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Interlaminar Bond Strength of Thermoset Composites

Delamination is one of the major areas of failure for laminated composite materials and ‘Interlaminar stresses’ are the source of this failure. These ‘stresses’ can be caused by a number of factors including (but not limited to): internal voids / volatiles that are not properly removed during the laminating process, improper curing of resin, and the environmental conditions in its final application. Understanding the bond between laminations and amongst the resin molecules of thermosetting materials is essential in understanding how these composites are tested, and what applications are best for each.

Perhaps the largest factor(s) when looking at bond strength characteristics of composites are the base material and the resin system that it is laminated with. As you can see in the table of Lamitex® grade composites, the interlaminar bond strength differs among each Lamitex grade by material/resin composition. This is important in determining the correct material for your application. For instance; Lamitex grade G7 is a phenomenal thermal insulator with a temperature index of (220 °C), however, it will not be the best material for an application where it will be exposed to extreme tensile stresses pulling the material apart.

The ASTM D229 test method is one of the most common specifications called for to determine the bond strength of a composite sheet material. This test defines bonding strength as:

“ bonding strength, n—the force required to split a prescribed specimen under the test conditions specified herein.”

The results show the weight(in pounds or kilograms) necessary to split the sample. One factor to keep in mind is that the results are dependent on the sample thickness, so only samples of the same thickness should be compared. In this instance, ASTM D229 specifies that the sample tested is 0.500”.

Bond Strength of Lamitex ® Grade Composites

Grade Test Method Material/ Resin Sample thickness Result
XX ASTM D229 Paper/ Phenolic 0.500" 900 lbs.
XXX ASTM D229 Paper/ Phenolic 0.500" 1,000 lbs.
CE ASTM D229 Cotton/ Phenolic 0.500" 1,800 lbs.
MFG ASTM D229 Cotton/ Phenolic 0.500" 1,800 lbs.
LE ASTM D229 Linen/ Phenolic 0.500" 1,600 lbs.
G3 ASTM D229 Glass/ Phenolic 0.500" 1,570 lbs.
G5 ASTM D229 Glass/ Melamine 0.500" 1,570 lbs.
G9 ASTM D229 Glass/ Melamine 0.500" 1,700 lbs.
G7 ASTM D229 Glass/ Silicone 0.500" 650 lbs.
G10 ASTM D229 Glass/ Epoxy 0.500" 2,500 lbs.
G11 ASTM D229 Glass/ Epoxy 0.500" 2,200 lbs.

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