Failure Analysis Testing

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Failure analysis is a broad term covering the analysis of failures of small parts to large systems. Metallurgical Consulting uses its training and experience in the fields of metallurgy and materials, fracture mechanics, corrosion, stress analysis, and welding to provide an engineering analysis of failures. The goal is to go beyond identification of fracture or corrosion modes and determine what the design or service factors were that set up conditions for failure. This requires an understanding of the system and how it works, and most importantly, how it actually worked. A strong background in engineering and 35 years of experience in consulting engineering failure analysis has enabled Dr. Clarke of Metallurgical Consulting to evaluate failures in a wide range of areas using this fundamentals based approach.

Dr. Clarke uses a rigorous approach to analyzing mechanical component failures. This method requires an estimate of the stresses or forces at the time of failure from the fracture surfaces. The fracture surface stress estimate must then be compared to design stress or known stresses/forces at the time of failure. If the two estimates do not agree, the two must be reconciled. It has been frequently found using this type of approach that actual forces at the time of failure were actually in excess of design.

Accurately determining stresses from the fracture surface requires estimating stresses from fracture surface features such as fracture mode, fatigue striation spacing, and net section ductility. Linear elastic fracture mechanics is used for brittle materials. Tearing instability has also been used for steel pressure vessels. Plastic analysis has been used to estimate failure stresses on ductile materials. (See papers on saw arbor and automotive axle failures.)

Dr. Clarke's extensive experience with fatigue and electron microscopy has enabled him to successfully estimate stress ranges at the start of fatigue crack growth by measuring fatigue striation spacing. Many of These analyses were independently validated. Metallurgy sometimes can play a strong role in failure stresses as shown in the article on the pole gin failure. Finally, this approach can quantitatively address the question of whether an imperfection is a true defect or was just exposed by the failure. The article on the tree stand failure describes two cases where the imperfections were serious but failure still occurred at loads well in excess of the tree stand rated capacity.

Corrosion failures usually require a thorough understanding of operating conditions. Factors that are often important for corrosion are usually over looked by plant operators or others involved in the problem. Actual in situ corrosion data on site is valuable because published data give only average values. Dr. Clarke has used DC potentiodynamic corrosion testing in situ for years to acquire real time data. This data frequently shows very transient corrosion conditions. Stress corrosion cracking conditions have been identified with startup conditions in batch reactors. The resultant changes in start-up conditions have avoided the need for more resistant (and expensive) alloys. Potentiodynamic corrosion testing on rain water plus salt was used to develop data showing that a clogged drain after a hurricane could produce corrosion rates high enough to account for rapid metal loss in the canopy on a gas station and its eventual collapse during a heavy rain.

Failure analyses involving weldments and castings frequently identify imperfections as the cause of failure without addressing stresses. Metallurgical Consulting has extensive experience with the analysis of weldments and castings with imperfections and/or code discrepancies. Steel weldments in particular can often contain imperfections and yet be adequate for intended service. Metallurgical Consulting has used different types of stress analyses (including strain gauges) to evaluate weld and casting failures to determine the actual forces/stresses involved in the failure. Results have frequently revealed stresses well in excess of design at failure.

For more information on all of Metallurgical Consulting failure analysis capabilities, see the table below or contact Metallurgical Consultants directly.


Failure Analysis Testing Capabilities

Analysis/Capability
Metallurgy
Materials Science
Welding
Corrosion
Fracture Mechanics
Stress Analysis
Strain Gauge Testing
Fatigue Analysis
Full Root Cause Failure Analysis
Product Areas
Trucks
Automobiles
Aircraft
Ships
Pressure Vessels
Machinery
Paper Industry
Chemical Process Industry
Power Generation
Litigation
Experience
Since 1973
Equipment
Large Chamber Scanning Electron Microscope
X ray with SEM
Hydraulic Ram for full Scale Tests
Load Cells
Multi-Channel High Speed Data Acquisition
DC&AC Corrosion Testing Equipment
Optical Microscopes
Specimen Preparation Equipment
Information to Work from
Engineering Drawings & Specifications
Service History
Exemplars or Similar Situations
Service Environment
Failure Determination
Fracture Surface Analysis
Microstructural Analysis
Non-Destructive Evaluations
Stress Analysis
Industry Focus
Litigation
Chemical
Paper
Ship Building
Aircraft
Power Generation
Oil Field
Types of Experience
Full scale testing
Pressure Vessel
Coatings
Automotive Welds
Ugly Looking Welds
Tree Stands
Tube Rolling in Heat Exchangers
Automotive Axles
Mechanical Equipment
Fire Related
Bolting / Fasteners

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