DESIGN AND TEST OF A WITNESSING COUPON FOR FILAMENT-WOUND TOW PREG CFRP PARTS
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Georg SÖLLINGER (AUSTRIA), Matthias REDL (AUSTRIA), Matthias LECHNERAbstract :
Due to high complexity, expensive filament-wound CFRP products such as Composite-Overwrapped Pressure Vessels (COPVs) require appropriate witnessing coupons, such that in case of deviating manufacturing processes the part can be released for use by verifying the structural behavior of the witnessing coupon via destructive testing.
The requirements for the witnessing coupon are both to allow a similar manufacturing process by filament winding and autoclave curing, and to allow structural testing of both fiber- and resin-dominated properties.
To answer these requirements, a tubular coupon with a +/-45 degree cross-ply layup, filament-wound over a 50 mm stainless steel core was designed. A tensile test allows to determine the in-plane properties (0.2 % offset shear strength, shear modulus) in a similar way as standardized for flat coupons as per ASTM D3518. The ends of the tube are reinforced locally with hoop layers outside of the gage section, allowing homogeneous load introduction to the gage section similar as tabs for flat coupons, see Fig 1. Two mechanical locking assemblies clamp the tube during tensile testing and allow load transmission through friction, resulting in no need of any machining operation prior to test. Each tube is instrumented with a 3-grid strain gauge rosette applied in axial direction in the middle of the gage section.
The manufacturing takes place in the same winding robot configuration as for the product, allowing a manufacture of the witnessing coupon right before or after the product. Both the product and the witnessing coupon get cured in the same vacuum bag. Hence, the entire manufacturing process is equal for both parts.
Considering these aspects, the design of this coupon allows the verification of
> Winding angle (fiber-dominated)
> Temperature and consolidation during cure cycle (resin-dominated)
> Resin batch quality (resin-dominated)
A statistical basis is required for the verification in the serial production, which was determined by testing 13 coupons.The results show a similar stiffness (4.8 GPa vs. 4.4 GPa) and 0.2 % offset shear strength (A-Basis of 46.0 MPa vs. 46.7 MPa) compared with the resulting properties as per ASTM D3518 on flat coupons of the same material. This comparison shows that the as-designed coupon is also able to replace material tests on flat coupons, which are more difficult to produce and are not entirely representative of filament-wound parts.
During manufacture, the cure of one coupon needed to be repeated due to an abortion of subsequent cure cycle. This was caused by a vacuum loss in the early stage of the cure prior reaching the gel time due to a faulty vacuum bag. An evaluation of the shear modulus and the shear stress vs. shear strain curve showed that this coupon is significantly behaving out-of-family, see Tube 7 (red line) in Fig 2. This result confirms that the as-designed witnessing coupon is capable to reveal deviations of the curing cycle due to its structural response.
The requirements for the witnessing coupon are both to allow a similar manufacturing process by filament winding and autoclave curing, and to allow structural testing of both fiber- and resin-dominated properties.
To answer these requirements, a tubular coupon with a +/-45 degree cross-ply layup, filament-wound over a 50 mm stainless steel core was designed. A tensile test allows to determine the in-plane properties (0.2 % offset shear strength, shear modulus) in a similar way as standardized for flat coupons as per ASTM D3518. The ends of the tube are reinforced locally with hoop layers outside of the gage section, allowing homogeneous load introduction to the gage section similar as tabs for flat coupons, see Fig 1. Two mechanical locking assemblies clamp the tube during tensile testing and allow load transmission through friction, resulting in no need of any machining operation prior to test. Each tube is instrumented with a 3-grid strain gauge rosette applied in axial direction in the middle of the gage section.
The manufacturing takes place in the same winding robot configuration as for the product, allowing a manufacture of the witnessing coupon right before or after the product. Both the product and the witnessing coupon get cured in the same vacuum bag. Hence, the entire manufacturing process is equal for both parts.
Considering these aspects, the design of this coupon allows the verification of
> Winding angle (fiber-dominated)
> Temperature and consolidation during cure cycle (resin-dominated)
> Resin batch quality (resin-dominated)
A statistical basis is required for the verification in the serial production, which was determined by testing 13 coupons.The results show a similar stiffness (4.8 GPa vs. 4.4 GPa) and 0.2 % offset shear strength (A-Basis of 46.0 MPa vs. 46.7 MPa) compared with the resulting properties as per ASTM D3518 on flat coupons of the same material. This comparison shows that the as-designed coupon is also able to replace material tests on flat coupons, which are more difficult to produce and are not entirely representative of filament-wound parts.
During manufacture, the cure of one coupon needed to be repeated due to an abortion of subsequent cure cycle. This was caused by a vacuum loss in the early stage of the cure prior reaching the gel time due to a faulty vacuum bag. An evaluation of the shear modulus and the shear stress vs. shear strain curve showed that this coupon is significantly behaving out-of-family, see Tube 7 (red line) in Fig 2. This result confirms that the as-designed witnessing coupon is capable to reveal deviations of the curing cycle due to its structural response.