Assessment of the impact performance of repaired thermoplastic laminates
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Luigi SORRENTINO (ITALY), Giuseppe LAMA , Letizia VERDOLOTTI , Maria OLIVIERO , Fabienne TOUCHARD (FRANCE), David MELLIER (FRANCE), Paweł MALINOWSKI (POLAND)Abstract :
Thermoplastic composites with 45% by volume of fibres were prepared by using poly(lactic acid) as matrix and basalt woven fabrics as reinforcement. The effects of a repairing process of laminates after non perforating single or consecutive impact events was investigated. The impact energy ranged from 1 J to 16 J, and the number of impacts on the same target position ranged from 1 to 4.
A difference was detected between the first impact of a series and the following ones. The former was characterized by a lower value of the peak force, and more gradual increase of the response. The latter showed a higher peak and a steeper rise of the force but the responses of consecutive impacts were almost superimposed. The laminates showed some energy dissipation after the first impact at all energy levels, while only at 8 J successive impacts showed additional dissipation phenomena.
A selection of samples was first repaired by means of a hot plate press and then impacted again on the previously targeted positions. Scanning laser Doppler vibrometry (SLVD) and microtomography (microCT) were used to assess the laminate damaging. Two repairing temperatures were used, one above the glass transition temperature (100 °C) and the other above the melting temperature of PLA (175 °C). The laminate planarity was recovered after the treatments at both temperatures, but visible signs of damaging (after the first impact series) were erased only after the melting of the matrix at 175 °C (Figure 1). The SLDV analysis was very effective in detecting damages occurring in the solid phase, and its integration with microCT allowed to correctly discriminate damages occurred to the matrix from those occurred to fibres. Additonally, SLDV results showed that a relatively low signal frequency suffices for making the inspection without the need for megahertz range of traditional ultrasonic wave-based approaches. Both SLDV and microCT analyses (Figure 2) confirmed that the use of a temperature higher than the matrix melting point successfully restored the matrix continuity and the woven fabric planarity. This was fostered by the use of a thermoplastic matrix for the composite, which exploited polymer melting to almost completely recover all damages occurring in the matrix and at matrix/fibres interface.
A second campaign of multiple impacts performed on repaired laminates on the same previously impacted points confirmed the effectiveness of the repairing process. In fact, both indentations and force peaks of the laminates repaired above melting temperature were almost superimposed to those showed by pristine laminates, demonstrating a complete recovery of the impact performance.
A difference was detected between the first impact of a series and the following ones. The former was characterized by a lower value of the peak force, and more gradual increase of the response. The latter showed a higher peak and a steeper rise of the force but the responses of consecutive impacts were almost superimposed. The laminates showed some energy dissipation after the first impact at all energy levels, while only at 8 J successive impacts showed additional dissipation phenomena.
A selection of samples was first repaired by means of a hot plate press and then impacted again on the previously targeted positions. Scanning laser Doppler vibrometry (SLVD) and microtomography (microCT) were used to assess the laminate damaging. Two repairing temperatures were used, one above the glass transition temperature (100 °C) and the other above the melting temperature of PLA (175 °C). The laminate planarity was recovered after the treatments at both temperatures, but visible signs of damaging (after the first impact series) were erased only after the melting of the matrix at 175 °C (Figure 1). The SLDV analysis was very effective in detecting damages occurring in the solid phase, and its integration with microCT allowed to correctly discriminate damages occurred to the matrix from those occurred to fibres. Additonally, SLDV results showed that a relatively low signal frequency suffices for making the inspection without the need for megahertz range of traditional ultrasonic wave-based approaches. Both SLDV and microCT analyses (Figure 2) confirmed that the use of a temperature higher than the matrix melting point successfully restored the matrix continuity and the woven fabric planarity. This was fostered by the use of a thermoplastic matrix for the composite, which exploited polymer melting to almost completely recover all damages occurring in the matrix and at matrix/fibres interface.
A second campaign of multiple impacts performed on repaired laminates on the same previously impacted points confirmed the effectiveness of the repairing process. In fact, both indentations and force peaks of the laminates repaired above melting temperature were almost superimposed to those showed by pristine laminates, demonstrating a complete recovery of the impact performance.