expert system for testing mechanical properties of aluminum and aluminum alloys

Pergamon Expert Systems With Applications, Vol. 12, No. 4, pp. 473-482, 1997

© 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0957-4174/97 $17.00 + 0.00

Plh S0957-4174(97)00007-9

Expert System for Testing Mechanical Properties of Aluminum and Aluminum Alloys

IGOR EMRI AND DRAGO KOVACI~

Center for Experimental Mechanics, University of Ljubljana, Cesta na Brdo 49, SI- 1000 Ljubljana, SIovenia

Abstract--This paper presents the TENSALUM expert system for computer-assisted testing of aluminum and aluminum alloys according to nine different standards. The expert system comprises testing conditions and comparative values for mechanical properties that the tested material should meet for nine different standards. The corresponding database, which at present contains roughly half a million data, is modularly structured and can be updated or modified with any ASCII editor without entering the source code of the program. Practical experience shows that using the expert system shortens the testing time by approx. 300-400% in comparison to other programs available on the market. © 1997 Elsevier Science Ltd

1. INTRODUCTION

THE TENSILE TEST i s one of the most common methods of testing the mechanical properties of materials. Condi- tions and measurement procedures for every specific material are explicitly defined by standards [ASTM B 557M/84, ASTM E8/87, BS 18/87, BS 1470/87, DIN 1745/83, DIN 1788/83, DIN 50145/75, DIN EN10002/ 91, ISO 6892/84, JUS C.C4.020/70, JUS C.C4.025/63, JUS C.C4.120/70, JUS C.A4.002/85, NF A50-451/81, NF A50-471/81], set by the appropriate national or international organizations. These standards also determine the mechanical properties (comparative values) that the tested material should meet. Testing conditions and comparative values differ between the various standards. In the case of aluminum and aluminum alloys, the testing conditions (speed, specimen length and pre-tension force) change in addition within the chosen standard according to specimen geometry (thickness). For example, the DIN standard alone defines 52 testing conditions.

Despite the computerization of modem testing systems which enables fast and reliable measurements, the selection of testing conditions and the evaluation of the results according to the selected standard are still left to the operator. With each new specimen geometry, the operator must determine the set of conditions for performing the test according to the chosen standard. In addition, he must find the comparative values that depend on geometry, extensibility criteria and the material's type and quality. This procedure is time consuming and unreliable due to the human factor.

As an illustration, Fig. 1 shows the course of the

operator's activities while performing measurements on a computer-assisted testing system, controlled by a typical commercial software package. His activities are

COMPUTER PARALLEL ASSISTED ACTIVITIES ACTIVITIES

( START )

-~ INPUT OF GENERAL

DATA

~ - INPUT OF DIMENSIONS

I'~ INPUT OF l 1 TESTING 14- I c°NDm°Ns 1

EXECUTION OF EXPERIMENT

.~ MEASUREMENT I OF SPECIMEN DIMENSIONS

CLAMPING OF I SPECIMEN

t SELECTION OF A STANDARD

.I DETERMINATION OF V1, V2, Lo

ANALYSIS OF I DETERMINATION / RESULTS I OF MECHANICAL I '"

I PROPERTY LIMITS /

I I COMPAR,SON I i

I

FIGURE 1. Course of operator's activities.

473

474 L Emri and D. Kova(i6

I BS 1470 [ I ASTM a 209M I

I JUS c.c4.120 I I JUS C.C4.025 I JUS C.C4.020 I

I DIN 1788 I DIN 174~ I I NF A50-471 I I NF AS0-4Sl I I I I I l i I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i i 11411 i i i i i i i i i i i i i i i i i i i i

0 66 INTERVALS OF THICKNESSES

FIGURE 2. Classes of specimen's geometry and standards.

divided into 'computer assisted' and 'parallel activities'. The latter are needed in order to complete the test according to the selected standard.

The operator measures the specimen geometry and inserts the sample into the grips of the measuring system (this operation can be automated as well). Next he enters general data on measurement and specimen geometry. He then selects the standard according to which he intends to perform the experiment, and checks if this particular standard covers the geometry of his sample (not all standards cover all geometries). The specimen geometries (thicknesses) can be divided into 66 groups (classes or sub-intervals of thicknesses) as shown by the scheme in Fig. 2. The boundaries of these classes are subject to standards, types of material and quality of thermal treatment (temper). The scheme in Fig. 2 shows the ranges covered by the individual standards contained in the expert system.

If the geometry is covered by the selected standard, the testing conditions for the particular sample geometry have to be found manually in the appropriate standard. The parameters are: vj, the speed of loading up to the elastic limit; v2, the speed of loading beyond the elastic limit, i.e. in the plastic region up to fracture, and L0, the gauge length. The selected data are entered into the computer by the operator. The testing procedure that follows, along with the statistical data evaluation, proceed automatically. Usually the test is completed with the print-out of the protocol which contains all the necessary data on measuring, a diagram of the measurements and a table of the results.

The obtained results now still need to be compared with certain values set forth by the standard: the tensile strength, Rm, the elastic limit, Rpo.2, the hardness, HB, and two values for the elongation, A. Again, the operator has to find these values manually. In a standard these mechanical property limits are given in the form of tables as a function of thickness, type of material, quality of thermal treatment and extensibility (deformability). For nine standards, i.e. DIN 1745, DIN 1788, ASTM B 209M/90, BS 1470, NF A50-451, NF A50-471, JUS C.C4.020, JUS C.C4.025 and JUS C.C4.120, the number of these data comes to about 76,000. On the basis of a comparison of the measured and standardized values, the operator concludes whether a material conforms with

regulations or not. Experience obtained in practice indicates that manual

searching for standardized data requires about four times more of the operator's time than the automated testing procedure itself. Furthermore, manual searching of standardized data is also unreliable because of the possibility of human error.

This paper presents the TENSALUM expert system, a novel system with regard to the software packages currently offered by leading manufacturers of tensile testing systems (ZWICK, SCHENCK, INSTRON, MTS, FRANK . . . . ). The most important advantage of the TENSALUM expert system over others is that the operator does not need to determine or enter the set of testing conditions and comparative values determined by the standard; these data are sent to the expert system's measuring unit by its central unit. A description of both units follows.

The TENSALUM expert system 'leads' the operator through the separate phases of the complete testing procedure, and supervises the performance of testing and communication with the database that stores all the information specified by the chosen standard. The measurement is completed by the protocol print-out which consists of a graphic display of the measured values, a table of analysed and statistically evaluated results and, most important, the conclusion. This is derived by the expert system on the basis of the comparison of results with the reference values prescribed by the chosen standard.

2. EXPERT SYSTEM STRUCTURE

Tensile testing of aluminum and alminum alloys can be performed with any software package that comes with commercial universal testing machines. In this case, however, a vast number of information, i.e. testing conditions and comparative values, should be gathered from the standard according to which the experiment is being performed. This is a very time-consuming and unreliable procedure, fraught with 'human error'. We decided to obviate these tedious procedures by develop- ing an expert system that would limit the operator's activity to proper clamping of the specimen, and input of its geometry and some general information, i.e. excluding any time-consuming communication with standards.

The expert system is essentially built around the database that comprises information and/or data contained in different standards. As standards vary in their organization, the first and most essential step in the development of the expert system was definition of the database structure that would allow unified treatment of all possible standards. The database structure is shown in Fig. 4 and is described in detail below. In principle we have followed steps of an operator getting the necessary information from standards, when performing the experiment 'manually' using a conventional software, as

TENSALUM Expert System 475

MEASURING UNIT PREPARATION OF

MEASUREMENT ( START )

I

t ]RO Fol ARSn ARY IISTA.O CONOrrlo.s IIMEASUREMEVr I I I

MEASUREMENT l__ r

ANALYSIS IIANALYSlS, PRIN'I ANOPRINT II CONCLUSION

I -

UNIT

FIGURE 3. Structure of the expert system.

described in the introduction. The expert system structure is schematically shown in

Fig. 3. It consists of three independent units: (1) the measuring unit (MU), (2) the central unit (CU) and (3) the database (DB).

2.1. The Measuring Unit

The measuring unit can also be used as an independent program for 'manual' performance of measurements; in this function it is essentially not different from other commercial tensile testing software packages. When 'measurement according to standard' is chosen, testing is fully automated. The structure of the MU is schematically shown in Fig. 3. It consists of three modules: (1) preparation of measurement, (2) measurement and (3) analysis and print-out.

The first module, preparation of measurement, performs the function of communication with the operator. It is a user interface which enables entry of test conditions, general data on the measurement and on the material to be tested. When the operator chooses 'manual' testing he enters the testing conditions by means of a keyboard ('manually'). He can still perform measurements according to standard, but then he must find the testing conditions in the selected standard and this, as mentioned earlier, is a time-consuming procedure. The second option represents fully automated measurement according to selected standard. The central unit of the expert system delivers the testing conditions prescribed by the standard to the MU. The CU leads the operator through individual phases of the test, provides him with the data specified in the selected standard and supervises his decisions.

The second module, measurement, performs the test in conformity with the parameters provided by the first module. The tensile testing machine is controlled via communication interface IEEE 488 or RS232. Similar to

all commercial programs, the module, prior to test, checks all connected sensors and the status of the measuring system. The operator can follow the testing procedure on the screen in the form of a diagram and a display of current force and deformation values. The coordinates of the diagram can be arbitrarily adjusted prior or after the test.

As the test is completed, the measured values are delivered to the third module, analysis and print-out. This module again provides two options that are automatically activated, as shown in Fig. 3. The first option is active when 'manual' testing is being performed. In this case the module performs the analysis of the measured values, their statistical evaluation and prints a protocol containing the entered data on measurement and material, a graphic survey (diagram) of the measured values, as well as a table of the results. The operator himself composes the conclusion based on the comparison of measured and standardized values that have to be looked up in the standard. Looking for mechanical property limits is again a time-consuming procedure requiring an operator who is well-acquainted with the standards.

The second option is activated when automatized 'measurement according to standard' is performed. In this case, the conclusion is reached by the central unit upon reading the mechanical property limits from the database. It is printed automatically as an integral part of the protocol.

2.2. The Central Unit

The central unit represents the 'brain' of the expert system. When the operator selects 'measurement according to standard', the CU takes over the supervision of the measuring performance: It controls and supervises the operator's work, directs the procedure and performs communication between the database and the MU. It


delivers the standardized testing conditions to the MU. After completion of the test, it creates a conclusion based on the comparison of the measured values and the standardized mechanical property limits. It is practically impossible to perform an incorrect measurement provided that the specimen has been inserted and clamped correctly. The logic incorporated into the CU will be explained in the following section which presents the structure of the database.

2.3. The Database

The third part of the expert system is the database which consists of several different files. These files contain all data on the measurement specified by standards, i.e. the testing conditions and mechanical property limits, as well as the information required by the central unit to

I-

supervise and guide the operator through the test. The structure of the DB is shown schematically in Fig. 4 which indicates its complexity and thus shows indirectly how demanding it is to determine these parameters manually.

The first file, named 'standard', contains information on the classes of thicknesses ('input of dimensions') covered by a particular standard (there are 66 altogether). Based on the specimen geometry entered and the information stored in this file, the CU provides the operator with a list of standards covering the particular geometry. The specimen geometry and the selected standard determine the conditions of testing stored in the file named 'testing conditions'. The next three files store groups of data on 'types of material', 'form of product' and 'elongation'. All of these are functions of the test- sample geometry and the selected standard. This means

TEST ~ ( TYPEOF ~ ( FORM ~IDITIONS] MATERIAL ] OF THE PRODUCT

TEMPER

DATA BASE

FIGURE 4. Structure of the database.


that each of these files contains 594 groups of data. The sixth file, 'temper', contains groups of qualities of thermal treatment of the materials. These again are functions of the geometry, standard, type of material and form of product. There are 266,112 of these groups altogether. 'Types of material' and 'form of products' contain the information on which the selection of the quality of thermal treatment, 'temper', is based.

'Temper', together with 'elongation' lead to the file containing the 'mechanical property limits' which is thus a function of all the previously mentioned parameters. For the nine standards that are at present included in DB this means 532,224 different groups of mechanical property limits altogether. This calculation also takes into account as data the information that certain standards do not cover some values. Excluding this information reduces this number appropriately.

The individual files are of modular structure and thus allow subsequent addition and correction of separate standards. This is especially important in cases when so- called internal standards in use by a given company need to be added.

3. USING THE EXPERT SYSTEM

The TENSALUM software package is written for the DOS operating system and requires a VGA graphic card. This means that it will run on practically all up-to-date personal computers. Other requirements for the computer are: A CPU 286 or higher and at least 4 MB of hard disc space. The use of a mouse is recommended. A mathemat- ical co-processor is not required. A software version for the Windows environment is in preparation. Commu- nication with the testing apparatus (tensile testing machine) is provided via an IEEE 488 or RS232

interface, depending on the particular tensile testing machine used. The course of the program's use is displayed in the flowchart presented in Fig. 6. This consists of three parts: (1) Preparation of measurement, (2) measurement and (3) analysis and print-out.

3.1. Preparation of Measurement

The use of the program is protected by a password. By means of an ancillary program the supervisor (e.g. head of laboratory) can define a personal code for each operator. The CU will keep a separate time record for each operator. This information is accessible only to the supervisor.

After entering the password a screen displaying a company logo and the main menu will open, shown in Fig. 5. The structure of the main menu is identical with that of all Microsoft programs for the DOS environment. The first pull-down menu named 'LEM' (an abbreviation of the name of the author's laboratory) allows exit from the program and resetting of all previously entered parameters (see flowchart diagram in Fig. 6). The second menu named 'general' enables entry of general data on the measurement and material to be tested. The entry of these data is optional, with the exception of the number of specimens to be tested. From the third pull-down menu, 'geometry', the shape of the test specimen (i.e. rectangular, cylindrical or arbitrary) is selected first. All standards cover the rectangular specimen shapes, except ASTM which also covers cylindrical shapes. Choosing the shape and dimension of a specimen which is covered by standards, automatically selects 'measurement according to standard'. The CU immediately takes over supervision of the performance of measuring. It deactivates selection of the other two geometries and opens the

FIGURE 5. The main menu.


window for entry of the dimensions. Based on the dimensions entered, in the following pull-down menu 'standards' the CU activates those standards which cover the geometry in question. Figure 7 shows an example of a pull-down menu for a specimen geometry of a=2.10 mm and b=20 mm.

The selection of an arbitrary specimen shape in the 'geometry' menu forces the 'standards' menu to activate the 'arbitrary test conditions' entry. This requires the performance of 'manual testing' which necessitates manual entry of all testing conditions via a submenu. In this case the 'material' pull-down menu remains inactive. This method of using the TENSALUM expert system does not essentially differ from the use of other commercial programs and thus does not require further elaboration.

We continue with the way of implementing 'measurement according to standard'. When the standard according to which the test is to be performed is selected from the 'standards' menu, the CU first deactivates all other standards and in the 'materials' pull-down menu activates the files for selection of the elongation, form of product and types of material. The selection of these parameters is mutually independent. The menu for the selection of the quality of thermal treatment ( ' temper ') remains inactive until the material has been selected. After selection of ' temper' , the CU deactivates the menus for form of product and type of material, reads the standardized mechanical property limits from the DB and displays them in the 'list of selected parameters' window. This window can be activated from the 'measurement' menu; the same menu also contains an

PREPARATION OF MEASUREMENT

START ) ( END ) /

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FIGURE 6. Flowchart diagram of TENSALUM.


FIGURE 7. Pull-down menu for selection of standards.

option to start the testing procedure. This option remains inactive as long as the operator does not confirm the testing conditions and mechanical property limits defined by the selected standard. It frequently happens that a company sets stricter conditions than the standardized ones. In that case the Ctrl+F9 key combination activates the fields for the entry of the mechanical property limits and testing conditions in the 'list of selected parameters' window. Thus corrections can be made. Figure 8 shows an example of such a window showing the mentioned active fields. As soon as the standardized or corrected parameters are confirmed by

pressing the F8 key, the central unit activates the measurement start-up option.

The following pull-down menu, 'print-out', enables printing of the protocol covering the last measurement, selection of automatic print-out after a series of measurements has been completed and resetting the parameters of the coordinate system. This system can be modified as desired either before each measurement or before each print-out. The last pull-down menu, 'IMPOL' (the name of the company for which the software package had been developed), contains some handy utilities: A calendar, a clock and a calculator.

FIGURE 8. Window with data on testing conditions.

480 L Emri and D. KovaEi?

Laboratory for Experimental Mechanics University of Ljubljana A~,ker6eva 6 61000 LJUBLJANA, SLOVENIA

Telephone: (386 61) 1261 310 Telefax: (386 61) 218 567 Telex: 32240 FAKSTR

Ljubljana, 28.01.1994

CERTIFICATE OF QUALITY No.

PROCEDURE: TESTING MECHANICAL PROPERTIES ACCORDING TO STANDARD ASTM B 209M

INPUT DATA:

TYPE OF MATERIAL: 5052 TEMPER: O H ORDER No.: D N 143 TYPE OF PRODUCT: SI ' IBET DIMENSIONS [mm]: 6x2000~000 CHARGE: 3 CUTTING DIRECTION: PARALLEL GAGE LENGTH [mml: 50

MECHANICAL PROPERTY LIMITS"

TENSILE STRENGTH [MPa]: 170-215 YIELD STRENGTH [MPa]: >65 ELONGATION [%]: > 19 HARDNESS liB]: 40 SPEED OF TESTING [mm/min]: 0.5 NUMBER OF SPECIMENS [n]: 5 TEMPERATURE [C]: 23.0 PRODUCER: IMPOL-SL Bistrica

REMARKS:

RESULTS OF MEASUREMENTS:

O 3

O3

200 i i ! | | i i !

180

160

140

120

100

80

60

40

20

0 0 10 20 30 40

STRRIN rY.]

N a S Rm Rp.2 A HARDNESS ERICHSEN EARRING mm mm 2 MPa MPa % HB mm %

6.081 121 .934 181.7 71.4 31.5 40 6.083 122 .522 180.7 71.5 30.0 40 6.083 122 .522 179.9 70.9 29.5 40 6.078 121 .739 178.1 68.2 27.3 40 6.080 121 .934 182.5 70.7 27.6 40

Xsr 6 . 0 8 1 122 .130 180.6 70.5 29.2 40

CONCLUSION: Material FULFILLS conditions of standard!

OPERATOR: Marjan Pohl OUALITY CONTROLLiqR: Drago Kova~i~

FIGURE 9. Example of protocol.


3.2. Measurement

The start-up option initiates the communication between the computer and the tensile testing machine. A flowchart of the measurement phase is shown in Fig. 6. It starts with checking the testing machine (checking the connected sensors and the machine status), followed by setting the measuring parameters. On concluding these procedures the program displays a notice for manually starting the testing machine (newest models of testing machines may provide for computer start-up as well). This now initiates the testing process itself, which can be monitored on the screen in the form of a diagram and a display of the current force and deformation values. After the measurements have been completed and confirmed, the measured values are stored on a hard disc. The operator may then enter the geometry of a new specimen in the series to repeat the measuring phase.

3.3. Analysis and Print-Out

After the last test sample in the series has been measured, the program analyses the measured data and calculates certain statistics. In the case of manual measurements this is immediately followed by the print-out of the protocol. The conclusion on the test(s) has to be made by the operator himself, based on a comparison with the standardized mechanical property limits. This, as mentioned before, is a time-consuming and error-prone procedure. By contrast, when automatized 'measurements according to standard' were performed, the conclusion is reached by the central unit instead of the operator and is printed automatically as part of the protocol, shown in Fig. 9.

4. LIMITATIONS OF THE EXPERT SYSTEM

The expert system was originally developed for testing of aluminum and aluminum alloys using specimens with a square cross section. However, if data from the appropriate standards are added to the existing database, in conformity with its structure detailed in Section 2.3, the expert system may be used for testing other materials. In preparation are database add-ons for polymeric materials, wood, textile fabrics and leather.

The ES supports communication with a testing machine and a printer via IEEE 488 and RS232 ports, using appropriate drivers. At present drivers for testing machines 'Zwick ' and printers 'EPSON' and 'Hewlet Packard' are available. The driver for 'Instron' machines is under preparation and will be very likely available at the time when this paper is printed.

The ES was written for the DOS environment and is not aimed to be used in a 'multi tasking' Windows 3.x environment. During the test the ES has to have total control over the testing machine, which can be lost if several programs are running in parallel, thus causing damage to the testing machine. Using the DOS environ-

ment also permits utilization of any 'out-of-date' PC with a 286 processor or higher. We are planning to prepare a Windows '95 version of the ES which will allow 'background' communication with testing machine. However, in this case a PC with a much faster processor should be used, particularly if several more demanding programs are used simultaneously.

All texts for the user interface menus are saved in ASCII format files and can be easily translated to any language using a text editor, e.g. simple DOS-Editor, without entering the main source code of the ES. The same is true for the entire database.

5. C O N C L U D I N G R E M A R K S

The TENSALUM expert system has now been used in industry for over two years. Analysis based on this experience has shown that the time saved with regard to the computer-assisted standard ( 'manual ' ) method of measuring is between 300 and 400%, depending on how many different series of measurements are performed daily. During the 'manual ' experiment the operator has to consult standards for testing conditions and comparative values of mechanical properties that the tested material should meet, for each specimen with a new dimension. This tedious procedure requires a very accurate and skilled operator and is completely avoided when the ES is used. An important fact is that, with the use of TENSALUM the number of errors due to incorrectly read values specified by the standard is reduced to zero. The only thing that the operator has to concentrate on is the proper and accurate clamping of the specimen.

Again, it has to be stressed that the DB is of a modular structure and thus can be modified and updated with standards for tensile testing of any other materials.

Acknowledgements--The authors acknowledge the contribution of Professor N. W. Tschoegl, of California Institute of Technology, whose comments have greatly improved the quality of the paper. The authors are also indebted to Mr Mirko Gregori~, dipl. ing., and Mr Slavko Kranj~i~ for undertaking the task of examining and testing the TENSALUM expert system, and they gratefully acknowledge support of this work by the IMPOL Company under Contract 5/75-93 and by the Slovene Ministry of Science under Grant 32-5667-0782-93.

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thickness over 0.35 ram; properties. Strips and sheets of wrought aluminium and aluminium alloys, with

thicknesses from 0.021 to 0.350 mm; properties.

482 1. Emri and D. Kova(i(

Testing of metallic materials; tensile test. Metallic materials; tensile testing; part 1; method of testing (at ambient

temperature). Metallic materials, tensile testing. Aluminium sheets and strips. General requirements for manufacture

and delivery. Aluminium foils. General requirements for manufacture and delivery.

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expert system for testing mechanical properties of aluminum and aluminum alloys

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