Lecture 6Lecture 6The sandwich effect

Lecture 6Lecture 6

FAILURE MODES IN SANDWICH STRUCTURES

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

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FAILURE MODES IN SANDWICH STRUCTURES1.Face yielding

fracture

2.Core shear failure

3.Face wrinkling

4.General buckling

5.Shear Crimpling

6.Local indentation

Lecture 6Lecture 6

FAILURE MODES IN SANDWICH STRUCTURES

Lecture 6Lecture 6FAILURE MODES IN SANDWICH STRUCTURES

Face yielding fracture

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

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The maximum principal stress in the faces of a panel is according to Mohr’s circle of stress;

21

2

2

22

fxy

fyfxfyfxf

Since in the face sheet we have that z=xz=yz=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

fyy

fyyfyfx

x

fxxfx D

zEM

D

zEM ˆdirection-yin and ˆ

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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

fyfyfxfx ˆ and ˆ

Lecture 6Lecture 6FAILURE MODES IN SANDWICH STRUCTURESCore shear failure

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

21

21

2

2

22

2 and

2

cyz

cycyzcxz

cxcxz

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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 cx = cy = 0 and the maximum shear stress can be written as

cyzy

cyzcxzx

cxz d

T

d

T ˆ and

Lecture 6Lecture 6Face wrinkling

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;

33 5.0 5.0 cycyfyfycxcxfxfx GEEandGEE

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Face wrinkling

The actual failure can occur in two ways:

a. 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

b. 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

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FAILURE MODES IN SANDWICH STRUCTURES

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

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Shear Crimping

The shear crimping failure is actually the same as the limit of the general buckling mode.

The critical face stress is hence

ff t

S

2

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Face Dimpling

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

3.0for 5.2

f

fff a

tE

Lecture 6Lecture 6Core Indentation

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;

cz

PA

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Typical mechanical properties of face materials

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Cell Configuration for Honeycomb core