By July 3rd, the Southeastern Pennsylvania Transportation Authority (SEPTA) had to remove 115 out of 120 of its newest Silverliner V cars due to a welding flaw that led to fatigue cracks. SEPTA is the major transit provider for Philadelphia, and the U.S.’s 6th largest rapid transit system by ridership. Shortly after midnight on July 1st, a staff engineer noticed that one railcar tilted more than the railcar it was linked to. When the usual protocol didn’t fix the lean, the railcar was taken to the workshop, where workers realized there was a ten-inch crack running through the length of the nine-foot equalizer beam. The crack began at the weld, leading engineers to conclude that the crack was caused by a flawed weld.
The beam was completely severed from the railcar, and if it weren’t for the pressure holding the cars together, “…a serious accident – even a derailment – could have resulted,” said Jeff Knueppel, General Manager of SEPTA. SEPTA inspected the remaining Silverliner V cars and found smaller, less severe cracks in all but five of the railcars. The railcars were pulled off the line since federal railway guidelines mandate that cars be removed from service when critical problems like these are identified.
The cracks found on the heavy-duty steel equalizer beams are known as metal fatigue cracks. Fatigue is a result of wear and tear that occurs in metals that are exposed to repeated, fluctuating loads. For example, in the case of SEPTA, metal fatigue could have occurred as a result of passengers getting on and off the railcars, as well as the movement of the trains as they move from one section of the track to the next. In SEPTA’s case, each of the 146,000-pound cars have four equalizer beams, which help distribute the weight evenly across eight wheels – two wheels per beam. However, fatigue can eventually lead to microcracks when there are flaws present in the weld. These flaws function as stress risers that cause more wear and tear to occur in one particular area of the component. For SEPTA, the microscopic fatigue cracks could have originated in eight places, on each of the ends where the equalizer beams rest on the wheel bearings. The precise welding flaw that led to the microcracks remains unclear.
The Importance of Fatigue Testing
According to the article, “The Mechanics of the SEPTA Crisis: Why Cracks Can Form,” metal fatigue has haunted mechanical engineers for as long as they have worked with metal. In fact, the very first instance of metal fatigue was noted as far back as 1842, when several disastrous accidents involving train axels led engineers to perform destructive tests in order to understand this phenomenon. Since then, sophisticated fatigue tests have been developed in order to measure the fatigue life of components, as well as identify any design flaws before they lead to costly cracks or catastrophic failures.
Even though the precise cause for SEPTA’s fatigue cracking is uncertain, a welding flaw that results in metal fatigue could have been exposed with the right destructive tests. Hyundai Rotem, the supplier for the Silverliner V railcars, stated that the equalizer beams were tested, but they weren’t subjected to repeated fatigue tests in order to simulate the pressure of years of use. Both SEPTA and Hyundai Rotem admit that performing the appropriate fatigue tests might have revealed the weld flaws.
Representatives of Hyundai Rotem stated that the company will review its design and inspection process. The company is currently performing multiple fatigue tests on two replacement beams. However, Hyundai Rotem also stressed that any company could be susceptible to such an error due to competitive pressures, along with the limited number of companies qualified to do heavy industrial steel work. “There’s no doubt in my mind that if someone else had done the work, they might have run into the same issues we have,” stated representatives of Hyundai Rotem. While they’re reviewing their design and inspection process, the company has begun ordering replacement equalizer beams for all the SEPTA railcars. Once testing is over, Hyundai will allow SEPTA to choose between keeping the replaced beams or switching over to the new design that has been fatigue tested. The company will absorb all costs of manufacturing associated with fixing and safeguarding SEPTA’s railcars.
Negative Repercussions for SEPTA
SEPTA officials have acknowledged that “if ignored, the cracks could have led to serious rail accidents.” With only 5 of their 120 cars in service, SEPTA is doing everything they can to keep up with the city’s transit demands with considerably less cars. The transit authority is currently using its own money to lease 28 railcars and 4 locomotives from other transportation agencies. Despite its efforts, the agency has lost one third of its clientele due to overcrowding and a limited train schedule. Matt Mitchell, Vice President of the Delaware Valley Association of Rail Passengers, estimated that it would take up to four years before SEPTA regains all the riders it has lost in recent weeks. “This is a very upsetting situation. It’s all hands on deck,” stated Knueppel. “The railroad has grown in ridership and we want to make sure our customers come back.”
From the SEPTA crisis, it’s clear that metal fatigue is, at best, an incredibly expensive issue that can negatively impact an agency’s public reception. In most cases, however, the issue can be avoided all together with destructive testing that subjects the component to the kinds of loads it will endure when in service. Unlike nondestructive testing, destructive testing methods involve the irreversible alteration of the metal in order to measure how that component will perform under in-service conditions. Destructive tests also allow material scientists to identify any design flaws that may lead to catastrophic failures and costly repairs. It is possible that this whole issue could have been avoided had the appropriate fatigue tests been performed on the equalizer beams.
AWS is aware of the importance of performing destructive tests to ensure public safety. That’s one of the reasons why AWS Learning is launching its first online Destructive Testing course on August 12th. With 17 short modules, the course presents the principles of several destructive tests in an engaging, easy-to-understand format. The testing methods covered are: strength, hardness, fatigue, toughness, soundness, chemical properties, corrosion, and metallographic. AWS Learning offers a complete suite of courses designed specifically for welders and welding professionals. Click here to learn more.