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In-plane fiber waviness is one of the defects that can occur from the stamp-forming process of thermoplastic composite (TPC) parts. The influence of this defect on the mechanical performance of multidirectional composites is not yet fully understood. The main challenge in determining the influence on mechanical properties lies in reproducing the waviness in test coupons that can subsequently be subjected to testing. This paper describes an experimental approach to reproduce representative in-plane waviness defects, specific for TPC, by reverse-forming of V-shape parts of various bend angles and inner radii. Characterization results show that this method enables the manufacturing of localized in-plane waviness in flat 24-ply quasi-isotropic C/PEEK composites with no voids. Furthermore, laminates having varying levels of maximum waviness angle ( θ max ), between 14° to 64°, were successfully produced in this work. By comparing the θ max value with the examples of industrial stamp-formed parts, it can be concluded that the developed coupon manufacturing method can reproduce waviness from TPC part production reasonably well. Finally, all of the produced laminates have defective region lengths smaller than 20 mm, localized within a predefined location which makes them well suited for standard compression test coupons. [ABSTRACT FROM AUTHOR]

A model is proposed to optimise the processing parameters for the consolidation of glass/polyphenylene sulphide (PPS) laminates using a film stacking procedure. In a split approach, the heating and consolidation phase are treated separately. The heating phase is modelled using the one-dimensional heat conduction equation with variable thermal diffusivities. The model shows good agreement with experimental results. The consolidation phase is modelled using Darcy's law to predict the bundle impregnation time. The model predicts an impregnation time in the order of seconds, which is significantly shorter than the typical consolidation time of approximately 15 min used in practice. The impregnation model is validated in a comprehensive experimental programme, which included optical microscopy and mechanical testing. The experiments show that the consolidation time can indeed be shortened significantly for the glass/PPS system under consideration. [ABSTRACT FROM AUTHOR]

The influence of in-plane fiber waviness on the first ply failure of quasi-isotropic thermoplastic composite laminates was investigated. The failure mode and stress at first failure were studied by using a four-point bending test and an end-loaded bending (ELB) test. The experimental results confirm that no undesirable stress concentration due to load introduction occurred in the ELB test, thereby supporting the use of this test as an alternative method to measure stress at failure. The experiments demonstrated that waviness severity affects stress at first failure and compressive damage development. The stress at first failure initially decreases with increasing maximum waviness angle and levels off as the angle starts exceeding 20°. These results reinforce the hypothesis from previous research which suggests that wavy ply compressive strength is less sensitive to changes in severity at larger angles. Furthermore, it was found that kinking failure was the dominant failure mode for maximum waviness angle up to about 45°. No kink band was observed in wavy regions with maximum angles above 45° when there was another wavy region with a lower maximum angle in the same specimen. This means that failure is not necessarily initiated at the location where waviness is most severe. [ABSTRACT FROM AUTHOR]

The effect of the degree of crystallinity on the fracture toughness of titanium–PEKK interfaces was investigated experimentally. The level of crystallinity at the interface was varied by employing different processes commonly used in aerospace, namely autoclave consolidation, press-forming and annealing. The fracture toughness was assessed via the Double Cantilever Beam test, while the polymer degree of crystallinity was evaluated via Differential Scanning Calorimetry. Fracture surfaces were analyzed using confocal microscopy, SEM and AFM, to correlate the degree of crystallinity to the failure mechanisms and the toughness. The samples with a high degree of crystallinity exhibited a lower fracture toughness and a dominant cohesive failure, consisting of a combination of brittle fracture of the spherulites, and ductile fracture of the amorphous regions between the spherulites. Lowering the degree of crystallinity led to a higher fracture toughness, due to extensive plastic deformation of the amorphous polymer. In addition, fractography showed a transition from cohesive to interfacial failure in the case of a low degree of crystallinity. Our results show that the crystalline structure of the polymer has to be taken into account when optimizing the performance of metal–composite hybrid joints based on thermoplastic matrices. • Titanium and PEKK bonded via typical aerospace processes: autoclave, press-forming, annealing. • The induced PEKK morphology influenced toughness and failure modes of titanium/PEKK interfaces. • Decrease crystallinity increased toughness and changed failure from cohesive to interfacial. • Cohesive failure consisted of brittle (spherulite core) and ductile (spherulite edge) failure. • Interfacial failure involved extensive plastic deformation of the amorphous PEKK. [Display omitted] [ABSTRACT FROM AUTHOR]

The influence of in-plane fiber waviness defects on the compressive properties of quasi-isotropic (QI) carbon polyether-ether-ketone (C/PEEK) composites was investigated experimentally. Specimens with localized waviness were manufactured using a stamp forming process resulting into laminates featuring multiple wavy plies, i.e. from one to three wavy 0 ° plies in a 24-ply QI composite and a range of maximum waviness angle between 23 ° to 60 °. No significant influence of the waviness was found on the global laminate stiffness. Compression tests coupled with high-speed camera monitoring were performed to study the failure process. It was confirmed that the waviness defects act as a trigger for the initiation of damage, predominantly by the kinking mechanism, resulting into an early failure and significantly lower ultimate strength than the baseline when loading in 0 ° direction. Furthermore, it was found that all specimens with waviness and with the same layup have a similar strength, indicating that the maximum waviness angle within the range studied in this work did not significantly influence the ultimate compressive strength. However, the presence of waviness in multiple plies clearly affected the strength. It was found that the ultimate compressive strength decreased proportionally to the percentage of plies oriented in the loading direction that is wavy. [ABSTRACT FROM AUTHOR]