ISO 16808:2022

Overview

ISO 16808:2022 - "Metallic materials - Sheet and strip - Determination of biaxial stress-strain curve by means of bulge test with optical measuring systems" specifies a standardized bulge-test method to determine the biaxial true stress–true strain curve of metallic sheets (thickness < 3 mm) in pure stretch forming. The method uses a fluid-driven bulge (hydraulic bulge) and an optical, full-field measurement system to record surface geometry and calculate curvature, true strains and thickness evolution up to fracture. Compared with uniaxial tensile tests, the bulge test enables the assessment of higher strain states representative of sheet forming processes.

Key topics and technical requirements

• Scope and principle
  • Circular blank clamped between die and blank holder; fluid pressure forms a dome while pressure and surface deformation are recorded.
  • True stress at the dome apex is calculated assuming a thin-walled spherical pressure vessel; true strains and thickness follow from optical measurement and incompressibility assumption.
• Test equipment
  • Die, blank holder and fluid chamber with sufficient rigidity; recommended lock bead to prevent material flow.
  • Fluid pressure measurement with precision of ~1% from 20% of maximum measured pressure.
  • Recommended die diameter to thickness ratio d/t ≥ 33.
• Optical measuring system
  • Multi-camera optical-deformation-field system recording X, Y, Z coordinates of a grid over at least half the blank diameter.
  • Local grid spacing requirement: g_max ≤ d_die / 50.
  • Shape/height accuracy target: z-coordinate RMS error ≈ 0.015 mm (area concentric to blank).
  • Strain measurement accuracy targets: rms(ε1) = rms(ε2) ≈ 0.003.
  • Missing measurement points: < 5% allowed within the central measurement area.
• Specimen and grid
  • Flat blanks; lock bead recommended; grids may be deterministic (squares, dots) or stochastic (speckle).
  • Grid application methods: electrochemical/photochemical etching, offset printing, grid transfer, sprayed speckle patterns.
• Data processing
  • Procedures for curvature evaluation, strain calculation at the pole, conversion to biaxial stress–strain curves, and test reporting are defined.
  • Informative annexes cover quality checks of the optical system, response-surface curvature computation, and determination of equi-biaxial yield/hardening points.

Applications and practical value

• Provides reliable biaxial formability data for metallic sheets used in stamping, deep drawing and stretch forming.
• Supplies material input for finite element forming simulations and constitutive model calibration where biaxial loading is critical.
• Enables evaluation of higher strain regimes than tensile tests, useful for advanced high-strength steels, aluminum alloys, and lightweighting programs.
• Supports quality control, material development and R&D in automotive, aerospace, appliance and metal‑forming industries.

Who should use ISO 16808:2022

• Materials testing laboratories and certification bodies
• Forming and process engineers in automotive/aerospace OEMs and suppliers
• R&D teams developing sheet alloys and forming simulations
• Quality engineers requiring standardized biaxial mechanical data

Related information

• ISO 16808:2022 is a second edition (minor revision) prepared by ISO/TC 164/SC 2. The standard includes informative annexes for measurement-system checks and computation methods. Keywords: ISO 16808:2022, bulge test, biaxial stress-strain curve, optical measuring systems, sheet and strip, metallic materials testing.