Frequently asked questions about i3D®

For which materials is the i3D® method suitable?

The method is designed for plastically deformable metallic materials that form a characteristic pile-up at the rim of the indentation. The grain size must be at most one third of the indentation diameter. Materials with pronounced anisotropy, porous microstructures, or a strong tension–compression asymmetry (e.g., lamellar graphite) are not suitable.

What is the objective of the i3D® test?

To determine stress-strain curves and mechanical comparison values—e.g., Rᶦₚ₀.₂ (yield) and Rᶦₘ (tensile)—in a low-damage, local, and efficient way, as an alternative or complement to the conventional tensile test per DIN EN ISO 6892-1.

How are stress-strain curves obtained with i3D®?

Via an inverse, FEM-based approach that compares the measured 3D indentation geometry with simulated results to precisely compute the plastic material properties.

Are there limitations regarding microstructure?

The method assumes a largely homogeneous microstructure. Very coarse grains, highly porous materials, or microstructures causing tension–compression anisotropy may bias results.

Can it be used on materials with property gradients?

Only to a limited extent: with through-thickness or strong local gradients, a single point is not representative. Multiple measurement points are required for a meaningful picture.

What does “quantitatively direction-independent results” mean?

Results are generally independent of specimen orientation provided no significant anisotropy or gradients are present—an advantage over tensile testing where specimen orientation matters.

Can anisotropic stress-strain curves be determined?

With shallow indentation depths and tailored evaluation, indicative statements on directional properties are possible—yet this does not fully replace dedicated anisotropic tensile testing.

Is calibration required?

Material- and specimen-specific calibration can be added to further increase accuracy—especially for unusual material behavior or surface conditions.

Which standard is used for comparison?

i3D® values serve as comparison metrics versus the tensile test per DIN EN ISO 6892-1 (room-temperature tensile testing).

Can very thin or small specimens be tested?

Yes—provided requirements for grain size, surface condition, and indentation depth are met. Point-level access is a key strength of i3D®.

How does the i3D® method work in principle?

A diamond indenter is pressed force-controlled, normal to the surface. The test force is held and subsequently unloaded. The residual 3D deformation (indent) is then captured with high resolution.

What happens after measuring the indent?

A simulation model reproduces the material response under load. The FEM model includes friction, boundary conditions, and the material’s plastic behavior.

How are material properties derived from the indent?

An optimization algorithm adjusts material parameters until the simulated indent best matches the measured one—this inverse approach yields the stress-strain curve.

What is the goal of the inverse procedure?

To determine mechanical properties—such as the stress-strain curve, Rᶦₚ₀.₂, and Rᶦₘ—without destroying the component. The process ends when convergence between real and simulated indents is achieved.

What does i3D® output?

Full plastic stress-strain curves and comparison values aligned with tensile results per DIN EN ISO 6892-1.

Why is the method innovative?

It combines low-damage testing with modern 3D metrology and simulation-driven evaluation. This enables fast, precise, and local strength assessments—directly on real parts.

What are the main components of an i3D® system?

A hardness tester, specimen holder (e.g., table), indenter, 3D measurement device, evaluation software with FEM simulation, and suitable computer hardware.

What is the task of the test machine?

It provides force-controlled relative motion between specimen and indenter, must apply the defined force accurately, and meet DIN SPEC 4864 requirements.

How is the indentation geometry measured?

With a device for areal 3D scanning of the indent that offers high resolution and measurement accuracy.

What is the role of software?

Based on the real indent, the software computes the stress-strain curve and properties via the inverse approach by matching FEM simulations to the measurement.

What are the site requirements?

The installation site must be free of vibration and shocks to ensure the necessary topography accuracy.

What about the test area?

Provide sufficient space for specimens and calibration tools, e.g., load cells.

How should the surface be prepared?

The surface must be smooth, flat, and free from scale, debris, and lubricants. Avoid thermal or cold-work alterations to the material properties.

Are lubricants allowed for reactive metals?

Yes, e.g., for titanium. A suitable lubricant (such as kerosene) may be used—its use must be documented in the test report.

What is the minimum specimen thickness?

At least 10× the residual indentation depth, unless it can be demonstrated that a lower thickness does not influence the result.

Flatness and alignment requirements?

The test force must be applied orthogonally (max. 2° deviation). Surface and support must meet ISO 2768-L tolerances.

How large must the test volume be?

Large enough to form a complete plastic indent without boundary or backing influences.

Surface roughness by force range?

Small (10–<300 N): Sa ≤ 0.2 µm — fine grinding, optional polish Medium (300–<1000 N): Sa ≤ 0.5 µm — pre-grind, mill, turn Large (1000–2500 N): Sa ≤ 1 µm — cut, coarse grind

Is special specimen prep always required?

Only if the surface condition does not meet the requirements; otherwise additional machining can be omitted.

What is the required indenter shape?

Spherical or spheroconical, with rotational symmetry.

Which types are preferred?

Preferred types are: Type A: 350 µm spherical radius, tangential transition into a 45° cone. Type B: 100 µm spherical radius, tangential transition into a 45° cone.

What is the maximum depth?

Approximately 300 µm. This depth, together with the 45° angle, defines the cone length.

How must indenters be marked?

Each diamond indenter must carry a unique ID, ideally on the indenter shank.

What temperature range is used?

Room temperature between 10 °C and 35 °C. For controlled conditions: (23 ± 5) °C. Deviations must be documented in the report.

How to deal with temperature fluctuations?

Significant temporal gradients increase measurement uncertainty; the lab must assess environmental influence.

What must be checked daily?

Before first use of any force range, perform the daily system check per Annex A. Also check indenter condition.

What if components are changed?

After replacing/removing the indenter or fixture, perform and discard two tests, then run the daily system check.

Indenter prerequisites?

Use the same indenter as in the last indirect verification. For first-time use, check with two reference materials of different strength.

How to position the specimen?

The surface must be orthogonal to the force direction and clamped securely to prevent slipping.

Required spacing between indents?

At least 3× the indent diameter between centers of adjacent indents; at least 2.5× to any specimen edge.

1) Create the test indent

The indenter is driven orthogonally and force-controlled into the surface. This is minimally invasive and requires no classical specimen machining.

2) 3D measurement of the indent

Optical topography (e.g., white-light interferometry) captures the indent in 3D with high precision.

3) Inverse evaluation to obtain the stress-strain curve

FEM results are matched to the real indent; optimization yields the underlying parameters (e.g., yield, tensile, hardening).

4) Stress–strain curve & comparison values

The plastic stress-strain curve provides technical stress–strain curves and comparison values such as Rᶦₚ₀.₂ and Rᶦₘ—analogous to tensile testing.

How is the force F applied?

Smoothly, without shocks, overshoot, or overload, and orthogonally to the surface. Dwell times follow the standard to ensure uniform plastic deformation.

What about unloading?

Reduce force in a controlled manner to avoid altering the residual indent—no shocks or relative motion of specimen/indenter.

Recommended forces

49.03 N, 98.07 N, 147.10 N, 294.20 N, 588.40 N, 1,471.00 N. Selection depends on strength, target indent size, and thickness.

Can the cycle be repeated?

Yes. A second indent under identical conditions improves reproducibility and reduces standard deviation, especially in series testing.

How is the i3D® system calibrated?

Two stages: direct and indirect verification. Both must be performed at least annually per DIN SPEC 4864.

What does the direct check cover?

• Test forces• Indenter assembly• Test cycle (force-time control)• 3D metrology for indent measurement

What is the goal of the indirect check?

To validate results against calibrated reference specimens with known tensile strengths; useful as a regular in-house routine check.

How often is full calibration required?

At least once every 12 months—starting with direct, ending with indirect verification.

Form-measurement device requirements?

Expanded uncertainties (95%): Angle: ±0.1° Radius: ±5 µm

Indenter geometry requirements

Conical section 90° ± 0.35°. Spherical tip: Type A: 350 µm radius (±15 µm single, ±10 µm mean), max. 2 µm local deviation Type B: 100 µm radius (±15 µm single, ±10 µm mean), max. 2 µm local deviation

How is visual inspection done?

Use a loupe with ≥10× magnification to check for damage or contamination of the tip.

How often should the indenter be replaced?

Every ~2 years is recommended, as even microscopic changes can affect results.

Purpose of the indirect check

To ensure the i3D® system delivers reliable results by testing certified reference materials with known tensile strengths.

What is certified reference material?

Material with round-robin validated properties and homogeneous mechanical behavior—serving as a standardized comparison basis.

How many specimens and tests?

Two specimens per alloy group with sufficiently different tensile strengths (≥ 500 MPa for steels; ≥ 100 MPa for aluminum). Perform five tests per reference specimen.

Required statistics

Compute systematic deviation (absolute & relative to nominal) and the relative scatter of single tests; check against Table A.1.

Steels: Rᶦₚ₀.₂: ±8% | Rᶦₘ: ±5% | scatter: Rᶦₚ₀.₂ = 3%, Rᶦₘ = 2% Aluminum alloys: Rᶦₚ₀.₂: ±10% | Rᶦₘ: ±10% | scatter: both ≤ 5%

Restrictions for reference-specimen test areas

Testing is permitted on one surface side only. Specimens must be homogeneous and free from surface defects.

What is the user’s responsibility?

Use certified reference specimens only and document full compliance with the requirements.

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