Abstract
The polyethylene terephthalate(PET)FRP is a new type of FRP composite.It can be made from the waste plastic bottles,which is environmentally friendly.Owing to the fact that this new material has a tensile rupture strain of more than 5%,it is also called the largerupture-strain(LRS)FRP composite.Besides,it has a bilinear stress-strain relationship,which is different from the linear stress-strain relationship exhibited by the conventional FRP(carbon FRP,glass FRP,aramid FRP and basalt FRP).When this material was in the initial linear stress-strain portion,it could provide the strength enhancement for the reinforced concrete(RC)structure with the relative large elastic modulus.When it was in the second linear stress-strain portion,it has a great energy absorption capacity with a relative small elastic modulus and a large tensile strain.Therefore,some studies have reported to date on the seismic strengthening of the RC structure with the LRS FRP as the external jackets[1-8].Nevertheless,the RC structures suffer not only the seismic loading but also the impact loading such as the vehicle impact in the bridge pier during their service life[9].The investigations of the impact resistance of the RC structure strengthened with FRP are necessary for the application of the PET FRP in the impact-resistant strengthening of the RC structure.
Considering the fact that the FRP composite mainly bears the tension in the strengthening of the RC structure and the fiber bundle is the main load-carrying element in FRP composites.Therefore,it is of vital importance to investigate the dynamic tensile mechanical properties of the PET fiber bundle.
In this study,a total of 20 PET fiber bundle specimens with a gauge length of 25 mm(Figure 1)were tested at the displacement rate of 1,2,3 and 4 m/s using an Instron drop-weight impact system,as shown in Figure 2(a).The corresponding strain rates were calculated with the value of 40,80,120 and 160 s-1 according to the definition of the strain rate that is the ratio of the displacement rate to the corresponding gauge length of the specimen.For the comparison purpose,5 specimens were tested at the displacement rate of 2.5 mm/min with the corresponding strain rate of 1/600 s-1 using an MTS testing machine,as shown in Figure 2(b).
Figure 1 Schematic diagram of the PET fiber bundle
Figure 2 Testing machine
Figure 3 shows the failure modes of the PET fiber bundle specimens at the strain rate of 1/600(quasi-static loading)and 160 s-1(dynamic loading).As can be seen in the figure,the fiber bundle failed in a chaotic manner under quasi-static loading whereas the failure locations of the filaments were relatively concentrated with a trim fractography.This phenomenon might be explained as follows.Under the quasi-static loading,the defects of the filament could be fully developed,followed by the successive rupture of the filament.Under the dynamic loading,there was no enough time for the defects to develop,resulting in the failure of the fiber bundle as a whole at the weakest part.
Figure 3 Failure modes at different strain rate
Figure 4 shows the stress-strain curves at the strain rate of 1/600 and 160 s-1.The curves exhibit a bilinear stress-strain relationship,initialing with a linear elastic stage.Then,the curve went to the second linear stage until the peak of the curve with the slope of the second linear portion smaller than that of the former.After the peak of the curve,the curve had a drop from peak to zero,which shows a brittle failure of the PET fiber bundle.As can be seen in the figure,the descending portions of the curve under the quasistatic and dynamic loading were different from each other:the former had a relative slow descending portion whereas the latter had a sharp one.This is due to the fact that the successive failure of the filament could be well recorded under quasi-static loading whereas under the dynamic loading,it is difficult to be recorded due to the sudden rupture of the specimen.
Figure 4 Stress-strain curves at different strain rate
From the stress-strain curve in Figure 4,the tensile strength,failure strain,elastic modulus and toughness can be obtained,whose average values are listed in Table 1.The tensile strength is the peak stress of the curve and the failure strain is the strain corresponding to the peak stress.When the strain rate increased from 1/600 to 160 s-1,the tensile strength had an increase from 708 MPa to 941 MPa whereas the failure strain decreased from 13.0%to 9.11%.The elastic modulus has two values,including the initial elastic(E1)and second elastic modulus(E2).The former is defined as the slope of the initial linear elastic portion,and the latter is defined as the slope of the second linear portion.The initial and second elastic modulus increased from 11.0 GPa and 5.83 GPa to 17.3 GPa and 10.8 GPa,respectively,with an increase in the strain rate from 1/600 to 160 s-1.The toughness is defined as the area enclosed by the stress-strain curve and strain axial.An increase in the strain rate from 1/600 to 160 s-1 led to a decrease in the toughness from 57.1 MPa to 46.7 MPa.
Table 1 Dynamic mechanical properties at different strain rates
