The US Naval Air Systems Command is funding a process that could make aircraft titanium hydraulic system tubes more resistant to surface flaws. This could lead to a reduction in maintenance costs.
Hydraulics, operating at pressures of up to 345bar (5,000lb/in2), must be reliable because they power the actuators for the flight controls, landing gear and other aircraft systems. Titanium tubing is used for its low mass, but can have its useful life significantly reduced by very small scratches.
These can happen during routine handling and maintenance, and surface flaws are the main cause of all material failure. Present US military practice is to replace any titanium hydraulic line that has a visible scratch to assure the highest level of safety.
To counter this the US Naval Air Systems Command is funding Connecticut company Flightware to develop a surface protection process called "inverse autofrettage".
Autofrettage, typically used for gun barrels, uses fluid pressure to introduce residual compressive stresses on to the interior surface of metallic tubes to make them damage resistant. It does not need computer control, unlike alternate processes such as laser shock and waterjet cavitation peening.
These peening processes also introduce residual compressive stresses on to surfaces by sweeping high-pressure water jets across the surface or using a high energy pulsed laser. Flightware has filed a provisional patent for inverse autofrettage, a batch process that the company says enables lower unit costs.
Source: Flight International
