03/29/2024
By Danielle Fretwell

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a Doctoral Dissertation defense by Nahal Aliheidari on "Long-Term Environmental Durability Assessment of Fiber-Reinforced Composite/Adhesive Joints."

Candidate Name: Nahal Aliheidari
Degree: Doctoral
Defense Date: April 8, 2024
Time: 1-3 p.m.
Location: Ball Hall 313 and virtual via Zoom. Please email advisor amir_ameli@uml.edu for link.

Committee:

  • Advisor Amir Ameli, Assistant Professor, Plastics Engineering, University of Massachusetts Lowell
  • Davide Masato, Assistant Professor, Plastics Engineering, University of Massachusetts Lowell
  • Akshay Kokil, Assistant Teaching Professor, Plastics Engineering, University of Massachusetts Lowell

Brief Abstract:

Adhesives such as epoxies are extensively used as structural components in wind turbines. As the service life of a wind turbine is in the range of decades and it can experience harsh environments (e.g., hot, humid, saline), the long-term durability and reliability of these structural adhesives are vital. Water ingress over this long time causes degradation in composite/adhesive joints such that adhesive failure has become one of the leading causes of the blade's malfunction. Therefore, understanding and quantification of long-term durability in adhesive joints of wind turbine blades (WTB) are vital.

In this research, a framework that enables the assessment and lifetime prediction of in-service joints, the optimization of new joint designs, and the prediction of joint strength upon environmental degradation was proposed. The overall procedure incorporated fracture toughness data from aged, degraded open-faced ENF specimens, adhesive water diffusion characteristics obtained from diffusion modeling, and the concept of the exposure index to estimate the residual strength of degraded closed joints directly. To establish this framework, several major tasks have been done: a) substrate surface preparation, b) hygrothermal aging characterization by gravimetric measurement and diffusion modeling, c) evaluation of the mechanical performance of the aged adhesives, d) empirical modeling using Minitab software for bulk adhesives, e) establishment of the fracture specimen making, f) Aging and testing for fresh and aged joints and g) measuring properties needed for FE CZM model, h) FE CZM modeling integrated with hydrothermal exposure data, and i) aging & testing of closed joints to verify the FE model.

A response surface methodology was used to analyze the flow rate, power, and time factors of plasma surface treatment. Surface free energy (SFE) of treated glass fiber-reinforced composites showed a strong quadratic dependence on flow rate, power, and time, with significant interaction between time and power. Next, the aging experiments were performed considering various periods at three levels of RH (43, 75, and 96%) and three temperatures (40, 50, and 60°C). Results reveal that water absorption follows the simple Fickian model. To systematically assess the effect of moisture concentration and diffusion coefficient on mechanical performance, statistical models were developed for modulus and UTS. The developed model showed a remarkable alignment between the experimental results and the model predictions. A MATLAB code was fully developed for the Exposure Index (EI) concept for absorption and desorption. Further, the EI values were calculated for a long time of the exposure until there was no significant drop in fracture toughness at 9 exposure conditions. The TSL parameters were determined using a bilinear CZM model for adhesive in open faced specimen by inverse method. The resultant load displacement from the model matches with experiment, only 2% difference was observed at maximum load for the fresh joint. The fitting curve for GIIC vs. EI was performed, and an R-sq of 93% was achieved. The results of the closed-joint specimens confirmed the model's predicting capability.