A study was conducted to evaluate the adhesion of Carbon and Kevlar reinforcements applied to PETG sheets. This material is commonly used in the manufacturing of orthopedic braces; however, it presents several critical issues, primarily related to durability and comfort. These braces often experience premature failures, which compromise the longevity of the device. The use of reinforcement patches could, on one hand, improve resistance to early damage, and on the other, enhance user comfort by allowing the use of thinner PETG sheets. This is made possible through the localized application of reinforcements in the most critical areas.

Goal: verify that the adhesive DP490 employed to attach Carbon and Kevlar reinforcement patches onto PETG beams endures repeated cyclic loading, thereby confirming its suitability for everyday use in orthopedic brace applications.

Method: to evaluate the performance of the adhesive used to bond the reinforced patches onto the PETG specimens, a durability test under force control was carried out through cyclic loading using a three-point bending (3PB) setup. The analysis was performed by G. Francabandiera under the supervision of B. Rossetti, using the Litem BYC 200 machine available at the Department of Civil Engineering and Architecture (DICAR) at the University of Pavia. The research activity was coordinated by G. Alaimo, S. Marconi, and F. Auricchio. Two specimens were tested: the first consisted of a 3 mm thick PETG sample with two-layer carbon fiber reinforcement patches bonded to both ends (resulting in a total thickness of 5.25 mm), and the second consisted of a 3 mm thick PETG sample with two-layer Kevlar reinforcement patches bonded to both ends (resulting in a total thickness of 4.8 mm), both specimens were 130 mm long and the adhesive was left to cure for 16 hours. The specimens were clamped at both ends and at the center, with the central clamp acting as the movable grip.

Figure 1: tested samples.

Figure 2: left: Carbon-reinforced sample test set-up; right: Kevlar-reinforced sample test set-up

The specimens were subjected to a maximum number of 50’000 loading cycles at a frequency of 5 Hz. As a preliminary analysis, to determine the range of maximum and minimum forces for the test, three-point bending displacement controlled tests were conducted on similar specimens (figure 3 and 4).

Figure 4: 3PB test load versus displacement results for the Kevlar-reinforced specimens.

The maximum and minimum loads of the cycle were chosen to be equal to half of the force required to reach the lowest yield strength among the materials constituting the laminate, in order to simulate realistic yet non-destructive operating conditions. Following these considerations, the maximum and minimum forces for the specimen reinforced with Carbon fiber patches were found to be ±110.5 N, and for those reinforced with Kevlar patches ±61.6 N.

Results: an analysis of the maximum and minimum displacement as a function of the applied loading cycles showed that these values remained constant throughout the entire test. This outcome confirms the durability of both the materials and the bonding system under cyclic loading conditions.

Figure 5: top: displacement versus number of cycles for the Carbon-reinforced specimen; bottom: load versus number of cycles for the Carbon-reinforced specimen.

Figure 6: top: displacement versus number of cycles for the Kevlar-reinforced specimen; bottom: load versus number of cycles for the Kevlar-reinforced specimen.