In this article, metallurgist, Dawn DiMarco provides an overview of the processes involved in a typical metallurgical failure analysis. Her discussion describes various properties of metals that can be examined, some of the tests and processes that are used in an analysis, as well as precautions that should be taken when handling or receiving failed components.
The experts at Robson Forensic frequently perform failure analyses on metals and other materials. Contact our experts to discuss your case and how a failure analysis may be relevant to your case.
Typical Elements of a Metallurgical Failure Analysis
There are numerous ways in which a component may fail. 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.
The first step in a failure investigation involves gathering background information. This includes material, manufacturing process, circumstances surrounding the failure, engineering drawings, associated specifications and other background data. In addition to the failed component, it is also helpful to have an intact, unused, “exemplar” for examination. An experienced failure analyst can assist in the discovery process to obtain relevant documentation.
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. The temptation to put fracture surfaces back together should be avoided as it can damage fracture features. Because metallic parts are prone to oxidation, a reaction between the metal and the oxygen in the air, failed components should ideally be examined as soon as possible.
First and foremost, a test protocol must be developed and agreed to by all parties. The protocol is usually separated into nondestructive and 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. Again, details of all testing, destructive and non-destructive must be shared and agreed to by all involved 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) and Ultrasonic Testing (UT).
Mechanical testing determines properties of a material when force is applied, therefore indicating its appropriate use in mechanical applications. The mechanical properties of metals can be expressed in numerous ways: strength, ductility, hardness, toughness, etc. The hardness and the strength of a material are closely related. Hardness is useful for estimating wear resistance and approximate strength; and is defined as the resistance of a material to surface indentation. Rockwell hardness is the most widely used method for determining hardness and several different Rockwell scales are utilized for materials of a variety of hardness ranges. Aluminum alloys, brass, and soft steels are often measured on the Rockwell B scale or HRB, whereas harder steels and titanium are measured on a Rockwell C scale, or HRC.
Other ways to obtain mechanical properties include: tensile testing, compression testing, impact testing, fatigue testing, and fracture toughness testing, etc., depending on the application and performance requirements of the component.
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 origin of cracking. Examination of the origin by the experienced failure analyst may reveal the cause and determine any contributing factors toward failure.
Microscopic examination is usually performed in a Scanning Electron Microscope or SEM. A Scanning Electron Microscope is a high-power magnifying and imaging instrument that uses an accelerated electron beam as a light source. 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. Semi-quantitative chemical information can also be gathered while in the SEM by Energy Dispersive X-Ray Analysis.
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. Examination reveals microstructural characteristics such as porosity, cracks, grain size, segregation, distribution of phases, and any non-metallic inclusions present. Microstructural examination may also reveal prior mechanical or thermal treatment. Metallography can be enhanced by chemical etching to reveal particular metallurgical characteristics relevant to the Metallurgist.
A chemical analysis is sometimes performed by the Metallurgical Expert 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, to name a few.
Subsequent to performing an analysis of the findings and possibly conducting a test under similar conditions, 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.
Metallurgist & Materials Science Expert
Dawn DiMarco is a career Metallurgist with extensive experience in failure analysis, metallurgical consulting, electron microscopy and materials testing on a wide range of materials and industries including aerospace and automotive. Dawn has over 25 years of experience analyzing failures in aerospace components, rotating hardware, and automotive parts. She has conducted over 1300 different material evaluations and is well-qualified to investigate a broad range of product and material failures.