The instrumented guillotine impact testing apparatus

The instrumented guillotine impact testing apparatus M. Jordan (University College Dublin, Ireland)

Significant progress has been seen in recent years in the development of adhesives for use in applications of structural assembly. While the broadening of the range of applications and of adhesives available warrants constant updating of test methods, these have not kept pace with the industry. This paper presents a new procedure for the destructive testing of structural adhesives at high rates of strain and an instrumented testing apparatus is described in detail. Specimen preparation test procedure and data reduction techniques are also described, and sample results shown.

Key words: adhesive-bonded joints; impact testing; instrumented guillotine impact testing apparatus; specimen preparation; test procedure; data reduction

With the development of new adhesives and production methods, adhesive bonding technology offers potential advantages in structural assembly. As well as providing more uniform load distribution and better fatigue and damping properties than conventional methods, adhesive bonding provides a smooth surface finish, reduces component weight, provides sealed, filled joints and facilitates simpler joint design. This ultimately offers reductions in costs in high volume production areas such as the automobile industry and high cost areas such as the aerospace industry. While adhesives have been used in less severe applications in the automobile industry and other areas, the potential applications of specialized adhesives have not been fully exploited. Industries have failed to rely on adhesives as a primary structural bonding agent in severe operating environments. As a result, they have failed to avail of their advantages. Improvements in the quality and range of adhesives over the last decade have helped to establish adhesives as reliable and cost saving means of structural joining 1-3. Where the primary method of joining is through the use of adhesives, the ability to withstand complex loading patterns is of paramount importance, especially where the safety of occupants is concerned. The determination of the response of the adhesives to these loading patterns is therefore essential before application becomes practical. While the evaluation of the physical, chemical and other properties is of use in predicting the reliability and performance of the

adhesive bond 4"5 the need for more rigorous testing in a practical simulated service environment is apparent. Due to the dependence of the adhesive performance on the loading rate, reliable dynamic performance cannot accurately be deduced from quasi-static type tests 5 and testing procedures have been developed in order to ascertain quantitatively the performance of bonded structures under severe operating conditions such as are present in situations of impact loading. The standard impact testing procedure for testing adhesives (ASTM Method D950/72 and BS 5350 Grp C4) uses a conventional pendulum-type device, with the only indication of energy absorbed by the bonded joint being provided by the swing-through of the pendulum. Alternative test methods, generally adapted from impact testing of plastics and predominantly of the drop-tower configuration with single lap joint specimens6, have been devised to overcome the shortcomings of this standard test and the full-scale crash testing of adhesive bonded vehicles has also been reported7. The present paper reports on the instrumented guillotine impact testing apparatus (IGITA), the testing methodology and experimental proceedure used in a specific testing programme to evaluate the comparative dynamic performance of mild steel structures joined by adhesive and conventional techniques.

Scope of test programme The test programme for which this test facility was designed was initiated with the objective of evaluating

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the comparative performance of adhesive-bonded joints (structural epoxy and acrylic) in high strain-rate dynamic loading environments and to compare that with the performance of pop-riveted and spot-welded joints. In particular, a new line of structural acrylic adhesives was tested in order to optimize the content of the toughening rubber modifier. The specimens, all of the same general configuration, consisted of four groups according to whether the joint was made by: • single-part heat cure epoxy adhesive; • rubber-toughened acrylic adhesive; • pop-riveting; or • spot-welding

Of the rubber-toughened acrylics (RTA), there were eight groups according to the rubber content (0, 7.5, 14.0, 23.0, 37.5, 54.0, 57.5, 60%) prepared on cosmetically ideal surfaces, and one further group with a fixed rubber content (23.0%), but prepared with cosmetically non-ideal adherend surfaces. The structural epoxy specimens were divided into two groups depending on the condition of the plate surfaces, again cosmetically ideal and non-ideal. Finally, two further batches of specimens contained the conventionally bonded (ie, pop-riveted and spot-welded) specimens.

Testing apparatus A testing facility, the instrumented guillotine impact testing apparatus, was specifically designed for the purpose of this testing programme. Designed for use in a materials testing laboratory, this apparatus (shown schematically in Fig. 1), while of the basic drop-tower configuration, differs from other such devices in two

Upper assembly and hoist U Cable

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housing

~

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~

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1

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Fig. 1

Schematic representation of test rig

40

INT.J.ADHESION A N D .,~,DHESIVES J A N U A R Y 1 9 8 8

respects: (1) that a dedicated onboard electronic module is provided for immediate display of quantities of interest, and (2) a greater range of test energies is available. Overall the test is 2.2 m high and consists of four vertical hardened steel bars, which provide support for the upper assembly and also guide the impactor during its vertical travel. The upper assembly consists of a framework which stabilizes the four columns and supports a geared airmotor which hoists the dropweight. The drop-weight onto which the impacting head is mounted comprises rigid tubular steel framework constrained to travel on the vertical columns by a linear recirculating ball bearing fixed to each corner of the assembly, thereby virtually eliminating the shudder problem associated with other drop-towers. On the underside of the structure is attached the impacting head, consisting of a hardened and half-ground 300 mm length of steel shaft 25 mm diameter, fixed in a horizontal position. Onto this shaft is fixed the force transducer, described below. The drop-weight is hoisted by means of the airmotor, through a cable and harness, to a pre-arranged height where it is held while the specimen is loaded by a simple quick release mechanism, consisting of a frictionless drawbolt unit. The specimen rests on two round steel supports firmly fixed to a baseplate (700 × 700 × 25 mm), so that loading is of the simple three-point bending type. The design of these supports and the drop-weight assembly is such as to facilitate free deformation of the specimen. The mass of the slider is nominally 20 kg. which provides energies up to 400 J (drop height 2.0 m). This mass is variable up to 40 kg hence doubling the energy capacity. For energies lower than that practical by the basic configuration, eg, energies less than 100 J, a counter balance arrangement is used so that more precise control of the drop energy is provided. Thus, by means of effective mass and height variability, this test rig is capable of infinite control over a wide range.

Instrumentation The instrumentation is composed of three distinct stages, shown in block diagram form in Fig. 2. The primary stage, the force transducing stage, consists of a Bruel & Kjaer accelerometer type 4333. A piezoelectric accelerometer housed in a titanium case, this transducer has a voltage sensitivity of 17.9 mV g - 1 a flat response up to 10 kHz and, as a general purpose accurate transducer with low mechancal impedance, is ideally suited for impact applications. The accelerometer is mounted on the ground steel impacting head and connected to the next stage by matched coaxial cable secured to the hoisting harness and kept taut to limit triboelectric effects. The secondary stage consists of the signal processing circuitry, where there is a dual configuration. Firstly, onto the control panel of the rig is mounted a unit which takes the output of the accelerometer, amplifies and processes it by analogue electronic techniques and displays the peak acceleration encountered during the test, the peak rate of change of acceleration during the test, the velocity of the mass and the deformation of the specimen. This unit is battery powered and is used

Stage

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Fig. 2

Blockdiagram of instrumentation system

primarily to give rapid information on the foregoing test as a source of reference for the ensuing test, but can be used as a stand-alone processor where more detailed or hardcopy information is not required. The parallel system is composed of a Bruel & Kjaer conditioning amplifier type 2626, a charge amplifier which gives a voltage at its output proportional to the charge at its input 8, a Gould digital storage oscilloscope type OS4100 which displays the voltage v s time output of the acce!erometer, and finally a JJ 3cy' flatbed plotter type PL100 on which the displayed output is plotted, providing a hardcopy record of the test. In the third and final stage of this system, the output plot of voltage v s time is read into a VAX 11/ 780 mainframe computer using a Houston digitizer model DT11. Here, a Fortran 77 reduction program is run which, in response to prompts from the keyboard, outputs plots of parameters of interest on a Calcomp 945 plotter.

Experimental details Specimen preparation Irrespective of the type of bond used, a joint will perform better if subjected to either shear o r tensile stress. However, in structural assembly applications, the tendency of adherends to deform in service causes the bond to be exposed to a combination of such stresses and also to the more damaging peel-type stresses 9. Joint design is concerned with minimizing the development of complex loading patterns so that deformation of flexible adherends either does not change the pattern of load application to the bond or is limited to certain acceptable levels which do not overactivate stress singularities. Redesign of joints to accommodate adhesives, expensive and possibly presenting assembly problems, would be seen as a distinct disadvantage over existing joining methods. The need therefore arises, for structural adhesives with sufficiently high tensile strength and the toughness and flexibility to withstand these loading patterns for use in the sheet metal fabrication industries.

While a discussion on the development of such adhesives is beyond the scope of this paper 3, the trend is to modify existing epoxy and acrylic adhesives 5 by the addition of rubber polymers 1°. Adhesives tested in this schedule consisted of a new line of acrylic adhesives toughened by the addition of polymers such as chlorosulphonated polyethylene, and epoxy adhesive toughened with liquid nitrile rubber ]l. Specimens were made up by bonding mild steel plates in a sandwich configuration, as shown in Fig. 3. Prior to assembly, samples of the mild steel plates were polished on alumina paper and analysed by X-ray fluorescent spectroscopy, and the results compared with alloy steel standards so as to limit the effect on test results of variance in plate properties. Steelbonder rubber-toughened acrylics are available as a single-part adhesive for use with a solvent-based surface activator and, while it is normally only necessary to apply adhesive to one surface and activator to the other j°, components were mixed prior to application in a stationary screw device attached to the output of the respective containers and then applied to one surface only. The plates were centred, slightly rotated to achieve an even bond, re-centred, clamped, and, in order to create conditions similar to those existing in paint baking processes, cured for 30 min at 185°C in a Gallenhamp oven. The singlepart toughened epoxy is applied to one adherend surface, rotated, centred, e t c , in the same way and inserted in an upright position in the oven, where curing again takes 30 min at 185 °C. The plates whose surfaces were cosmetically ideal for adhesive bonding were first degreased in trichloroethane at 80°C and then wiped with tissue to remove excess carbonization on both surfaces. The plates were then Astrolan-coated prior to application of the adhesive. Small spacers were used to maintain constant bond thickness at 0.5 mm, and excess adhesive was not removed after the curing process in order not to develop areas of stress concentration at the root of the beads. The spot-welded specimens were assembled according to BS 1140 and rivets to BS 4620 were used to construct the pop-riveted specimens (see Fig. 4).

275

Fig. 3 Adhesive-bondedspecimenconfiguration and dimensions {in mm)

INT.J.ADHESION AND ADHESIVES JANUARY 1988

41

[~

275

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51

Fig. 4 Rivetedand spot-welded specimens (dimensions in mm); pitch of rivets and spot-welds 25 mm, centred at 10 mm from small plate e d g e

Each batch was fully and separately tested before progressing to the next batch in order to reduce the possibility of any confusion arising during the test schedule. Energies were selected at discrete points in the range available so that each batch underwent the same series of tests, thereby facilitating comparison. Each specimen was tested only once and the testing of each group started at the lower end, progressing to higher energies until failure (see data analysis and evaluation) was deemed to have occurred. At this point further progression was suspended and, where enough specimens remained, half-interval searching was employed to determine, as a secondary method, the energy absorbed at the onset of what was deemed to be failure. Each test was recorded on the oscilloscope and the output routed to the plotter. The subsequent plot was annotated with the oscilloscope and charge amplifier settings, the drop height and weight, as well as values obtained from the onboard electronic module and the specimen identification number.

Test procedure

Data analysis and evaluation

Each specimen is centred on the base supports which are greased, to reduce friction, and set at a distance of 200 mm apart so that the lower plate is supported by the upper plate through the joint and otherwise unconstrained. The drop-weight, having been set to provide the desired impact energy, is released by the quick release mechanism and the impactor impinges on the upper plate (Fig. 5), causing it to deform and consequently causing the bond to undergo a high rate of strain in both tensile and shear modes (Fig. 6).

Two direct sources of i n f o r m a t i o n are available from

each test: first, the actual specimen in its deformed state, which may be subject to further non-destructive testing, and second, the acceleration/time trace which may be processed to obtain quantitative data. The deformed specimen is examined visually to determine whether or not damage has been sustained and if no damage is evident a series of non-destructive tests is carried out to determine whether or not cracks have been initiated along regions of stress singularities.

Fig. 5

Impact test showing drop-weight assembly and specimen undergoing test

42

INT.J.ADHESION AND ADHESIVES JANUARY 1988

60

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Briefly, the specimen is subjected to moderate transverse flexure beneath a binoccular microscope which reveals cracks as they open and close as the specimen deforms. In the event that this fails to reveal the existence of cracks, the dye penetrant technique is used. After wiping the surface clean, penetrant dye is applied which travels by capillary action into cracked regions. This process can again be accelerated by moderate flexure of the specimen. After some minutes (5-15) the surface is then cleaned with a solvent which removes active dye, and a developing agent is applied. This draws the dye back to the surface and reacts with it to show cracks as bright regions against the adhesive background. In the event that no cracks appeared with this test, the specimen was deemed to have withstood the impact without failure. In the present series of tests, failure refers to the state whereby damage has been sustained, however slight, although this would not constitute failure in all engineering situations. Nonfailure applies to the state whereby no evidence could be detected of any cracks appearing in the bond. Thus failure covers a variety, of states from crack initiation to catastrophic failure where the lower plate came away from the upper plate without sustaining any permanent deformation. In order to facilitate specimen comparison, it was necessary to use a damage severity index (DSl). Based on the observation that damage sustained differed in the degree of separation of the two plates, the DSl was calculated as the difference in bend angles between the plates measured in radians. Where angles differed on both sides of the centre of impact, the average DSI was taken. The plot of acceleration vs time (Fig. 7) is read into the VAX 11/780 and the OSl (where failure occurred), identification number, scale data, drop height, effective impactor mass, peak deceleration, peak rate of change of deceleration, velocity, and deformation from the onboard unit, which provided a check on the reduction process at the various stages of analysis, are entered from the keyboard. The energy equation, taking into account wind resistance and bearing friction, is: 1Amv 2 + 1ApAv2 + 4 F H = mgh

where v is the impact velocity (m s-I), h is the drop height (m), t9 is the density of air (kg m-3), A is the exposed area of the drop weight, F is the friction force of each bearing (estimated from manufacturer's data)

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and m is the drop weight (kg). This equation may be re-arranged to give: (gm-4F I v 2 = 2gh \ g m + g p A h /

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The voltage/time trace from the flatbed plotter is converted to an acceleration/time trace using the following system equation: a ( t ) = {V(t)/S}

where V(t) is the voltage function (mV), S is the accelerometer sensitivity (mV g-i) and a(t) is the

INT.J.ADHESlON A N D ADHESIVES JANUARY 1 9 8 8

43

acceleration (m s-2). The force exerted by the impact mass, F(t). is given by the equation:

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where zero subscript refers to time of contact, and v~ is velocity of impact. Further integration yields the displacement of the falling mass from the time of triggering of the oscilloscope: F

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which holds as long as the impactor is in contact with the specimen. At any time the change in total ener~' of the falling mass is equal to the energy transferred to the specimen. (Some of the energy lost by the falling mass will be dissipated by extraneous effects, such as mechanical deformation of the test rig. However. in the case of such deformations, since the rig is more rigid than the specimen, the loss is assumed negligible.) This quantity, given by:

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where the specimen mass is assumed to be negligible, may also be plotted against time. Considering the plots in Fig. 8 (where the direction of gravity is taken as positive, ie. a(t) negative for to < t < tc), the striker experiences resistance from the specimen up to a peak level at time (a). After this point, the striker continues the downward motion until the velocity reaches zero. This time (b) is the time at which deflection of the specimen is at a maximum. After time (b), the specimen continues to exert an upward force on the striker, causing it to accelerate in the upward direction and increase velocity' until this force is equalized by gravity and thereafter bouncing develops. After time (b), the deflection of the specimen decreases as the striker moves back up until time (c) is attained. As soon as the specimen fails to exert a force on the mass, time (c), we assume that specimen and mass separate until contact is again resumed on a subsequent bounce. The specimen may continue to relax after time (c), but since the drop mass is no longer in contact with the specimen, the accelerometer output may no longer be used to quantify this, and visual inspection is needed. In all cases, permanent deformation is incurred, and we can calculate the energy absorbed through the work done to deform the specimen (including bond breakage) as the energy lost by the mass at time (c). Zoller 12 uses an energy, transfer ratio ( E T a ) to quantify this energy: R(%) = [I00 X (energy transfer at time (c) )/(impact energy)]

44

INT.J.ADHESlON A N D ADHESIVES J A N U A R Y 1 9 8 8

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0.5 1.0

1.5 2.0 215 3'.0 3.5 4.0 4.5 5.0 Time (ms)

Fig. 8

Typical acceleration,velocity and deformation histories

After all data have been input, parameters of interest may be plotted where overall interpretations are desired. For example, as shown in Fig. 9, m a x i m u m G level (or m a x i m u m deformation) may be plotted against rubber content for unfailed specimens: for failed specimens, os] may be plotted against rubber content for fixed impact energy, or for fixed rubber content, G level against DSI. Discussion

Referring to the acceleration/time histories of failed specimens, it may reasonably be assumed that failure is initiated at the point of m a x i m u m G level as this corresponds to the time of exertion of m a x i m u m force by the ~pecimen. Although detailed mechanisms of failure L and energy absorption 14 are beyond the scope of the present paper, further dynamic analysis =5 than that presented here may be used to calculate the peak forces which initiated delamination in the outer edges of the lower plates at time (a). The tests did determine that a rubber content of 14.5% optimized the performance of the acrylic adhesive, which was found to be relatively independent of cosmetic effects, as in the case of toughened epoxy. General performance of the acrylic adhesive was better than that of the epoxy

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in that, for unfailed specimens and similar energies, the acrylic endured lower G levels suggesting more uniform absorption, while for failed specimens, damage was more severe in the epoxy for similar G levels. Neither adhesive endured impact forces as high as pop-riveted or spot-welded specimens. However, as might be expected, the adhesives gave more uniform absorption curves and. even when failure occurred, the resultant bend angle was smaller than that of unfailed pop-riveted and spot-welded specimens.

The comprehensive kinematic and dynamic information yielded by the computer assisted instrumented impact test, and the overall insight it gives into the behaviour of the bond, make it a necessary test in the complete evaluation of any structural adhesive. High speed dataloggers such as the Magus may be coupled directly to the conditioning amplifier, allowing the trace to be stored on disc, or analysed immediately, the subsequent results then stored on disc or plotted on a dedicated peripheral

INT.J.ADHESlON A N D ADHESIVES J A N U A R Y 1 9 8 8

45

plotter. Alternatively, the trace may be output to a microcomputer from the storage oscilloscope and subsequently processed through the use of readily available programmable controller/datalogger software.

Acknowledgements

The author wishes to thank Dr Des Moore of the Tribology Design Centre for his helpful advice, and Loctite R&D Ireland Ltd.

References 1

Patrick. R.L. "Structural Adhesives with Emphasis on Aerospace Application" (Marcel Dekker Inc, New York, USA, 1976)

2

Charnock, R.S. 'Structural acrylic adhesives for the sheet steel fabrication industry' paper presented at International Adhesion Conference, Nottingham, UI< 1984

3

Lees, W.A. 'Toughened structural adhesives and their uses' Int J Adhesion and Adhesives 1 No 5 (July 1981) pp 2 4 1 - 2 4 7

4

Adams, R.D. and Wake, W.C. "Structural Adhesive Joints in Engineering" (Elsevier Applied Science, London. UK, 1984)

5

Shields, J. "Adhesives Handbook' (Butterworths, London, UK, 1970)

6

Beavers, A. and Ellis, M.D. 'Impact behaviour of bonded mild steel lap joints' Int JAdhesion and Adhesives 4 NO 1 (January 1984) pp 13-16

46

INT.J.ADHESION AND ADHESIVES JANUARY 1988

7

'Adhesive bonded car body repairs withstand crash tests' Int J Adhesion and Adhesives 3 No 3 (July 1983) pp 1 2 1 - 1 2 2

8

Oliver, B.M. and Cage, J.M. "Electronic Measurements and Instrumentation" (McGraw-Hill, New York, USA, 1971 )

9

Mostovoy, S. and Ripling, E.J. "Effect of joint geometry on the toughness of epoxy adhesives" J Appl Polym Sci 1 S (1971 ) pp661-673

10

Chamock, R.S. 'Toughened adhesives of sheet metal bonding' paper presented at The LA V.D. Conference, Geneva, Switzerland, 1985

11

Wake, W.C. "Adhesion and the Formulation of Adhesives" (Applied Science, London, UK, 1971 )

12

Zollar, P. 'Impact testing of plastics' Polym Testing 3 (1983) pp 1 9 7 - 2 0 8

13

Altus, E., Haber, O. end Tirosh, J. 'An engineering failure envelope for adhesive joints' Exptl Mech (September 1986) pp 2 6 7 - 2 7 4

14

Harris, J.A. and Adams, R.D. "Energy absorption and strength of adhesive bonded lap joints under impact loading' papers presented at 2 I st Annual Conference on Adhesion and Adhesives, London, UK 1984

15

Goland, M. and Reissner, E. "Stresses in cemented joints' J Appl Mech, Trans ASME 66 (1944) pp A17-A22

Author

Mr Jordan is a Research Engineer at the Tribology Design Centre, University College Dublin. Mechanical Engineering Department, Upper Merrion Street. Dublin 2, Ireland.