Fatigue Analysis of Piping System using CAESAR II

The continuous cycling of peak-stress concentrations may cause a crack in the piping system that propagates and results in leakages. This failure mechanism is known as fatigue, and damage done during the fatigue process keeps increasing and usually is unrecoverable. When the combinations of static and dynamic stresses exceed allowable values, it results in the piping fatigue.

Let’s have a look at the factors affecting the fatigue behavior of piping system:

Types and nature of loads

Size of piping components

Distribution of stress and strain

Mean stress corrections

Stress/strain concentration

Environmental effects

Machine induced vibration

Metallurgical and material properties

Strain-rate and high-frequency effects

Failure Criteria in Fatigue Analysis

Low Cycle Fatigue (LCF): The number of loading cycles applied in the piping design is very low in the order of a few thousand. It is characterized by repeated plastic deformation in each period.

High Cycle Fatigue (HCF): The number of loading cycles to failure is high for HCF and it involves little or no plastic action as it is stress governed. Elastic deformations characterize it.

The fatigue curve (S-N curve) is derived from experimental tests that correlate by applying cyclic stress to the number of cycles to failure for every material.

Fatigue failure can be prevented by careful attention at the design stage by ensuring that the number of load cycles (N) is sufficiently low, then the amount allowed in endurance or S-N curve. However, practical service conditions often subject a piping system to alternating stresses of different magnitudes. As the curves are based on constant-stress, such changes in amplitudes make the direct use of fatigue curves inapplicable.

Below listed are some main clues to the essential nature of fatigue based on the tests of materials and structures:

Most metals and alloys that exhibit some ductility in static tests show fatigue failure under repeated stress, much lower than the ultimate tensile strength (UTS)

The ranges of stress cause fatigue failures applied rather than the speed of the loading

Some metals, including ferrous alloys, possess a safe range of stress, known as ‘endurance or fatigue limit,’ failure does not occur regardless of the number of loading cycles

The increase in the mean tension stress of loading cycles decreases the stress range needed to produce failure in a fixed quantity of periods

The fatigue analysis is generally performed in existing plants to examine the actual cause of failure. It is necessary to be ready with the following data before starting the study:

Piping material fatigue curve

Enough process data for calculating the number of cycles throughout the design life cycle of the piping system.

Steps for fatigue analysis using Caesar II:

To assign the fatigue curve data to the piping material. Caesar II provides some commonly used curves for accurate fatigue analysis.

Define the static or dynamic load cases. For this purpose, the FAT stress type is already defined in the CAESAR database.

Caesar II automatically calculates the fatigue stress as it calculates the stress intensity.

Determine allowable fatigue stress

Determine the permissible number of cycles

Reporting the analysis results

If you’re looking for a piping system flexibility analysis using CAESAR-II for your engineering project, consider hiring an experienced piping engineering service provider.

Judith Morrison is an expert in the field of industrial engineering and writes articles related to piping, civil, equipment engineering related articles.