Impact-Fatigue of Self-Reinforced Polyethylene Terephthalate

1. Introduction

In self-reinforced polymers, the polymer matrix is reinforced with high-tenacity fibres or tapes from the same polymer family [1]. These materials are emerging as environmentally competitive alternatives [1], as the energy required to produce raw materials is lower compared to composites reinforced with glass or carbon fibres [2,3]. Additionally, the thermoplastic nature of both the matrix and fibres facilitates efficient mechanical recycling. Moreover, the stamping of these composites, combined with the over-moulding of ribbed structures or other functional elements, makes them highly suitable for productive sectors such as the automotive industry.

Self-reinforced PET (srPET) stands out compared to other self-reinforced composite materials, such as polypropylene (srPP), due to its wider range of service temperatures and its more developed recycling system. The quasi-static properties [4], creep [5], and fatigue [6] properties of srPET have been reported in the literature, but there is a significant gap regarding its impact behaviour. Carbon and glass fibres can be damaged under low energy impact loads and, consequently, the residual properties of the composites are reduced [7], whereas thermoplastic fibre composites are less sensitive to damage due to their plasticity [8].

This paper aims to elucidate the repeated low-velocity impact properties of srPET composites. A systematic low-velocity impact testing program using an instrumented drop weight was conducted. The characteristic peak load, absorbed energy, and permanent deflection were measured to evaluate the impact performance and damage resistance.

2. Materials and testing

2.1. Materials

The self-reinforced PET composite investigated here (supplied by Comfil ApS) is a balanced sheet made of 2/2 twill fabric. The fibres are made of high-tenacity PET with a melting point of 260 °C, while the matrix consists of an amorphous chemically modified PET with a melting point of 170°C [9]. The samples used for the falling weight impact tests were square, measuring 100 × 100 mm, with a thickness of 1.1 mm.

2.2 Impact characterisation

Low-velocity impact tests were conducted using a falling weight machine (Fractovis-Plus, Ceast) equipped with a 20 kN load cell attached to a 20 mm diameter hemispherical striker, which recorded the contact force history. The absorbed energy-time curves were calculated by integrating these force/time curves. The laminates were clamped into an annular ring with an inner diameter of 40 mm, and immediately after impact, the striker was caught by a pneumatic clamp to prevent rebound. By releasing the 2.045 kg striker from heights ranging from 100 to 1000 mm, an impact energy range of 2 J to 20 J was achieved. All tests were carried out at room temperature, with a minimum of three samples tested for each impact energy. Based on the damage threshold energy, also known as critical energy, impact events were categorised into two main types; subcritical for values below this threshold and supercritical for those above. For all energy levels, laminates were subjected to up to one hundred impacts, or fewer if perforation occurred.

3. Results and discussions

3.1. Single- impact test

Single low-velocity falling weight impact tests were conducted to understand the impact response of the composite samples and to select the impact energies for the repeated impact study. It is noteworthy that, for any given energy, the experimental data were quite repeatable, and therefore, average data are considered in the following graphs. Figure 1 shows three representative load–time curves from three impact tests. The lower energy impacts (2 J) are subcritical in nature. The impacts carried out with intermediate energies (12 J) are supercritical, as confirmed by the permanent central deflection. The rounded shape of the impact curve and the absence of sudden load drops suggest that plastic deformation of the tape, rather than cracking, delamination, or fibre breakage, is the cause of this deflection. The impact curve of the test with complete perforation (15 J) shows a sharp load peak associated with the failure of the tape.

Figure 1. Load-time representative impact curves.

The energy plot, as shown in Figure 2, includes the dissipated energy (E_dis) curve and the 1:1 line of available incident kinetic energy (E₀). Dissipated energy increases with impact energy up to the perforation threshold, which is identified when the absorbed energy first equals the impact energy (13.9 J). Note that the last two data points correspond to complete laminate perforation, where the impact energy exceeds the absorbed energy, and the excess energy is retained in the striker for post-perforation motion.

Figure 2. Energy plot of srPET.

3.2. Repeated impact behaviour

The number of impact events before failure, depending on whether the incident energy is subcritical or supercritical, provides fundamental information for characterizing the impact-fatigue behaviour of the composites. To compare the performance of srPET with a well-established self-reinforce polypropylene composite (srPP), we use the ratio between the incident energy and the perforation threshold of each material, rather than the incident energy alone. The data for srPET are original to this paper, while the data for srPP have been taken from one of our previous works [10]. The first significant result was that srPET could withstand more than 100 impacts up to 60% of its perforation threshold, whereas srPP could endure 100 impact events only if the incident energy was below its perforation limit. Additionally, the drop in srPET was more gradual compared to srPP, which experienced a sudden drop.

Figure 3. Impact-fatigue curves for srPET and srPP (data for srPP from [10]).

Figure 4 illustrates the evolution of load-time curves from repeated impact experiments at 11.5 J. Up to the fourth impact, the curves retained a rounded shape, suggesting that the primary damage mechanism was the plastic deformation of both fibres and matrix. However, from the fifth impact onward, the curves transitioned from a rounded shape to a load drop after the peak, indicating a shift from plastic deformation to fibre breakage, which ultimately resulted in specimen perforation.

Figure 4. Evolution of load-time curves with increasing number of impact events at 11.5 J.

4. Conclusions

The results of the impact characterization, covering the range of incident energies from subcritical to perforation, show that the main deformation mechanism is plastic deformation followed by tensile failure of the PET fibres. The penetration energy threshold for a 1.1 mm thick specimen is 13.9 J. In single impact scenarios, srPET has a specific penetration threshold of 3.24 J/g, whereas srPP has a higher threshold (5.49 J/g) [10], indicating higher energy expenditure for srPET. However, if the environmental goal is to reduce waste volume, srPET is a better option since PET has a recycling fraction of 18.2%, compared to 2.7% for PP [11]. Additionally, srPET guarantees a lifespan of 100 impacts for incident energies up to 60% of its penetration threshold, while srPP cannot exceed 40%. Furthermore, the fatigue life loss of srPET is more gradual, which is a key aspect from the perspective of structural integrity.

Acknowledgment

The authors thank the Basque Government for providing financial support (IT883-16) for this study.

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