Development of penetrant testing (PT) methods in Belarus started about 25 years ago in the Laboratory of Capillary Phenomena in Physical-Technical Institute (after 1980 - in the Institute of Applied Physics (IAPh)) of National Academy of Sciences (NASB). In the eighties belarussian researchers took the leading place in the area of physical and theoretical principles of PT among well-known teams engaged in studying of this kind of NDT in Russia (VIAM, NPO "Spectr", TSNIITMASH) and Ukraine (Welding Institute of NASU, KPI).
The widely used liquid penetrant testing is a reliable, high-sensitive non-destructive method to detect surface discontinuities. The method is based on a whole complex of physical processes. Therefore, both to optimize the technological process of PT and to develop new, high-sensitive product families (penetrant, remover and developer) one needs to establish quantitative description of physical correlation between these processes. We developed general principles of PT theory about 15 years ago [1,2]. The last experimental and theoretical results we summarized in our recent book . It is possible to estimate the dimensions of the smallest defects, which can be detected by given product family, using this theory. Our approach for sensitivity evaluation is based on the comparison of the volume of penetrant entrapment in defect's cavity and the volume required for the formation of an indication on the outer surface of developer layer. The minimum dimension of defect's opening, which can be visualized at prescribed defect's depth, characterizes the sensitivity of penetrant testing for given product family. The methods to determine structural properties of the developers, necessary for PT sensitivity evaluation, have been developed as well .
The influence of the most important physical phenomena on PT characteristics has been studied and explained at the IAPh during the last 15 years. These are gas diffusion and dissolution in a liquid , the interaction of two liquids in a capillary , two-side filling of defects with a penetrant , the dimension-dependent viscosity effect and some other phenomena . The results of experimental researches of liquid penetration into the porous layer from one-side closed capillary are presented in . In certain cases an air bubble appears in the region of capillary outlet and cuts off a direct contact between liquid column and porous layer. It is established that in spite of this fact the liquid's extraction from a capillary (preliminary filled with a penetrant) into porous layer takes place. The physical mechanism of such a process is explained in the frame of theoretical model of film flow, based on the concept of disjoining pressure.
The role of ultrasound in some stages of PT procedure has been studied at the IAPh as well . The strong influence of ultrasound on the process of defect's filling is based on two main factors. First of all, it is the ultrasound cleaning. The strong ultrasonic impact both removes all the residues and contaminants from the surface and also leads to an improved wetting of the surface. Secondly, an ultrasound creates an excess pressure in the liquid, which causes a faster and deeper filling of the defects due to so called capillary ultrasonic effect. The principle of capillary ultrasonic effect is based on an increase of the depth and rate of liquid's penetration into the capillaries under the action of ultrasound in the regime of cavitation .
The conventional methods usually cannot be used for porous material, as the pores open to the surface evoke fluorescence, which may be stronger than the indication. Field of application of porous non-conductive materials is increasing year by year the last time. Since magnetic, electromagnetic and electric methods are not applicable for these materials (ceramics, some composite et cetera), penetrant testing is the most promising method. As a result the problem of the development of effective methods and means for PT of porous materials becomes especially relevant and important
The method and the means of PT method for porous materials (including ceramics) have been developed at IAPh [2,10-12]. In  PT method for porous material was proposed by using the capillary ultrasonic effect. A buffer liquid is applied on the material and washed out afterwards using ultrasound. As this liquid is removed by cumulative jets from the defects faster than from the pores, the first are filling with penetrant deeper than the pores.
Later we proposed the method of penetrant testing of porous materials without ultrasound application [2,10,12]. Penetrant testing of porous material with the use of pre-wetting to eliminate the background luminescence by closing the pores has been applied in  for porous nitride ceramic samples. The pair "pre-wetting liquid-penetrant" must be chosen so, that the liquids are not dissolving one in another and their interface in the opening of the pore forms a meniscus concave to outwards. The pre-wetting liquid with these properties penetrates both into the pores and into the cavities of the defect. However in the defect the porous walls absorb the liquid and the cavity becomes at least partly free of penetrant. As a result a liquid cannot penetrate into the pores but imbibes the defects. Special means for PT of porous materials has been developed as well.
The ways of increasing sensitivity of penetrant testing procedure with a use of thermal effects are considered in our paper . It was established that two kinds of thermal effect (both at the stage of penetrant application and further heating of inspected parts during a development process) result in reaching the optimal results in terms of sensitivity threshold and duration of penetration stage. The described ways of increasing sensitivity are simple for realisation, they do not take of essential additional equipment and can be easily used for any penetrant testing procedures
Nontoxic sensitive product families were developed at IAPh. For example luminescent penetrant LZHT (III level sensitivity) and dye penetrant "Pion" (II level sensitivity) are used at many industrial enterprises in Belarus.
Sensitivity and efficiency of PT in a large degree depend on a quality of penetrant systems being used. The evaluation and periodic quality control of penetrant systems are the essential condition of reliable penetrant inspection. The differences in the outcome of various product families are determined by the properties of the product family components, technological stages and conditions of realization of penetrant testing, characteristics of tested object. Therefore a final choice of product family from the list recommended for the given application, should be carried out by the results of evaluations of the influence of these factors on defect's detection. Known methods of quality evaluation of penetrant systems have a comparative character. The equipment and means used for PT do not completely eliminate the subjective factor from the evaluation process.
The method of quality evaluation of penetrant systems with the use of automated recording system, processing and analysis of penetrant inspection outcome is developing now at the IAPh. It enables to eliminate the majority of disadvantages of usually used methods and reliability of inspection results.
In 1999 National Standard 1172-99 "Penetrant testing. General requirements", harmonized with ISO 3452-1, was worked out at IAPh and approved by National Organization for Standardization (BELSTANDART) of Republic of Belarus.
All specialists in PT methods to be certified in Belarus have to pass the training course at IAPh. The only Training Center for PT methods in the country was organized 5 years ago at IAPh. Up to present about 280 specialists passed the training course.
Belarus has succeeded in industrial development of such branches as motor industry, manufacture of tractors, various automobiles (trucks, trailers, buses etc) and agricultural engineering, machine-tool industry, manufacture of bearing and other branches of industry. Taking into account highly developed industrial infrastructure of Belarus, the scale of an application for PT methods and means is very wide and the potential of PT methods to improve a quality of industrial production seems promising.
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