The capability of a computer program which can be part of a larger computer program for a fully automated multiaxial testing facility is described. The program was designed to process test data from tubular or flat specimens made from isotropic or anistropic materials, including high modulus fiber composites. Data from a large number of strain gages and combinations of applied loads can be used. Options are provided for single element, 90-degree, rectangular or Delta rosettes, or any combinations of these types of strain gages. Options are provided for strain gage transverse sensitivities. The program outputs include: structural axes strains and stresses, initial and strain-dependent elastic constants, shift of principal strain direction with load, and local curvatures from back-to-back strain gages, and either Calcomp or microfilm plots. The computer program is described with respect to its flow chart, input/output, embedding or linking with other programs

Chamis, C. C.

Kring, J.

Sullivan, T. L.

English

NASA Technical Reports Server

NASA TECHNICAL
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AUTOMATED TESTING DATA REDUCTION COMPUTER PROGRAM
by C. C. Chamis, J. Kring, and T. L. Sullivan
Lewis Research Center
Cleveland, Ohio
TECHNICAL PAPER proposed for presentation at
Conference on Analysis of Test Methods for High
Modulus Fibers and Composites sponsored by the
American Society for Testing and Materials
San Antonio, Texas, April 12-13, 1972
https://ntrs.nasa.gov/search.jsp?R=19720015506 2020-03-23T10:32:09+00:00Z
AUTOMATED TESTING DATA REDUCTION COMPUTER PROGRAM
# f *
• by C. C. Chamis, J. Kring, and T. L. Sullivan
• . National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
ABSTRACT .
The capability of a computer program which can be part of a larger
computer program for a fully automated multiaxial testing facility is
described. This computer program is designed to 'process test data from
tubular or flat specimens made from isotropic or anistropic materials in-
cluding high modulus fiber composites. The program can receive data from
a large number of strain gages and combinations of applied loads. Options
are provided-for single element, 90-degree, rectangular or Delta rosettes,
or any combinations of these types of strain gages. Options are provided
for strain gage transverse sensitivities. The program outputs include: struc-
tural axes strains and stresses, initial and strain dependent elastic constants,
shift of principal strain direction with load, and local curvatures from back-
to-back strain gages, and either calcomp or microfilm plots. The computer
program is described with respect to its flow chart, input/output, embedment or
linking with other programs and its possible utility in a fully automated
testing system.
Key words: Computer program, testing data reduction, automated testing,
strain gage data, angle shift, local curvatures, fiber composites, stress
analysis, computer plotting.
' Aerospace Technologist.
"•"Physical Scientist.
INTRODUCTION
In fully automated testing for characterization of materials in general .
and in particular fiber composites, a computer program is required. It is
the function of the computer program to: receive input signals from the
testing machine, convert them to engineering data, .perform the necessary compu-
tations, carry out the comparisons with, the predetermined test-program-profiles,
and issue new commands for the next testing step. Thus, the computer program
together with the predetermined test-program-profile.can control the testing
and make fully automated testing a practicality. Fully automated testing will
be of great utility in testing of advanced fiber composites where the load
path effects, load rate, multitude of load conditions and their history of
application must be precisely controlled and recorded. Specific cases of
automated testing are described in [l] and [2].
The subject of this paper is to describe the capability of a computer
program which can be a part of a larger computer program for a fully automated
multiaxial testing facility. However, the computer program described herein
has not been used in such a facility as yet. This computer program is designed
to process test data from tubular or flat specimens made from isotropic or
anisotropic materials including high modulus fiber composites. The program
can receive data from a number of strain gages and combinations of applied
loads. Options are provided for single element, 90-degree., rectangular or
Delta rosettes, or any combinations of these types of strain gages. Options
are provided for strain gage transverse sensitivities.
-'•The italic number in the brackets refer to the list of references
appended to this paper. •
2• The inputs to the computer program are digital strain gage and load cell
data. The program outputs include: structural axes strains and stresses,
initial and strain dependent elastic constants, next load increment extrapo-
lated strains, extrapolated failure strains and shift of principle strain
direction with load. The program has provisions for analysis of data from
back-to-"back strain gages at a point and the computation of local curvatures
at this point. Plotting options of the output are also provided.
The capability of this computer program is described with respect to its
flow chart, input/output, and its embedment or 'linking with a laminate analysis.
Its possible utility in fully computer controlled testing is discussed.
CAPABILITY OF COMPUTER PROGRAM
The computer program is organized as is -depicted in the flow chart in
Figure 1. A brief description of the functions of the.various boxes^in the
-Flow Chart (Fig. l) follows.
Input Data
The input data for the program consists of the specimen parameters listed
in Table 1, the test parameters and options listed in Table 2,.and the strain
gage raw data. The strain gage raw data, as .used in this program, consists of
digital strain readings on paper tape. The data from the paper tape is punched
on cards in a format acceptable to the program. The input data is read in as
depicted in the first three blocks of the Flow Chart (FLg. 1).
Convert Input Data to Stress and Apparent Strain
The portion of the program represented by block (h) in the Flow Chart
3(Fig. l) converts the strain gage scaled digital strain readings to apparent
strains. The average stress in the specimen is computed using the load cell
reading input and the specimen cross section dimensions. .
In this portion of'the program, a table of input data information is
constructed and the raw strain gage data is 'converted to apparent strains.
This table serves two purposes: • (l) it facilitates subsequent computations,
and (2) it serves as a standard for the input data in cases which have different
data input formats than the one presently used and described in the previous
section.
/
Compute Actual Strains
The apparent strains are corrected for transverse sensitivity in the
portion of the computer program denoted by block 5 in the Flow Chart (Fig. l).
Two sets of equations are used for thes'e corrections. The number of constants
•required for the transverse sensitivity corrections depend on both the type
of gage and the supplier. For example, for the case of three element rosettes,
.one option requires three constants and the Poisson's ratio of the material
on which the manufacturer's gage factor was measured [3]. The other option
requires only one constant. In either case, the constants are provided by .
the gage manufacturer. The corrected strains are stored in a table and are
.available for subsequent computations.
Transformation of Actual Strains to Structural Axes
The actual strains described in the previous section are determined with
respect to a local gage axes, that is, the longitudinal and transverse gage
uaxes. For useful engineering data, the strains need to be transformed to the
structural axes of the specimens. The structural axes of the specimen, or
any component for that matter, refer to the overall specimen geometry and
usually coincide with the principal external loading directions.
The transformation of the strains is accomplished in the portion of the
program represented by block 6 in the Flow Chart (Fig. l). The transformation
is performed using the well known rotation transformation relations [U] and
the input gage orientation with respect to. structural axes (load direction)
information.
The structural axes strains are stored in a table and are available for
subsequent computations and for input to the Multilayered Fiber Composites
Analysis Computer Code [5] for the laminate stress analysis as will be described
later. The data transfer takes place in the part of the computer program
denoted by block 7 in the Flow Chart (Fig. l).
Shift of Principal Strain Axes with Load
Several factors contribute to changes in angle between gage axes and
structural axes. Some of the important ones are: gage misalignment, load
eccentricity, fiber misalignment and fiber relative rotation during loading.
So far as the .gage is concerned, the combined effect of these factors is
reflected as increments in the input gage orientation-angle.
This angular increment can be readily determined by satisfying known
conditions of the strain state on the structural axes. For the case of
coincident axial load and fiber direction, for example, the known strain
• 5 •
condition on the structural axes is that the shear strain should be zero.
Using this condition, the change in the angle can be obtained from the strain
transformation equations and the structural axes strains. Corresponding con-
ditions for other loadings are derivable from the rotation transformation
equations.
The angular shift is computed in the portion of the program indicated
by block 8 in the Flow Chart (Fig. l)'. The results are stored in a table .
and are available for subsequent laminate stress analysis computations.
Smoothing Routines for Extrapolation,
Interpolation and Plotting
The structural strain gage data is fitted with a smooth curve which can .
be used for -interpolation, extrapolation, plotting and.for computing derivatives
at desired points. The smooth piecewise function is'generated in the part
of the computer program denoted by block 9 in the Flow Chart (Fig. l).
In these computations, the structural axes longitudinal strain (E ) is
.xx
taken as the independent variable in generating the piecewise smoothing
function for curve fitting. The dependent variables are stress, the structural
axes transverse strain (e ), and the structural axes shear strain (e ).yy xy'
Engineering Constants and Failure Load
Extrapolated Values
Engineering constants (moduli, Poisson's ratio, etc.), and'failure load
parameters (stress and strains) are readily computed from the piecewise smooth
functions described in the last section. The longitudinal modulus'(e ) equals
6the slope of the function longitudinal stress (a ) versus longitudinal.strain
XX
(e ) evaluated at the desired strain (e ). The major Poisson's ratio
XX • XX
v equals the slope of the function-transverse strain (e ) versus longitudinal
xy H yy
strain (e ) evaluated at e . The coupling term between longitudinal strain
XX XX
and shear strain equals the slope of the function shear strain (e ) versus e
xy xx
evaluated at e . Other stress-strain or strain-stress relationships can be
obtained in a similar fashion. These computations are performed'in the portion
of the program designated by block 10 in the Flow Chart (Fig. l).
The values for the failure strains for each gage are obtained by extra-
polation when only the failure load is known. The corresponding failure strains
are calculated using the smoothing function (block 11, Flow Chart in Fig. l).
The results of these computations are stored in a table and are available
for laminate analyses with load dependent stress-strain or strain-stress
relationships. The computations for interpolated slope values are performed
in the part of the program denoted by block 1^ and block 15 in the Flow Chart
(Fig. 1). '
Plotting Routines
The computer program has options for plotting the data reduction results.
The plotting is accomplished using the smoothing functions described previously
in conjunction with available .plotting routines. This is accomplished in the
part of the computer program denoted by blocks- 12 and 13 in the Flow Chart
(-Fig. 1). .
7The plotting can be done either oh the calcomp or on microfilm. Samples .
of calcomp plots are.shown in figures 2, 3, and k for stress, transverse strain,
(e ) , and shear strain (e ) versus longitudinal .strain (e ).yy • xy xx
Displacement Matrix
Bending in the test specimens is measured by back-to-back rosettes.
Values read from these gage's can be used to compute reference plane strains
and local curvatures .if the assumption is made that plane sections before
and after deformation remain plane. In this computer program, these computa-
tions are performed in the part of the program denoted by block l6.in the
Flow Chart (Fig. l). Subsequently, they are used as input data for .the
laminate stress analysis. See block 17 Flow Chart (Fig. l).
V
Computer Program Output
The output of the computed results described previously is printed in
the part' of the program- denoted by block 18. Here, several tables are printed
out.. The headings of these tables are listed in Table 3.
LINK WITH MULTILAYER FIBER COMPOSITE ANALYSIS
The laminate stress analysis using the strain gage data is performed
by the Multilayer Fiber Composite Analysis Computer Code [5]. The linking
between the. two programs is simply accomplished through the aid of temporary
storage (block 19, Flow Chart, Fig. l).
Laminate stresses are computed either for all or for selected gage values
by transferring the desired strain values from temporary.storage to multi-
layered fiber composite analysis code. The additional input data required,
8the mechanics of the computer code for laminate stress analysis, and the
output are described in detail in [5].
DISCUSSION ON COMPUTERIZED CONTROLLED TESTING
The computer program-described herein has been used extensively in the
reduction -of the strain gage data reported in [6]. It became evident in that
effort that a computer program of this type has great utility.
In general, coupling data reduction programs with multilayered laminate
analysis programs to control fiber.composite testing requires a real-time
• operation. This means either instantaneous access to time sharing systems
or a small computer [2] attached to and used only with the testing machine.
Real time programming of small computers [7] is a highly specialized
profession and can be quite expensive. It usually results in rigid format
software which can only be modified by real-time programmers. This leads to
the following two distinct disadvantages:
1. Operator does not have control of the program beyond that provided
by the rigid format.
2. Limited.flexibility in carrying out computations required for fiber
composite laminate stress analysis.
One alternative to the previously described disadvantages is a cross-
compiler [8]. The cross-compiler translates a FORTRAN IV program, for example,
into the machine language acceptable to the smaller computer. Programming
for computer controlled testing can be done in FORTRAN, or any other high
level language, and then translated and loaded in the smaller computer.
9With either the availability of cross-compilers or an instantaneous
access to a time sharing system, the linked system of.programs described
herein could supply the major portion of all the software required to control
automated testing. .
CONCLUDING REMARKS
The results of this investigation lead to the following statements,
1. An operational computer program has been generated to reduce the
strain gage data generated in manual or automated testing of fiber composites.
2. This computer program can handle readily .a large number(greater than
one-hundred)and various types of strain gages, their transverse sensitivity,
different specimen geometries, and isotropic or anistropic material.
3. The changes in local curvatures are.computed from .back-to-back strain
gages. The change in fiber direction with load is computed via suitable
strain.transformation.
4. This computer program is designed to provide engineering data in
terms of tables, calcomp plots, or microfilm.
5• This computer program can be linked with an available multilayered
fiber composite computer code [5]. In this form the package can be used with
an on-line computerized testing facility.
10
REFERENCES
[l] Skoe, R. E., and Matson, G. L. , Materials Research and Standards, MTRSA,
Vol. 11, No. 2, 1971, p. 8.
[2] Wu, E. M., and Jerina, K. L., Materials Research and Standards, MTRSA,
Vol. 12, No. 2, 1972, p. 13.
[3] Anon, "Transverse Sensitivity Errors," Technical Note TN-137, Micro-
Measurements ' •
[U] Leknitskii, S. G. , "Theory of Elasticity of-an Anisotropic Elastic Body,"
Holden-Day Series in Mathematical Physics, San Francisco, CA, 19&3.
[5] Chamis , C. C., "Computer Code for the Analysis of Multilayered Fiber
Composites - Users Manual," NASA TN D-7013, 1971.
[6] Sullivan, T. L., and Chamis, C. C., "Some Important Aspects in Testing
High Modulus Fiber Composite Tubes Designed for Multiaxial Loading,"
(To be presented'-at this -conference). ' . ' ', • v . '
[7] Anon, Introduction to-.Programming. Digital Equipment Corporation,
19.69. • • . ' . • • : ' ' • • : ' ' '
[8] Danko, D. M., and Svarovsky, J. R., "Dedicated Computer Controlled
Testing Machines," Gilmore Industries, Inc., Cleveland, Ohio,
Oct. 19, 1971. ' .
11
'TABLE 1—Specimen parameters.'
Type of specimen ' • - flat or tube
Type of material - isqtropic, anisotropic
Transverse sensitivity - one or four gage
manufacturer constants
Specimen gage section - diameter or width
Specimen gage section - thickness
TABLE 2—Test parameters and options.
Number of gage positions
Number of load increments
Type of gage and orientation
Failure load
Back-to-back gage positions
Positions to be interpolated
Positions to be plotted
12
TABLE 3—List of output table headings.
Input data
Apparent strains
Actual strains
Structural axes strains
Strain principal axes
Extrapolated strains to failure load
Slope values - engineering constants
evaluated at load increments
Interpolated slope values - engineering
constants evaluated at any point
READ INPUT PARAMETERS
& OPTIONS
READ GAGE DATA &
ELECTRIC SIGNAL DATA
CONVERT INPUT DATA TO
STRESS & APPARENT STRAINS
COMPUTE SHIFT IN
ORIENTATION ANGLE
CURVEFIT DATA FOR:
EXTRAPOLATION
INTERPOLATION PLOTTING
COMPUTE SLOPES & STRESS
STRAIN RELATIONSHIPS
PLOT STRESS-STRAIN
RELATIONSHIPS (MICROFILM)
INTERPOLATE STRAINS
FOR SELECTED STRESSES
TRANSFORM ACTUAL STRAINS
TO STRUCTURAL AXES STRAINS
COMPUTE DISPLACEMENT MATRIX
FOR MATCHING GAGE POSITIONS
TRANSFER DISPLACEMENT
MATRIX TO DISK
PRINTOUT
TABULATED RESULTS
STOP)
INPUT DATA FOR
M.F.C.A. PROGRAM
CS-62648
Figure 1. - Strain gage data reduction computer program flow chart.
STRUCTURAL flXES STRfllNS
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~ £
ITS"
C"!
S^»|
«l
.00 .02 .PS .U9 .10 .12 ,„.., .13GflGf: POS NO. 01 STRHIN EXX (X10"Z r
Figure 2. - Structural axes strains.
STRUCTURRL flXES STRfllNS
:06 .09 .10 .12 ,13
C-fiGf: POS NO.04 STRflIN EXX I X I ° '
Figure 3. - Structural axes strains.
.19
CS-62670
s, STRUCTURflL flXES STRfllNS
a:
cr
.05 .1'8 .10 ' . 1 2
GfiH: POS NO.04 5TRHIN EX)
Figure 4. - Structural axes strains.
CS-62671
NASA-Lewis-Com'l


Automated testing data reduction computer program

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