The basic features of a terfenol-d actuator contribute to its reliability.


Construction

Bonding hardened end caps onto each end of the terfenol-d rod significantly increases its fracture resistance.

• A bonding agent between the rod and its end caps acts
  • To distribute stress more uniformly.
  • As a cushion.
  
  
• Ferromagnetic end caps reduce magnetic flux divergence at rod ends.
  
  
• Mechanical interface design features can be included in end caps.

Because magnetic field needs no medium to transmit, the solenoid coil need not contact the rod.

• No contact between the terfenol-d and its source of energization, the coil, eliminates the possibility of fatigue cracking caused by local field gradients and/or differential expansion between two different materials.
  
  
• The solenoid coil may therefore be mechanically fixed and immovable.
  
  
• A gap between the coil and rod permits flow of suitable fluid for temperature conditioning if desired.

Operation

Mechanical rod limits are

• Strength. "Standard" terfenol-d has a compressive strength of -805+/-34 MPa, a tensile strength of 28 MPa, and a bending strength of about 40 MPa.
  
  Sources
  
  • Handbook of Giant Magnetostrictive Materials , Page 108
    
  • Load Capacity of Giant Magnetostrictive Materials
  
  
• Column buckling.
  
  
• Shock impact reloading.
  
  
• Compression fatigue. Terfenol-d rods inside actuators are generally operated within the compressive stress range of -5 to -150 MPa. Billions of stress cycles of individual rods within, for instance, power ultrasonic and Navy sonar actuators have been run without detectable degradation. These actuators were run at stress cycles less than this test data:
  
  Stress Cycle: -250 ±250 MPa @ 10 Hz
  Runout Point: >1 × 10^6 Cycles
  
  Source Load Capacity of Giant Magnetostrictive Materials

Magnetic. The coil and power supply limit

• Magnetic field magnitude and its time rate of change.
  
  
  • Within all known data, terfenol-d appears immune to magnetic fields beyond saturation.
  
  
• Magnetic field polarity.
  
  
  • A permanent magnet will stick to terfenol-d but the alloy itself cannot be permanently polarized or permanently depolarized.

Heat.

• Terfenol-d magnetostriction occurs within the range of -40C to 357C.
  
  
  • If the terfenol-d has not melted (~1,300C), performance is restored upon cooling below its Curie temperature of 357C.
  
  
• The coil is typically the major heat dissipator. Conduction and convection cooling requirements depend on design and operation details.
  
  

Incorporating awareness of the above makes a magnetostrictive actuator simple, durable, robust, survivable, and reliable.