Weld Failures - Part II
by, Dr. Jesse A. Grantham, Forensic Welding Engineer
Part 2. Welds
Weld failures are not just accidents or acts of God. Weld failures are the result of human errors originating from unrealistic delivery schedules, oversight, carelessness, ignorance or greed. With the advance of design sophistication and fast-track methodologies also came the proliferation of metal failures. Early savings in design and construction costs can easily boomerang as later and larger costs of repair and litigation. The vulnerable metal structures of the late 20th century can provide “bread and butter” to Forensic Experts of the 21st century.
Carbon Steel is a metal alloy that contains specific elements which are added to the iron matrix. Most of the alloying elements are added to the iron in small amounts, but these alloying elements have a tremendous effect on the final properties of the steel. When alloying elements are added to a metal matrix, the metal’s properties are enhanced without decreasing other desirable properties.
The ability of the metal to be welded in a production environment, with mechanical soundness and the serviceability of the resulting weldment is affected by the metal alloying elements.
Most welding, brazing or soldering processes require metallurgical compatibility that can be achieved by using clean base metals, controlled thermal conditions, and the addition of filler metal by skilled personnel.
In the old days, designers worked empirically with welders, who used past experiences and a practical approach to welded fabrication. This practice became self-defeating by invariably resulting in excessively heavy designs and high fabrication costs. Today, designs are based on computer generated calculations intended to maximize efficient use of metal properties. Engineered safety factor analysis methods are readily available to assist with the prediction of metal fatigue conditions and avoid metal fracture conditions.
The metals used in welded products and welding processes are described by a variety of accepted standards and specifications. The Forensic Expert knows to look for quality deviations in all aspects of metal processing and fabrication. Tests and inspection methods are specified by mortals and may be totally irrelevant to confirming the suitability of a metal component for the intended service. The actual metal used in a product must achieve the designer’s intent.
Applications of metal assessment models and computer simulations must fit real-world choices. The Forensic Expert knows to develop and study the computer analysis as well as consider the published, historical requirements for metals to determine quality, uniformity or interchangeability. These documents may be codes, standards, specifications and professional association with experienced Forensic Experts.
Managers, supervisors and attorneys don’t have experience or understand the history of past weld failures. After a new failure occurs, there are a “hurry-up meetings”, examinations of a dirty, broken welded pieces, and the resounding questions, “Why did the weld fail and who is to blame?”
Engineering Catastrophes and Metals Investigations include the causes and effects of major accidents. Several of these accidents are:
- Challenger Space Vehicle that disintegrated upon re-entry to the earth’s atmosphere the catastrophic Space Vehicle launch attributed to O-rings on the fuel container.
- Loss of the RMS Titanic ship, brittle steel fracture at rivets.
- Semi-submersible oil rig (Alexander L. Kielland) collapse from fatigue failure at the toe of a fillet weld in a diagonal brace.
- Offshore platform (Piper Alpha) in water depth of 474 feet experienced an explosion, fire during a safety audit.
- Liberty ships during WW II, design of hatch corners on the top deck.
- Bridges, buildings and civil structures, cracked members and in-service weld failures
- Water heater boiler explosions – one per day in the 1800’s which began of the ASME boiler and pressure vessel rules and regulations by Insurance companies.
- Macondo weld blowout in the Gulf of Mexico.
Weld failures have received more attention over the past 40 years, primarily because of concerns related to public safety, environment and equipment reliability. In addition, weld failures have a strong impact on economic changes and pressure exerted by expanded competition on the global market. In the United States, increases in litigation have forced commercial entities to investigate weld failures and identify the root causes to avoid repeated events and potential liability
It is a common statement in the welding business that the steel complied with the specifications until someone welded on it.
Weld life prediction and fitness-for-service analyses have achieved wider acceptance by the engineering profession and the legal community. The economic advantages of avoiding business interruptions, extending equipment life and saving time required to build replacement components justifies the use of advanced forensic techniques.
The standard mechanical laboratory tests are still used to accurately assess welds and metal properties in bending, tension, toughness, hardness and micro-structures. Nondestructive tests provide accurate surface and sub-surface flaw detection and defect sizing.
Many experienced engineers know that a great deal of important information and failure experience is locked up in the engineering files of companies fearing adverse publicity of their products. This is unfortunate since this unavailable data could be a great benefit to future engineering designers, metal selection considerations, fabrication techniques and general “know-how”.
Welds are classified in a variety of ways that include: appearance, dimensions, weight, magnetic properties, chemical composition, mechanical properties, thermal coefficients and electrical conductivity. There are numerous accepted methods used to select and identify welds and metals.
Fatigue of metals refers to changes in properties resulting from the application of cyclic loads and encompasses many engineering disciplines. Fatigue of metals is a major area of scientific and applied research. The term, fatigue, is widely accepted terminology in engineering vocabulary for the damage and failure of metals under cyclic loads. There are many forms of fatigue. Several these terms are: mechanical fatigue, creep-fatigue, thermo-mechanical fatigue, corrosion fatigue, sliding contact fatigue, rolling contact fatigue, and fretting fatigue.
The field of fracture mechanics was virtually nonexistent prior to 1940. Fracture mechanics emphasizes material testing as well as mathematical derivations. Fracture mechanics includes dimensional analysis, linear elastic and elastic-plastic considerations, fracture toughness, crack propagation, crack growth, micro-alloying mechanisms and standard material properties.