Compensation of edge effects in picture frame testing of continuous fiber reinforced thermoplastics

In picture frame testing, influences of the clamping and fiber misalignment have been already studied for dry fabrics. Due to the presence of solid regions near the clamps in picture frame tests of thermoplastic continuous fiber reinforced composites at processing conditions, the influences of the clamping are amplified. This study examines the effects of the presence of unmelted material regions near the clamps on picture frame tests both analytically and experimentally for a polypropylene glass fiber composite, using a custom built picture frame which allows for an inward and outward repositioning of the clamps. The results show the presence of major strains in the composite samples when unmelted material is present near the clamps which can lead to forces up to 6.5 times higher than for supposedly ideal kinematic conditions. An overadjustment of the clamps by an up to 0.5% of the joint distance of the picture frame to allow for a minor amount of compression in the composite has shown favorable results which are comparable to correctly corrected measurements. This setting is hence recommended to generate more reliable and comparable results for intra play shear properties of thermoplastic composites. Further research is necessary to additionally account for fiber misalignment.