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photographed before tensile testing and identified using a Leica DMLM microscope with 50 9 magni- fication and OptiMOS monochrome sensor camera from QImaging and using literature as a reference (Ilvessalo-Pla¨ffli 1995). The objective was to accept only pulp fibers without axial twisting and visible fractures to the tests. All fibers that were clearly identified as earlywood (springwood) fibers and the fibers with axial twisting, damage, or improper gluing were rejected. After tensile testing, cross-section images revealed that some of the tested fibers were thin-walled (earlywood) fibers, which was not possible to determine from the side-view image. Due to the laborious test procedure, the tensile results of the individual thin-walled and thick-walled fibers were grouped separately.
smoothed. Then image analysis software was used to measure the width of the fibers (pixel by pixel along the fiber length) and calculate the average fiber width. A reference image of a ruler was used for defining the length scale. The averages of the calculated fiber widths are presented in Table 2.
Cleavage test of pulp fibers
A cleavage test using hydrochloric acid (HCl) treat- ment developed by Ander et al. (2005, 2008) and slightly modified by Zeng et al. (2012) was used as an estimate for the number of fiber dislocations in the pulps and viscose fibers. The treatment presumably cuts the fibers at the dislocation areas, which have higher accessibility to the acid and can be considered as weak points in fibers. The arithmetic (AR) fiber lengths that were utilized in the calculation of cleavage index were measured using the STFI Fiber- Master after setting the lower length limit of fibers to 44 from200 l m. The cleavage results are presented in Table 3.
Determination of single fiber dimensions from microtome cross sections
The cross sectional area (A cross ), fiber width (w f ), and fiber thickness (t f ) of single fibers were measured in the dry state from light microscope images, see Figs. 3b, 4b, 5b and 6b. After tensile testing, the fibers were glued onto paper tabs with a free end protruding out and embedded in a resin. Once the resin was cured, the fibers were cut with a Leica RM 2255 microtome with a cut thickness of 3 l m and imaged according to Lorbach et al. (2014). However, it was not possible to do the microtome analysis for all fibers due to the too short length of the broken pieces. In the case of successfully cut fibers, the images were binarized and the area analyzed using Matlab R2014b. From the images (presented in electronic supplementary material), it was possible to divide the tested untreated BSKP fibers into thick-walled and thin-walled fibers using the microtome photographs and a cross-sectional area of 190 l m 2 as a limit.
Single fiber tensile testing
The single fiber tensile testing was performed using the fiber bond tester at TU Graz (Fischer et al. 2012; Lorbach et al. 2014; Jajcinovic et al. 2016). Off-set force was measured prior to the test and then a minor preliminary loading was applied to fiber. A displace- ment measurement system (shown in Fig. 2) was attached to the fiber bond tester in order to measure load-elongation data of the single fiber tensile tests. The displacement of the tension support was measured using a laser displacement sensor (Micro-epsilon OptoNCDT LD 1605-10, with a resolution of 3 l m). The displacement data was recorded using a National Instruments data logger system. The velocity of the moveable support of the fiber bond tester turned out to be very stable within the measurement accuracy of the laser sensor and therefore the linear fit of each measured displacement as a function of time was used in order to decrease the noise of the displacement data. The load data recorded by the fiber bond tester was complemented by the displacement data as a function of time in the data analysis afterwards. Both data were measured at 1 kHz frequency. Sample holder mounting was performed as described by Lorbach et al. (2014) and Jajcinovic
Determination of single fiber width using fiber images
Due to the too short length of the broken pieces, it was not possible to do the analysis for all fibers. The image analysis of width–length direction photographs, see Figs. 3a, 4a, 5a and 6a, was used for producing the fiber width data for all the measured fibers. The photographs were first changed to black and white (B&W) images and the fiber edges were slightly
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