This experiment closely follows ASTM D7332/D7332M-07
Test Specimen Requirements:
At least test and prepare 3-6 test specimens. More is always preferable so we can distinguish the different modes of failure present
Test pieces to be manufactured in the dimensions 84mm x 84mm (length and width), and height is depended on the sandwich stack of the 2 CF layers + divinycell foam core + another 2 CF layer.
Equipment Needed:
Universal Testing Machine
Calipers
Fastener Pull Through Strength Fixture
Image below is one type of fixture design used to execute this experiment. Note, there are different types of fixtures that overall serve the same purpose of shearing the bolt out of the CF panel. The fixture would be attached to the Universal testing machine.
Procedure:
Condition the specimen, ensuring that the coupon used has been manufactured in the proper dimensions and contains no defects. If the specimen needs to be placed in a conditioned/environment chamber prepare the test pieces prior to the shear out test.
Measure the specimen dimensions using a caliper and taking note of imperfections in the sample. Install the fastener onto the specimen by using a calibrated torque wrench at a specific torque. Within this experiment, a baseline 125N tensile preload was applied onto the fastener to ensure that they are secure.
Place the test fixture onto the UTM and making sure that the fastener is held in place tightly around the fixture. Depending on the fixture used, follow the instructions on how to properly attach it to the UTM machine.
Attach a deflectometer if UTM does not already measure displacement and force. Check fixture and specimen if they are all aligned before a force is applied.
Apply a tensile force at a crosshead speed of 0.5mm/min. The test should be stopped when the load has dropped to 50% of its max load. In addition, a graph of load against displacement should be attained.
Variable | Quantity |
|---|---|
Head displacement speed | 0.5mm/min |
Tensile preload on fastener | 125N |
Load displacement readings | Every 0.2s throughout test |
An equation below will allow you to find the fastener pull through work by calculating the area under the curve.
Integrating this equation will get you the energy done by fastener pull through.
Different types of Failure:
Fastener Pull Through: mainly occurs with countersunk bolts (Bolt that is flush to the surface). This type of failure mainly depends on the thickness of the material, fiber pattern, and layup sequence.
Bearing Failure: crushing at the bolt hole contact interface and the material moves out of plane. This type of failure usually occurs when the bolt to width ratio of the specimen is large. The accumulation of the cracks, internal delamination and fiber buckling is a direct result of this type of failure.
Net tension Failure: opposite of bearing failure where the ratio of width to bolt is too small. The failure in this type of mode is suddenly and occurs at 90% of the failure load. Delamination and cracks in the matrix cause the displacement curve to be non linear.
Shear Out Failure: A special case of bearing failure due to a short end distance. This failure mode was only sudden and warning of final failure occurred at 87% of the final failure load. Matrix cracks, delamination, and fiber microbuckling were also present in its failure.
Bolt/Fastener Failure: This type of failure occurs as a subset of bearing failure. A considerable amount of damage of bearing failure must occur initially, then the final failure occurred through a bolt failure. Bolt would normally fail at its minimum cross sectional area near the root of the head.
Image Below shows a great depiction of what these failures look like.
