Standard Test Method for Constant Amplitude of Force Controlled Fatigue Testing of Acrylic Bone Cement Materials - PDF

ASTM F2118 Vise Grip – Universal Grip Data Sheet (PDF)

 

ASTM F2118 is the standard test method for constant-amplitude, force-controlled fatigue testing of acrylic (PMMA) bone cement. Molded dumbbell specimens are cycled in fully reversed tension and compression in 37 °C saline until they fracture or reach 5 million cycles, and the results are plotted as an S-N curve. Bone cement makers and orthopedic companies use it to compare cement formulations, mixing methods and additives, because fatigue cracking of the cement mantle is a leading cause of implant loosening.

Specimen Requirements

• Cylindrical dumbbell with a straight gauge section 5 mm in diameter and 10 mm long.

• Ends substantially larger than the gauge diameter, with blended radii, so failure happens in the gauge section.

• Molded directly in silicone molds: the cement is mixed as the manufacturer directs and injected during the dough phase (a 150 mL syringe with a 10 mm nozzle is suggested). Porosity from mixing has a large effect on fatigue life, so the mixing method must be reported.

• Inspect specimens and discard any with visible voids or flash in the gauge section.

Test Conditions

• Loading: sinusoidal, constant force amplitude, fully reversed tension–compression (R = −1).

• Environment: specimens immersed in phosphate-buffered saline (PBS) held at 37 °C.

• Frequency: low enough that the cement does not self-heat; labs commonly use 5 Hz or less.

• Stress levels: at least three, with duplicate tests at each, reported as the maximum applied stress.

• Runout: 5 million cycles.

• Alignment: grips and load train checked per ASTM E466 at both maximum tension and maximum compression.

Test Procedure

1. Mold, de-mold and age the specimens as specified, then measure the gauge diameter.

2. Mount the specimen in backlash-free grips inside the saline bath and let it reach 37 °C.

3. Set the force amplitude for the chosen stress level (force = stress × gauge area, 19.6 mm² for a 5 mm gauge).

4. Cycle at constant amplitude until fracture or 5 million cycles, recording cycles to failure.

5. Repeat at the other stress levels and plot stress against cycles to failure.

Results

Report an S-N diagram of maximum stress against cycles to failure, the median fatigue life at each stress level (the cycle count at which half the specimens have failed), and the fatigue strength at 5 million cycles. Example force setting: for ±15 MPa on a 5 mm gauge, the force amplitude is 15 × 19.6 = ±295 N.

Grips and Fixtures for F2118

Universal Grip F2118 grips clamp the enlarged ends of the bone cement dumbbell without play, so the specimen can be cycled through zero force in tension and compression without backlash. They are sized for the 5 mm gauge specimen, and fit servo-hydraulic and electrodynamic fatigue frames. Contact us for saline-bath compatible materials and adapters for your frame.

ASTM F2118 FAQ

What does ASTM F2118 test?

The fatigue life of acrylic (PMMA) bone cement under constant-amplitude, force-controlled, fully reversed tension–compression loading. It tests the cement material, not implants or devices.

What specimen does ASTM F2118 use?

A molded cylindrical dumbbell with a straight gauge section 5 mm in diameter and 10 mm long, with larger ends so it breaks in the gauge section.

What environment and runout does F2118 use?

Specimens are tested immersed in phosphate-buffered saline at 37 °C, and a test is stopped as a runout at 5 million cycles.

What grips are needed for F2118?

Backlash-free grips that clamp the enlarged ends without any play, aligned to ASTM E466, because fully reversed loading passes through zero force every cycle and any slack shows up as error and bending.