Primary Stress Failure and Secondary Stress Failure

Primary and Secondary Stress Failure in Engineering Systems

In mechanical and structural engineering, failures do not happen suddenly without warning. Most failures are the result of incorrect design assumptions, material limitations, or changes in operating conditions. One of the most important topics engineers must understand is stress failure. Stress failure is mainly divided into two types: Primary Stress Failure and Secondary Stress Failure.

This article explains these two types of failures in very simple language, using practical examples and step-by-step explanations. The goal is to help students, engineers, and general readers clearly understand why systems fail and how such failures can be avoided.

What Is Stress in Engineering?

Stress is the internal force experienced by a material when an external load is applied. These loads can come from:

• Weight of equipment
• Internal pressure
• Temperature changes
• Wind or earthquake forces
• Movement restrictions

If the stress inside a component becomes too high, the material may deform, crack, or even fail completely. Engineers classify these stresses to better understand and control them.

Primary Stress Failure

What Is Primary Stress?

Primary stress is caused by loads that must be supported continuously by a structure or component. These loads do not go away unless the system is shut down or the load is removed.

Common sources of primary stress include:

• Weight of valves, pipes, and equipment
• Internal pressure inside vessels and pipelines
• External forces like wind or seismic loads

Primary stress is dangerous because it can lead to immediate and catastrophic failure if the material strength is exceeded.

Real-Life Example of Primary Stress Failure

In one real engineering case, springs were used to support the weight of a valve operating on a piping system. Unfortunately, the springs were not designed properly to carry the actual weight of the valve.

When the system was tested with cold water (hydrostatic testing), everything looked fine. The pipe and supports could handle the load at room temperature, and no visible problem appeared.

However, when the system was heated during actual operation, the situation changed. At higher temperatures, materials lose strength. Because the springs were already overloaded, the reduction in strength caused serious deformation.

Within less than 30 minutes, the valve sagged downward, and the guardrail attached to it was crushed. This happened because the springs could no longer support the load at operating temperature.

Steps That Led to Primary Stress Failure

1. Incorrect Weight Calculation:
   The actual weight loads were not properly considered during design. As a result, the primary stresses became too high.
2. Loss of Material Strength:
   At high operating temperatures, the strength of the spring material dropped significantly.
3. Large Deformation:
   The springs began to deform almost immediately until they reached a point where no more movement was possible. This is known as spring bottoming out.

This type of failure shows why primary stresses must always be kept below allowable limits with proper safety margins.

Secondary Stress Failure

What Is Secondary Stress?

Secondary stress is caused by deformation or movement restrictions rather than direct loads. These stresses usually occur due to:

• Thermal expansion
• Temperature changes
• Misalignment
• Pipe restraints and anchors

Unlike primary stress, secondary stress is often self-limiting. This means that once the material yields slightly, the stress does not keep increasing indefinitely.

Real-Life Example of Secondary Stress Failure

In another case study, a pressure vessel operated successfully for over 12 years without any issues. During a routine inspection, engineers found fatigue cracks near the pipe nozzle connection inside the vessel.

Further analysis revealed that the operating temperature of the nearby piping system had been increased. At the same time, pipe restraints were relocated to suit the new operating conditions.

These changes caused thermal expansion stresses to exceed allowable limits. Even though the calculated stress at the junction was more than 470,000 psi, the joint did not fail immediately.

The reason was that thermal stress is self-relieving. Also, the system experienced very few temperature cycles—less than a dozen over two years. Because of this, the fatigue damage took many years to develop.

Steps That Led to Secondary Stress Failure

1. Thermal Limits Were Exceeded:
   The expansion allowables were exceeded due to design changes.
2. Repeated Loading:
   After several cycles of excessive thermal stress, cracks formed on the inner surface of the vessel near the nozzle.

This example shows that secondary stress failures usually occur slowly and are related to fatigue rather than sudden collapse.

Difference Between Primary and Secondary Stress

Primary Stress	Secondary Stress
Caused by weight and pressure	Caused by thermal expansion
Can cause sudden failure	Usually causes fatigue failure
Not self-limiting	Self-limiting in nature
Checked against yield strength	Checked against fatigue limits

Engineering Compliance and Safety Checks

To prevent both types of stress failure, engineers must carefully follow design codes and standards. Piping systems should always be checked for both primary and secondary stresses.

Basic Steps in Stress Compliance

1. Calculate Primary Stresses:
   These include sustained loads such as weight and pressure, as well as occasional loads like wind and earthquakes.
2. Calculate Stress Range:
   Determine how much stress changes due to temperature variation and expansion.
3. Compare With Allowables:
   Primary stresses are compared with allowable stresses based on yield strength.
4. Check Fatigue Limits:
   Expansion stress range is checked against fatigue limits based on the number of cycles.

Important Engineering Insight

Primary and secondary stresses have completely different failure mechanisms. Therefore, they must always be evaluated separately. Because of this, most engineering codes do not recognize a single term called “operating stress.”

Understanding this concept is critical for safe and reliable design.

Conclusion

Primary and secondary stress failures are common causes of mechanical and structural damage. Primary stress failures usually occur due to poor load estimation or material strength reduction, while secondary stress failures develop slowly due to thermal expansion and fatigue.

By properly calculating loads, considering temperature effects, and following design codes, engineers can greatly reduce the risk of failure. A good design is not just about strength—it is about understanding how a system behaves under real operating conditions.

This knowledge is essential for students, professionals, and anyone interested in mechanical and structural safety.