Process Modeling of Thermoset Composites used for Wind Blade Manufacturing

A computational process modeling framework, informed by accurate material characterization, is presented for virtual manufacturing of wind energy thermoset composites. Process modeling simulations of composite microstructures are carried out to predict in-situ matrix property evolution and performance-altering residual stress generation. To achieve this, comprehensive material characterization effort is carried out. A novel material property dataset for a widely-used wind energy thermoset system is generated as a function of the temperature and curing. Informed by these material properties, the ability of the process model to reliably estimate manufacturing-induced residual stresses is highlighted. For a prescribed cure cycle, in-situ elastic modulus evolution, chemical and thermal strains, and random fiber distribution are shown to significantly influence residual stress generation. The results also show that a full process modeling analysis that includes the complete cure cycle (instead of the standard approach of just considering post-processing cool-down) is necessary to accurately predict manufacturing-induced residual stresses.

OPTIMIZATION OF PROCESS PARAMETERS DURING CO-CURE OF HONEYCOMB SANDWICH STRUCTURES

Honeycomb sandwich structures are co-cured to bond partially cured thermoset prepreg facesheets with an adhesive layer to both sides of the honeycomb core under a pre-defined pressure and temperature cycle in an autoclave. High dependency of the co-cure process on the materials and process parameters makes it susceptible to defect such as poorly consolidated facesheet and highly porous bondline which can cause premature failure of the structure. The temperature and pressure in the autoclave and pressure in the vacuum bag are the parameters that describe the cure cycle of the process. In this work, an optimization of the process cycle for the co-cure process of sandwich structures that maximizes the fiber volume fraction within the prepreg and reduces the porosity is presented. The objective function is constructed to reflect the quality of both the facesheet consolidation and bond-line porosity. The Surrogate Optimization Algorithm is employed to find the cure cycle resulting in the highest facesheet consolidation level and the lowest porosity within the bond-line.

Joining Titanium by Means of Ceramic Adhesives

Ceramic adhesives are an interesting alternative to traditional methods to join metal to ceramics such as fastening, vacuum brazing and gluing. Ceramic adhesives are made of an inorganic matrix with a filler (alumina, zirconia, silica, etc.), and they require a thermal cure cycle in order to establish adhesion. In this work, the adhesion between two different adhesive and Ti6Al4V is studied in details and the influence of the curing cycle is analyzed. Two different adhesives have been used, the first made of a phosphate matrix with an alumina filler, the second made of a silicate matrix wit an alumina filler. The results indicates that in the case of the first adhesive a high temperature cure it is necessary in order to establish a strong adhesion with the metal; on the contrary the second adhesive is capable to create a strong bonding already at low temperature.

Cure cycle modification for efficient vacuum bag only prepreg process

Over the past decade, increasing use of continuous fiber-reinforced polymer composites has created a demand for manufacturing methods with lower costs, higher production rates, and improved processing efficiency. To meet the growing demands, vacuum bag only (VBO) prepreg processing has been proposed and implemented in industrial settings. However, in the absence of high consolidation pressure, VBO prepreg must undergo compaction for longer durations during cure and requires use of more elaborate processing schemes to conform to complex geometries. The main objective of our cure cycle modification was to reduce overall manufacturing time for more efficient processing, while maintaining robust part quality. This study demonstrates the effect of cure cycle on formability and part quality of three complex-shaped composite structures, a bulkhead, fuselage, and I section frame, featuring drop-offs, corners and sandwich areas consisting of less than 10 plies. Three different cure cycles were chosen: Reference cure cycle 1, Modified I and Modified II cure cycle. The reference was modified based on resin cure kinetics/viscosity modeling results and “effective flow number” to shorten the overall cure cycle time while maintaining robust part quality. To compare the quality of manufactured parts, destructive test and digestion method were used. For the bulkhead parts, Modified I was proven to be more effective in meeting the commercially acceptable part criteria (void content, ply wrinkle, resin ridge, and surface resin starvation), whereas Reference failed to meet the requirements, showing pervasive presence of porosity in drop-offs, corners and sandwich areas. The fuselage and I section frame parts produced with Modified I and Modified II were shown to meet the part quality requirements, with slight improvements in surface quality observed with the Modified II method.

Epoxy Based Blends for Additive Manufacturing by Liquid Crystal Display (LCD) Printing: The Effect of Blending and Dual Curing on Daylight Curable Resins

Epoxy-based blends printable in a Liquid Crystal Display (LCD) printer were studied. Diglycidyl ether of bisphenol A (DGEBA) mixed with Diethyltoluene diamine (DETDA) was used due to the easy processing in liquid form at room temperature and slower reactivity until heated over 150 ° C. The DGEBA/DETDA resin was mixed with a commercial daylight photocurable resin used for LCD screen 3D printing. Calorimetric, dynamic mechanical and rheology testing were carried out on the resulting blends. The daylight resins showed to be thermally curable. Resin’s processability in the LCD printer was evaluated for all the blends by rheology and by 3D printing trials. The best printing conditions were determined by a speed cure test. The use of a thermal post-curing cycle after the standard photocuring in the LCD printer enhanced the glass transition temperature T g of the daylight resin from 45 to 137 ° C when post-curing temperatures up to 180 ° C were used. The T g reached a value of 174 ° C mixing 50 wt% of DGEBA/DETDA resin with the photocurable resin when high temperature cure cycle was used.

Cure-induced residual stresses for warpage reduction in thermoset laminates

The paper addresses the role played by the cure stage of a vacuum assisted resin transfer molding process in residual stresses generation. The Airstone 780E epoxy resin and Hardener 785H system broadly used in the wind turbine blade industry has been used in this study. The viscous–elastic properties of the resin have been characterized and implemented in a thermo-mechanical FE model. The model has been validated against manufactured [0/90]4 asymmetric laminates. Analysis of residual stresses generation highlighted that compressive stresses generation occurs when the cure is shrinkage dominated and tensile stresses when expansion dominated in the 0° plies. The finding points out that 10% reduction in warpage and 33% reduction in process time can be obtained by selecting cure cycle parameters that allow tensile stresses development during the cure process in the 0° plies.