Date of Defense
3-6-2025 2:30 PM
Location
F1-1043
Document Type
Dissertation Defense
Degree Name
Doctor of Philosophy in Mechanical & Aerospace Engineering
College
COE
Department
Mechanical and Aerospace Engineering
First Advisor
Prof. Abdel-Hamid I. Mourad
Keywords
Slow crack growth, High density polyethylene, Plasticization, Micromechanism, Fatigue testing, Crack layer theory, Lifetime prediction.
Abstract
This dissertation investigates the slow crack growth (SCG) behavior of High-Density Polyethylene (HDPE) under various mechanical and environmental conditions. The study combines experimental analysis and computational modeling to enhance the understanding of SCG mechanisms in HDPE, particularly in pressurized pipes and under the exposure to hydrocarbons. A novel Crack Layer (CL) theory-based SCG model is developed and validated through experimental data, offering a predictive framework for HDPE failure assessment. The main objective of this dissertation is to quantify and model the viscoelastic-viscoplastic behavior of HDPE under monotonic and cyclic loading conditions while addressing SCG kinetics in structural applications. The study explores the effects of strain rate, temperature (23°C to 95°C), and hydrocarbon exposure on HDPE’s behavior. Additionally, a viscoplastic constitutive model is calibrated to capture thermo-viscoplastic responses, enabling accurate predictions of material deformation and crack propagation. To achieve these objectives, SCG experiments are conducted on stiff constant-K (SCK) specimens and pressurized HDPE pipes with circumferential and buttfusion joint cracks. A parametric study examines the influence of key factors such as stress intensity factor, transformation energy, and crack front kinetics. Computationally, Green’s functions, thermodynamic forces, and time-marching simulations are utilized to extend CL theory for SCG modeling in HDPE components. Furthermore, a diffusion-assisted SCG framework is introduced to assess the plasticization effects of hydrocarbons on fracture behavior and lifetime predictions. The study successfully validates the CL-based SCG models with experimental results, demonstrating their accuracy in predicting failure times, crack growth rates, and discontinuous crack jumps in HDPE pipes. Findings reveal that external circumferential cracks in thin-walled pipes (SDR > 20) experience a 20–40% reduction in lifetime compared to internal cracks, highlighting the need for conservative design criteria. Additionally, hydrocarbon-induced plasticization accelerates SCG up to 5 times, significantly altering viscoelastic properties, including the glass transition temperature.This dissertation makes important contributions to SCG analysis, computational modeling, and failure prediction of HDPE materials. Future work should focus on high-pressure insitu testing and extended SCG validation to further refine HDPE lifetime predictions and ensure enhanced reliability in critical infrastructure applications.
COMPUTATIONAL AND EXPERIMENTAL STUDY OF SLOW CRACK GROWTH IN HIGH DENSITY POLYETHYLENE
F1-1043
This dissertation investigates the slow crack growth (SCG) behavior of High-Density Polyethylene (HDPE) under various mechanical and environmental conditions. The study combines experimental analysis and computational modeling to enhance the understanding of SCG mechanisms in HDPE, particularly in pressurized pipes and under the exposure to hydrocarbons. A novel Crack Layer (CL) theory-based SCG model is developed and validated through experimental data, offering a predictive framework for HDPE failure assessment. The main objective of this dissertation is to quantify and model the viscoelastic-viscoplastic behavior of HDPE under monotonic and cyclic loading conditions while addressing SCG kinetics in structural applications. The study explores the effects of strain rate, temperature (23°C to 95°C), and hydrocarbon exposure on HDPE’s behavior. Additionally, a viscoplastic constitutive model is calibrated to capture thermo-viscoplastic responses, enabling accurate predictions of material deformation and crack propagation. To achieve these objectives, SCG experiments are conducted on stiff constant-K (SCK) specimens and pressurized HDPE pipes with circumferential and buttfusion joint cracks. A parametric study examines the influence of key factors such as stress intensity factor, transformation energy, and crack front kinetics. Computationally, Green’s functions, thermodynamic forces, and time-marching simulations are utilized to extend CL theory for SCG modeling in HDPE components. Furthermore, a diffusion-assisted SCG framework is introduced to assess the plasticization effects of hydrocarbons on fracture behavior and lifetime predictions. The study successfully validates the CL-based SCG models with experimental results, demonstrating their accuracy in predicting failure times, crack growth rates, and discontinuous crack jumps in HDPE pipes. Findings reveal that external circumferential cracks in thin-walled pipes (SDR > 20) experience a 20–40% reduction in lifetime compared to internal cracks, highlighting the need for conservative design criteria. Additionally, hydrocarbon-induced plasticization accelerates SCG up to 5 times, significantly altering viscoelastic properties, including the glass transition temperature.This dissertation makes important contributions to SCG analysis, computational modeling, and failure prediction of HDPE materials. Future work should focus on high-pressure insitu testing and extended SCG validation to further refine HDPE lifetime predictions and ensure enhanced reliability in critical infrastructure applications.