2.2.1 Compression-to-tension method
Prepreg samples with the size of 50mm × 50mm are pasted onto the surface of the fixture through double-sided adhesive tape. The tack between prepreg and prepreg is expressed by prepreg / prepreg tack, and the tack between prepreg and other material is expressed by prepreg / other material. For the prepreg / prepreg tack test, two prepreg samples are pasted on the upper and lower surface of the fixture, respectively. And for the prepreg / metal tack test, only one prepreg sample is pasted on the lower surface of the fixture. The testing fixture consists of two square stainless steel plates, with a specification size of 50mm × 50mm. The test fixture is installed on the universal testing machine. The up and down movement of the fixture is controlled by the universal testing machine to conduct a compression and tension test. The experimental device and schematic diagram are shown in Fig. 1.
According to the stress state of the prepreg, the entire experimental process can be divided into three period: compression (loading), holding, and tension (unloading). In a standard test cycle, the compression rate is set to 1mm/min in the compression period, an the prepreg sample is compacted to a pressure of 20N, keeping the pressure constant for a duration of 30 seconds during the holding period. During the tension period, the tensile rate is 3mm/min. The load-displacement curve is recorded to study the changes of prepreg tack, as shown in Fig. 2. All the averages measured tensile strengths were obtained from five prepreg specimens for every test.
For the quantitative evaluation of prepreg tack under different materials and testing conditions, this paper proposes a parameter of Compression Tack Index (CTI), which describes the ratio of the output energy (i.e. tensile energy) of the prepreg stack during tensile unloading to the input energy (i.e. compressive energy) of the prepreg stack during compressive loading. It can be expressed by Eq. (1):
Where Wadh is the tensile energy and Wpre is the compressive energy. Wadh and Wpre can be expressed by Eq. (2) and Eq. (3), respectively:
In equations (2) and (3), x represents displacement, and f (x) represents tensile or compressive force.
According to Eq. (1), the larger the CTI, the greater the prepreg tack. In the previous studies[4–10], only considering separation force and separation energy without considering the contribution of compression energy during the compression process to prepreg tack. For example, the increase in separation energy caused by the large compression energy storage during the compression process does not necessarily indicate the high tack of the prepreg. Therefore, the introduction of CTI to quantitatively characterize and evaluate the prepreg tack can fully consider the two processes of tack formation and separation, and can more objectively and accurately characterize the prepreg tack.
2.2.2 Vertical metal plate method
The vertical metal plate method is a qualitative testing method for prepreg tack. Reference standard HB 7736.8–2004 (in China) [3], for the prepreg / prepreg tack test, one prepreg specimen with a specification size of 300mm × 300 mm is pasted to the center of the stainless steel plate, and the other specimen is laid in the direction of pasting perpendicular to former specimen, that is, the two specimens are pasted in 0 °/90 ° direction, with the front and back facing each other. Rubber rollers are used to roll out the air, and the adhesion and non-destructive separation of the two specimens are observed. The prepreg tack difference can be divided into five different levels from this test method. The first level has the best laying process for prepregs. When the prepreg tack is in the fourth level, the prepreg itself cannot stick to each other and cannot stick to stainless steel plates, indicating that the prepreg has lost its laying process. The tack between prepreg and steel plate is observed the separation after 1 hour of bonding between prepreg and steel plate.