Masters Theses

George Jacob

Date of Award

5-2001

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

John F. Fellers

Committee Members

Roberto Benson, Joseph Spruiell

Abstract

The energy absorption capability of a composite material is important in developing improved human safety in an automotive crash. In passenger vehicles the ability to absorb impact energy and be survivable for the occupant is called the "crash worthiness" of the structure. The vehicle must be designed such that, in the event of an impact at speeds up to 35 mph with a solid, immovable object, its occupants do not experience a resulting force that produces a net deceleration greater than 20g. Subjection of the occupants to decelerations greater than 20g can cause serious internal injury, particularly brain damage. Energy absorption in these composite materials are dependent on many parameters like fiber type, matrix type, fiber architecture, specimen geometry, processing conditions, fiber volume fraction, and testing speed. Changes in these parameters can cause subsequent changes in the specific energy absorption of composite materials up to a factor of 2. Through ongoing research programs a considerable amount of experimental data on the energy absorption characteristics of polymer composite materials have been generated. They have been found to be efficient energy absorbers and suitable for crash worthy structural applications. But there are a lot of other criteria, in addition to a material being crash worthy, that need to be met before one can begin the use of a particular composite as a crash energy absorber in automobiles. Some of the primary ones being, low costs involved in their manufacture, the raw materials being readily available and many more. Once a composite material is identified to meet the above necessary requirements, one ought to study the effect all the controllable parameters (like fiber volume fraction, specimen geometry etc.) will have on its energy absorption capabilities, in an attempt to design the most crash worthy structure. The ACC (Automotive Composite Consortium) is interested in investigating the use of chopped fiber reinforced composites as crash energy absorbers primarily due to the low costs involved in their manufacture thus making them cost effective for automotive applications. While many scientists have investigated the energy absorption characteristics in various continuous fiber reinforced composite materials, there is no literature available on the energy absorption and crushing characteristics of chopped fiber reinforced composite materials. Therefore the primary goal of this project was to determine the crash worthiness of a chopped carbon fiber composite material system and to see how it compared with that of other fiber resin systems like graphite/epoxy cross-ply laminates, a graphite/epoxy braided material system and a glass-reinforced continuous strand mat (CSM). To meet this goal first an experimental set up was developed for discerning the deformation behavior and damage mechanisms that occur during the progressive crushing of composite materials. The composite material systems studied were chopped carbon fibers reinforced in an epoxy resin system, graphite/epoxy cross-ply laminates, graphite/epoxy triaxial braids with 0/+30°/-30° fiber orientation and glass/polyurethane continuous strand mat. Quasi-static progressive crush tests were then performed on these composite plates to identify and quantify their energy absorbing mechanisms. An attempt was made to understand in great detail the effect of various material (fiber volume fraction, fiber length, fiber tow size) and test (specimen width, loading rate, profile radius, constraint condition) parameters on their energy absorption capability by varying these parameters during testing. After having identified which combination of fiber volume fraction, fiber length, fiber tow size and specimen width will yield the highest energy absorbing material, the quantity of this material needed to ensure passenger safety in a mid-size car traveling at various velocities was calculated. The specific energy absorption, SEA, of the chopped carbon fiber composite material, CCF, was the highest compared to that of all the other composites investigated herein. The 2 inch wide specimens belonging to panel group CCF5 having fiber tow size 150 gsm (grams per square meter), a fiber length of 1 inch and 50% fiber volume fraction recorded the highest SEA equal to 28.11 kJ/kg when tested at 5 mm/min crushing speed under the tight constraint condition using a profile block of radius 0.635 cm. It was calculated that only 4.27 kg of it would need to be placed at specific places in the car to ensure passenger safety in the event of a crash at 35 mph. This clearly led to an important practical conclusion that only a reasonable amount of this composite material is required to meet the necessary impact performance standard. The CCF composite tested at 5 mm/min crushing speed met both the criteria that need to be satisfied before a material is deemed highly crash worthy: A high magnitude of energy absorption and a safe allowable rate of this energy absorption. All the experimental data and test observations generated from the above work was used to support the modeling efforts conducted by the Computer Science and Mathematics division at ORNL in their pursuit to develop analytical material damage models.

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