Evaluation of the tensile stress-strain properties in the thickness ...

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influence the mechanical properties of paper when testing thin paper sheets (Byrd et al. 1975). The irregular thickness of paper and irregular mass distribution also causes non-uniform strain distributions over the area of the test pieces as pointed out by Van den Akker (1952). These aspects became more relevant for thinner test pieces. The aim of the present paper was to develop a testing procedure for the measurement of the stress-strain properties in the Z-direction of paper enabling the extraction of the Z-directional tensile strength, strain at break and elastic modulus for paper. The penetration of adhesive and its influence on the performance of paper in the thickness direction were defined. A selected number of papers were tested. The relations between the mechanical properties and the paper structure, in terms of structural density, were obtained. Material and Methods Materials One thermo mechanical pulp (TMP) and one chemical pulp were tested. The pulps were either unbeaten or beaten in an industrial refiner to different CSF levels, Table 2. Handsheets were made according to SCAN-C 26:76 with the exception that the grammage of the tested handsheets varied from 10 g/m 2 to 300 g/m 2 . Key in-plane mechanical properties for 180 g/m 2 sheets are given in Table 2. Structural thickness and structural density were evaluated by SCAN-P88:01 and in-plane tensile properties by ISO 1924-3. Table 2. Materials used in the present investigation and their in-plane properties for 180 g/m 2 handsheets. Pulp CSF Structural In-plane In-plane Strain ml density tensile tensile at kg/m 3 index stiffness break kNm/kg index MNm/kg % TMP 1 Unbeaten softwood TMP 2 325 407 38 4.3 2.1 Beaten softwood Chem1 Bleached chemical pulp, 210 484 47 5.0 1.6 mix of birch, eucalyptus and softwood, lightly beaten Chem2 Bleached chemical pulp, 538 682 50 6.5 3.4 mix of birch, eucalyptus and softwood, highly beaten 236 871 73 8.3 3.6 Methods Preparation of the paper for testing The handsheet was fastened to circular 10 cm 2 metal platens by means of a photo mounting tissue (Beinfang ColorMount ® adhesive). The sum of platens, adhesive and paper is referred to as the test piece. The adhesive and the paper are henceforth referred to as the sandwich. The test piece was first subjected to 0.23 MPa pressure at 110°C in a hot air oven. After one hour curing time, 50 Nordic Pulp and Paper Research Journal Vol 22 no. 1/2007 the test piece was removed from the oven and conditioned under pressure at 23°C and 50% RH. The conditioning time was set to at least 12 hours which was found adequate to obtain the equilibrium of moisture content. Additionally bare handsheets underwent the same procedure in order define the change of the thickness due to the applied stress. The structural thickness of the handsheets after pressing, named thandsheet, and the grammage of the handsheet, whandsheet, were used to determine the structural density of the pressed handsheets, which is written as whandsheet ρ = handsheet t [1] handsheet The sandwich consisted of three areas, which could be delimited by imaginary horizontal planes, as shown in Fig 1. After curing, the handsheet was penetrated by the adhesive on both upper and lower surface. The mix area consisted both of the adhesive left outside of the paper and the part of the paper penetrated by the adhesive. The stress-strain properties of the paper calculated in the present investigation referred to the center part of the handsheet, not penetrated by the adhesive. Fig 1. Schematic picture of the structure of the test piece and adhesive penetration into a handsheet with definitions of three thicknesses. Procedure for the measurement of adhesive penetration into the handsheets, two-handsheets technique The evaluation of the mechanical properties in the thickness direction required an estimation of the penetration grammage, which was the quantity of the handsheet affected by the adhesive. The penetration grammage was assessed by testing two-handsheets sandwiches for each pulp type. Two handsheets with equal grammage were piled, pressed together and fastened each on one surface to the platens by performing the already described preparations procedures. The two-handsheets sandwich was subjected to a tensile stress by the same experimental set up used for the measurements of the mechanical properties of the paper. The grammage of the two handsheets was varied in order to obtain different degrees of adhesive penetration. As long as the grammage of two handsheets was higher than the penetration grammage, no separation force was expected. As soon as the grammage of the handsheets was lower than the penetration grammage, the opposite adhesive layers could make contact and consequently a separation force was measured. The separation force was likely to increase as the grammage of the handsheets was reduced. The penetration grammage, w penetration, was defi-
ned as the intercept at zero force in a plot relating the separation force to the single handsheet grammage. Thickness definition of the constituents The combined effect of adhesive penetration and nonrecoverable compression did not allow the direct evaluation of the thickness of the mix, t mix, and of the thickness of the paper, t paper. An indirect assessment of these quantities was therefore necessary. The only measurable quantity was the sandwich thickness, t sandwich. This measurement, required ad hoc prepared test pieces. Aluminum sheets were placed between the platens and the adhesive so that the sandwich could be easily removed from the platens. The thickness of the sandwich could be determined after removing the aluminum sheets from the sandwich. The thickness of the paper (see Fig 1 for its geometrical definition) was calculated for each tested grammage as the difference between the thickness of the handsheet after pressing and thickness of paper penetrated by the adhesive, Eqs (2)-(4). In Eq (2) the penetration grammage and the density of the pressed handsheets for each grammage are used. Eq (2) can also be written using Eq (1) as ⎛ 2w t = t ⎜1− paper handsheet ⎜ ⎝ w penetration handsheet The thickness of the mix could then by calculated as t mix t − t = 2 sandwich paper Description of the testing apparatus A schematic drawing of the testing apparatus is shown in Fig 2. The rod was first screwed onto the test piece. Successively, the test piece and the rod were screwed onto the load cell. These actions were performed without subjecting the sandwich to undesired loading. The rod was finally secured to the connector by the upper pin. Three sensors, fastened to the upper platen and displaced at 120 degrees from each other, measured the displacement between the lower and the upper platen. The loading was performed by means of a MTS servohydraulic testing machine. The load was applied under displacement control in order to enable the measurement of the post-peak behavior of the material. The load was transferred to the rod via a universal joint system, consisting of a connector, an upper and lower pins crossing each other. The point contact between the pins avoided the bending moment caused by possible misalignments between the centre of the test piece and the point where the force is applied. The value of the Z-directional tensile strength obtained with the testing apparatus was compared to the value obtained by a commercial apparatus following SCAN P80:98. The latter method used tape to fasten the test pieces to the metal platens. ⎞ ⎟ ⎠ [2] [3] [4] Fig 2. A schematic drawing of the testing apparatus. Elaboration of the stress-strain data The data collected during the experiments were the displacements between the two platens measured by the sensors and the corresponding applied force measured by the load cell. The displacement at the centre of the test piece was calculated from the sensors measured displacements, as explained in Appendix I. The displacement at the centre of the circular test piece, called δ sandwich, was taken as representative for the deformation of the whole sandwich. The elastic modulus of the sandwich was evaluated by a linear regression of the stress-strain curves. The thickness of the sandwich t, the instantaneous force F and the tested area A were used for the determination of the elastic modulus according to Eq (5). E sandwich = δ F sandwich A t sandwich The displacement of the mix, δ mix, was determined by assuming a linear elastic behavior of the mix using Eq 6 where the elastic modulus of the mix was obtained from the slope of the stress-strain curve of handsheets which were completely penetrated by the adhesive. The displacement of the paper was calculated as the difference between the displacement of the sandwich and the displacement of the mix: Consequently the strain in paper was calculated as The value of the elastic modulus in the thickness direction of paper was determined by combining the elastic [5] F δmix mix E A t = [6] mix δ = δ −2 δ paper sandwich mix [7] ε paper ⎛ δ δ ⎜ paper ⎝ = = t paper sandwich F E A t − 2 t paper mix Nordic Pulp and Paper Research Journal Vol 22 no. 1/2007 51 mix ⎞ ⎟ ⎠ [8]
Page 1: Evaluation of the tensile stress-st
Page 5 and 6: Testing speed Paper material and ad
Page 7 and 8: Fig 16. Strain at break versus dens

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