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Book ChapterDOI

A computer-controlled automated test system for fatigue and fracture testing

01 Jan 1989-

AbstractA 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 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. 24 refs., 6 figs.

Summary (2 min read)

INTRODUCTION

  • The automation of materials testing equipment is certainly not a recent concept.
  • This is evident from the relatively narrow technical field for which this symposium was developed.
  • The characteristics of automated systems are as diverse as the applications, reflecting the particular needs of the user.
  • 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.

BACKGROUND

  • Automated testing for evaluation of fracture resistance was largely spurred by developments in elastic-plastic fracture mechanics.
  • The unloading compliance test procedure requires excellent test control, high-precision data acquisition capability, and rapid calculation.
  • 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.
  • Dc-pd is an important aspect of the testing system and analysis procedures described herein.
  • The dc-current programmer consists of a 12-bit digital-to-analog converter and a transformer-isolated output amplifier and can set the output of the dc-current supply over a range of 0 to 100 A.

DESCRIPTION OF TEST SYSTEM

  • Function generator ramp outputs (used in J-R testing) are obtained by using a 12-bit digital-to-analog converter to set input current to an operational amplifier integrator; this results in ramp outputs that are smooth and stepless even at low ramp rates.
  • The test controller incorporates photo-isolation on all input/output lines to the computer and programming lines to the dc-current supply, the extensometer is electrically isolated from the specimen by using Teflon spacers between the razor blades and the specimen, and the specimen grips are electrically isolated from the load frame by ceramic inserts in the load train ball joints.
  • The single-point ground chosen is that of the servohydraulic test machine electronics.
  • Comparison of the fits revealed that first order coefficients for low and high temperatures varied less than 4% from that at room temperature.

ELASTIC-PLASTIC FRACTURE MECHANICS TESTING

  • The specimen is similarly cycled a third time to obtain the initial value of compliance which, together with the previously entered estimate for initial crack length, is used to calculate an effective modulus (as specified in ASTM E 813).
  • The initial slope of the load-displacement trace is used to determine the 95% secant offset line as described in ASTM E 399.
  • During the hold period, the computer acquires and stores 60 readings of both pd-active and pd-reference voltages with full current and zero current.
  • The amount of unloading is dependent on the accuracy of the compliance determined during the previous unloading (a statistical analysis of the unloading data is performed).

ANALYSIS PROCEDURES

  • The length of the fatigue precrack and the final crack length are measured, as prescribed in ASTM E 813, using a digital measuring microscope.
  • The computer analysis routine uses the measured initial crack length value and the compliance determined at the initial test unloading to calculate an effective modulus (the equation in ASTM E 813 is used) which is used for the balance of the analysis.
  • This procedure differs from that described by Lowes and Fearnehough, 12 and used by others, 13 " 15 and investigations to determine which of the procedures gives the more reliable results are continuing.
  • Final measured crack length is compared to those predicted by unloading compliance and dc-pd analyses, and the differences in crack extension printed.
  • The routine displays the results and the power-law fit on the CRT and prompts the operator to accept the fit; if the fit is rejected, any suspect test results may be excluded and the regression repeated.

FATIGUE CRACK GROWTH TESTING

  • For each pd-voltage sampling, the load, pd-active and pd-reference voltage waveforms are read twice, once with full current and once with zero current.
  • The software contains logic to determine if a data point is significant based on the change from the last crack extension value.
  • The testing performed so far has yielded very promising results.
  • Software for the test system and analytical techniques described is being developed for various types of FCG tests (constant load, constant K, decreasing K, constant K^-increasing R ratio, etc.), and specific test results will be reported separately.

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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™*^,, „»,«
authored by a contractor ol tha U.S.
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K fT% U.S. Govarnmant rataina a nonaxckisiva.
purpcuat.

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

Citations
More filters

Journal ArticleDOI
Abstract: Commercially available unalloyed molybdenum (Low Carbon Arc Cast (LCAC)), Oxide Dispersion Strengthened (ODS) molybdenum, and TZM molybdenum were subject to fracture toughness testing following neutron irradiation at temperatures of nominally 244 °C, 407 °C, and 509 °C to neutron fluences between 1.0 and 4.6 × 1025 n/m2 (E > 0.1 MeV). All alloys exhibited a Ductile to Brittle Transition Temperature that was defined to occur at 30 ± 4 MPa m . The highest post-irradiated fracture toughness values (26–107 MPa m ) and lowest DBTT (100–150 °C) was observed for ODS molybdenum in the longitudinal orientation. The results for ODS molybdenum are anisotropic with lower post-irradiated toughness values (20–30 MPa m ) and higher DBTT (450–600 °C) in the transverse (T-L) orientation. The results for ODS molybdenum are better than those for LCAC molybdenum (21–71 MPa m and 450–800 °C DBTT). The fracture toughness values measured for LCAC and T-L ODS molybdenum at temperatures below the DBTT were determined to be 8–18 MPa m . The role of microstructure and grain size on post-irradiated fracture toughness was evaluated.

9 citations


Book ChapterDOI
01 Jan 2001
Abstract: This paper describes the fracture toughness characterization of annealed 304L and 316L stainless steels and precipitation hardened Alloy 718, performed at the Oak Ridge National Laboratory as a part of the experimental design and development for the Accelerator Production of Tritium (APT) target/blanket system. Materials were irradiated at 25 to 200C by high-energy protons and neutrons from an 800-MeV, 1-mA proton beam at the Los Alamos Neutron Science Center (LANSCE). The proton flux produced in LANSCE is nearly prototypic of anticipated conditions for significant portions of the APT target/blanket system. The objective of this testing program was to determine the change in crack-extension resistance of candidate APT materials from irradiation at prototypic APT temperatures and proton and neutron fluxes. J-integralresistance (J-R) curve toughness tests were conducted in general accordance with the American Society for Testing and Materials Standard Test Method for Measurement of Fracture Toughness, E 1820-99, with a computer-controlled test and data acquisition system. J-R curves were obtained from subsize disk-shaped compact tension specimens (12.5 mm in diameter) with thicknesses of 4 mm or 2 mm. Irradiation up to 12 dpa significantly reduced the fracture toughness of these materials. Alloy 718 had the lowest fracture toughness in both the unirradiated and irradiated conditions. All irradiated specimens of Alloy 718 failed by sudden unstable crack extension regardless of dose or test temperature. Type 304L and 316L stainless steels had a high level of fracture toughness in the unirradiated condition and exhibited reduction in fracture toughness to saturation levels of 65 to 100 MPa/m. The present reduction in fracture toughness is similar to changes reported from fission reactor studies. However, the currently observed losses in toughness appear to saturate at doses slightly lower than the dose required for saturation in reactor-irradiated steels. This difference might be attributed to the increased helium and hydrogen production associated with irradiation in the high-energy, mixed proton/neutron spectrum.

8 citations


Cites methods from "A computer-controlled automated tes..."

  • ...The J-integral-resistance (J-R) curve toughness tests were conducted in general accordance with the American Society for Testing and Materials (ASTM) E 1820-99 [5] Standard Test Method for Measurement of Fracture Toughness, with a computercontrolled test and data acquisition system [6]....

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Journal ArticleDOI
Abstract: This paper investigates the effect of load shedding steps on constant-ΔK testing. ASTM fatigue crack growth specimens have been used in constant-ΔK tests for many fatigue investigations. In the tests, load levels need to be gradually stepped down to keep the stress intensity factor constant. This work points out that care should be taken in choosing the length of the step in order to control ΔK variation. Formulas for ΔK as well as crack growth rate variability are derived. The experimental application of these techniques is also presented.

2 citations


25 May 2013
Abstract: Recent advances in fracture toughness have led to employment of the Weibull statistic to model scatter of fracture toughness in the transition region of low-alloyed ferritic steels. This methodology uses a concept of the universal temperature dependence of fracture toughness in the transition region, the so-called “master curve”. The current physical background for this methodology suggests that it is applicable to a wide variety of ferritic bcc steels, including tempered ferritic-martensitic steels. Those steels are structural material candidates for fusion reactors. However, irradiation embrittlement (shift of the ductile-to-brittle transition region) of these steels is one of the main concerns for application purposes. Yet, the transition fracture toughness data for this class of steels are rather sparse. In this study, two types of fracture toughness specimens of a F82H steel were tested to verify the master curve concept. Specimens were tested at several temperatures in the transition region and at least four specimens were tested at each temperature allowing for application of the Weibull statistic/master curve analysis procedure. The largest specimens were 1T compact specimens. Broken halves of 1T specimens were later used to machine and test smaller, 0.4T, size specimens which could be more suitable for irradiation experiments. It was shown that scatter of fracture toughness of this material was rather high relative to scatter of low-alloyed steels but it was similar between larger and smaller specimens.

Cites methods from "A computer-controlled automated tes..."

  • ...Transition Range, with a computer-controlled test and data acquisition system (Nanstad [1])....

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