Failure Investigation
Two of the critical goals in a failure investigation are to identify both the failure mechanism(s) and failure mode(s) that are present. Failure mechanisms are a key component in categorizing damage and failures.
Stages of a Failure Analysis
Depending on the nature of the failure and the availability of physical evidence or background information, there are stages that are common to all successful failure analysis. Steps may be followed by assessment of the damage and conditions leading to failure. These stages may differ depending on whether fracture, corrosion, and/or wear conditions are being investigated.
Generally, a failure analyst will start with a broad range of possible explanations but over time will narrow and refine the existing possibilities. Often, a likely theory develops during the course of the investigation. This can be helpful, but only if the investigator does not let the theory influence his or her objectivity.
The Principal Task of a failure analyst during a physical cause investigation is to identify the sequence of events involved in the failure. Technical skills and tools are required for such identification, but the analyst also needs a mental organizational framework that helps evaluate the significance of observations.
The basic steps are:
1 Collect data
2 Identify damage modes present
3 Identify possible damage mechanisms
4 Test to identify actual mechanisms that occurred
5 Identify which mechanism is primary and which is/are secondary
6 Identify possible root causes
7 Test to determine actual root cause
8 Evaluate and implement corrective actions
Collection of Background Data and Samples
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
Information about the failure:
Date and time of failure, temperature, and environment
Extent of damage, sequence of failure, and injuries
Stage of operation when failure occurred
Blueprints, photographs, or sketches of the failure and adjacent areas
Any service deviations that might have contributed to the failure
Opinions of operating personnel regarding the failure
But for the analysis of some components, it may be impractical or impossible for the failure analyst to visit the failure site. Under these circumstances, data and samples may be collected at the site by field engineers or by other personnel under the direction of the failure analyst. A field failure report sheet or checklist can be used to ensure that all pertinent information regarding the failure is recorded.
Visual Examination
The failed part, including all its fragments, should be subjected to a thorough visual examination before any cleaning is undertaken. For example, traces of paint or corrosion found on a portion of a fracture surface may provide evidence that the crack was present in the surface for some time before complete fracture occurred. The preliminary examination should begin with unaided visual inspection. The unaided eye has exceptional depth of focus, the ability to examine large areas rapidly and to detect changes of color and texture. If required, macroscopic examination is performed to document the main features by using low‐power magnification. 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. Documentation of damage and cracking surrounding a fracture, including damage patterns and crack origins, scores, scuffing, dents, distortions, evidence of plastic deformation, and fractures is an important step in assessing relevance to final failure.
Testing
First and foremost, a test protocol must be developed and agreed to by all parties. The protocol is usually separated into non‐destructive and destructive evaluation. Destructive evaluation, in the forensic sense, includes any process that alters the evidence. Testing also includes cleaning and some types of “non‐destructive” 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.
Non‐Destructive Evaluation/Non‐Destructive Testing (NDE/NDT)
Depending on the metal alloy, various types of non‐destructive inspection can be performed. Non‐destructive 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). All these tests are used to detect surface cracks and discontinuities. Radiography is used mainly for internal examination. A photographic record of the results of non‐destructive inspection is a necessary part of record keeping in the investigation.
Mechanical Testing
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.
Macroscopic Examination
Macroscopic examination is an extension of the visual examination and evaluates quality and homogeneity of the part. It is used to determine the origin of the failure and the type of fracture such as ductile, brittle, torsion or fatigue. Microstructural features can be used to assess internal quality, presence of hydrogen flakes, chemical segregation, hard cases, flow lines, and welds. A stereomicroscope (1–50×) is often utilized for the macroscopic examination. 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”