NASA 

Technical Memorandum 10204 1 

AVSCOM 

Technical Report 89-C-002 

A Data Acquisition and Control Program for 

Axial-Torsional Fatigue Testing 

Sreeramesh Kalluri 

Sverdrup Technology, Znc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 

and 

Peter J. Bonacuse 

Propulsion Directorate 

U. S. Army Aviation Research and Technology Activity-A VSCOM 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

Symposium on the Applications of Automation Technology 

to Fatigue and Fracture Testing 

sponsored by the American Society for Testing and Materials 

Kansas City, Missouri, May 22-23, 1989 

a 

US ARM 

AVlATlO 

SYSTEMS COMMANO 

AVIATION RBT ACTIVITY 

(NBSA-T8-102041) B DATA ACQUISXTXON AND N89-230 29 

COBTROL PROGRAn FOR AXIAL-TORSIONAL FATIGUE 

TESTXNG (NASA. Lewis Research Center) 23 P CSCL 20K Unclas 

G3/39 0224991 
A DATA ACQUISITION AND CONTROL PROGRAM FOR AXIAL-TORSIONAL FATIGUE TESTING 

Sreeramesh Kal 1 uri 

Sverdrup Technology, Inc. 

NASA Lewis Research Center Group 

Cleveland, Ohio 44135 

and 

Peter J. Bonacuse 

Propulsion Directorate 

Lewis Research Center 

Cleveland, Ohio 44135 

U.S. Army Aviation Research and Technology Activity - AVSCOM 

SUMMARY 

A computer program was developed for data acquisition and control of the 

axial-torsional fatigue experiments at NASA Lewis Research Center. The multitasked, interrupt-driven program was written in Pascal and assembler. This 

program is capable of dual-channel control and six-channel data acquisition. 

It can be utilized to perform inphase and out-of-phase axial-torsional isother0 mal fatigue or deformation experiments. The program was successfully used to 

conduct inphase axial-torsional fatigue experiments on 304 stainless steel at 

room temperature and on Hastelloy X at 800 "C. This paper describes the 

details of the software and gives some of the results generated to date. 

m 

h 

-4. 

I 

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INTRODUCTION 

Failure of engineering components due to fatigue is a common occurrence in 

the aerospace and automotive industries. It is therefore necessary to characterize the fatigue behavior of materials so that the engineering components can 

be operated safely and reliably. The fatigue behavior of a material is usually 

determined by conducting axial (tension/compression) and sometimes torsional 

fatigue experiments at the service temperature. In reality, however, engineering components are subjected to loads in multiple directions, and these loads 

produce complex states of stress and strain within the component. The fatigue 

life is governed by the state of stress and strain within the component. 

Indeed, the often detrimental effect of multiaxial states of stress and strain 

on fatigue life has been well documented in the literature (refs. 1 to 3). 

Fatigue life prediction models developed primarily with axial or torsional 

fatigue data can lead to erroneous life estimates when they are used to predict fatigue life for multiaxial stress and strain states. The reason for this 

error is that under complex multiaxial states of stress and strain the mechanisms causing fatigue failure can be completely different from those causing 

fatigue failure under either axial or torsional loading conditions. As a 

result, fatigue life prediction models must be developed with data generated 

under multiaxial loading conditions SQ that the fatigue lives of components can 

be estimated accurately. 

Generation of fatigue data under multiaxial states of stress and strain 

is not only expensive but is also very labor intensive. Fatigue testing under 
multiaxial loading has been limited by the complexities involved as well as by 

the lack of suitable equipment to automate such experiments. Hence, only a 

limited amount of fatigue data is available in the literature for multiaxial 

conditions, compared with the large amount of axial and torsional fatigue data. 

experiments to document the fatigue behavior of a material under specific 

multiaxial states, such as combined axial-torsional testing and biaxial tension 

testing (refs. 4 and 5). Even under these seemingly simple multiaxial states 

I of stress, substantial experimental difficulties can be encountered. For 

example, in axial-torsional fatigue (unlike simple axial or torsional fatigue) 

two waveforms of loading must be controlled simultaneously. In addition, 

these axial and torsional waveforms can be applied either inphase (proportional 

no phase difference exists between the axial and torsional waveforms, whereas 

during out-of-phase loading the torsional waveform lags behind the axial waveform by a predetermined phase angle. 

out-of-phase axial-torsional waveforms are shown in figure 1. Under these cirthe testing procedures. 

The axial and torsional stress-strain hysteresis loops, and the variation 

of axial and torsional stress and strain with time have to be recorded for the 

duration of an axial-torsional fatigue test. These data are required to characterize the fatigue life and the hardening-softening behavior of a material 

under the imposed loading conditions. In the past, these tasks were performed 

by using X-Y recorders and strip chart recorders, respectively. The testing 

procedures can be simplified considerably by automating both the control of the 

axial-torsional fatigue experiment and the acquisition of the required data. 

This automation is possible with digital computers and appropriate hardware 

and software. Automation with a digital computer can considerably reduce the 

amount of human attention required to monitor the fatigue experiment. 

tion, data stored on the computer can easily be accessed for further analysis. 

I More often than not, researchers have chosen to conduct multiaxial fatigue 

I loading) or out-of-phase (nonproportional loading). During inphase loading, 

Schematic illustrations of inphase and ~ 

I 

I 

cumstances, computer data acquisition and control are necessary to simplify 

In addiI A computer program was written in Pascal at NASA Lewis Research Center to 

I control as well as to acquire data from either an inphase or an out-of-phase 

I to interface with test control and data acquisition hardware. Specific attenI 

axial-torsional isothermal fatigue experiment. Assembler routines were written 

tion was given to speed of execution and efficiency in these routines. 

program can generate either sine or triangular axial-torsional control waveforms that can be used under either load control or strain control. The capabilities of the software, the equipment required to run the software, and some 

of the data acquired are presented in this paper. 

The 

I BACKGROUND 

Automation of combined axial-torsional testing was first reported during 

the 1970's by Penn, Fong, and Kearsley (ref. 6). Penn et al. described the 

computer software and hardware that were necessary to control and acquire data 

from axial-torsional experiments. Their test facility consisted of a servocontrolled biaxial test machine and a computer system for test control and data 

acquisition. The software was written primarily in BASIC with some subroutines 

written in assembly language. Penn et al. conducted computer-controlled axialtorsional deformation experiments on thin tubular specimens made of polyvinyl 

2 
chloride (PVC). The results of the deformation experiments were used by Penn 

and his coworkers to characterize the biaxial yield and ultimate strength of 

PVC for different axial-to-torsional load ratios. However, only a limited 

amount of fatigue testing results were reported by Penn and his coworkers 

(ref. 6). The reported fatigue results were confined to pure axial fatigue 

and axial fatigue with a static torque. No combined axial-torsional fatigue 

testing results were reported. 

In general, combined axial-torsional loading of metals and alloys can 

activate failure mechanisms that are not present in either axial fatigue or 

torsional fatigue tests. Hence, it is necessary to conduct combined axialtorsional fatigue testing to understand the different types of mechanisms that 

are likely to cause failure under these conditions. 

In order to simplify the experimental procedure required for performing 

the combined axial-torsional fatigue tests, automation of the control and data 

acquisition of the fatigue testing system is essential. The advances in computers and electronics, and the subsequent reductions in cost, have brought 

the task of automating axial-torsional fatigue tests within the reach of experimentalists. The set of instructions required to automate an axial-torsional 

tes can be encoded into a computer program that can be used to control the 

fat gue test under the combined axial-torsional loading and to acquire the 

des red data simultaneously. 

are described in the following section. 

The desirable features of such a computer program 

AXIAL-TORSIONAL FATIGUE TESTING SOFTWARE REQUIREMENTS 

In general, the goal of automating any system is to minimize the number 

of instructions required while maximizing the computer-controlled aspects of a 

test program. The control of axial and torsional loading waveforms and simultaneous acquisition of data require several features in the software. These 

features can be divided into four major areas: 

(1) Determination of elastic and shear moduli 

(2) Dual-channel command waveform generation 

(3) Data acquisition from multiple channels 

(4) Program interruption capability at predetermined intervals 

In the following paragraphs, the importance of each of these features is 

described in detail. 

Determination of Elastic and Shear Moduli 

The elastic modulus of a material (either axial or shear) is a very useful piece of information in fatigue analysis. During fatigue experiments, it 

is common to control either load or strain. However, while analyzing the 

experimental data, most strain-based fatigue 1 ife prediction models require the 

separation of the total strain into elastic and inelastic components (refs. 7 

and 8). The elastic strain component is usually calculated by dividing the 

stress by the elastic modulus. The inelastic strain is then obtained by subtracting the elastic strain from the total strain. Thus, for axial-torsional 

fatigue data analysis, the Young's and shear moduli must be determined for 

3 
each specimen. In addition, the variation in these properties within the same 

heat of material can also be documented by determining the Young's and shear 

moduli for each specimen. 

Dual-Channel Command Waveform Generation 

The primary goal of the axial-torsional test control and data acquisition 

software is to generate independent, yet synchronized, axial and torsional 

waveforms of loading at a desired frequency. The software should enable the 

experimentalist to apply these axial and torsional waveforms of loading either 

inphase or with a prescribed phase difference (e.g., 0", 30", 60", and 90"). 

In addition it would be useful if the software could generate different types 

of waveforms: namely, triangular (constant rate) and sinusoidal waveforms. 

At room temperature, most metals and alloys exhibit a deformation response 

which is independent of the type of waveform of loading. However, the type of 

waveform can influence the deformation response of these materials significantly at elevated temperatures. For a given amplitude and frequency, the 

material spends more time at higher stress during a sinusoidal waveform than 

it does during a triangular waveform. Thus, at elevated temperatures a sinusoidal waveform is more likely to introduce time-dependent creep deformation 

in the material than is a triangular waveform. The software should also enable 

the experimentalist to conduct the fatigue test in either load control or 

strain control. 

Data Acquisition From Multiple Channels 

The second goal of the axial-torsional test control software is to acquire 

load and deformation data from both the axial and torsional controllers while 

the test is in progress. The functions of controlling the fatigue test and 

simultaneously acquiring the data can be accomplished through a feature known 

as "multitasking," which is commonly used in the real-time control of processes. This feature allows two independent "threads" of code to be executed 

in apparent concurrency by a single central processing unit (CPU). 

data are written to a file in American Standard Code for Information Interchange (ASCII) format, they can be manipulated with commercially available data 

analysis packages to construct axial and torsional stress-strain hysteresis 

loops and to characterize the hardening and softening behavior of the material 

under investigation. The axial and torsional hysteresis loops generated thus 

can be used subsequently to estimate the fatigue life under combined axialtorsional loading conditions. 

Once the 

Program Interruption Capability at Predetermined Intervals 

Crack initiation in fatigue usually occurs on the surface of a specimen 

(ref. 9). In the absence of internal defects, the cracks tend to originate at 

the surface of the specimen and to propagate either into the specimen or along 

the surface as the number of cycles increases. Replication of the specimen 

surface with cellulose film is one of the techniques used to monitor the initiation and growth of fatigue cracks (ref. 10). By interrupting the program and 

replicating the surface of the specimen at periodic intervals during a fatigue 

test, a quantitative measure of the crack growth rate can be obtained. This 

4 
technique also provides qualitative information on the mechanisms prevailing 

during crack initiation and propagation. In addition, it is sometimes necessary to interrupt the fatigue tests well before failure either to study the 

damage mechanisms or to evaluate the extent of damage to the specimen. Metallographic techniques are usually employed to characterize the damage mechanisms, while nondestructive techniques such as dye penetration and radiography 

are used to assess the extent of damage in the specimen. The necessity of 

periodic evaluations of cracks and damage make program interruption capability 

a requirement for the software. 

Features in the four major areas that have been described should be 

included in the axial-torsional fatigue test-control and data acquisition software. However, this set of features is by no means complete. For example, 

temperature control, which is required in a thermomechanical fatigue test, has 

not been addressed. The current program automates only isothermal axialtorsional fatigue testing. Nonisothermal axial-torsional fatigue testing could 

be automated by adding another control channel and a data acquisition channel 

for temperature. 

EXPERIMENTAL SYSTEM 

The experimental system consists of an axial-torsional servohydraulic 

fatigue test rig, a minicomputer, digital-to-analog (D/A) and analog-to-digital 

(AID) hardware, and computer software. Since the software development depends 

on the machine interface hardware available and the computer system, these two 

systems are described first. The details of the software are presented at the 

end of this section. 

Axial-Torsional Fatigue Rig 

The axial-torsional fatigue rig (fig. 2) at NASA Lewis is a dual-post 

test machine, with hydrocollet grips designed for smooth-shank fatigue specimens. The geometry of a thin-walled, tubular axial-torsional fatigue specimen 

is shown in figure 3. The upper grip of the test rig is rigidly attached to 

the axial-torsional load cell, which in turn is bolted to the cross-head of 

the machine. The lower grip is capable of executing a helical motion. Thus, 

tension and compression, and clockwise and counter-clockwise twist, can be 

imparted by the movements of the bottom grip. Axial and torsional strains are 

measured with a commercially available high-temperature extensometer. A photograph of the axial-torsional extensometer is shown in figure 4. The mounting 

technique and the operational details of the extensometer are presented in reference 11. A linear actuator imparts the axial loads to the specimen, while a 

rotary actuator imparts the torsional loads. The displacement and rotation of 

the axial and rotary actuators are measured by a linear and a rotary variabledifferential transformer, respectively. The axial and torsional load, strain, 

and stroke specifications of the system are listed in table I. The test rig 

is also equipped with a 50-kW audiofrequency induction heating system to perform elevated temperature fatigue tests. 

The axial and rotary actuators are driven by two separate servovalves, 

each with a 5 gallmin capacity. The axial and torsional servovalves are control led by two separate servocontrol lers. Each of these control lers i s capable 
of operating in any one of the following control modes: load, strain, or 

stroke. Each servocontrol channel (on either the axial or torsional servocontroller) can operate at four different range settings: that is, 100, 50, 20, 

and 10 percent of the rated capacity. All of the servocontrol channels are 

designed to operate between 210 V regardless of the range setting of the servocontrol channel. The test rig can be operated in axial mode, torsional mode, 

or combined axial-torsional mode. 

Computer Hardware 

The computer system used in this program consists of a Data General S/20 

minicomputer (Data General, Westborough, MA) interfaced to a D/A converter and 

a multiplexed A/D converter. The S/20 minicomputer uses a multiprocessing, 

multitasking operating system that enables multiple tasks to be serviced by a 

single CPU. This particular feature of the computer system permits simultaneous waveform generation and data acquisition. 

tions of the computer system and the test interface hardware. 

Table I1 contains the specificaThe computer system was used to generate two separate command waveforms 

that served as the input signals for the axial and torsional servocontrollers. 

Data acquisition from the axial and torsional load, strain, and stroke transducers and the subsequent output of these data to the hard disk was also performed with the computer system. The interface between the axial-torsional 

test rig and the computer system is shown schematically in figure 5. 

Once the fatigue test was completed, the data were transferred via a highspeed multiprocessor communication system from the S/20 minicomputer to a Data 

General MV/4000 computer (Data General, Westborough, MA). Data were archived 

and analyzed on the MV/4000 system, which was also used as an environment for 

test-control software development. 

The details of the computer network of NASA Lewis' High Temperature 

Fatigue and Structures Laboratory are described in reference 12 by McGaw and 

Bartolotta. 

Computer Software 

The axial-torsional fatigue test-control and data acquisition software was 

written in Pascal language for two principal reasons. First, Pascal is a 

well-structured (with respect to modularity and readability) and "strong-typed" 

(all variables and their data type must be explicitly declared) programming 

language. Second, implementation of multitasked procedures for the existing 

operating system on the SI20 minicomputer is less cumbersome with Pascal than 

with either BASIC or Fortran languages. The D/A, A/D, and programmable interval timer (PIT) device driver procedures were written in assembly language. 

The software package was developed to meet the fatigue testing software 

requirements discussed in the earlier sections. In the following paragraphs 

the main features of the axial-torsional fatigue testing software are 

described, and flow diagrams are presented to indicate the control path and 

data flow. Examples of applications of the software are presented in the next 

sect i on. 

6 
Figure 6 shows a flow diagram for the logic of the axial-torsional 

fatigue testing software. The relevant test data are input to the program 

through an initialization procedure. This procedure queries the user for the 

following parameters: (1) the name of the engineer, (2) the name of the material and the specimen number, (3) the inner and outer radii of the tubular 

test specimen, (4) the ambient and test temperatures, (5) the mean coefficient 

of thermal expansion, and (6) the names for the data storage files. The procedure then computes the gage length, the cross-sectional area, and the torsional 

moment of inertia of the specimen at the specified test temperature. 

The data for the determination of the axial and shear moduli are input 

through a second data inout procedure. Initially, the axial and torsional 

load, strain, and stroke range settings for the servocontroller are input 

through this procedure. The procedure then assigns the appropriate engineering 

units to each of the ranges from the calibration constants of the transducers 

which have been "hard-coded" into the program. This procedure requires an 

estimate of the axial yield strength of the material at the test temperature 

and the frequency of elastic loading waveforms. The program estimates the torsional yield strength of the material from the axial yield strength with the 

von Mises yield criterion. The'procedure then computes the upper limits for 

the axial and torsional loads by using only 25 percent of the estimated axial 

and torsional yield strengths and the specimen geometry constants generated 

for the specified test temperature. Limiting the axial and torsional loads to 

about 25 percent of the respective estimated yield strengths, ensures that no 

plastic deformation occurs in the specimen during the determination of the 

elastic moduli. 

The elastic modulus determination is performed under load control with 

triangular waveforms. A schematic illustration of the waveforms used for the 

elastic loadings is shown in figure 7. Each of the triangular waveforms is 

incremented in multiple steps. The 12-bit D/A converter is capable of analog 

output values ranging from -10 to 10 V with a resolution of 1 part in 4096. 

The elastic moduli procedure is programmed to calculate the digital equivalents 

of the previously computed axial and torsional load limits. The waveforms are 

divided into 500 increments each, with the time interval between each increment 

controlled by a PIT interrupt. 

1 kHz. Thus, if the frequency of the waveform of loading is 0.1 Hz, and the 

waveform has 500 increments; the interval between each PIT interrupt is 20 msec 

(20 PIT cycles). At each increment of loading, the digital value of the waveform is computed by the procedure, and the corresponding analog value is output 

by the D/A converter to the appropriate servocontroller. At the same time, the 

program resets the A/D multiplexer to the first channel and proceeds to take 

data sequentially on six channels. Because the interval is small between conversions (about 17 psec per conversion), data from all six channels are considered to be acquired simultaneously. As shown in figure 7, the axial loading is 

applied first. Once the axial elastic waveform is applied to the specimen, a 

linear regression is performed on the acquired axial stress and strain data to 

determine the Young's modulus. At this stage, the procedure is repeated with 

the torsional waveform of loading, to determine the shear modulus. The axial 

and torsional load, strain, and stroke data acquired by this procedure are 

stored in the elastic compliance data file. In addition, the elastic moduli 

can be checked to ensure that the axial-torsional test system is performing as 

The PIT is set to operate at a frequency of 

7 
desired, so that the axial-torsional fatigue experiment can 

confidence. 

be started with 

After the elastic moduli have been successfully determ ned, the input for 

the axial-torsional fatigue experiment is queried by anothe procedure. The 

software allows the user to perform the axial-torsional fat gue experiment in 

either load or strain control. The amplitudes, the type of waveforms (triangular or sinusoidal), the frequency, and the phase difference, if any, between 

the axial and torsional command waveforms are entered into the program. The 

phase difference between the axial and the torsional waveforms can vary from 

0" to 90". The digital equivalents of the axial and torsional amplitudes are 

then computed by the software. At this stage, all the specifications of the 

axial-torsional fatigue test and the results from the elastic moduli experiment are written concisely to a test specification file on the hard disk. An 

example of the test specification file is shown in the appendix. The fatigue 

test control procedure also initiates a separate task which writes the data 

collected from the axial and torsional load, strain, and stroke channels to a 

fatigue data file (fig. 6). 

Figure 8 shows a flow diagram that depicts the execution of the axialtorsional fatigue experiment. At the beginning of the experiment, the PIT is 

initialized and the D/A and A/D converters are cleared. If a phase difference 

between the axial and torsional waveforms of loading is specified, the axial 

waveform is ramped to create the required initial phase angle. The amplitudes 

of both the axial and torsional waveforms are then gradually increased over a 

period of five cycles to the full amplitudes required by the test. This experimental technique prevents the extensometer from slipping out of the indentations in the specimen. However, these five cycles are not counted towards the 

fatigue life of the specimen, and no data are collected during this portion of 

the experiment. In both triangular and sinusoidal types of waveforms, the 

axial and torsional cycles are divided into 500 increments. The PIT is used to 

create the required time interval between two successive digital output increments. These digital outputs from the SI20 computer are fed to the D/A converters, the output of which is used to control both the axial and torsional 

servocontrollers simultaneously. 

The digital data are collected from the axial and torsional load, strain, 

and stroke in a multitasking mode. These data are collected continuously during the first 10 cycles and logarithmically thereafter. At the beginning of a 

test, an array is generated which contains the cycle numbers for which the data 

is to be stored. The data are continuously acquired for every cycle during the 

axial-torsional fatigue experiment and stored alternately in two data arrays. 

If the data from a particular cycle are required to be saved, then they are 

transferred from the data array to a permanent data file on the hard disk in a 

compact format during the test. Two data arrays are utilized so that one array 

can be used to store the data while the data in the second array are being 

transferred to the hard disk. 

required to be saved, then the data stored in the data arrays are written over 

by data acquired in subsequent cycles. 

If the data of a particular cycle are not 

If the test is required to be paused for surface replication, the execution of the test-control and data acquisition software is periodically halted 

after a predetermined number of cycles. If no interruption is specified, the 

8 
test continues until the specimen fails. The test program also can be interrupted at any moment from the computer keyboard. The axial-torsional fatigue 

test control software has a programmed specimen failure detection scheme. If 

the difference between the command digital signal and the feedback digital signal of either the axial or torsional control channel exceed a programmed limit, 

then the test specimen is considered to have failed, and the generation of the 

axial and torsional command waveforms is terminated. The data from the last 

two fatigue cycles are transferred from the data arrays to the compact data 

file. Finally, the digital data stored in the compact format are unpacked and 

converted into SI units. These data in SI units are finally written to a 

fatigue data file in ASCII format. The execution of the axial-torsional 

fatigue test control software is terminated after all of the data is transferred to a ASCII data file. A portion of data from the fatigue data file is 

shown in table 111. Each axial-torsional fatigue cycle is stored in an array 

of 500 by 7 elements. The first three columns contain data from the axial 

load, strain, and stroke channels; and the next three columns contain the corresponding data from the torsional channels. The seventh column contains a 

time stamp for the data in each row. At the end of the axial-torsional fatigue 

experiment, the data are transferred from the S/20 minicomputer to the MV/4000 

host computer for further analysis and archiving. Then the data are transferred cycle by cycle to IBM PC compatible systems, and they are analyzed with 

commercially available data analysis packages. 

The axial-torsional fatigue test software described meets all four of the 

requirements discussed earlier in this report. 

APPLICATIONS OF THE AXIAL-TORSIONAL TEST-CONTROL SOFTWARE 

The axial-torsional test control and data acquisition software described 

in this report was utilized in two separate programs. To date, only inphase 

axial-torsional isothermal fatigue testing has been performed with this software. The results of some of the experiments conducted with the software are 

presented in this section. 

Inphase axial-torsional fatigue tests were conducted on 304 stainless 

steel at room temperature in strain control as part of a round-robin program 

initiated by the ASTM task group on multiaxial fatigue research. Fatigue tests 

with and without surface replication pauses were conducted with the software 

(ref. 11). The axial and torsional hysteresis loops of one of the tests are 

presented in figure 9. These hysteresis loops are constructed from the data 

collected with the software for a near half-life cycle. Figure 9 also shows 

"cross-plots" of the axial strain versus torsional strain and the axial stress 

versus torsional stress for the same cycle. If the axial strain and torsional 

strain feedback signals are inphase, then a cross-plot would show no hysteresis. However, it is interesting to note that the axial strain and torsional 

strain feedback signals are slightly out-of-phase even though the axial and 

torsional strain command signals were inphase. This is, most likely, due to 

the small differences in the gains of the axial and torsional servocontrollers. 

The cross-plot of axial stress versus torsional stress exhibits substantial 

hysteresis compared with the cross-plot of axial strain versus torsional 

strain. This can be attributed to the large amount of inelastic deformation 

occurring in the specimen. The data acquired from the axial-torsional fatigue 

test software allow these comparisons to be made with relative ease. The 

9 
hardening-softening curves for the axial and torsional stresses are shown in 

figure 10 for the same inphase axial-torsional fatigue test. 

and a 

any g be ut 

characterize the constitutive behavior of a material under biax 

Such experiments were conducted by Kanazawa et al. (ref. 13) on 

Cr-Mo-V steel to study the flow behavior of that material. The 

and data acquisition for such out-of-phase axial-torsional defo 

ments would be simplified substantially by the software package 

this paper. 

The software presented in this report was developed mainly for controlling 

cquiring data from out-of-phase axial-torsional fatigue experiments with 

iven phase difference between 0 and 90". However, this software can also 

ilized to conduct out-of-phase axial-torsional deformation experiments to 

a1 conditions. 

1 percent 

test control 

mat i on exper i - 

described in 

In the second program, the Young's modulus and the shear modulus were 

determined at different temperatures for Hastelloy X (Cabot Corporation, 

Waltham, MA). This is a nickel-base superalloy which is used as a combustorliner material in gas turbine engines. Strain-controlled, inphase axialtorsional fatlgue experiments were also conducted on Hastelloy X at 800 "C. 

single specimen was used to generate the moduli at 100 OC increments starting 

with the room temperature. At each temperature, the axial-torsional fatigue 

test-control software was initialized, and the elastic modulus determination 

procedure was executed. The software execution was stopped after el as t 1 c 

moduli were determined. The results (fig. 11) are in close agreement with the 

data available for this superalloy (ref. 14). The axial and torsional hysteresis loops from one of the inphase axial-torsional fatigue experiments on 

Hastelloy X at 800 OC are presented in figure 12. The hardening and softening 

curves for the axial and torsional stresses are shown in figure 13 for the 

same fatigue test. 

A 

The axial-torsional test-control and data acquisition software presented 

in this report simp1 ifies the procedures for conducting complex axial-torsional 

fatigue experiments. 

in a format that eases some of the burden on the data analyst, and it also 

allows some analysis which would be cumbersome, if not impossible, with data 

acquired from traditional experimental techniques. 

In addition, it acquires the data from these experiments 

I 

The axial-torsional test-control and data acquisition software presented 

in this report is intended primarily for isothermal, continuous-cycling, 

inphase and out-of-phase fatigue experiments. Clearly, nonisothermal fatigue 

experiments, such as bithermal and thermomechanical axial-torsional fatigue 

experiments, would require temperature waveform control. For example, in 

bithermal axial-torsional fatigue experiments (ref. 15>, the axial and torsional mechanical loading waveforms must be activated at appropriate moments 

with respect to the temperature waveform. 

trolled by using additional data input and output channels. 

The temperature waveform can be conThe test-control and data acquisition software will also have to be modified to perform isothermal creep-fatigue axial-torsional experiments at elevated temperatures. Creep strain can be introduced by either a stress hold or 

a strain hold in both axial and torsional directions. In order to maintain the 

synchroniza,tion of the axial and torsional waveforms of loading, creep should 

be introduced at the same time and for the same duration for both waveforms. 

10 
Since creep is time-dependent deformation, the data acquisition rate can be 

reduced during this portion of the cycle. 

CONCLUSIONS 

1. A general purpose computer program to control and acquire data from 

inphase and out-of-phase axial-torsional isothermal fatigue experiments has 

been developed. This software has greatly simplified the procedures for conducting axial-torsional fatigue experiments and the analysis of the resulting 

data. 

2. The computer program utilizes the multitasking capabilities of the computer system to generate two separate command waveforms and to acquire data 

from six different channels. 

3. Inphase axial-torsional fatigue experiments were successfully conducted 

with the computer program on 304 stainless steel at room temperature and on 

Hastelloy X at 800 "C. 

4. The same computer program can also be used to control and acquire 

data from inphase and out-of-phase axial-torsional isothermal deformation 

experiments. 

11 
APPENDIX - TEST SPECIFICATION FILE 

SPECIFICATION FILE FOR MAX PROGRAM 

COMPLIANCE CHECK DATA FILE NAME : HX8-EMOD.DAT 

FATIGUE DATA FILE NAME : HX8.DAT 

NAXE OF THE ENGINEER : PETE AND RAMESH 

MATERIAL AND SPECIMEN NUMBER : HAST-x NO. a 

AMBIENT TEMPERATURE = 23.0 C 

TEST TEMPERATURE = 800.0 C MEAN VALUE OF ALPHA = 0.00001600 [mm/mm/C] 

THE FOLMWING VALUES WERE COMPUTED AT TEST TEMPERATURE 

INNER RADIUS = 11.190 [mm] 

OUTER RADIUS = 13.161 [m] 

CROSS SECTIONAL AREA = 150.797 [m**2 

TORSIONAL MOMENT OF INERTIA = 22499.900 [mm**4 

MEAN RADIUS = 12.175 [mm] 

ELASTIC COMPLIANCE DATA 

ESTIMATED AXIAL YIELD STRENGTH = 230.0 [MPa 

ELASTIC MODULUS = 149.503 rGPa REGRESSION COEFF. - 0.99816-- ~ 

= 53.187 [GPa] - 0.99970 SHEAR MODULUS 

REGRESSION COEFF. 

CONSTANT RATE FATIGUE GATA 

CONTROL MODE - STRAIN 

AXIAL CONDITIONER SETTINGS : MAD : 2 

STRAIN : 3 

STROKE : 2 

MAD : 2 

TORSIONAL CONDITIONER SETTINGS : 

STRAIN : 2 

STROKE : 2 

AXIAL STRAIN LIMITS = +/- 0.00250 

TORSIONAL STRAIN LIMITS = +/- 0.00433 AXIAL STRAIN RATE 

TORSIONAL STRAIN RATE 

AXIAL VOLTAGE LIMITS = +/- 1.25526 

TORSIONAL VOLTAGE LIMITS = +/- 2.07016 

TRIANGULAR WAVEFORM FREQUENCY = 0.20 [Hz] 

= 0.00100 [sec-11 

- 0.00173 [sec-l] 

PHASE SHIFT - 0.00 [Degrees] 

TEST STARTED : Friday January 13, 1989 10:35:07 AM 

LAST CYCLE = 1500 
REFERENCES 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

Brown, M.W.; and Miller, K.J.: Proceedings of the Institution of Mechaa 

cal Engineers, Vol. 187, No. 65/73, 1973, pp. 745-755. 

Krempl, E.: 

Structural Materials: A Literature Survey and Interpretive Report, ASTM 

STP-549, ASTM, Philadelphia, 1974, pp. 1-46. 

The Influence of State of Stress on Low Cycle Fatigue of 

Garud, Y.S.: Journal of Testing and Evaluation, Vol. 9, No. 3, May 1981, 

pp. 165-178. 

Brown, M.W.; and Miller, K.J.: Journal of Testing and Evaluation, Vol. 9, 

NO. 4, July 1981, pp. 202-208. 

Lohr, R.D.; and Ellison, E.G.: Fatigue of Enqineering Materials and 

Structures, Vol. 3, 1980, pp. 19-37. 

Penn, R.W.; Fong, J.T.; and Kearsley, E.A.: in Use of Computers in the 

Fatigue Laboratory, ASTM STP-613, H. Mindlin and R.W. Landgraf, Eds., 

ASTM, Philadelphia, 1976, pp. 78-93. 

Morror, J.: in Fatigue Design Handbook, SAE Advances in SAE Advances in 

Engineering, Vol. 4, J.A. Graham, Ed., 1968, pp. 21-28. 

Lesse, G.E.; and Morrow, J: in Multiaxial Fatigue, ASTM STP-853, 

K.J. Miller and M.W. Brown, Eds., ASTM, Philadelphia, 1985, pp. 482-496. 

Fuchs, H.O.; and Stephens, R.I.: Metal Fatigue in Engineering, John Wiley 

and Sons, 1980, p. 29. 

Bannantine, J.A.: "Observations of Tension and Torsion Fatigue Cracking 

Behavior and the Effect on Multiaxial Damage Correlations," University of 

Illinois at Urbana-Champaign, Co 

Bonacuse, P.J.; and Kalluri, S.: 

Fatigue Experiments on 304 Stain 

Aeronautics and Space Adrninistra 

McGaw, M.A.; and Bartolotta, P.A 

July 1986, pp. 79-95. 

lege of Engineering, Report No. 128, 

"Results of Inphase Axial-Torsional 

ess Steel ,I' NASA TM-101464, National 

ion, Washington, DC, 1989. 

: in 4th Annual Hostile Environments and 

High Temperature Measurements Conference Proceedings, Society for Experimental Mechanics Inc., Bethel, 1987, pp. 12-29. 

Kanazawa, K.: Miller, K.J.; and Brown, M.W.: Fatigue of Engineering Materials and Structures, Vol. 2, 1979, pp. 217-228. 

"Hastelloy Alloy X," Cabot Wrought Products Division, Cabot Corporation, 

Kokomo, IN, 1984, pp. 1-15. 

Halford, G.R.; McGaw, M.A.; Bill, R.C.; and Fanti, P.D.; in Low Cycle 

Fatigue, ASTM STP-942, H.D. Solomon, G.R. Halford, L.R. Kaisand, and 

B.N. Leis, Eds., ASTM, Philadelphia, 1988, pp. 625-637. 

13 
TABLE I . . SPECIFICATIONS OF THE 

AXIAL-TORSIONAL FATIGUE RIG 

Axial load. kN ......... t223 

Axial strain. a percent ..... 210.0 

Axial stroke. mm ........ 2127 

Torsional load. k -m ...... 22.26 

Torsional stroke. deg ...... 250 

Torsional strain.! percent ... 24.2 

aBased on a gage length of 25 mm . bEquivalent to a twist amplitude of 5" 

within a 25-mm gage section . 

TABLE I1 . . SPECIFICATIONS FOR THE COMPUTER SYSTEM 

(a) 16-bit minicomputer (Data General S/20 

satell i te computer) 

Clock speed. MHz ................. 2 

RAM. kbytes .................. 1024 

Winchester disk capacity. Mbytes ......... 5 

Floppy drive capacity. Mbytes ......... 1.2 

(b) Digi tal-analog convertera 

Resolution. bits ................. 12 

Number of channels ................ 2 

Conversion time. psec .............. 7 

(c) Analog-to-digital converter and mu1 tiplexer 

Resolution. bits ................. 12 

Number of channels ................ 16 

Conversion time. psec .............. 7 

Multiplexer switching time. psec ......... 10 

Aperture. psec .................. 5 

(d) 32-bit superminicomputer (Data General MV/4000 

host computer) 

RAM. Mbytes ................... 4 

Winchester disk capacity. Mbytes ........ 354 

Drive capacity 

Tape drive. bitdin ............ 800/1600 

Streaming tap drive. bitslin ......... 800 

Floppy drive. E Mbytes ............ 1.2 

Multiprocessor communication subsystem ...... 1 

aTwo di gi tal -to-anal og converters are avai 1 ab1 e . bTwo floppy drives are available . Each has a 1.2-Mbyte 

capaci ty . 

14 
~~ ~~ ~~ ~ ~~ ~~~~~ ~~ ~~ 

TABLE 111. - EXAMPLE OF DATA STORED IN AXIAL-TORSIONAL FATIGUE DATA FILE 

Strain Stroke, 

deg 

-0.00299 -0.6348 

-.00301 -.8789 

-.00305 -.6470 

-.00304 -.9277 

-.00306 -.8789 

-.00305 -.8423 

-.00302 -.go33 

-.00299 -.7690 

-.00296 -.7690 

-.00294 -.go33 

Axial data 

sec 

655.00 

655.01 

655.02 

655.03 

655.04 

655.05 

655.06 

655.07 

655.08 

655.09 

Stress, Strain 

MPa I 

-1 92.0 

-192.3 

-192.7 

-193.4 

-193.0 

-193.0 

-193.4 

-192.7 

-192.7 

-192.3 

.006 1 

-0.00 17 1 

-. 001 75 

-. 00175 

-. 00177 

-. 00178 

-.00180 

-.00179 

-. 00 179 

-. 00 180 

-.00178 

Stroke, 

mm 

-0.03 1 0 

-. 1550 

-.2791 

- ,2480 

-.2170 

-. 0620 

-.0310 

- .1240 

-.2170 

-.2170 

Torsional data I Time, 

Stress , 

MPa 

-104.4 

-104.4 

-104.4 

-104.4 

-103.5 

-102.9 

-101.1 

-98.7 

-97.2 

-96.0 

1 - AXIAL STRAIN 

TORSIONAL STRAIN 

- ,006 0 5 10 15 20 0 5 10 15 20 

TIME, SEC 

(A) INPHASE LOADING. (B) OUT-OF-PHASE LOADING. 

FIGURE 1. - INPHASE AND OUT-OF-PHASE AXIAL-TORSIONAL WAVEFORMS OF LOADING. 

15 
FIGURE 2. - AXIAL-TORSIONAL FATIGUE RIG. 

FIGURE 3. - GEOMETRY OF THIN-WALLED TUBULAR AXIAL-TORSIONAL FATIGUE SPECIMEN. ALL DIMENSIONS ARE GIVEN IN 

MILLIMETERS. 
FIGURE 4. - HIGH-TEMPERATURE AXIAL-TORSIONAL EXTENSOMETER, 

17 
SIX-CHANNEL 

\,DATA ACQU I SIT I ON 

\ 

MULTIPLEXER 

,-AXIAL LOAD, STRAIN, 

\AND STROKE SIGNALS 1 

\ 

\ ANALOG-TO-DIGITAL 

SATELLITE (S/20) 

SERVOCONTROLLER 

DIG ITAL-TO-ANALOG / CONVERTER 

/ 

SERVOCONTROLLER 

COMMAND WAVEFORMS 

AND STROKE SIGNALS 

/ 

FIGURE 5. - SCHEMATIC OF INTERFACE BETWEEN AXIAL-TORSIONAL FATIGUE RIG AND COMPUTER SYSTEM. 

INPUT 

SPECIMEN 

ENGINEERING 

DATA 

i 

INPUT 

EST IMATED 

AXIAL YIELD 

STRESS ELASTIC 

1 COMPLIANCE 

DATA FILE 

EXECUTE 

ELASTIC TEST 

SPEC IF I CATION .COMPLIANCE + COMPUTE 

CHECK E, G, AND +QE 

4- REGRESSION 

COEFFICIENTS 

INPUT 

FATIGUE TEST 

DATA 

FATIGUE EXECUTE WRITE 

FATIGUE - DATA TO --.) DATA FILE 

TEST DISK 

FIGURE 6. - FLOW DIAGRAM FOR AXIAL-TORSIONAL 

FATIGUE TEST-CONTROL SOFTWARE. 

18 
CLEAR D/A AND 

AID CONVERTERS 

I I 

I I 

I 

UPPER LIMIT (+) 

AXIAL LOAD (KN) 

0 

LOWER LIMIT (-1 

CALCULATE NEXT OUTPUT NEW I1 I 

COMMAND II COMMAND 

WAVEFORM VALUES (TWO I I 

VALUES CHANNELS) 

RAMP AXIAL 

COMMAND TO 

INITIAL PHASE 

SHIFT 

GRADUALLY 

INCREASE 

AMPLITUDE 

OVER FIVE 

CYCLES 

I I' 

I I' TAKE DATA 

LOWER LIMIT (-) 

* TIME 

FIGURE 7. - SCHEMATIC OF AXIAL AND TORSIONAL 

WAVEFORMS USED FOR THE DETERMINATION OF ELASTIC MODULI. 

~ 

1 

77 _LEP-UN_TIL INTERRUPTION/FAILURE - ___ 

ON SIX 

CHANNELS 

TIYES 4) ACT1 VATE 1 

I 

I 

I I STORED 

I 

I I 

DISK WRITE 

I 

I 

I 

--1 I 

I 1Ew LOOP X 500 + I 

I (1 CYCLE) I ------------_ l r------- 

II 

WAIT FOR PIT 

INTERRUPT 

'I 

'I 

'I 

FIGURE 8. - FLOW DIAGRAM DEPICTING EXECUTION OF AXIAL-TORSIONAL FATIGUE TEST 

19 
-1002 li5E 

-.010 -.005 0 .005 .010 

c 

0- 

E 

400 r I 

c 

E 

n 

AXIAL STRAIN 

d 

v) 

v) 

W 

oc 

c v) 

-I 

9 05 -.005 OlzCLu 

e -.010 

-.006 -.004 -.002 0 ,002 .004 .006 

TORSIONAL STRAIN 

AXIAL STRAIN AXIAL STRESS, MPA 

FIGURE 9. - HYSTERESIS LOOPS FOR INPHASE AXIAL-TORSIONAL FATIGUE TEST; 304 STAINLESS 

STEEL AT ROOM TEMPERATURE; CYCLE 1000. 

0 AXIAL 

0 TORSIONAL 

c 

600 

g 400 I- l=D=-w v, 

200 

1 00 101 1 o2 1 o3 lo4 

CYCLE NUMBER, N 

FIGURE 10. - CYCLIC HARDENING-SOFTENING 

BEHAVIOR OF INPHASE AXIAL-TORSIONAL 

FATIGUE TEST; 304 STAINLESS STEEL AT 

ROOM TEMPERATURE. 

250 a 

n 

. 200 

150 

100 

50 

a 

t 

0 

v) 

L 

4 

LL 

0 

3 

0 

aE 

n 

0 

0 EXPERIMENTAL 

0 EXPERIMENTAL 

YOUNG'S MODULUS 

SHEAR MODULUS 

I- - HANDBOOK VALUES 

4 (REF. 14) 

I- L 

200 400 600 800 1000 

TEMPERATURE, OC 

FIGURE 11. - VARIATION OF YOUNG'S AND 

SHEAR MODULI WITH TEMPERATURE FOR 

HASTELLOY X. 

20 
I. 

v) 

v) 

W 

CL 

ma. c-a 

d 5: 

x 

4 

5 

-.005 0 .005 

AXIAL STRAIN 

*ookzk5 d 

e 

E2 

gt;; E= 

I- 

-. 005 

-.003 -.002-.001 0 .001 ,002 .003 

AXIAL STRAIN 

200 

a 

, 100 

B 

v) 

v) 

W cc 

t;; 0 

2 z 

5 -100 cc 

0 + 

-200 

-.005 0 .005 

TORSIONAL 

STRA I N 

*OO r 

-200 

-300 -200 -100 0 100 200 300 

AXIAL STRESS. MPA 

FIGURE 12. - HYSTERESIS LOOPS FOR INPHASE AXIAL-TORSIONAL FATIGUE TEST: HASTELLOY X AT 

800 OC: CYCLE 532. 

0 AXIAL 

0 TORSIONAL 

a 

. 400 

e 

W LD

L Q 

cr I 

0 

100 

z 

101 102 103 104 

CYCLE NUMBER, N 

FIGURE 13. - CYCLIC HARDENING-SOFTENING 

BEHAVIOR OF INPHASE AXIAL-TORSIONAL 

FATIGUE TEST: HASTELLOY X AT 800 OC. 

21 
National Aeronautics and Space Administration 

- 7. Key Words (Suggested by Author@)) 

Axial-torsional fatigue; Computer-controls; Data acquisition; 

Inphase test; Out-of-phase test; Software development; 

Multitasking; Hysteresis loops 

Report Documentation Page 

18. Distribution Statement 

Unclassified -Unlimited 

Subject Category 39 

2. Government Accession No. NASA TM-102041 

AVSCOM TR 89-C-002 

1. Report No. 

4. Title and Subtitle 

A Data Acquisition and Control Program for Axial-Torsional Fatigue Testing 

9. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 

Unclassified Unclassified 22 

7. Author(@ 

Sreeramesh Kalluri and Peter J. Bonacuse 

22. Price' 

A03 

9. Performing Organization Name and Address 

NASA Lewis Research Center 

Cleveland, Ohio 44135-3191 

and 

Propulsion Directorate 

U.S. Army Aviation Research and Technology Activity-AVSCOM 

Cleveland, Ohio 44135-3 127 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

and 

U.S. Army Aviation Systems Command 

St. Louis, Mo. 63120-1798 

3. Recipient's Catalog No. 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No 

E-4790 

10. Work Unit No. 

505-63- 1 B 

11. Contract or Grant No 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Prepared for the Symposium on the Applications of Automation Technology to Fatigue and Fracture Testing 

sponsored by the American Society for Testing and Materials, Kansas City, Missouri, May 22-23, 1989. 

Sreeramesh Kalluri, Sverdrup Technology, Inc., NASA Lwis Research Center Group, Cleveland, Ohio 44135; 

Peter J. Bonacuse, Propulsion Directorate, U.S. Army Aviation Research and Technology Activity-AVSCOM. 

A computer program was developed for data acquisition and control of the axial-torsional fatigue experiments at 

NASA Lewis Research Center. The multitasked, interrupt-driven program was written in Pascal and assembler. This 

program is capable of dual-channel control and six-channel data acquisition. It can be utilized to perform inphase 

and out-of-phase axial-torsional isothermal fatigue or deformation experiments. The program was successfully 

used to conduct inphase axial-torsional fatigue experiments on 304 stainless steel at room temperature and on 

Hastelloy X at 800 "C. This paper describes the details of the software and gives some of the results generated to 

date. 

16. Abstract