R.H.W. ten Thije

Tool‐Ply Friction In Composite Forming

Friction between fibre composite laminates and rigid tool materials is an important phenomenon in composite forming processes and the resulting product geometry, fibre orientations and fibre stresses. Pull‐out experiments are presented and compared with a novel meso mechanical model based on Reynolds’ equation for thin film lubrication. The film thickness is derived from this analysis, rather than postulated as in earlier publications. Good agreement is found between the experiments, the meso mechanical model results and earlier empirically determined master curves.

Forming of Thermoplastic Composites

Design and production guidelines for UD reinforced thermoplastic composites are highly desirable. Therefore, forming experiments and simulations with a realistic complex shaped product were conducted. Thermoforming experiments with quasi-isotropic UD carbon/PEEK and 8HS woven glass/PPS composites showed a clear difference in formability. Many wrinkles develop near doubly curved areas for the considered UD composites, whereas significant in-plane shear is observed for the woven composites. Forming prediction tools can be utilised to optimise the product design with respect to formability. A forming prediction methodology is shown, which encompasses finite element modelling in combination with material models that describe major deformation mechanisms. Characterisation methods were developed to describe inter-ply friction and in-plane shear. Forming simulations are able to indicate the critical areas for the UD composites, as is concluded from the comparison of wrinkling and in-plane shear distributions within the formed specimens. Forming experiments and predictions match qualitatively well and this tool can successfully be utilised in the product design phases.

Friction in Forming of UD Composites

Inter‐ply and tool/ply friction play a dominant role in hot stamp forming of UD fiber‐reinforced thermoplastic laminates. This research treats friction measurements of a PEEK‐AS4 composite system. To this end, an in‐house developed friction tester is utilized to pull a laminate through two heat controlled clamping platens. The friction coefficient is determined by relating the clamp force to the pull force. The geometry of the gap between the clamping platens is monitored with micrometer accuracy. A first approach to describe the relation between the geometry and frictional behavior is undertaken by applying a standard thin‐film theory for hydrodynamic lubrication. Experimental measurements showed that the thin‐film theory does not entirely cover the underlying physics. Thus a second model is utilized, which employs a Leonov‐model to describe the shear deformation of the matrix material, while its viscosity is described with a multi‐mode Maxwell model. The combination of both models shows the potential to...

Intra-ply shear locking

Intra-ply shear locking results in unrealistic fibre stresses and spurious wrinkling in composite forming simulations. Three remedies are investigated: aligning the mesh, applying reduced integration and using multi-field elements. The bias extension simulation is used to test several triangular and quadrilateral elements on their capability to avoid locking. Their performance under large deformations is tested as well. The new multi-field element seems to be the best locking free element in random meshes.

Simple Tests as Critical Indicator of Intra-Ply Shear Locking

Standard finite elements can exhibit the numerical artifact of intra-plyshear locking during forming simulations. The displacement fields of elementsare piecewise continuous and cannot correctly capturediscontinuities in the shear field. This shear locking is illustrated insimulations of bias-extension experiments with an unaligned mesh. Two simpletests were developed as a critical indicator of intra-ply shear locking intriangular elements. A single-element-test shows the origin of the locking anda pull-out test indicates locking caused by small misalignments of theelements.