Porous Aluminum Motorcycle Brake Fractures–and Fails
Plastics often get a bad rap for being flimsy and cheap., But big hunks of metal aren’t immune to failing catastrophically.. Ken Russell, a forensic engineer and professor of materials science at MIT, describes the disastrous effects when monkeys substituted poorly cast aluminum for forged steel in a motorcycle brake. The SEM micrograph shows crack-inducing pores and oxide film in an aluminum casting with hydrogen porosity of the kind he describes..
“One of my forensic metallurgy cases involved a motorcycle owner who went for a spin with a friend on the buddy seat. The operator tried to brake for a traffic stop and the brake pedal snapped. The bike continued into the intersection and was struck by an oncoming car, killing the passenger and mutilating the operator. The pedals broke so frequently that the manufacturer issued a recall. The problem arose because forged steel had been replaced by cast aluminum to save one dollar per pedal. My scanning microscope study showed the fracture surface to be mostly hydrogen porosity so that the pedal had a fracture strength far below that of the expected sound material. Hydrogen pickup is the bane of aluminum casting. The reactive molten metal captures the oxygen from water vapor to form aluminum oxide and injects the hydrogen into the melt where it stays until forming bubbles in the solidifying ingot. There are several ways to remove hydrogen from molten aluminum, none of which was used here. What we had here was a serious failure to communicate. User and foundry must cooperate closely to produce satisfactory castings. There was no back and forth in this case and in addition the foundry had never produced a structural member. Hydrogen porosity that might have been tolerable in such ornamental objects as porch light brackets was disastrous in brake pedals. Prevention of the problem would have been both simple and cheap. The company probably did not need a full time metallurgist, but hiring a consultant on an ongoing, day a month basis would have cost about $10k/year. Such a consultant would have nipped the pedal problem in the bud and provided needed expertise on metallurgical issues in general So, how did this case cost out? The manufacturer was producing a few tens of thousands of bikes a year at the time of this case. I figure that over several years it probably produced a total of a couple of tens of thousands of the model with the defective pedal. There were easily dozens of suits with payoffs of perhaps $1 million each. So, saving few tens of thousands of dollars on a cheap pedal turned into a liability loss of tens of millions, a ratio of one thousand to one. Today the company is producing quality motorcycles under different ownership.””– K. C. Russell