ISO 18608:2017

Overview

ISO 18608:2017 specifies a standardized method to classify ceramic matrix composite (CMC) materials by their notch sensitivity and to determine an equivalent fracture toughness (Keq) where applicable. The standard uses tensile tests on double-edge notched specimens with varying notch depths and compares results to tensile tests on un‑notched specimens. ISO 18608 applies to CMCs with continuous fibre reinforcement (unidirectional, bidirectional, tridirectional) loaded along a principal reinforcement axis.

Keywords: ISO 18608, ceramic matrix composites, CMC, notch sensitivity, equivalent fracture toughness, notched specimens, tensile tests, fracture toughness.

Key topics and requirements

• Test principle: perform tensile tests on notched and un-notched specimens; compare net-section strength as a function of notch depth.
• Specimen geometry: double-edge notched specimens with defined ligament (width b) and notch depth (a). Cross-sectional dimensions between notches equal to un‑notched specimens.
• Measured parameters: maximum tensile force (Fm), un‑notched tensile strength (σr), notched tensile strength (σn), and the stress ratio (σn/σr).
• Classification: the standard distinguishes CMCs that are notch-sensitive from those that are notch-insensitive. For notch-sensitive materials the document provides a procedure to determine Keq, defined as the fracture toughness of an equivalent homogeneous material with the same sensitivity to crack propagation.
• Test apparatus & measurement: requirements for test machines, load trains/grips, data recording, micrometers and ligament measurement devices are outlined.
• Data analysis: plotting a notch-sensitivity diagram (σn/σr vs. notch depth) and calculation methods for Keq and class-dependent behaviour are specified.
• Limitations & significance: explains why traditional fracture mechanics parameters (K, G, J) are problematic for heterogeneous CMCs due to microcracking and damage zones at notch tips; motivates use of Keq for material comparison.

Applications

• Material selection: compare CMCs for tolerance to notches, holes and other stress concentrators.
• Design & qualification: support structural design and failure assessment in aerospace, automotive, energy and defense components made from CMCs.
• R&D and quality control: evaluate processing routes, reinforcement architectures (1D/2D/3D) and manufacturing variations affecting crack propagation sensitivity.
• Benchmarking: use Keq to compare CMC behaviour with monolithic ceramics or other engineering materials.

Who uses this standard

Materials engineers, mechanical test laboratories, composite designers, standards and certification bodies, academic researchers and industrial R&D teams working with advanced ceramics and ceramic matrix composites.

Related standards

• ISO 15733 - tensile properties of ceramic composites
• ISO 3611 - micrometers (dimensional measurement)
• ISO 7500-1 - calibration/verification of tension testing machines

For implementation, pair ISO 18608 test procedures with ISO 15733 tensile protocols and appropriate machine calibration per ISO 7500‑1.