On the origin of start-up effects in ply-ply friction for UD fiber-reinforced thermoplastics in melt

Rens Pierik*, Wouter Grouve, Sebastiaan Wijskamp, Remko Akkerman

*Corresponding author for this work

Research output: Chapter in Book/Report/Conference proceedingConference contributionAcademicpeer-review

6 Citations (Scopus)
39 Downloads (Pure)


Hot press forming is an attractive production technology to fulfil the increasing demand for complex fiber-reinforced thermoplastic parts. Over the years, process simulation tools on press forming have shown to be very helpful in facilitating the design stage for defect free parts production. One of the important deformation mechanisms considered in process simulations is the relative slip of successive plies or ply-ply friction, of which the underlying principles need to be better understood in order to improve the overall predictive simulation quality. In particular the use of steady-state friction values, neglecting the transient response, is questionable as experiments showed that shear stress overshoots can be as high as three times the long-time value. The phenomenon of the overshoot at start-up shear is analyzed. Possible explanations include nonlinear viscoelasticity and a slip relaxation effect giving rise to wall slip, which are discussed using relevant ply-ply friction measurements carried out on a dedicated friction test set-up. Experimental results on UD C/PEEK show that the shear stress build up and subsequent relaxation comply with nonlinear viscoelasticity. However, the long-time shear stress fails to match the matrix material's viscosity, possibly due to a yield stress. The flow curve corrected for a yield stress resembles the effects of wall slip. A transient model according to these findings will enhance the accuracy of press forming simulation software.

Original languageEnglish
Title of host publicationESAFORM 2021 - 24th International Conference on Material Forming
PublisherPoPuPS (University of LiFge Library)
ISBN (Electronic)9782870193020
Publication statusPublished - 2021
Event24th International Conference on Material Forming, ESAFORM 2021: Friction and wear in forming processes - Online Event, Belgium
Duration: 14 Apr 202116 Apr 2021
Conference number: 24


Conference24th International Conference on Material Forming, ESAFORM 2021
Abbreviated titleESAFORM 2021
CityOnline Event
Internet address


  • Hot press forming
  • Inter-ply slip
  • Ply-ply friction
  • Shear stress overshoot
  • Thermoplastic matrix composites


Dive into the research topics of 'On the origin of start-up effects in ply-ply friction for UD fiber-reinforced thermoplastics in melt'. Together they form a unique fingerprint.

Cite this