r¼
F
A
ð1Þ
whereFis the peak crushing force recorded and Ais the
initial contact area.
The radial tensile strength or longitudinal strength (rT)
was determined by subjecting the pellet to diametrical
compression [22, 23] (Fig.1b), i.e., perpendicular to the
cylindrical axis and by the following equation
rT ¼
2F
pDL
ð2Þ
whereDis the diameter of the pellet. In a compressive
strength test, the fracture occurs through the weakest plane
in the pellet; while the fracture occurs through a diametrical cross section of the pellet in a tensile strength test,
hence tensile strength generally reflects an average strength
of a pellet.
In addition to the strengths, the porosityeof the pellets
was determined by the following calculation
e¼1
qC
qT
ð3Þ
whereqC is the bulk density of the pellet at maximum
compaction andqT
is the true density of the powder. The
elastic rebound,ER, of the pellets was calculated from
ER¼
LL0
L0
100% ð4Þ
The influence of the following parameters on the pellet
strength was investigated:
Compaction pressure
Each powder size was compacted at pressures ranging from
130 to 200 MPa. The compaction speed and hold time were
preset at 100 mm/min and 3 s, respectively.
Compaction speed
The effect of the compaction speed was studied at a
compaction pressure of 164 MPa with zero holding time.
The compaction speed was varied from 1 mm/min to
100 mm/min. Thereafter, the formed pellet was ejected at a
speed of 10 mm/min and tested at a compression speed of
10 mm/min.