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

5.2.1 Experimental Study of the STT Specimens
Static and fatigue tests were conducted on STT specimens to study the residual lifetime of
debond damaged sandwich X-joints. Fifteen foam cored STT specimens with glass/polyester
face sheets were manufactured for static and fatigue tests. The polyester resin is Polylite 413-
575, which is specially designed for vacuum injection due to its low viscosity. The sandwich
faces consist of four DBLT quadraxial mats from Devold Amt, each of a thickness of 0.75 mm
and a dry area weight of 850 g/m2 and the fibre directions relative to the longitudinal direction of
the specimen [90,45,0,-45], where the -45 degree ply is placed closest to the core. The core
materials applied include Divinycell PVC foams of the types H45, H100 and H250 with nominal
densities of 45, 100 and 250 kg/m 3 , respectively. The core thickness is 50 mm. The properties of the
core and face materials are given in Table 5.1. Face sheet material properties are obtained from the
tests conducted on samples from the face sheet.
Table 5.1: Face and core material properties.
Material E (MPa) G (MPa)
Face sheet 19400 7400 0.31
Core: H45 50 15 0.33
Core: H100 130 35 0.33
Core: H250 300 104 0.33
A selection of manufactured STT specimens is shown in Figure 5.2. All specimens were reinforced
by wooden inserts at the ends to avoid crushing of the core when mounting them in the STT test
rig. The debond defect was introduced during the manufacturing process by inserting a sheet of
0.025 mm thick Airtech release film on the core and sealing the edges with resin before vacuum
injection. The panels were resin injected molded and cured with vacuum consolidation. The STT
specimens were cut from the manufactured panels. The release film was placed along 480 mm of
the specimen length so that the crack only propagates in one side, see Figure 5.2. The reason for
not just testing simply a half part of the specimen where the crack propagates is that as the crack
propagates the membrane forces in the face sheet becomes larger, generating harmful side forces
on the testing machine actuator. The side loads can therefore be decreased by carrying the loads
by the tension in the part of the face sheet which is not glued to the core.
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