paper for parari17 rl - unsw.adfa.edu.au · in finite element analysis (fea) using ls/dyna. ......

Approved for Public Release 13th PARARI, 21~23, Nov. 2017

Mechanical Failure Prediction of Composite Rocket Cases

Roger Li1

Weapons and Combat Systems Division, DST

This paper demonstrates mechanical failure analysis of a pressurised 10” rocket case

made of epoxy carbon fibre composite. The mechanical failure modes were simulated

in finite element analysis (FEA) using LS/DYNA.

The example rocket motor case uses a quasi-isotropic layup similar to 0°/90°/-

45°/+45°, and is well-suited to cope with the different failure modes, and particularly

in-plane shear failures which cause fibre tow splitting in the dome areas. The material

properties of the quasi-isotropic composite were captured by experimental coupon

tests and test simulation in LS/DYNA. Then these quasi-isotropic composite material

properties were used in the FEA models of the rocket case to predict the failure modes

and burst pressure. This paper will discuss the simulation results, based on a rocket

case of 3 mm thickness made from T700 grade carbon fibre composite.

Keywords: Solid Rocket, Composite Case, Failure Prediction Quasi-isotropic

Laminate.

1 Introduction

Rocket performance is characterised by thrust generated by high speed, high pressure

fluids from its nozzle. The propulsion parameters include the ratio of nozzle exit

pressure to the ambient pressure. Higher is the rocket case pressure, higher is the

propulsion performance.

Advanced rocket cases can employ filament winding or 3D woven composite

technologies to integrate cylindrical and dome parts, and even bring nozzle throat and

expansion cones together into a single composite structure without the need for bolted

or bonded joints. Such design features, married with the reduced weight offered by

carbon, Kevlar or poly(p-phenylene benzobisoxazole) (PBO) fibres, can produce high

performance rocket cases with a structure weight rather than propellant less than 10%

of total rocket weight.

1 Email: [email protected]


Figure 1 A DST 24” composite rocket case demonstrator (igniter side up)

However, such an integrated structure also introduces multiple failure modes to the

pressurised case, which can appear during hydroburst proof tests and live firings.

They could be carbon fibre tensile failure in the cylinder area caused by hoop stress,

and particularly in-plane shear failures which cause fibre tow splitting in the dome

areas. The key in optimisation case design minimising case structure weight is to

avoid excessive design safety margin in each of failure modes. Efficient and

optimised rocket cases must possess failure modes which occur at roughly the same

case pressures. There is thus a critical requirement to accurately predict these failure

modes and loads.

This paper demonstrates mechanical failure analysis of a pressurised 10” rocket case

made of epoxy carbon fibre composite. The mechanical failure modes were simulated

in finite element analysis (FEA) using LS/DYNA.

The example rocket motor case uses a quasi-isotropic layup similar to 0°/90°/-

45°/+45°, and is well-suited to cope with the different failure modes, and particularly

in-plane shear failures which cause fibre tow splitting in the dome areas. The strength

of quasi-isotropic carbon fibre composite was determined by coupon tests. The

coupon tests were subsequently simulated in LS/DYNA, thereby capturing the whole

set of quasi-isotropic material properties of the composite derived from such a layup

sequence.

By subsequently feeding the quasi-isotropic composite material properties into the

LS-DYNA FEA model of the rocket case, the failure modes and burst pressure of the

all-up case were predicted.

2 Composite coupon tests and FEA models

2.1 ASTM D2290 ring split disk tensile testing

ASTM D2290 ring split disk tensile testing was chosen as the coupon test to capture

the material properties of composite rocket cases because the coupons for this test are

cut from composite cylinder made of similar quasi-isotropic layup, as shown in Figure

2. Cut from a cylinder, these ring specimens are more representative to composite


rocket motor cases than flat coupons normally used in dumbbell tensile or three point

bending tests.

Figure 2 5 3/4” mandrel and composite tube for ASTM D2290 split disk ring tensile testing

A quasi-isotropic layup similar to 0°/90°/-45°/+45° has four or more fibre orientation

angles, is well-suited to cope with the different failure modes, particularly in-plane

shear failures which cause fibre tow splitting in the dome areas. The strength of

composite used for rocket motor cases was determined by ASTM D2290 split disk

ring tensile testing by repeating the same quasi-isotropic layup of carbon fibre

orientation as much as possible, as shown in Table 1.

Table 1 Quasi-isotropic layups in this work

Orientation angles Hoop tow Helical tow Polar tow

D2290 ring ±89° ±37° ±9°

24” case ±88° ±35° ±17°

10” case ±88° ±35°(50%±25° and 50%±45°) ±18°

For such quasi-isotropic composites with at least quadriaxial fibrous orientation

angles, Hart-Smith [1] has proposed a "Ten-Percent Rule" that each 45° or 90° ply

was considered to have one-tenth of the axial stiffness and strength of a reference 0°

ply, based on his empirical discoveries that each 45° ply has about 12%, 90° about 8%

of the axial stiffness and strength of a 0° ply. This "Ten-Percent Rule" also considers

that each 0° or 90° ply has only one-tenth of the in-plane shear stiffness and strength

of an equivalent ±45° ply. In other words, the in-plane shear strength of a cross-plied

laminate depends on greatly whether there is fibre tows aligning with maximum shear

direction (normally ± 45°).

Typical loading curves of ASTM D2290 split disk ring tensile testing of T700

composite rings are shown in Figure 3.


Figure 3 Loading curves of ASTM D2290 split disk ring tensile testing of T700 composite rings

Having subtracted the system compliance from the loading displacement, the ASTM

D2290 testing results are shown in Table 2.

Table 2 Results of T700 quasi-isotropic composite disks

Fibre type Maximum tensile stress/MPa Tensile extension at maximum load/mm

T700 574.739 3.12

2.2 ASTM D2290 ring split disk tensile testing simulation

A number of LS/DYNA models were created to simulate the ASTM D2290 ring split

disk tensile testing with various material failure algorithms. One of them is shown in

Figure 4. The model was built with 4032 brick elements and 5054 nodes.

Figure 4 Simulation of ASTM D2290 split disk tensile test of quasi-isotropic composite ring

Two material models were found able to reproduce the test results in LS/DYNA

simulation as shown in Figure 5. They are Types 022 Composite_Damage and 040

Nonlinear_Orthotropic material models [2], referred as “Damage” and “Burst”


respectively in Figure 5. When the material fails, its strength is downgraded to 10% of

pristine material in the ”Damage” model, and to zero (the failed elements are deleted

from the mesh) in the “Burst” model respectively. The former can reproduce the post-

failure “load versus displace” detail while the latter looks more realistic in instant

post-failure load drop in time scale.

Figure 5 Test and FEA comparison of No.2 specimen in ASTM D2290 test

With the coupon test results were simulated in LS/DYNA, the whole set of quasi-

isotropic material properties of the composite can be derived either in Types 022

Composite_Damage or 040 Nonlinear_Orthotropic material model formats [2].

2.3 10” composite rocket case hydroburst testing simulation

A number of LS/DYNA models were created to simulate the 10” composite rocket

case hydroburst testing with various material failure algorithms. One of them is

simulating one quarter of 10” composite rocket case using two symmetric conditions,

as shown in Figure 6. The model was built with 9342 brick elements and 13879 nodes.

This case model has a 3 mm uniformly thickness, is different from realistic composite

rocket cases whose two domes always have increased thickness from the cylinder to

openings because of fibre tow build-up.

Figure 6 Simulation of 10 inch rocket case made of quasi-isotropic composite in LS/DYNA


Both the types 022 Composite_Damage and 040 Nonlinear_Orthotropic material

models [2] have been used in the 10” composite rocket cases. They predict very close

burst results in hydroburst test simulation. In the following sections of this paper, only

the “Damage” model results are presented.

3 Results and discussions

3.1 Failure predictions

Using the materials of the quasi-isotropic derived from above exercise, the implicit

solution of LS/DYNA predicts the sample 10” composite case, excluding the two

fibre tow built-up areas around two openings, would fail at a burst pressure of 17.5

MPa, as shown as time = 175 in Figure 7. At this failure point, the maximum hoop

stress is 737.8 MPa for the cylinder area.

Figure 7 Tensile stress of the sample 10” composite rocket case excluding the tow build-up areas

at the predicted failure point

However, if the two fibre tow built-up areas are included, the maximum tensile stress

is 2753 MPa there at the same failure point, more than three times higher than 737.8

MPa for the cylinder area, as shown in Figure 8.

Figure 8 Tensile stress of the sample 10” composite rocket case including the tow build-up areas

at the predicted failure point


Nevertheless a true composite case would not fail around the openings by tension due

to the facts that there are the thickness would be more than tripled by fibre tow built-

up, and the fibre tows is more aligned with the maximum principal stress direction so

that the strength in that direction should be much higher than the quasi-isotropic one

derived from the ASTM D2290 ring tension test.

3.2 Case deformations

At the same failure point, the maximum longitudinal and radical deformations are 6.8

and 2.0 mm respectively (X and Z here are the global coordinate longitudinal and

radical directions of composite rocket case).

Figure 9 Deformation of the sample 10” composite rocket case at the failure point

Such case deformation cause about 0.5 mm dislocation of case opening against the aft

boss, as shown in Figure 10. This dislocation requires a highly elastic shear ply in

place to seal the gap between the case and the aft boss, or there will be a shear-out

failure before the predicted 17.5 MPa pressure by hoop stress in the cylinder area.

A B

Figure 10 Relative movement of case opening against aft boss (A: unloaded, B: at the failure

point)

At the same failure point, the maximum in-plane shear stress of composite case is

29.9 MPa (X and Y here are two in-plane directions of materials coordinate of

composite brick elements). This shear stress level is only half of normal 10 ksi shear

strength of matrix resin, The quasi-isotropic layup here does smooth the stress down

and prevents the tow splitting shear failure of composite in the dome areas.


Figure 11 In-plane shear stress of composite case at the failure point

4 Conclusions

Using the materials of the quasi-isotropic derived from ASTM D2290 test, LS/DYNA

predicts the sample 10” composite case would fail at a burst pressure of 17.5 MPa by

high hoop stress in the cylinder area.

At the same failure point, the maximum longitudinal and radical deformations are 6.8

and 2.0 mm respectively, causing ~ 0.5 mm dislocation of case opening against the aft

boss. This dislocation requires a highly elastic shear ply between the composite case

and aft boss, or there will be a shear-out failure before the predicted 17.5 MPa

pressure by hoop stress in the cylinder area.

At the same failure point, the maximum in-plane shear stress of composite case is

29.9 MPa, is only half of normal 10 ksi (69MPa) shear strength of matrix resin,

demonstrating the advantage of a quasi-isotropic layup for prevent the tow splitting

shear failure of composite in the dome areas.

5 References

1. Hart-Smith, L. J., “The ten-percent rule for preliminary sizing of fibrous composite structures” AA(Douglas Aircraft Co., Long Beach, CA) Publication, Weight Engineering (ISSN 0583-9270), vol. 52, no. 2, p. 29-45. 1992.

2. LSTC, “LS-DYNA® Keyword User’s Manual Vol II: Material Models”, Livermore Software Technology Corporation (LSTC), 05/26/16 (r:7647).

6 Acknowledgments

The author sincerely appreciates DST Weapons Propulsion Group staff, in particular

Dr Ian Johnston for various professional advice, and Dr Greg Yandek of US Air Force

Research Lab for S&T advice in composite rocket case design and manufacture.

paper for parari17 rl - unsw.adfa.edu.au · in finite element analysis (fea) using ls/dyna. ......

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