Z. Liu | TU Delft Repository

Experimental investigation on FRP-reinforced surface cracked steel plates subjected to cyclic tension

Journal article (2020) - Zongchen Li, Xiaoli Jiang, Hans Hopman, Ling Zhu, Zhiping Liu, Weiguo Tang

Surface cracked steel plates reinforced with single-side Fiber-Reinforced Polymer (FRP) subjected to cyclic tension are experimental studied. The main purpose is to analyze the effect of FRP reinforcement on the crack growth. The failure modes and their effects are analyzed as well. Given the single-side reinforcement, reinforcing the cracked surface significantly prolonged the fatigue life, while reinforcing the reversed side resulted in the opposite consequence. Most specimens did not encounter debonding failures, indicating such failures are avoidable by improving the reinforcement quality. The results also indicate the bond layer number is an insensitive factor–an optimum number is existed. ...

Applications of red-green-blue normalisation and interframe difference matrix in complex surface by eddy current pulsed thermography

Journal article (2018) - Zhiping Liu, Yong Du, Chao Zhu, Xuan Li, Xiaoli Jiang

As a result of the influence of surface characteristics, it is difficult to identify cracks on complex surfaces using the conventional processing of a single-frame image that misses important infrared thermal image sequences. In this paper, the red-green-blue normalisation method and the interframe difference method are used to make full use of the sequences and to characterise infrared thermal images of cracks on complex surfaces. An experiment to prove the effectiveness of the detection method was carried out on weld steel and carbon fibre-reinforced polymer (CFRP) steel plate. The selected infrared thermal image was divided into an objective area and a background area according to the relationship between three parameters for red-green-blue and objective temperature. The targeted and interfering information of the objective area were separated and processed based on the interframe difference method. The index of crack detection was extracted from the interframe difference matrix, which was constructed from two selected infrared thermal images. This method takes into account the local and global spatial information of the image and the maximum crack length measurement error was approximately 6%. ...

Design and experimental study of a novel excitation coil based on pulsed eddy current thermography

Journal article (2017) - Zhiping Liu, Fang Guihua, Jiang Lei, Xiaoli Jiang, Gabri Lodewijks

Active infrared detection technology is widely used in fields such as aerospace, shipbuilding, bridges and chemical pipelines. In the defect detection of complex-shaped metals, pulsed eddy current thermography (PECT) has attracted much attention due to its non-contact, fast, safe and intuitive features. Current PECT applications are broad; however, research on PECT excitation device performance has been lacking. The electromagnetic induction coil is an important component of the electromagnetic excitation system. Its function is to generate a transient pulse current and induce a magnetic field in the member under inspection. While simple in form, its large power consumption and low efficiency do not permit great portability. The objective of this paper is to study steel crack detection. A coil with a U-shaped magnetic core was used for the PECT system. The magnetic force lines winding around the magnetic core produced excitations. Excitation performance was then analysed through a finite element simulation and verified in a comparison of experimental results. The excitation of the magnetism-gathering device has many advantages, such as higher energy transfer efficiency and better uniformity. Moreover, the heating power has a smaller effect on the lift-off distance, hence better detection of metal surface defects can be achieved. More importantly, PECT detection adds a greater degree of portability to in-service equipment. ...

Crack monitoring method for an FRP-strengthened steel structure based on an antenna sensor

Journal article (2017) - Zhiping Liu, Kai Chen, Zongchen Li, Xiaoli Jiang

Fiber-reinforced polymer (FRP) has been increasingly applied to steel structures for structural strengthening or crack repair, given its high strength-to-weight ratio and high stiffness-to-weight ratio. Cracks in steel structures are the dominant hidden threats to structural safety. However, it is difficult to monitor structural cracks under FRP coverage and there is little related research. In this paper, a crack monitoring method for an FRP-strengthened steel structure deploying a microstrip antenna sensor is presented. A theoretical model of the dual-substrate antenna sensor with FRP is established and the sensitivity of crack monitoring is studied. The effects of the weak conductivity of carbon fiber reinforced polymers (CFRPs) on the performance of crack monitoring are analyzed via contrast experiments. The effects of FRP thickness on the performance of the antenna sensor are studied. The influence of structural strain on crack detection coupling is studied through strain–crack coupling experiments. The results indicate that the antenna sensor can detect cracks in steel structures covered by FRP (including CFRP). FRP thickness affects the antenna sensor’s performance significantly, while the effects of strain can be ignored. The results provide a new approach for crack monitoring of FRP-strengthened steel structures with extensive application prospects. ...

Image processing algorithms for crack detection in welded structures via pulsed eddy current thermal imaging

Journal article (2017) - Zhiping Liu, G. Lu, X. Liu, Xiaoli Jiang, Gabri Lodewijks