5.SMA

Smart Materials and Structures 5. Shape Memory Alloy Materials 111DaLian University of Technology Dr. Song 11 5. Shape Memory Alloy Materials • Introduction and History • How SMA Works • Transformation Hysteresis • Stress-Strain Relationship • Superelasticity • Types of Shape Memory Alloy Materials • Shape Memory Alloy Properties • A Constitutive Model of SMA Smart Materials and Structures 5. Shape Memory Alloy Materials 222DaLian University of Technology Dr. Song 22 Introduction Example of Shape Memory Alloys (SMA's) Smart Materials and Structures 5. Shape Memory Alloy Materials 333DaLian University of Technology Dr. Song 33 Introduction • Shape Memory Alloys (SMA's) are novel metal materials which have the ability to return to a predetermined shape when heated. • When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to its original shape. • If the SMA encounters any resistance during this transformation, it can generate extremely large forces. This phenomenon provides a unique mechanism for remote actuation. SMA Spring After being Elongated at Cold SMA Spring After being heated For example, the SMA spring shown in the figures can be easily elongated when it is cold, but the SMA spring returns to its original shape once heated. Smart Materials and Structures 5. Shape Memory Alloy Materials 444DaLian University of Technology Dr. Song 44 History • 1932 Chang and Read noticed reversibility of transformation in gold- cadmium (AuCd alloy). • 1938 The transformation was observed in brass (copper-zinc). • 1951 Read and co-workers observed Shape Memory Effect (SME) in bent bar of gold-cadmium (AuCd). • 1962 Buehler and coworkers at Naval Ordnance Laboratory (NOL) discovered SME in Ni-Ti alloys (NiTiNOL). • 60’s-70’s many other types of SMAs were found and several products hit market. • 80’s – 90’s “Smart” and “Intelligent” materials become a developing research and development area. SMA materials in various shapes are commercially available. SMA are utilized in medical applications. DARPA setup programs to demonstrate capacity of smart materials for military applications. SMA micro devices emerge. Smart Materials and Structures 5. Shape Memory Alloy Materials 555DaLian University of Technology Dr. Song 55 How SMA Works Shape Memory Effect (SME) (One Way) The shape memory effect is a unique property of certain alloys exhibiting martensitic transformations. These materials can be deformed in the low temperature phase, and they will recover their original shape by the reverse transformation upon heating to a critical temperature called the reverse transformation temperature. This shape change is due to a change in the atomic crystal structure of the alloy. Heat High Temperature Cool Remove Force Force Force Low Temperature Deformed SMA Spring Deformed SMA Spring SMA Spring Deform One Way Shape Memory Effect of a SMA Spring. Smart Materials and Structures 5. Shape Memory Alloy Materials 666DaLian University of Technology Dr. Song 66 Austenite and Martensite: The high temperature crystal structure is called austenite and is cubic and strong. When cooled, the material transforms to a structure called martensite, with a monoclinic lattice structure which looks like a parallelogram in two dimensions and it is weak. High Temperature Cubic Structure - Austenite Low Temperature Structure - Martensite How SMA Works (con’t) Nitinol Crystal Structures: Austenite and Martensite Smart Materials and Structures 5. Shape Memory Alloy Materials 777DaLian University of Technology Dr. Song 77 How SMA Works (con’t) Twinning Process: When a piece of shape memory material containing many atoms is cooled below a transformation temperature, the atoms do not all tilt in the same direction. Instead, the atoms form alternating rows of atoms tilting either left or right (shown in the figure). Any four atoms in the low temperature structure have the martensite parallelogram shape. The alternating rows in the figure is called twinning, because the atoms form mirror images of themselves, or twins, through a plane of symmetry. High Temperature Low Temperature Twinned Martensite Twinning Process: Nitinol Atomic Rearrangement upon Cooling Smart Materials and Structures 5. Shape Memory Alloy Materials 888DaLian University of Technology Dr. Song 88 De-twinning Process: When a stress is applied to the twinned low temperature SMA, the stress will deform, or accumulate strain, as the twins are reoriented so they all lie in the same direction. This is called de-twinning, and in shape memory alloys, the stress required to reorient twins is relatively low. This de-twinning process is shown in the figure. How SMA Works (con’t) As Cooled Deformed by Applied Force Force Force De-twinning Process: Deformation of Low Temperature Nitinol Structure Smart Materials and Structures 5. Shape Memory Alloy Materials 999DaLian University of Technology Dr. Song 99 Return to Austenite Upon Heating Heating the material above a certain temperature will cause the deformed martensite to return to austenite and the original shape of the piece will be obtained. This occurs because the original atomic positions are always maintained in the austenite phase. How SMA Works (con’t) Phase Transformation of Nitinol Shape Memory Alloy High Temperature Austenite Low Temperature Martensite Twinned Structure Deformed Low Temperature Martensite Detwinned Structure Deform HeatingCooling Smart Materials and Structures 5. Shape Memory Alloy Materials 101010DaLian University of Technology Dr. Song 1010 Phase Transformation of the SMA Spring (Macro and Micro Views) How SMA Works (con’t) Heat High Temperature Cool Remove Force Force Force Low Temperature Deformed SMA Spring Deformed SMA Spring SMA Spring Deform Cubic Form Twinned Parallelogram Form Detwinned Parallelogram Form Detwinned Parallelogram Form Smart Materials and Structures 5. Shape Memory Alloy Materials 111111DaLian University of Technology Dr. Song 1111 Heat High Temperature Cool Remove Force Force Force Low Temperature Deformed SMA Wire Deformed SMA Wire SMA Wire Deform Cubic Form Twinned Parallelogram Form Detwinned Parallelogram Form Detwinned Parallelogram FormA B C D Phase Transformation of the SMA Wire (Macro and Micro Views) Smart Materials and Structures 5. Shape Memory Alloy Materials 121212DaLian University of Technology Dr. Song 1212 Two-Way Shape Memory Effect (SME) Two-way shape memory effect: the shape memory material will return to a low temperature shape on cooling, as well as to a high temperature shape on heating. But the recovery stress of a two-way SMA is much lower than that of a one-way SMA. In both the one-way and two-way shape memory effects, only during heating work can be generated. During cooling with the two-way effect, the material simply recovers its low temperature shape and cannot provide a force to external mechanical components. Heating Cooling Deformation One-Way SME Cooling Heating Two-Way SME The one-way shape memory effect requires a force to deform the material while it is cool, but will recover its shape when heated. Smart Materials and Structures 5. Shape Memory Alloy Materials 131313DaLian University of Technology Dr. Song 1313 Transformation Hysteresis of SMAs • The phase transformation of SMAs exhibits hysteresis, i.e., transformations on heating and on cooling do not overlap. • Hysteresis, a nonlinearity, adversely affect precision control of the structures activated by SMA actuators. • To design control methods to compensate for the nonlinearities associated with SMA actuators poses a challenge for control engineers and researchers. Heating Wire Contracts Cooling Wire Extends Weight Shape Memory Alloy Wire Actuator C ur re nt C ur re nt C ur re nt An SMA Wire Actuator Temperature L en gt h M ar te ns ite % 100 0 Austenite Start Austenite Finish Martensite Start Martensite Finish Transformation Hysteresis As Af Mf Ms Mf < Ms < As < Af Smart Materials and Structures 5. Shape Memory Alloy Materials 141414DaLian University of Technology Dr. Song 1414 Hysteresis of an SMA Wire Actuator • A programmable current amplifier is used to electrically heat the SMA wire. • A linear variable differential transformer (LVDT) is placed aganst the slider to detect the actuator’s displacement. • The electrical heating of the wire causes a phase transformation, which is seen as a contraction of the wire. The wire’s contraction places additional tension on the spring. Once the current is cut off and heat is removed, the bias spring will pull the SMA wire actuator back to its cold length. Shape Memory Alloy Wire Actuator LVDT Position Sensor Current Amplifier Bia Spring Linear Bearing • A Nickel-Titanium shape memory alloy wire (30.48 cm in length and 0.381 mm in diameter) is used. • In this SMA test stand, the shape memory alloy wire is attached between two wire supports. One wire support is attached to a slider that is free to slide through a linear bearing. The slider is attached to a biasing spring which pretensions the shape memory alloy wire. Experimental Setup Smart Materials and Structures 5. Shape Memory Alloy Materials 151515DaLian University of Technology Dr. Song 1515 Motion Obtained by the SMA Wire Actuator SMA at low temperature. L1 Stretch the wire at low temperature by the bias spring. L2 Remove force, new length at low Temperature. Apply heat, regain original length L1 Smart Materials and Structures 5. Shape Memory Alloy Materials 161616DaLian University of Technology Dr. Song 1616 0 0.5 1 1.5 2 2.5 3 3.5 4 0 2 4 6 8 10 12 14 Displacement v/s Voltage Voltage (volt) D is pl ac em en t (m m ) • The shape memory alloy wire is excited using a sinusoidal signal. Though input voltage is pure sinusoidal, the displacement is not. • The hysteresis loops observed have an average width of 2 volts. The curves are not very smooth, and this can be attributed to the uncontrolled ambient conditions. • The shape memory alloy wire actuator is not fully repeatable due to the uncontrolled ambient condition. 0 50 100 150 200 250 300 0 0.5 1 1.5 2 2.5 3 3.5 4 Applied Voltage and Current - Training Signal .....Voltage _____Current Time (sec) V ol ta ge ( vo lt) a nd C ur re nt ( am p) 0 50 100 150 200 250 300 0 2 4 6 8 10 12 14 Displacement - Training Signal Time (sec) D is pl ac em en t ( m m ) The Applied Sinusoidal Voltage and Measured Current Displacement of the SMA Wire Actuator Relationship between the Applied Voltage and Displacement Hysteresis Loops Hysteresis Loop: Smart Materials and Structures 5. Shape Memory Alloy Materials 171717DaLian University of Technology Dr. Song 1717 Stress-Strain Relationship of SMA • The Stress-Strain relationship of shape memory alloys shows strong temperature dependence, because of the reversible Austenite to Martensite transformation. • This figure shows the stress-strain relationship of a shape memory alloy at or below the Mf temperature. It is assumed that the SMA is cooled from the Austenite without applying stress. Stress Strain Detwinning Elastic Region Elastic Region Plastic Deformation O BA CStress-strain Relationship at or below Mf OA: The initial curve segment represents elastic deformation and the microstructure consists of randomly oriented Martensite twins. A: Detwinning starts. At this point, the stress level is sufficient to start the twins to reorient according to the applied stress field. AB: Detwinning. The twins reorient until they all lie in the same crystallographic direction. B: Detwinning is complete at point. BC: The Martensite undergoes mostly elastic deformation again in segment BC. At point C: The stress level is sufficient to start plastic deformation of the Martensite. Beyond C: The shape memory effect is destroyed or severely diminished by plastic deformation of the Martensite. Smart Materials and Structures 5. Shape Memory Alloy Materials 181818DaLian University of Technology Dr. Song 1818 Stress Strain Stress induced Martensite Elastic RegionElastic Region Plastic Deformation O BA CStress-strain Relationship above Af below Md Md: temperature at which non-elastic deformation is due to slip (plastic yielding) at stress induced Martensite. O: The material is fully austenitic. OA: Elastic deformation. A: Martensite begins to form from the austenite, this material is referred to as stress induced martensite. AB: Stress induced Martensite.. BC: Represents elastic deformation. C: Plastic deformation starts to occur. AB: When the material is unloaded in this segment with stress induced Martensite, the Martensite becomes unstable and the material returns to austenite and its original shape. The material can experience 8% of strain change. Superelasticity occurs. Smart Materials and Structures 5. Shape Memory Alloy Materials 191919DaLian University of Technology Dr. Song 1919 At temperatures above Md, non-elastic deformation is entirely due to plastic yielding, and no stress induced Martensite is formed. Stress Elastic Region Plastic Deformation Strain Stress-strain Relationship above Md Smart Materials and Structures 5. Shape Memory Alloy Materials 202020DaLian University of Technology Dr. Song 2020 Example: Stress-Strain Memory Effect of an SMA Wire Stress Strain heating Deformed state w wHigh temperature Austenite curve Low temperature Martensite curve sA sM eA eM When heated contracts to high temperature state Smart Materials and Structures 5. Shape Memory Alloy Materials 212121DaLian University of Technology Dr. Song 2121 Superelasticity • Definition: the behavior of certain alloys to return to their original shape upon unloading after a substantial deformation has been applied. • The superelastic mode takes place under constant temperature conditions. • When a shape memory alloy is deformed above Af and below Md (the temperature above which stress-induced martensite can no longer be formed). stress-induced martensite is formed. When the material is unloaded, the martensite becomes unstable and the material returns to austenite and its original shape. Superelasticity occurs. The stress-strain relationship is shown in the figure. The unloading curve occurs at a lower stress due to transformational hysteresis which is closely related to the thermal hysteresis in shape memory behavior. The loading plateau is the result of the martensite accommodating the applied stress by forming the crystallographic twin variant most favorably inclined to the applied stress field. Unloading plateau Loading plateau STAIN STRESS Sl: loading stress Su: unloading stress et: total strain Sl Su et Smart Materials and Structures 5. Shape Memory Alloy Materials 222222DaLian University of Technology Dr. Song 2222 Superelasticity: Effect of Temperature on loading and Unloading Stresses Loading and unloading stress increase with increasing temperature within the “Superelastic window.” Unloading plateau Loading plateau Temperature Stress Material Elastic Strain Steel 0.8% Cu-Zn-AI 5.0% Ni-Ti 10.0% Smart Materials and Structures 5. Shape Memory Alloy Materials 232323DaLian University of Technology Dr. Song 2323 Superelastic Window • In the left portion of the figure, the plastic strain is large and due to the Martensitic transformation associated with the shape memory event (i.e. it can be recovered by heating above Af,). • To the right of the minimum point there exists a relatively flat portion which defines the “superelastic window” since the permanent plastic strain is small. • To the right of the “superelastic window” the permanent plastic strain increases dramatically and is therefore not acceptable for superelastic applications. % set after 8% Strain Shape memory zone (recover strain by heating) Superelastic + plastic Deformation (permanent set) Temperature Superelastic zone • This figure shows the useable temperature range for superelastic behavior, commonly referred to as the "superelastic window". Smart Materials and Structures 5. Shape Memory Alloy Materials 242424DaLian University of Technology Dr. Song 2424 Narrow Superelastic Window Limits Application • An approximately 40°C window, starting at the Af temperature, can be obtained by strengthening the alloy--through a combination of cold work, aging, and annealing. • Still, this functional temperature range is too narrow for most industrial and consumer applications. Automobile springs, for example, generally require elasticity from -30° to 200°C. Moreover, the stiffness of a superelastic device changes with temperature according to the Claussius-Clapeyron equation, at a rate of approximately 4-8 MPa/°C. • The variability of superelasticity with temperature, and therefore its narrow superelastic window, limits the general use of superelastic materials. Superelastic Window (con’t) Smart Materials and Structures 5. Shape Memory Alloy Materials 252525DaLian University of Technology Dr. Song 2525 Types of Shape Memory Alloy Materials • There are many known alloy systems which exhibit the shape memory effect, but only three have shown promise for commercial applications. They are Nickel-Titanium (Ni- Ti), Copper-Zinc-Aluminum (Cu-Zn-Al), and Copper-Aluminum-Nickel (Cu-Al-Ni). • The copper-zinc-aluminum alloys have a typical composition of 15 – 25 weight percentage Zn / 6 – 9 weight percentage Al / balance Cu. Cu-Zn-Al alloys are lower in cost than nickel titanium, but they possess some inferior characteristics. Transformation temperatures can drift slightly during cycling (particularly at service temperatures greater than 100 oC) and to a significant extent if the alloy is not processed properly. These alloys are susceptible to stress corrosion cracking when exposed to certain corrosive agents. • The copper-aluminum-nickel alloys have a typical composition of 13 – 14 weight percentage Al / 3 – 4 weight percentage Ni / balance Cu. Cu-Al-Ni alloys possess lower ductility than either Ni-Ti or Cu-Zn-Al. Their corrosion resistance is inferior to Ni-Ti and their cost is higher than Cu-Zn-Al. Cu-Al-Ni alloys undergo less degradation in shape memory properties than Cu-Zn-Al, after exposure to temperatures in the 100 to 350O C range. In addition, Cu-Al-Ni alloys have the highest transformation temperatures of the three alloys. Smart Materials and Structures 5. Shape Memory Alloy Materials 262626DaLian University of Technology Dr. Song 2626 Nitinol • The nickel titanium alloys (Nitinol) they have typical compositions of approximately 50 atomic percentage Ni / 50 atomic percentage Ti, and may have small additions of copper, iron, cobalt, or chromium. • Nickel-titanium is about four times the cost of Cu-Zn-Al alloys. • It has several advantages over Cu-Zn-Al and Cu-Al-Ni: – greater ductility – more recoverable motion –