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PhD Defence Ramona Sitohang | Effect of in-plane waviness defects on compressive failure of thermoplastic composite laminates

Effect of in-plane waviness defects on compressive failure of thermoplastic composite laminates

Due to the COVID-19 crisis the PhD defence of Ramona Sitohang will take place (partly) online.

The PhD defence can be followed by a live stream.

Ramona Sitohang is a PhD student in the research group Production Technology (PT). His supervisors are dr.ir. L. Warnet and prof.dr.ir. R. Akkerman from the Faculty of Engineering Technology (ET).

Defect-free manufacturing of parts and structures based on advanced materials can be challenging. Thermoplastic composites form no exception. In particular, process-induced in-plane fiber waviness is worrisome as it is often difficult to prevent, while it may have a detrimental effect on the mechanical performance. Often in practice, the part is rejected when the effect of the defect is unknown. A proper understanding and quantification of the effect of in-plane waviness on the mechanical performance are therefore required. Earlier studies, mainly on unidirectional composites, showed that the compressive strength is especially sensitive to waviness. To confirm whether this translates to multidirectional thermoplastic composites, this thesis investigates the effect of in-plane waviness on the compressive failure of quasi-isotropic laminates.

The objectives of this work are i. to develop a methodology to measure the effects of waviness on the mechanical performance, ii. to identify the mechanisms leading to failure, and iii. to quantify the effect of in-plane fiber waviness on the laminate's compressive strength. Reverse forming and press consolidation were used to manufacture well-defined test coupons, having waviness with a wide range of severities that reflect those seen in manufacturing. Uniaxial compression and bending tests were used to study the compressive damage development and to quantify the strength.

Localized in-plane waviness in the plies that are oriented in the loading direction can have a pronounced adverse effect on compressive strength due to early damage initiation which is governed by kinking failure. Two parameters, namely the maximum waviness angle and number of wavy plies in the loading direction, play an important role in the compressive damage development and strength. It was experimentally demonstrated that the compressive strength initially decreases with increasing maximum waviness angle and tends to level off for angles larger than 20°. Increasing the number of wavy plies in the loading direction has a similar degrading effect on the strength. The reduction is linearly proportional to the fraction of wavy axial plies, which means that the intact axial plies govern the ultimate strength. Between the two waviness parameters, the latter is the most influential on the ultimate strength. Lastly, the implication on the design of aircraft structures was evaluated by comparing the results from this work with the reference open-hole compressive strength retention. This work shows that, for the material system and the severity studied, the existing design value derived from open-hole compressive strength can accommodate the reduction due to waviness.