forensic engineering society australia metallurgical failures

FORENSIC ENGINEERINGSOCIETY AUSTRALIA

METALLURGICAL FAILURES

CAUSES OF METALLURGICALFAILURES

• Single event overload failures caused by extreme stress

• Affects of residual stress caused from cold rolling or shaping of the metals at room temperature

• Notch ductility issues from poor manufacturing and tight radius bending

• Cyclic (dynamic) or repeated loading

• Residual stress caused by welding with superimposed static and dynamic loading

• Thermal cyclic loading (expansion and contraction)

RESIDUAL STRESS CAUSED BYCOLD ROLLING

• Steel tubular SHS and RHS sections made from flat sheet and cold formed to produce a square or rectangular profile. The corner radius of a cold rolled section is work hardened by plastic deformation. This has the effect of rearranging the microstructure resulting higher hardness and strength. However, the steels ductility is reduced.

RESIDUAL STRESS CAUSED BYWELDING

• Welded fabrications / components which are restrained from distortion develop residual stress.

• Residual stress occurs as the expanded molten weld metal cools. The hot metal contracts and as so, tries to pull or distort the adjoining parent materials.

• Where the shape of the weldment is rigid (inflexible), residual stress is developed in the center of the weld and extend to the weld toes and the Heat Affected Zone (HAZ).

• The stresses are by and large tensile in nature and will remain unrelaxed for the life of the weldment without post-weld heat treatment.

RESIDUAL STRESS INWELDMENTS

FATIGUE CRACKINGFAILURE

Notch defect from brake pressfold, tight radius and reducedmaterial thickness

BRITTLE FRACTUREFAILURE

Notch defect from tight bendradius and reduced thickness, Cracking caused by poorrolling control

WELDING RELATEDWINCH DRUM FAILURE Circumferentially welded

drum flange, cracked due to high residual stress

WELDING RELATEDTRAILER AXLE FAILURE

Crack initiation

Fatigue striations(beach marks)

Fracture tracks toe of weld (HAZ)

MECHANICAL FAILURESDUE TO LOOSENED NUTS

Dual set of truck wheelsDislodged from hub

Reamed holes from loosecontact with wheel studs

MECHANICAL FAILURESDUE TO LOOSENED NUTS

Impact bruising fromrotating contact with brake caliper

MECHANICAL FAILURESBY MANUFACTURING FAULTS

Normal thickness socket head

Reduced thickness socket head

FORENSIC CONSULTINGCONSIDERATIONS

• Is the work within your field of expertise?

• Has the client been transparent about the failure?

• Carefully consider the clients motives for undertaking the work?

• Is the failure subject to litigation?

• How to handle forensic investigations which are conflicting to your own?

• Always try to solicit a second opinion

• Resist pressure from the client to embellish the facts

• Keep concise records of your observations and retain evidence until the dust has settled (may take years)