Failure analysis testing performed by MSi Testing & Engineering on two ductile iron brake heads did not identify any evidence of detrimental material conditions contributing to the component fractures.

1. Based upon the performed steel failure analysis, it is our opinion the cracking on both samples initiated from common as-cast surface conditions of the outer rib of the brake shoe arm, and inside pocket location of the head. Oxidation (corrosion) of exposed fracture surfaces prohibited interpretation of the original fracture features. It is our opinion that the failure of Sample #1 was not related to any detrimental material defect conditions, such as internal porosity (gas or shrinkage cavities), excessive dross or surface stringers, degenerated graphite, or intercellular carbides.

2. Sample #1 conformed to the specified requirements for Ductile Iron Grade 65-45-12 per ASTM A536. Tensile and hardness testing of material removed adjacent to the fracture verified the casting conformed to the tensile requirements for Grade 65-45-12. Un-notched Charpy impact results provided additional support of properly manufactured ductile iron.

3. Visual examination of exposed and cleaned fractures revealed similar sites of initial cracking along the inner pocket radius location of the head and arm. The fracture features of Sample #1 exhibited a chevron pattern from the pocket radius, which indicated the area was the site of initial cracking. The exposed fracture surface of Sample #2 revealed no informative evidence due to oxidation (corrosion).

4. Metallographic examination of sections through the fracture of Sample #1 revealed an “as-cast” surface condition along the cast surface that consisted of some magnesium-silicate stringers (dross inclusions) to a depth of .002”. The amount and depth of the dross stringers were not considered excessive. However, the stringers potentially serve as stress-concentrators during rapid loading and could serve as sites for fatigue cracking.

5. The matrix microstructure of Sample #1 consisted of 7 – 10% pearlite in a matrix of ferrite, which was characteristic of Grade 65-45-12 ductile on the lower end of the typical hardness range of 156 – 217 HBW. The graphite morphology consisted of 90% minimum Type I and II graphite, primarily sizes 6 and 7. The nodule count varied between 75 – 100/mm2. No evidence was observed of any intercellular carbide, carbide network, or any other detrimental material defect conditions that could have contributed to the failures.

6. The chemical composition results of Sample #1 indicated a carbon equivalent (CE) value of 4.54. In our opinion, the silicon content of 2.90% was aimed for a predominantly ferritic microstructure with some pearlite, which was observed. No alloying elements were utilized to promote increased amounts of pearlite in the microstructure. Small amounts of copper or tin or combinations of both could be utilized to safely increase the amount of pearlite in the microstructure to 10% or greater.

1. Examination of the submitted brake head castings revealed a complete fracture of Sample #1 and cracking of Sample #2 from the brake shoe arm and the inner pocket location of the head. The amount of oxidation of the fracture was significant and after cleaning masked some of the fracture features. (See Photos 1 and 3)

2. Visual examination of exposed and cleaned fractures revealed similar sites of initial cracking along the inner pocket radius location of the head and arm. The fracture features of Sample #1 exhibited a chevron pattern emanating from the pocket radius, which indicated the area was the site of initial cracking. The exposed fracture surface of Sample #2 revealed no informative features due to corrosion. (See Photos 2 and 4)