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Failure Analysis




Failure analysis is the process of collecting and analyzing data to determine the cause of a failure. QCML’s failure analyses are conducted by experienced metallurgical engineers with the support of our highly competent technical staff. We utilize sophisticated laboratory equipment to generate the test data when conducting failure analysis. Laboratory equipment used for chemical analysis and fractography include SEM/SEM-EDS, GC, ICP, FLAA, FTIR, OES and Digital Microscopes with quantitative imaging software. Mechanical and metallography tests include tension, compression, impact testing, microhardness, Rockwell and Brinell hardness testers. Our detailed failure analysis reports include all of our testing data, photographs, discussion of the test results along with the engineer’s evaluation that includes the mode of failure and practical solutions to prevent future failures.

Failure Analysis Process

Failure Analysis Process

Failure Investigations It’s More Than Broken Parts







SEM Microscope with Bruker EDS X-ray

Failure analysis has evolved into more than evaluating parts that have failed in the field. It can include assisting in

  • Comparison of similar parts between suppliers/competitors to determine why one is performing better or differently then the other.
  • Parts/components not performing as expected.
  • Evaluation of unexpected staining and corrosion issues.
  • Are parts wearing out prematurely
  • Evaluation of maintenance parts to reduce production down time.
  • Parts failing in the field.
  • Litigation pending and need failure analysis.

Typical Root Cause Failure Analysis Tests

Each failure investigation is unique and there is no one size fits all when determining the types of testing required to conduct a root cause failure analysis. Listed below are typical tests conducted in a routine failure analysis.

  • Chemical analysis by OES, FLAA, ICP, SEM-EDS, XRF to determine the chemistry of the material(s).
  • Mechanical tests such as; tensile, charpy impact, Brinell, Rockwell, and Microhardness (HK, HV).
  • Microhardness on welds, HAZ, base material, case depth analysis.
  • Microstructure
  • Fractography by SEM to evaluate fracture surface.
  • Analysis of corrosion products by SEM-EDS.
  • Weld evaluation by microstructure and digital optical microscope.

Fractography’s Role In Failure Analysis

Fractography is the study of fracture surfaces of materials and is a key element in any failure analysis. It requires an experienced metallurgical engineer to evaluate a fracture surface and identify the various characteristics associated with specific types of fractures (e.g. fatigue, stress corrosion cracking, hydrogen embrittlement). Q.C. Metallurgical Laboratory Inc. has been specializing in failure analysis for over 30 years. Our metallurgical engineers and technicians have conducted countless root cause failure analysis. Q.C. Metallurgical Laboratory Inc. A2LA Scope of Accreditation includes failure analysis along with the mechanical and chemical testing required to conduct a complete root cause failure analysis. Fracture surfaces are evaluated by optical microscope and by Scanning Electron Microscopy (SEM). Once the mode(s) of fracture are identified, the metallurgical engineer evaluates all of the additional information from historical usage, mechanical, metallographic and chemical testing to determine the mode of failure.




Arrow indicates origin of fracture



Mag 1000X, Two modes of fracture are identified, ductile dimple and brittle transgranular cleavage



Microstructure Mag 100X
Arrow indicates corrosion pit where the branched transgranular cracks initiated.



SEM 1000X, Arrow indicates micro striations in fatigue fracture

Types of Fractures

Types of FracturesTypical Fracture Characteristics
Ductile Cup and Cone
Dimples
Dull Surface
Inclusion at the bottom of the dimple
Brittle Intergranular Shiny
Grain Boundary cracking
Brittle Transgranular Shiny
Cleavage fractures
Flat
Fatigue Beachmarks
Striations (SEM)
Initiation sites
Propagation area
Zone of final fracture


Modes of Failure

Once the following failure characteristics are established; the mode of fracture(s), origin of the crack and cause of crack initiation, the engineer will determine mode of failure. Listed below are various modes of failure.

  • Fatigue
  • Creep
  • Rupture
  • Cracking
  • Embrittlement
  • Erosion Corrosion
  • Oxygen Pitting
  • HIC
  • Hydrogen Embrittlement
  • Hydrogen Assisted Cracking
  • Stress Corrosion Cracking
  • Corrosion Fatigue
  • Caustic Cracking
  • Caustic Embrittlement
  • Stress Corrosion
  • Sulfide Stress Cracking
  • Stress Accelerated Corrosion
  • Hydrogen Stress Cracking

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A2LA Accredited for Failure Analysis



Mag 10X, Fracture surface of bolt


Mag 500X, Failure mode is a mixture of brittle transgranular cleavage and ductile dimples


Mag 1500X, Arrows indicate intergranular fracture along grain boundaries



Failure Mode
Hydrogen Embrittlement

The following photographs are from a bolt that failed when torqued to the appropriate ft-lb. The mode of fracture is a mixture of brittle intergranular, brittle cleavage and ductile dimples that is characteristic of hydrogen embrittlement. High strength steels are susceptible to hydrogen embrittlement when exposed to process that may allow hydrogen to diffuse into the steel. Manufacturing processes such as cathodic protection, electro-coating, phosphate plating and acid pickling can result in the entrapment of hydrogen in the steels.

Ductile Dimples and Brittle Transgranular Cleavage

The brittle transgranular cleavage facets are typically flat or angular and require more energy to rupture than the intergranular fracture but typically less than the ductile mode. The ductile dimples are cup-like features caused by microvoid coalescence during tensile rupture that require the most energy to fracture.

Intergranular Fracture

The intergranular mode shows cracking along the prior austenitic grain boundaries where hydrogen can become entrapped. Intergranular fracture is the least desirable fracture mode. This is because it requires the least amount of energy to fracture when compared to the transgranular and dimple modes that are indicated in the above photographs.

Failed spindle shaft, fracture occurred within the reduced section of the shaft. The change in diameter concentrates the stress in the fillet area.

Failure Mode
Fatigue Fracture

The following photographs are from a shaft that failed on a spindle. The cause of the failure was due to a reversed bending fatigue cracking, caused by cyclic loading at the outer surface where there was a change in diameter of the shaft.

Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands (PSBs), and grain interfaces. Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture.

Ratchet Marks

Mag 10X, Arrows indicate ratchet marks on the outer surface

Ratchet marks are a good indication of fatigue fracture. These marks originate when multiple cracks, nucleated at different points, join together, creating steps on the fracture surface.

 

Mag 100X, Microstructure is pearlite (dark) with grain boundary ferrite (light). There is secondary cracking (arrow) near the fracture surface.

Photograph of failed shaft, the two dark areas located to the left and right is the initiation site of the failure.

This microstructure is not the most desirable to prevent fatigue failures. The grain boundary ferrite network can cause lower ductility and toughness than a normal ferrite and pearlite structure.

 

This area shows primarily brittle cleavage facets with some dimples.

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