Enhancing sustainability and structural performance of fiber-reinforced isotactic polypropylene
Sten van den Broek defended his PhD thesis at the Department of Mechanical Engineering on December 4th.
Fiber-reinforced plastics are all around us, from car parts to construction materials, due to their strength, light weight, and versatility. However, predicting how these materials perform over time, particularly under varying stresses and temperatures, remains a significant challenge. Manufacturers often add extra material to ensure safety, which raises costs and impacts environmental sustainability. For his PhD research Sten van den Broek focused on fiber-reinforced isotactic polypropylene (iPP), a composite material with widespread applications. By improving predictive models for its behavior, particularly its long-term performance, his findings can help create safer, more cost-effective, and environmentally friendly products in fields such as automotive manufacturing, aerospace, and construction.
A central breakthrough was the development of a predictive model based on the principle of factorization. This approach characterizes temperature- and strain-rate dependencies and the impact of fiber orientation separately, then combines them multiplicatively into a cohesive model. This enables accurate predictions of both short-term and long-term material behavior under various conditions, including dynamic stresses. This research also led to the creation of a 3D constitutive simulation model validated with a multi-axial puncture test. The model provides engineers with tools to simulate complex product designs and load conditions in three dimensions, expanding its applicability across industries.
Understanding failure from macro to micro
Delving into the microstructural origins of failure, the researcher used representative volume elements to analyze localized stress around fibers. The study revealed that fiber debonding is a primary cause of macroscopic failure. This insight suggests that improving the bond between fibers and the matrix could enhance material performance without necessitating longer fibers, simplifying manufacturing processes.
Additionally, the research addressed a key manufacturing challenge: balancing fiber content and molecular weight for ease of production. By examining how molecular weight influences fatigue crack propagation, a model to predict crack growth under diverse conditions was developed, paving the way for more durable and high-performance materials.
A new standard for fiber-reinforced plastics
In summary, this research of Sten van den Broek significantly advances the characterization process for fiber-reinforced iPP by applying a factorized approach, enabling predictions of short- and long-term behavior under various stresses and temperatures. Beyond one-dimensional applications, the findings are integrated into a 3D simulation model validated with real-world tests, providing a versatile tool for engineers.
At the microstructural level, this work highlights the role of fiber debonding in failure and offers strategies to optimize material properties without compromising manufacturing ease. Furthermore, the developed model for crack growth kinetics in high-molecular-weight iPP provides a practical solution for predicting long-term durability.
Ultimately, these contributions offer industries innovative tools to design safer, longer-lasting, and more sustainable fiber-reinforced plastics, reducing costs and improving reliability across applications ranging from medical devices to aerospace structures.
Research School: EPL (Eindhoven Polymer Laboratories).
Title of PhD thesis: . Supervisors: Prof. Leon Govaert, and Associate Prof. Tom Engels.