Nitinol for Mobile Device Protection – Kellogg’s Research Labs

1. Purpose: Nothing is more frustrating than dropping a mobile device onto a hard surface.  The last time I bought a new phone, I had it less than two weeks before I dropped it onto a concrete floor, causing the screen to crack and look like a spider web.  The sudden impact, or shock, is the primary cause of screen fracture in mobile devices.  Dissipating these forces allows larger screen sizes, thinner devices, and better overall customer experience.  Plastics and rubbers are excellent dampers, but they lack structural strength.  The Figure at right shows an acceleration curve demonstrating the difference between an aluminum sample and a nitinol sample.
   1. A Simple Test for Shock Damping: There is a very simple test to qualify how effectively a metal damps shock—the drop test.  If the metal returns a sharp ring, that means that it is supporting standing wave vibrations—the super destroyer.  If the metal returns a dull thud, then the metal is very effectively damping vibrations.  Aluminum returns a pronounced ring.  Magnesium returns a less pronounced ring.  Nitinol returns a dull thud.
2. Background
   1. What is Nitinol: Nitinol is the trade name associated with the family of alloys based on the mixture of nickel and titanium which exhibit the shape memory effect (SME).  The name nitinol is synthesized from Nickel-Titanium Naval Ordinance Laboratory—the organization that discovered it.  The shape memory means that it can be deformed at a low temperature and then the shape is recovered upon heating.  This apparently magical property is caused by the existence of a solid state phase transformation.

The low temperature phase, martensite, has bonds that easily rotate, absorbing deformation without breaking.  This causes the material to feel soft and malleable, similar to solder.  The high temperature phase, austenite, is a rigid cubic structure.  Since the bonds were not broken in the martensite phase, they very forcefully snap back to the original matrix form when heated.

The temperature at which this transformation occurs can be tuned to meet application requirements.  For binary nitinol alloys, the range of possible transition temperatures lies from -100°C to 100°C.  By adding in a third metal, the range of possible transition temperatures broadens out to -200°C to 250°C and, if materials like platinum fit the budget, the upper limit can be pushed up to 600°C.

When nitinol operates above its transition temperature, a different phenomenon is observed: superelasticity.  Superelasticity has two basic properties that are notable: very high strain recovery and plateau stress.  The plateau stress means that, once the deformation reaches a high enough value, the force supplied by the nitinol element changes very little with the increasing or decreasing deformation.  A superelastic nitinol element can elastically recover up to 50% strain. 

• Shock Reduction: One of the side effects of the shape memory effect and superelasticity is that nitinol is very effective at hardening structures against mechanical shock.  The switching of the material from one microstructure to the other and then back again results in dissipating enormous amounts of energy.  Most materials that are effective at shock hardening are very soft—meaning that they cannot be used in structural applications.  Nitinol, on the other hand, has the structural strength of titanium.  This means that a part of the cost of shock hardening with nitinol can be written off by not needing an additional structural material.  This is especially important for space restrictive applications.
  • Shock Hardening Metals: It is important to note that nitinol is not he only metal that is useful for shock hardening.  Magnesium is very effective at shock hardening due to the high degree of twinning in the microstructure.  The impact causes the microstructure to detwin, creating internal friction, converting the   However, a nitinol damper is far more effective at shock hardening than magnesium (as well as having some side benefits).  This increased effectiveness comes from higher degrees of twinning and the hystereitic nature of nitinol’s microstructure.
  • Nitinol and Mobile Devices: Mobile devices continue to revolutionize how people around the world interact with each other and how they conduct their daily lives.  One of the major weaknesses of mobile devices is the frailty of the screens.  While Gorilla Glass from Corning has helped to dramatically improve the fracture resistance, the greatest cause of fracture in screen glass is drops onto hard surfaces, where the device lands on its corner.  In this case, all of the impact energy is concentrated on the corner of the device, giving the greatest probability of fracture.    
    • Nitinol Frame or Frame Components: By creating a single piece nitinol frame that passes throughout and supports the entire device, this ensures the maximum amount of shock hardening possible.  It also ensures that impacts from the widest range of possible angles
    • Perceived Benefits: Without a deep understanding of the possible benefits of implementing nitinol shock hardeners, our perceived benefits include but are not limited to:
      • Larger screen size: By having screens less prone to fracture, larger screen sizes can be used. 
      • Thinner devices: With greater shock damping capabilities built into the device, the device does not require as much structural strength in order to provide adequate protection.
      • Customer Experience: With more durable devices, customers will be more satisfied with their devices, potentially increasing brand loyalty.
      • Obsolescence of device cases: With increased shock damping capabilities, built into the device, it is possible that device cases will no longer be necessary.  This would eliminate a large amount of bulk from the device and allow the user to have the most sleek possible design.
    • Historical Experience: Two case studies have been selected here for comparison.
      • Consumer Electronic Device: In support of work done for a major consumer electronics customer, nitinol dampers were used to reduce the probability of screen fracture and other internal damage.  During the specified drop test, the device experienced 17.7g accelerations before nitinol.  After nitinol was introduced, the device experienced just 0.7g acceleration.
      • Civil Engineering: KRL has done shock hardening work for earthquake applications together with the University of Malaysia.  In this case, the test structure failed at 7.6 magnitude earthquake.  After installing the nitinol, the same test structure survived a 9.2 magnitude earthquake.  That is a 40x increase in energy and the structure survived!
• Problem Statement: The nitinol frame or frame component should have the following properties:
  • Energy Dissipation: Maximize based on energy dissipation per gram
  • Ambient Temperature: -20°C – 45°C
  • Fatigue Life: >10,000 impacts
  • Maximum Height: Customer determined
  • Maximum Weight: Customer determined
  • Other Geometrical Requirements: Customer determined
  • Other Material Requirements: Customer determined
  • Attachment Method: Customer determined

Proposed Solution: There are two ways in which nitinol could be integrated into mobile devices.  The first is to make the entire frame from nitinol.  With nitinol passing throughout the entire device, impacts from any direction would be dissipated by the nitinol frame.  A concept sketch of this use case is shown in Figure 1.  However, it is possible that this method is cost prohibitive.  The second method would be to create corner blocks to address the worst case impact scenario and use a lower cost material, such as magnesium, for the remainder of the frame.  It is possible that this would provide the bulk of the value at a fraction of the cost.  It is also likely that, for impacts not involving a corner, much of the impact energy would be transmitted by the frame to the nitinol corners, where it is dissipated harmlessly.

Prototypes: Since the cost/benefit ratio of using a single piece frame as compared to a frame where only the corners are made of nitinol has not yet been established, it is recommended that prototypes of both methods be evaluated.  Initial shock testing can be conducted at KRL in accordance with customer specifications.  As the project progresses, KRL is able to provide custom melting, characterization, optimization and a vast array of other services that ensure that the frame has the best possible damping characteristics.