# Experimental Study on the Evolution of Mechanical Properties and Their Mechanisms in a HTPB Propellant Under Fatigue Loading

## TL;DR

This study examines how the mechanical properties of HTPB propellant degrade under repeated stress, identifying key phases and mechanisms of deterioration.

## Contribution

The study introduces a novel application of WLF theory to model residual elongation decay in HTPB propellants under fatigue.

## Key findings

- Residual elongation decreases in three distinct phases as fatigue cycles increase.
- SEM analysis revealed mesostructural damage like dewetting and particle fragmentation.
- A generalized model based on WLF theory accurately predicts material degradation.

## Abstract

In this study, we explored the evolution of mechanical properties in hydroxyl-terminated polybutadiene (HTPB) propellants under fatigue loading by performing fatigue tests with varying maximum stresses and cycle numbers, followed by uniaxial tensile tests on post-fatigue specimens. Residual elongation was used as a key parameter to characterize mechanical behavior, while scanning electron microscopy (SEM) provided insights into the mesostructural morphological changes that occur under different loading conditions, revealing the mechanisms responsible for variations in mechanical properties. The results show that, as the number of loading cycles increases, residual elongation decreases, with three distinct phases of decline—slow change, gradual decline, and rapid deterioration—depending on the stress levels. SEM analysis identified damage mechanisms such as “dewetting” and particle fragmentation at the mesostructural level, which compromise the material’s structural integrity, leading to reduced residual elongation. A novel aspect of this study is the application of Williams–Landel–Ferry (WLF) theory to construct a master curve describing residual elongation decay. This approach enabled the development of a generalized model to predict the material’s degradation under fatigue loading, with experimental validation of the fitted evolution model, offering a new and effective method for assessing the long-term performance of HTPB propellants.