Overview
ISO 20975-2:2018 defines a standardized flexural test method to determine the through-thickness (out-of-plane) tensile properties of carbon fibre‑reinforced plastic (CFRP) laminates. It applies to unidirectional CFRP (UD‑CFRP) laminates and covers measurement of through‑thickness tensile strength, fracture strain, and elastic modulus, plus quantitative assessment of Weibull size effects through calculation of an effective volume. The procedure uses a rectangular specimen loaded in three‑point bending to infer out‑of‑plane tensile stresses and strains.
Key Topics
- Test principle: Three‑point flexural loading with the specimen’s longitudinal axis oriented through the laminate thickness; deflect until rupture and derive through‑thickness stress–strain behavior.
- Measured properties: Through‑thickness tensile strength (σf), fracture strain, and modulus (E) derived from the linear region of the stress–strain curve.
- Weibull analysis & effective volume: Procedures to determine the Weibull modulus (m) and calculate the specimen’s effective volume (Veff) to quantify size effects on strength.
- Specimen geometry & preparation:
- Typical specimen width: 10 ± 0.5 mm; thickness example shown as 10 mm (span length L must satisfy L/t > 25).
- Machining of specimens from test plates per ISO 1268 and ISO 2818; thickness and width tolerances within central third.
- Conditioning & environment: Conditioning per ISO 291 (standard atmosphere 23/50 unless otherwise agreed); testing under controlled temperature and humidity.
- Apparatus & instrumentation:
- Testing machine per ISO 5893 with constant speed.
- Fixture parameters include support and loading‑nose radii (example values: supports ≈2 mm, loading nose ≈5 mm).
- Displacement transducer and data acquisition to record force vs. deflection.
- Calculations & reporting: Standardized formulas for through‑thickness stress, strain, modulus, Weibull modulus, effective volume and recommended expression of results and significant figures.
Applications
ISO 20975-2:2018 is intended for:
- Composite material test laboratories validating out‑of‑plane mechanical performance of UD‑CFRP.
- Material engineers and R&D teams assessing laminate durability and failure risk related to thickness direction loading.
- OEMs and suppliers in aerospace, automotive, wind energy and sporting goods where through‑thickness strength affects structural integrity and damage tolerance.
- Certification bodies and quality control groups requiring reproducible comparatives and size‑effect analysis for design validation.
Related Standards
- ISO 291 - Conditioning atmospheres
- ISO 1268‑1 / ISO 1268‑4 - Production of test plates (prepregs)
- ISO 2818 - Specimen preparation by machining
- ISO 5893 - Test machine requirements
- ISO 20501 - Weibull statistics for strength data
- Reference comparisons: ASTM D6415 and ASTM D7291 (discussed in the ISO introduction)
Keywords: ISO 20975-2:2018, through-thickness properties, CFRP, unidirectional laminate, flexural test, Weibull modulus, effective volume, through-thickness tensile strength, through-thickness modulus.