In safe-life design, products are intended to be removed from service at a specific design life.
Safe-life is particularly relevant to simple metal aircraft, where airframe components are subjected to alternating loads over the lifetime of the aircraft which makes them susceptible to metal fatigue. In certain areas such as in wing or tail components, structural failure in flight would be catastrophic.
The safe-life design technique is employed in critical systems which are either very difficult to repair or whose failure may cause severe damage to life and property. These systems are designed to work for years without requirement of any repairs.
The disadvantage of the safe-life design philosophy is that serious assumptions must be made regarding the alternating loads imposed on the aircraft, so if those assumptions prove to be inaccurate, cracks may commence prior to the component being removed from service. To counter this disadvantage, alternative design philosophies like fail-safe design and fault-tolerant design were developed.
One way the safe-life approach is planning and envisaging the toughness of the mechanisms in the automotive industry. When the repetitive loading on mechanical structures intensified with the advent of the steam engine, back in the mid-1800s, this approach was established (Oja 2013). According to Michael Oja, “Engineers and academics began to understand the effect that cyclic stress (or strain) has on the life of a component; a curve was developed relating the magnitude of the cyclic stress (S) to the logarithm of the number of cycles to failure (N)” (Oja 2013). The S-N curve because the fundamental relation is in safe life designs. The curve is reliant on many conditions, including the ratio of maximum load to minimum load (R-ratio), the type of material being inspected, and the regularity at which the cyclic stresses (or strains) are applied. Today, the curve is still consequential by experimentally testing laboratory specimens at many different continuous cyclic load levels, and detecting the number of cycles to failure (Oja 2013). Michael Oja states that, “Unsurprisingly, as the load decreases, the life of the specimen increases” (Oja 2013). The practical limit of experimental challenges has been due to frequency confines of hydraulic-powered test machines. The load at which this high-cycle life happens has come to be recognized as the fatigue asset of the material (Oja 2013).
The safe-life design philosophy is applied to all helicopter structures. In the current generation of Army helicopters, such as the UH-60 Black Hawk, composite materials make up as great as 17 percent of the airframe and rotor weight (Reddick). Harold Reddick states that, “With the advent of major helicopter composite structures R&D projects, such as the Advanced Composite Airframe Program (ACAP), and Manufacturing Methods and Technology (MM&T) projects, such as UH-60 Low Cost Composite Blade Program, it is estimated that within a few years composite materials could be applied to as much as 80% of the airframe and rotor weight of a helicopter in a production program” (Reddick). Along with this application it is the essential obligation that sound, definitive design criteria be industrialized in order that the composite structures own high fatigue lives for economy of ownership and good damage tolerance for flight safety. Safe-life and damage-tolerant criteria are practical to all helicopter flight critical components (Reddick).
Oja, Michael (2013-03-18). "Structural Design Concepts: Overview of Safe Life and Damage Tolerance". Vextec.com | Reducing Life Cycle Costs From Design To Field Service. Retrieved 2019-06-11.
"Fatigue (material)", Wikipedia, 2019-06-04, retrieved 2019-06-11
Reddick, Harold. "Safe-Life and Damage-Tolerant Design Approaches for Helicopter Structures" (PDF). NASA. Retrieved June 11, 2019.