Abstract
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.
Highlights
Hot press forming is an attractive processing technology for thermoplastic composites due to the short processing cycles, which allows for cost-effective manufacturing of complex parts with continuous fiber reinforcements [1]
Ply-ply friction tests were performed on unidirectional C/PEEK tape to investigate the nature of the transient response, which exhibits a shear stress overshoot
nonlinear viscoelasticity (NLVE) seems to comply with most of the experimental observations, but the long-time values do not correspond well with the matrix viscosity. This deviation can be solved for the full velocity range by means of a changing film thickness or at low velocities by introducing a yield stress
Summary
Hot press forming is an attractive processing technology for thermoplastic composites due to the short processing cycles, which allows for cost-effective manufacturing of complex parts with continuous fiber reinforcements [1]. Most press-formed parts are relatively simple and are based on woven fabric reinforced blanks with a uniform thickness. The industry is moving towards unidirectional (UD) ply-based components with thickness variations and more complex geometries [3]. This shift comes with new challenges, as forming trials on UD plybased laminates showed the occurrence of defects, like fiber wrinkling, that were not present with woven fabrics [4]. More advanced process simulations tools are needed to enable defect-free-manufacturing
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