A TESTING METHOD FOR MACHINED WORKPIECES

Abstract: The invention refers to a method for determination of the presence, if any, of detrimental surface changes, such as increased hardness or residual stress in machined workpieces. For achieving a very reliable test result, the invention suggests that the determination is made with the aid of an eddy current probe of absolute-type.

Description: A testing method for machined workpieces The present invention refers to a method for determination of the presence, if any, of detrimental surface changes, such as increased hardness or residual stress in machined workpieces. For such checking of the presence of machining damages, if any, in rotary parts of aircraft engines, various methods and equipments have been used since long. When it comes to parts made of titanium there has been carried out a so- called blue-etch of the surface of the material for checking machined surfaces. There are, however, two problems in connection with this method when inspecting, namely on one hand that this method is a visually-based one, which requires that one can view the surface satisfactorily (and preferably at right angles) , which causes difficulties in connection with deep holes but does not constitute an obstacle to the inspection of the hole edges including a portion of the hole wall closely ajacent to the edge, and on the other hand that there is a risk that the method is not perfectly reliable when foreign substances might be present in the surface. This might be the case e.g. in connection with a tool breakdown where substances from the tool such as Si, Cr, Ni and Fe or from the environment (oxygen etc) can diffuse into the matrix. Another possibility is to try to find cracks in the material, if any, by means of a fluorescent penetrating liquid. However, the machining damages which might be present, do not always cause cracks to occur until the part has been loaded and normally this does not occur during the production but only later in service. A third frequent inspection method is eddy current testing with a differential probe. In carrying out this test two coils are used and the signal difference between the coils is observed. Said method is also well suited for localising cracks while long-term changes in the materials such as the machining damages aimed at in the present case, may fade away. A supplementary checking method over the blue-etch-, the differential probe and the penetration methods would be highly desirable in order to provide for an establishment with certainty whether there are machining damages present due to local over-heating by worn tools, tool breakdown or the like. According to the present invention it has been found that the use of an absolute-probe optimized for the present use, implies a very effective possibility to investigate holes made in workpieces of titanium and its alloys, super- alloys and steel for applications in rotary aircraft parts. Should there occur a detrimental heating action of the material during the machining as a consequence of e.g. worn or damaged tools, it is easy to discover with such a probe residual stresses, diffused impurities or composition variations or a more or less heavy deformation of the surface microstructure . The invention will be further described below with reference to the accompanying drawing, in which Fig. 1 discloses a diagrammatical and partly sectioned side elevational view of an absolute probe used according to the invention and introduced into a hole, while Fig. 2 discloses two examples of various microstructures of titanium material . As is evident from Fig. 1 the absolute probe used in connection with the present invention is of simplest possible design. More closely, it has the shape of a body with a cross section adapted to the geometry of the inspected surface and a coil wound around said body. This design makes the probe sensitive to everything having influence on the electrical conductivity or magnetic permeability of the material tested. It is also sensitive to everything which changes the relationship between the surface of the material and the probe. The result thereof is that also minor material changes might be measured such as changes in residual stress, dissolved foreign atoms in small percentages as well as composition variations etc. The probe according to the invention is connected to an eddy current instrument and introduced with a uniform speed through the hole in a workpiece which is to be investigated. The signal from the instrument is digitalized and stored in e.g. a PC. Measuring data are then treated in order to facilitate the evaluation. It might also be added that the cylindrical probe body preferably is made of plastic and has a sufficient length for allowing a simple attachment for automatic measuring. The diameter is adapted such that the probe guides itself through the hole . In experiments with the inventive probe there have been found damages which exhibit both a heavily deformed zone and a layer having increased percentages of i.a. iron and oxygen. The hardness in the area has been increased and cracks have occured in parts having been in service. Should said cracks not be discovered in routine maintenance they might propagate sufficiently to cause a breakdown. The inventive absolute probe also makes it possible to identify surfaces of the workpiece which have been damaged in the manufacture, where the damaging action is very small. Furthermore, the testing might be automized and the inspection time will be short compared to other testing techniques. The evaluation can be automized and measuring data might be stored for being traced and evaluated later.

Claims: C l a i m 1. A method for determination of the presence, if any, of detrimental surface changes, such as increased hardness or residual stress in machined workpieces, c h a r a c t e r i z e d i n that the determination is made with the aid of an eddy current probe of absolute-type . AMENDED CLAIMS [received by the International Bureau on 12 March 1999 (12.03.99); original claim 1 replaced by amended claim 1 (1 page)] A method for determination of the presence, if any, of detrimental surface changes, such as increased hardness or residual stress, in machined workpieces of titanium and its alloys, c h a r a c t e r i z e d i n that the deter- mination is made with the aid of an eddy current probe of absolute-type .

Inventors: BRANDER LARS, ECKERBOM LENA, FRANSSON ANDERS, BRANDER, LARS, ECKERBOM, LENA, FRANSSON, ANDERS
Applicants: BRANDER LARS, ECKERBOM LENA, FRANSSON ANDERS, VOLVO AERO CORP, BRANDER, LARS, ECKERBOM, LENA, FRANSSON, ANDERS, VOLVO AERO CORPORATION
Application year: 1997
Main classification: G01N27
IPC codes: G01N27