An Automated Analysis System for a Tensile Tester - ACS Symposium

12 A n Automated Analysis System for a Tensile Tester T. T. Gill and Mark E. Koehler Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 27, 1986 | doi: 10.1021/bk-1986-0313.ch012

Glidden Coatings and Resins, SCM Corporation, Strongsville, OH 44136 An Instron Tensile Tester Model TM was interfaced to a micro-computer for data collection and transmission to a minicomputer. A FORTRAN program was developed to allow data analysis by the minicomputer. The program generates stress-strain curves from the raw data, calculates physical parameters, and produces reports and plots. The Instron automation provides an easy and rapid means for acquiring accurate analyses of tensile properties. Sample preparation is now the rate limiting step as opposed to data analysis. Tensile t e s t i n g i s an important part of the physical characterization of free f i l m coatings. The fundamental properties measured relate d i r e c t l y to performance properties of the coating. Because of the time required to obtain and analyze t e n s i l e data, a laboratory which routinely performs t e n s i l e tests may find that an automated system i s needed. Although commercial packages are a v a i l a b l e , i t i s f e a s i b l e to develop an in-house system with r e l a t i v e l y l i t t l e expense. This paper describes one such system as implemented at Glidden Coatings and Resins with very s a t i s f a c t o r y r e s u l t s . System Configuration A dedicated microcomputer i s interfaced to the Instron instrument i n order to c o l l e c t the raw data. The microcomputer consists of an 8080A microprocessor, 32K bytes of memory, A/D converter, s e r i a l I/O for communication, p a r a l l e l I/O for d i g i t a l control and sensing, a real-time programmable clock, and vectored interrupt c o n t r o l . The microcomputer i s f i r s t i n i t i a l i z e d by means of a DIALOG program on the minicomputer which transmits c a l i b r a t i o n information, sample i d e n t i f i c a t i o n , and data c o l l e c t i o n rates. The microcomputer c o l l e c t s data at two operator selectable rates, i n i t i a l and f i n a l . The s t a r t i n g rate i s set faster to provide better i n i t i a l resolut i o n , with t y p i c a l i n i t i a l and f i n a l rates of 100 pts/sec and 10 pts/sec, respectively. The duration o f the i n i t i a l period can also be varied, with a usual length of 1000 points. Once the microcomputer has received t h i s information, a ready l i g h t i s turned on and data a c q u i s i t i o n can be begun by pushing a s t a r t button. A busy l i g h t then flashes to indicate that data c o l l e c t i o n i s i n progress. After the sample breaks, the operator 0097-6156/86/0313-0123$06.00/0 © 1986 American Chemical Society

Provder; Computer Applications in the Polymer Laboratory ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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COMPUTER APPLICATIONS IN THE POLYMER LABORATORY

signals the end of data c o l l e c t i o n by pushing a stop button. The microcomputer then transmits the data t o the minicomputer and r e - i n i t i a l i z e s for another run* Meanwhile, the minicomputer stores the data i n a unique job f i l e * D e t a i l s regarding the minicomputer system have been previously reported O ) .

Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: June 27, 1986 | doi: 10.1021/bk-1986-0313.ch012

Data Analysis Program Instron data f i l e s which have been saved on the minicomputer can be analyzed at any time using the STRESS program* After reading the s p e c i f i e d raw data f i l e , the STRESS program f i r s t converts the voltages t o stress values based upon the data c o l l e c t i o n rates and c a l i b r a t i o n parameters, according t o Equation 1*

S(i)

WW-Co) WT(Cl-Ch)

s

( 1 )

In Equation 1, S i s the stress at point I i n PSI, V i s the raw voltage data f o r point I i n m i l l i v o l t s , F i s the f u l l scale load i n pounds, W i s the sample width and T i s the thickness i n inches, and Co, CI, and Ch are c a l i b r a t i o n data f o r zero, load, and sample hanger i n m i l l i v o l t s * The program then locates the s t a r t i n g point of the s t r e s s - s t r a i n curve* The most dependable method f o r i d e n t i f i c a t i o n o f the s t a r t i n g point w i l l vary with sample and instrument behavior* For our coatings work, we search f o r a datum greater than zero followed by two successively higher values* Variants o f t h i s approach have yielded inaccurate s t a r t i n g points which cause considerable error i n the Young's Modulus ( i n i t i a l slope) computation* Next, s t r a i n values are calculated according t o the following equations* Equation 2 i s used i n the i n i t i a l c o l l e c t i o n rate period and Equation 3 i s used i n the f i n a l period*

E i ( I )

.

CTi(I-Is) 60000L

Ef(i)

= C(Tf(I-Ir)+Ti(Ir-Is))

( ) 2

( 3 )

60000L In Equations 2 and 3, E i and Ef are s t r a i n s at point I i n the i n i t i a l and f i n a l c o l l e c t i o n rate areas, I s i s the s t a r t i n g point and I r i s the rate t r a n s i t i o n point, T i and Tf are the i n i t i a l and f i n a l c o l l e c t i o n rates i n sec/point, C i s the crosshead speed i n inches/sec, and L i s the sample length i n inches* After these c a l c u l a t i o n s , a l e a s t squares f i t i s done on the f i r s t 250 milliseconds o f data t o obtain the i n i t i a l slope, which i s Young's Modulus* This represents 5% or less o f the data f o r a run o f at least 2*4 seconds at an i n i t i a l data c o l l e c t i o n rate of 100 points per second* STRESS searches for the Y i e l d Strength (maximum s t r e s s ) , then continues on t o f i n d the greatest drop i n stress* From there i t reverses d i r e c t i o n t o look for a peak or a point 0.004 L/Lo p r i o r , whichever comes f i r s t . This point i s taken t o be the Break Point.


12.

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Automated Analysis for a Tensile Tester

GILL AND KOEHLER

The value of 0.004 L/Lo was chosen e m p i r i c a l l y since most breaks do not exceed t h i s range. After the curve i s characterized, the Work at Break (Wb) i s calculated as the i n t e g r a l of stress (S) as a function of s t r a i n (E) from the s t a r t point (1) t o t o break point (n).


Wb =

/

S (E)dE

(4)

I f r e p l i c a t e runs were made, STRESS w i l l repeat the preceeding c a l c u l a t i o n s f o r each data set. Fracture

Analysis

The Instron analysis program also can perform a Fracture Analysis i f multiple runs are made as a function of flaw length. To perform a Fracture Analysis, the operator indicates t h i s i n the DIALOG session. The DIALOG w i l l ask f o r the flaw lengths f o r each run. The STRESS program recognizes t h i s f l a g and calculates fracture energy a f t e r a l l runs have been processed. Two r e l a t i o n s h i p s are u t i l i z e d , depending on whether the sample e x h i b i t s e l a s t i c or v i s c o e l a s t i c behavior. The l a t t e r i s defined as materials with a s t r a i n at break greater than 2%. The following equations show fracture energy t o be equal t o the slope of a l i n e having an intercept of zero, based upon the work of G r i f f i t h (2).

Wb =

a e

—

+ 0

(5)

^L. • 0

(6)

AY o

2

=

a v

2

AY 2

Y = 1.99 - 0.41(A/W) + 18.7CA/W) 38.48(A/W) + 53.85(A/wr

(7)

J

Where a equals t e n s i l e strength, A equals one h a l f flaw length, Y i s an edge correction (3)» and a equals fracture energy. Slope i s computed by l e a s t squares"~analysis with c o r r e l a t i o n c o e f f i c i e n t o f fit. Outlier Identification Tensile t e s t i n g i s susceptible t o i n v a l i d runs due t o sample flaws, poor mounting, or many other other sources of e r r o r . I t i s therefore e s s e n t i a l that o u t l i e r s be i d e n t i f i e d and removed. After a l l runs have been analyzed, STRESS calculates means and standard deviations for each parameter. I t also performs Student t - t e s t s on


126


a l l r e s u l t s i n order t o help i d e n t i f y o u t l i e r s . The Student t-score i s calculated using Equation 8. t

=

(8)


V In Equation 8, V i s the value t o be tested, Xv i s the mean, and n i s the number of values. The t-score i s converted t o a p r o b a b i l i t y by a subroutine which calculates the incomplete Beta function. This could also be accomplished using an i n t e r n a l look-up t a b l e . Any r e s u l t s with a p r o b a b i l i t y o f deviation i n the 5-10% range are highlighted with one a s t e r i s k . Those with a p r o b a b i l i t y of deviat i o n of 0-5% are highlighted with two a s t e r i s k s . A second method used t o v e r i f y the v a l i d i t y o f the r e s u l t s i s v i s u a l inspection o f the graphic data. After the tabular r e s u l t s are presented, the operator can c a l l a subroutine which p l o t s the s t r e s s - s t r a i n curves. Anomalous curves can usually be e a s i l y i d e n t i f i e d i n t h i s manner. E d i t i n g and Reporting C a p a b i l i t y After inspecting the tabular and graphic data, the operator i s allowed to remove runs which appear t o be o u t l i e r s . Any run can be deleted or restored i n any order, and the comparative s t a t i s t i c s are recalculated with each operation. By comparing the standard deviation before and a f t e r d e l e t i n g a run, the e f f e c t of that run can by determined. The e d i t i n g process can continue i n d e f i n i t e l y u n t i l the operator i s s a t i s f i e d with the v a l i d i t y o f h i s r e s u l t s . Once s a t i s f i e d with the r e s u l t s , the operator can p r i n t out a report giving experimental parameters and r e s u l t s o f the a n a l y s i s . A sample report i s given i n Figure 1. Plotting Capability Any p l o t which i s formed during data analysis can be accumulated and a l l accumulated p l o t s can be printed out a f t e r the analysis i s complete. An example of a standard run i s shown i n Figure 2. For fracture studies, the fracture data can also be plotted along with the least squares l i n e which yielded the Fracture energy. Conclusions The Instron data c o l l e c t i o n and analysis system described has several strong features. F i r s t , i t i s t a i l o r e d t o our needs and designed t o take maximum advantage of e x i s t i n g system u t i l i t i e s , such as the p l o t t i n g package and the subroutine f o r obtaining exact p r o b a b i l i t i e s from the t - s t a t i s t i c . I t i s amendable t o a l t e r a t i o n or expansion i f needs should change or grow. Only the addition o f a robotic sample preparation and loading system would be required t o completely automate our Instron analyses(JO. Operator i n t e r a c t i o n i s minimized wherever p o s s i b l e , yet the program helps the operator to make judgement c a l l s regarding data v a l i d i t y . The automated system improves the e f f i c i e n c y o f the instrument and operator and enhances the aesthetic impression of Instron t e s t i n g .


12.

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GILL AND KOEHLER


Experimental Parameters — J o b Number:

Sample ID: Operator ID: I n i t i a l Period: Crosshead Speed: Sample Length: Temperature:

879C602G Control PJM 10ms 2.000in/min l.OOOin 72.00°F

Run

Thickness (in)

1 2 3 4 5 6

0.003700 0.004600 0.005000 0.003700 0.004400 0.003600

Run Date: Final Period: F u l l Scale Load: Sample Width: Humidity:

16-Apr-85 100ms 20.1bs 0.750in 50.001

Results of Analysis

Tun Number

Youngs Modulus

Yield Strength (PSI)

Stress At Break (PSI)

Strain At Break (X Lo)

Work At Break (InLb/CuIn)

1 2 3 4 5 6

0.410E+05 0.397E+05 0.400E+05 0.451E+05 0.437E+05 0.456E+05

0.279E+04 0.334E+04 0.295E+04 0.257E+04 0.269E+04 0.298E+04

0.279E+04 0.334E+04 0.295E+04 0.257E+04 0.269E+04 0.298E+04

37.80 51.13 44.50 33.77 39.10 42.57

0.709E+03 0.976E+03 0.812E+03 0.627E+03 0.685E+03 0.757E+03

Mean Std. Dev.

0.425E+05 0.261E+04

0.289E+04 0.269E+03

0.289E+04 0.269E+03

41.48 0.604E+01

0.761E+03 0.123E+03

Figure 1.

Instron report generated by stress program.



128


F i g u r e 2.

I n s t r o n p l o t g e n e r a t e d by s t r e s s program.


12.

GILL A N D KOEHLER


129

Literature Cited 1.


2. 3. 4.

Niemann. T. F., Koehler, M. E., and Provder, T., "Microcomputers Used as Laboratory Instrument Controllers and Intelligent Interfaces to a Minicomputer Timesharing System" in "Personal Computers in Chemistry"; Lykos, P., Ed., John Wiley and Sons, New York, 1981. G r i f f i t h , A. A . , P h i l . Trans., A221, 163 (1966). Brown, W. F., and Strawley, J . E . , ASTM STP410 (1966). Scott, R. L . , Advances in Laboratory Automation - Robotics 1984, Zymark, Hopkinton, MA (1984) p. 151.

RECEIVED November 14, 1985


An Automated Analysis System for a Tensile Tester - ACS Symposium

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