# Experimental Analysis of a Novel GFRP Stiffened Pipe
## 4 Results and Discussion
### Failure Mode  
#### 4.3.1 Plain Pipes Compared with the Outer Stiffened Pipes  
The damaged areas of loaded stiffened specimens with diameter 400 mm at different stages of 3%, 15%, 20%, and 30% diametric deflections are shown in Figure 14. As the test continued, the failure happened in the outer layer, characterized by fibers broken at 90\({}^{\circ}\) points, and the applied compressive load on the specimens reached 15% diametric deflection (as shown in Figure 14b). By increasing the applied compressive load, the outer fiber continuously fractured until the test was terminated, and the applied compressive load reached more than 30% diametric deflection (as shown in Figure 14d). As illustrated in Figure 14d, the inner structural layer was damaged and the correspondingly applied compressive force was measured as 19,621 N. Although according to ASTMD2412-2002 [41], the tests should have terminated when the applied compressive load on the specimens reached 20% diametric deflection, it was observed that the bearing load continued to increase as the increase of diameter deflection reached 30%, and large areas of inner structural layer breakage happened near 90\({}^{\circ}\).  
Figure 14: Deformation of outer stiffened pipes during failure phase: (a) 3% diametric deflection; (b) 15% diametric deflection; (c) 20% diametric deflection; (d) 30% diametric deflection.  
Figure 15 shows the damage areas of the plain pipe specimens during compression tests at different stages of 5%, 15%, 20%, and 30% diametric deflection. During the experiment, no visible evidence of failure was observed but there were sounds of slight fractures before 15% diametric deflection was reached, with delamination failure first occurring at the upper loading part (as shown in Figure 15b). With the increase of applied compressive load, delamination failure appeared in the bottom loading part and the areas continued to expand (as shown in Figure 15c). When the applied compressive load was almost 30% of the diametric deflection, all layers separated from each other immediately, the test was terminated with a loud bang from the tested specimen, and it was observed that the strain gauges at points of 45\({}^{\circ}\) and 90\({}^{\circ}\) had fallen off (as shown in Figure 15d).  
As the outer stiffened layer was supported by the inner structural layer, and was full of empty spaces due to the spacing arrangement windings, the outer fibers receiving the greatest force were ductile fractured when subjected to the compressive load. The crush failure of the stiffened pipe happened in the outer web, different from the delamination failure observed for the plain pipe, indicating a full use of the maximum load capacity of the proposed pipe. Furthermore, one should note that the cost of the two types of pipes was almost the same (the pipes of different types used the same weight of fiber and resin), while the stiffnesses of the stiffened pipes increased by an average of 38.7%, indicating great potential for cost reduction while achieving the same pipe stiffness.  
Figure 15: Deformation of plain pipes during failure phase: (**a**) 3% diametric deflection; (**b**) 15% diametric deflection; (**c**) 20% diametric deflection; (**d**) 30% diametric deflection.