Study Of The Dry Compaction Behavior Of Unidirectional Warp Knitted Fabric For Wind Turbine Blades

Unidirectional composites are mainly used in wind turbine blade spar caps, due to the characteristics of vacuum infusion molding process, the thickness control becomes very difficult.In the vacuum infusion molding process,the preform is firstly compressed in the dry condition. The thickness control of the composite material has an important relationship with the preform’s dry compression characteristics. It’s necessary to study the dry compression characteristics of the unidirectional warp knitted fabric.This paper selects four commonly used unidirectional glass fiber fabric on the market. Under the vacuum bag compression process the impact of pressure, number of layers, loading cycles and holding time on the dry compression characteristics of the unidirectional warp knitted fabric were studied. Then,use the resin fixed the fabric geometry in the different compression stages,and analysis the microscopic deformation of the perform in the different compression stages.In order to study the difference between the vacuum bag compression process and the flat compression process,the thickness of the fabric in the two processes were collected. By comparing the results of the two groups, respectively, from the the force and the thickness trend of the fabric in compression process to explain why there are differences in the two processes.Then chose the vacuum bag compression process to study the dry compression characteristics of the uniaxial warp knit fabric.In order to better evaluate the dry compression characteristics of the uniaxial warp knit fabric.This paper, except using the fiber volume fraction, than the average thickness of a single layer of fabric and compression rate, also defined the different stages of the turning point: the critical point, and the compression initial angle indicating the degree of difficulty of compressing of the perform.This paper, use the same ply angle and the same number of layers of the four unidirectional glass fiber fabric, analysis the different thickness under the different pressure; Study the effect of the number of layers on the dry compression characteristics of the fabric, the number of layers was seted as one layer, four layer, eight layer, twelve layer and twenty layer,then found that with theincreasing of layers,the fabric fiber volume fraction increased gradually, and the average layer thickness gradually reduced, however, it tends to be stable up to a four number.Study the thickness variation of the fabric under the first cycle, the second cycle, the third cycle, the fifth cycle and the tenth cycle. Then found that when increasing the number of loading cycles can increase the volume fraction of fibers of the fabric, and reducing the average layer thickness of the fabric;Study the thickness variation when the holding time is zero minutes, two minutes, five minutes,ten minutes and twenty minutes. Then found that with the increasing of the holding time, the fabric of fiber volume fraction increased gradually, and the average layer thickness gradually reduced, when the holding time reach 10 minutes, the thickness of the fabric is not changed.For a more intuitive understanding of the deformation of preform in the process of compression, this paper use resin to fix the fabric’s status under the different pressure, then obtained the cross-section under the different pressure, observing the variation of tow shape,length, thickness and the gap area. This will provide a basis for statistical data analysis for more intuitive fabric compression process. Then found that the tendency of the shape of the yarn bundle with pressure increasing tendency from the semi-oval to track shape to rectangular; the length of the yarn bundle with little change in pressure increases; The thickness of the yarn bundle and the void area between the beams gradually decreases as the pressure increases.