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What is a peel test?

  • 3rd March 2021
  • Valentina Guerra
  • Reading time: about 5 minutes
Illustration showing equipment used to perform a floating roller peel test on a composite material

What does a peel test measure?

Peel tests measure the strength of the bond between a substrate and the material being peeled off (known as the adherend). This peel strength is expressed as the load required to separate the adherend from the substrate, per unit width of the bond.

As you might imagine, understanding this property is important in a wide variety of applications, wherever two (or more) materials are being bonded together. For composite materials this could, for example be the bonding between skins and core in a sandwich laminate, or it could be composite-to-metal bonding, in a hybrid structure.

Are all peel tests the same?

The short answer is no. An important thing to understand is that the peel strength for a given combination of substrate and adherend will vary, depending upon the peeling angle. There are, therefore, different peel tests, depending upon the peel angle you want to use. Common peel tests angles are:

  • 180° peel test – typical for single coated adherend
  • 90° peel test – typical for single coated film-adherend on a surface
  • T-peel test- typical for flexible bonded materials
  • Floating roller peel test – typical for a flexible adherend on a rigid substrate
180° Peel Angle 90° Peel Angle
T-Peel Floating Roller Peel

How do I carry out a peel test?

Peel tests are typically carried out using universal testing machines. Given that these machines can only pull (or push) in one direction, different jigs and grips are required to suit the type of peel test being carried out (see section above), and to enable the material to be pulled at a steady rate and controlled angle without slippage.

The Universal test machine records the applied force and crosshead displacement, with the results typically displayed as a force – displacement curve. The force applied is the minimum force needed to peel the adherend off the substrate whereas the displacement is the correspondent length of adherend peeled.

Peel Test force-displacement graph
Example of force-displacement curve as obtained from a peel test

The initial peak of the curve is due to the “adjustment” phase of the equipment-specimen system as the tester starts to pull the adherend from the substrate. The average load should be calculated in the constant region (e.g. from a displacement of 20mm in the chart above). The peel strength is therefore defined as:

An example: Determining the peel strength of a thermoplastic tape bonded to a metal substrate using the floating roller peel test

The idea of using hybrid structures – where different materials are combined in order to give tailored properties (e.g. mechanical strength, electrical and/or thermal conduction, chemical/corrosion resistance etc.) – is growing. Thermoplastic tape bonded to a metal substrate is one such example.  In this application, where we have a relatively flexible adherend (the thermoplastic tape) bonded to a rigid substrate (the metal), the floating roller peel test is the most suitable means of determining the peel strength.

Specimens are typically 25 mm wide and at least 200mm long (ASTM D 3167). The prepared specimen is inserted in the floating roller jig with the substrate facing upward and the tape facing downward. The unbonded end of the tape (at least 30mm) is clamped in the lower grip of the testing machine. As the test starts, the crosshead of the tester pulls the floating roller jig away, causing the tape to be peeled away from the metal. The illustration below shows the floating roller peel test setup for a specimen of carbon fibre reinforced thermoplastic tape adhered onto an aluminium substrate as performed at Coventive Composites:

Summary

Due to their versatility, ease of setup and cost efficiency, peel tests (and especially floating roller peel tests) are ideally suited to characterising hybrid composites (e.g. metals and polymers).

How we can help

At Coventive we use peel tests alongside a wide range of other testing facilities as part of our material & process development and characterisation services. If you have any enquiries about peel tests, thermoplastic tapes or hybrid composites, please contact us.


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