Geosynthetic Tensile Creep Tester (20 kN)

Theory and Method

Polymers deform continuously under sustained load. Their long molecular chains slide and reorganize over time, so a geogrid carrying constant tension keeps stretching long after the initial elastic response has finished. That behavior is viscoelastic creep, and it means the strength measured in a short-term tensile test overstates what the material can carry for a hundred years. A reinforced wall designed on index tensile strength alone will deform, and in the worst case the reinforcement ruptures at a load it survived easily in the laboratory.

The test applies a constant tensile force set as a fraction of the material's ultimate tensile strength and records strain as it develops. At modest load levels the specimen creeps toward a stabilizing strain and the run produces a creep curve. At higher levels the specimen eventually breaks, and the run produces a rupture time instead. Testing several load levels builds a set of points relating applied load to time at rupture, which plots as a straight line against logarithmic time for most reinforcement polymers. Extrapolating that line to the design life gives the creep-limited strength, and dividing short-term strength by it yields the creep reduction factor that goes into the design calculation.

Running four stations at once is what makes the method practical. A creep program needs several load levels, and each level runs for a thousand hours or more, so sequential testing on a single frame would take years to produce one curve. The TCT's 800 mm creep space leaves room for the gauge length plus the substantial elongation an extensible geogrid accumulates before rupture, avoiding a run that ends at the limit of travel rather than at failure. Displacement resolution of 0.03 mm captures the small strain increments that separate one decade of logarithmic time from the next, which is where the slope of the creep curve is actually determined.