What is Failure Analysis
Failure analysis is the process of collecting and analyzing data to determine the cause of a failure, often with the goal of determining corrective actions & advise preventive actions.
Carrying a failure analysis is being like a detective. Important clues and footprints are discovered throughout the investigation that offers insight into what may have caused the failure, attributed to the failure and what other contributing factors may have been involved.
The failure analyst is aided by a broad knowledge of materials, process, and damage mechanisms in general. Success is more likely if the analyst is aware of the failed material’s mechanical and physical properties and its fabrication and historical performance characteristics.
A component is considered to have failed when it has deteriorated to the point at which it is unsafe or only marginally capable of performing its intended function. For an item to be classified as a failure it need not be completely broken.
Steps for the failure analysis
Step 1: Determine when, where and how the failure occurred
Determine when, where and how the failure occurred / Sample collection for laboratory examination / Taking pictures for future reference.
1. Visit the failure site in the field if possible.
2. All operators involved in the failure should be interviewed personally.
3. Determine the conditions were at the time of failure.
4. Were there prior indications suggesting failure was about to occur?
5. Was the failure gradual or catastrophic?
6. Was the part protected after failure?
7. How was the fracture handled?
Samples selected should be characteristic of the material and contain a representation of the failure or corrosive attack.
Sampling handling is a paramount issue on which the whole remaining analysis depends.
Fracture surfaces must be protected from damage during shipment by rigorously careful packaging.
Photographs should be taken of the failed piece of equipment including the samples to be removed and their surroundings. These should show the relationship of the questioned area to the remainder of the piece of equipment.
The dimensions of the sample, the date the failure occurred, and the date of the photographs should be noted. Consider the use of video recording if complex disassembly is required.
Step 2: Visually and NDT examination of sample
Visually examine the sample. Examine the sample with unaided eye, hand lens and/or low magnification field microscopes.
Note the condition of the accessible surface documenting all sorts of anomalies, searching for cracks, corrosion damage, the presence of foreign material, erosion or wear damage, or evidence of impact or other distress.
Non-destructive technique such as radiography, magnetic particle, ultrasonic, liquid/dye penetrant, eddy current, leak, etc. could be employed for sample examination.
Step 3: Confirm material composition
Confirm material composition and identify contaminants through EDS analysis. EDS (Energy-Dispersive Spectroscopy) is an analytical method based on the differences in energy of the characteristic x-rays emitted by the various elements. It is used in conjunction with scanning electron microscopy (SEM) to identify the elements present at a particular spot on a sample. Advantages of EDS are that it is easily performed and is reliable as a qualitative method. Limitations are that it is only marginally useful as a quantitative method.
Step 4: Determine the type of failure
The major types of failures likely to be encountered by metals in service are:
B. Brittle, and
C. Fatigue fractures
A. Ductile fractures are characterized by tearing of metal accompanied by appreciable gross plastic deformation. The microstructure of the fracture surface is quite complex and may include both transgranular and intergranular fracture mechanisms.
Ductile fractures in most metals have a gray fibrous appearance and may be flat-faced (tensile overload) or slant-faced (shear). The specimen usually shows considerable elongation and possible reduction of cross-sectional area as well. Whether a part fails in a ductile or brittle fashion depends on the thickness of the part, temperature, strain rate and the presence of stress-raisers.
B. Brittle Fracture
Brittle fractures are characterized by rapid crack propagation without appreciable plastic deformation. If brittle fractures occur across particular crystallographic planes they are called Tran crystalline fracture. If along grain boundaries they are called intergranular fracture. Brittle fracture is promoted by:
• thicker section sizes,
• lower service temperatures, and
• increased strain rate.
A material’s tendency to fracture in a brittle mode can be determined by measuring its notch ductility. The most common test for this is the Charpy V-notch test.
C. Fatigue fracture
Fatigue is a progressive localized permanent structural change that occurs in a material subjected to repeated or fluctuating stresses well below the ultimate tensile strength (UTS). Fatigue fractures are caused by the simultaneous action of cyclic stress, tensile stress, and plastic strain, all three of which must be present. Cyclic stress initiates a crack and tensile stress propagates it. The final sudden failure of the remaining cross-section occurs by either shear or brittle fracture. Striations on the crack surface are the classic sign of fatigue fracture.
Step 5: Report writing
After all analysis has bee completed, it is very important to compile all findings and conclusion in the form of failure investigation report.
Recommendations to avoid similar failure should be included in the report.
Example of failure analysis case study
People on the Case
Mr. Philips is an American Bridge Society consultant and has a Masters in materials engineering.
Mrs. Chloe is the Branch Chief for Element Materials Testing Branch with 30 years’ experience as an engineer.
Dr. Stephen who is a Element consultant with 32 years of experience with metallurgy and failure analysis.
Steps for the failure analysis:
Visual Observation which is non-destructive examination (NDE). This revealed sign of brittleness with no permanent plastic deformation before the bars broke. Cracks were shown which were the final breaking point of the shear bars. The engineers suspected hydrogen was involved in producing the cracks.
Scanning Electron Microscopy which is the scanning of the cracked surfaces under high magnification to get a better understanding of the fracture. The full fracture happened after the rod couldn’t hold under load when the crack reached a critical size.
Micro Structural Examination where cross-sections were examined to reveal more information about interworking bonds of the metal.
Hardness Testing using two strategies, the Rockwell C Hardness and the vickers Microhardness which reveals that bars was not heat treated correctly.
Tensile Test tells the engineer the yield strength, tensile strength, and elongation was sufficient to pass the requirements. Multiple pieces were taken and performed by Galsco Inc.
Charpy V-Notch Impact Test shows the toughness of the steel by taking different samples of the rod and done by Glasco Inc.
Chemical Analysis was the Final Test also done by Glasco Inc. which met the requirements for that steel.
Conclusion of the failure analysis Case Study
The bars failed from hydrogen embrittlement which was susceptible to the hydrogen from the high tensile load and the hydrogen already in the material. The bars did not fail because they did not meet the requirements for strength in these bars. While they met requirements, the structure was inhomogeneous which caused different strengths and low toughness.
This study shows a couple of the many ways failure analysis can be done. It always starts with a nondestructive form of observation, like a crime scene & a detective working on it. Then pieces of the material are taken from the original piece which are used in different observations. Then destructive testing is done to find toughness and properties of the material to find exactly what went wrong.