A COMPUTER-CONTROLLED AUTOMATED TEST SYSTEM
FOR FATIGUE AND FRACTURE TESTING*
R. K. Nanstad, D. J. Alexander, R. L. Swain,
J. T. Hutton,
+
and D. L. Thomas*
Metals and Ceramics Division
Oak Ridge National Laboratory CONF-8905112—3
Oak Ridge, Tennessee 37831-6151
DE90
ABSTRACT
A computer-controlled system consisting of a servohydraulic test machine,
an in-house designed test controller, and a desktop computer has been developed
for performing automated fracture toughness and fatigue crack growth testing both
in the laboratory and in hot cells for remote testing of irradiated specimens.
Both unloading compliance and dc-potential drop can be used to monitor crack
growth. The test controller includes a dc-current supply programmer, a function
generator for driving the servohydraulic test machine to required test outputs,
five measurement channels (each consisting of low-pass filter, track/hold
amplifier, and 16-bit analog-to-digital
converter),
and digital logic for various
control and data multiplexing functions. The test controller connects to the
computer via a 16-bit wide photo-isolated bidirectional bus. The computer, a
Hewlett-Packard series 200/300, inputs specimen and test parameters from the
operator, configures the test controller, stores test data from the test
controller in memory, does preliminary analysis during the test, and records
'Research sponsored by the Office of Nuclear Regulatory Research, U.S.
Nuclear Regulatory Commission, under Interagency Agreement DOE 1886-8011-9B with
the U.S. Department of Energy under contract DE-AC05-84OR21400 with Martin
Marietta Energy Systems, Inc.
instrumentation and Controls Division. -T*, *&«„* m™*^,, „», b««
authored by a contractor ol tha U.S.
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DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States
Government. Neither the United Slates Government nor any agency
thereof,
nor any of their
employees, makes any warranty, express or implied, or assumes any legal liability or responsi-
bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or
process disclosed, or represents that its use would not infringe privately owned rights. Refer-
ence herein to any specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-
mendation, or favoring by the United States Government or any agency
thereof.
The views
and opinions of authors expressed herein do not necessarily state or reflect those of the
United States Government or any agency
thereof.
2
sensor calibrations.specimen and test parameters, and test data on flexible
diskette for later recall and analysis with measured initial and final crack
length information. During the test, the operator can change test parameters
as necessary.
KEY WORDS
Computer, fatigue, crack growth, fracture toughness, test controller,
compliance, dc-potential, J-integral, multichannel, J-Rcurve, analog-to-digital,
photo-isolation,
CMOS,
clip gage, automation, interface.
INTRODUCTION
The automation of materials testing equipment is certainly not a recent
concept.
Researchers have applied various degrees of automation over the years
with the general objectives of increasing productivity, efficiency, and
consistency. The advent of desktop laboratory computer systems capable of
machine control and data acquisition was the real catalyst in this area, and the
result has been an explosion of computer automation. This is evident from the
relatively narrow technical field for which this symposium was developed. The
applications for computer automation span the entire range of technology. The
characteristics of automated systems are as diverse as the applications,
reflecting the particular needs of the user. Some are hard, dedicated,
inflexible systems which perform precisely the same set of tasks for every
operation, while others have a high degree of flexibility and adaptability. The
needs of the fracture and fatigue testing community span that range. The rapid
3
evolution of sophisticated desktop computers, peripheral devices, and other
electronic hardware, in terms of speed and memory capacity, has likewise allowed
for increasingly greater flexibility and capability in the software for test
control,
data acquisition, storage, and analysis.
The Fracture Mechanics Group of the Metals and Ceramics Division at
Oak Ridge National Laboratory began a computer automation activity in 1978 for
the purpose of conducting elastic-plastic fracture mechanics tests. The system
has evolved markedly since then, particularly in terms of speed. The need for
test control and rapid data acquisition during fatigue crack growth testing
spurred the development of a high-speed, multichannel test controller. This
paper describes the computer-automated system, test and analysis procedures, and
some test results.
BACKGROUND
Automated testing for evaluation of fracture resistance was largely spurred
by developments in elastic-plastic fracture mechanics. Starting with the concept
of the J-integral by Rice
1
and the description of a practical means for
estimating J vs crack extension in test specimens by Rice et al. ,
2
the advantages
of computer involvement were apparent. It was the development of the unloading
compliance test method,
3
however, which forced the incorporation of computers in
tesc:
systems. The unloading compliance test procedure requires excellent test
control,
high-precision data acquisition capability, and rapid calculation. The
use of computers for automated unloading compliance testing has been described
by a number of researchers.*-
5
-
6
4
Although the unloading compliance technique is an accepted procedure for
determining J
Io
and J resistance (J-R) curves in ASTM standards, obtaining
accurate and consistent test results is not an easy task. Because the unloading
compliance technique involves a fairly large number of periodic unloading cycles,
usually with a hold period at the start of each cycle to allow for load
relaxation in the system, the testing time can be on the order of one hour. In
many instances, especially those involving remote testing of irradiated specimens
in a hot
cell,
the high expense of facilities and equipment mandate that all
feasible reductions in testing time be effected. Because the unloading
compliance test requires high-precision measurements of displacement during each
unloading cycle, the extensometer (usually a clip-on displacement gage) is very
important. The extensometer must be carefully calibrated and must be seated in
such a way that effects of error sources such as friction and vibration are
minimized. Testing at low and high temperatures adds temperature shifts in
extensometer calibration as another source of error. The ability to accurately
infer crack length without resorting to unloading the specimen (with the
associated extensometer and sources of error) has made the dc-potential drop (dc-
pd) method for determining the crack length a widely used technique for both
fracture mechanics
(J-R)*
and fatigue crack growth
(FCG)
7
tests. Dc-pd is an
important aspect of the testing system and analysis procedures described herein.
DESCRIPTION OF TEST SYSTEM
Figure 1 shows a block diagram of the major components of the interactive
fracture mechanics test system. The computer is a Hewlett-Packard series 200/300
with 4 MB of random-access memory and Hewlett-Packard technical BASIC operating
