Creep behaviour of natural fibre reinforced unsaturated polyester composites

1. Creep behaviour of natural fibre reinforced unsaturated polyester composites Department of Mechanical and Design Engineering PO1 3DJ, U.K. H N Dhakal Z Y Zhang M O W Richardson

2. Where is Portsmouth? Located in Southern England

3. Advanced polymer and composites (APC) research group http://www.port.ac.uk/composites

4. Outline • Introduction • Objectives • Experimentation • Results and discussion • Creep behaviour at low stress levels and low temperature • Creep behaviour at high stress levels and high temperature • Conclusions

5. Introduction A comprehensive study on the applications of thermoset and thermoplastic matrices relative to natural fibre based products is supported on the many distinctive aspects of natural fibre composites by number of publications and reviews (Bledzki et al. 2002, Joseph et al. 1999, Saheb and Jog 1999, Mohanty et al. 2001, Robson et al. 1993). Thus, it is well documented that natural fibre reinforced composites is an emerging alternative to synthetic fibres as a reinforcement in composite materials.

6. Objectives • To fabricate unsaturated polyester composites reinforced with hemp fibre • To investigate the effect of hemp reinforcement on creep behaviour

7. Some advantages of NFC • Natural plant fibres are renewable resources • They are inexpensive • They have high specific properties compared to conventional fibres • They have good reinforcement effects in polymer composites • Important materials for sustainable composites • The effect of various levels of reinforcement of hemp fibre on creep behaviour of UPE/hemp composites were investigated

8. Typical reported tensile properties Table: Typical reported tensile properties of important plant fibre composites. (MA) was used as compatibilising agent, Silane treated, for comparison

9. Materials Matrix: Unsaturated polyester • Cheap • Low viscosity • Low shrinkage • Good mechanical properties Reinforcement: Hemp

10. Applications of natural fibres/composites • Transport • Construction (insulating materials) • Packaging Plant fibres applications in the Mercedes-Benz E-class (Gayer and Suhuh 1996) [1] Aerospace industries: a potential use Door inner trim panels moulded using mats of 60 wt% natural fibre in a Baypreg polyurethane resin

11. Sample fabrication process

12. Creep testing Creep is the increase in the strain or deformation of a material with time when a material is subjected to a constant load for an extended period of time. The creep and recovery behaviour of the hemp fibre reinforced samples was evaluated using a TA instrument Q 800 Dynamic Mechanical Analyser (DMA) in a three-point bending mode with a constant span of 48 mm.

13. Creep testing A schematic diagram of three-point bending mode and creep test configuration of a DMA creep test The time period used for the creep test was kept constant at 5 hours and was followed by 3 hours recovery. The specimens were placed onto the bottom support and the top probe was adjusted to just contact the specimen. The oven was set to the desired temperatures (25 oC and 50 oC)

14. Creep model Combination of spring and viscous dashpot elements can be represented to model the time dependant, viscoelastic behaviour of polymeric materials. Among the numerous available viscoelastic creep models, the four-element Burger body as shown is applicable for this study.

15. Creep results for 5 MPa at 25 °C The strain/time curves for these specimens are found to have a typical form showing an instantaneous elastic strain on loading followed by a period of slow linear deformation. The deformation, following application of constant stress, can be divided into instantaneous deformation and creep.

16. Creep results for 5 MPa at 25 °C At the start point, the sudden increase in strain reflects the existence of constant elasticity. Likewise the recovery can be divided into instantaneous contraction and creep recovery. After unloading the sample, a constant deformation remains, which indicates that the instantaneous deformation consists of both plastic and elastic deformation. The recovery curves of these composites show an instantaneous contraction (by an amount equal to the initial elastic strain) followed by a period of slow recovery after unloading.

17. Creep results for 15 MPa at 25 °C The creep strain of the hemp fibre composites and its polyester matrix increases with increase in stress. As can be seen, the increased stress has an effect on the overall creep behaviour.

18. Creep results for 5 MPa at 50 °C As can be seen, creep strain significantly increases with increase in temperature. The strain for UPE sample increases almost 10 times compared to the same load at a temperature of 25 °C. It can be seen from figure above that the higher the temperature the greater the creep elongation and creep rate.

19. Creep results for 5 MPa at 50 °C When the temperature increases from 25 °C to 50 °C (stress level 5 MPa) the deformation response increases for all samples. It is evident from the figure above that the deformation response is larger at higher temperature for unreinforced samples compared to hemp-reinforced samples.

20. Creep results for 15 MPa at 50 °C It can be seen from this figure that the strain rate increases with the increase in temperature. An increase in stress from 5 to 15 MPa at 50 °C causes an increase in both the initial compliance and the height of the creep curve.

21. Creep behaviour 1. Instantaneous deformation: mainly elastic 2. Primary/transient: Slope of strain vs. time decreases with time 3. Secondary/steady state creep: rate of straining is constant 4. Tertiary: rapidly accelerating strain rate up to failure

22. Creep behaviour With increasing stress of temperature: The instantaneous strain increases The steady-state creep rate increases The time to rupture decreases

23. Creep behaviour Stages of creep.

24. Creep mechanisms Different mechanisms are responsible for creep in different materials and under different loading and temperature conditions. The mechanisms include if it is metal: Stress-assisted vacancy diffusion Grain boundary diffusion Grain boundary sliding Dislocation motion

25. Conclusions • Short-term flexural creep behaviour of hemp fibre reinforced unsaturated polyester composites have been successfully investigated. • Creep strain is higher with unreinforced polyester specimens than with hemp fibre reinforced specimens. • The results show that by incorporation of hemp fibre by increments of volume fraction, the creep resistance of the composites is significantly enhanced. • The creep test results show that the creep behaviour of 3 layered hemp reinforced specimen (15% fibre volume) at a temperature of 25 °C is comparable to the creep behaviour of 21% fibre volume chopped strand mat E-glass reinforced specimens.