Structural Integrity Life Assessment

"Structural Life Valuation" is an approach that determines whether a piece can safely withstand the forces in its operational and expected life-span. In these assessments, parts must be subjected to various tests such as creep test, fatigue test, tensile stress testing, thermal tests, etc. in order to determine their strength in the working conditions. Structural Integrity Assessment methods can also be helpful in ranking and re-checking existing equipment.


1- Creep analysis

Creep is plastic and permanent deformation of materials under a constant load that is applied at a high temperature and for a relatively long time. The amount of deformation is a function of the load, loading time and applied temperature. Creep is a concern for engineers in many cases, as it can lead to fracturing and fragmentation before it develops fatigue. For example, a gas turbine blade creep can cause blistering with the shell and, consequently, break the blade.


2- Fatigue Analysis

Sudden fracture by cyclic loading or fluctuating fatigue. Due to dynamic loading, fracture failure occurs in tension less than the final stress. Failure due to fatigue occurs suddenly without any precedent and is therefore dangerous. In many industrial pieces of equipment such as axial flow compressors, fatigue is the determining factor in rotating parts fracture. In the laboratory for testing fatigue, parts are used from a variety of devices in which the piece is under repeated or oscillating stress and the test continues until the complete fracture happens.


3- Low cycle fatigue

In low cycle fatigue (LCF), the fragment is disrupted under a low number of cycles and in high tensions (and consequently high strain). In other words, in low cyclic fatigue, the number of cycles that a piece can withstand before failure is much less than normal fatigue. Thermal stress due to expansion or contraction of materials can be effective in the appearance of low cycle fatigue characteristics.


4- Thermomechanical fatigue

When faced with thermal and mechanical stress cycles and alternating simultaneously, we face thermomechanical fatigue (TMF). Thermomechanical fatigue should be considered when designing gas turbines. In general, three factors are involved in thermomechanical fatigue: 1) high temperature creep 2) fatigue due to intermittent loading 3) oxidation due to environmental factors


5- residual stress analysis

Residual stress is a tension that occurs during the process of welding, machining, forging, heat treatment, etc. This tension can be in some cases useful and in some cases harmful. There are many ways to measure residual stresses that are categorized as destructive, non-destructive, and semi-destructive. Nondestructive methods include XRD, ultrasonic, and electromagnetic testing.