Analysis of a Brake Cylinder Failure
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CASE HISTORY—PEER-REVIEWED
Analysis of a Brake Cylinder Failure B. A. James • S. I. Lieberman
Submitted: 1 September 2010 / in revised form: 7 February 2011 / Published online: 1 March 2011 ASM International 2011
Abstract A gray cast iron inlet boss was observed to have fractured following an accident in which a three-wheeled parking enforcement vehicle reportedly lost its braking ability and crashed on a hill in San Francisco, CA. Metallurgical and fractographic examination as well as exemplar testing conclusively determined that the inlet boss broke before the accident, and that the fracture occurred by overtorquing of the inlet hydraulic line during the vehicle servicing that occurred just prior to the incident. Keywords Cast iron Failure analysis Engineering expert litigation
Background A parking enforcement vehicle unfortunately crashed at the bottom of a hill in San Francisco, resulting in significant injuries to the driver. The vehicle operator reported that she lost braking ability, resulting in the crash. Accident scene investigators did report a puddle of liquid consistent with brake fluid on the day of the accident, but fluid samples were not collected for further analysis. The front-wheel brake cylinder was observed to have fractured at the brakeline inlet, as shown in Fig. 1. The subject vehicle was reportedly serviced the day before the accident, including removal of the front wheel, which required the removal and re-tightening of the brake line at the subject brake cylinder. Our analysis was conducted as a result of the litigation that ensued as a result of the accident. B. A. James (&) S. I. Lieberman Exponent Failure Analysis Associates, Menlo Park, CA, USA e-mail: [email protected]
Analysis An image of the fractured brake cylinder on the accident vehicle is shown in Fig. 1. The subject brake cylinder is shown in Fig. 2, and the fractured brake-line boss is shown in Fig. 3. As observed in the photos, spotty corrosion product was observed on the fracture surface upon our receipt of the evidence. Three fractures were observed on the subject brake cylinder: the transverse fracture that caused the boss to separate from the cylinder, and two cracks that extended radially into the boss from the threaded brake-line hole. The threaded portion of the brake line, which screws into the cylinder boss is shown in Fig. 4. An exemplar brake cylinder is shown in Fig. 5. All the testing processes of the subject brake cylinder were required to be non-destructive, and hence metallographic inspection, hardness testing, and chemical analyses were not conducted. Stereomicroscopic examination of the transverse and radial fracture surfaces indicated that no large-scale casting defects were present, which could have contributed to the failure, shown in Fig. 6. No evidence of cross-threading was observed. Fractographic examination using scanning electron microscopy (SEM) confirmed that no significant casting defects were observed at the fracture surface, as shown in Figs. 7 and 8. Owing to the graphite flake mo
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