Tear/Tensile or Fracture
Fracture toughness
Fracture toughness describes the
ability of the material to resist the propagation of
a pre-existing crack.
It is an important material property since the occurrence
of flaws (cracks) is not completely avoidable in
the processing, fabrication, or service of a material/component.
Fracture toughness is often determined as the critical
stress intensity factor, and sometimes in material
mechanics it is referred to as “tenacity”.
In paper physics, fracture toughness has been conventionally
measured as “fracture energy index” (Jm/kg),
which equals the critical fracture energy release
rate divided by the paper grammage. It is the critical
point beyond which if the fracture energy release
rate increases, the crack will start to propagate.
Fracture toughness of paper is difficult to measure
due to the elastic, visco-elastic and plastic properties
of paper. The conventional linear elastic fracture
mechanics (LEFM) approach is not directly applicable
to paper fracture toughness. Different methods have
been developed for measuring paper fracture toughness.
The most commonly used measuring methods are “J-integral”,
which has been incorporated into the SCAN-test standard; “In-plane
tear energy”, which shows a good agreement with
the “J-integral”; and “EWF-essential
work of fracture”. In some cases, paper fracture
toughness is directly calculated with tensile strength
index and stretch of the paper.1 The fracture toughness
applied in material mechanics equals the geometric
mean value of paper fracture toughness (fracture energy
index) times specific elastic modulus.
Despite being
difficult to measure, fracture toughness has attracted
more and more attention from the paper
industry in recent years since the conventional strength
properties have often proved inadequate for evaluating
the paper web breaks in its manufacturing and end-use
processes. The most commonly used strength properties
for the evaluation of pulp and paper strength in industry
have been tensile strength and Elemendorf or Brecht-Imset
tearing (out-of-plane) resistance.
Tensile strength
gives the maximum tension-carrying capacity of paper.
It is known that tensile strength
only measures the weakest point in a test strip while
the weakest point is a random event. It is hard to
relate the random strength to the web resistance to
breaks. It has been shown that web breaks normally
occur at web tensions that are much lower than the
tensile strength of paper. No reports have shown that
there is a good correlation between web break frequency
and tensile strength directly. (Web breaks have been
shown to correlate with other morphological properties
such as length and coarseness however, which do in
turn affect tensile strength.) To reach high in-plane
strength properties, the sheet generally requires a
high degree of bonding. This requirement is invariably
negatively correlated with “out-of-plane” tearing
resistance since the tearing resistance decreases with
increased bonding after a maximum, usually corresponding
to a very low in-plane strength, is reached. What is
more is that the out-of-plane tearing mode makes the
tearing resistance less relevant for many end-use applications. When paper fails by crack propagation initiated by
defects in a web, the mechanism underlying the failing
process is clearly more likely to being understood
using a fracture toughness approach than simply a tensile
strength approach. It is never likely to be understood
using an out-of-plane tearing resistance approach.
Unlike tensile strength, paper fracture toughness is
a fundamental material property, which always measures
the resistance against the crack propagation along
the crack line and almost not affected by the weak
point in the test specimen. In fact, it has been shown
that the paper fracture toughness either measured directly
or calculated from tensile strength and stretch is
the only strength property that correlates to the paper
runnability in press-house.
1) equation derived by Seth, Fracture toughness = 0.6
(Tensile (N)0.74 + Stretch(%)0.58) |
