Since the defects distribute quite inhomogeneous in the materials, the results of
mechanical measurement are often irreproducible. One can arbitrarily introduce the
defects at a specific location to make the measurement reproducible, for instance, to
measure thenotched impact strength. The Charpy’s method makes two ends of the
sample fixed, and impact the middle part where the defect is notched. The Izod’s
method makes only one end fixed, and impact the other end, as illustrated in Fig.6.22.
Assuming the weight of the hammer isW, the length of the hammer handle isl,sothe
loss of impact energy depends on the loss of angles fromatob,asgivenby
A¼WlðcosbcosaÞ (6.82)The sectional area at the notched place isS, so the impact strengthI¼
A
S
(6.83)
with the unit of J/m^2.
Blending polymer is similar to alloying metal, to obtain the complementary
advantages of two materials. For example, polystyrene (PS) appears hard and brittle,
while polybutadiene (PB) appears soft and tough. By adding amphiphilic
Fig. 6.21 Illustration of (a) the crazing initiated inside polymer solid materials; and (b) the
microstructure of adhesives peeling from a solid substrate
Fig. 6.22 Illustration of
Charpy method to measure
the notched impact strength
122 6 Polymer Deformation