Lecture 6

1. Lecture 6 The sandwich effect

2. Sandwich panels can fail in several ways • The faces and core can yield plastically or fracture depending on the nature of the materials from which they are made. • Each of failures giving one constraint on the load bearing capacity of the sandwich • Depending on the geometry of the sandwich and the loading, different failure modes become critical and hence set the limits for the performance of the structure • The most common failure modes in sandwich structures are schematically illustrated in Figure 7.1 Lecture 6 FAILURE MODES IN SANDWICH STRUCTURES

3. Face yielding fracture • Core shear failure • Face wrinkling • General buckling • Shear Crimpling • Local indentation Lecture 6 FAILURE MODES IN SANDWICH STRUCTURES

4. Lecture 6 FAILURE MODES IN SANDWICH STRUCTURES

5. Face yielding fracture Lecture 6 Depending on the materials used and on the chosen fracture criterion one will consider the face and the core to have failed either if yielding occurs or if the component has actually fractured Hence for every material component there will be a maximum allowed stress, whether this stress is a yield or a fracture stress The criterion for failure is then when the maximum stress in the component reaches the allowable stress FAILURE MODES IN SANDWICH STRUCTURES

6. The maximum principal stress in the faces of a panel is according to Mohr’s circle of stress; Lecture 6 Since in the face sheet we have that sz=sxz=syz=0. In fact, the direct stresses in the faces are usually orders of magnitude higher than the shear stress in the core and faces. For a sandwich beam subjected to bending the failure criterion can be reduced to

7. A similar criterion could be stated for the core, but such a failure criterion is very seldom used since most core materials have higher yield and fracture strain than the faces, implying that tensile and compressive failure or yielding will occur in the faces long before anything happens to the core. If the load is in-plane tension or compression, the criterion is simply Lecture 6

8. Core shear failure Lecture 6 The core material is mainly subjected to shear and carries almost the entire transverse force However, the direct stresses in the core could be of the same magnitude as the shear stresses The maximum transverse shear stress in the core is FAILURE MODES IN SANDWICH STRUCTURES

9. Which is used as a fracture criterion The allowable could now also be either a yield or a fracture stress Assuming a beam with a weak core, Ec << Ef, then scx = scy = 0 and the maximum shear stress can be written as Lecture 6

10. Face wrinkling Lecture 6 Face wrinkling can in practical cases occur in a sandwich either when subjected to an in-plane compressive buckling or in the compressive face during bending, or in combination of those. The criterion stating that wrinkling will occur in a face when the compressive stress in that face reaches the wrinkling stress suggested as follows;

11. Face wrinkling • The actual failure can occur in two ways: • A wrinkle that becomes unstable causes an indentation in the core if the compressive strength of the core is lower than the tensile strength of the core and adhesive joint • The wrinkle causes a tensile fracture if the tensile strength of the core or the adhesive joint is lower than the compressive strength of the core Lecture 6

12. Lecture 6 Altough buckling itself sometimes does not damage a structure, it must still be avoided since s structure which has buckled may have lost its capability of fulfilling its purpose The actual buckling load may also be the ultimate load bearing capacity of the sandwich since in its buckled shape it may not sustain any more load FAILURE MODES IN SANDWICH STRUCTURES

13. Lecture 6 The shear crimping failure is actually the same as the limit of the general buckling mode. The critical face stress is hence Shear Crimping

14. Lecture 6 Another instability phenomenon that may occur in sandwich structures with honeycomb or corrugated cores is dimpling or intercellular buckling For a square cell honeycomb this buckling stress equals Face Dimpling

15. Lecture 6 Indentation of the core occurs at concentrated loads, such as fittings, corners, or joints. Practically they can be avoided by applying the load over sufficiently large area This area can be roughly estimated; Core Indentation

16. Lecture 6 Typical mechanical properties of face materials

17. Lecture 6 Cell Configuration for Honeycomb core