As a test engineer for a well-known manufacturer of lawn and garden equipment, I have seen my share of product failures. The following product failure illustrates the use of hardness testing as a failure analysis tool and how material hardness can be used as a quality control or strength indicator.
In the late 90's, our plant had totally re-designed our 46-inch mowing deck for improved cutting, mulching, and bagging performance for the six most prominent regional grass types. The redesign incorporated a modified deck shape for improved flow, a new blade profile for improved flow and lift and a different blade steel for reduced cost and extended life. Roughly two months into the spring mowing season, reports started to trickle in that the new 46-inch deck was throwing broken blades and that this phenomenon was occurring generally in the Northeast region of the country. In one extreme instance, a six-inch long blade segment was thrown through a customer's car door. Fortunately no one was injured. Our plant initiated a full-scale failure investigation.
Both the broken blades and associated mower decks were collected and shipped to our plant for inspection. Initial inspection revealed fractures through the blades where they are bolted to the blade spindle. Further evaluation showed crack initiation and progression from damaged blade surfaces directly underneath the mounting washer. Fine brown debris, in this case oxidized steel powder and commonly referred to as "cocoa", was also found in the vicinity of crack initiation. These two observations indicated that the blades failed through fretting fatigue, which is normally brought about by intimate contact and slight motion between the two parts due to operating vibration.
Next, a small section of blade was cut, mounted in cross-section and polished. Material hardness measurements were taken from one blade surface to the opposite side in very small increments. The Vickers micro-hardness technique was used to make the measurements since each indentation is small. Smaller indentations allowed hardness variation measurements over very small distances. Results of the test indicated that the blade surfaces were much softer than the core. The loss of surface carbon during heat treatment was thought to have caused this problem and was later confirmed through a micro-structural and chemical composition evaluation. In this case, lower carbon content leads to a softer, lower strength blade surface which is more prone to wear, grooving, and eventual crack initiation.
The failure investigation finally concluded that softer blades, first identified through hardness testing, coupled with a deck that tended to clog with wet Fescue grass, caused the blade fractures.