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This work aims to better comprehend the mechanical behaviour, including failure sequence and failure modes, of differently surface-treated CFRP T-joints when subjected to out-of-plane loadings. Specimens consisted in two CFRP adherends – skin and stiffener – adhesively bonded with a structural film adhesive. The adherends' untreated and treated (by peel ply or abrasion) surfaces were firstly characterized using scanning electron microscopy (SEM), profilometry and wettability measurements. Stiffener pull-off tests (SPOT) were then experimentally conducted under quasi-static loading. The fractured surfaces were analysed by fractography to identify the failure mechanisms. The peel ply treatment has led to the worst results in terms of maximum load, while the abrasion-treated specimens performed the best of the three series. Surface characterization and fractography analysis were properly correlated with the SPOT results. Fracture damage typically initiates as mode I within the tip of the stiffener's flange, but as the fracture extends along the stiffener's foot, it does it under predominant mode II. • The mechanical performance of differently treated T-joints bonded with a brittle adhesive was evaluated. • Some results from the surface characterization failed to effectively predict the quality of bonding. • The abrasion treatment led to the best results in terms of maximum load of the bonded joint. • Large voids and a predominance of cohesive failure were found in the fractured surfaces of the peel ply-treated specimens. • Failure mechanisms and modes were identified and confirmed by fractographic analysis. [ABSTRACT FROM AUTHOR]

Composite T-joints bear low-velocity impact in service. The characteristics of their impact response and residual tensile performance are crucial to structural safety but have not been researched in sufficient detail. This study applies the Z-pin pre-hole insertion process in concert with two fillet radii to manufacture four types of carbon fiber reinforced plastics (CFRP) T-joints. The joints were then characterized by using robust low-velocity impact tests at different energies as well as post-impact quasi-static tensile tests. The results show that increasing the size of the deltoid improved the impact resistance of the joints but the deltoid itself was more vulnerable to damage, which reduced its influence on the residual tensile performance of the joints with increasing impact energy. The Z-pin reinforcement in the non-impact region had a limited influence on the low-velocity impact resistance, while its influence on the residual tensile performance of the T-joints was positively correlated with the degree of damage to them after impact. Moreover, the difference in influence between the deltoid and the Z-pin on the structural properties and damage tolerance of the T-joints was closely related to their respective reinforcement mechanisms, and can contribute to the lightweight design of the T-joint. We also used infrared thermography to observe the evolution of damage to the T-joints during the loading process. The significant fracture of the joint was predicted successfully by comparing different infrared characteristics. [ABSTRACT FROM AUTHOR]

• Images of multiple fracture progression in composite T-joints were experimentally captured by a high-speed camera. • The recent augmentation method achieved a high-fidelity modeling of multiple fracture progression in T-joints, together with cohesive model. • It was revealed that noodle cracking induced interface delamination is responsible for the final failure. • The augmentation method was coupled with binary model to quantify the improvement in delamination resistance of Z-pinned T-joints. This paper reports a combined numerical and experimental study on the multiple fracture evolution in carbon fiber reinforced polymer (CFRP) T-joints under pull-off load. The experimental study successfully captured the sequential fracture initiation and evolution involving a complex interplay among noodle cracking, noodle-skin and skin-stiffener delamination, and their correlation with the load-displacement curves. Numerically, two independent numerical methods, (a) the extend finite element method (X-FEM) available in ABAQUS, and (b) the augmented finite element (A-FEM) method, are used to cross-check their predictive capability in modeling progressive fracture evolution in the T-joints. It is found that, although the nonlinear load-displacement curves predicted by both methods are consistent with experimental data, their predictions on fracture progression responsible for final failure are different. The A-FEM prediction is in good agreement with experimental record while the X-FEM's prediction is inaccurate. Finally, the validated A-FEM model is coupled with the Binary Model to quantify the improvement in fracture resistance using the Z-pin reinforcement technique. The simulation results, which are validated by previously reported experimental data, show that properly arranged Z-pin reinforcement can improve the fracture tolerance and significantly delay the final failure of T-joints. Image, graphical abstract [ABSTRACT FROM AUTHOR]