End Tabs

End Tabbing of Composite Test Specimens

Why use end tabs?

End tabbing of composites is necessary if wedge grips are being used at a wedge angle of 10⁰ because an untabbed material will experience a much higher through-thickness clamping force.
This will cause a stress concentration at the edge of the grips.
To combat this, materials are usually dog-boned in shape (pictured below) but, this is not possible for composites.

Even the grips can cause damage to the composite since the grips can break through the surface and cause damage to the fibres underneath the resin.
The diagram below illustrates the clamping force due to wedge action (the wedge grips principle).

End tabbing isn’t always necessary though, as a rule of thumb given in the article, the more orthotropic a material is (orthotropic materials have material properties at a particular point, which differ along three mutually-orthogonal axes, where each axis has twofold rotational symmetry (https://en.wikipedia.org/wiki/Orthotropic_material)), and the higher the strength, the greater the need for end tabs.
The stronger a material gets, the bigger the need for end tabbing and the design of the end tabs becomes more important.
For example, fiberglass doesn’t need end tabbing because it has relatively low strength and irregular fibre distribution.
However, once you have a unidirectional composite, end tabbing is necessary.

(unidirectional composite)

What material to use?

The choice of material for end tabs can influence a lot from the measured properties to the ease of use during future tests.
Metallic end tabs have been used for composite tests for a long time.
Carbon steel and aluminum are the preferred materials but, both materials have issues.
- For example, the high toughness of steel means that when machining the specimen with steel tabs pre-bonded onto the composite, the tabs will pop off.
- Although you can bond the steel tabs onto the test specimen after machining, this process is said to be very tedious and time-consuming.
- Aluminum tabs don’t suffer from the same issue but, the only way to actually bond them onto the composite, you must chemical etch the metal before bonding, this is because oxide film forms on aluminum.
- You must then use chromic acid or phosphoric acid to remove this film and there are extreme health concerns when using chromic acid. Use with extreme caution.
The most commonly used tabbing material is a fibreglass printed circuit board material ideally in a +/- 45⁰ orientation.
This material is easily bonded to composite panels and doesn’t pop off during machining. Also, fibreglass has a very low stiffness in this orientation which helps minimize the tab termination stress concentrations.
Since fibreglass is also quite soft, the grips can bite into the surface of the tab.
This material is also relatively cheap and available in many thicknesses.
It was noted that “Glass fabric/epoxy should be considered as a tabbing material unless there is a clear reason why another material is preferred”

What adhesive to use?

The adhesive should have a suitably high shear strength.
An old CRAG standard said that the adhesive should have a shear strength of more than 30 MPa.
Other than that, the choice is up to the user and the Tg (glass-transition temperature?) of the material being tested.
Film adhesives are often easier to use but, they require an elevated temperature and pressure to cure but, they do give a controlled bond-line thickness.
Sometimes, the cure temperature of the material is greater than the Tg of the test material which means the adhesive is worthless and another should be used.
Another type of adhesive that can be used is a two-part epoxy paste adhesive.
It doesn’t require any extra equipment and can be cured at room temperature but, steps must be taken to control bond-line thickness.

Design of the End Tabs

This is a diagram showing the various terms used in the following notes on end tab designs.

Generally, the tab should be the same width as the specimen.
For tensile testing, the tab should be at least 40mm long and up to 90mm long. This length depends entirely on the length of the wedges in the grips of the machine that we are using.
Tabs with a termination angle between 10-15⁰ should be used because they can increase the tensile strength up to 18%.
Shallower angles of 10⁰ should not be used because it can lead to peeling off during testing.
During compression test, however, it is recommended to not taper the tab termination area since buckling has more of an influence on final results than stress concentration caused by the tab termination region.
The thickness of both the adhesive and the tabbing material has an effect on the results.
This is because both materials act as a cushioning layer.
The adhesive has the greatest effect since it is closer to the material.
A rule of thumb is that the tabs should be 1.5-2mm thick and the bond line should be 0.5-1.3mm thick, these measurements give optimum results and avoid the additional cost of using more material.