Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 149608

149608 - A Novel Experimental Technique to Determine the Tack Development During Automated Placement of Uncured Thermoset Carbon/epoxy Tows 

In-situ bond strength toughness (IBST), commonly referred to as tow tackiness, is a first order property that influences the quality of adhesion and the occurrence of defects such as wrinkles and folds during the automated tow placement (ATP) of uncured thermoset polymer matrix composite (PMC) tows. Tow-tow IBST develops over a characteristic millisecond timescale (t_c <= 50 ms) due to the rapid tow placement velocity of ~ 1 m/s, under the application of temperature and pressure. This research work introduces an experimental approach to assess the tow-tow mode-I IBST on this short millisecond timescale, utilizing the principles of fracture mechanics to analyze traction-separation behavior. The test specimen consists of two commercially available Hexcel’s IM7-G/8552 carbon/epoxy prepreg tows, each 40 mm long and 6.35 mm wide and a pre-crack to induce crack initiation. The two tows are attached to the two steel platens using a Loctite 426 adhesive. A 0.0127 mm thick optically clear FEP film, made of Teflon fluoroplastic is used to create a pre-crack of length 25 mm resulting in a bonding contact area of 15 x 6.35 mm². Experiments are performed for both long and short timescales at a constant debonding rate of 5 mm/s, contact pressure of 0.23 MPa and bonding temperature at 40° C. In addition, a 100 mm macro lens and two extension tubes of 14 mm and 7 mm each attached to a high-speed Photron Fastcam SA-X2 is used for two-dimensional digital image correlation (2D-DIC) to visualize and measure tow-tow interfacial deformation during debonding. The fracture mechanics-based traction separation relationship is determined in terms of the crack tip stress and crack initiation displacement, with the area under the curve representing the critical energy release rate. The peak traction and apparent energy release rate are found to significantly decrease at the short millisecond timescale contact hold times compared to longer (i.e., second) timescale.  For a 1 s contact hold time, the peak traction is determined as 0.232 MPa, which reduces to 0.099 MPa at 50 ms and further to 0.087 MPa at 30 ms. This represents a substantial decrease of 62.5% from 1 s to 30 ms. For a 1 s contact hold time, the apparent critical energy release rate is determined as 72.0 J/m2, which decreases to 30.75 J/m2 at 50 ms and to 28.40 J/m2 at 30 ms. This represents a substantial decrease of 60.6% from 1 s to 30 ms. This also indicates a transition from cohesive failure during long contact times to adhesive failure during short contact times.

 

Presenting Author: Debrup Chakraborty University of South Carolina

Presenting Author Biography: My journey in the field of composite materials began with my undergraduate final year project at SRM Institute of Science and Technology in India. Leading this project, I worked on improving the fracture toughness and stiffness of Kevlar-based nanocomposites, resulting in a publication in the Journal of Applied Polymer Science. During my MSc at Queen’s University Belfast, UK, I investigated the crashworthiness of composite laminates by studying composite tension-absorbing bolted joints. My research contributions include chapters on metal matrix nanocomposites in sustainable energy and bio-composites in packaging.

Motivated by my interests in developing fundamental knowledge/understanding of mechanisms during composite material manufacturing, I am currently pursuing a Ph.D. in Mechanical Engineering at the University of South Carolina, focusing on automated composites manufacturing. My research goal is to improve our understanding of the fundamentals of in-situ bond-strength toughness (IBST) (commonly referred as tackiness) development during the contact and adhesion of uncured polymer-rich layers, which is crucial for enhancing material quality and minimizing defects during automated tow placement (ATP) of fiber-reinforced uncured thermoset polymer matrix composite tows. To achieve this goal, my approach is to establish an experimental technique combining compressive/shear crack closure and StereoDIC to measure millisecond-timescale uncured tow-tow IBST across various temperatures and pressures. This technique will help quantify the measurement of both short milliseconds and long seconds timescale uncured tow-tow IBST, informing cohesive zone modeling and thermo-mechanical multi-physics finite element process models of the tow-tow interface.

Recently, I developed a novel experimental technique to determine millisecond timescale tow-tow mode-I IBST of uncured thermoset tows using fracture mechanics-based traction-separation relationships. Using IM7-G/8552 carbon/epoxy prepregs and high-speed two-dimensional digital image correlation (2D-DIC), I captured interfacial deformation during debonding. This work was presented at the ACM6 Conference in Columbia, SC, in March 2024, and published in the Manufacturing Letters journal. The broader impact of this research will provide guidelines for the ATP manufacturing process, reducing defects, manufacturing cycle times, and manual inspections, while enhancing understanding of ATP processing for complex geometries in lightweight composite structures for aerospace applications.

Authors:

Debrup Chakraborty University of South Carolina
Karan Kodagali University of South Carolina
Sreehari Rajan Kattil University of South Carolina
Dennis Miller University of South Carolina
Subramani Sockalingam University of South Carolina
Michael A. Sutton University of South Carolina