11/07/2025
By Danielle Fretwell

The Francis College of Engineering, Department of Mechanical Engineering, invites you to attend a Doctoral Dissertation defense by Alex Krueger on: "Experimental and Computational Study of Dynamic Mechanical and Transient Thermal Response of Thermoplastics For Impact Protection Applications."

Candidate Name: Alex Krueger
Degree: Doctoral
Defense Date: Tuesday, November 18, 2025
Time: 9:30 - 11:30 a.m.
Location: Olsen 412

Committee:

  • Advisor: Alireza Amirkhizi, Professor, Department of Mechanical Engineering, University of Massachusetts Lowell
  • Daniel Schmidt, Principle R&T Scientist, Luxembourg Institute of Technology
  • Joey Mead, University Professor, Department of Plastics Engineering, University of Massachusetts Lowell
  • Scott Stapleton, Professor, Department of Mechanical Engineering, University of Massachusetts Lowell
  • Christopher Hansen, Department Chair, Department of Mechanical Engineering, University of Massachusetts Lowell

Abstract:
In impact protection applications, glassy thermoplastics are often studied due to their favorable combination of ductility, toughness, strength, and low manufacturing cost. However, limited thermoplastic chemistries are employed in these systems because their complex physical behavior under dynamic loading makes accurate characterization challenging, costly, and time-consuming. Under high-velocity impact, thermoplastics exhibit unique mechanisms, including localized and large-amplitude strains, spatially varying high strain rates that promote a transient stiffening response, and adiabatic heating that leads to significant temperature rise and thermal softening, with potential for additional capacity for energy dissipation before ultimate failure. Developing thermoplastic chemistries with superior impact resistance and dynamic mechanical properties, therefore requires a comprehensive understanding of their coupled mechanical and thermal responses under high-rate loading grounded in both polymer physics and solid mechanics.

In this work, the dynamic response of glassy thermoplastics is investigated from a structure–property–performance perspective through three main objectives. First, the structural response is examined to define how cooperative molecular chain dynamics near the glass transition influence macro-scale impact resistance. Second, the coupled mechanical and thermal behavior under controlled uniaxial loading is characterized to evaluate the transient thermomechanical response at high strain rates. Finally, finite element models incorporating adiabatic heating are developed and experimentally validated to quantify the influence of structural characteristics and constitutive parameters on deformation mechanisms and overall impact performance of selected polymeric systems. Together, these efforts establish a coupled mechanistic and polymer physics framework for linking molecular mobility, thermal dissipation, and continuum-level deformation; thus, enabling more accurate modeling and improved material design for next-generation impact-resistant polymers.