Tensile Stress-Strain Properties and Fracture Resistance of Paper and Paperboard T. Yokoyama and K. Nakai Department of Mechanical Engineering, Okayama University of Science 1-1 Ridai-cho, Okayama 700-0005, Japan ABSTRACT The tensile stress-strain properties and fracture resistance of two kinds of commercial paper and paperboard were determined in a tensile testing machine equipped with a non-contacting optical extensometer. PPC (or copy) paper, UKP-sack paper and paperboard were tested at about 25 °C. and 25% ~ 55% relative humidity. A dumbbell-type or necked-down specimen specified in the JIS Z 2201 for sheet materials was used in the tension tests, instead of a constant-width strip specimen specified in the JIS P 8113 (ASTM D828). Tension specimens were cut in three directions from each type of paper and paperboard on a paper cutter, following a steel template. The thickness of paper was carefully measured with a high-precision digital micrometer under a constant pressure. The effects of specimen shape and size, test direction, moisture content and strain rate on the tensile properties and fracture resistance were examined in detail. 1. INTRODUCTION The increasing diversity of end-use applications of paper and paperboard requires accurate assessment of their mechanical properties in various aspects. Tensile properties and fracture resistance are the most important characteristics to be determined in the fabrication process and in the physical characterization of paper and paperboard. A test method for determining the tensile properties of paper and paperboard has been specified in the JIS P 8113 (ASTM D828) . In the test method, the paper tensile strength (S) is defined as the maximum load per unit width on a constant-width strip specimen, because of difficulties associated with measurement of the real thickness of paper. However, the paper tensile strength (S) is not a true tensile strength, which makes it impossible to make a direct comparison with tensile properties of other engineering materials. In the present work, we attempted to characterize the tensile stress-strain behavior and fracture resistance of two types of commercial paper and paperboard. They were tested at room temperature and two different relative humidity. The stress-stain curves were accurately determined in a tensile testing machine equipped with a non-contacting optical extensometer on dumbbell-type (necked-down) specimens. The influences of specimen shape and size, test direction, moisture content and strain rate on the tensile properties and fracture resistance were investigated. 2. MATERIALS AND SPECIMEN PREPARATION Machine-made papers were selected for test: copy paper, sack paper made from unbleached kraft pulp (or UKP-sack paper), and paperboard. They are manufactured in such a way that the axes of the fibers tend to be aligned parallel to the flow of the paper through the paper machine (see Fig.1a). Dumbbell-type (or necked- down) specimens were cut from each type of paper and paperboard in the three directions (see Fig.1b). on a paper cutter, following 1-mm thickness steel templates of JIS Z 2201 specimens 5 and 6  as shown in Fig. 2. The specimen’s ends were over-wrapped with chip board doublers to prevent localized end failure. Their basic properties in the machine and cross-machine directions are given in Table 1. Note that paper whose basis 2 weight larger than 200g/m is classified as paperboard in the paper industry. It is very difficult to determine the real thickness of paper because of its roughness surface texture. In this study, the thickness of paper was carefully measured with a digital micrometer Mitutoyo: GMA-25 DM under a constant pressure of 31kPa with an accuracy of ±4µm. Furthermore, the surface roughness Ra (center-lined average roughness) was measured in both MD and CD with a surface roughness tester (Mitutoyo: MST-301). The paper tensile strength (S) given in Table 1 was determined on constant-width strip specimens of 25mm in width by 250mm in length adopted as an ASTM D828 Standard for paper testing. Obviously, the paper tensile strength (S) is not a true tensile strength. Z MD (machine direction) CD (cross-machine direction) direction of paper flow through paper machine (a) (b) Fig. 1 (a) Principal material directions in paper; (b) three directions in which tension specimens are cut. The longitudinal direction in paper ordinarily corresponds to the machine direction. Table 1 Basic properties of two kinds of commercial paper and paperboard tested Roughness Tensile strength Basis weight Thickness Apparent density Ra (µm) S (kN/m) (g/m2) t (µm) ρ (kg/m3) MD CD MD CD Copy paper (64)* 85 (90) 753 2.5 2.7 3.9 2.2 UKP-sack paper (85) 125 680 4.0 3.8 6.6 3.4 Paperboard (390) 750 520 5.3 5.5 23.2 9.2 *Note: values in parentheses are taken from catalogues by paper manufacturers 200 SPECIMEN 5 50 60 (DIMENSIONS IN MM) 130 SPECIMEN 6 25 40 Fig. 2 Geometry of JIS Z 2201 sheet specimens 5 and 6 (dumbbell-type specimens) 3. TESTING APPARATUS WITH NON-CONTACTING OPTICAL DEVICE It is virtually impossible to mechanically measure the deformation of paper specimen in a conventional universal testing machine with a strain-gaged extensometer, and hence we used a tensile testing machine JT Tohsi Inc.: Little Senstar LSC-1/30 equipped with a non-contacting optical extensometer using a CCD camera (see Fig.3). The applied load was measured with a load cell of 1kN capacity and the elongation over a 50 mm gage length marked on a reduced section of paper specimen was determined with the optical extensometer. The resulting tensile load-elongation relation was then converted to the nominal tensile stress-strain curve. The crosshead velocity was set at 3mm/min and 30mm/min. The data processing was performed on a personal computer (Dell: Optiplex GX280). Tension tests were conducted at about 25 °C and 25% ~ 55% relative humidity. CCD TESTING MACHINE CAMERA PERSONAL COMPUTER Fig. 3 Picture of tensile testing machine equipped with optical extensometer and personal computer 4. RESULTS AND DISCUSSION 4.1 Effect of Specimen Shape and Size In an effort to examine the effect of specimen shape on paper properties, the stress-strain curves for copy paper in MD from dumbbell-type specimen 5 and the constant-width strip specimen were measured and shown in Fig.4. Before testing, a small pretension load of 1 N was applied to straighten the specimen. The dotted-solid line indicates the stress-strain curve from the constant-width strip specimen whose strain was determined from the crosshead movement. Comparison indicates that the constant-width strip specimen does not provide the accurate stress-strain data when the deformation over the gage length is not measured using a non-contacting extensometer. Typical stress-strain curves for copy paper in both MD and CD from dumbbell-type specimens 5 and 6 are given in Fig. 5. The size effect  is clearly observed, that is, the tensile strength of copy paper greatly decreases, and the strain-to-fracture increases in CD with increasing specimen size. This is because of the increased statistical probability of the occurrence of a failure-induced flaw in the specimen. Therefore, it is required to evaluate fracture toughness of paper and paperboard from a fracture mechanics point of view. Taking account of the actual size of paper and paperboard used, all the tensile stress-strain data shown in Figures given below were obtained from the larger size of dumbbell-type specimen 5 of a 50 mm gage length. 100 100 COPY PAPER DUMBBELL (JIS SP.5) COPY PAPER 0 t = 85µm (GAGE MARK) 90 (CD) 80 STRIP 80 JIS Z 2201 SPECIMEN . 55%. RH (CLAMP DISP.) t = 85µm : FRACTURE 55% RH No. 6 . -3 60 : FRACTURE 60 ε=10 /s VCH= 3mm/min No. 5 MD w = 15mm GL = 25mm 40 40 No. 6 No. 5 20 w = 25mm 20 GL = 50mm 0 0 0 2 4 6 8 10 0 2 4 6 8 10 TENSILE STRAIN ε (%) TENSILE STRAIN ε (%) Fig.4 Comparison of tensile stress-strain curves for Fig.5 Effect of specimen size on tensile stress-strain copy paper in MD from dumbbell type-specimen and curves for copy paper in MD and CD strip specimen 4.2 Effect of Test Direction Typical tensile stress-strain curves for paper and paperboard in three different test directions are shown in Figs. 6 to 8. It is observed that Young’s modulus and tensile strength decrease, and the strain-to-fracture and absorbed energy increase with increasing angle from MD. A large difference in paper tensile properties in different test directions is due to the fact that machine-made paper and paperboard have more fibers aligned in MD. The paperboard with the highest thickness has the lowest strength properties in each test direction. This is because the fiber strength of paperboard is different from that of other two papers. The tensile data are summarized in Table 2, where each value indicates the average and standard deviation of the five tests. The tensile properties of UKP- sack paper in both MD and CD are consistent with those given in . Figure 9 shows fracture appearance of paper and paperboard specimens, indicating that fractures occurred inside the extensometer gage length. Note that fractures in the 45 degree paper and paperboard specimens took place at planes at around 15 deg to 40 deg to the loading axis, which do not suggest shear failure. The fracture direction corresponds nearly to the CD direction in which less fibers are aligned. 100 100 COPY PAPER 0 (MD) UKP-SACK PAPER 0 (MD) JIS Z 2201 SPECIMEN 5 45 JIS Z 2201 SPECIMEN 5 45 80 80 t = 85µm t = 125µm 90 (CD) 90 (CD) 55% RH 55% RH : FRACTURE : FRACTURE 60 . 60 . -3 ε=10 -3 /s (VCH= 3mm/min) ε=10 /s (VCH= 3mm/min) 40 40 20 20 0 0 0 2 4 6 8 10 0 2 4 6 8 10 TENSILE STRAIN ε (%) TENSILE STRAIN ε (%) Fig. 6 Effect of test direction on tensile stress-strain Fig. 7 Effect of test direction on tensile stress- curves for copy paper strain curves for UKP-sack paper 100 PAPERBOARD 0 (MD) JIS Z 2201 SPECIMEN 5 45 80 t = 750µm 90 (CD) 55% RH : FRACTURE 60 . -3 ε=10 /s (VCH= 3mm/min) 40 20 0 0 2 4 6 8 10 TENSILE STRAIN ε (%) Fig. 8 Effect of test direction on tensile stress-strain curves for paperboard -3 Table 2 Summary of tensile properties of paper and paperboard tested at a strain rate of nearly 10 /s in MD and CD Young's modulus Tensile strength Strain-to Tensile energy Proof strength -fracture absorption (GPa) σ 0.2 (MPa) σ (MPa) ε f (%) 3 f (MJ/m ) MD 6.54 0.64 34.8 3.8 49.3 2.4 1.7 0.2 0.56 0.08 Copy paper CD 3.13 0.54 10.2 2.0 25.3 1.6 6.3 0.7 1.18 0.18 MD 7.74 0.75 35.2 3.4 54.7 2.0 1.7 0.2 0.61 0.07 UKP-sack paper CD 2.71 0.13 13.0 2.7 28.2 0.8 4.3 0.3 0.83 0.09 MD 4.96 0.52 16.2 1.6 30.3 0.6 1.9 0.2 0.38 0.05 Paperboard CD 1.44 0.16 6.8 1.3 12.7 0.1 4.9 0.2 0.50 0.03 COPY PAPER UKP-SACK PAPER PAPERBOARD MD 45 CD MD 45 CD MD 45 CD Fig. 9 Fracture appearance of dumbbell-type specimens of paper and paperboard 4.3 Effect of Moisture Content The effect of moisture content  on the tensile stress-strain curves for the two types of paper in the three different test directions is shown in Figs.10 and 11, respectively. It is found that the tensile strength greatly increases and the strain-to-fracture decreases with decreasing moisture content from 55 % to 25% relative humidity in each test direction. The decrease in moisture content has the same effect on paper tensile properties as the increase in strain rate does. 100 100 COPY PAPER 0 (MD) UKP-SACK PAPER UKP-SACK PAPER 0 (MD) JIS Z 2201 SPECIMEN 5 45 JIS Z 2201 SPECIMEN 5 45 80 80 JIS Z 2201 SPECIMEN 5 t = 85µm 90 (CD) t = 125µm 90 (CD) t = 125µm 25% RH 25% RH : FRACTURE : FRACTURE 60 60 55% RH 25% RH ε.=10 -3 /s (Vch = 3mm/min) 55% RH ε.=10 -3 /s (Vch = 3mm/min) 55% RH 25% RH 25% RH 40 40 55% RH 55% RH 25 25% RH 55% RH 20 20 0 0 0 2 4 6 8 10 0 2 4 6 8 10 TENSILE STRAIN ε (%) TENSILE STRAIN ε (%) Fig.10 Effect of moisture content on tensile Fig.11 Effect of moisture content on tensile stress-strain curves for copy paper in three test stress- strain curves for UKP-sack paper in directions three test directions 4.4 Effect of Strain Rate In order to study the effect of strain rate on paper properties, the tensile stress-strain relations for copy paper are determined at two different strain rates and shown in Fig. 12. Clearly, the deformation stress increases slightly with increasing strain rate in both MD and CD, whereas the strain-to-fracture hardly varies with strain rate. The similar strain rate dependence of the 100 deformation stress is reported in . From Figs. 6 to 8, COPY PAPER 0 it is conceivable that machine-made paper and JIS Z 2201 SPECIMEN 5 90 (CD) 80 t = 85µm paperboard are sheet materials with orthotropic 55% RH : FRACTURE anisotropy as well as viscoelasticity . 60 . -2 ε=10 /s . -3 ε=10 /s 4.5 Fracture Resistance of Paper 40 . The resistance of paper to crack propagation in the ε=10 -2 /s . tensile mode is an important sheet property, because 20 ε=10 -3 /s of the presence of flaws or defects in the sheet. A device for measuring fracture resistance is shown in 0 0 2 4 6 8 10 Fig.13. Two alignment rods are firmly attached to the ε (%) TENSILE STRAIN lower clamp. The upper clamp moves on the Fig. 12 Effect of strain rate on tensile stress-strain curves for copy paper in MD and CD alignment rods. These maintain parallelism between the clamp faces during loading. Fracture resistance of 200 COPY PAPER 0 (MD) t = 85µm 90 (CD) 150 VCH= 1mm/min SEN SPECIMEN 100 a 0 = 33mm l a0 Δa= 12.4mm b b = 100mm l = 50mm 50 Δa= 12.6mm 0 0 0.5 1 1.5 2 CROSSHEAD DISPLACEMENT δ (mm) Fig. 14 Typical tensile load-elongation curves for quasi-static crack propagation for copy paper in MD and CD 200 UKP-SACK PAPER Fig. 13 Picture of device for measuring fracture 0 (MD) t = 125µm 90 (CD) resistance using single-edged specimen with 150 constant wide width VCH= 1mm/min Δa= 20.7mm Table 3 Typical values of fracture resistance for 100 Δa= 19.5mm copy paper and UKP-sack paper in MD and CD SEN SPECIMEN a 0 = 33mm Fracture resistance G c (kJ/m2) 50 l a0 b = 100mm b MD CD l = 50mm Copy paper 9.1 (5) 14.2 (5) 0 0 0.5 1 1.5 2 UKP-sack paper 13.0 (5) 11.5 (5) CROSSHEAD DISPLACEMENT δ (mm) Note: figure in ( ) indicates the number of tests Fig. 15 Typical tensile load-elongation curves for quasi-static crack propagation for UKP- sack paper in MD and CD copy paper and UKP-sack paper was evaluated on a single-edged notched (SEN) specimen with a100mm- wide width, under the assumption of small scale yielding. It is accepted that fracture resistance depends on the strip specimen geometry and the initial crack length to specimen width ratio ao/b. Following Seth’s article , the crack length ratio ao/b was set to be 0.33. The initial crack was inserted into wide-width strip specimens on the paper cutter. Typical tensile load-elongation relations from the SEN specimens of copy paper and UKP- sack paper in both MD and CD are presented in Figs.14 and 15. The crack extension length ∆a was accurately measured with an Image Sensor (Keyence: CV-3000). Fracture resistance is calculated by dividing the area ∆W under the tensile load-elongation curve by generated crack area. Typical values of fracture resistance for the two papers are listed in Table 3. Note that Gc of copy paper is lower than that of UKP-sack paper in MD, whereas Gc of copy paper is higher than that of UKP-sack paper in CD. 5. CONCLUSIONS The in-plane tensile stress-strain behavior and fracture resistance for copy paper, UKP-sack paper and paperboard have been characterized in the testing machine equipped with the optical extensometer on the dumbbell-type specimens and the SEN specimens. Tension tests and fracture resistance tests were performed in both MD and CD. From the experimental results, we can draw the following conclusions: 1. The stress-strain characteristics are very sensitive to the specimen geometry and size. 2. The stress-strain characteristics are significantly affected by the strain rate and moisture content. 3. Machine-made paper and paperboard show orthotropic, anisotropic and viscoelastic behavior. The degree of anisotropy for them can be ranked as: paperboard > UKP-sack paper > copy paper 4. Fracture resistance varies, depending on the type of paper as well as the test direction. Acknowledgements This research program has been supported in part by a FY 2004 fund for Improvement of Private School Facilities provided by the Ministry of Education, Culture, Sports, Science and Technology of Japan. We wish to thank T. Odamura, Y. Yoshida and M. Terada for their technical assistance with the experimental work. References  JIS P 8113: JIS Handbook-32, Paper and Pulp, Japanese Standards Association, p. 218 (2003).  JIS Z 2201: JIS Handbook-1, Steels, Japanese Standards Association, p. 121 (1994).  Setterholm, V.C. and Kuenzi, E.W.: Method for determining tensile properties of paper, Tappi J., Vol. 40, No. 6, 197A-204A (1957).  Tanaka, A. and Yamauchi, T.: Crack propagation of paper under fracture toughness testing, J. Pack. Sci. Technol., Vol. 6, No. 6, 324-332 (1997).  Benson, R.E.: Effects of relative humidity and temperature on tensile stress-strain properties of kraft linerboard, Tappi J., Vol. 54, No. 5, 699-703 (1971).  Mark, R.E. (Editor): Handbook of Physical and Mechanical Testing of Paper and Paperboard, Vol. 1, Marcel Dekker Inc., New York, p. 255 (1983).  Seth, R.S.: Measurement of fracture resistance of paper, Tappi J., Vol. 62, No. 7, 92-95 (1979).