ammonium perchlorate

Journal of Energy and Chemical Engineering Aug. 2014, Vol. 2 Iss. 3, PP. 94-105

- 94 -

Effect of the Formulation of Ingredients and the

Process Parameters on the Fracture Toughness of

HTPB Based Composite Solid Propellant CH Devi Vara Prasad*

1, V. Arunachalam

2, V. Ranganathan

3

*1Scientist, Solid Propellant Plant, Satish Dhawan Space Centre, Sriharikota, A.P., India-524124

2Deputy General Manager, Solid Propellant Plant, Satish Dhawan Space Centre, Sriharikota, India 3Chief General Manager, Solid Propellant Plant, Satish Dhawan Space Centre, Sriharikota, India

*[email protected]

Abstract-Composite solid propellants (CSPs) based on hydroxyl-terminated polybutadiene (HTPB) resin are the commonest

contemporary solid propellants for launch vehicle and missile applications. This paper signifies the effect of the formulation of propellant

ingredients and the process parameters on the fracture toughness of the composite solid propellant by assessing the failure from the non-

linear stress-strain behavior of a uniaxial tension specimen, which is useful for characterizing the composite solid propellant material.

The influence of the process parameters such as premix temperature, cure time and temperature, sequence of solid ingredients addition,

final mixing time and the nature of propellant ingredients such as curative type, hydroxyl content of HTPB binder, concentration of

chain modifiers and their relative ratio etc. have significant roles in deciding the fracture toughness of the specimen that is explained

here in detail with more elaborative way by experimentation. The stress-strain behavior is one of the best methods of representing the

specimen failure. The stress-strain behavior on a HTPB-based propellant with the variation of process parameters and the nature of

propellant ingredients also discussed here in detail. It is proven that the fracture toughness is one of the finest tools in deciding the

capability of CSP to withstand severe stresses from various conditions.

Keywords- Composite Solid Propellant; Fracture Toughness; HTPB; Mechanical Characterization; Nature of Propellant Ingredients;

Propellant Processing Parameters; Stress-Strain Behavior

I. INTRODUCTION

Solid propellants are structural materials and processing of solid propellants, especially highly filled system with 85-90 %

solids in 10-15 % liquid, poses complex technological problems. Processing involves the thorough understanding of propellant

chemistry, rheology, particulate technology, manufacturing techniques and safe handling of explosives and hazardous materials.

Today's rocket motors must be designed to meet variety of mission applications, with severe demands on the structural capability

of propellant grain [1, 2]. The loading environment, which a propellant grain must survive, includes thermal cycling, handling,

vibration, ignition, pressurization, and acceleration. Quantitative measurements of the propellant's mechanical characteristics have

become very important to the designer [3-7].

CSP grain must hold its shape over an extended temperature range, and must withstand the stresses and strains imposed on it

while handling ignition, and firing in a rocket. In the design of rocket motors, great importance is attached to the mechanical

behavior of solid propellant grains [8-10]. This is because the presence of cracks in the propellant during the ignition stage may

lead to irregularities in the burning process and lead to mal-function of the rocket motor [11]. Stress cracking in the propellant may

occur either during the storage period as a result of shrinkage or thermal stresses, or during the ignition period in which the

pressure in the motor chamber increases rapidly to its equilibrium or operating value that depends upon ballistic property

requirements, or during the combustion period dues to the action of the combustion pressure or that of the acceleration forces [12-

14].

CSPs are known to exhibit properties which depend strongly on temperature and strain rate [15]. Many reports discussed that

the theory of linear visco-elasticity is commonly used to describe such behaviors [16, 17]. The evaluation of structural integrity of

the grain at ignition, where it is subjected to critical loads, is therefore one of the critical activities to be performed to satisfy

requirements of safety and reliability of operation. Significant mechanical testing efforts are generally needed to evaluate the

material quality after manufacturing by changing the propellant ingredients and the nature of those [18]. Fracture mechanics of

elastomers and propellants, theoretical approaches were published by Knauss [19] and have been completed in a general theory by

Shapery [20]. Once the induced stresses and strains are predicted, it is necessary to decide whether they exceed the mechanical

capability of the propellant and to formulate a measure of the grain’s structural reliability in terms of a safety factor or a probability

of failure at ignition [21]. Thermal cycling can propagate small cracks until a critical size is reached for the motor [22] if the

mailto:[email protected]


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propellant is brittle or made brittle by chemical aging. Fracture-mechanical properties of a poly-butadiene-acrylonitrile (PBAN)

binder system and composite-modified double-base propellants were measured by Beckwith and Wang [23]. Bencher et al. [24]

characterized an inert formulation based on a HTPB binder system at three different temperatures and slow strain rates. Most

experimental work on the fracture mechanics of ammonium perchlorate (AP)-HTPB-based solid propellants has been performed by

Liu [25] on experimental crack-resistance curves, pointing out the dependence of these curves on temperature and strain rate at

typical application temperatures and slow-to-moderate strain rates. Studies on time-temperature dependence of poly-butadiene

acrylo-nitrile (PBAN) propellant have been carried out by Manjari Rajan and Mohandas [26].

For the characterization of propellant, the conditioning of the specimens at 50-60 % RH conditions is an important factor. The

stress and/or strain at a specific point in the visco-elastic material may vary significantly with time and temperature even though

the applied forces are constant. These time-temperature dependent phenomena may have a considerable effect on the stress

distribution developed in a solid propellant grain subjected to prescribed loads acting successively or simultaneously [27-29]. The

propellant must not crack or become brittle at subzero temperatures or soften and deform at higher temperatures (~60°C). The

propellant has to perform according to the stringent specifications, sustaining the rapid pressurization during ignition. Hence

mechanical characterization of the propellant has to be done to know whether it can withstand the forces and perform its intended

function [30].

II. MATERIALS AND METHODOLOGY

In CSPs, liquid portion consists of a five component system which contains a long-chain diol such as HTPB, a curing agent

such as toluene diisocyanate (TDI), a low molecular weight chain-extender such as 1, 4 butane diol (BDO), a crosslinker such as

trimethylol propane (TMP) and plasticizer are mixed together with other solid ingredients- like inorganic oxidizer (AP), metallic

fuel (Al), burn-rate modifier, cure catalyst etc. The plasticizer is organic oil depositing itself between the binder chain segments,

facilitating mutual shearing through weakening of the Vander Waals bonds which exist between different atoms of the chain

segments. The propellant mixing is usually carried out in two phases. In the first phase, all the ingredients except the curing agent

are mixed for an optimized time which is termed as premixing followed by second phase (final mixing) in which the curative is

added, and the subsequent mix time and temperature are governed by the reactivity of the curative and the duration to get the slurry

viscosity amenable for casting.

A 4 kg capacity horizontal sigma blade mixer was used for experiments with a batch size of 3 kg. The propellant slurry was

vacuum-cast into cartons and cured at 60°C for 5 days/ 50°C for 8 days. These tests are also used to obtain constant stress and

strain to failure data, creep data, fatigue data etc. The Universal Testing Machine (UTM) was used for determining the propellant

mechanical properties and fracture toughness of the cured propellant. The equipment used in this present investigation is a 50 KN

capacity Instron UTM model 4469. A schematic of the UTM is shown in Fig. 1. Specimen effective gauge length is taken as 45

mm instead of 33 mm to account for the extrusion effect of propellant in gripping jaws. The thickness and the width of the

dumbbells were measured at three regions of the gage length, and the minimum cross sectional area was computed.

Fracture toughness is a property that describes the ability of a material to resist fracture, and is one of the most important

properties of any material for many design applications. Fracture toughness (FE) also indicates the strain capability of the

propellant. The propellant grain experiences a lot of stresses during firing, vehicle acceleration etc. For the composite solid

propellants, fracture toughness could be the convenient tool to express its mechanical behavior because propellant system needs

not only the tensile strength but also the sufficient strain (% elongation). The fracture toughness considers both tensile strength and %

elongation which is well suited to the propellant system which is using for the case bonded rockets. From the experiments, the

range of the fracture toughness required could be determined. It is expressed in J/cm2 (when stress is expressed in kg/cm

2). It is

derived from the area under the stress- strain curve and obtained by multiplying the area under the curve by 0.098 (for unit

conversion) and the gage length (centimeter).

Fracture toughness (J/cm2) = 0.098 x area under the stress-strain curve x gage length (cm) --- (1)

In the present work, special emphasis is given to investigate the influence of various parameters and formulations on the FE

that can be a useful tool to make a comprehensive study to optimize the process conditions and also to correlate the theoretical

estimates with the test results of a strip uniaxial tension specimen using stress-strain curves for a HTPB based propellant. Uniaxial

tension tests are primarily used for quality assurance testing to obtain tensile strength and elongation at maximum stress/ failure,

and for evaluating the fracture toughness of the composite solid propellant which was derived from the area under the stress-strain

curve which is derived under a constant strain rate of 50 mm/min (constant strain rate of 0.0185 sec-1

) at a constant temperature

maintained under conditioned atmosphere for computation of tensile stress & strain and fracture toughness [31-33].


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Fig. 1 Schematic view of Universal Testing Machine

The process variables studied in the investigations on the mechanical properties of the propellant are: Cure time (2-6 days),

cure temperature (500C and 60

0C), premix temperature (40-60

0C) and the sequence of addition of solid ingredients; similarly, the

influences of the following composition and formulation variables on the fracture toughness are also studied. These include Coarse

to fine ratio of AP, the effect of stoichiometry (r- value), effect of chain modifying agents (TMP: BDO ratio and their relative

concentration) and the nature of curative type.

III. RESULTS AND DISCUSSION

The effect of various process parameters, formulation and the nature of ingredients on fracture toughness are described in detail

and as follows:

A. Effect of Hydroxyl Value of HTPB

Drastic changes in properties of the CSP can be induced by varying the molecular weight of the pre-polymer and the crosslink

density. Hence, an attempt was made to investigate the effect of hydroxyl content of HTPB on fracture toughness. HTPB resin

manufactured by free-radical polymerization using a peroxide initiator, with varying molecular weights and hydroxyl values, was

used in propellant formulation experiments with a view to study its influence on the resultant propellant properties [34]. It is seen

that HTPB resins with a wide range of hydroxyl values could be effectively utilized in propellant formulations. Also, propellants

with higher strain capability and chain flexibility could be produced from lower hydroxyl value resins. High strain capability is a

critical requirement for realizing case bonded solid propellant grains used in satellite launch vehicles. The margin of safety of the

propellant grain in the rocket motor is determined based on a host of parameters such as equilibrium modulus, % elongation, bond

strength etc. However, the propellant formulation is optimized based on four basic mechanical properties viz., tensile strength, %

elongation, initial modulus and hardness. In this present investigation, the influence of hydroxyl value of HTPB was varied and

evaluated the fracture toughness of the propellant. It was evident from Fig.2 that on using the low hydroxyl value HTPB for

propellants, the fracture toughness was found higher (1.92 J/cm2) in comparison to the higher hydroxyl value HTPB based

propellants (1.73 J/cm2). However, the criteria for the hydroxyl value for HTPB is based on the mechanical and interfacial

properties obtained for the case bonded propellant system.

0.0 0.1 0.2 0.3 0.4 0.5

0

1

2

3

4

5

6

7

8

9

10

11OH value=35 mg KOH/g

Tens

ile s

tren

gth

(kg/

cm2 )

Tensile strain

OH value=42 mg KOH/g

Fig 2 Effect of hydroxyl value of HTPB on stress strain behavior of CSP


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B. Nature of Curative

In HTPB propellants, TDI is conventionally used as curative and the propellant slurry has a shorter pot life i.e. is the period up

to which the propellant slurry is amenable for easy processing, of 4-5 hours. Processing of large solid motor demands longer pot

life for making defect free grains. Using curatives having sufficiently low reactivity over a wider range of temperatures can solve

the problem. IPDI (Isophorone di-isocyanate) and MDCI (Methylene dicyclo hexyl isocyanate) are slow reactive curatives, which

can give larger pot life of 15-20 hrs for HTPB propellants [35]. These curatives have low volatility and hence low toxicity

compared to TDI. . Cure catalysts are chemicals such as iron octoate, ferric acetyl acetonate (FeAA) which can increase the speed

of cure reaction. It was evident from the Fig. 3 that the IPDI propellant offers the chain flexibility while TDI propellant offers the

higher tensile strength. The FE observed is higher (1.82 J/cm2) for TDI based propellant when compared to IPDI based propellant

(1.67 J/cm2).

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0

1

2

3

4

5

6

7

8

9

10

IPDI based propellant

Tens

ile s

tren

gth

(kg/

cm2 )

Tensile strain

TDI based propellant

Fig. 3 Effect of the nature of curative on stress strain behavior of CSP

C. Effect of Cure Catalyst

The rate of viscosity build-up is slower for IPDI propellant slurry in comparison to TDI propellant slurry, despite a higher

process temperature. HTPB-IPDI propellant system has got greater flexibility to meet any contingency especially during casting of

large sized segments of propellant [36]. IPDI cured HTPB based composite propellant system has considerably longer pot life

because of relatively slower cure kinetics and also it takes longer time for curing. Hence, an attempt was made to add a cure

catalyst which can enhance the cure kinetics. FeAA was added as a cure catalyst for IPDI based propellant system to reduce the

cure time. It was evident from the Fig. 4 that the required properties were achieved in shorter duration in comparison to the non-

catalysed system. The FE observed was lower for the catalysed system (1.58 J/cm2) when compared to the non-catalysed system

(1.72 J/cm2). However, to reduce the time duration of propellant curing, the cure catalyst concentration is the deciding factor for

slow-reacting curatives used for propellant processing. The concentration of catalyst could be optimised from the fracture

toughness data and also by the pot life of the propellant slurry.

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0

1

2

3

4

5

6

7

8

9

10 IPDI propellant curing

with cure catalyst; 6 days/ 600C

Tens

ile s

tren

ghth

(kg/

cm2 )

Tensile strain

IPDI propellant curing

16 days/ 600C

Fig. 4 Effect of cure catalyst on stress strain behavior of CSP (for IPDI propellant system)

D. Effect of Reactant Stoichiometry

The mechanical properties of HTPB-AP based composite solid propellants depend on the variations in crosslink density, which

is predominantly determined by the molar ratio of diisocyanate to the total hydroxyl content of the binder and chain modifiers. By


- 98 -

optimizing the [-NCO] / [-OH] ratio (which is termed as “r”-value) and diol/ triol equivalent ratios in the propellant formulation,

cross link density can be varied and the desired mechanical properties could be achieved [37, 38]. The fracture toughness of the

propellant was so sensitive to the “r” value when the HTPB used is having a lower hydroxyl value in comparison to make a

propellant.

TABLE 1 EFFECT OF “R” VALUE ON FRACTURE TOUGHNESS

“r” value Fracture toughness

0.95 1.93

0.94 1.90

0.916 1.47

0.887 1.16

From the table 1, the fracture toughness followed a systematic change with “r” value when it is in the range of 0.95-0.887.

However, further increase in “r” value leads the propellant brittle (since the fracture toughness diminishes with high “r” values)

and the propellant is no longer suits the requirement as shown in Fig. 5. Based on the mechanical property data and fracture

toughness, the optimum reactant stoichiometry would be chosen.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0

1

2

3

4

5

6

7

8

9

10

r = 0.95

r = 0.94

r = 0.916

Tens

ile s

tren

gth

(kg/

cm2 )

Tensile strain

r = 0.887

Fig. 5 Effect of reactant stoichiometry on stress strain behavior of CSP

E. Effect of AP Coarse-to-Fine Ratio

To make a closely packed dense propellant, the bi-modal distribution of oxidizer is chosen in the current practice. The fracture

toughness of the propellant used with AP coarse-to-fine ratio (AP C/F) of 2:1 was found higher (1.94 J/cm2) in comparison to the

propellant used with AP C/F ratio of 4:1 (1.78 J/cm2). It was clearly evident from Fig.6 that the use of more AP fine content, there

is an increase in fracture toughness and mechanical properties as well. But the usage of finer AP in propellant formulations is

regulated by the viscosity of the propellant slurry after the end of final mixing since the viscosity is increasing with fine content of

AP [39]. To prepare the propellant without defects the viscosity of the propellant slurry should be amenable to casting.

0.0 0.1 0.2 0.3 0.4 0.5

0

2

4

6

8

10 AP C/F=2:1

Tens

ile s

tren

gth

(kg/

cm2)

Tensile strain

AP C/F=4:1

Fig. 6 Effect of AP C/F ratio on stress strain behavior of CSP


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F. Effect of the Relative Ratio and the Concentration of Chain Modifiers

The chain modifier used in the present investigation, is a mixture of triol (TMP) and diol (BDO), the former for cross linking

and the latter for chain extension. Therefore, their relative amounts and concentration will have a pronounced effect on the

mechanical behaviour and FE of the propellant. The propellant formulation so far described had TMP and BDO in 1:2 proportions

and the quantity of the mixture was 0.12 wt %. Therefore in the present investigations, the effect of two ratios of TMP and BDO

ratio 1:2 and 2:1 with the same amount of the mixture (0.12%) was studied. From Fig. 7, the fracture toughness was found lower

for the TMP: BDO ratio of 2:1((1.71 J/cm2) in comparison to the TMP: BDO ratio of 1:2 (1.83 J/cm

2).This is because of the

reduction of chain flexibility due to the formation of the rigid polymeric network structure by the excess cross linker. Hence, the

fracture toughness is so sensitive to the relative ratio of chain modifiers and the choice is based on the target properties.

Fig. 7 Effect of relative ratio of chain modifiers on stress strain behavior of CSP

Fig. 8 Effect of concentration of chain modifiers on stress strain behavior of CSP

In another experiment, the amount of the mixture was raised to 0.18% with TMP: BDO ratio of 1: 2. In this experiment, HTPB

is compensated for chain modifiers. On using the higher concentration of chain modifiers it was observed from Fig.8 that there is

an increase in the fracture toughness (from 1.48 to 1.66 J/cm2). Hence to get the desired mechanical properties, increase in the

concentration of the chain modifiers is the best choice.

G. Effect of Sequence of Addition of Solid Ingredients

Since coarse & fine ammonium perchlorate and aluminium are the major ingredients in propellant composition, studies were

done to investigate the effect of the sequence of addition of these major ingredients on the rheological and mechanical behavior of

the propellant. For all the additional sequences, first the weighed mass of HTPB and other minor ingredients were added into the

mixer and mixed thoroughly for 10 minutes, and thereafter the major solid ingredients were added in various following sequences

of addition namely: (a) If the solids were added in the sequence of aluminium followed by coarse AP in two lots and then fine AP

in two lots with an interval of 10 minutes after each addition and is represented as A+2C+2F; (b) If the solids were added in the

sequence of coarse AP in two lots followed by fine AP in two lots and then aluminium with an interval of 10 minutes after each

addition is represented as 2C+2F+A. (c) If the solids were added in the sequence that half of the coarse AP followed by fine AP

and then the remaining coarse AP and fine AP followed by aluminum in two lots with an interval of 10 minutes after each addition

is represented as C+F+C+F+2A [39]. From the table 2, the sequence 2C+2F+A has given higher fracture toughness compared to

other sequences of additions (Fig.9). Hence, the better sequence of addition could be opted from the fracture toughness data.

-10123456789

10

-0.05 0.05 0.15 0.25 0.35 0.45 0.55

ten

sile

str

ess

(kg/

cm2)

tensile strain

TMP:BDO=2:1

TMP:BDO=1:2

-2

0

2

4

6

8

10

0 0.2 0.4 0.6

ten

sile

str

engt

h

(kg/

cm2)

tensile strain

TMP+BDO=0.18%

TMP+BDO = 0.12%


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TABLE 2 EFFECT OF SEQUENCE OF ADDITION ON FRACTURE TOUGHNESS

Sequence of addition Fracture toughness (J/cm2)

A+2C+2F 1.74

2C+2F+A 1.86

C+F+C+F+2A 1.59

0.0 0.1 0.2 0.3 0.4 0.5

0

1

2

3

4

5

6

7

8

9

10

A+2C+2F

2C+2F+A

Tens

ile s

tren

gth

(kg/

cm2)

Tensile strain

C+F+C+F+2A

Fig. 9 Effect of sequence of addition on stress strain behavior of CSP

H. Effect of Premix Temperature

The rheological behavior of HTPB based CSP varies significantly with respect to solid loading, oxidizer particle size

distribution, and aluminum content. In addition to these parameters, the understanding the effect of process temperature is also of

considerable importance from the point of view of processability [40]. The premix operation was carried out at three temperatures

of 400C, 50

0C and 60

0C. But the final mixing operation was done at 40

0C (because of the high reactivity of TDI leading to very low

pot life at higher temperatures like 500C and 60

0C). After completing the premix process, the slurry was cooled down to 40

0C to do

the final mixing operation. From the Fig. 10, it was evident that premix processing at 600C is advantageous for propellant

processing. This can lead to better flow of polymer and hence the better wetting of the binder with the solid ingredients.

TABLE 3 EFFECT OF PREMIX TEMPERATURE ON FRACTURE TOUGHNESS

Premix temperature Fracture toughness (J/cm2)

400C 1.69

500C 1.73

600C 1.82

0.0 0.1 0.2 0.3 0.4 0.5

0

1

2

3

4

5

6

7

8

9

10

Premix @ 600C

Premix @ 400C

Tens

ile s

tren

gth(

kg/c

m2 )

Tensile strain

Premix @ 500C

Fig. 10 Effect of premix temperature on stress strain behavior of CSP


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I. Effect of Final Mixing Time

Propellant process time is one of the most important parameters which are having a significant effect on the propellant

rheological and mechanical behavior of solid propellants. An investigation has been done on evaluating the easiness in Propellant

processing. Since the cure reaction sets in after the addition of diisocyanate curative in HTPB based propellants, the time available

after the addition of the curative is limited. However, the propellant slurry was followed a thixotropic behavior even after addition

of curative while mixing up to 100 minutes, a study has been carried out to evaluate the influence of final mix time on propellant

properties such as viscosity and mechanical properties and also to check the reproducibility in mechanical properties as well. These

trials ensure the better wettability of solid ingredients with the liquid binder and reflected in improving the rheological and

mechanical behavior of solid propellants. There was a significant increase in fracture toughness was observed from Fig.11, when

the final mix time was raised from 40 minutes (FE: 1.4 J/cm2) to 60/80 minutes (FE: ~ 1.78 J/cm

2). The final mix time optimization

could be done on considering the value of fracture toughness.

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0

1

2

3

4

5

6

7

8

80 minutes

60 minutes

tens

ile s

tren

gth

(kg/

cm2 )

Tensile strain

40 minutes

Fig. 11 Effect of final mix time on stress strain behavior of CSP

J. Effect of Cure Time

As the cure time progresses, the tensile strength increases, the elongation decreases and the initial modulus increases and

stabilizes. From the fracture toughness data, the optimum cure time for propellant could be determined. However, excess time

could be given to give a positive margin. The optimum cure time for propellant is around 4-6 days when the propellant cured at

600C and 7-8 days when the propellant cured at 50

0C. Hence the recommended cure time was 60

0C for 5 days (or) 50

0C for 8 days

to give a positive margin. From Fig. 12, the fracture toughness was found an increase with cure time (from 1.47 J/cm2 at 60

0C

curing for 2 days) and finally stabilized at ~ 1.74 J/cm2. The optimum cure time could be derived from the stabilized value of

fracture toughness.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

1

2

3

4

5

6

7

8

95/ 6 day curing

4 day curing

3 day curing

Tens

ile s

tren

gth

(kg/

cm2 )

Tensile strain

2 day curing

Fig. 12 Effect of cure time on stress strain behavior of CSP

K. Effect of Cure Temperature

From the Figure, it can be seen that the rate of curing at 600C is higher than that at 50

0C. From Fig. 13, there was no significant

change in mechanical properties and in fracture toughness when the propellant was cured at the two optimized conditions viz.,


0C for 5 days. Thus, the recommended cure condition can be either 50


0C for 5 days.

The mechanical properties, fracture toughness of the propellant cured at these two conditions are nearly the same (~ 1.74 J/cm2),


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indicating that the cure temperature of 600C or 50

0C can be used based on the process convenience. Since the cure temperature

strongly effects the cure time, the cure temperature is decided by the operational convenience [41, 42]. However, in this present

comprehensive study it was opted for 600C/5 days curing to evaluate mechanical properties.

Fig. 13 Effect of cure temperature on stress strain behavior of CSP

IV. DISCUSSION

Currently, the structural design of a solid rocket motor (SRM) is based on the concept of a mechanically weak solid propellant

grain cast into a stronger metallic or composite case. The outer case provides the needed resistance for maintenance and

operational loads; whereas the inner propellant grain’s low strength is used to transmit loads from the grain surface to the outer

case. It is well known that, under loading conditions, cracks can develop in solid propellants due to the excessive loads. The

mechanical properties of solid rocket propellants are very important for good functioning of rocket motors. During use and storage,

the mechanical properties of rocket propellants are changing due to chemical and mechanical influences such as thermal reactions,

oxidation reactions or vibrations. These influences can result in malfunctioning, leading to an unwanted explosion of the rocket

motor. An increase in burning surface of the propellant grain due to undesired cracks and voids can result in malfunctioning or, in

worst case, explosion of the rocket motor [43]. For reliability in terms of the safety factor, if the value of the safety factor is lower

than one during the lifetime of the solid-propellant grain, including motor firing, the solid propellant is considered as being failed.

The methodology to predict a margin of safety involves a lot of technical disciplines: the study of mechanical properties of

propellant and bonding, stress analysis, failure criteria, aging etc.

The evaluation of structural integrity of the grain at ignition, where it is subjected to critical loads, is therefore one of the

critical activities to be performed to satisfy requirements of safety and reliability of operation set by the end user. Once the induced

stresses and strains are predicted, it is necessary to decide whether they exceed the mechanical capability of the propellant and to

formulate a measure of the grain’s structural reliability in terms of a safety factor or a probability of failure at ignition. Solid

propellant grain behavior under loading is complicated, including interfacial debonding, micro via formation, temperature and

pressure dependence, large deformation, stress softening, etc.

The propellant of each grain actually has individual mechanical properties because of batch-to-batch variations and different

aging conditions, which changes both the constitutive and the failure properties of the propellant. Mechanical aging, possibly

enhanced by chemical aging, reduces the capability of the propellant, finally generating a crack in the grain. Thermal cycling

(occurring during day–night and seasonal temperature variations or captive carriage) can propagate small cracks until a critical size

is reached for the motor if the propellant is brittle or made brittle by chemical aging. Grain design for a solid-propellant rocket

motor frequently necessitates compromises among the conflicting requirements of ballistic performance, structural integrity,

mission reliability, and geometric constraints [44]. The mechanical damage of the propellant material leading to the development

of conductive combustion is followed by a process of transition from combustion to detonation in this porous medium and final

detonation.

Failure is initiated on a microscopic scale by cavitation in the matrix near the particles or at the matrix–filler interface; this is

typical of heterogeneous materials with a finite adhesion stress between a more rigid phase, such as the oxidizer particles for a

propellant and the stone filler in concrete, and a softer phase constituting the matrix. The presence of a state of hydrostatic pressure

in the material improves the failure properties by delaying or suppressing the onset of cavitation. Another effect influencing failure

properties of elastomers is pre-strain, which produces orientation of the binder molecules in the load direction, thereby changing

the mechanical properties [44].

00.5

11.5

22.5

33.5

44.5

55.5

66.5

77.5

88.5

9

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

TS(k

g/cm

2)

tensile strain


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Damage induced softening is irreversible and caused by cavitation (micro crack formation caused by de-wetting of the filler

particles or cohesive failure in the binder near the filler), which in turn is generated by disentanglements and relative displacements

on a molecular scale, between chain segments of different lengths (which causes micro-fractures directly in the binder), or by direct

scission of chain segments and strong bonds between binder and oxidizer [44].

The fracture energy increases as well, because the strain capability for a HTPB based solid propellant increases before dropping

sharply in the proximity of glass transition. Therefore, the fracture toughness of the material follows the trend of the bulk fracture

energy: it is low at high temperatures and slow strain rates, when the fracture proceeds mostly in the binder by disentanglements

and suppression of weak van der Waals bonds, and it increases when the relaxation processes slow down, at lower temperature. At

these conditions, the failure stress increases by an order of magnitude and the strain increases too, reaching a peak; the fracture

proceeds by destruction of strong bonds at the bonding agent or in the binder and requires a maximum of energy. After the strain

drops sharply across glass transition, the fracture energy drops and the fracture proceeds by direct chain scission of the molecular

segments randomly oriented in the direction of the load in a very small region close to the crack tip [44].

A grain must hold its shape over an extended temperature range, and must withstand the stresses and strains imposed on it

during handling, ignition, and firing in a rocket. It seems unlikely that a universally applicable failure criterion can exist since

sometimes motor failure will result from excessive deformation of the propellant; in others, failure results from over pressurization

due to propellant cracking and more frequently the motor fails as a result of the casing burn through due to premature exposure of

the insulation because of the grain structural failure or the propellant-insulation motor casing bond failure. In general, these

different types of motor failure occur under clearly defined conditions; for example, excessive slump deformation is associated

with large motors under long storage or with upper stages when booster stage is operating under high accelerations, whereas

propellant cracking is usually associated with motors operating at low temperatures [45-47].

In turn over pressurization can happen due to the propellant failure because of excessive hoop strain developed due to the

higher grain web thickness for a faulty structural design of the rocket motor grain. In a solid propellant rocket motor under

pressurization, since the propellant material is under hydrostatic compression, the criterion of grain failure is usually taken as the

critical tensile hoop strain.

Solid rocket propellants exhibit nonlinear viscoelastic (NLVE) behavior, that is, their deformation/fracture properties are highly

strain-rate and temperature dependent, and only a portion of the applied strain energy is recoverable. The material nonlinearity of

solid propellant behavior is due in large to damage processes, such as volume change/de-wetting of the oxidizer particles from the

polymeric binder and, to a lesser extent, thermo-mechanical coupling and modulus strain sensitivity [48]. It is also well known that

partial recovery occurs in damaged composite propellants when they are left in an unstrained condition over a period of time.

The propellant grains which were made with PVC binder developed cracks and the propellant system failed during flight due to

the lower fracture energy. PVC propellants do not have the rubbery and high elongation characteristics needed for case bonding;

they are adapted to small, free-standing grains. For the future it was then necessary to develop cross-linked binder systems based

on the condensation of liquid functional pre-polymers. The PVC based propellant system is brittle in nature and is not having

adequate strain capability to withstand severe stresses. Brittle propellant system with lower elongation results in failure due to de-

bonding with the rubber insulation. Composite propellants are very well suited for case bonded grain applications because of their

mechanical behavior: low modulus and high-elongation capability in a wide temperature range [43].

V. CONCLUSION

For the solid rocket motor to perform its mission successfully, it is necessary for it to retain its integrity under a wide variety of

mechanical loads, which are imposed on it during storage and operational phases. The effect on the fracture energy by the various

process parameters and the nature of propellant ingredients were studied in detail and it was found that the fracture energy was

influenced significantly. It has been demonstrated that the propellant mechanical behavior i.e. fracture toughness is susceptible to

the nature of the ingredients and also by the process conditions maintained. The premix temperature, influence of relative ratio of

chain modifiers, the premix temperature and sequence of ingredients addition are having moderate effects on the fracture toughness.

The effect of hydroxyl content of HTPB, the effect of cure time, the concentration of chain modifiers, reactant stoichiometry,

nature of curative, the cure catalyst on slow reacting curatives, AP C/F ratio and final mixing time are having significant effect on

the fracture toughness. The effect of cure temperature is having negligible effect on the fracture toughness.

ACKNOWLEDGEMENTS

The authors would like to thank Director, SDSC/SHAR and also the propellant manufacturing/characterization team for their

kind support in providing the data to publish this article.


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REFERENCES

[1] B. Gondouin, “Structural Analysis of Propellant Grains,” in: Solid Rocket Propulsion Technology, A. Davenas, ed., Pergamon, Oxford,

England, K, pp. 215–302, 1993.

[2] J. E. Fitzgerald, W. L. Hufferd, “Handbook for the Engineering Structural Analysis of Solid Propellants,'' CPIA Publications, p. 214, 1971.

[3] “Structural Assessment of Solid Propellant Grains,” AGARD Rept.AR 350, Neuilly-sur-Seine, France, 1997.

[4] K. N. Ninan, T. L. Varghese, “A Composite Solid Propellant Composition Based on Hydroxyl Terminated Polybutadiene and a Process for

Making the Same,” Indian Patent Number IN 217041, 1999.

[5] R. F. Landel, “Under Tensile Testing of Composite Propellants,'' report No. 66, Jet Propulsion Laboratory, California Institute of

Technology, 1958.

[6] Anon, “STANAG 4506: Explosive Materials, Physical/Mechanical Properties, Uniaxial Tensile Test,” NATO Military Agency for

Standardization, Brussels, 2000.

[7] A. Gocmez, C. Erigken, U. Yilmazer, F. Pekel, and S. Ozkar, “Mechanical and Burning Properties of Highly Loaded Composite Propellants,”

Journal of Applied Polymer Science, p. 67, 1998.

[8] A. S. D. Wang, H. Yuan, T. Tung, H.C. Wang, and Chung-Shan, “Mechanical behavior of a polyester composite propellant,” AIAA/ 11th

Propulsion Conference, vol. 75.

[9] J. W. Jones, W. G. Knauss, “Propellant Failure Criteria,” AIAA Paper, pp. 65-157, 1965.

[10] K. Renganathan, R. B. Nageswara, and M. K. Jana, “Failure Assessment on a Strip Biaxial Tension Specimen for a HTPB-Based Propellant

Material,” Propellants, Explosives, Pyrotechnics, vol. 24, pp. 349-352, 1999.

[11] A. Davenas, J. Thepenier, “Recent Progress in the Prediction and Analysis of the Operation of Solid Rocket Motors,” International

Astronautical Federation, pp. 98-102, 1998.

[12] K. Renganathan, R. B. Nageswara, and M. K. Jana. “Failure Pressure Estimations on a Solid Propellant Rocket Motor with a Circular

Perforated Grain,” International Journal of Pressure Vessels and Piping, vol. 76, pp. 955–963, 1999.

[13] J. D. Ferry, Visco-elastic Properties of Polymers, 3rd ed., Wiley, New York, 1980.

[14] A. J. Kinloch, R. J. Young, Fracture Behaviour of Polymers, Applied Science Publishers, London, 1983.

[15] Shekhar Himanshu, A. D. Sahasrabudhe, “Maxwell Fluid Model for Generation of Stress-Strain Curves of Visco-Elastic Solid Rocket

Propellants,” Propellants, Pyrotechnics and Explosives, vol. 35, pp. 321-325, 2010.

[16] S. K. Ho, “High Strain-Rate Constitutive Models for Solid Rocket Propellant,” Journal of Propulsion and Power, vol. 18, pp. 1106–1112,

2002.

[17] A. J. Kinloch, R. A. Gledhill, “Propellant Failure: A Fracture-Mechanics Approach,” Journal of Spacecraft, AIAA-80-1180R, vol. 18, 1981.

[18] G. Herder, F. P. Weterings, and W. P. C. De klerk, “Mechanical analysis on rocket propellants,” TNO Prinsaurits Laboratory, The

Netherlands, Journal of Thermal Analysis and Calorimetry, vol. 72, pp. 921-929, 2003.

[19] W. G. Knauss, On the Steady Propagation of a Crack in a Viscoelastic Sheet: Deformation and Fracture of High Polymers, Plenum, New

York, pp. 501–54, 1974.

[20] R. A. Shapery, “A Theory of Crack Initiation and Growth in Viscoelastic Media, Part 2,” International Journal of Fracture, vol. 1, no. 3, pp.

369–388, 1975.

[21] Hung-Ta Chu, Jung-Hua Chou, “Poisson Ratio Effect on Stress Behavior of Propellant Grains under Ignition Loading,” Journal of

propulsion and power, vol. 27, no. 3, 2011.

[22] V. Yang, T. B. Brill, and W. Z. Ren, “Solid Propellant Chemistry, Combustion, and Motor Interior Ballistics,” AIAA Progress Series, vol.

185, AIAA, Reston, VA, 1999.

[23] S. W. Beckwith, D. T. Wang, “Crack Propagation in Double Base Propellants,” 16th AIAA Aerospace Sciences Meeting, AIAA Paper 78-

170, 1978.

[24] C. Bencher, R. Dauskardt, and R. Ritchie, “Microstructural Damage and Fracture Processes in a Composite Solid Rocket Propellant,”

Journal of Spacecraft and Rockets, vol. 32, no. 2, pp. 328–334, 1995.

[25] C. T. Liu, “Crack Growth Behavior in a Composite Propellant with Strain Gradients,” Journal of Spacecraft and Rockets, vol. 27, no. 6, pp.

647–652, 1990.

[26] R. Manjari, C. V. Mohandas, and K. V. Sreekumar, “On the Properties of HTPB-Based Propellant,'' Propellant Group, Vikram Sarabhai

Space Centre, Trivandrum, India, Report No. PCM/PRG/05/85, 1985.

[27] M. K. Jana, K. Renganathan, and Rao G. Venkateswara, “A Method of Nonlinear Visco-Elastic Analysis of Solid Propellant Grains for

Pressure Load,” Computers and Structures, vol.52, no.1, pp. 61-67, 1994.

[28] T. L. Cost, C. H. Parr, “Analysis of the Biaxial Strip Test for Polymeric Materials,'' Journal of Materials, vol.4, pp. 312-323, 1969.

[29] G. V. Sakovich, “Design principles of Advanced solid propellants,” Journal of Propulsion and Power, vol.11, no 4, 1995.

[30] K. Renganathan, B. Sharma, “Effect of Ageing on Structural Integrity of a Composite HTPB Solid Propellant Rocket Motor Grain,”

Propellants, Explosives and Pyrotechnics, vol. 9, pp. 20-23, 2011.


- 105 -

[31] S. S. Bhagawan, S. S. Rao, and K. N. Ninan, “Viscoelastic Behavior of Solid Propellants Based on Different Polymeric Binders,” 7th

National seminar on high energy materials, HEMSI, internal report, 1994.

[32] ASTM D 412, Standard test method for mechanical characterization of polymers.

[33] G. V. Sakovich, “Design Principles of Advanced Solid Propellants,” Journal of Propulsion and Power, vol. 11, 1995.

[34] R. Manjari, “Structure Property Relationship of HTPB Based Propellants: Effect of Hydroxyl Value of HTPB Resin,” Journal of Polymer

science, vol. 48, pp. 271-278, 1993.

[35] Y. Y. Rena, Adicoff Arnold, “Polymerisation Kinetics in Propellants of the HTPB-IPDI Systems,” Journal of Applied Polymer Science, vol.

20, pp. 1117-1124, 1976.

[36] Y. Pelipenco, M. Biagioni, “Improvement in Propellant and Processing for Arian 5 Boosters,” AIAA-98-3847, 1998.

[37] Ozgur, Lu Tu, Layo, and Zbelge Fikretpekel “Fine-Tuning the Mechanical Properties Composite Solid Propellants by Varying the NCO/OH

and Triol / Diol Ratios,” Journal of Applied Polymer Science, vol. 84, pp. 2072–2079, 2002.

[38] R. Manjari, U. Somasundaran, V. Joseph, “Evaluation of crosslink density with HTPB polyurethanes,” Journal of Applied Polymer Science,

vol. 48, pp. 271-278, 1993.

[39] C. P. Reghunadhan, Devi Vara Prasad CH, K. N. Ninan, “Effect of Process Parameters on the Viscosity of HTPB/AP/Al Based Propellant

Slurry,” Journal of Energy and Chemical Engineering, vol. 1, pp. 1-9, 2013.

[40] R. M. Muthiah, V. N. Krishnamurthy, B. R. Gupta, “Rheology of HTPB Propellant: Development of generalized correlation and evaluation

of pot life,” Propellants Explosives and Pyrotechnics, John Wiley, vol. 21, p. 186-192, 1996.

[41] R. M. Muthiah, B. John, and U. Somasundaram, “Room Temperature Cured Polyurea-Urethane Propellant with Unique Mechanical

Properties,” Propellants Explosives and Pyrotechnics, John Wiley, vol. 8, pp. 203-205, 1983.

[42] Z. Wirpsza, Polyurethane: Chemistry Technology and Applications, New York, 1993.

[43] Alain Davenas, “Development of Modern Solid Propellants,” Journal of Propulsion and Power, vol. 19, no. 6, 2003.

[44] Giuseppe Sandri Tussiwand, Victor E. Saouma, Robert Terzenbach, and Luigi T. De Luca, “Fracture Mechanics of Composite Solid Rocket

Propellant Grains: Material Testing,” Journal of Propulsion and Power, vol. 25, no. 1, 2009.

[45] G. Herder, F. P. Weterings, and W. P. C. de Klerk, “Mechanical analysis on rocket propellants,” Journal of Thermal Analysis and

Calorimetry, vol. 72, pp. 921-929, 2003.

[46] H. C.Yıldırım, S. Ozupek, “Structural assessment of a solid propellant rocket motor: Effects of aging and Damage,’ Aerospace Science and

Technology, vol. 15, pp. 635–641, 2011.

[47] G. Bandyopadhyay, R. M. Muthiah, and K. N. Ninan, “Failure behavior of a HTPB-based Composite Propellant Damage,” Internal report,

MS 1235/ VSSC/05-172.

[48] Sook-Ying Ho, “High Strain-Rate Constitutive Models for Solid Rocket Propellants,” Journal of Propulsion and Power, vol. 18, no. 5, 2002.

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