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De-lamination Strength Testing

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De-lamination is the separation of the layers in a laminate. Laminate is a sheet of material that has been created by stacking thin layers, or plies, and bonding them together. De-lamination can occur locally, in a small area of the laminate, or it can affect the entire laminate.

In laminated materials, repeated cyclic stresses, impact, and so on can cause layers to separate, forming a mica-like structure of separate layers, with significant loss of mechanical toughness. A wide range of structural applications are possible; however, amongst other considerations the performance of these materials under crash and impact loading is a primary concern.

Nondestructive evaluation has an increased industrial interest especially in the plastic industry. One of the challenging tasks is the detection of de-laminations in plastic weld joints. Weak bonding is a major cause of fiber pull-out (a form of failure) and de-lamination. Therefore, de-lamination is a dangerous kind of failure as it develops inside of the material, without being obvious on the surface, much like metal fatigue. Examples of the types of laminated materials to be tested are identity and credit cards that are produced with a laminated cover.

A climbing drum test is a typical peel testing method used to determine the de-lamination characteristics of adhesives used to bond different materials together. This test covers the determination of the peel resistance of adhesive bonds between a relatively flexible adherent and the relatively flexible facing of a sandwich structure and its core.

When the adhesive used is relatively brittle, the peel resistance can often indicate the degree of cure, or to cause a material to bond by permanently cross-linking its molecules. Adhesives cure as they solidify, that is, higher values than normally realized may indicate an incomplete cure. The test involves a measurement to determine the strength of a laminate's bond to a substrate material. It is equal to the tensile load applied divided by the sample width multiplied by the sample's thickness.

TEST STANDARDS:
• ASTM C363 / C363M - 09 Standard Test Method for Node Tensile Strength of Honeycomb Core Materials
• ASTM D1781- Standard Test Method for Climbing Drum Peel for Adhesives