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Electrochemical Fatigue Sensor (EFS)
The Electrochemical Fatigue Sensor (EFS) was invented in 1992 by Dr. Campbell Laird and Dr. Yuanfeng Li. It is a patented non-destructive crack inspection and monitoring technology that determines if a growing crack exists at an inspection location of a metal infrastructure like steel bridges. It can be used on existing cracks to determine their activity under ambient loading or at similar fatigue susceptible details to determine the existence of growing cracks that can be missed by conventional inspection methods. The EFS can also immediately determine if repairs and retrofits are effective by being able to detect precursors to crack initiation and propagation. Similar in concept to a medical EKG, which is used to determine if actively growing fatigue cracks are present. An EFS sensor is first applied to the fatigue sensitive location on the bridge or metal structure, and then is injected with an electrolyte, at which point a small constant voltage is applied.

The technology behind EFS was devised by researchers from the U.S. Air Force and the University of Pennsylvania for use in the aerospace industry. This technology was revamped the technology for bridge inspections between 2002 and 2006. The original research was aimed at developing a technology for detecting problem cracks in airframes and engines. Grade 5, also known as Ti6Al4V, Ti-6Al-4V or Ti 6-4, is the most commonly used alloy in the aerospace industry, e.g. internal combustion engine connecting rods. It has a chemical composition of 6% aluminium, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen, and the remainder titanium. It is significantly stronger than commercially pure titanium while having the same stiffness and thermal properties (excluding thermal conductivity, which is about 60% lower in Grade 5 Ti than in CP Ti). Among its many advantages, it is heat treatable. This grade is an excellent combination of strength, corrosion resistance, weld and fabricability. Generally, it is used in applications up to 400 degrees Celsius.

(Grade 5 has a density of approximately 4420 kg/m3, Young’s modulus of 110 GPa, and tensile strength of 1000 MPa. By comparison, annealed type 316 stainless steel has a density of 8000 kg/m³, modulus of 193 GPa, and tensile strength of only 570 Mpa and tempered 6061 aluminium alloy has a density of 2700 kg/m3, modulus of 69 GPa, and tesnsile strength of 310 MPa.)

The EFS system subsequently monitors changes in the current response that results from the exposure of fresh steel during crack propagation. The system consists of an electrolyte, a sensor array (one reference and one crack measurement sensor) and potentiostat for applying a constant polarizing voltage between the bridge and sensor, as well as data collection and analysis software.

The current response from the sensor array indicates quickly and clearly, whether a growing crack exists at the inspection location. Data is presented in both the time domain and the frequency domain. An algorithm, specifically written for this system, automatically indicates the level of fatigue crack activity at the inspection location. Cost-Saving Benefits: • Increases the safety of the infrastructure and the efficiency of bridge management through better and more timely fatigue crack detection. • Replaces “wait and see” approach by allowing immediate detection of growing cracks at known and unknown locations, as well as at repairs. • More accurate assessment of condition ratings – extends the life of the structure through early identification and repair of growing cracks. • Determines which cracks need immediate attention and which repairs can be deferred or eliminated; helps bridge owners utilize repair and rehabilitation funds more effectively. • Repairs / retrofits can be verified immediately – no re-inspection needed.

EFS appears to be the only truly definitive metal fatigue assessment technology in the world. Until EFS, little progress had been made in gauging crack growth and the extent of this insidious and largely invisible strength-sapping phenomenon. With EFS it is possible to conduct the appraisal of steel structural members using a technique which does not require any knowledge of past structural or loading history for the object to be monitored. This provides an answer to the metal fatigue problems of a broad array of industries, such as bridges and aerospace as well as ships, cranes, power plants, nuclear facilities, chemical plants, mining equipment, metal windmills and “heavy iron”.