Evaluation of microdamage initiation in Z-pinned laminates by means of automated RVE computations

Abstract

Z-pinning was originally designed to improve the delamination toughness and the impact resistance of composite laminates. However, there is extensive experimental evidence that this improvement is accompanied by a reduction of the in-plane properties. The main mechanisms responsible for this deterioration are the local change in fiber content, fiber distortion, and the inclusion of resinrich regions near the Z-pin. The shape of these geometrical features strongly depends on the laminate stacking sequence and on pin parameters such as pin diameter, pin content, and initial pin inclination angle. Their shape complexity challenges analytical modelling approaches which are currently used to generate RVE geometries for simulations. A computational approach is presented to generate such geometrical models. Resin-rich regions are modelled by initially straight discretized lines which are gradually shaped by a set of geometrical operations mimicking pin insertion, pin rotation and fiber deflection. Fiber distortion is modelled in a post-processing stage in cross-sections accounting for the preservation of the amount of fibers. These models are then transformed into finite element mechanical models in order to investigate how local fiber volume fraction changes, fiber misalignment,

Automated generation of 3D orthogonal woven composites RVEs including yarn cross-section variations

“…Local fiber volume fractions have already been shown in fiber-reinforced composites to act as stress concentration and damage initiation regions [50,51,55,65,66]. A similar trend can also be observed in this work, by comparing the contour plots of the regions of stress concentrations in the surface weft and binder yarns illustrated in Fig.…”

“…A node P of a line i (see Fig. 6) which interpen- etrates a line j is to be moved along its normal D P on the line j to suppress the interpenetration [48,50]. The movement x P of the node P can further be regulated by a parameter q :…”

“…> is the MacAuley bracket defined as < x >= 1 2 (x + |x|), σ nn and σ tn are the normal and tangential stress component, respectively, at the interface and X nn and X tn are the corresponding mode-1 and mode-2 interfacial strengths (see Table 4). A 97-percentile of each damage initiation indicator (as defined in [50]) will further be used to compare the local damage initiation indicator distribution between the different models. a) b) Figure 14: Generated mesh models for: (a) the non 3D woven model, (b) the 3D woven model.…”

“…A locally distorted zone in the surface weft yarns should then be adopted to account more properly for fiber volume fraction variations. Local fiber-reinforced distorted zone models similar to [50,51] for z-pinned and stitched laminates can therefore be adopted. The quantitative effect of these assumptions should be mechanically investigated and are assumed in this work to fall in the experimental range.…”

“…The discretised lines are shaped by straightening operations until an equilibrium between fiber-bundle movements is obtained, with a contact treatment accounting for the line interactions. This geometrybased framework was adopted later on to generate Z-pinned and stitched laminate unit-cell models [50,51]. Potential resin-rich regions and out-of-plane undulations in the laminae are hereby represented by initial straight discretised lines while the pin and stitching yarn are represented initially by a single discretised line.…”