ASTM D7791 Uniaxial Fatigue Testing of Plastics

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ASTM D7791 Uniaxial Fatigue Testing of Plastics

ASTM D7791 describes how to determine the dynamic fatigue properties of plastics in uniaxial loading. 

What Is ASTM D7791?

The ASTM D7791 standard is a test for tension-tension fatigue in plastics. ASTM D7791 applies to rigid and semi-rigid plastics. Uniaxial tension and compression fatigue test machines are employed to create stress and strain levels below the proportional limits of the material where the strains and stresses are relatively elastic.

Why Perform Fatigue Tests on Plastics?

These tests help determine the effect of processing, surface condition, stress, and other factors, on the fatigue resistance of plastic material. The results help guide the selection of plastic materials for service under conditions of repeated tension and/or compression stresses.

Important Considerations When Testing per ASTM D7791

Testing is needed because not all plastics have the same material response to fatigue. If a plastic does not have an elastic range, it will cause the amplitude of the programmed load or deformation to change during testing. This can happen as a result of fatigue testing and cause plastic deformation. Caution should be taken when using the results for design as they are generally not indicative of the true fatigue properties of the material.

The results of tests are suitable for application in design only when the specimen test conditions realistically simulate the actual service conditions.

ASTM D7791 Test Methods

Procedure A Tension Fatigue Tests

A rectangular or circular cross section test specimen is cyclically loaded in tension to a specific stress or strain level at a selected frequency until the specimen fails. Fatigue strengths are determined at specific cycle counts. Four different stress or strain levels are tested to construct a stress versus number of cycles to failure (S-N) curve or a strain versus number of cycles to failure (r-N) to determine the endurance limit of the material in tension. Specimen dimensions, shape, surface conditions, and limitations need to match ASTM D638.

Procedure B Compression Fatigue of Rigid Plastics

A rectangular or circular cross section test specimen is cyclically loaded in compression to a specific stress or strain level at a selected frequency until the specimen fails. Fatigue strengths are determined at specific cycle counts. Four different stress or strain levels are tested to construct a stress versus number of cycles to failure (S-N) curve or a strain versus number of cycles to failure (r-N) to failure (r-N) to determine the endurance limit of the material in compression. Specimen dimensions, shape, surface conditions, and limitations need to match ASTM D695. An anti buckling fixture may be used.

Dynamic Test Machines for ASTM D7791

Our 800 series fatigue test machines serve both static and fatigue testing applications. The 800 series is capable of performing plastics fatigue tests at speeds up to 15 hz with load capacities up to 50 kN (11,250 lbf). The 800 series is based on all electric dynamic actuators. All of our fatigue testers are capable of load, strain and position control.

What Test Fixtures Are Recommended for ASTM D7791?

Wedge action grips provide high force capacity and are available in mechanical or manual, pneumatic and hydraulic action. When under tension, their two jaws are drawn tighter together in a self tightening mode. For this application, consider our G175 Pneumatic Wedge Grip.

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