Visualization of material defects - modern approaches in acoustical and electrical NDE-methods
(2008)
Increasing demands in materials quality and cost effectiveness have led to advanced
standards in manufacturing technology. Especially when dealing with high quality
standards in conjunction with high throughput quantitative NDE techniques are vital to
provide reliable and fast quality control systems. Fast NDE-systems using a high degree
of automatisation can be used for both determining the degree of integrity of the
components under test and indicating a change of production parameters as well.
However, independently of the applied NDE method and the underlying physical
principle a reliable visualisation of hidden defects within the component under test is
based on a sufficient high signal to noise ratio (SNR) and a high spatial resolution. In this
talk we illuminate two standard NDT methods such as Ultrasonic Testing and Eddy
Current Testing and show their physical principles also discussing the interaction
between sound waves or induced eddy currents with different kinds of material defects.
This introduction substantiates the attainable SNR and spatial resolution of both methods
with respect to defect sizing and defect classification. As a first future prospect we report
on the SAFT-algorithm to improve SNR and spatial resolution paving the way for a flaw
sizing approach in ultrasonic inspection. As a second modern NDE approach we
represent the use of small magnetoresistance sensor arrays for EC testing of Al-laser
welds or for testing superconducting wires. The high sensitivity and small extent of GMR
sensors results in a remarkably SNR and spatial resolution offering new visualisation
techniques for defect localisation, defect characterization and tomography-like mapping
techniques.
Introduction
Due to the long history of railroad companies around the world, varying means of examination according to the specifications for the different railroad components have been carried out based on the general industrial progress of the country [1-3]. For more than 50 years the application of customized ultrasonic techniques for the examination of railroad components has increased, in comparison to conventional ultrasonic techniques used until to the end of 20th century. In this traditional atmosphere some accidents have disturbed the silent harmony of companies involved in the production of critical parts. More than ever high speed modern trains require modern and advanced examination techniques [4-8]. For practical application new ultrasonic techniques have been developed and optimized in co-operation with the German Railroad Company. In the present contribution the inspection of railroad axles will be described. The ultrasonic technique used for the examination is the phased array technique. The optimization of the phased array probes for the different geometries of the axles given was completed using a computer model for directivity calculations. In relation to the limited accessibility between wheel and brake angles of incidence for shear waves in the range between 28° and 72° are necessary.
During the examination of the axles a a considerable amount of ultrasonic data must be stored (storage of the digitized A-scans). However, due to economic aspects the evaluation of this data should be carried out in a very short time. The whole examination of one axle should not exceed 5 minutes. Therefore we have developed a special evaluation program adapted to the particular problematic of the axle inspection. The build up of the whole inspection system was done with partners from the industry like Krautkramer and a division of the Research Department of the Deutsche Bahn AG. In this consortium the Research Department of the Deutsche Bahn AG was responsible for the specification of the inspection problem, Krautkramer for the organization of the whole technical follow up and BAM for the development and optimization of the ultrasonic inspection technique as well as for the evaluation software. Some examples concerning the optimization process will be described in the present contribution.