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A universal rig for mechanical testing of thin specimens using a novel loading assembly controlled by computer

This content has been downloaded from IOPscience. Please scroll down to see the full text. 1997 Meas. Sci. Technol. 8 178 (http://iopscience.iop.org/0957-0233/8/2/011) View the table of contents for this issue, or go to the journal homepage for more

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Meas. Sci. Technol. 8 (1997) 178–183. Printed in the UK

PII: S0957-0233(97)74610-1

A universal rig for mechanical testing of thin specimens using a novel loading assembly controlled by computer G H Rubiolo†‡ and G M Benites† † Laboratorio de Prop Mecanicas de Pol´ımeros y Materiales Compuestos, Departamento de F´ısica, FCEyN-UBA, Pab 1, Ciudad Universitaria (1428), Buenos Aires, Argentina ‡ Departamento Materiales, CNEA-CAC, Avenida del Libertador 8250 (1429), Buenos Aires, Argentina Received 20 May 1996, in final form 27 September 1996, accepted for publication 24 October 1996 Abstract. This paper describes an apparatus which is able to perform tensile true constant stress or strain rate, stress-relaxation and creep tests on a thin strip specimen up to a maximum load of 20 N, in vacuum or in an inert atmosphere and at high temperature. The machine applies the load on the specimen by means of a balance beam, whose counterweight is partially submerged in a container filled with oil. The load, strain and temperature are measured with a load cell, a linear variable differential transformer and two thermocouples, respectively, and they are monitored by a personal computer through an analogue–digital interface. The computer-based control and data acquisition software was written in QBasic language and it works on the power stage through the digital outputs. The power stage consists of two pumps that change the fluid level in the container. The temperature on the specimen is achieved by using an electrical furnace controlled by a PID controller. The performance of the machine is illustrated with results on aluminium 3003 foil alloy and Fe73.5 Si13.5−x Alx B9 Cu1 Nb3 amorphous alloys.

1. Introduction The small universal rig for mechanical testing described in this paper was developed to investigate the mechanical properties of amorphous and nanocrystalline ribbons, but it is suitable for any similar specimen. The several devices for testing small specimens which have already been described in the literature were primarily intended for thin foil specimens (Menter and Pashley 1959, Marsh 1961, Hoffman 1965, Kuhlman-Wilsdorf and Srinivasa Raghavan 1962), for electron microscope (transmission or scanning) specimens (Stephens and Hoeppner 1988, Schenk et al 1990) or for specially shaped and sized specimens (Taub and Spaepen 1979, Taub and Luborsky 1981, Deng et al 1993, Shinohara et al 1993). The oldest were wholly electro-mechanical in construction, open-loop controlled and with analogue data acquisition. The new generation use sophisticated and expensive new electronics and computer control which gives flexible and high-performance operation. Also, in the last few years, some commercial machines with these characteristics have appeared, for example the c 1997 IOP Publishing Ltd 0957-0233/97/020178+06$19.50

DMA7 dynamic mechanical analyser from Perkin-Elmer Corporation. We describe here an apparatus comprising a reliable mechanical arrangement of novel design for the loading assembly with simple, stable and cheap electronic and computer control. Besides a detailed description of our set-up and its performance, we present some results on the stress–strain behaviour at room temperature of amorphous alloys. 2. Design principles Figure 1 is a diagram illustrating the basic principles of the machine. A load is applied to the specimen by a balance beam. This load is transmitted across the balance beam from a counterweight suspended from the other arm of the beam and partially submerged in a container filled with oil. Then, the load applied to the specimen is proportional to the difference between the weight of the counterweight and the buoyancy force. The buoyancy force depends on the fluid level in the free volume between the counterweight and the container.

A mechanical rig for thin specimens

Figure 1. A schematic diagram of the loading assembly.

Figure 2. The main features of the rig for mechanical testing.

In order to drive the buoyancy force, two miniature centrifugal pumps circulate the oil back and forth between the container and a reservoir. The maximum load and load rate of the machine depend at least as much on the detailed design of the loading system components as they do on the general principles given above. In particular they depend on the flow rate of the pumps and the ratio between the counterweight volume and the free volume. This loading system was chosen because of its simplicity, stability and low cost from the viewpoint of the control unit design, as can be seen below.

3. A description of the apparatus The main features of the tensile machine are illustrated in figure 2. The basic structure is the base-plate A which lies on three levelling screws mounted on a frame B. A double cantilever load cell C is attached to the left arm of the balance beam and a universal joint D (ball and socket) is fastened on the free end of the load cell. The other end of the universal joint is a threaded rod which screws into the threaded end of the tensile tube E; a plane nut register ensures that the tensile tube is on the axis of the joint. The tensile tube runs the length of the balance height and goes further down through a cylindrical hole in the 179

G H Rubiolo and G M Benites

Figure 3. The grip holder for mounting the specimens.

base-plate. The lower half of the loading assembly is a ‘quick-connect’ fixture F attached to the lower surface of the base-plate. This fixture allows the installation or removal of the specimen, as is described below. The specimen grips are jeweller’s collet chucks with accurately plane ground spring collets. The chucks end in ground rod registers that fit into corresponding registers in the tensile tube and in the ‘quick-connect’ fixture and are fastened in place by means of pins Elongation of the specimen is measured by monitoring the displacement of a reference plate G, which forms part of the tensile tube, immediately above the upper surface of the base-plate. A miniature positioning stage (MPS) H, mounted on A, holds the body of a linear variable differential transformer (LVDT) I and a lever mechanism J transmits the displacement of the reference plate to the core of the LVDT. A second MPS K carries a fork-shaped peg that is used together with the reference plate to hold

rigid the tensile tube during the installation and removal of the specimen. The assembly of the container L and the reservoir M consists of an outer cubic tank and an inner cubic tank bonded at one face. Thus, two volumes are defined; the outer one is the reservoir and the inner one is the container. Two tubes for each tank, attached at one face near the bottom of each tank, are used to feed the oil back and forth between them. The pumps N are connected close to this ensemble. The counterweight P is a hollow cylinder lidded at both ends. Considering the base-plate as a reference plane, the upper part of the apparatus is placed inside a bell jar Q that can be evacuated to 10−2 Torr. The lower part is sealed with a quartz tube mounted in a conflat flange machined onto the main body of the ‘quick-connect’ fixture F. Also, two type K thermocouples, housed in a 0.5 mm diameter AISI 304 cover and threaded in thermocouple hollow feedthroughs, pass through holes made in the latter part and are welded to it. The thermocouples are placed close to both ends of the specimen’s gauge length. The furnace R consists of a long alumina tube which is externally wound with Constantan wire and coated with a refractory cement that ensures long-term operation of the winding up to 1000 ◦ C. The control element is a type K thermocouple placed in the middle part of the winding. This ensemble is placed inside a metallic can and the space between them is filled with alumina silicate fibre matting. Three masses S, hanging from three pulleys T mounted on the frame B at a 120◦ angle to each other, counterbalance the furnace, thereby allowing it to be placed gently anywhere along the tensile ensemble. The overall dimensions of the rig are 900 mm × 500 mm × 500 mm, approximately. The pumps for circulating the fluid between the container (L) and the reservoir (M) should have no sealed parts or volatile lubricants. We chose ones designed for automotive

Figure 4. The stress–strain behaviour of polycrystalline aluminium 3003 foil alloy.

180

A mechanical rig for thin specimens

Incorporated into the electronic design are two signal conditioners used for conditioning of load cell and LVDT signals. Four operating modes are available: true constant stress rate (CSSR), true constant strain rate (CSNR), creep (CR) and stress relaxation (SR). In the CSSR and CR modes, the command signal is a stress ramp programmed over a specific range of time, after which the stress is held constant. The CSNR and SR modes have a similar command signal built with the strain. The feedback signals are built on-line using the following equations:   1l F ε = ln 1 + − (1) l0 kl0 σ =

F exp(ε) A0

(2)

where ε is the true total strain, 1l is the total elongation of the specimen plus machine (LVDT signal), F is the load on the specimen, k is the spring constant of the machine and l0 and A0 are the initial gauge length and cross sectional area of the specimen, respectively. The exact equation for the true stress is σ =

Figure 5. The creep behaviour of polycrystalline aluminium 3003 foil alloy. See the text for more details.

applications. We used oil for rotary vane vacuum pumps from a local provider. The LVDT is from Lucas Shaewitz, model DC-E 500. The load cell came from a commercial electronic scale with 50 N maximum load. 4. Description of the control unit The machine is controlled by a computer equipped with a 16/32 bit microprocessor (Intel 80386) and 2 Mbytes of random access memory (RAM), of which 640 kbytes are conventional memory. A 12 bit ADC (Keithley DAS8/PGA), with eight analogue input and four digital output channels, is used to switch on/off the motors of the pumps and to digitize the data with 25 µs conversion time. The card is also provided with a software-programmable amplifier with nine full-scale ranges. The digital control circuit is as follows. The command signal is a digital ramp generated by the control software, the feedback signal is built by the control software using the digital conversion of the out-of-balance voltage coming from the load cell and LVDT, the difference between both signals serves as input for the digital controller that operates, through the digital ADC outputs, on two optoelectronic switches. These switches drive the current between a 9 V DC power supply and the pump motors. The optoelectronic switches can also be activated manually for the set-up of the test.

F exp(ε) exp[−(1 − 2ν)εe ] A0

(3)

where ν is Poisson’s modulus and εe is the elastic deformation of the specimen. Using equation (2) instead of (3) introduces a maximum relative error of about 0.4%. The main program providing the user interface and supervising all functions is written in QBasic. Several subroutines, also written in QBasic, do all time-critical tasks like control, data acquisition and real-time data monitoring and graphic display. The data are stored in a virtual disk on the extended RAM memory and are automatically copied to the hard disk when the former is full. The temperature is controlled by feeding the furnace coil with the power output of a temperature PID controller that uses the control thermocouple of the furnace as feedback. 5. The calibration and testing procedure A calibration sub-routine for the load and the elongation is provided by the software. The load cell is calibrated by clamping the balance beam with a peg mounted on the balance foot and hanging calibrated masses from the tensile axe. The elongation is calibrated using a dial micrometer mounted on an arm that pivots around and moves along the balance foot. The dial micrometer reads the displacement of the reference plate G, which can be manually driven with the MPS K. After running the load and elongation calibration sub-routines, both calibration constants are automatically stored and are available to the main program every time it is started, so these sub-routines need not be executed every time. Careful measurement of the nonlinearity of the load and elongation scale showed that it agreed closely with the calibrated errors of 0.05% full scale from AD conversion. The attachment to facilitate the placement of specimens without mechanical damage is sketched in figure 3. The 181

G H Rubiolo and G M Benites

Figure 6. The stress-relaxation behaviour of polycrystalline aluminium 3003 foil alloy. The initial loading corresponds to the curve for ε˙ = 1 × 10−6 s−1 at T = 508 K in figure 4.

Figure 7. The stress–strain behaviour of Fe73.5 Si13.5−x Alx B9 Cu1 Nb3 amorphous alloys. The alloys are labelled by their aluminium content. The curve corresponding to the 0% Al content is shifted along the x axis for clarity. Young’s moduli are shown.

ground rod registers of both grips are clamped rigidly on the V grooves of the holder, thereby fixing the position of the grips while the specimen is being placed. By tightening or loosening the screws, the grips can either be clamped at the desired length of the specimen or be released when grips and specimen are clamped in position for a tensile test. In this way it is very simple to achieve closely reproducible gauge lengths and alignments of the specimens. Also, this holder avoids any accidental stretching of the specimen during its mounting in the machine. After mounting the specimen in the holder, the ‘quickconnect’ fixture F must be removed to give access to the lower end of the tensile tube. Then the upper grip is fastened in position with its pin, the screws of the holder are released and the grips are freed. Finally, on coupling 182

the ‘quick-connect’ fixture F, the lower grip is fastened with its own pin. 6. Performance The following characteristics of the machine have been determined: maximum load rate maximum load (Fmax ) maximum elongation (1lmax ) spring constant (k) maximum operative temperature absolute error in the load absolute error in the elongation

3 × 10−2 N s−1 20 N 2 mm 720 ± 7 N mm−1 773 K 10−2 N 10−3 mm.

A mechanical rig for thin specimens

From equation (1) and the measurement errors given above, an error in the deformation of the order of 5 × 10−5 can be estimated. The error in the stress depends on its actual values; however, the minimum relative error is 0.4%. The errors originating from the control system also depend on the actual values of the signals. The following typical relative errors have been determined: CSNR mode: CSSR mode: SR mode: CR mode:

0.1% 0.1% 0.02% 0.01%

for for for for

ε˙ = 1 × 10−6 s−1 σ˙ = 7 × 10−2 MPa s−1 ε = 5 × 10−3 σ = 40 MPa.

These errors were determined from the standard deviation of the least squares fit of the feedback signal as a function of time. So far, the machine has been tested in air atmosphere between room temperature and 505 K with polycrystalline aluminium 3003 foil alloy (Mn 1.2%, Cu 0.12%, Si 0.6% maximum and Fe 0.7% maximum) and three Fe73.5 Si13.5−x Alx B9 Cu1 Nb3 amorphous alloys with 0%, 1% and 2% Al content. The aluminium specimens were cut, after annealing, from a 70 µm thick foil in the form of ribbons 1 mm wide and the amorphous specimens about 0.7 mm wide and 20 µm thick were cut from ribbons obtained by using the melt-spinning technique. Figure 4 shows the effect of temperature on stress– strain curves at constant true strain-rate of aluminium specimens. Strain-rate dependences at constant temperature on the same material are also shown in figure 4. The straight line through the origin shows the linear range with a slope of 70 GPa. The second straight line, parallel to the first one, is used to determine the values of the flow stresses σ0.2 . Unloading/re-loading cycles show the hardening of the material. Tensile creep tests on aluminium specimens have been carried out at two stresses and 505 K; the results are shown in figure 5(a). The post-processing of these data shows the period of constant creep rate and its dependence on the applied stress (figure 5(b)). Stress relaxation tests have also been carried out at the same temperature and the result is shown in figure 6. Finally, figure 7 shows the room temperature tensile stress–strain curves up to rupture for Fe73.5 Si13.5−x Alx B9 Cu1 Nb3 amorphous alloys. The stress–strain behaviour is essentially linear; the possible deviations from linearity are very slight at this temperature. From figure 7 it can be seen that Young’s modulus hardly changes between the specimen with no Al content and that with 1% Al content, whereas it increases

considerably for the specimen with 2% Al content; however, the tensile strength shows no significant variations among these alloys. 7. Conclusions A small universal rig for mechanical testing with a PC closed-loop controller has been described together with performance figures. The apparatus is simple to construct, easy to operate and cheap. It is capable of modification to include multifarious sample types and test geometries. Anelastic dynamic mechanical and fatigue tests at low frequencies can be performed with a small modification in the control software. It was developed mainly for tensile testing of amorphous and nanocrystalline ribbons, but may well have more general applications in testing fibres and polymeric foils. Acknowledgments This work was supported by the Fundaci´on Antorchas (Argentina) and the University of Buenos Aires (Argentina). References Deng D, Zheng F, Xu Y, Qi G and Argon S A 1993 Creep and structural relaxation in Pd40 Ni40 P20 glass Acta Metall. 41 1089–107 Hoffman R W 1965 Physics of Thin Films vol 3 (New York: Academic) Kuhlmann-Wilsdorf D and Srinivasa Raghavan K 1962 New tensile testing machine for thin specimens Rev. Sci. Instrum. 33 930–3 Marsh D M 1961 Micro-tensile testing machine J. Sci. Instrum. 38 229–34 Menter J W and Pashley D W 1959 Structure and Properties of Thin Films (New York: Wiley) p 111 Schenk A, Berka L and Vasina P 1990 A new tensile tester for micro-scale strain measurements in the scanning electron microscope Meas. Sci. Technol. 1 136–8 Shinohara A H, Sato T, Saito F, Tomioka T and Arai Y 1993 A novel method for measuring direct compressive properties of carbon fibres using a micro-mechanical compression tester J. Mater. Sci. 28 6611–6 Stephens R R and Hoeppner D W 1988 New apparatus for studying fatigue deformation at high magnifications Rev. Sci. Instrum. 58 1412–9 Taub A I and Spaepen F 1979 Isoconfigurational flow of amorphous Pd82 Si18 Scri. Metall. 13 195–8 Taub A J and Luborsky F E 1981 Creep stress relaxation and structural change of amorphous alloys Acta Metall. 29 1948–81

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