It follows that the curve will decrease more steeply with increasingLfor a
smaller value ofL^0 cr.
The fracture propagation stress may thus very strongly depend on
notch length. Figure 17.9 also contains a curve for themodulusas a function
of notch length for the same material as for curve 2. It hardly decreases with
increasingL. Since moduli are measured at very small strains, the ratio
between stress and strain—i.e., the modulus—will virtually be constant
throughout the test piece. This illustrates thatinhomogeneities in a sample
have a far greater effect on strength than on modulus.
The latter phenomenon is also involved in the shape of curve 3, which
is of a quite common type. It is frequently observed thatsprdoes not
decrease with increasingLuntil it reaches a certain value that is greater than
L^0 cr. The length scale at which the curve starts to decrease is commonly
FIGURE17.9 The effect of the length of a notchLin a test piece on the stressspr
needed for fracture propagation in the piece. Configuration as depicted in Figure
17.7a;wis test piece width. The vertical scale is linear and arbitrary. Curve 4 shows
the modulusEas a function ofL. Meant to illustrate trends.