Experimental Setup. Experiments are performed on
three types of bent steel pipes with the outer diameter of 70 mm,
pipes length of 3000 mm, wall thickness of 3.5 mm, bends radius
of 100 mm, three about 925mm straight part, and bends angles of
90 deg (as shown in Fig. 4). All the bent pipes have two bends,
including space-Z, plane-Z, and U, which are bended by the hot
bending method. In this experiment, the circumferential cracks
located in the first and second elbows and beyond the second
elbow are investigated, respectively. Besides, the circumferential
defects in different areas in elbows are also investigated, and as
shown in Fig. 5, the defect B is in intrados of the elbow and the
defect A is in extrados of the elbow. All the defects are circumferential
through-wall cracks with 1-mm width, which are machined
artificially. The defect rates (the defect rate is the cross section
between the defect and the pipe) of defect B and defect A are
23.2% and 21.8%, respectively. The defect rate of the defect
located beyond the second elbow is 26.4%. In this experiment, the
six integrated pipes are first investigated in the previous experiment
to explore the penetration rates for different kinds of pipes
configurations. In addition, the damage-induced wave packages
can be easily distinguished by comparing the benchmark signals.
After that, cracks beyond two elbows are introduced to explore
the detecting ability of guided waves in different pipe configurations
after traveling across two bends. Then, the cracks in the second
bends are made to study the propagation characteristics