Getting More from Fatigue Testing

John J. Kutz
Applications Engineering
Lightwave Division

Fiber optic sensors have a great combination of simple installation, repeatability and fatigue endurance that make them well suited for use in fatigue testing. Characterizing fatigue behavior of parts or structures is important in applications where parts are subjected to cyclic loading. such as fan or turbine blades commonly used in aerospace, automotive, power generation and the HVAC industries.

Getting the most out of fatigue testing requires measuring strain at specific points on your part. The foil strain gage is the most common tool available for measuring point strains. However, fatigue life of resistive strain gages rapidly drops off when cyclic strain amplitudes reach 2000 µstrain (See Vishay TN-5081). Before they fail, they drift, reporting erroneous strain measurements as they are exposed to increased loading cycles. Without the ability to make local strain measurements under fatigue loading, stresses are inferred from load and deflection, giving a single average result with limited usefulness.

Measuring strain on parts during fatigue testing can give a better understanding of performance over a test article lifespan, especially when things get really interesting – as the part gets close to failure. The measurements acquired during such tests can be used to improve numerical simulations (such as the modal FEA shown below). Simulation saves money by speeding up part optimization and enabling virtual testing.

Three Reasons why optical fiber is better

1. Repeatability

You can trust fiber measurements – they don’t drift! At Luna, we have seen fiber sensors give consistent, repeatable measurements up to 27,000 cycles at +/- 4000 με cyclic loading (see Engineering Note EN-FY1314[GM1] ). The figure below shows peak strain of the optical fiber and a foil strain gage. We see significant drift of the strain gage whereas the fiber holds steady.

2. Endurance

Fiber-optic sensors have superior fatigue endurance over electrical strain gages: Our optical sensors with similar properties to our distributed fiber optic strain sensors were tested up to 17,725,000 cycles at 0-4000 micro-strain cyclic loading with no signs of degradation (Zetterlind, et al.). Nearly all strain gages failed when exposed to the same loading.

3. Simplicity

Fiber-optic sensors give you more data with less hassle. Instrumenting your part with optical fiber is simpler than bonding electrical strain gages and many of the same techniques are used. The same adhesives designed for strain gages can be used with fiber-optic sensors. Strain gages require 2-3 electrical connections for a single measurement point. Our systems give you up to 800 software reconfigurable measurement points per meter of fiber with a single optical connection.

The Micro-Structure Advantage

Why does fiber perform so much better in fatigue loading? Brittle materials (like fused silica glass in optical fibers) always return to their original shape until failure (which occurs around 30,000 με). Ductile materials (like a metal strain gage grid) reach a “point of no return”, called their elastic limit or yield strength. When strained past this point, they plastically deform and don’t return to their original shape. Micro-scale plastic deformation leads to fatigue, the underlying effect that causes drift in foil strain gages. Optical fiber never experiences plastic deformation, so it has a clear advantage when making strain measurements under fatigue loading.

The bottom line

Ultimately, fiber-optic strain sensors save you time, money and give you the data you need.