Surface cracking

This paper compares experimental data for aluminium and steel specimens with two methods of solving the forward problem in the thin-skin regime. The first approach is a 3D Finite Element / Boundary Integral Element method (TRIFOU) developed by EDF/RD Division (France). The second approach is specialised for the treatment of surface cracks in the thin-skin regime developed by the University of Surrey (England). In the thin-skin regime, the electromagnetic skin-depth is small compared with the depth of the crack. Such conditions are common in tests on steels and sometimes on aluminium. 

At sufficiently high frequency, the electromagnetic skin depth is several times smaller than a typical defect and induced currents flow in a thin skin at the conductor surface and the crack faces. It is profitable to develop a theoretical model dedicated to this regime. Making certain assumptions, a boundary value problem can be defined and solved relatively simply leading to rapid numerical calculation of eddy-current probe impedance changes due to a variety of surface cracks. 

N. Flarfield and J. R. Bowler, Theory of thin-skin eddy-current interaction with surface cracks, J. Appl. Phys., 82(9), 4590 - 4603, 1997. 

J. R. Bowler and N. Flarfield, Evaluation of prohe impedance due to thin-skin eddy-eurrent interaction with surface cracks, IEEE Trans. Mag., accepted. 

A method has been worked out for eddy current testing of surfaces and surface cracks or corrosion under dielectric or non-magnetic metal layer of up to 10 mm. The method is based on excitation of eddy currents by a coil with U - type core and information reading by a sensitive gradientometric element located on a axis of symmetry of the core (fig. 1). 

Fig. 4 Godographs of voltage from surface cracks in nonmagnetic samples.

Measuring surface crack depth is performed by calibration samples made of the same material like the object being tested. Calibration samples are the plates having narrow grooves like slits of various depth 0.2 mm, 0.5 mm, 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm and made by electric erosion method. The samples have dimensions 50 mm X 150 mm x 6 mm and 25 mmx 150 mm x 6 mm and are made of magnetic... 

This enables to locate both small surface cracks from 0.1 mm deep and undersurface defects with one and the same eddy-current converter. This also allows to reveal corrosion between sheets while suppressing the influence of variation of clearance between them. 

High selective performance of the sensor also enable to detect surface cracks in ferromagnetic and non-magnetic welded connections under rust or dirt layer without prior cleaning. 

Depending on the requirements, C-scan or a cascading display are used. These types of display facilitate tests for internal damage below the surface with low-frequency eddy currents in addition to high-frequency tests for surface cracks. 

High sensitivity for both small surface cracks and bigger inner fractures and corrosive defects under the layer of a non - magnetic metal to 7 mm, also through the skins. 

When heat-electric generating station turbine rotors are under operation there is a probability of forming surface cracks in axial canal and heat grooves. Evolution of the above defects can cause a serious crash. Therefore, in accordance with safety standards periodic inspection of the rotor component parts is required. 

Figure 2 a) A deep sub-surface crack in a half-space conductor b) the utilized transducer... 

Figures Absolute e.m.f. dependence of the probe vs. position for different sub-surface crack depths - theoretical and experimental results...

Surface Crack Detection Using Magnetic Particle Techniques and Penetrant Inspection Method. 

Sizing of 3-D Surface Cracks with Complex Cross-Sections and Different Orientations Using Leakage Field. 

In this paper, computations are performed for sizing of surface cracks with different orientations with respect to the applied field, complex cross-sections, and unknown shapes by using the algoritlim from It is shown that the algoritlim allows to perform sizing of cracks with complex cross-sections independent of the shape of the crack for orientation angles not exceeding 45°. 

A surface crack with right-angular parallelepiped shape is illustrated in Fig.l. A schematic drawing of the positioning of this crack at the surface plane (xOy) is shown in Fig.2. The crack is oriented at an angle O with respect to the direction x of the applied field, and the applied field is considered to be magnetic field for simplicity. 

Fig.l Surface crack with right-angular parallelepiped shape. (xOy ) is the surface plane. 

The properties of a single crystal creeping wave probe The a single crystal creeping wave probe is suitable for testing various artificial defects such as surface cracks, FBH, columned hole and SDH etc. and its distance amplitude cruve is shown in Fig.6... 

Ohtsuka K. (1986) Generalized G-integral and three-dimensional fracture mechanics. Surface crack problems. Hirosima Math. J. 16 (2), 327-352. 

Microstructural examinations disclosed highly branched, predominantly transgranular cracks originating on the internal surface. Cracks of this form are typical of SCC in austenitic stainless steels. 

Below about 0.75 T, polymers are brittle (Fig. 23.9). Unless special care is taken to avoid it, a polymer sample has small surface cracks (depth c) left by machining or abrasion, or caused by environmental attack. Then a tensile stress tr will cause brittle failure if... 

Kic = fracture toughness a, = size of widest microcrack (crack width for surface crack crack half-width for buried crack). 

Fracture mechanics analysis requires the determination of the mode I stress intensity factor for a surface crack having a circular section profile. Here the circular section flaw will be approximated by a semi-elliptical flaw. 

J. C. Newman, Jr. A Review and Assessment of the Stress Intensity Factors for Surface Cracks," Part-Through Crack Fatigue Life Prediction, ASTM STP 687, J.B. Chang, Ed, American Society for Testing and Materials, 1979, pp. 16 42. 

---