Fracture Characterization of Sandwich Face/Core Interfaces

Sandwich structures are nowadays widely used in lightweight structural applications because oftheir superior stiffness/weight and strength/weight ratios compared with traditional metallic as well as monolithic structures made from composite materials. A major limiting factor of wider application of sandwich structures is defects that are introduced in the manufacturing process.

It is inevitable that areas of the face sheets will not fully adhere to the core resulting in defects known as “debonds”. Debonds can also be induced in-service due to e.g. localised impact loading or overloading. As the means of load transfer between the faces and the core layer is lost, the debonds are considered as primary damage initiators.

Under fatigue loading the debonds may evolve into cracks that cause a reduction in structural performance and consequent failure. At present most structural design is based on “life-time expectancy” so that construction, integrity and functionality are maintained within a defined life-span. This design philosophy often leads toinherently conservative designs that are on average much heavier and stronger than is actuallyrequired, and therefore not cost competitive.

An alternative, and at present underutilised, designphilosophy is the “fail-safe” or damage tolerant approach where damage evolution is permitted. Here it is accepted that debonds and damage evolution are inevitable, so features are included in the construction that control the ultimate size or severity of the damage.

This approach accounts for damage development from debonds and thereby reduces the need for over conservative design. Indesigning composite sandwich structures it is therefore essential to establish the maximum tolerablede bond size and thereby the damage tolerance of the structure. In order to achieve such result it is important to devise new experimental and analytical techniques to establish the multi-mode fracture characteristics of sandwich plate structures and accordingly develop methods to inhibit defect propagation.

This thesis deals with characterization of fracture between face and core of sandwich structuresusing a combination of numerical and finite element simulations as well as experimental testing of avariety of sandwich specimens. The mixed mode bending test rig and specimen was used to simulate mixed mode I/II loading conditions at the crack tip of artificially debonded sandwich samples.

A number of sandwich materials were tested (GFRP/foam cores and CFRP/Nomex) bothin static and fatigue. A linear elastic fracture mechanics model was used to determine the analyticalexpression of compliance which allowed to calculate automatically the crack length. In combination, a finite element model of the MMB specimen was used to determine the mode-mixityangle.

A new testing methodology called G-control was developed in order to enable cyclic crack growth testing of sandwich structures at a constant level of cyclic energy release rate, which is not possible with traditional testing methods such as displacement or load controlled. Such approach guarantees stable crack propagation speed which provides more reliable and faster crack growth rates measurements, resulting in less specimens and time consuming tests.

The G-control method was applied to experimental fatigue crack growth testing of different types of sandwich samples atdifferent mode mixities and ΔG levels, and results in agreement with previous studies were obtained suggesting that the developed testing method is a reliable tool for the study of face/core interface debonded sandwich structures.