PSI - Issue 68

Andreas J. Brunner et al. / Procedia Structural Integrity 68 (2025) 1266–1272 Brunner et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction FRP composites are the material of choice for structures or components in many fields. These range from transportation (e.g., aircraft, cars), infrastructure (e.g., pipelines, storage vessels, bridges) and power generation (e.g., wind rotor blades, high-voltage insulators), to protective wear (e.g., bullet-proof vests), to sports and leisure equipment (e.g., bikes, golf clubs), and medical devices (e.g., implants, prosthetic limbs). Many of these applications operate under complex service conditions, both with respect to mechanical loads as well as environmental exposure, see, e.g., Nakai and Hiwa (2002) or Bender et al. (2021) or Brunner et al. (2023).

Nomenclature AE

Acoustic Emission

ASTM CFRP DCB

American Society for Testing and Materials Carbon Fiber-Reinforced Polymer

Double Cantilever Beam

ELS ESIS FEP

End Loaded Split

European Structural Integrity Society

Fluorethylen-Propylen Fixed Ratio Mixed Mode Fiber-Reinforced Polymer

FRMM

FRP

GF

Glass Fiber

GFRP GF-EP1 GF-EP2

Glass Fiber-Reinforced Polymer first type of Glass Fiber Epoxy second type of Glass Fiber Epoxy

ISO JIS

International Organization for Standardization

Japanese Industrial Standard

MMB Mixed Mode Bending NASA-TM National Aeronautics and Space Administration–Technical Memorandum PEEK poly-ether-ether-ketone UD unidirectional

For long-term, safe operation of FRP composite structures, understanding the damage mechanisms, their sources and the resulting damage initiation and accumulation is essential. This provides the basis for damage-tolerant structural design, where damage development and hence inspection intervals are predictable as discussed by, e.g., Jones and Kinloch (2020). Fracture toughness data, in principle, allow for estimating delamination propagation behavior in FRP composites. The precision of such estimates, however, is limited by repeatability and reproducibility between 10-20% of toughness data, see Pascoe (2021) or Brunner (2022). Fiber bridging in FRP test specimens affects test data whereas structural elements with multidirectional fiber lay-up show these to a much lesser extent, see Yao et al. (2024). Interactions between multiple delaminations, see Pascoe et al. (2013) or Khudiakova et al. (2021a, 2021b) as well as from variable service loads and variable environmental exposure are difficult to account for, see Brunner et al. (2023). Micromechanical modelling can provide deeper insight, but requires information about microscopic defect types, their distributions and interactions, see Qi et al. (2022). In-situ AE monitoring of standard quasi-static fracture tests yield estimates of micro- or meso-scale defect sizes. A simple approach took the total delamination area between tip of the starter crack film and delamination tip at the end of the test and divided that by the number of AE signals recorded. This indicated matrix crack diameters of an average around 110 micrometers. As a lower bound estimate, this average is consistent with analysis of video recordings of projection X-ray radiography yielding diameters around 200 micrometers, see Brunner (2016). Potential corrections apply for delaminated area and number of AE signals and their effect on defect size estimates. Defect diameter distributions derived from AE signals may be useful for detailed micromechanical modelling of delamination initiation and propagation in FRP composite structures.