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Abstract
Considerable developments in metal failure analysis techniques and reliability are emerging with legal community acceptance of traditional methodologies in mechanical tests and nondestructive tests. Mechanical tests assess metal properties such as ductility, tensile strength, toughness and weldability. Nondestructive tests provide surface and sub-surface flaw detection, location and features. Verification of metal properties by actual tests is just a good policy for the Forensic Engineer.
Introduction
One test is better than a thousand expert opinions. The seasoned Forensic Engineer knows that quality satisfies design and quality cannot be inspected into a part. During manufacturing a metal item, the responsibility of management is to evaluate all costs  and benefits of production activities, including exposure to product-liability issues because of their decisions.
Extending service-life and fitness-for-service analyses are achieving wider interest by governments, engineers and the legal community. The economic advantages of avoiding business interruptions, personal injuries, and extending service life justify the use of traditional mechanical tests and nondestructive tests. The Forensic Engineer often makes decisions using precise fundamental principles, while answers to ethical questions are often a shade of gray.
The Forensic Engineer knows to accurately examine failed metals, the designer’s intent must be understood, then confirm that the proper metals and processes were used. The experienced Forensic Engineer also applies many bits of practical information that are seldom taught in schools or found in handbooks.
Many industries use mechanical tests and nondestructive tests to eliminate defective incoming material from manufacturing processes. During production, routine metal tests are used to check for flaws while processing an item, eliminating wasted time and material when defects are located at the earliest possible stage in the cycle. When the part is finished, customer satisfaction is the primary goal of all interim metal tests.
 Metal classifications include appearance, dimensions, weight, magnetic properties, chemical composition, mechanical properties, thermal coefficients, corrosion resistance, electrical conductivity, and others. There are numerous methods used to identify and confirm that the proper metal, without defects, is used in the item being produced.
Personnel performing mechanical tests and nondestructive tests need to be qualified by education, experience and routine training. Inspectors must pass written examinations and hands-on proficiency tests to obtain recognized certification that is acceptable in the legal community.
During an examination, the Forensic Engineer must consider the statistical probability of flaw detection, which is defined as the probability that, using a given inspection procedure, a trained inspector will detect a flaw if it exists. The degree of confidence in the probability of detection may refer to the ability to estimate from a limited sample the probability of detection representative of large-scale inspection. When examining an item for unacceptable flaws, the Forensic Engineer considers many topics, including the type of test, confidence level, flaw type, flaw characteristic range and interval, flaw-free parts, sample size requirements, test procedures, and evaluation decisions. During a metal failure analysis, proper assessment methods, flaw description and reliability of tests are the Forensic Engineer’s responsibility. Validation of metal properties by proper tests is just a good, standard policy for the Forensic Engineer.
Mechanical Tests
 The Forensic Engineer relies on technical reports about metal tests containing terms that have different meanings to different people. The Forensic Engineer’s reports attempt to make complex topics simple to understand by the non-technical, legal community.
Several mechanical tests used by the Forensic Engineer involve “direct measurement” of metal properties. The Annual Book of ASTM Standards Volume 03.01 contains descriptions of standards and specifications for mechanical tests. Several of these tests are listed:
- Bends
- Hardness – Rockwell, Brinell, Vickers, Knoop
- Tensile – Yield, Ultimate, elongation reduction of area
- Charpy V-notch - toughness at specified low temperatures, DBTT (Ductile to Brittle Transition Temperature)
- Alloy Analysis by Optical Emission Spectroscopy (OES).
- Weldability
- Other mechanical tests
Mechanical tests encompass much more than direct or indirect measurement of dimensions or visual judgment of the appearance of a product. Inevitably, there is some vital characteristic of almost every manufactured part that cannot be measured directly without rendering the part unfit for use.
Nondestructive Tests
Most of the vital characteristics of an item can be determined indirectly or can be inferred from the measurement of some other characteristic. Often, such nondestructive measurements can be made without harm to the part. Nondestructive tests (NDT) are widely used to indirectly examine metal in an attempt to locate surface discontinuities or subsurface indications. NDT is used for many reasons, including:
 to ensure product reliability- to prevent accidents
- to make a profit for the user
- to ensure customer satisfaction
- to aid in better product design
- to control manufacturing processes
- to lower manufacturing costs
- to maintain a uniform quality level
NDT is a powerful tool for reducing costs, improving quality and maintaining quality levels. The prime reason for using NDT is not the threat of catastrophic failure of an item rather it is a basic responsibility of management to operate at a profit. Profits can only be ensured by effective programs leading to reduced material and production costs; elimination of wasted labor and machine time; and assurance of quality that eliminates rejects and avoids expensive litigation.
The choice of NDT methods depends on the type of flaw anticipated, as well as the nondestructive inspection location, in the shop or field. Visual Tests (VT) is the most common NDT and least costly inspection method for locating discontinuities on base metal, welds and adjacent surfaces. VT may be used with the unaided eye in ambient light or with magnification tools and fluorescent lighting. Surface discontinuities are typically located by these methods;
- Visual in adequate light
- Magnetic Particle Tests (MT)
- Liquid Penetrant Tests (PT)
- Other NDT methods
Finished products, such as rolled plate, forgings, castings, and welds, may be examined for sub-surface (volumetric) discontinuities by these methods;
- Ultrasonic Tests (UT)
- Radiographic Tests (RT)
- Other NDT methods
Use caution when indirectly assessing quality by NDT methods. Indirect examinations and measurements require interpretation, which introduces a factor of human subjectivity usually not present when quality is measured directly. There must be a known relationship between the characteristic actually being sought by NDT and the quality feature that is believed to be a primary cause of metal failure. NDT results provide excessive information that may have no substantive meaning in a case.
NDT implies a technique for finding “defective” parts while avoiding damage to good parts. In this context, the word “defective” means failure to possess one or more of the specified quality characteristics, in other words, nonconformance to specifications. In recent years, the words “defective” and “defect” have been interpreted as meaning unfit-for-service, and their use is declining in favor of terms like discontinuity, flaw, imperfection, indication and nonconformance. All these terms are synonymous, although ”nonconformance” refers only to specified characteristics whereas the other terms are more general.
Test Results
The Forensic Engineer recognizes the need for accreditation of each mechanical test laboratory as a necessity for acceptance of test data for the litigious matters. Laboratory accreditation is addressed in the ASM Handbook Volume 8, Mechanical Tests and Evaluation. Laboratory accreditation is a procedure by which an authoritative body gives formal recognition that a laboratory is competent to carry out specific tasks. There is a list of minimum requirements for agencies performing NDT contained in a standard practice, ASTM E543. This practice can be used as a fundamental basis to evaluate and qualify testing agencies. The work- product of a metals test laboratory is data and confidence in accuracy of the data.
The effectiveness of test applications depends upon the capabilities of the personnel who are responsible for and perform the mechanical and nondestructive tests. There are standardized recommended practices available in industry to establish guidelines for the qualification and certification of test personnel whose specific jobs require appropriate knowledge of the technical principles underlying the tests they perform, witness, monitor or evaluate. It is recognized that guidelines vary by industry applications and between employer circumstances. When selecting a test laboratory, the Forensic Engineer should review the required test procedures and personnel records to validate the technicians, as necessary, to meet his project’s specific needs.
Accuracy and repeatability of measurement devices used in metals tests are vital to the Forensic Engineer. Test equipment may be affected by continued use, and maximum intervals between calibrations should be specified in written quality assurance procedures of the test laboratory. Verification standards and technician proficiency should be traceable to a recognized industry or accredited agency. The Forensic Engineer periodically reviews and audits the calibration and personnel records of each test laboratory used for failure analysis.
The Forensic Engineer gathers metal samples from the failure site or the failed component for alloy analysis, mechanical tests and nondestructive tests. Comparison of a failed metal piece with the engineered design documents and specifications often reveals clues to the cause of a metal failure event. When possible, the Forensic Engineer studies an exemplar item, exactly like the subject item that failed.
The Forensic Engineer knows that anticipated quality of metal and welds varies between industries. Metal failures are often the result of inadequate design considerations based upon cost considerations and human errors originating from carelessness. With the advance of design sophistication and fast-track methodologies, metal failures have the potential to increase. Early savings in design and construction costs can easily boomerang later as litigation costs. The vulnerable metal structures sometimes built today provide work for the Forensic Engineer.
When the Forensic Engineer examines a failed part it is extremely useful to identify part failures that are the result of lot inspections of only a few pieces versus 100% inspection of every item.
Acceptance sampling or lot acceptance by inspecting samples is common in high volume manufacturing operations. The cost to inspect every part in a high volume manufacturing operation is often prohibitive.
Ethics
 Forensic Engineers take seriously their responsibility for ethical investigations and the welfare of the public. The guiding principles for the Forensic Engineer, a Registered Professional Engineer, are contained in the National Society of Professional Engineer’s Code of Ethics and the Engineer’s Creed. Forensic Engineers often make decisions using precise engineering principles, while answers to ethical questions are often varying shades of gray.
Forensic Engineering is an important and learned profession. As members of this profession, engineers are expected to exhibit the highest standards of honesty and integrity. The Forensic Engineer is dedicated to the determination and presentation of the truth. The Forensic Engineer performs under a standard of professional behavior that requires adherence to the highest principles of ethical conduct. The Forensic Engineer provides services only in his area of competence.
The Forensic Engineer knows that mechanical tests and nondestructive tests by a competent, independent laboratory is the only sure way to confirm the type of base metal or weld metal contained in failed items. It may be necessary for a number of test laboratories to perform tests to statistically validate the results and establish a statistical confidence level for the investigation.
The Forensic Engineer presents detailed technical information to the legal system and therefore he must select, use and interpret test methods selected for proper evaluation of a failed part or assembly. The Forensic Engineer’s decisions about desirable or undesirable flaws must be consistent with the product designer’s intent.
The Forensic Engineer’s final work-product may be an oral testimony or a written report. The Forensic Engineer is charged with the responsibility to find the truth, know the truth and tell the truth. One test is worth a thousand expert opinions.
References:
- CHOICES, by Jesse A. Grantham, P.E., NAFE 597M
- ASM Handbook Volume 8, Mechanical Tests and Evaluation.
- ASM Handbook Volume 11, Nondestructive Testing.
- ASNT(American Society of Nondestructive Testing) SNT-TC-1A Recommended Practice.
- NSPE (National Society of Professional Engineers) Code of Ethics for Engineers.
- The Annual Book of ASTM Standards Volume 03.01 Standard Specifications, Metals - Mechanical Testing.
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