Polarized light technique clearly defines the fatigue zones in metal for measuring and photographing. White light is passed through a vertical polarizing filter and then is reflected onto the surface of the fracture specimen

Walsh, F. D.

English

NASA Technical Reports Server

April 1967	 Brief 67-10082 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Fatigue Zones in Metals Identified by Polarized Light Photography 
Polarizing Filter
Light Source 
Rellectorl 
efractor
Specimen 
The problem: 
While fatigue zones in metal where surfaces frac-
ture are often evident to the naked eye, particularly 
under properly oriented light, photographing or meas-
uring these zones has proven difficult. 
The solution: 
A polarized light technique that clearly defines the 
fracture surfaces for measuring and photographing. 
How it's done: 
White light is passed through the vertical polarizing 
filter whose plane of polarization is parallel with the 
upper edge of the reflector/refractor. Light passed by 
this vertical filter is plane-polarized, vibrating at right 
angles to the plane of the illustration, and is reflected 
downward without rotation onto the surface of the 
fracture specimen. Upon striking the surface, each 
ray in the plane-polarized beam will behave optically 
in one of three ways: 
(1) If the surface at a particular prominent point 
is met at a very low angle of incidence, the light ray
will be scattered outside of the optical system. 
(2) If the surface is met perpendicularly, at a flat 
spot, the light ray will be reflected back into the 
optical system without rotation. 
(3) If the surface is met obliquely at a large angle 
of incidence, the light ray is reflected at least twice 
and suffers some change in direction as well as a 
change in polarization from plane to elliptical. 
Following reflection from the specimen surface, the 
rays represented by items 2 and 3 in the right figure 
are refracted without rotation through the reflector/ 
refractor to impinge on the horizontal polarizing 
filter. Because this filter's plane of polarization is 90 
from that of the vertical filter, all rays unrotated at 
the specimen surface (as in item 2) will be extin-
guished. In contrast, the light rays that undergo multi-
ple reflections from oblique surfaces (as in item 3) 
become elliptically polarized and result in transmittal 
of two planar segments of each light wave 180° apart 
through the horizontal filter into the camera system. 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19670000082 2020-03-11T20:43:39+00:00Z
The plane-polarized light obtained from elliptically-
polarized light is used to obtain photographs by the 
crossed-polar technique. The degree of shading or 
contrast obtained is apparently a measure of the rela-
tive strength of plane-polarized rays and elliptically-
polarized rays from each photographically discrete 
zone in the fracture surface and which, in turn, then 
becomes a measure of the relative statistical distribu-
tion of reasonably flat surfaces and variously oblique 
surfaces. 
Note: 
Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Western Support Office 
150 Pico Boulevard 
Santa Monica, California 90406 
Reference: B67-10082
Patent status: 
No patent action is contemplated by NASA. 
Source: Frank D. Walsh

of The Boeing Company
under contract to

Western Support Office

(WOO-286) 
Brief 67-10082
	
Category 02


Fatigue zones in metals identified by polarized light photography

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