EN 6031 Determination of In-Plane Shear Properties of Fiber-Reinforced Plastics (Plus/Minus 45 Degree Tensile Test)

The European standard EN 6031 Aerospace series - Fibre-reinforced plastics - Test method - Determination of in-plane shear properties (plus/minus 45-degree tensile test) describes a procedure for determining the in-plane shear strength and modulus of fibre composites. This procedure is based on the uniaxial tensile stress-strain response of a 45-degree laminate laminated symmetrically about its mid-plane. This standard is applicable to composite laminates manufactured with unidirectional tape or woven fabric reinforcement.

This standard does not provide any instructions for meeting health and safety requirements. It is the responsibility of the user of this standard to consult and determine appropriate health and safety measures.

For the purposes of this standard, the following terms and definitions apply:

The tensile stress at a specific moment during the test refers to the longitudinal tensile load experienced by the test specimen at a specific moment during the test, divided by the initial unit cross-sectional area within the gauge length.
The shear stress occurring at a given moment during the test is defined as the in-plane shear stress whose main direction is plus/minus 45 degrees below the direction in which the tensile load is applied, and its magnitude is half the tensile stress.
Shear strength is the maximum shear stress that occurs during testing.
Tensile stress refers to the change in the longitudinal or transverse distance between points within the measuring length of the test specimen. It is produced by the tensile load and is expressed relative to the initial distance between points (the value of the longitudinal stress is positive, the value of the transverse stress of the tensile specimen is negative).
Shear modulus refers to the slope of a line on a shear stress/strain diagram passing through points corresponding to two longitudinal stress limits.

Using relationships derived from laminated plate theory, expressions are presented that allow the construction of an in-plane 0 degree shear stress-strain curve from a uniaxial tensile test of a plus/minus 45 degree laminate. Experimental test data from a tensile test are used to establish shear strength and modulus.

Major equipment used in testing includes:

Tensile testing machine with accuracy up to 1 percent in the load range used.
Flat surface micrometer with accuracy to the nearest 0,01 mm.
Caliper with accuracy to the nearest 0,1 mm.
This procedure requires load-strain data in both the longitudinal and transverse directions. This is accomplished by equipping the specimen with longitudinal and transverse strain gauges (5 mm to 10 mm gauge length). Gauges, surface preparation, and binders should be selected to ensure optimum performance on the subject material, and appropriate automatic strain recording equipment should be used. If possible, a duplex extensometer or a combination of longitudinal and transverse extensometers can be used. Extensometers should be accurate to within 1 percent of the applied load range.
Temperature measuring equipment with an accuracy of plus/minus 0,2 degrees at the applied test temperature.
Vacuum bag sealing material.
Timer or clock.

The test specimen may consist of unidirectional tape or woven fabric. Eight layers are applied, oriented at plus 45 degrees and minus 45 degrees to the longitudinal axis. To prevent distortion and induced bending, the layers must be completely symmetrical. The dimensions and tolerances of the test specimen shall be as specified in the standard.

During test specimen preparation, specimens are cut from the plates. The coefficient of variation in thickness measurements must be less than 2 percent per plate. The placement must be symmetrical and alternate at +45 degrees and minus 45 degrees. The fiber orientation must be accurate to within +/- 2,5 degrees. Carbon plates must be manufactured in accordance with EN 2565, and glass plates in accordance with EN 2374. Process parameters such as blades, curing temperature, and time must conform to the applicable technical specifications. All specimens must have a placement of +/- 45 degrees relative to the specimen axis. Precautions must be taken to prevent nicks, undercuts, roughness, or uneven surfaces after processing.

If strain gauges are applied for modulus determination, they should be attached to the centre of the specimen in the longitudinal (0 degrees) and transverse (90 degrees) directions.

The sample should be tabbed using pre-cured tabs made from two layers of fabric (folded plus/minus 45 degrees) or bonded together using a suitable prepreg system. The pre-cured tabs are bonded to both sample faces with an adhesive system that meets temperature and aging requirements.

Unless otherwise specified in the applicable technical specifications, five specimens are tested per test condition. If tests are performed after aging or at a temperature other than room temperature, care must be taken to ensure that room-temperature/dry reference specimens processed from the same plate as the specimen under investigation are also tested.

In post-immersion tests, conditioning must be carried out in accordance with EN 2489. In tests performed after exposure to a humid atmosphere, conditioning must be carried out in accordance with EN 2823.

Storage and testing of cured samples must be carried out in accordance with EN 2743 at a temperature of 23°C plus/minus 2°C and a relative humidity of 50% plus/minus 5%. Aged samples must be tested immediately after the aging procedure (a maximum of 8 hours at 23°C plus/minus 2°C is permitted).

Before aging and mechanical testing, the thickness and width of the sample are measured and recorded at three points in the non-grip area. A micrometer is used for the thickness, and a vernier caliper or micrometer is used for the width.

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