**1.2. Failure-modes of sandwich sheets**

Sheets with vibration damping qualities can be made of steel sheets enclosing a viscoelastic plasticcorelayer (see Figure 17). [2] The vibration energy of the oscillating coversheet is converted into heat.[3] [4] For automotive lightweight constructions, sandwich sheets with a noise-absorbent behavior are highly useful in the engine bay. [5]

Parts of sandwich sheets with viscoelastic layers excel with a higher security against cracking. [6]

Due to the relocatability of the outer and the inner sheet, leakage of e.g. an oil pan in contact with the subsoil or in case of a crash is unlikely for this sandwich in contrast to a comparable single layer sheet. The cover-sheets remain undamaged, but the bond can fail during forming. Under load, during and after the forming process special effects and failures occur. [7]

**Figure 2.** Failure modes of during forming of sandwich sheets (viscoelastic, shear transmitting and shear fixed)

Mainly displacement and delamination of the cover sheets occur as failure modes (see Figure 2). Wrinkling at the inside surface is detected especially by vibration-damping composite sheets with long non-deformed legs and small thickness of the inner layer. Plastics and adhesives creep under load. [8] Because of the residual stresses in the cover sheets or due to temperature influences, the composite delaminates often after hours or days, respectively.[9] To decay noises effectively, the viscoelastic interlayer has to be as thin as possible (chapter 5). In contrast to this, lightweight sandwich sheets achieve great stiffnesses with thick synthetic cores. [10] As well as by shear weak connection, the cover layer of shear transmitting sandwiches usually does not crack. Now, additive failures like core-failure, local dents and wrinkling of cover sheets limit the forming capabilities. [11] Typically, laminates which are jointed shear fixed crack on the brittle side. [12]
