Datasheet

www.vishaymg.com [e-mail] Document Number: 11304
60 Revision: 05-Sep-06
RS-200
Vishay Micro-Measurements
Milling Guide
for Residual Stress Measurements
INTRODUCTION
A predominant factor contributing to the structural failure of
machine parts, pressure vessels, framed structures, etc., may
be the residual "locked-in" stresses that exist in the object prior
to its being put into service. These residual stresses are
usually introduced during manufacturing, and are caused by
processes such as casting, welding, machining, heat treating,
molding, etc.
Residual stress cannot be detected or evaluated by
conventional surface measurement techniques, since the
strain sensor (strain gage, photoelastic coating, etc.) can only
respond to strain changes that occur after the sensor is
installed.
The most widely used practical technique for measuring
residual stresses is the hole-drilling strain gage method
described in ASTM Standard E837. With this method, a
specially configured electrical resistance strain gage rosette is
bonded to the surface of the test object, and a small shallow
hole is drilled through the center of the rosette. The local
changes in strain due to introduction of the hole are measured,
and the relaxed residual stresses are computed from these
measurements. Vishay Micro-Measurements Tech Note
TN-503, Measurement of Residual Stresses By The
Hole-Drilling Strain Gage Method, presents a detailed
discussion of the theory and application of this technique.
The hole-drilling method is generally considered semi-
destructive, since the drilled hole may not noticeably impair the
structural integrity of the part being tested. Depending on the
type of rosette gage used, the drilled hole is typically 0.062 or
0.125in (about 1.5 or 3.0mm), both in diameter and depth. In
many instances, the hole can also be plugged, if necessary, to
return the part to service after the residual stresses have been
determined.
The practicality and accuracy of this method is directly related
to the precision with which the hole is drilled through the center
of the strain gage rosette. The Vishay Micro-Measurements
RS-200 milling guide provides a practical means to accomplish
this task.
RS-200 MILLING GUIDE
The RS-200 Milling Guide is a precision fixture for accurate
positioning and drilling of a hole through the center of a special
strain gage rosette. Principal features and components of the
milling guide assembly are shown in the photos below. When
installed on the workpiece, the guide is supported by three
leveling screws footed with swivel mounting pads to facilitate
attachment to uneven surfaces.
Alignment of the milling guide relative to the strain gage rosette
is accomplished by inserting a special-purpose microscope
into the guide's centering journal, and then positioning the
guide precisely over the center of the rosette by means of four
X-Y adjusting screws. The microscope assembly, consisting of
a polished steel housing with eyepiece, reticle, and objective
lens, permits alignment to within 0.0015in (0.038mm) of the
gage center. The microscope is also used to measure the
diameter of the hole after it is drilled. An illuminator attaches to
the base of the guide to aid in the optical alignment procedure.