White paper: Weld defects and failures

How finite element mathematical simulations can mitigate risk and cut costs

Heat exchanger with multiple cracks

Coupling spacers and sockets are susceptible to vibration-induced fatigue, stress corrosion cracking, severe mechanical loading, and torsional failures, each of which can contribute to pipe leakage and the release of volatile gasses. This article shares insight about managing risk specific to weld failures.

By Dr. William J. O’Donnell, Founder, O’Donnell Consulting Engineers, Inc.

The predictive power of finite element mathematical simulations can often prevent the loss of millions of dollars in downtime, industrial catastrophes, injuries and loss of life.

Cyclical stress factors

In materials science, fatigue refers to failure that results from cyclical stress. Most often, such failures manifest themselves in some form of mechanical or thermal fatigue, often as a result of vibration, loading and unloading, or repetitive fluctuations in temperature. These failures are often blamed on the designer or fabricator, when the real cause is transient operating conditions. For instance, operators often increase applied loads and temperatures in an effort to increase productivity; as a result, they inadvertently push the limits of fatigue damage and increase the risk of failures.

As a point of reference, it is estimated that fatigue contributes to approximately 90% of all mechanical service failures. An increasing demand for high performance industrial systems
has exacerbated the likelihood of structural fatigue.

Fatigue as the cause of weld failures

Weld failures present one of the most serious financial, safety and reputational threats across many sectors including: chemical, petrochemical, aerospace, automotive, construction and energy. Any component that exceeds its fatigue or endurance level can trigger a weld failure. They can occur suddenly, causing catastrophic failures that could
have been avoided by prior analysis and repair.

The nuclear industry was the impetus to develop Fitness-for-Service (FFS) practices. In particular, the ability to quantify the tolerance of weld defects played a key role in validating the safety of nuclear vessels.

Even today, however, few industries outside of the chemical and nuclear are well-versed on the availability and exactness of FFS procedures. As a result, many companies fail to verify fatigue tolerance and safety margins before making the costly decision to replace components that are evidencing surface damage.

By employing FFS measures, many companies could have the capacity to design and safely implement repairs to restore fatigue and fracture-safety margins, even given an operational environment of cyclical stresses. Making an informed decision to repair or replace the compromised component requires a multi-faceted investigation that yields quantifiable results.

To read the full article by Dr. William J. O’Donnell, please contact the Editor.

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