Automatic Decreasing Stress-Intensity Fatigue Crack Growth Rate Testing Using Low-Cost Circuitry

To run decreasing-K tests to very low growth rates, around 10-7 mm/cycle, it has previously been necessary to use a digital computer controlled system, or to manually shed the load at prescribed crack lengths. In this work, low-cost, off-the-shelf electronic circuitry was used to control decreasing-K tests so that very low crack growth rates were achieved. This was done automatically, using the compliance method of crack measurement for the electronic control inputs. By switching to constant load limit control, an increasing-K test may be conducted using the same specimen. This predetermined, decreasing-K technique was verified by comparing decreasing and increasing-K data at low growth rates for an alloy steel.