The demise of liquid penetrant testing has been predicted frequently during the past twenty five years yet it persists in defying such predictions and remains one of the most widely used methods of non-destructive testing in a wide range of industries. The passing of the millennium will doubtless produce more gloomy predictions.
Despite its somewhat unglamorous image liquid penetrant testing remains vitally important in very sensitive industries including aerospace and nuclear engineering for the inspection of the majority of non-porous, solid workpieces at some stage in their lives from their preparation as raw materials through to their use. While it is unusual for components to be accepted as fit for purpose solely on the information gained from penetrant inspection in these industries failures are detected at a stage where wasteful further work can be avoided.
It is more than fifty years since the first modern penetrants became available and around thirty years since the various commonly applied methods of application became established. In this paper two aspects will be discussed, these being the penetrant materials themselves and ways in which improved performance can be achieved and the future pressures which appear likely to influence their continued use.
2. The materials
During the years since modern liquid penetrant testing became established formulation chemists have developed the materials to such an extent that their potential performance far exceeds the actual results which are achieved in practise in industrial use. It is now possible to inspect distinctly unforgiving surfaces with very bright penetrants and achieve excellent indication to background contrast. This is reflected in the fact that penetrants of higher potential sensitivity associated with their intrinsic brilliance are routinely used where previously materials of a lower classification were specified. Some of the penetrants which show lower levels of sensitivity are becoming almost redundant and of academic interest only. While efforts will continue to improve the formulations of the materials to produce better flaw finding capabilities this is not the primary area which will occupy the formulation chemist's efforts for some years. In section 3 below the opportunities to gain greater performance from existing materials is discussed in some detail.
With the continued public interest in industrial chemicals and their management it is in this aspect of penetrant testing that greater development is needed. A recent example which illustrates this is the loss of the volatile solvent 1,1,1, trichloroethane which was the solvent of choice for vapour degreasing in preparation for penetrant testing and was widely used as a solvent remover in one of the widely applied penetrant techniques, furthermore is was the carrier liquid in many non aqueous liquid penetrant developers. This material had become very important since its characteristics are ideal in many ways for the purposes in question. It is non flammable, has ideal solvency characteristics, a very acceptable drying time, and offered the most acceptable health and safety profile of the volatile chlorinated solvents. It was found to be an ozone depleting chemical and was phased out rapidly. An interim solution has been adopted with the use of related compounds which are not implicated in the depletion of the ozone layer in some applications, notably vapour degreasing and by flammable materials such as ketones, alcohols, esters, or mixtures of these compounds for
penetrant removers and carrier fluids for non aqueous developers. There remains pressure on all volatile organic compounds (VOCs) and the increasing interest in aqueous degreasing is a reflection of the awareness of this threat. Further there remains great interest in the use of all types of industrial chemical with a range of surfactants becoming classified as marine pollutants and the possible requirement to avoid hydrocarbon distillates. There are many ways in which these challenges can be met with a number of possibilities for the formulation of penetrants which contain no hydrocarbon distillates and surfactants unrelated chemically to those classified as marine pollutants are readily available. Detergent removers could replace organic solvents in the "solvent removal penetrant process", and the problem posed for non-aqueous liquid developers should the volatile organic solvents become unavailable can be resolved. Thus at present the challenge to our formulation chemists is to satisfy the predicted environmental and health and safety requirements rather than to increase the potential performance of the materials.
3. Application of the materials
It is the application of the penetrant materials which presents the greatest scope for improvement in the performance of penetrant processing. The application of the process can be achieved with very basic equipment and, if sufficient care taken, excellent results achieved with three aerosol cans. At the other end of the scale full automation even as far as the individual processing of components is possible.
The definition of sensitivity in all forms of non-destructive testing is difficult to quantify and remains semi-qualitative. In recent years a more attractive method for classifying methods and even techniques in probability of detection (POD) studies has become established. These studies relate the sizes of defect which can be expected to be discovered with the method in question as either a percentage or a limit of dimension. This approach has indicated that there is great potential for improving the performance of penetrant testing if the details of the application of the materials and the process are better controlled.
While it is recognised that manual processing can produce better performance than automatic processing the attraction of automatic processing is in its consistency. Individual operators produce variation in their level of performance and the variation from one operator to another in always a significant consideration.
Great improvement in the manual application of the penetrant processes has been achieved following greater attention to operator training. This must be supported by regular and frequent quality auditing to ensure that all equipment and materials are maintained in an acceptable state. There remains however a considerable difference between the levels of POD which can be achieved in a laboratory and those which are seen in practice.
When the number, type, and value of workpieces justifies the capital outlay for a fully automated installation it is often the preferred option. Disappointment is sometimes encountered initially when insufficient attention is given to presentation of the components to the process chemicals. Frequently components are placed into baskets which shield them from the process chemicals. In such cases the baskets or carriers are processed better than the components. In such instances the advantages enjoyed by virtue of uniformity of processing are largely offset by the difficulties of processing. Significant improvement in the performance can be achieved by careful attention to the design of jigs and fixtures. Properly designed fixtures can minimise the contact areas with the components and reduce the shielding of the surfaces to a negligible level.
Penetrant testing still offers the most efficient NDT method for testing for surface defects when large surface areas, particularly if they are of even mildly complex shape, and if large numbers of workpieces are involved. Penetrant testing alone is not usually taken as evidence of fitness for purpose but a high proportion of defective items can be identified at an early stage. At present the levels of POD which can be achieved in a laboratory are significantly higher than those produced day by day in a factory environment. A great deal of improvement can be introduced even in the manual application of the method by close attention to training and quality control. A combination of automation and fixture design can lead to further improvements in the effort to raise the levels of POD achieved in practical applications to approach those which are possible in a laboratory.
A combination of efforts to achieve improvements in the application of the penetrant method together with the attention to formulations which minimise any environmental or health and safety aspects indicates a healthy future for penetrant testing as we enter the new century.