Torsion system proves exploratory data on accelerated creep due to multiple stress reversals. Torsional testing of tubular specimens is best suited for reversed stress creep tests since large strains are obtainable while maintaining specimen geometry

Lilienthal, P. A.

English

NASA Technical Reports Server

May 1969	 Brief 69-10147 
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. 
Torsion System for Creep Testing with Multiple Stress Reversals 
.
A torsion system has been devised and operated to 
provide exploratory data on accelerated creep due to 
multiple stress reversals. These data should be of in-
terest to metallurgists engaged. in the development of 
construction materials since conventional creep and 
relaxation testing is static and offers little information 
on the cyclic creep behavior of metals. A constant-
torque pulley and dead weight system, used to re-
peatedly reverse shear stress on tubular specimens, 
proved to be slow and complex. Push—pull equipment 
has also been used but was inherently limited to small 
strains due to such deformational instabilities as neck-
ing and buckling. 
Torsional testing of tubular specimens appears best 
suited for reversed stress creep tests since large strains 
are obtainable while maintaining specimen geometry. 
Although truly uniform stress distribution is sacrificed, 
the necking, subsequent instability, and the nonuni-
formities in stress and strain encountered in other tests 
are reduced. 
In this test system, torsional loads are measured by 
using the elastic response of a shaft. The amount of 
windup, directly proportional to the torque, is meas-
ured by a pair of linear variable differential trans-
formers (LVDT's) mounted in a holding fixture. Elec-
tronic functions are provided by a load controller. 
Torque is applied by a hydraulic rotary actuator con-
nected to a torque cell and mounted in a load frame. 
Fluid flow to the actuator chambers is controlled by a 
servovalve, and pressure variations at the valve mani-
fold are minimized by an accumulator. By measuring 
the relative angle of rotation of two specimen cross 
sections, a measure of the shearing strain is obtained. 
The operation involves two collars, clamped around 
the specimen, which fasten arms to the specimen sec-
tions of interest. These arms are connected to two 
coaxial cylinders that are free to rotate and translate 
about and along each other and the bearing race on the 
torque cell. Relative rotation of the two section clamps 
is translated into a one-to-one relative rotation of the 
coaxial cylinders. Such displacement is measured with 
a rotary variable differential transformer (RVDT), 
while transducer excitation and readout are performed 
by an amplifier—demodulator. 
Either stress in terms of torque, or strain in terms 
of relative angle of twist may be controlled in a closed-
loop mode. The load controller generates a current 
that drives the servovalve to force the load or relative 
angle of twist to assume values dictated by a com-
mand signal. Feedback signals generated by the torque 
cell and relative angle of twist device are scaled and 
fed into a controller for comparison with the com-
mand signals. 
For elevated-temperature tests, the specimen is 
heated by a three-zone radiant heater in conjunction 
with three chromel-alumel thermocouples which sense, 
respectively, temperatures at the specimen center and 
its outer extremities within the heater. Two differen-
tial transducers measure the temperature differences 
between the center and outer portions of the specimen. 
Independent control of each of the three heater zones 
affords some control over axial thermal gradients. 
Note: 
Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP 69-10147
(continued overleaf) 
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=19690000147 2020-03-16T17:57:23+00:00Z
Patent status: 
No patent action is contemplated by NASA. 
Source: Paul A. Lilienthal 
University of Illinois 
under a grant 
from NASA Headquarters 
(HQN- 10039)
. 
. 
Brief 69-10147	 Category 03


Torsion system for creep testing with multiple stress reversals

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