The main objective of this study was to determine the
effects of the moisture content, loading velocity and compression position
on the force, specific deformation, absorbed energy and required power
to achieve the initial rupture of the walnut shell. Iran is the third
walnut producing country in the world. Separation of walnut kernel from
the shell is a very time consuming and costly task. A proper cracking
machine for walnut has not yet been manufactured. It seems that the determination
of mechanical properties of different walnut varieties is the pre-requisite
step for this purpose. Thus, 108 local walnut samples, (a local cultivar)
obtained from the seedling trees of Urmia district, with almost equal
dimensions were selected. These samples were compression loaded by an
Instron test machine until the shell rupture was initiated. During this
experiment, rupture force, specific deformation, required energy and power
for shell rupture, in 9, 21 and 27% (w.b.) walnut moisture levels, 50,
200 and 500 mm min-1 loading velocities and at 3 major compression
directions namely, length (X), suture (Z) and width (Y), were determined.
The results of this experiment indicated that, the highest force, energy
and power values, required for walnut rupture, occur when the force is
applied at the length direction of the fruit and the lowest of these values
were required at either width or suture line. Specific deformation of
walnut shell increased with increasing loading velocity, regardless of
its moisture content and loading direction. Furthermore, moisture content
of walnut shell increased the specific deformation for 21% moisture, but
did not cause further changes at the higher level of 27% of moisture.
However, the strain values (specific deformation) appeared to be the same
at all loading position. Increasing the loading velocity, although increased
the required rupture energy and power, but reduced the fracture force.
Furthermore, in higher loading velocities (about 500 mm min-1),
the specific deformation increased which indicates more flexibility in
walnut shell. Flexible shell may protect the fruit against fracture in
the walnut cracking process.