An experimental method for fatigue testing cast iron water pipes using combined internal water pressure and bending loads

Background

Investigations into the fatigue failures mechanism of Grey Cast Iron (GCI) water pipes are inhibited by the lack of a lab-based method to conduct extensive high-cycle biaxial fatigue test programmes.

Objective

The work presented in this paper developed and tested a novel experiment capable of causing controlled fatigue failures of GCI pipe specimens in the high-cycle fatigue regime using bending and internal water pressure fatigue loading.

Methods

A novel four-point bending and internal water pressure fatigue testing system was developed to apply constant amplitude out-of-phase biaxial loading to 58 mm diameter GCI pipes at 1.7 Hz. To verify the ability of this equipment to apply known stresses and repeatable loads to pipe specimens a series of tests were conducted. A finite element model of the pipe specimen was used to estimate the strains and displacements applied by the equipment.

Results

Experimental strains and displacements were mainly within ± 10% of the estimated values and the pressure amplitudes measured over 103 cycles were within ± 3% of the average. Dynamic load effects occurred at higher bending loads, but these were quantified and accounted for. Trial destructive tests revealed that the lifespan of leaking fatigue cracks in GCI pipes with uniform wall-loss subject to combined internal pressure and bending fatigue loads is less than 1% of the total cycles-to-burst.

Conclusions

The experimental method developed was able to apply combined, out-of-phase internal pressure and bending fatigue loads accurately and consistently to small-dimeter GCI pipes, and cause these pipes to develop high-cycle fatigue regime failures.