Othmane Benafan and Ronald D. Noebe

Glenn Research Center, Cleveland, Ohio

Timothy J. Halsmer

Jacobs Technology, Cleveland, Ohio

Shape Memory Alloy Rock Splitters (SMARS)—A 

Non-Explosive Method for Fracturing Planetary 

Rocklike Materials and Minerals

NASA/TM—2015-218832

July 2015
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Othmane Benafan and Ronald D. Noebe

Glenn Research Center, Cleveland, Ohio

Timothy J. Halsmer

Jacobs Technology, Cleveland, Ohio

Shape Memory Alloy Rock Splitters (SMARS)—A 

Non-Explosive Method for Fracturing Planetary 

Rocklike Materials and Minerals

NASA/TM—2015-218832

July 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

Funding from the NASA Space Technology Mission Directorate (STMD), Center Innovation Fund (CIF) is gratefully acknowledged. 

Manufacturing and imaging support from Grant E. Feichter, D.J. Brinkman, B.R. Caswell, and D. Brown are gratefully 

acknowledged. Material development support from (ARMD) Transformational Tools & Technologies (TTT) project is gratefully 

acknowledged. The authors thank S.A. Padula II, G.S. Bigelow, A. Garg, and D.J. Gaydosh, (shape memory alloy team) for helpful 

discussions. The authors also thank C.M. Creager and P.B. Abel for support in the SLOPE facility. 

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NASA/TM—2015-218832 1

Shape Memory Alloy Rock Splitters (SMARS)—A Non-Explosive 

Method for Fracturing Planetary Rocklike Materials and Minerals

Othmane Benafan and Ronald D. Noebe

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Timothy J. Halsmer

Jacobs Technology

Cleveland, Ohio 44135

Abstract

A static rock splitter device based on high-force, high-temperature shape memory alloys (HTSMAs) 

was developed for space related applications requiring controlled geologic excavation in planetary bodies 

such as the Moon, Mars, and near-Earth asteroids. The device, hereafter referred to as the shape memory 

alloy rock splitter (SMARS), consisted of active (expanding) elements made of Ni50.3Ti29.7Hf20 (at.%) that 

generate extremely large forces in response to thermal input. The preshaping (training) of these elements 

was accomplished using isothermal, isobaric and cyclic training methods, which resulted in active 

components capable of generating stresses in excess of 1.5 GPa. The corresponding strains (or 

displacements) were also evaluated and were found to be 2 to 3 percent, essential to rock fracturing and/or 

splitting when placed in a borehole. SMARS performance was evaluated using a testbed consisting of a 

temperature controller, custom heaters and heater holders, and an enclosure for rock placement and 

breakage. The SMARS system was evaluated using various rock types including igneous rocks (e.g., 

basalt, quartz, granite) and sedimentary rocks (e.g., sandstone, limestone).

1.0 Introduction 

Solar system exploration has been reliant on state-of-the-art space technologies ranging from 

navigation systems, to propulsion, to scientific instruments. These missions often include close contact 

with planetary bodies (e.g., the Moon, Mars, asteroids and comets) where in-situ environmental sampling 

is sought. In such cases, scientific instruments are built to detect, collect and characterize samples from 

the atmosphere, dust particles, soil, rock samples, or aeolian deposits amongst others. This provides 

researchers with geologic and climate history for a better understanding of near-Earth planets, asteroids 

and the evolution of the objects as a whole. However, certain areas of interest such as large rocks or hard 

craters are often not examined due to the site size, rock rigidity or inadequate capability of on-board tools

for extraction of appropriate samples. This is mainly due to the restrictions in physical space and weight 

to carry necessary equipment to space, inability to use typical tools from the lack of gravitational forces, 

or for mission safety reasons (e.g., carrying explosives onboard spacecraft and rovers). Consequently, 

only surface sampling (e.g., through brushing), dust (e.g., through boring and drilling) or small, loose 

rocks can be examined. Moreover, based on revolutionary discoveries by previous space missions, it is of 

interest not only to analyze matter on the planetary bodies, but also to return soil and rock samples to 

Earth for more detailed studies and investigations. As a result, rock breaking capability onboard 

spacecraft can be enabling for the examination of internal structures or compositions of larger geologic 

structures, and if appropriate, transfer of relevant samples back to Earth for more rigorous studies.
NASA/TM—2015-218832 2

Rock breaking and splitting is a common task on Earth, accomplished using many different methods. 

Dynamically, explosive (blasting) methods have frequently been used to break large rock formations in 

mines and drilling operations. Static methods have also been employed such as fluid pressure cells, 

chemical agents, and hydraulic wedges. Most of these methods, however, are not suitable for space 

applications due to the large size and weight of the equipment. Moreover, demolition techniques generate 

dust, noise, vibrations and flying debris that can interfere with space vehicle components (e.g., sensors, 

detectors, cameras) and pose safety concerns. Even in the case when static agents and pressure fluids are 

used, these methods are time-consuming, risk contamination of the environment, and do not guarantee 

chemical reactions with certain rocks, especially unknown rock structures on distant planets. As a result, 

there is a need for compact, reliable and cost-effective methods for static rock splitting.

One way of accomplishing this task is by taking advantage of shape changing materials known as 

shape memory alloys (SMAs). SMAs are materials that possess the unique ability to recover large 

deformations and generate high stresses in response to thermal or mechanical stimuli. This behavior 

occurs by virtue of a crystallographically reversible martensitic phase transformation between a high 

symmetry austenite phase and a low symmetry martensite phase. In general, when this type of material is 

deformed, the induced deformation can be recovered by applying heat above a certain threshold 

temperature referred to as the austenite start (As) temperature. On the other hand, when the material is 

deformed and constrained from recovering, it can generate extremely large forces upon the application of 

heat. This high power/weight ratio characteristic makes SMAs suitable for large force applications, such 

as static rock breaking.

SMAs for rock breaking concepts have been proposed in the past for ground-based applications. 

Nishida et al. (Refs. 1 to 4) have demonstrated the concept using conventional NiTi alloys where a 

maximum recovery force of 100 kN (566 MPa) was attained when the SMA elements was heated to 

approximately 90 °C with hot water. Carosio et al. (Refs. 5 and 6) have also used NiTi (As = 95 °C) and 

NiTiCu (As = 66 °C) alloys where stresses of about 800 and 650 MPa were obtained, respectively. Jing et al.

(Refs. 7 to 9) have disclosed a method for directional fracture of rocks in which NiTi, iron-based, or copperbased shape memory alloys were used as the pressure transfer body. In this case, the rock breaker 

embodiment (i.e., the SMA shapes and placement) was similar to the work done by Nishida et al. (Ref. 1).

An and Nam (Ref. 10) utilized ternary Ti-Ni-Cu alloys with compositions ranging from 5 to 20 at.% Cu

with activation temperatures below 100 °C. This approach utilized the expansion stresses of the SMA rods 

along with additional plates for maximizing the load transfer. Lee (Ref. 11) developed a static rock crusher 

where the load transfer is accomplished by converting the vertical displacement of an expanding SMA into 

horizontal stress using wedge-shaped metals. Glushchenkov et al. (Ref. 12) developed a jaw breaker where 

the shape memory material is used for thrust rods and reset springs. Chevakin, et al. (Ref. 13) made a power 

actuator where the load-bearing casing of the actuator body is made of a material such as a NiTi alloy to 

reduce the space of the chamber, forcing the working medium to exert a force on the actuating member. 

Yamauchi and Sato (Ref. 14) used a Ti-Ni alloy consisting of 49.0 to 51.0 at.% Ni that was aged at 400 to 

500 °C to produce a SMA rock crusher predeformed by compression at room temp and has thermal 

restoring function at ≥50 °C. Watabe (Ref. 15) developed a static crushing agent made of SMA pieces in 

conjunction with a hydrate inflating agent and an admixture of sand. The mixture is filled in a hole with 

water where expansion takes place once heated. In a more general conception, Coenen et al. (Ref. 16) have 

originated the idea of placing some expanding elements (such as an SMA) as part of a fracturing tool to 

exert a circumferentially varying pressure against the borehole wall. This notion was targeted for fracturing 

underground formations for the production of hydrocarbon fluids. Pelgrom (Ref. 17) has also considered a 

method for placing a body of shape memory metal within a tube targeted for subsurface formations for the 

production of hydrocarbons. Both of these concepts employ SMAs to generate a force on the borehole walls 

upon the application of heat. A comparison of these SMA-based methods and other techniques such as 

blasting for breaking rock can be found in Table 1 and Table 2, respectively.
NASA/TM—2015-218832 3

TABLE 1.—COMPARISON OF ROCK SPLITTING METHODS AND

APPLICATIONS BASED ON SHAPE MEMORY ALLOYS

Methods Principle of operation Example applications References

Non-Explosive Methods

Shape memory alloy 

rock splitter (SMARS)

NiTiHf high temperature shape 

memory alloy

Foreign planets (e.g., Mars), Martian volcanic 

rocks, asteroids, comets, proppants, spacers, 

fragile geological sampling, fossils, structure 

reinforcement, mining, oil drilling

*As = 156 °C, **σrec = 1.5 GPa

Current 

work and 

18

Actuator for rock 

splitting

NiTi and NiTiCu actuators Quarrying industry

As = 95 °C, σrec = 0.8 GPa (NiTi)

As = 66 °C, σrec = 0.65 GPa (NiTiCu)

5, 6, and 

19

Static rock breaker TiNi-shaped memory alloy Mining, civil and construction engineering

As = 90 °C, σrec = .566 GPa

1 to 4

Shape memory alloy 

Stone Breaker

NiTi, Fe-based, Cu-based Stone exploitation, tunnels, side slopes, 

presplitting blasting and demolition blasting

7 to 9

Static stone breaker 

using ternary SMA

Ti-Ni-Cu alloy (5 to 20 at.% Cu) Stoner breaker

As < 100 °C

10

Stress transfer device Wedge-type static rock crusher 

using SMAs

Rocks 11

Apparatus for breaking 

monolithic objects

Jaw breaker using shape memory 

material for thrust rods and reset 

springs and liners for recovery

Monolithic objects 12

Power actuator Load-bearing casing of the 

actuator body are made of a “form 

remembering” material such as a 

NiTi alloy

Mining and construction for working rock 

quarries, fracturing of oil reservoirs and gasbearing strata, presses, jacks, guillotines

13

Method and tool for 

fracturing an 

underground formation

Placement of elements (such as an 

SMA) to exert a circumferentially 

varying pressure against the 

borehole wall.

Production of hydrocarbon fluids, oil and gas 

production,

16

Method for placing a 

body of shape memory 

metal within a tube

SMAs generate a force on the 

borehole walls upon the 

application of heat

Subsurface formation for the production of 

hydrocarbons

17

Shape memory alloy for 

rock crusher

Ti-Ni alloy consisting of 49.0 to 

51.0 at.% Ni aged at 400 to 500 °C

rock crushing

Thermal restoring function at ≥50°C

14

Rock crushing method Shape memory alloy rods and 

heaters

Rock crushing, reduction of noise and oscillation 20

Static Crushing Agent A crushing agent made of a SMA 

formation piece with a hydrate 

inflating agent and an admixture of 

sand

Holes

Restoring temperature of 50 to 100 °C

15

*As: Austenite start temperature, **σrec: Recovery stress
NASA/TM—2015-218832 4

TABLE 2.—COMPARISON OF ROCK SPLITTING METHODS AND

APPLICATIONS THAT DO NOT INVOLVE SHAPE MEMORY ALLOYS

Methods Principle of operation Example applications References

Non-Explosive Methods

Heat source Heaters placed in a series of drilled small 

holes; the heat transmitted to the rock 

causes splitting

Splitting of granite, marble, and other 

rocks

21

Thermal expansion Metal reinforcement bars within concrete 

slabs are heated to cause thermal expansion 

for cracking

Ferroconcrete bodies, concrete 22

Thermal expansion Attaching and heating of two metals with 

dissimilar coefficient of expansion to cause 

distortion for splitting

Splitting of stone, either in quarrying or on 

a smaller scale

23

Ripping Large teeth attached to the back of a 

bulldozer/tractor

Populated areas, soft to moderately firm 

rocks

24

Foam Injection High-pressure foam Rock excavation or secondary breakage 

(mining and tunneling)

25

Flame jet Ultra hot hydrogen flame (~4000 °C) Mining industry 26 and 27

Water injection Hydraulic Mining industry 28

Plasma blasting Copper-oxide/aluminum thermite reaction 

initiated by an electrical charge 

Mining industry 29

Cardox tube High pressure gas (carbon dioxide) Rock and concrete breakage, deep sea 

excavation, tunneling and shaft sinking 

and trenching and excavation

30

Nonex High pressure gas (propellant burning) Slate mines, mine shafts, 31

Penetrating Cone 

Fracture (PCF)

High pressure gas (propellant burning) Mines, clearing of block grizzlies, 

crushers or chute (oversize)

30

Pneumatic Fracturing Gas (typically air or nitrogen) Mining industry 32

Thermal (cryogenic) 

fracturing

Thermal stresses from fluids colder/hotter 

than the reservoir

Mining industry 32

Chemicals Chemical reaction/pressure Rock trenching, granite boulder breaking, 

rock removal, concrete demolition

32

Explosive Methods

Explosive blasting Explosive charges Canals, Watercourses, harbors, quarries, 

rock fracturing, dams, construction sites

33

Presplitting Create a secondary fracture plane before 

production blasting (light to heavy charges)

Competent, hard to extremely hard rock 34

Trim (cushion) 

blasting

Light charges placed in single row of holes 

along the excavation line

Contoured slopes with benches or other 

slope variations

35

Contour (smooth) 

blasting

Decoupled charges fired simultaneously in 

closely spaced holes

Mining industry 36

All of the abovementioned applications and concepts utilized SMAs, but to the best of the authors’ 

knowledge, all of the SMAs incorporated in these devices exhibited activation temperatures below 100 °C 

and generated stresses of ≤800 MPa through isothermal (compressive or tensile) loading. When dealing 

with space applications, however, the harsh space environment poses added constraints and requirements 

for the design of space mechanisms. Constraints such as weight, size, operational temperatures, and ease 

of operation amongst others have always been of prime importance. SMARS, designed as part of this 

work is capable of activation at temperatures above 100 °C and generating nearly twice the stress of these 

other systems. It is also ideal for use in planetary rock drilling/sampling operations, where flying debris 

from blasting methods can destroy the rock formation of interest, pose safety concerns to the astronauts, 
NASA/TM—2015-218832 5

or cause damage to the costly nearby equipment (e.g., rover mirrors and sensors). In addition, static 

SMARS requires little setup and activation time compared to other static methods such as chemical 

agents that can take up to a few days to react with some hard and unknown rock formations. Mission 

reliability is another benefiting factor since SMARS operates based on a material response and only 

requires heat input to activate without the need for complex valve systems or hydraulic fluids, making it

extremely simple and essentially fool proof to operate. The small volume and extremely low weight of the 

SMARS reduces payload launch costs and transportation hazards when compared to heavy hydraulic 

wedges and dangerous explosive materials and chemicals. 

Thus, the goal of this work was to explore high-force, high-temperature SMAs, referred to as 

HTSMAs, for use as a static rock breaker where higher activation temperatures and greater force 

generation can be achieved. The SMARS device described herein employs Ni-rich NiTiHf alloys, which 

have shown promising results regarding actuation and stability (Ref. 37), as the expanding members. 

Along with custom heaters and transportable controller, SMARS key components and performance 

parameters were evaluated during trials using several rock types. In addition, new material conditioning 

methods were developed and optimized for prestraining the shape memory elements, which are simple 

enough that they could also be utilized in space to reset the actuators.

2.0 Principle of SMARS Operation 

SMARS is a static rock breaking device developed with the objective to provide controllable rock 

splitting without any demolition damage to the sample or surrounding environment, compared to the 

alternative dynamic explosive or blasting approaches. It consists of (i) an expanding element made of 

NiTiHf alloys, (ii) a heating system that fits over the HTSMA member, (iii) end-tips used to contact the 

rock surface consisting of various end geometries (i.e., conical, spherical, cylindrical, helical or flat ends), 

and (iv) a temperature controller and thermocouples for device process operation (Figure 1).

Figure 1.—SMARS components showing a schematic of the control box, the heater assembly and an expanded view 

of the heater internal construction and SMA placement.
NASA/TM—2015-218832 6

SMA conditioning: The actuator material (i.e., the NiTiHf alloy) in the form of a cylindrical pellet is 

initially conditioned, henceforth referred to as “training”, to produce desirable forces once activated. The 

training methods used are described in the forthcoming sections. After training, the material exists in the 

low temperature martensite phase where the end-conditions are mechanically attached or directly 

machined on the trained material. The effective length from tip to tip is made equal to a predrilled 

borehole diameter for proper operation (Figure 2(c)). 

Heaters and SMARS placement: Custom heaters, made of Nichrome wires and stainless steel housing, 

are placed over the trained HTSMAs and inserted into a predrilled borehole in the area of interest within a 

rock or pre-existing cracks (Figure 2(d)). Multiple assemblies can be placed within the rock to magnify 

the force or provide additional displacement necessary for complete rock splitting, especially of larger 

samples.

SMARS activation: The final step is to deliver a power signal to the heaters using a DC power supply 

or other power source to activate the device. On heating, the trained material undergoes a phase 

transformation to the high temperature austenite phase, where in the unconstrained condition a shape 

recovery would normally occur (i.e., expansion in this case). However, since the device is confined by the

borehole wall, the SMA cannot initially expand; instead it exerts a force at the contact area resulting in 

rock fracture (Figure 2(e)). After the rock splitting, the device can be recovered and reused for successive 

employments, given that a new SMA piece will be used, or the same SMA member can be retrained.

Figure 2.—Principle of SMARS operation: (a) assembled and exploded view of SMARS components. (b) Rock 

sample before drilling. (c) Hole drilling corresponding to the SMA length. (d) SMARS placement within the 

borehole. (e) SMARS activation after heating to above the austenite finish (Af) temperature.

Shape memory 

alloy 

Tip

Heater

(b) Rock sample (c) Drill hole (d) Insert SMARS (e) Activate SMARS

(a) SMARS components 

Tip
NASA/TM—2015-218832 7

3.0 Materials and Procedures

3.1 Material

The material used in this work was a ternary Ni-rich Ni50.3Ti29.7Hf20 (at.%) alloy produced by vacuum 

induction melting. The casting was vacuum homogenized at 1050 °C for 72 h, followed by extrusion at 

900 °C through an area reduction ratio of ~7:1. SMARS pellets and other test specimens were machined 

from the extruded rods (Ext. 189) and subjected to an aging treatment of 3 h at 550 °C followed by 

furnace cooling. Various physical and thermomechanical properties of heat-treated alloys of similar 

composition are available in the literature (Refs. 37 and 37).

3.2 Thermomechanical Testing and Training

Thermomechanical testing was conducted using a servohydraulic load frame equipped with a hightemperature extensometer and an Ameritherm (Ambrell) induction heating system. Prior to any testing, 

each specimen was gripped on the frame and subjected to two stress-free (<1 MPa) thermal cycles 

between 30 and 300 °C to relieve any residual stresses resulting from the sample machining and handling.

The transformation temperatures: martensite start (Ms), martensite finish (Mf), austenite start (As), and 

austenite finish (Af) were determined from the stress-free, strain-temperature response and were 141, 126, 

156, and 170 ± 2 °C, respectively.

In order to achieve the anticipated forces and displacements necessary for effective SMARS 

operation, several training processes were used. Training is a process used to produce a desirable shape 

memory/superelastic response by performing selected mechanical, thermal and/or thermomechanical 

loading procedures. Training is typically performed to stabilize the cyclic response of an SMA and reduce 

dimensional instability (Refs. 39 and 40), to shape set a specific form (Ref. 41), or to introduce a 

particular behavior such as the two-way shape memory effect (TWSME) (Ref. 42). In the case of 

SMARS, training was used to attain the highest possible blocking forces (i.e., the forces generated during 

the constrained heating, also known as recovery forces) with the highest possible displacements. For this 

purpose, two different training methods were investigated, referred to as isothermal and isobaric training 

routines.

Isothermal training: This method consisted of deforming the material in the martensite phase at room 

temperature to a specific strain (or stress) level. The resulting blocking stresses were determined by 

holding the strain constant (before unloading) while thermal cycling between 30 and 300 °C. 

Isobaric training: In this method, the material was thermomechanically cycled twice under a constant 

stress between 30 and 300 °C. To measure the blocking force generated by this technique, the material 

was unloaded to zero stress at the end of the second isobaric thermal cycle, the remnant strain was held 

constant and the constrained sample was again thermally cycled between the temperature limits of 30 and 

300 °C.

Repeated isothermal training and evaluation cycles were used to verify the ability to reuse/retrain 

NiTiHf elements for SMARS operation. During this process, the material was deformed in the martensite 

phase at room temperature to a specific stress value, followed by unloading to zero stress. At this point, 

the blocking force was measured by holding the remnant strain constant while cycling between 30 and 

300 °C. This procedure was repeated again multiple times, i.e., loading and unloading followed by 

constant strain thermal cycling, to determine the viability of the alloy to be used in repeated SMARS 

applications. 
NASA/TM—2015-218832 8

TABLE 3.—MECHANICAL AND PHYSICAL PROPERTIES OF SELECTED ROCK MATERIALS

DATA PRESENTED IN THIS TABLE WAS OBTAINED FROM REFERENCES 32 AND 33

Properties Igneous rocks Sedimentary rocks

Basalt Granite Quartz Sandstone Limestone

Density, g/cm3 2.8 to 3.0 2.6 to 2.7 2.6 to 2.8 2.2 to 2.8 2.3 to 2.7

Uniaxial compressive strength, MPa 341 250 300 170 250

Tensile strength, MPa 10 7 to 25 5 to 20 4 to 25 6 to 25

Elastic modulus, GPa 40 to 80 30 to 70 150 to 300 15 to 50 20 to 70

Strain at failure, % 0.35 0.25 0.2 0.2 ----------------

Fracture mode I toughness 0.11 to 0.41 >0.41 >0.41 0.027 to 0.041 0.027 to 0.041

Porosity, % 0.22 to 22.1 1.02 to 2.87 0.40 to 0.65 1.62 to 26.4 0.27 to 4.10

Drilling rate index, DRI 20 to 75 30 to 80 25 to 80 15 to 90 30 to 100

Hardness, Vickers 450 to 750 725 to 925 1060 550 to 1060 125 to 350

3.3 Rock Selection and Properties

Several categories of rocks were used for the assessment of the SMARS performance. The rock 

selection was made based on the types that are typical to lunar and Martian igneous and sedimentary 

deposits, and other forms that can be found in solar system planets, asteroids or comets. The igneous 

rocks used were basalt and granite. Sandstone and limestone were used as examples of sedimentary type 

rocks. Basic properties of these rocks can be found in Table 3. Cylindrical-shaped boreholes were drilled 

in each rock sample using diamond core drill bits with an outer diameter of 19.05 mm (0.75 in.).

4.0 Prototype Hardware Design

End-tips: In addition to the HTSMA used as the expanding member, several other components were 

indispensable to the design of SMARS. One of these components is the end-tips. The end-tips directly 

attach to the HTSMA member and were designed for two purposes: first was the interchangeability of tips 

depending on the rock types or hardness, and second was to compensate for any gaps between SMARS 

and the rock surface in the case of an irregularly shaped borehole. Several end-tips were designed 

including conical, spherical, cylindrical, and flat ends (Figure 3). The flat ends were the simplest form and 

were designed for plane borehole walls or pre-existing cracks in the vicinity of the rock of interest. The 

conical ends were considered for acute penetration into the rocks to help with the crack initiation and 

ultimately propagation to fracture. Both the spherical and cylindrical ends were designed for maximizing 

the contact surface area within the borehole. In these cases, the end tip forms were machined to match the 

borehole diameter. Once the SMARS is used, presetting the form was achieved using custom platens that 

matched each of the end-tips (in the case of fixed tips). In the current study, a servohydraulic frame was 

used to apply resetting loads, but a hand pump equipped with a heater can also be used if resetting is 

required on remote sites.

Pushers: Due to the limited strain recovery of most SMAs (approximately 2 to 5 percent strain), the 

strains (or displacements) produced are occasionally insufficient for complete rock splitting, particularly 

for small boreholes. Nonetheless, it is noted that a complete splitting is not necessarily the goal in all 

cases. Often, only initial cracking is desired near wells or other critical areas such as fragile fossils. When 

complete splitting is required, SMA pushers were designed to provide additional displacement after the 

main HTSMA members have been activated. The pushers were made in the form of helical springs
NASA/TM—2015-218832 9

(Figure 4) with axial spring rates ranging from 175 to 875 N/mm (1000 to 5000 lbf/in) with displacement 

exceeding 10 percent from the solid height (fully compressed). The pushers are placed in the top position 

of the heater assembly shown in Figure 1, and are heated sequentially after the bottom and/or middle

HTSMA members are fully expanded. As a result, these pushers are not required to provide high blocking 

forces, but provide additional displacements to fully split the rock.

Heaters: Three types of heating systems were considered for SMARS operation. The heater design 

shown in Figure 1 was custom made using a metallic sleeve, adhesive cement, and Nichrome wires 

housed in the heater assembly. This design was compact in size and capable of heating to very high 

temperatures (exceeding 400 °C) in a relatively short period of time (seconds to 1 min). The second 

design consisted of using commercial flexible Kapton (DuPont) or Thermofoil(Minco Products, Inc.)

heaters that are wrapped around and bonded to the HTSMA members. Although this was the lightest and 

least power-consuming heating method (10 W/in2

), it was very time consuming and in some cases the 

desired temperatures were not reached due to the small surface area of the HTSMA members in contact 

with the heaters. The final method consisted of using induction heating. This technique is preferred for 

large, dry boreholes. Induction heating yields the fastest heating rates, but requires much higher power 

consumption (e.g., 120 V AC) and footprint (a separate power supply is needed to power the induction 

unit).

Controller: SMARS operation was controlled using a custom-built controller box comprised of a 0 to 

20 mA temperature indicator with retransmission, a 24 V DC power supply, selector switches to trigger

the individual heaters either simultaneously or sequentially, connectivity for type-K thermocouples, and a 

main power switch to activate the chosen heaters. A circuit diagram of the controller components is 

shown in Figure 5. 

Figure 3.—End-tip conditions and method for gap adjustment.

End-tip conditions Adjustable length end-tips

Spherical Conical Cylindrical

Side view Top view

Attached Machined

Gap Gap 

corrected
NASA/TM—2015-218832 10

Figure 4.—Shape memory alloy helical pushers. End-tips can also be attached to this component.

ID

OD

L

C (borehole) C

Section C-C
NASA/TM—2015-218832 11

Figure 5.—Simplified controller circuit diagram. 

5.0 Training Results and Discussions

5.1 Isothermal Training

The isothermal stress-strain response of the Ni50.3Ti29.7Hf20 alloy in uniaxial compression at room 

temperature is shown in Figure 6. In addition to the reference load-unload cycle to –1000 MPa (solid 

line), individual samples were compressed to the indicated strain levels (open symbols) at which point the 

strain was held constant followed by thermal cycling. It was shown in previous studies that no plastic 

deformation attributed to slip was observed during the initial loading to –1 GPa (Ref. 37), however an 

increasing amount of inelastic deformation due to martensite variant reorientation/detwinning occurs with 

increasing stress (Ref. 37).

To determine the effect of this training procedure on the blocking stress, once the strain imposed at 

room temperature was held constant, each specimen was thermomechanically cycled twice between room 

temperature and 300 °C as shown in Figure 7. Note that two sample geometries were used for this 

purpose. The data presented in Figure 7(a) to (d) was produced from threaded, dogbone specimens 

5.08 mm (0.2 in.) in diameter and 15.24 mm (0.5 in.) in gauge length. For these tests, it was possible to 
NASA/TM—2015-218832 12

Figure 6.—Compressive stress-strain response of the 

Ni50.3Ti29.7Hf20 alloy deformed at room temperature. The open 

symbols indicate the prestrain level imposed for each specimen 

(isothermal training) prior to temperature cycling shown in 

Figure 7.

allow the load to reverse to slightly positive values during thermal cycling as indicated by the dashed 

lines. For higher strains where buckling may become an issue with this sample geometry, compression 

samples 5 mm in diameter and 10 mm long were used to produce the data in Figure 7(e) and (f). In this 

case however, it was not possible to allow the load to reverse above the zero load line with the 

compression setup used. As a result, the lower cycle limit for temperature cycling was increased from 30 

to ~50 °C resulting in a slightly compressive load (≤ –10 lbf) at the lower end of the thermal cycle in

order to always maintain contact with the platens (Figure 7(e)).

In either geometry, the data exhibited similar trends. During the initial heating cycle, the material 

exhibited a transient behavior at around 150 to 170 °C where the stress went up briefly followed by a 

sharp decrease to a minimum value at the upper cycle temperature (300 °C). This behavior is only 

observed on the first cycle and it occurs at approximately the same temperature regardless of the applied 

strain. The second (and all succeeding cycles—not shown) exhibited typical hysteresis loops identified by 

the four characteristic temperatures. Because of the transient behavior during the initial cycle, the four 

transformation temperatures corresponding to the 2nd cycle are summarized in Figure 8. It is noted that 

accurate determination of the Af temperatures was rather challenging due to the non-linear nature of the 

curves. Both the As and Mf transition temperatures, as determined by the intercept method, are shown to 

slightly decrease with increasing level of the applied strain imposed during thermal cycling. On the other 

hand, the Af and Ms transition temperatures shifted to higher values as a function of applied strain. 

The important feature relevant to the SMARS device is the magnitude of the stresses that can be 

generated after the trained and subsequently constrained sample is heated (Figure 7). The initial thermal 

cycle, in particular, simulates the operation of the SMARS device after isothermal training. In addition to 

the phase transformation, the material’s thermal expansion also contributes to the maximum stress 

generation, which is prevalent in the linear region after the Af temperature. It has also been found in 

previous studies (Ref. 43) and other unpublished work, that even after the apparent Af temperature, there 

could be remnant martensite that continues to transform with increasing temperature. Nonetheless, both 

the continued phase transformation and the thermal expansion favor the stress generation in samples 

constrained in compression. However, the thermal expansion works against samples constrained in 

tension resulting in relaxation of the stress (Ref. 43).
NASA/TM—2015-218832 13

Figure 7.—Stress–temperature responses of the Ni50.3Ti29.7Hf20 alloy corresponding to the isothermal training of 

Figure 6.
NASA/TM—2015-218832 14

Figure 8.—Transformation temperatures determined from 

the 2nd cycle during the constant-strain, thermal cycling 

conditions.

Figure 9.—End stresses measured in the austenite phase 

at 300 °C after the isothermal training as a function of 

the applied strain.

The maximum stresses generated, measured at the upper cycle temperature of 300 °C, are plotted as a 

function of applied strain level in Figure 9. For extended cycles (not shown), a decrease in blocking stress 

with cycling was evident for applied strains higher than –0.8 percent. This is largely due to yielding of the 

austenite phase once critical stresses are reached resulting in permanent plastic deformation as shown by 

Coughlin et al. (Ref. 44) and Benafan et al. (Ref. 37). However, only the first thermal cycle is pertinent 

to SMARS operation, and the largest stress is always generated during the first cycle. The maximum 

blocking stresses achieved through this training method was –1.3 GPa using an applied strain of 

–2 percent.

5.2 Isobaric Training

The strain–temperature response of the Ni50.3Ti29.7Hf20 alloy in uniaxial compression is shown in 

Figure 10. Each specimen was thermomechanically cycled under a constant stress of –100, –200, –300 or 

–400 MPa, for two complete cycles. As opposed to the isothermal training method where the martensite 

was textured isothermally through mechanical loading at room temperature, in this isobaric training 

routine the martensite is preferentially textured via the initial loading and most predominantly through 

thermomechanical cycling under the constant stress. For example, under a constant stress of –200 MPa 

(Figure 10(b)), the initial loading resulted in ~0.35 percent strain, while a complete thermal cycle reduced 

the size of the sample by an additional 1.55 percent strain. Note that since this alloy is dimensionally 

stable, one single cycle is sufficient to reorient the martensite phase, as the two cycles overlap except for 

the initial heating cycle. But for examination purposes, two cycles were conducted in each trial. At the 

end of the second cycle, the material was unloaded to zero stress.

To determine the level of blocking stress developed by this training technique, the sample was fixed 

in strain at its new shape and the sample thermally cycled under constant strain. The corresponding 

blocking stress results are shown in Figure 11. Since the martensite is mostly oriented as part of this 

training method, the typical transient response as observed during isothermal training (Figure 7), did not 

occur. In this condition, blocking stresses built up on the first heating cycle followed by a stress relaxation 

close to zero upon cooling. Similar to the isothermal training, the nature of the transformation 

temperatures is analogous to the previous results.
NASA/TM—2015-218832 15

The blocking stresses obtained in this training process were higher compared to the isothermal 

training method. Yet, the stress levels used in the isobaric training process (Figure 10) were much lower 

than the conjugate stresses attained during the prestrain (training) cycle, shown in Figure 6. For instance, 

isobaric training at –400 MPa (Figure 11(c)) resulted in a blocking stress of –1.5 GPa. On the other hand, 

a prestrain to –2 percent strain (conjugate stress of –2 GPa (Figure 6)) resulted in blocking stress of 

–1.3 GPa on the first cycle. Consequently, isobaric training is a more effective method for reorienting the 

microstructure for optimum blocking stress generation. 

Figure 10.—Constant stress, strain-temperature response of the Ni50.3Ti29.7Hf20 for the indicated stress level (isobaric 

training).
NASA/TM—2015-218832 16

Figure 11.—Stress–temperature responses of the Ni50.3Ti29.7Hf20 alloy corresponding to the isobaric training of 

Figure 10. The blocking stresses are generated during constant strain, thermal cycling after different training levels 

indicated in each plot.

A summary of the blocking stresses measured at 300 °C is shown in Figure 12. It may even be 

possible to obtain higher blocking stresses using this training method by additional martensite 

reorientation. The –400 MPa isobaric thermomechanical cycle yielded a transformation strain of 

–2.4 percent (Figure 10(d)), but it is known from previous work (Ref. 37) that additional transformation 

strain is possible with higher applied stresses. However, the load limit for the mechanical test setup was 

reached and could not accommodate much larger blocking stresses. Regardless, a significant 

improvement in blocking stress generation was achieved using this isobaric training process in 

comparison with the previous isothermal method. A blocking stress of nearly –1.5 GPa was attained by 

this technique, with a training stress of only –400 MPa.
NASA/TM—2015-218832 17

Figure 12.—End stresses measured in the austenite phase at 

300 °C after the isobaric training as a function of the

applied training stress.

Figure 13.—Compressive stress-strain response of the Ni50.3Ti29.7Hf20 alloy deformed to -1 GPa at room temperature. 

(a) The cyclic testing sequence, and (b) the 9 sequences showing the change with cycling. 

5.3 Ability to Reuse/Retrain Ni50.3Ti29.7Hf20 SMA Elements for SMARS Operation

Cyclic operation of the Ni50.3Ti29.7Hf20 alloy was investigated to determine the effect of retraining on 

the behavior of the SMARS elements. During this process, the material was loaded, similar to the 

isothermal training routine except to a target stress instead of target strain level, to –1 GPa followed by 

unloading to zero stress. The remnant strain of approximately –0.8 percent was held constant while 

thermal cycling between 30 and 300 °C (Figure 13(a)). After the constant-strain cycling, the specimen 

was thermally cycled twice under 0 MPa, and the procedure was repeated. The stress-strain curves 

corresponding to each sequence (a sequence is defined by isothermal loading and unloading, constantstrain thermal cycles, and finally two stress-free thermal cycles) are shown in Figure 13(b). There is an 

apparent softening as shown by the inset, which is associated with an increase in blocking stresses 

(labeled 1 to 9). The stress-temperature responses corresponding to each of these sequences are shown in 

Figure 14. To better follow the trend, the data was plotted all together in the middle of Figure 14, while 

the individual responses are shown for sequences 1 to 8 labeled on the upper right corner of each plot. It 

is clearly shown that the blocking stresses gradually increase reaching approximately –1 GPa at the end of 

the 9th sequence (summarized in Figure 15). This additional increment in blocking stress is indicative of 

further martensite reorientation after each sequence.
NASA/TM—2015-218832 18

Figure 14.—Stress–temperature responses of the Ni50.3Ti29.7Hf20 alloy corresponding to the cycling of Figure 13. For 

comparison purposes, all the responses were plotted together in the middle figure, while the individual responses 

are shown for sequences 1 to 8 labeled on the upper left corner of each plot. 
NASA/TM—2015-218832 19

Figure 14.—Concluded.
NASA/TM—2015-218832 20

Figure 15.—End stresses measured in (a) and(c) the martensite phase at 30 °C and (b) and(d) the austenite phase at 

300 °C. The data is plotted as a function of cycle number in (a) and (b), and as a function of the testing sequences 

in (c) and (d).
NASA/TM—2015-218832 21

This behavior is further examined by considering the stress-free thermal cycles performed at the end 

of each sequence. The post strain-temperature behavior for each case is shown in Figure 16 where the 

data was combined in the middle of the figure, and the individual responses are shown for sequences 1 to 

8 labeled on the upper left corner of each plot. Initially, the remnant strains are recovered and the no-load 

response is typical of that observed for as-extruded material with the strain change due to the uniaxial 

response of the volume change due to the martensitic transformation (Ref. 37) (Figure 16(a)). With 

repeated reloading and cycling, it is apparent that the random variant structure is being replaced by a 

structure consisting of compressive martensite variants resulting in a typical compressive straintemperature response even without application of an external load (Figure 16(h)). This progression takes 

place by way of interplay between martensite variant reorientation and formation of lattice defects that aid 

the process, both of which are experimentally demonstrated by the generation of additional blocking 

stresses and the slight accumulation of residual strain, respectively.

Regardless of the microstructural mechanisms, it is noted that the cyclic operation of the 

Ni50.3Ti29.7Hf20 elements actually resulted in an increase in blocking stresses and an improvement in 

performance. This actually demonstrates the benefit of retraining of the SMAs after use in SMARS and 

the advantages of the NiTiHf alloy used in this work. 

6.0 SMARS Trials

The SMARS concept was evaluated using a custom testbed as shown in Figure 17(a). Predrilled rocks 

were positioned in an enclosure along with the supporting hardware as discussed previously. A variety of 

SMAs with different end-conditions were also fabricated as shown in Figure 17(b). Depending on the nature 

of the rock and desired outcome (i.e., cracking, or complete splitting), appropriate SMA elements (SMA 

plus end-tips) were placed within the heater assembly, which was then inserted in the drilled hole for testing. 

Over 35 trials were conducted on four rock types (basalt, granite, sandstone and limestone). The results, 

cracking and/or complete fracture of the rock samples are documented in Figure 18 to Figure 23.

In Figure 18, a Ni50.3Ti29.7Hf20 sample, trained using the isobaric method under –200 MPa, was used 

to break a basalt rock. In this test, one SMA sample was placed 2.5 in. deep inside the borehole, with a

thermocouple attached to the surface. Although cracking noises were audible at around 170 °C, the first 

visible crack was observed at approximately 198 °C. After that, the cracking continued until the SMA 

element was fully expanded at about 289 °C. The entire test from SMARS activation to full cracking took 

less than 1 min. Because of thermocouple placement the actual temperature of the SMA was lower but 

tracked that of the thermocouple.

Another basalt test was attempted using a binary Ni49.9Ti50.1 alloy element isobarically trained under 

–300 MPa (Figure 19). Previous attempts using isothermal training on binary NiTi were unsuccessful and 

did not result in any cracking. However, the isobaric training technique was proven to be more efficient 

(Secs. 5.1 to 5.2), and when utilized in this test resulted in full fracture of the basalt sample in just 38 sec. 

But it should be noted that the maximum blocking stresses obtained in this material were only 700 MPa 

as opposed to 1.5 GPa in the Ni50.3Ti29.7Hf20 alloy. This means that less material can be used in the case of 

the Ni50.3Ti29.7Hf20 alloy or higher stresses can be generated to break much larger specimens, which is

typically an advantage for space missions. 
NASA/TM—2015-218832 22

Figure 16.—Strain–temperature responses of the Ni50.3Ti29.7Hf20 at 0 MPa stress. Each two cycles were performed 

subsequent to the constant strain, thermal cycling. For comparison purposes, all responses were plotted in the 

middle figure, while the individual responses are shown for sequences 1 to 8 labeled on the upper right corner of 

each plot. 
NASA/TM—2015-218832 23

Figure 16.—Concluded.
NASA/TM—2015-218832 24

Figure 17.—SMARS prototype testbed and enclosure with temperature controller. (b) Prototype 

kit of several SMAs with different end-conditions, heaters, and resetting platens.
NASA/TM—2015-218832 25

Figure 18.—Basalt test with Ni50.3Ti29.7Hf20 trained using the isobaric method under 

–200 MPa. The sample was placed ~2.5 in. deep within the borehole.

T = 209 °C T = 289 °C

Cracking start T = 25 °C T = 198 °C
NASA/TM—2015-218832 26

Figure 19.—Basalt test with Ni49.9Ti50.1 trained using the isobaric method under –300 MPa. 

The sample was placed ~0.5 in. deep within the borehole.

t = 31 s t = 38 s

Cracking start t = 0 s t = 28 s
NASA/TM—2015-218832 27

Granite rocks were tested using Ni50.3Ti29.7Hf20 samples trained using the isobaric method under 

–300 MPa (Figure 20) and the isothermal method to –1 GPa followed by unloading to zero stress 

(Figure 21). In the first example, the isobarically trained SMA element was placed 2.5 in. deep within the 

borehole and resulted in a sudden and complete fracture of the rock sample. Similar to the basalt rocks, 

cracking was audible before the break, which took place at approximately 249 °C. In the second example, 

the isothermally trained SMA element was placed only 2.0 in. deep within the borehole and resulted in a 

more controlled cracking starting at 291 °C. Although the granite in this example did not fully split, this 

case serves to demonstrate a more controlled fracture process by regulating the SMA placement within 

the borehole.

Figure 20.—Granite test with Ni50.3Ti29.7Hf20 trained using the isobaric method under 

–300 MPa. The sample was placed ~2.5 in. deep within the borehole.

T = 250 °C T = open

T = 25 °C T = 249 °C
NASA/TM—2015-218832 28

Figure 21.—Granite test with Ni50.3Ti29.7Hf20 trained using the isothermal method to 1 GPa 

(followed by unloading). The sample was placed ~2 inches deep within the borehole.

Limestone (Figure 22) and sandstone (Figure 23) rock samples were tested using binary NiTi 

elements isobarically trained under –200 and –100 MPa. Due to the nature of the rocks, cracking was 

achieved in most cases but a small hand pressure was necessary to split the rocks in half. It was observed 

that these kind of rocks crumble at the SMA/rock contact area requiring more strain for a full fracture. 

The SMA pushers (Figure 4) were designed for this purpose where additional displacement is needed, at 

stresses beyond what you can do by hand.

T = 302 °C T = 320 °C

T = 25 °C T = 291°C Cracking start
NASA/TM—2015-218832 29

Figure 22.—Limestone test with Ni49.9Ti50.1 trained using the isobaric method under 

–200 MPa. The sample was placed ~0.75 inches deep within the borehole.

T = 113 °C

t = 32 s

T = 130 °C

t = 39 s

Cracking 

start

T = 25 °C

t = 0 s

T = 107 °C

t = 26 s
NASA/TM—2015-218832 30

Figure 23.—Sandstone test with Ni49.9Ti50.1 trained using the isobaric method under 

–100 MPa. The sample was placed ~2.5 inches deep within the borehole.

7.0 Potential Applications

The SMARS device is targeted for static rock breaking, particularly when in situ extraction and 

analysis are imposed. It provides new solutions for sample extraction with significant reduction in weight, 

volume, and design complexity that would otherwise be deemed impractical for flight. The compactness 

and small footprint of the SMARS device makes it a possible candidate for future instrument packages

onboard spacecraft and rovers (e.g., Mars missions, etc.). The application of SMARS is fast (seconds 

from activation to rock splitting), non-disturbing to the surrounding environments (e.g., no adverse 

vibration effects), and a clean process (no dust or flying debris). Analysis of rocks in planet Mars is one 

potential application for this device. SMARS enables the splitting of a rock or crater formation of interest 

T = 104°C

t = 31 s

T = 125 °C

t = 36 s

Cracking start

T = 25 °C

t = 0 s

T = 99 °C

t = 29 s
NASA/TM—2015-218832 31

where in situ investigation is targeted. The inner structure of rocks that have not been exposed to the 

elements can hold clues to the past or presence of key minerals. It is envisioned that SMARS can be made 

part of an excavation tool comprised of drill heads, SMARS, and grippers (Figure 24). Another potential 

application is asteroid sampling as part of the NASA’s Asteroid Redirect Mission (ARM). Once the 

asteroid mass is redirected to orbit the Moon, SMARS can be used (either by robots or astronauts) to 

break a piece of the asteroid and bring it back to planet Earth for detailed analyses. SMARS is 

advantageous in this case over other methods since gravitation forces are not required to activate the 

device. While rock splitting is the concept in hand, SMARS can also be used in non-rock related 

applications. For example, the concept can be used as a proppant or spacers to unjam a trapped 

component.

Figure 24.—Future prospects and applications of SMARS in space missions.
NASA/TM—2015-218832 32

The SMARS device development can also be extended to the applicability of ground-based systems 

such as oil drilling, mining, fossil collecting and retrieval of other fragile geologic samples, proppants, 

civil engineering and other fields requiring compact but large static forces (Figure 25). It can also be used 

for structure reinforcement and corrective force applications to structural members and other engineering 

components.

Figure 25.—Future prospects for SMARS in terrestrial applications. 
NASA/TM—2015-218832 33

8.0 Conclusions 

A static shape memory alloy rock splitter (SMARS) along with supporting hardware including a 

custom heating system, detachable end-tips, and a modular controller was designed and tested using a 

precipitation strengthened, Ni-rich Ni50.3Ti29.7Hf20 (at.%) alloy.. The device was built particularly for 

space-related applications where sampling geological deposits is required in extraterrestrial environments 

including planetary bodies such as the Moon, Mars, and near-Earth asteroids. The distinctive 

characteristics of this work are summarized below:

• The Ni-rich Ni50.3Ti29.7Hf20 alloy exhibited inherently good force generation capability with 

activation temperature above 100 °C. This heat treatable, precipitation strengthened alloy was 

capable of generating stresses in excess of 1.5 GPa. 

• Several training methods were used to maximize the force generation including isothermal and 

isobaric methods. Isobaric training was found to be the most efficient method and yielded the 

highest blocking stresses.

• Repeated cycling of the Ni50.3Ti29.7Hf20 alloy elements indicated that recurrent use did not degrade 

the operability of the SMA elements and actually resulted in greater blocking stress capability 

with continued application. 

• SMARS functionality was demonstrated on multiple rock types including igneous rocks (i.e., 

basalt, granite) and sedimentary rocks (i.e., sandstone, limestone), all of which were successful.

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