Analysis And Characterization Of Shear And Drape Behavior Of Glass Warp Knitted Non-crimp Fabrics During Forming

This thesis is focused on the main mechanisms of fabric deformation during draping.The chosen fabric is from fiberglass rovings arranged in the multi-axial multi-layer warp knitted structures.The fabric shear behavior was experimentally characterized based on two methods;uniaxail bias extension(UBE)and picture frame fixture.Also,the draping behavior was also experimentally characterized using double-dome punching geometry.The fabric samples were produced in PGTEX~?company for the investigations in this project.They were classified as Warp Knitted Non-Crimp Fabrics(WKNCFs).Besides two experimental tools were set up and manufactured;the picture frame fixture for in-plane shear testing and the double-dome assembly for fabric draping investigation.In-plane shear rigidity modulus was characterized based on bias extension experimentation arrangement.The objective of this stage was to compare the shear behaviour of those relatively high areal density fabrics.With the aim of characterization the shear properties,stress equations were established to deliver the in-plane modulus of shear rigidity as a function to shear angle.The effect of stitching pattern on shear behaviour was also analyzed.It was found that the shear force for uni-directional(UD)and 0/90?bi-axial(LT)fabrics with tricot stitching pattern have the same trend.However,the difference in values was according to the weight distribution between the two layers of those two fabrics.It was higher in UD fabric where most of the weight come from one layer and was lower in LT where the layers had nearly equal weight.In the case of chain stitching pattern for±45?bi-axial(DB)fabric,the magnitude of shear modulus in parallel to chain direction was about ten times bigger than it was in perpendicular to chain direction.For tri-axial fabrics with tricot or chain stitching,just one direction could be tested by UBE method,where it was no roving in that direction.As tricot stitching had chains on the technical face,the modulus for the tri-axial 0/±45?(DBL)was many times lower compared to DB fabric.The easy shear deformation of tricot pattern was because of the sliding of longitudinal rovings out from both sides of the sample and the slackness of stitches under compression in DBL.Nevertheless,the 45/90/-45°tri-axial(DBT)modulus was much higher due to the tension of stitching chains.Also,the shear deformation mechanics was experimentally investigated using Picture Frame testing apparatus equipped to a universal testing machine.The fabric samples were tested for four cycles inside the frame.The aim was to characterize and compare the shear behavior of relatively high areal density fabrics.Energy normalization theory was used to get the normalized shear force from testing machine data,and then the shear stress against shear angle was fitted by cubic polynomial regression equations.The results achieved from that derivation of equations provide the in-plane shear rigidity modulus associated to shear angle.Also,the effect of shearing cycles and stitching pattern on shear resistance was also analyzed.It is proved that determining the modulus by this method is simpler than other methods like bias extension-based method and it depends on pure shear mechanism occurring through the sample in the picture frame.This part of the study has shown the simple procedure of defining shear modulus equation depending on pure shear mechanism applied by picture frame apparatus.And also,it was found that the shear rigidity for fabric with tricot pattern increase by increasing shear angle and the fabric follow the same trend for next cycles but with lower shearing force.For chain stitching pattern under compression,the shear rigidity is decreasing in initial stage of shearing then increasing rapidly in the first cycle but it is generally decreasing in the next cycles.On the other hand,chain pattern in tension shows increasing in shear rigidity up to breaking some stitches then it is decreasing rapidly.The formability of WKNCFs over double-dome geometry was investigated.Fabric wrinkles and local out-of-plane bucking of yarns were the fundamental defects in focus.The objective of this part of the project was to experimentally investigate and compare the draping behavior of WKNCFs.The tested fabrics included two stitching patterns;tricot and chain.A double-dome punching test was performed to implement draping for each fabric then the defects were detected and characterized.Punching load-displacement curves were also recorded.The structure and the number of fabric axes,stacking sequence and stitching pattern all contribute to defect formation during draping.However,some fabrics(UD and DB)have well drapeability in all tested configurations.Whereas,some other fabrics(the 0/±45/90?qudri-axial(QX)and DBL)have bad drapeability with severe defects.Nevertheless,some fabrics like(LT)has good drape behavior(no defects)in one direction and bad drape behavior(whether defects are small or large)in other directions.The studied configurations in this chapter can help in studying the simulation of each fabric and open the doors to a method by which draping defects can be minimized and even eliminated.