ISO 24370:2005

Overview

ISO 24370:2005 defines a standardized laboratory test method - the chevron‑notched beam (CNB) method - for determining the fracture toughness of monolithic and macroscopically homogeneous whisker- or particulate‑reinforced ceramics at room temperature. The method measures a fracture toughness value (K_I,CNB) by fracturing a chevron‑notched beam under four‑point flexure and calculating the stress intensity from the fracture load and a geometry‑dependent coefficient. The standard is intended for fine/advanced ceramics with fracture toughness generally less than about 12 MPa·√m and is suited to materials with a flat crack‑growth resistance curve (and may be applied to some R‑curve materials). It is not applicable to continuous‑fibre reinforced ceramic composites.

Key Topics and Requirements

• Test principle: four‑point flexure of a chevron‑notched beam; record force vs displacement, backface strain or time to detect stable crack growth.
• Specimen geometry: rectangular beams with defined tolerances (example recommended cross‑section and dimensions are provided in the standard); notch tip and chevron dimensions controlled precisely.
• Notch preparation: chevron notch cut with fine diamond grit; notch thickness (kerf) ≤ 0.30 mm and notch planes must meet within 0.06 mm; narrow notches encourage stable crack extension.
• Apparatus:
  • Universal test machine meeting ISO 7500‑1 accuracy requirements.
  • Flexure fixtures complying with ISO 14704, with specified inner/outer spans and rolling bearings.
  • Micrometer (resolution 0.002 mm) and optical microscope for notch and specimen measurements.
  • Stability detection equipment (backface strain gauges or local extensometers preferred).
• Data and calculations: compute K_I,CNB from measured fracture load and minimum stress intensity coefficient Y*; follow the standard’s formulas and validity checks.
• Validity: suitable for materials with relatively low-to-moderate toughness and where stable crack growth from the chevron notch can be achieved.

Applications and Users

ISO 24370 is used for:

• Material development and comparison of monolithic advanced ceramics.
• Quality assurance and batch verification in ceramic production.
• Generation of fracture toughness data for design, reliability analysis and failure investigations. Typical users:
• Materials and ceramics engineers, R&D and metrology labs, quality control teams, and standards bodies involved in fine/advanced ceramics testing.

Related Standards

• ISO 7500‑1:2004 - Verification of static uniaxial testing machines (force measurement).
• ISO 14704:2000 - Test method for flexural strength of monolithic ceramics (fixture requirements).
• ISO/TC 206 - Technical committee for Fine ceramics (development context).

Keywords: ISO 24370, CNB method, chevron‑notched beam, fracture toughness, K_I,CNB, monolithic ceramics, fine ceramics, test method, four‑point flexure, notch preparation.