Weld Failures - Part III
by, Dr. Jesse A. Grantham, Forensic Welding Engineer
Part 3. Forensic Welding Engineer
Weld failures are the result of mechanical effects and thermal effects.
Features associated with weld failures are cracks, micro-fissures, lack of ductility, non-metallic inclusions, laminations, brittle heat affected zones, degradation of micro-alloying elements, grain boundary precipitation of micro-alloy elements, hydrogen dissipation, cold-forming stresses, and fatigue stresses.
The Forensic Welding Engineer must have background, education, experience and certifications in all aspects of weld failure assessments.
One test is worth a thousand expert opinions. The Forensic Welding Engineer knows to conduct testing and to observe the movement of stresses and microscopic dislocations within the metal matrix.
The Forensic Welding Engineer studies the weld, adjacent metal and conditions involved in each weld failure. He must determine the mechanical effects and thermal effects for crack initiation site, propagation driving forces, and corrosion effects. These are common assessments for a forensic expert’s analysis of a failed weld. The forensic expert knows that though the failure was in a pipe, ship, bridge or airplane, these fundamental weld metal analysis techniques must be applied to truly evaluate the failure event.
The Forensic Welding Engineer is experienced in weld failure assessment techniques, investigations, analysis, reconstruction, causality, prevention, risk assessments, and consequences. From the inception of a weld designation, there are legal aspects of the failure of an engineered product.
The Forensic Welding Engineer must ask questions and listen to responses, take photographs, write reports, and understand welding standards, specifications and codes.
Accident investigation and failure analysis in the Forensic Welding Engineer framework requires background research, documentation of the incident and accumulation of evidence.
In the laboratory measurements of metal behavior by testing an exemplar weld and welding simulations. Practical experience and education are the answer to the success of every forensic investigation. The cost of education and experience may be expensive, but the cost of ignorance is more.
Weld metallurgy is the science of welds and metals. The behavior of welds can be predicted. Forecasting the internal actions of metal (steel) during thermal cycles and welding is common. Metallurgists can predict what metal will do as it is heated, cooled and welded. Metallurgists are challenged to increase the strength of a metal without also increasing the metal’s brittleness.
Forensic weld failure investigations require complete focus on the details of a weld failure. Often, reverse engineering is used with consideration for the choices made by the original designers. Personal inspection of the part and surroundings for traces evidence as well as conversations with eye-witnesses. Practical working knowledge of appropriate industry standards for welding. A review of similar failures in historical records. Assessments of an exemplar item allow the expert to find out what could have happened. Careful examinations of the subject failed component and exemplars.
Numerous catastrophes are the result of incorrect material in the product. The forensic expert confirms the type of metal actually incorporated into a failed metal component, affirms design considerations, exposure to in-service conditions, as well as flaws that became defects.
The Forensic Welding Engineer knows to examine failed metals efficiently, consider the designer’s intent and confirm use of the best suited metals and connections. The Forensic Expert also uses many bits of practical information that are seldom taught in schools or found in handbooks. Common definitions associated with welds are:
Standard: A guide established by authority, custom or general consent as a model or example to be followed. Applies collectively to recommended practices, classifications, and methods for a welding process or applications. A standard may be a rule, which is a prescribed guide for conduct or action, an accepted procedure, custom, or habit having the force of a regulation.
Specification: a detailed precise, explicit presentation of something or a plan or proposal of something. Clearly and accurately describes the essential technical requirement for a material, product, system or service. It indicates the procedures, methods, qualifications or equipment by which it can be determined that the requirements have been met.
Code: A systematic statement of a body of law. The code is a specification that is intended to be mandatory and a requirement by the authority having jurisdiction. A systematically arranged, comprehensive set of rules, standards and specifications for welding applications, published to secure uniformity and to protect the public. Established and enforced usually by a public agency. Consists of a set of conditions and requirements relating to a particular subject and indicating appropriate procedures by which it can be determined that the requirements were met.
The Forensic Welding Engineer gathers metal samples from the failure site or the failed component for independent alloy validation, nondestructive tests and mechanical tests. Comparison of a failed metal piece with the engineering design documents, specifications and material test reports often reveals clues to the cause of a weld failure event.
Evidence shows that in most fields of human activity, and particularly those which are most advanced mechanically, there is a general improvement of safety with the passage of time and a proportional reduction in the accident rates.
We must accept the rising standards of welding technology and relinquish comfortable routines and practices rendered obsolete because they no longer achieve the standards for improved metal.
All welds must realistically address the designer’s intent, the installer’s capability and comply with the codes, standards and specifications.
A weld failure incident is of most interest when you are personally affected or inconvenienced by the costs of the weld failure event.