Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

Displacement controlled static tests with 1 mm/min loading rate were conducted on two MMB
specimens of each core type to determine the static crack propagation load. Typical load vs.
displacement curves from the static tests are presented in Figure 5.22. The point where the crack
starts to propagate is marked with an open circle (“”). The critical failure load is marked
according to the ASTM D6671/D 6671M-06 recommendation and complemented by visual
inspection.
Load (N)
80
60
40
20
0
0
0 1 2 3
0 2 4 6
Displacement (mm)
Displacement (mm)
(a)
(b)
Figure 5.22: Typical load vs. displacement curves (“” onset of crack growth) for the MMB
sandwich specimens with (a) H45 core and (b) H100 core.
Fatigue tests in displacement control with sinusoidal wave form were conducted on three
specimens of each core type at a frequency of 2 Hz and with a loading ratio R=0.1. Displacement
controlled testing was chosen for better servo-hydraulic control of the loading and to avoid any
unstable crack growth in the MMB specimens. To have a stable crack growth, 80% of the static
crack propagation load was chosen as the maximum fatigue load after testing a few trial
specimens. The crack length was determined every 50 cycles using the compliance of the MMB
specimen as
105
( 5.1)
Where c is the lever arm distance, L the half-span length, is the load partitioning parameter. C1,
C2 and C3 are compliances of the sub-beams according to Quispitupa et al. (2009) as introduced
in Chapter 3. For further details see Chapter 3. Moreover, visual crack length measurement was
performed by a calliper with an accuracy of ±0.05 mm. The maximum fatigue load (Pmax) and
the corresponding displacement (max) were used to determine the compliance of the MMB
specimens and subsequently the crack length. The MMB compliance in Equation (5.1) is a
function of the crack length. Knowing the maximum load (Pmax) and displacement (max) from
the testing machine, the compliance CMMB=/P can be calculated and subsequently the crack
length can be determined, see Chapter 3. Furthermore, since the MMB test rig has several hinge
connections and load introduction points, the deflections of the test rig during the fatigue tests
Load (N)
160
120
80
40