A US standard test block P3-10 UTT 1015 is used to determine the effectiveness of the developed system in inspection of surface cracks in ferromagnetic material. The standard test block has the dimension of 200×45×20 mm3 and has a fatigue crack in the middle of the long side. The crack is parallel to the AD side and has a width of 20 µm and a depth of 10 mm (see Figure 5-1).
The objective of the measurement is to test the feasibility and efficiency of the robot supported automated measuring system. The test block is scanned continuously in three-dimension. The scanning parameters such as scanning speed, scanning resolution, scanning mode (step by step or continuously) are adjusted according to the specific requirement of the measurement. The measurement result is displayed automatically after the scanning is finished.
Figure 5-1 Specimen with artificial defect
In order to get a relative uniform distribution of magnetic flux, the specimen is magnetized overall by an electromagnet using the magnetic flow technique. The hysteresis curve of the material is a wide loop and the remanence value of the material is around 0.7 T. It is close to the minimum magnetic flux density that has been specified for the carrying out of magnetic flux leakage testing. Furthermore, there is no sudden cut-off value, below which the MFL leakage testing does not work. There will only be a gradual diminution in flaw sensitivity if lower magnetic fluxes are used. Therefore the residual magnetism is used to carry out magnetic leakage testing. The magnetic field direction is perpendicular to the crack.
Before the measurement starts, support points are collected first, they can be used to determine the scan area on the specimen. The support points are collected by moving the sensor near the specimen manually with the “KUKA Robot Motion System” module, then, with the help of the “Search” function provided by the software module, the robot will automatically approach the surface of the specimen and stop immediately after the sensor contacts with specimen surface lightly, the location of the magnetic sensor becomes the support point and can be stored in the support points list in the “Robot Script Controller” module. After all the necessary support points are collected and stored, the test area on the specimen is determined. The scanning task is then described as a script in the “Robot Script Controller” module and the robot scan path is planned with these support points. The robot scan path is shown in Figure 5-2. Surfaces with crack are scanned continuously with a sequence settled in the script command. Scan task management is shown in Figure 5-3. Each line of the script corresponds to a scanning task. These lines are executed one after another.
Figure 5-2 Schematic description of scan path
For a scanning area of the same size, the scanning time depends on the resolution and the scanning velocity which are set at the beginning of the test. The resolution is determined based on considering several factors comprehensively such as estimated dimension of the defect, size of scanning area, the required accuracy and efficiency of each specific test. In the present confirmatory experiment, one of the scanning areas is 14 mm × 200 mm. The resolution is set to be 70×1000 scanning points. The scanning velocity is chosen according to the resolution selected. A proper velocity should ensure that the desired resolution is achieved. The current coordinates of the sensor from the robot controller can be updated every 12 ms, this update rhythm is determined by the communication property and the KUKA robot software packet, and can not be changed. The lower the scanning velocity, the longer the scanning time lasts and the more coordinates need to be recorded. In our testing, the scanning velocity is improved without decreasing the resolution by interpolating the coordinates recorded. For a continuous scan path with length of 200 mm, the resolution is set to be 1000, and the scanning speed is about 17 cm/s. The scan time used for a scan area of size 14 mm × 200 mm with a resolution of 70 × 1000 is about 2 minutes. The whole measuring process includes four steps: support point collection, scan task description, scanning process and result display, it can be finished within 5 minutes. Scanning time can be reduced further if the resolution is set lower, provided the defect can be detected in the reconstructed image.
After the scanning is accomplished, the measurement data from the magnetic sensor and its location obtained during scanning are analyzed and the result of the measurement is displayed in the developed three-dimensional measurement display module “3D Inspector”.
Figure 5-4 displays the measurement result obtained with the “3D Inspector”. The three-dimensional reconstruction of the three surfaces is demonstrated in the module. Cracks on the surface of the specimen can be observed directly and explicitly in the reconstructed image. The tendency of the magnetic leakage field distribution around the crack is also displayed. Location of the crack displayed in the reconstructed coordinate system is consistent with the crack position on the test block.
In order to analyze more explicitly the magnetic field distribution on a specific area of the reconstructed image, a 3D cursor function which is specially designed for the result display module can be used. If one moves the 3D cursor to the point of interest on the reconstructed image, and A-Scan results about this point on the corresponding surface are displayed on the left and top side of the three-dimensional reconstruction image. As shown in Figure 5-4, a cursor is put on the surface in the Y-Z plane of the coordinate
system, an A-Scan that is parallel to Y is displayed on the top side and an A-Scan that is parallel to Z is displayed on the left side. Surface noise due to finishing status of the surface and the non-linearity of the permeability is also seen in the image. Therefore, experimental investigation of the interacting effect of the two adjacent cracks is facilitated. The measurement result can be graphically displayed as a reconstructed image and saved in BMP format or JPEG format. It can also be saved as text format to enable further evaluation. Sensor lift-off can be precisely adjusted with the help of the integrated touch probe system according to the surface status of the specimen. Possible wear of the magnetic sensor and damage of the specimen can be avoided.
Figure 5-4 3D Display of the measurement result
Since the scanning parameters are programmable during the measurement, hot issues about the magnetic sensor based MFL testing which are mostly studied by simulation method can be effectively verified experimentally with the developed automated measurement system. For example, sensor lift-off, scanning speed and resolution are programmable for each specific measurement. The study of sensor lift-off effect and its
influence on the magnitude of the detected signal, the influence of the magnetic sensor speed during the measurement on the detected signal value, identification of the two cracks can be facilitated. Furthermore, highly accurate measurement results obtained from the precisely controlled measurement process can provide a basis for quantitative study of flaw characteristics based on the fact that peak-peak amplitude of the MFL signals indicates the defect depth and the peak-peak separation distance can be used to characterize the length of the flaw [Lord1978]