic, deformation. It provides more information than a single strength value: two regions may begin to yield at similar stresses but subsequently strain-harden at different rates.
Indentation analysis can provide local benchmark values for proof stress and tensile strength. The indentation-derived proof stress Rip0.2 is the model-based counterpart of the stress at 0.2% permanent strain. The indentation-derived tensile strength Rim is the corresponding tensile strength benchmark. The superscript “i” identifies a value derived from indentation.
These results depend on the measured indentation geometry and the material model used. The method is described in DIN SPEC 4864.
How are material properties obtained from an indentation?
An indenter is pressed into the prepared surface under controlled conditions. We then use optical 3D measurement to capture the residual shape of this indentation. An inverse finite element analysis adjusts the parameters of a material model until the calculated indentation matches the measured shape. The local plastic response is derived from this fit.
A measurement series across the weld cross-section assigns the results to their testing positions. This allows properties in the weld metal, HAZ and base material to be compared. Our indentation plastometry overview explains the method in more detail.
When are hardness testing or miniature tensile testing sufficient?
A hardness profile identifies hardening or softening across a weld. Under suitable conditions it can also support a tensile strength estimate. A hardness value does not, however, provide a complete flow curve. Southern and co-authors compare these limitations.
Test
Typical question
Points to consider
Hardness profile
Where is the joint harder or softer?
Strength conversions depend on material and condition. Hardness alone does not describe the full plastic response.
Micro- or miniature tensile test
How does an extracted small specimen deform under tensile loading?
A specimen must be obtainable from the required zone. Preparation and strain measurement can be demanding.
Indentation plastometry
How do local plastic flow curves and tensile benchmark values differ?
No separate tensile specimen is required; the zone and material must suit the sampled volume and model.
Hertelé and co-authors compare miniature tensile tests with optical strain measurement and hardness maps in a heterogeneous weld. For the practical use of hardness traverses, see weld hardness testing.
How narrow can the heat-affected zone be?
The key consideration is the volume of material mechanically affected by indentation. It extends beyond the visible imprint. If this volume includes several microstructural regions, they may all contribute to the result. Closely spaced measurement positions do not automatically mean that equally small regions can be evaluated separately.
Before testing, we consider zone width, grain size, indentation dimensions, edge distances and specimen thickness. Material directionality and behaviour in tension and compression must also suit the model. Particularly narrow HAZ regions therefore require assessment of the actual specimen. Okano, Takeuchi and Mochizuki investigate the relationship between indentation size and the region sampled.
How does a weld investigation at Testawell work?
Define the objective: For example, compare two welding parameter sets, investigate a particular HAZ region or obtain local material data.
Prepare the specimen: Select a suitable cross-section and create a flat testing surface. Sectioning, grinding and any required polishing are matched to the material and testing task.
Assign regions and positions: Examine weld geometry and microstructural transitions, then define measurement paths and repeat measurements.
Measure and evaluate: Produce indentations, measure their residual shapes using optical 3D measurement and derive the local mechanical properties. Reference measurements or hardness traverses can complement the investigation.
Interpret the results: Assess differences, scatter and the limits of assigning results to individual zones against the original testing objective.
Which results will you receive?
We agree the scope in advance. For a measurement series across a weld, useful outputs include:
an image of the specimen with measurement positions assigned to the investigated regions;
local plastic flow curves with the model and evaluated strain range identified;
indentation-derived proof stress and tensile strength for each position that can be evaluated;
comparisons between zones or welding conditions, including scatter where repeat measurements are performed.
Separate material data for weld metal, the HAZ and base material can support an FE calculation. Curve type, units and data format are agreed for the task, for example true stress against plastic strain. The material model must be assessed for suitability in the intended calculation.
Feasibility and quotation: Before accepting the work, we assess the cross-section or component, accessible test surface, zone width, specimen thickness, edge distances and required repeats. The quotation then defines specimen preparation, measurement positions, evaluation, report scope, schedule and price. Specific minimum dimensions are only confirmed after this assessment.
Traceable report: The agreed report identifies the specimen and measurement positions and documents the method, material model, evaluated strain range, local curves, benchmark values and known limitations. Required raw data or an agreed FE data format are defined before testing.
Case study: local strength of an S460M welded joint
The S460M publication by Anatoliy Zavdoveev, Peter Zok and their co-authors shows how welding parameters are reflected in local properties. The investigation combines mechanical indentation testing and analysis of optically measured 3D indentation shapes with complementary hardness and tensile tests. It compares, among other results, the distributions of local tensile benchmark values across the welded joint.
Our case study explains the research question, test arrangement and findings relevant to comparing welding processes. The published results relate to the specimens and welding conditions investigated. Original publication: 2023; published online in 2022.
DIN SPEC 4864:2019-11 covers flow curves and tensile benchmark values obtained from mechanically produced indentations, optical 3D measurement of their residual shapes and a material model. ASTM E3499-25 addresses indentation plastometry of metallic materials. The applicable document and requirements are agreed before testing; using one document does not automatically establish compliance with the other.
Elongation at fracture, toughness, freedom from cracks and fatigue life require other evidence. Welding procedure qualification and component acceptance remain subject to the prescribed tests. ASTM E3499 identifies limitations concerning fracture and strongly anisotropic materials in particular.
What should you include in your enquiry?
Please provide the material designation, welding process, component or specimen thickness and, where available, a photograph, drawing or existing cross-section. Describe the region or process change you want to investigate and whether the specimen may be sectioned and prepared. We use this information to agree the measurements, required specimens and report.
DIN SPEC 4864:2019-11 – Flow curves and tensile benchmark values from indentations, optical 3D measurement of their residual shapes and a material model.
ASTM E3499-25 – Standard Test Method for Indentation Plastometry of Metallic Materials.
Can every heat-affected zone be tested separately?
This depends on zone width, microstructure, indentation size and the volume of material mechanically affected by the test. If an indentation samples several regions, the result cannot be assigned unambiguously to one zone. We assess suitability from the component or cross-section before defining the measurement positions.
Does weld testing leave the component undamaged?
An indentation leaves permanent deformation. Testing through a weld cross-section usually also requires sectioning and metallographic preparation. The method avoids machining a separate tensile specimen, but this does not make it non-destructive.
Will I receive a stress-strain curve up to fracture?
Indentation plastometry determines plastic material behaviour through a model. It does not provide a tensile test measurement up to fracture or determine elongation at fracture. The model and evaluated strain range are therefore part of the interpretation of the results.
Is the test report accredited?
Testawell is currently not accredited to ISO/IEC 17025. If you require an accredited report or evidence for welding procedure qualification, please provide these requirements before placing an order.
