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

Testing speed
Paper material and adhesives exhibit a visco-elastic
behavior, i.e. their response in terms of strength will
change depending on the testing speed. Rate dependency
was not addressed in this paper. However the purpose
was to test the material in a time window in which the
strain rate had negligible influence on the material properties.
The influence of the strain rate was studied for
120 g/m 2 paper sheets prepared with chemical pulp 2.
Two different testing speeds were used, 0.025 mm/s and
0.0025 mm/s. The mean times to break were respectively
6.7 and 66 seconds. No influence on the Z-directional
tensile strength was found. The tests in the following part
of the article were therefore performed at 0.0025 mm/s
speed.
Stress strain curves
Fig 6 presents four typical stress-strain curves for 300
g/m 2 , one for each type of pulp studied in the present
investigation. The curves were calculated by Eq (8). The
elastic moduli presented in this investigation were obtained
by linear regression of the Z-directional stress-strain
curves. The limit at which linearity was good approximation
varied depending on the pulps. For TMP the elastic
moduli were calculated up to 50% of the strength. For
chemical pulp the calculation limit was set to 80% of the
strength.
Fig 6. True stress-strain curves for the investigated handsheets.
Sheets prepared from chemical pulps were stiffer than the
ones made of mechanical pulps. TMP was characterized
by a strain at break around 8%, whereas strain at break
for chemical pulp ranged from 1.4% to 1.8%. The postpeak
behavior changed drastically for different pulps. For
chemical pulp 2 the post-peak behavior was unstable at
any grammage. For chemical pulp 1, post peak instability
was also present for low grammage handsheets, but for
higher grammage the failure was stable. For TMP
handsheets the failure was always stable.
Z-directional tensile strength
In Figs 7 to 10 the Z-directional tensile strength is shown
as a function of grammage. The strength values are
obtained using two different techniques, i.e. measurements
performed by the custom built apparatus using
adhesive, and data measured using double adhesive tape.
Fig 7. Grammage versus Z-directional tensile strength plot for TMP1 obtained
using two different testing methods.
Fig 8. Grammage versus Z-directional tensile strength plot for TMP2 obtained
using two different testing methods.
Fig 9. Grammage versus Z-directional tensile strength plot for chemical pulp 1
obtained using two different testing methods.
Fig 10. Grammage versus Z-directional tensile strength plot for chemical pulp 2
obtained using two different testing methods.
Nordic Pulp and Paper Research Journal Vol 22 no. 1/2007 53