Introduction:

Polymer Composite Testing by split Hopkinson pressure barKennon Owens1,2, Shruti Nair1, and Shreyas Joglekar1, Dr. Mark Pankow1

1Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 276952Department of Mechanical Engineering, North Carolina A&T State University, Greensboro, NC 27401

kmowens@aggies.ncat.edu

Objective:

Background:

Results:

Conclusion:

Discussion:Methods:

Future Work:

Acknowledgements:This material is based upon work supported by the National Science Foundation under Grant No. 1460935. 

• Carbon fiber composites typically consist of fibers or nanoparticles (“fillers”) that have been cured in a resin (“matrix”).

• These components incorporate properties that mesh well and complement each other to create a material with many benefits and few weaknesses.

• To obtain a stronger understanding of how relevant properties of carbon fiber, such as: Young’s modulus (force needed to manipulate a material’s size and shape) and Poisson’s ratio (negative ratio of transverse strain to axial strain) are affected by a high strain and loading rate.

• The experiment shown utilizes a split Hopkinson pressure bar (SHPB) and the material testing techniques that it encompasses.

• Carbon fiber is a fiber-reinforced plastic, developed and utilized for its material property combination of extremely high-level strength and light weight.

• The first step would be calculating the specimen geometry and fabricating them.

• The specimen would be made to fit into the SHPB, while the connections with all measuring equipment are established.

• This setup leads into the performance of the test and gathering of data.

• This data would be analyzed and simplified through the use of MATLAB software.

• The applications of the composite form of these fibers can be found in the automotive and aerospace industries, being used as a competitively alternative material for the vehicle chassis.

• An SHPB, like the one utilized in this experiment, is a tool used to characterize the mechanical response of materials being made to deform at high strain rates.

• The strain rate range of an SHPB is 102 s-1 to 104 s-1, which is the same rate commonly faced in collision-related loading situations, such as a car collision.

A diagram depicting the layout of an SHPB is displayed below.

• Testing samples of carbon fiber on an SHPB proved to be troublesome due to their flexibility.

• We were forced to use specialized tools to assist with fabrication and loading of specimens.

• Failure to handle the specimens properly will result in premature failure and/or experimental failure along areas of stress concentration.

Above is a picture of the SHPB setup, including the camera used, lighting, and specimen.

Displayed above is a carbon fiber sheet

1. Xia, Kaiwen (2014). "Dynamic rock tests using split Hopkinson (Kolsky) bar system – A review“2. Carbon Fiber. (n.d.). http://www.acpsales.com/Carbon-Fiber.html3. Gyrostat (Wikimedia, CC-BY-SA 4.0) 4. Addressing the environmental impact of carbon fiber Scuttlebutt Sailing News. (2013).

http://www.sailingscuttlebutt.com/2013/10/17/addressing-environmental-impact-carbon-fiber/5. The Carbon-Fiber Future: It's About More Than Speed (Op-Ed). (n.d.). http://www.livescience.com/53995-carbon-fiber-

may-finally-be-coming-to-cars-everywhere.html6. Carbon Fiber Gear. (n.d.). http://

store.carbonfibergear.com/autoart-1-18-lamborghini-sesto-elemento-with-carbon-fiber-pattern7. Carbon Fiber Creations. (n.d.). http://www.botoxbeerbling.com/carbon-fiber-creations/8. Weinong W. Chen, Bo Song (2011). "Split Hopkinson (Kolsky) Bar Design, Testing and Applications“9. M Pankow, C Attard, A M Waas (2009). "Specimen size and shape effect in split Hopkinson pressure bar testing"

Test Specimen 1

Test Specimen 2

Test Specimen 3

Test Specimen 4

Some of the equations used to determine specimen design size are displayed at the right.

Shown above is a picture of the SHPB used to retrieve the data for this experiment.

• The next step for this lab would be to use the data found here to extrapolate the definite material response of these fibers, in terms of how there properties changed with respect to strain rate

• Future experiments should have a means of mitigating the easily damaged nature of the carbon fiber specimen.

• It is also recommended that any future experiments work to find a means of mitigating the threat of stress concentrations in the specimens.

Found below is another example of carbon fiber utilized in an automotive vehicle.

At the left is an example of carbon fiber utilized in an automotive vehicle.

At the left is an airplane utilizing carbon fiber in the skin of its wingspan.

Above is a picture of a few sheets, from which carbon fiber specimens were cut.

• An SHPB works by using a gas pressure gun to fire a striking rod into an impact with the incident rod.

• This impact sends a stress pulse through the rod, which is then sent through the specimen, causing failure.

• While in the specimen, the pulse is both reflected back into the incident rod and transmitted into the transmission rod, where it can be measured by strain gauges.

• The first specimen utilized a dog-bone shape.• As can be seen in the above picture, failure

occurred in the middle section of the specimen.• Further inspection showed signs of premature

failure during the experiment, thus leading to a small transmitted signal.

• The second specimen used a straight rectangular shape (no dog-bone).

• The experiment showed signs of failure initiation, however failure did not occur due to it being loaded into the SHPB incorrectly.

• A strong signal was transmitted despite not having failure.

• The third specimen was another straight rectangular one.

• As can be seen above, the type of failure that took place with this specimen was very irregular.

• This type of failure has not been seen in this lab before, however the signals were strong.

• The fourth specimen was that of a dog-bone.• The failure shown took place at the gripped

edge of the specimen, due to stress concentrations along the edges.

• This type of failure typically results in weak signals, as was the case here.

Shown at the left is the rear-view mirror of an automotive vehicle, which incorporates carbon fiber.Shown at the right is the front area and front left wheel of another vehicle with carbon fiber.

Shown below is a carbon fiber sheet.

• The data gathered appears to favor the straight rectangular specimen shape, as opposed to the dog-bone shape.

• Both dog-bone specimens gave weak signal readings, however previous experiments have seen these types of readings in straight specimens also.

• The reading distribution may be a coincidence.

• Based on these results, it would appear as though the strength of the carbon fiber tested, along with several other of it’s properties, experience significant change at higher strain rates.

• The difficulty in the experimentation itself leads us to believe that the SHPB was not designed with this type of specimen shape and flexibility in mind.

• In terms of the signals received, the experiment showed much greater strain pulses in the reflected signal than that of the transmitted signal, leading us to believe that the carbon fiber is well suited to block strain inducing force.

• The way in which the specimens failed leads us to believe that this type of carbon fiber doesn’t have a well-structured and even strength distribution.

• The tests conducted in this lab will go onto be used as a means to classify the properties of carbon fiber as they apply to conditions involving a high strain rate.