Determination of the unsaturated through‐thickness permeability of fibrous preforms based on flow front detection by ultrasound

Abstract

The permeability of fibrous preforms used as reinforcements for fiber‐reinforced polymeric composites is a decisive parameter for simulation‐based predictions of common defects that occur during the filling process. This study focuses on the transverse permeability which becomes important for high thickness parts. An instrument that detects the transverse flow front by ultrasound and determines the transverse permeability has been developed. In contrast to other approaches, the developed permeameter uses a nondeflecting (metallic) tool while the selected sensing method is nonintrusive to the material. Novel electromechanical features allow a wide range of measurable preforms. The remaining limitations of the system have been identified and the model that correlates raw ultrasound measurements with flow front has been verified by visual observations. The transverse permeabilities of 18 different preforms have been determined in an experimental scheme designed to isolate the influence of preform thickness. POLYM. COMPOS., 39:360–367, 2018. © 2016 Society of Plastics Engineers

Identification of the effect of typical curvatures encountered in RTM parts on localized permeability of fibrous preforms

“…repeating the exact same experiment three times) which normalized scatter and (b) focusing on higher FVCs during the experimental determination of draped permeability (~47 to ~63%) 10 and 11 focus on permeabilities (along the x-and y-axis, respectively) vs. FVC. To enable a comparison, the following quantities have been plotted: the planar permeability values for the RTM part (K i_p_RTM ), the theoretical approximation of the draped permeability values (K i_d_the ), the analytically calculated draped permeabilities (K i_d_cal ) (applicable only to K x due to restrictions of the model of equation (5)) and the draped permeabilities derived by numerical optimization (K i_d_opt ). Finally, the flow time increase along the x-axis relative to the theoretical approximation (t inc ) vs. FVC was plotted in order to provide a clear view of the quantitative benefit of the methods of this study (Fig.…”

“…tow shifting) may take place if injection pressure is higher than the compaction pressure. 5 Naturally, areas closer to the inlet where injection pressure is more significant are more probable to suffer from this deformation. More limitations (often contradicting) were finally involved in the selection of the injection pressure: injection pressure has to be low enough to avoid significant preform deformation/sensor shifting phenomena, high enough to keep gravitational effects negligible/allow unproblematic injection even at high FVC and constant for all preforms (a validity condition of the developed ANN).…”

“…To 5are met while all parameters are now known, allowing the analytical calculation of the x-axis permeability in draped zones. As discussed in Section Theoretical background, equation (5) is applicable only when the flow times correspond to flow along the i-axis. In this case, Δt i,d is measurable only along the x-axis therefore equation (5) can be used to calculate K x and not K y .…”

“…As discussed in Section Theoretical background, equation (5) is applicable only when the flow times correspond to flow along the i-axis. In this case, Δt i,d is measurable only along the x-axis therefore equation (5) can be used to calculate K x and not K y . Moreover, for the same reason the calculation can be achieved only for the sensor pair 1.1 and 1.2.…”

“…Their operation is based on flow front detection which leads to permeability by a forward calculation of existing flow models. [5][6][7] Alternatively, flow measurements can be utilized by an inverse method; an iterative optimization process where the difference between flow measurements and simulation outputs is minimized by redefining permeability. 8,9 Despite the fact that a realistic part includes curvatures and areas of draping, past studies that are using the forward or inverse method focus mainly on in-plane flow behavior.…”