Solid rocket motors (SRMs) are extensively used in defence and space applications. Com-
posite solid propellants (CSPs) are widely used as an energy source in SRMs due to their
excellent mechanical properties, stability, and high energy. Solid propellant grains in
SRMs are exposed to various thermal and mechanical loads during handling, transporta-
tion, storage, and operation. These loads may induce stress or strain higher than the
material’s capability since the propellant grain is the weakest structural component, caus-
ing crack formation in the propellant grain. Cracks lead to the development of additional
burning surfaces, thus increasing pressure and leading to combustion instability. It is thus
essential to study the mechanical response of CSPs under these loads to avoid such fail-
ures of SRMs. The propellant grain in the SRMs is subjected to a multi-axial stress
field. Biaxial tests are more accurate representations of the in-service loads, and these
are used to study the mechanical behaviour of the propellant grains. Also, these CSPs
can be assumed to be quasi-isotropic when they are cast as grains. However, at large de-
formations, micro-voids form, and it becomes anisotropic (damage-induced anisotropy).
It is essential to conduct the biaxial test to determine the mechanical properties of the
anisotropic material. Since CSPs are viscoelastic in nature, their behaviour depends on
various parameters like temperatures, displacement rates, and displacement rate ratios;
hence, studying the material response by considering these are essential. The primary
objective of the current research work is to study the mechanical response of CSPs under
biaxial loading at various displacement rates, displacement rate ratios (X:Y directions),
and temperatures. It is observed from the existing literature that biaxial testing of solid
propellants has been conducted only using a universal testing machine with a strip biaxial
sample, where different displacement or load ratios cannot be achieved in two mutually
perpendicular directions, which is a significant disadvantage of this testing technique.
Biaxial testing of the CSPs is conducted using cruciform specimens by applying different
displacement rate ratios, and the drawback of the strip biaxial test is addressed. Further,
uniaxial tension and compression tests at constant displacement rates are conducted, and
dilatation and Poisson’s ratio responses during these experiments are studied. 2D-DIC
(digital image correlation) and 3D-DIC are used to measure strain during tension and
compression tests, respectively. The mechanical response under uniaxial and biaxial
loading is compared to understand the effect of the biaxial nature of the load on the
material response of CSPs. Fractography and energy dispersive spectroscopy (EDS) of
uniaxially and biaxially loaded fractured surfaces at all experimental conditions using a
scanning electron microscope (SEM) are then studied to understand the damage growth
mechanisms and failure.