Method and apparatus for producing a uniform magnetic field in a test sample

An apparatus for producing a uniform magnetic field in a test sample having an angled end during a magnetic particle test procedure, includes a conductive, angled fixture having a contact surface and an overall shape corresponding to that of the angled end of the test sample, so that when a pulsed current is passed through the blade, the resulting uniform magnetic field permits detection of defect indications in the angled end.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to magnetic particle test 
procedures and, more specifically, to a fixture used in a magnetic 
particle test procedure which produces a uniform magnetic field. 
2. Description of the Related Art 
In the production of crucial components such as steam turbine blades it is 
very important to detect manufacturing defects which are not visible to 
the naked eye. Magnetic particle test procedures have been developed 
whereby a test sample, such as a turbine blade is pulsed with an electric 
current to generate a magnetic field therein. Then, a solution containing 
iron oxide particles is sprayed on the test sample and the particles align 
themselves in the direction of the magnetic field. 
Latent cracks produce localized variations in the magnetic field, and these 
variations are manifest in the distribution of iron-oxide particles from 
the sprayed-on solution. When the test sample is illluminated with a black 
light, the variations in the magnetic field attributable to latent cracks 
are manifest in observable changes in magnetic field. 
While magnetic particle test procedures have been used in the past to test 
for defects in turbine blades, the shape of the blade has been found to 
effect the ability to achieve a uniform magnetic field in the test sample. 
The problem of creating a uniform magnetic field is particularly acute for 
blades of the type known as triple pin blades in which two or more 
airfoils shrouded together at the top share a common root portion which 
has an angled end and two or more root "fingers". In particular, it has 
been found that the magnetic field does not adequately extend into the 
fingers and over the sloped surfaces of the end of the root portion. This 
problem can lead to the failure of the magnetic particle test procedures 
to detect defects in the root portion of the blade. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to provide an apparatus for 
producing a uniform magnetic field in the test sample as a pulsed current 
is passed through the test sample, whereby the uniform field permits 
detection of defect indications on blade areas which would otherwise be 
missed. 
Another object of the present invention is to provide an apparatus for 
producing a uniform magnetic field in a triple pin rotating blade, whereby 
the magnetic field imparted in the blade by a pulsed electric current 
passing uniformly through the blade and in particular between the fingers 
of the root portion and along the angled sides of the root portion. 
Another object of the present invention is to provide a fixture for use in 
a magnetic particle test procedure which is relatively simple in 
construction and cost effective to produce. 
These and other objects of the invention are met by providing an apparatus 
for producing a uniform magnetic field in a test sample having an angled 
end during a magnetic particle test procedure, including a conductive 
angled fixture having a contact surface and an overall shape corresponding 
to that of the angled end of the test sample, so that when a pulsed 
current is passed through the blade, the resulting uniform magnetic field 
permits detection of defect indications in the angled end. 
In another aspect of the present invention, a method of conducting a 
magnetic particle inspection of a test sample having an angled end 
includes placing a conductive angled fixture over the angled end of the 
test sample, clamping the test sample between an axially stationary 
headstock and an axially movable tailstock, whereby the conductive angled 
fixture is pressed against and conforms in shape to the angled end of the 
test sample, pulsing an electric current through the test sample to 
generate a magnetic field, spraying an iron oxide fluid on the test 
sample, and illuminating the test sample with a black light to observe 
defect indications. 
These and other features and advantages of the magnetic particle test 
fixture of the present invention will become more apparent with reference 
to the following detailed description and drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1 and 2, a fixture 10 is used to produce a uniform 
magnetic field in a test sample, such as a triple pin blade 12 which has a 
root portion 14, an airfoil portion 16 and a shroud portion 18. The root 
portion 14 has tow or more fingers 20 which are spaced apart by one or two 
axial grooves 22. Opposite sides 24 and 26 of the fingers 20 converge at 
an angle and terminate at a flat end 28. The airfoil portion -6 is formed 
from a single stock by electric discharge machining (EDM), while the root 
portion 14 and the airfoil portion 16 are formed by machining. 
After manufacturing by EDM, the present invention is used to check the 
blade 12 for latent defects. While magnetic particle inspection techniques 
have been used in the past for other types of turbine blades, the 
structure of the blade presented in FIG. 1 creates a problem in that the 
magnetic field imparted in the blade 12 during testing is frequently 
unevenly distributed, especially around the areas of the converging sides 
24 and 26 and in the grooves 22 of the angled end of the test sample (the 
test sample being a blade structure in FIG. 1 for example). 
According to the present invention, a fixture 10 is provided with a three 
part construction. A backing plate 30 is interconnected with a braided pad 
32, with the braided pad 32 being held on an inner surface of the backing 
plate 30 by bolts 34. Preferably, the backing plate and the braided pad 
are made of copper, which is electrically conductive. Prior to undertaking 
the magnetic particle test procedure, the fixture 10 is placed over the 
angled end of the blade 12, as depicted in FIG. 1. Then, the blade 12 is 
axially clamped between an axially movable headstock 36 and an axially 
movable stationary 38. The headstock 38 is driven axially by a hydraulic 
ram 40 so that the tailstock 38 abuts the shroud portion 18 of the blade 
12. At the opposite end, the flat end 28 of the root portion 14 abuts a 
flat portion 42 of the fixture 10. At the same time, arm portions 44 and 
46 of the fixture 10, which are angled at the same angle of convergence of 
the sides 24 and 26 of the fingers 20 so that the arm portions 44 and 46 
contact the sides of the fingers 20. Thus, the headstock and tailstock are 
brought together so that the fixture 10 is clamped therebetween and over 
the angled end portion of the blade 12. Then, a pulsed electric current is 
applied to the blade 12 so as to impart a magnetic field therein. 
Application of the pulse electric current can be effected by using any of 
the PG,7 various known techniques. Then, an iron-oxide particle fluid is 
sprayed on the blade 12 in the usual manner, using commercially available 
iron-oxide powder properly mixed. The fluid contains iron-oxide particles 
which are disbursed in the fluid. When the fluid is sprayed on the 
magnetized blade 12, the particles will align themselves according to the 
field lines of the generated magnetic field. Defects in the blade will 
cause variations in the magnetic field and thus the particles will align 
themselves accordingly. The alignment of the particles can be visualized 
by illuminating the blade with a black light. The aforementioned 
application of an iron-oxide fluid and the use of a black light are known 
techniques. However, it has been found that the magnetic field is not 
uniform in the area of the fingers, and generally, the angled end of the 
blade 12; 
The fixture 10 corrects the tendency for the magnetic field to be unevenly 
disbursed. The copper braided angled plate or fixture 10 is placed on the 
root end of the triple pin blade 12 and when an electric current is pulsed 
through the clamp assembly the pulsed current follows the braided pad - 
blade contact surface. This current flow will generate a circular magnetic 
field throughout the finger contact area. This magnetic field will provide 
the means to detect axial directed indications of a defect on all parts of 
the blade, especially between and on surfaces of the fingers. Normal 
clamping of only the end of the root fingers does no provide the field on 
all parts as would be required to obtain an adequate test. 
The length of the arm portions 44 and 46 of the fixture 10, as well as the 
angle of the arms relative to the flat portion 42 can be varied to fit 
blades having a variety of root finger shapes and sizes. It is understood 
that the fingers 20 of the blade 12 are used to mount the triple pin blade 
to a rotor by means of bolts which pass transversely through the end 
portions of the fingers 20. 
Numerous modifications and adaptations of the present invention will be 
apparent to those so skilled in the art and thus, it is intended by the 
following claims to cover all such modifications and adaptations which 
fall within the true spirit and scope of the invention.