What is the Hole Expansion Test?
The hole expansion test (HET) according to ISO 16630 is a test method for determining the formability of sheet metal edges. It primarily serves to characterize the so-called edge crack sensitivity of a material.
In contrast to classic cupping tests (such as the Erichsen cupping test), which test the elongation of the base material, ISO 16630 evaluates how a punched edge behaves under stretching. This is particularly critical with modern multi-phase steels (dual-phase, complex-phase steels), as the punching process locally hardens and damages the material structure at the edge (Shear Affected Zone).
Relevance of the Hole Expansion Test
In the automotive industry, chassis components (e.g. control arms) are often punched from high-strength sheet steel and subsequently formed by flange stretching. If the material cracks prematurely at the punched edge, the component fails—even though the elongation at fracture specified in the material data sheet may appear sufficient.
ISO 16630 provides the decisive parameter in this context: the hole expansion ratio λ.
While conventional tensile tests provide characteristic values for the base material, real components frequently contain punched holes. Punching causes strain hardening and micro-damage at the cut edge.
ISO 16630 therefore evaluates not only the material itself but the combined system property of material and cut edge condition. This is essential for designing forming processes and preventing edge cracking during production.
Relevance for AHSS/UHSS
The hole expansion test according to ISO 16630 is particularly important for processing advanced and ultra-high-strength steels (AHSS/UHSS), such as dual-phase and complex-phase steels, as these materials are more sensitive to cut edge quality.
Test procedure according to ISO 16630
From specimen preparation to evaluation
1. Specimen Preparation (Punching)
A hole (typically 10 mm in diameter) is punched in the center of a sheet specimen (usually square or round). The quality of the cut edge is critical and part of the test itself, since microcracks induced during punching influence the result.
Important: ISO 16630 specifies punching, not drilling. The cutting clearance must be approximately 12% of the sheet thickness. Only in this way can realistic edge damage (strain hardening) be simulated.
2. Clamping & Tooling
The specimen is securely clamped between die and blank holder to prevent material flow from the flange area.
The punching burr must generally face the die (burr-up or burr-down positioning must be recorded in the test report; the standard configuration is often with the burr opposite to the expansion direction).
A conical punch with a 60° angle is used as the forming tool.
3. Expansion & Crack Detection
The conical punch moves into the hole at a constant speed (typically 0.1 to 5 mm/s), expanding it.
Stop criterion: The test is stopped as soon as a crack through the entire sheet thickness (through-thickness crack) becomes visible. This is the most critical moment of the test, as crack detection on manual machines is often subjective. Modern ERICHSEN testing machines use the camera system HEXRASCAN with dedicated evaluation software to ensure maximum reproducibility.
4. Measurement & Calculation
After stopping the test, the final hole diameter (Dₕ) is measured. The characteristic value λ (lambda) is then calculated.
Results and Calculation
The central result of the test is the Hole Expansion Ratio (HER), commonly referred to as λ (lambda). It describes the percentage increase in diameter until crack initiation.
λ = (Dₕ − D₀) / (D₀ × 100)
- λ: Hole expansion ratio in %
- D₀: Original hole diameter (usually 10 mm)
- Dₕ: Hole diameter at crack initiation
The results of the hole expansion test are highly sensitive to process parameters. The following factors must be defined to ensure reproducible results:
Influencing Factors on the Test Result
Cutting Clearance During Punching
If the clearance is too small or too large, the degree of strain hardening at the hole edge changes. ISO 16630 recommends 12% ± 1%.
Timing of Crack Detection
The exact moment of crack detection has the greatest influence on result scatter. Automated camera systems such as the ERICHSEN HEXRASCAN significantly reduce operator influence.
Burr Orientation
Whether the punching burr faces the punch or the die influences when crack initiation occurs.
Microstructure
Hardness differences between phases (e.g., ferrite vs. martensite) promote crack formation at the damaged edge.
Automatic Crack Detection
Modern ERICHSEN testing machines use camera systems such as HEXRASCAN to detect cracks objectively and reproducibly, since visual detection by the operator can vary significantly.
Specimen after the hole expansion test
After completion of the hole expansion test, crack initiation typically originates from the cut edge. The preceding punching operation has impaired the formability of the edge.
Hole Expansion with ERICHSEN
Our universal sheet metal testing machines (e.g., Model 142 or Model 145) provide highly precise functions for most sheet metal forming tests, including the hole expansion test according to ISO 16630 – with precise punch control, high-resolution camera technology, and intelligent evaluation units.
ERICHSEN Universal Sheet Metal Testing Machines