A PROTOCOL FOR DETERMINATION OF ADHESIVE FRACTURE TOUGHNESS OF FLEXIBLE LAMINATES BY PEEL TESTING: MANDREL PEEL METHOD.

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1 A PROTOCOL FOR DETERMINATION OF ADHESIVE FRACTURE TOUGHNESS OF FLEXIBLE LAMINATES BY PEEL TESTING: MANDREL PEEL METHOD. An ESIS protocol June 2005 L F Kawashita, D R Moore, J G Williams 1 INTRODUCTION This protocol is a supplement to another entitled A Protocol For Determination Of The Adhesive Fracture Toughness Of Flexible Laminates By Peel Testing : Fixed Arm and T-Peel Methods [1] which relates to fixed arm peel and T-peel. For all peel tests on flexible laminates, adhesive fracture toughness (G A ) is obtained by measuring the external energy (G E ) required to conduct peel of the laminate whilst making allowance for the plastic bending energy in the peel arm (G P ). A global energy analysis then provides a determination of adhesive fracture toughness [1]: G A = G G (1) E P For many of the peel methods, including fixed arm and T-peel, the peel strength (peel force per unit width of specimen) is measured as well as the stress-strain behaviour of the peel arm [2,3]. This enables G E and G P to be calculated. G E is determined from peel strength (F/b) and peel angle (θ), and assuming negligible tensile deformation [2]: G E F = (1 cosθ ) (2) b G P is determined by the application of large displacement theory and the calculations are complex (although software for conducting the calculations is available [2, 3,4]). It involves the fitting of a function (either a bilinear or linear-power function) to the measured stress-strain behaviour of the peel arm in order to make the calculations tractable. Sometimes, this fit will not be perfect because of the nature of the deformation characteristics of the peel arm and therefore there is a possibility of inaccuracy. A mandrel peel procedure, where the peel arm is conformed to a circular roller (the mandrel) overcomes these complex calculations because a global energy analysis allows direct determination of both G A and G P [5,6,7,8]. 2 ANALYSIS OF A MANDREL PEEL TEST A mandrel peel method is shown in Figure 1, where the curvature of the peel arm is controlled by its conformation to a roller of radius R 1 [7,8]. This is achieved by applying a horizontal force D and in the case shown in Figure 1, this alignment force is at 90 o to the peel force. At the debonding point we have an angle θ and a force F as shown. For negligible friction, equating the moments around the axis we have, FR 1 = PR 1 i.e. F=P (3)

2 and horizontal equilibrium gives an expression for the force D, D 1 = P(cosθ + cosθ ) (4) P θ1 R 1 D F θ Figure 1 Mandrel peel method Thus and D cosθ = cosθ 1 P (5) F P D G = (1 cosθ ) = (1 + cosθ 1 ) b b b (6) Since θ 1 = 90 then P D G = ( GA+ GP ) = ( ) (7) b Two tests are conducted in the mandrel peel method; one with an unbonded specimen (G A = 0) and the other with a bonded laminate. For an unbonded specimen, the radius of curvature of the peel arm R 0, is large and conformity is guaranteed and data in the form of P 0 /b versus D 0 /b is shown in Figure 2 with a slope of unity (for zero friction) but displaced by G P (R 1 ) as shown. For the bonded specimen at D = 0, θ = 90 o so the value is as in a 90 o fixed arm peel test and usually R 0 < R 1 and P/b> G P (R 0 ) + G A > G P (R 1 ) + G A. As D increases θ decreases and so R 0 increases until R 0 = R 1 and conformity occurs as shown. For larger D values the lines should be parallel and displaced by G A.

3 P/b G(90 o ) Conformity R 0=R 1 Bonded specimen Unbonded specimen P 0 /b vs D 0 /b Reference line of slope (1+µ):1 G A G P (R 1 ) 0 D/b Figure 2 Analysis of mandrel peel. If the friction is negligible in these tests then the slopes of the lines will be unity. Generally the friction should be low (coefficient of friction µ ) but it can be calibrated out so that the slope is ( 1+ µ ):1 and the horizontal translation of the lines gives the G values 3 MANDREL PEEL TEST Mandrel peel specimens Two types of specimen must be tested in a mandrel peel procedure: (i) An unbonded specimen for the determination of the plastic bending energy. This will have the same substrates as those to be used in the bonded test but will be held together (and fixed to the mandrel table) via a bolt mechanism. No adhesive is used. (ii) A bonded specimen to be used for the determination of G A. The substrates are now joined together with the adhesive and the complete specimen is bolted to the mandrel table. An example of a generic specimen is shown in Figure 3; in practice some of the details may be different. Bolt holes for attaching specimen to mandrel table Base substrate Unbonded area to be placed in machine grip Peel arm Figure 3 General aspects of a bonded mandrel peel specimen

4 The unbonded specimen will have the same peel arm (material and thickness) as that for the bonded specimen. This peel arm can be fixed to the base substrate by a bolt that also fixes both parts to the mandrel table. Specimen dimensions will be as those for the bonded specimen. Bonded specimens for conducting mandrel peel should be in the form of rectangular specimens where the two parts of the laminate have already been adhered but where there is a region of unadhered material (of nominal length 50 mm), as shown in Figure 3. The overall dimensions of a peel specimen need not be rigidly defined but for many tests we have found that a length of 300 mm and width (10 20) mm proves to be quite satisfactory. Three specimens should be tested for each set of conditions. However, by using a long specimen it is possible to conduct more than one test on the same specimen. This is achieved by conducting the first test (i.e. obtaining a set of D, P data), unbolting the specimen from the mandrel table and relocating it in order to obtain another set of data. A re-location is necessary because the travel of the table with its linear bearing mechanism is limited. Peel arm Chain for attachment of alignment load (D) Mandrel roller with bearings Base substrate attached by bolts to table on a linear bearing Figure 4 A Mandrel Peel Instrument Mandrel peel equipment The choice of mandrel peel jig is not unique but the apparatus should incorporate a number of features. A successful kit is shown schematically in Figure 4. First, the jig is attached to an Instron or similar universal testing machine (a 10 kg load cell should

5 be adequate with a frame of 25kN capacity) such that as peel occurs the equilibrium peel force is maintained steady by the jig moving along a low friction linear bearing system. Second, the mandrel can rotate with minimal friction, achieved by the presence of bearings in the mandrel roller. Third, only one side of the laminate is allowed to be the peel arm in the test. Fixing the base substrate of the laminate to the peel table is a critical issue. If this layer can separate from the table during the test then the energy involved in that process will increase the measured adhesive fracture toughness value which will then be erroneously highl. The means of fixing the base substrate to the table should be reported. A bolt method can be successful. The friction in the system should be less than 5 % and the procedure will include a measurement of total friction. In assembling the mandrel peel jig there are a number of factors to consider: (i) The mandrel roller radius (R 1 ) is a variable of the test and typical values might be in the range (1-20) mm. A roller should be selected such that the peel arm curvature (R 0 ) in the test will be larger than the roller radius, particularly when the alignment load is large. This cannot be judged a priori but previous experience or test results from a 90 o fixed arm peel test [3] can define the value of R 0. If this condition (R 0 > R 1 ) is not satisfied, then conformance of the peel arm to the roller will not be achieved. (ii) Once the roller size has been selected it must be attached to the equipment so as to ensure proper operation. There are two adjustments to make. First, the horizontal adjustment should be made with a rectangular and long metal piece in the test machine top grip (e.g. a small metal ruler) in order to simulate a peel arm and to set the location of the roller. Second, the vertical adjustment of the roller is made in order to obtain a balance between the roller being as near to the specimen as possible but without the overall friction (µ) of the equipment being greater than 5%. (During the analysis of results µ can be measured as already mentioned in Figure 2). (iii) Attaching the specimen laminate to the mandrel table is best achieved with bolts. The bolt-holes in the base substrate must be counter sunk so that the bolt heads do not touch the mandrel roller during the peel process. Experimental method Prior to testing, the following dimensions should be obtained: Thickness of peel arm (5 measurements along the length of the specimen); the average value should be obtained (h). The width of the peel arm (5 measurements along the length of the specimen); the average value should be obtained (b). Thickness of base substrate (5 measurements along the length of the specimen); the average value should be obtained. Total thickness of the laminate (5 measurements along the length of the specimen); the average value should be obtained. This can be used to determine the adhesive bond-line thickness (h a ) The aim of the test on the bonded and unbonded specimens is to obtain a set (for each test) of peel force (P) and alignment force (D) data during peel of the bonded specimen and bending around the mandrel roller for the unbonded specimen. In both tests it is necessary to gently bend by hand, the peel arm around the roller and grip the peel arm in the test machine. If necessary, a re-gripping procedure can be used if the

6 initial length of peel arm is insufficient to adequately grip the peel arm (20 mm should be gripped). In both tests, small alignment loads should be used first and after the peel force has been measured, then the alignment force can be increased. In planning the alignment force to be used, the available length of displacement from the linear bearing trolley should be identified. For example, a 100 mm stroke linear bearing might be common. The gripping of the peel arm might use say 20 mm of displacement. Therefore, 80 mm of displacement remain available for peel or deformation around the roller. For each alignment load, 10 mm of displacement should be recorded whilst monitoring the peel or deformation force for bonded and unbonded specimen, respectively. Consequently, the number of alignment loads that can be used with 80 mm of available displacement will be 8. The smallest alignment load should be applied and the peel force measured and recorded. It is imperative that the maximum displacement of the linear bearing trolley is not exceeded, since this will damage the mechanism. Therefore, the table should be marked so that the extremity of displacement of the trolley is well signalled. For the example cited here, 8 sets of data (D, P) will be obtained for the unbonded and bonded specimens N N Peel Force (N) N 403 N 501 N 599 N Displacement (mm) Figure 5 An illustrative set of peel force versus displacement data for a bonded specimen (width 15 mm) with alignment loads in the range 300N to 750 N The initial alignment load will depend on the width of the specimen (b), the toughness of the adhesive (G A ), the thickness and material used for the peel arm and the roller radius. Therefore, some experience will be necessary. This can be obtained by some trial experiments or by specific instruction from a source of experience. It will be important to start at relatively small alignment loads but to generate high alignment loads by the end of the test. This needs to be achieved in some 6-8 steps. The ultimate application of high alignment loads is mandatory otherwise conformation of the peel arm to the roller will not be achieved. Moreover, several values of (D,P) should be consistent with conformance of the peel arm to the roller so that the appropriate

7 parallel portions for the unbonded and bonded specimens are achieved (as discussed in the text associated with Figure 2). An illustrative set of peel force versus peel displacement results are shown in Figure 5 for a range of alignment loads in the range (300 to 800) N. It is possible that alignment loads might be as large as 100kg and even for smaller alignment loads, care must be taken in ensuring the gentle application of these loads (as dead-weights). In addition, the chain mechanism must be able to hold such weights, but even so, care must be taken in ensuring that if chain failure occurs that injury to the operator does not occur. Using large lengths of chain so that the weights are near to the floor can be helpful, but this does not guarantee safety and therefore precautions should be taken AA 2024-T mm thickness Adhesive F Mandrel 20mm radius 60 P/b (N/mm) Bonded Specimen Unbonded Specimen D/b (N/mm) Figure 6 Illustrative P/b versus D/b results from a mandrel peel test for bonded and unbonded specimens (width 15 mm) using a 20 mm radius mandrel roller. The slope of the plots of P/b versus D/b should be parallel for cases when the peel arm conforms to the mandrel roller as shown in Figure 6. The slope of the curves should be measured and the coefficient of friction for the equipment should be obtained (µ). The horizontal displacement of the line for the bonded specimen and the line for the unbonded specimen provides a value for G A whilst the horizontal displacement between the line for the unbonded specimen and the reference line (see Figure 2) provides the value for G P. In addition, each data point can be considered individually and the horizontal displacements can be used to obtain a set of values for G A that can then be plotted against D/b, as illustrated in Figure 7.

8 6 5 AA 2024-T mm thickness, Adhesive F Mandrel 20mm radius Mandrel Peel 4 GA (kj/m 2 ) D/b (N/mm) Figure 7 An illustrative plot of G A versus D/b 5 PRESENTATION OF RESULTS Results should be presented for each test and for each specimen. A presentation of results should include a description of the equipment used (test machine and mandrel jig) and a description of the test laminate (substrate materials and adhesive). In addition, specimen dimensions should be given: Peel arm thickness (h) mm Base substrate thickness mm Bond-line thickness (h a ) mm Specimen width for peel arm (b) mm Specimen width for base substrate mm The following derived data should be reported: Adhesive fracture toughness (G A ) J/m 2 Plastic bending energy (G P ) J/m 2 Coefficient of friction µ A number of graphical presentations should be included in the report: (i) A plot of peel force versus displacement for each of the alignment loads (as shown in Figure 5). (ii) A plot of P/b versus D/b for both the bonded specimen and the unbonded specimen (as shown in Figure 6). (iii) A plot of G A versus D/b (as shown in Figure 7).

9 6 REFERENCES [1] Kinloch, A.J., Lau, C.C. and J G Williams, Int. J. Fract., 66, (1994). [2] Georgiou, I., Hadavina, H., Ivankovic, A., Kinloch, A.J., Tropsa, V., Williams, J. G. The Journal of Adhesion, 79, 1-27 (2003). [3] Moore, D.R., Pavan, A., Williams, J.G., eds "Fracture Mechanics Testing Methods for Polymers, Adhesives and Composites, Moore, D.R., & Williams, J.G., Ch 3 p 203 ISBN Elsevier, Oxford, [4] Imperial College website, peel test protocols- ICPeel [5] Gent, A. N., & Kaang, S.Y., J Adhesion, 24, (1987). [6] Breslauer, E., Trocynski, T., J.Adhesion Sci. Technol., 12, (1998). [7] Kawashita, L.F., Moore, D.R., Williams, J.G., The Journal of Adhesion, 80, 1-21, 2004 [8] Kawashita, L.F., Moore, D.R., Williams, J.G., A Comparison Of Peel Tests For Metal-Polymer Laminates For Aerospace Applications tbp The Journal of Adhesion 2005

KEYWORDS Adhesive fracture toughness, cohesive fracture toughness, fracture mechanics, peel tests, adhesive joint tests, interfacial fracture.

$KEYWORDS Adhesive fracture toughness, cohesive fracture toughness, fracture mechanics, peel tests, adhesive joint tests, interfacial fracture.$ Eng. Fracture Mechanics, 7, 6, - A CRITICAL INVESTIGATION OF THE USE OF A MANDREL PEEL METHOD FOR THE DETERMINATION OF ADHESIVE FRACTURE TOUGHNESS OF METAL-POLYMER LAMINATES L F Kawashita, A J Kinloch,