Case Histories


Cap Screw Failure – Hydrogen Embrittlement

Failure of a suspension bolt
Failure of a suspension bolt
Hydrogen embrittlement of a suspension bolt
Hydrogen embrittlement of a suspension bolt

Specimen: ¾-inch Cap Screw from Automobile Suspension System

Background: A metallurgical failure analysis was conducted for a number of suspension cap screws. The cap screws had been zinc plated (galvanized), and had originated from an off-shore manufacturer.

Service Life: Mileage at the time of failure ranged from 30,000 to 50,000 miles.

Findings: The forensic engineering failure analysis of the cap screw found that the re-occurring under-head catastrophic failures were caused by hydrogen embrittlement originating most likely from a poor cleaning and galvanizing procedure during the manufacture of the cap screws.

Cracking of Axle

Polished Transverse Cross-Section Showing Crack Initiation and Propagation from both the Top and Bottom of the Axle
High Magnified view of the thick iron oxide later lining the portion of the crack closest to the outer surface of the axle

Material: Chromium Steel Alloy

Environment: Motorized Vehicle

Background: The axle had been manufactured in Germany in 2005.. Visual inspections have revealed 36 out of 50 vehicles had a cracked front axle. The axle design has been used since the 1990s.

Service Life: Approximately 12 years

Findings: Stress corrosion cracking was not present for the failure. Cracking had initiated from both the outer surface of the top and bottom of the axle beam. The longitudinal character of the cracking, in the thickest section of the axle beam suggests a thermal/high temperature failing process. The series of “angular character” micro cracks adjacent to the main longitudinal crack suggest a high temperature rupture failure mechanism.

Filiform Corrosion of Coating on Aluminum Structure

Semi-Circular shaped corrosion sites had initiated at damaged area of the coating at the outer edge of the rivet
View of the Cavity beneath the 2-coat system indicating significant corrosion of the aluminum (scanning electron microscope)
Magnified view of Corrosion Site . Micro-cracking of the coating along with filiform tunnels (denoted by arrow)

Material: 3004 Aluminum Alloy in the H291 Temper.

Environment: Exterior of vehicle driven in the Prairies.

Background: Since Testlabs analyzed coating failures of the one coat system used in the past, the company has implemented a 2-coat paint system for the aluminum sheets.

Service Life: Ranged from 5-8 months.

Findings: The 2-coat paint system which specifies a total dry film thickness of 1.1- 1.3 mils has reduced the number of corrosion site/unit areas as compared to one-coat paint system.

  • With the 1-coat paint system there were 100+ corrosion sites / ft2
  • With the 2-coat paint system, there are 5 to 17 corrosion sites/ ft2