Failure Analysis and Fracture Mechanics

Edward Dalder is an experienced failure analysis engineer. His firm, Dalder Materials Consulting, Inc. offers failure analysis consulting and fracture mechanics services. He has conducted analysis of failures of welded aluminum alloy vacuum vessel, pressure vessels, and transportation vehicles. He has analyzed failures in bridges, structural steel lifting fixtures and cranes, and in stainless steel structures.


bent ship showing brittle fracture

In January 1943, the one-day old tanker SS Schenectady returned to the dock after completing sea trials. Without warning, the decks and sides of the vessel fractured aft of the bridge. The vessel jack-knifed and the center portion rose so that no water entered the hull. The bow and stern settled into the river-bottom.


Ed Dalder, as a failure analysis engineer, uses the techniques and methods of metallurgical analysis of failure in developing answers as to the cause(s) of failure of engineering parts and systems. “Metallurgical failure analysis” is the process by which a failure analysis engineer determines the mechanism that has caused a component to fail. Typical failure modes involve various types of corrosion and mechanical damage. Metal components fail as a result of the environmental conditions to which they are exposed to as well as the mechanical stresses that they experience. Often a combination of both environmental conditions and stress will cause failure.

A metallurgical failure analysis, conducted by a failure analysis consultant such as Ed Dalder, takes into account as much of this information as possible during analysis. Analysis of a failed part can be done using destructive testing or non-destructive testing. Destructive testing involves removing a metal component from service and sectioning the component for analysis. Destructive testing gives the failure analysis consultant the ability to conduct the analysis in a laboratory setting and perform tests, such as chemical analysis and various types of microscopic examination on the material that will ultimately destroy the component. Non destructive testing describes a collection of test methods that allows certain physical properties of metals, such as the presence of low-density regions caused by porosity and cracks, to be examined without taking the samples completely out of service. NDT is generally used to detect failures in components before the component fails catastrophically.

Ed Dalder uses forensic inquiry into the failed process or product as the starting point of failure analysis. Such inquiries are conducted using scientific analytical methods such as electrical and mechanical measurements, and by analysing failure data such as product reject reports or examples of previous failures of the same kind. The methods of material failure analysis are especially valuable in tracing product defects and flaws. These flaws may include fatigue cracks, brittle cracks produced by stress corrosion cracking or environmental stress cracking for example. Witness statements can be valuable for reconstructing the likely sequence of events and hence the chain of cause and effect. Human factors can also be assessed when the cause of the failure is determined. There are several useful methods to prevent product failures occurring in the first place, including failure mode and effects analysis (FMEA) and fault tree analysis (FTA), methods which can be used during prototyping to analyse failures before a product is marketed. Difficult to determine modes of failure can be attributed to oxidation, defective connections of electrical components, temporary shorts or opens in the circuits, software bugs, temporary environmental factors, but also to operator error.

Fracture Mechanics and How It Is Used in Failure Analysis

Fracture Mechanics is an area of technology that quantitatively describes the relationships between flaw-size, material-toughness, and stress(applied and residual). Knowledge of fracture-mechanics enables the material failure analyst to determine the relative importance of each of these factors during design.

flaw-size, material-toughness, and stress

The reluctance of large segments of the design community to incorporate fracture-mechanics into the design-process greatly contributes to structural failure. This results in a growing need for failure analysis consulting of the type that is performed by a failure analysis engineer such as Ed Dalder.

A fracture mechanics consultant, such as Ed Dalder, uses the principles of fracture-mechanics to relate the three prime variables of flaw-size, material-toughness, and stress in design to reduce the chance of failure. He also uses fracture-mechanics, along with many of the tools available to failure analysis engineers, to conduct failure analyses. of these factors during design, operation, and failure.

 


bridge

Silver Bridge before

On December 16, 1967, the “Silver Bridge” over the Ohio River between Point Pleasant, Va. and Gallipolis, Ohio, collapsed without warning.

fractured bridge

Silver Bridge after

Analysis of the possible causes of failure pointed towards corrosion of brittle, high-strength, steel eye-bars from which the road-way was hung. A combination of careful re-construction of the event, together with the use of fracture-mechanics, provided the answer to this tragedy.


Edward Dalder uses fracture-mechanics to relate the three prime variables of flaw-size, material-toughness, and stress in design to reduce the chance of failure. He also uses fracture-mechanics, along with may of the tools available to materials engineers, to conduct failure analyses.