A Novel Thermography-Based Dry Magnetic Particle Testing Method

Abstract

Magnetic particle testing (MT) has been widely used in nondestructive testing (NDT) of ferromagnetic materials. MT can be divided into wet method and dry method. The wet method is highly sensitive for microcracks and easy to operate. But the problem is that magnetic suspension liquid utilized in the wet method pollutes both test objects and environment. Dry magnetic particles utilized in dry method are recyclable and environmentally friendly. But the irrelevant magnet particles must be manually removed to reveal the magnetic particle indication (MPI). This operation limits both the detection efficiency and sensitivity of dry method. To increase the automation in MT, this article proposes a thermography-based dry MT method (TDMT). Specifically, the dry magnetic particles are heated before utilization and then thermal data are utilized for crack identification. TDMT inherits the economic and environmental advantages of the dry method. Additionally, the time-varying thermal data can provide more useful information than conventional MPI, which makes it possible to identify cracks without manual assistance. The mathematical model of TDMT is designed to prove its feasibility and a dimensionality reduction-based algorithm is applied for crack signal enhancement. Experiments on different specimens with artificial and natural cracks have been conducted. The notch with both a depth of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$15~\mu \text{m}$ </tex-math></inline-formula> on the standard test shim and a width up to 0.8 mm on the steel plate can be successfully detected. The high sensitivity and wide detection range can be seen from TDMT.