Metallurgical Failure Analysis 101 Expert Overview

A metal component can fail in numerous ways. Sometimes, failure involves fracture. Other times, failure can include loss of function, wear, corrosion, and distortion.

This article provides an overview of some typical failure analysis methods as well as the role of the Metallurgical Expert in a forensic evaluation. Depending upon the direction of the investigation, the sequence of the evaluation may vary.

Typical Elements of a Metallurgical Failure Analysis

Background

The first step in a failure investigation involves gathering background information, such as:

• Material(s) involved
• Manufacturing process(es)
• Circumstances surrounding the failure
• Engineering drawings
• Associated specifications
• Other available background data

In addition to the failed component, it is also helpful to have an intact, unused, “exemplar” component for comparison. An experienced failure analyst can assist in the discovery process to obtain relevant documentation.

Inspection

Visual examination

Visual examination is usually performed with low power magnification and controlled lighting. It is important at this stage to fully document the “as-received” condition and photograph overall fracture and position.

For proper evaluation it is important to prevent wear of the fracture surfaces. The failed pieces should not be placed back together, as that can damage fracture features.

Because metallic parts are prone to oxidation, which is a reaction between the metal and the oxygen in the air, failed components should ideally be examined as soon as possible or at least kept inside in dry conditions.

Testing

First and foremost, a test protocol must be developed and agreed to by all parties.

Nondestructive Evaluation/ Nondestructive Testing (NDE/NDT)

Depending on the metal alloy, various types of nondestructive inspection can be performed.

Nondestructive inspection can reveal discontinuities or additional cracking in the component. The most common types of NDE/NDT are:

• Liquid Penetrant Testing (PT)
• Fluorescent Penetrant Inspection (FPI)
• Magnetic Particle Inspection (MPI)
• Acoustic Emission Testing (AE)
• Radiographic Testing (RT)
• Ultrasonic Testing (UT)
• Computed tomography (CT)

Macroscopic examination

A stereomicroscope (1-50X) is often utilized for the macroscopic exam. It is during this step that the fracture surface is evaluated.

The first piece of information often observed is where the fracture initiated, that is, locating the fracture “origin” or “origins” and determining whether it/they are located at the surface or below the surface.

It is also important to note the fracture direction in relation to the normal or expected loading. Markings on the fracture surface formed during the initiation and propagation of the crack can be used to evaluate the fracture and determine the origin. Examination of the origin by the experienced failure analyst may reveal the cause and determine any contributing factors toward failure.

Fatigue Failure that occurred progressively, evident by the oxidation present on the fracture face. 20x magnification.

Microscopic examination

Microscopic examination is usually performed in a Scanning Electron Microscope or SEM. The SEM allows magnifications up to 50,000X and improves depth-of-field resolution.

A metallurgical expert can examine the fracture surface in the SEM and determine fracture topography while inspecting the origin area for anomalies. It is in the SEM that the Metallurgist can classify the fracture and determine the fracture type.

The elements present on the surface can also be gathered while in the SEM by Energy Dispersive X-Ray Analysis or EDS. This can aid in determining if there are contaminants or corrosive elements that may have contributed to the failure.

SEM image showing striations consistent with fatigue failure. 2500X magnification.

Destructive Evaluation

Destructive evaluation, in the forensic sense, includes any process that alters the evidence. This includes cleaning and some types of “nondestructive” testing. Significant care must be taken prior to any destructive testing. 

Mechanical testing

Mechanical testing determines properties of a material and its appropriate use in mechanical applications. Most metals have specified mechanical properties that can be evaluated through testing to determine if the component material meets specification.

Some ways to obtain mechanical properties include: tensile testing, hardness testing, compression testing, impact testing, fatigue testing, and fracture toughness testing, etc., depending on the application and performance requirements of the component.

Metallographic examination

Metallography is defined as the science of the constitution and structure (or microstructure) of a metal.

During metallographic inspection, the failure analyst or technician sections the area of interest usually through abrasive-wheel cutting and mounts the specimen without deformation.

The metallographic cross-section is then polished to a mirror-like finish. The sample is subsequently examined by the Metallurgist in a metallurgical microscope.

Microstructural examination may reveal cracks, non-metallic inclusions, segregation, porosity,  grain size or prior mechanical or thermal treatment. Metallography can be enhanced by chemical etching to reveal characteristics relevant to the Metallurgist.

Alloy 825 Cross Section before (left) and after (right) etching which reveals the crack propagates trans-granularly through the grains of the metal. 280x magnification.

Chemical analysis

Additionally, chemical analysis may be performed to determine bulk chemistry, local elemental concentration, surface corrosion products, and coating chemistry.

Chemical analysis can verify conformance to a standard or specification, detect impurities, identify alloys, and analyze trace elements. Analytical chemistry can be performed by a variety of techniques including optical emission spectroscopy, atomic emission spectroscopy, and inductively coupled plasma analysis.

Formulating a Conclusion

Subsequent to performing an analysis of the findings and possibly conducting a test to simulate the condition at failure, the metallurgist typically summarizes the evaluation including cause of failure and any contributing factors.

As detailed above, a metallurgical failure analysis can be technically demanding. It is valuable to get a metallurgical expert involved early in the legal process to maximize their contribution to multiple phases of litigation.

Materials Science / Metallurgy Investigations

The experts at Robson Forensic apply a rigorous scientific approach to determine the failure mode as well as how the failure occurred, how the failure appeared over time, and whether the failure was due to misuse, a material defect, a manufacturing defect, or a design defect. We conduct these investigations involving a broad range of materials, products, and industries.

For more information, submit an inquiry or call us at 800.813.6736.

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