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

the lowest values being for the Type C (triaxial DBL-800) laminates. The core materials are
Divinycell PVC foams of type H130 and H250, and Rohacell PMI foam of type 51-IG. The
properties for the core materials, taken from the manufacturers’ data sheets, are given in Table 3.3.
Core
type
Nominal density
(kg/m 3 )
Table 3.3: Material properties of the cores.
Compressive modulus
(MPa)
43
Shear modulus
(MPa)
Compressive strength
(MPa)
H130 130 170 50 3
H250 250 300 104 6.2
PMI 52.1 70 19 0.9
3.3 Characterisation of Face/Core Interface
The interface fracture toughness characterisation of the foam cored sandwich specimens was
performed using the Mixed Mode Bending (MMB) test rig and the MMB sandwich specimen
(Quispitupa et al., 2009 and 2010), as shown in Figure 3.2. The MMB test rig was originally
developed for mixed-mode fracture testing of monolithic composites (Reeder et al., 1990 and
Ozdil et al., 2000) and has recently become an ASTM standard test method D6671-01. The
MMB test rig allows adjustment of the mixed-mode ratio by changing the lever arm distance c as
shown in Figure 3.2. Quispitupa et al. (2009) further developed the MMB test rig for fracture
testing of sandwich structures. The standard MMB composite test specimen is a rectangular
unidirectional beam specimen with a predefined delamination located at the midplane. In the
MMB sandwich specimen the initial crack is located in the face/core interface at the top of the
specimen, see Figure 3.2. The compliance of the MMB sandwich specimen is determined by
(Quispitupa et al., 2009):
where and P are the deflection of the loading point and the applied load, respectively, c is the
lever arm distance, L the half-span length and is the load partitioning parameter at the left
support (see Figure 3.2) given by
3
a 1 a 1
3 D2
k G f h f Gxzhc
3
3
(3.2)
a 1 a 1 a 1 a 1
3 D k G h G h 3 D k G h
2
f
f
xz
c
1
f
f
where the subscripts 1 and 2 refer to the face sheet and the core, respectively, a is the crack
length, k is the shear correction factor, k=1.2, D2=D-B 2 /A, D1=1/(Efhf 3 /12), hf and hc are the face
(3.1)