Fracture toughness resistance curves for a delamination in CFRP MD laminate composites, Part II: Mixed-mode deformation

Mor Mega*, Orly Dolev, Leslie Banks-Sills

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The use of composite laminates continues to increase, especially in the aerospace industry, due to their high strength-to-weight ratio, resistance to corrosion, and surface degradation. However, in such materials, separation of plies, referred to as delamination, is common. Characterization of the driving forces causing delamination initiation and propagation is different in the case of mode I, mode II, or mixed-mode I/II deformations. This paper is Part II of a two-paper series. Results from calibrated end-loaded split specimens, producing nearly mode II deformation, were presented in Part I. Here, mixed-mode end-loaded split, quasi-static tests were performed. Two multi-directional carbon fiber reinforced polymer material systems are considered and are referred to as the ’wet-layup’ and ’prepreg’, which is attributed to their manufacturing process. Based upon results from three-dimensional finite element analyses in conjunction with the displacement extrapolation method, as well as with the conservative interaction energy or M-integral, stress intensity factors were determined. Based upon the obtained stress intensity factors, the in-plane and out-of-plane mode mixities were evaluated by means of phase angles as a function of the delamination extension through the specimen width. In addition, fracture toughness resistance curves, or R-curves, were generated as a function of delamination extension. The critical interface initiation and fracture resistance interface energy release rate values were determined based on the obtained stress intensity factors and compared with values obtained from the J-integral. Both methods are considered local. Additionally, the global experimental compliance method was employed. Differences of less than 4% for the prepreg and 12% for the wet-layup were observed between the results obtained using the global and local methods. Moreover, it was found that although the materials and interfaces tested are different, the average in-plane mode mixity, as measured by the in-plane phase angle, was found to range between 0.645 rad and 0.668 rad. This implies that the opening mode is dominant for these specimens. In addition, the critical values of the interface energy release rate for initiation Gic and steady state propagation Giss of the two materials were found to be relatively close although the failure mechanisms are not the same.

Original languageEnglish
Article number104583
JournalTheoretical and Applied Fracture Mechanics
Volume133
DOIs
StatePublished - Oct 2024

Funding

FundersFunder number
Ministry of Innovation, Science and Technology00040436000
Ministry of Innovation, Science and Technology

    Keywords

    • Composite laminate
    • Energy release rate
    • Fracture resistance curves
    • Fracture toughness
    • Mixed-mode deformation
    • MMELS

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