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Z-pinning Article

The figure on the right shows a Z-pin inserted in between the fibers of the material.  The pin spreads the fibers and creates an oval shaped gap that is filled with resin to prevent it from splitting.  When a load is applied the cracks will typically form along the line of the opening.

Z-pin inserted between the fibers of a composite

Uses

Z-pinning has been used in aircraft manufacturing to add strength.  The Z-pins can also be added to any material that would benefit from the added strength.  They are especially effective when used in materials that are subject to delamination, because the Z-pins can counteract this problem.  By Z-pinning the materials on an aircraft, such as the wings, it can have a much higher resistance to damage during flight.  Also, if the aircraft does suffer from a minor crack, the Z-pinning will prevent it from completely falling apart.  Z-pins can also be used for automotive applications. The pins can be inserted into carbon fiber parts to increase the strength of them.  If the front splitter of a car was constructed with Z-pins, it would be able to withstand significantly more impacts because the Z-pins would hold it together even with a minor crack. This allows the carbon fiber parts to remain light while still being strong.

Manufacturing

There are many methods of creating Z-pins.  One process consists of pulling a continuous-fiber tow through a bath of liquid resin using a pultrusion machine.  The fiber is then pulled out of the bath through the die which creates the shape and size of the pin.  The pin is next sandwiched in a vertical orientation in foam to finish the process.  The pin may be coated or treated as an additional step depending on the application. This process is one of the more efficient and cost effective ways of producing Z-pins because it can be easily adapted to different pin sizes.

Z-pins have many ways of being inserted. The most common method is a process using an ultrasonic hammer.  The hammer compresses the foam that encases the pins and pushes the pins into the material. The remaining pin and laminate above the surface are removed to create a smooth and even surface The surface can be finished with a coating to seal the Z-pins inside the material.

Testing

F. Lenzi, A. Riccio, A Clarke, and R. Creemers performed coupon testing to study the benefits of Z-pinning. The main purpose of the study was to see if Z-pinning could aid in preventing laminated composites from delamination, which is a big flaw in laminated composites type of material.  The material chosen for testing was a thermoset polymer matrix composite.  The composite contained carbon fiber yarn (HTA-12K/35) and CYTEC 977-2.  Through testing they found that Z-pinning the composite greatly strengthened it.  They also studied the effects of the size of the Z-pins and their location in the composite.  Larger pins were found to increase strength but they disrupting the fibers of the material more. In their testing, the Z-pins were inserted using a hand-held ultrasonic gun into the composite.  The remaining length of the pin protruding from the composite was cut off.  The composite was then laminated to seal the pin in. Two different size Z-pins (0.28 and 0.5 mm) were chosen for testing and placed in different locations of the sample.  It was found that a 1% increase in the size of the pin increased toughness by 6 to 25 times.  Overall the Z-pinning of the composite material led to it being significantly stronger than the unpinned sample.