Previous testing of such test pieces as fan blades utilized in gas turbine engines has involved application of paint markings to identify fragments upon fragmentation of the test piece. Such fragmentation causes debris which will obscure paint markings and the paint itself may create clouds of dust obscuring visual images of the test piece under test conditions. By applying an image pattern comprising a number of lingering image patches, typically in the form of thermal patches upon the test components, and arranging the test components to be within an enclosure which is evacuated it will be appreciated that these image patterns linger and can be viewed by an appropriate monitor over a period at least of the test conditions. Utilization of extra visual spectrum radiation images extends the period when clear images of the test piece under test conditions can be seen and analyzed.