From coronary stents to Space Shuttle engines, ship hulls to buried pipelines, steam turbines to microelectronics, the design, maintenance, and life extension of most engineered structures involves some form of life prediction or fitness-for-service (FFS) evaluation. Life prediction typically involves quantitative analysis of the effects of damage mechanisms such as fatigue, creep, wear, and corrosion on the structural integrity and serviceability of mechanical components. Whether for assessment of new designs, life extension of existing equipment, or evaluation of damaged components, Exponent maintains cutting-edge analysis tools and expertise needed to perform reliable life predictions and fitness-for-service evaluations.
Virtually all engineered structures suffer from some type of time-dependent degradation that can limit their useful life. In many instances, multiple mechanisms can act to limit the safe life of components and structures. Following is a list of just a few examples of life-limited structures and common mechanisms affecting them:
- Steam turbines - fatigue, creep crack growth, corrosion, temper embrittlement
- High-temperature piping - creep, creep crack growth, corrosion
- Transmission pipelines - fatigue crack initiation and growth, stress-corrosion cracking (SCC), general corrosion, impact damage
- Offshore structures- fatigue, corrosion
- Aircraft airframes - fatigue, corrosion, impact damage
- Electronic components - thermal fatigue
Some structures, such as implantable medical devices, must be designed to withstand a large number of fatigue loading cycles without cracking to help ensure safe operation in an environment where there is no option for in-service inspection. Coronary artery stents fall into this category. On the other hand, structures such as ships and aircraft are inspected periodically in an effort to detect and repair developing damage before it reaches a critical level. Exponent routinely applies state-of-the art life prediction analysis techniques to assist clients developing or maintaining both types of structures. The NASCRAC™ Software is one such tool developed by Exponent engineers to calculate crack propagation life for fatigue, creep, and stress-corrosion cracking mechanisms.
Reliable life prediction is critical in establishing appropriate re-inspection intervals for life-limited components. A high degree of reliability can often be achieved by designing a re-inspection interval that provides for multiple inspections within the time necessary for damage to progress from a detectable level to a critical level. Exponent engineers have developed both deterministic and probabilistic life simulation programs to help clients establish appropriate damage control programs for new designs and life extension of existing hardware.
Fitness-for-service evaluation typically refers to condition assessment and future life prediction for existing structures or components in which some type of damage has been identified through in-service inspection. While fitness-for-service evaluation methodologies have been published in standards and recommended practices such as British Standard BS 7910 and American Petroleum Institute (API) Recommended Practice RP 579, application of such methodologies can be challenging. Exponent engineers have extensive experience applying these and other evaluation techniques for determining the serviceability of damaged components.