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Recommendations on specimen bending are included in many composites tensile test standards, such as:

ASTM D3039 (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials) “good testing practice is generally able to limit percent bending to a range of 3 to 5% at moderate strain levels (>1000 µ€)”

ISO 527-4 (Test Conditions for Isotropic and Orthotropic Fibre-Reinforced Plastic Composites) and ISO 527-5 (Test Conditions for Unidirectional Fibre-Reinforced Plastic Composites); both recommend that the total bending strain is less than 3% at the mid-point of the modulus range (1500 µ€)

Limits on PBS are also defined in individual company test specifications and audit check lists, such as:

Nadcap AC 7101 (Materials Testing Laboratories Mechanical Testing) – 10% for static tests and 5% for cyclic tests

Nadcap AC 7122 (Non-Metallic Materials Testing Laboratories) – 8%

Satisfying alignment requirements requires a systems approach involving the test frame, load string, alignment device (e.g. Instron® AlignPRO), grips, and specimen. The discussion here is limited to the gripping requirements. In general terms, accurate and consistent alignment of composite specimens requires:

The means to accurately and repeatedly locate test specimens in the grips; this requires rigid specimen stops on both grips.

Grips with a symmetrical mechanism (e.g. wedge grips) are preferred over grips with independent mechanisms (e.g. dual side-acting hydraulic grips).

The jaw faces come into contact with the specimen in a repeatable manner. Moving body grips are preferred to moving face grips. In a moving body grip, the faces move in a direction normal to the specimen surface; whereas in a moving face grip, the faces move in a diagonal direction and friction effects can cause the faces to make an uneven contact, which results in misalignment.

For grips to maintain good alignment under load, the body design should be symmetrical. Asymmetrical designs (e.g. an open front /closed back grip body) will introduce specimen bending under load due to uneven deflections.

Other Test Types Composite materials are multi-phase (i.e. fibers and matrix) and anisotropic (i.e. properties that depend on direction), and along with tensile tests additional tests (e.g. compression, shear, flex) are usually required to fully characterize the material. Generally, these tests require specific test fixtures. Unless a laboratory is able to dedicate machines to specific test set ups, testing machines will need to be reconfigured when changing tests. The task of reconfiguring machines is made much easier by providing a means of mounting test accessories between the grips allowing the grips to remain in place when changing the test set up – this is a major advantage with heavy, high-capacity grips or grips that have previously been aligned and certified.

Gripping Solutions Manual Grips for Composites

The Instron® 100 and 250 kN Precision Manual Wedge Grips have been designed to provide reliable gripping of composites and other materials while meeting Nadcap alignment requirements when installed on a suitable frame. The grip is a moving body wedge design, which ensures accurate and consistent centering of the jaws. Actuation is via a compact worm gear driven by a front-mounted ratchet handle. The mechanism is capable of producing a high initial preload (5 kN for the 250 kN grip), which reduces the possibility of a specimen with a hard surface slipping before the wedge action takes over from the initial pre-load. The body is guided to maintain good alignment. The depth of the jaw faces used in 100 kN grips is 75 mm and in the 250 kN grips 100 mm – making the grips usable with specimens with long tabs.

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