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Author Topic: April 2017 Jeff Miller Composites presentation  (Read 146 times)
Posts: 82

« on: April 08, 2017, 09:18:36 AM »

Starting thread ahead of the presentation for a change.  

Rob Schwartz and I will be building some test panels for destructive testing at the upcoming meeting.  I will be posting my math here as I figure out how much they should take before failure.  

I have done a previous presentation on the topic of composite battery boxes, when we were prepping for the Porsche modification.  That presentation was light on math, and heavy on costs and concepts.  

This time around it will be more math and destruction.  Can't have destruction without the math.

As usual around here we will be using the Kevlar left over from the Porsche and Dakota.  KEVLAR 5oz from US Composites
Their spec sheet is seriously light so we have to go back to the manufacturer of the yarn it is made out of.  

Physical Properties   Metric   English   Comments
Density                   1.44 g/cc   0.0520 lb/in   
Water Absorption    3.5 %   3.5 %   As shipped; Typical moisture levels on yarn as shipped; they reflect values reached at normal, moderate temperature and humidity levels following fiber production, which is a wet process.
Moisture Absorption at Equilibrium    3.5 %   3.5 %   Equilibrium from Bone-Dry Yarn; Equilibrium values are determined by bone drying the fiber and condition at 75F (24C), 55% RH.
Mechanical Properties           Metric   English   Comments
Tensile Strength, Ultimate    3000 MPa   435000 psi   Breaking Tenacity
                                    3620 MPa   525000 psi   Epoxy-impregnated strands, ASTM D2343
Elongation at Break            2.4 %   2.4 %   
Tensile Modulus                    112 GPa   16300 ksi   
Tenacity                            2.08 N/tex   23.6 g/denier   
Poissons Ratio                      0.36   0.36   
Thermal Properties   Metric   English   Comments
Specific Heat Capacity    1.42 J/g-C
@Temperature 25.0 C   0.339 BTU/lb-F
@Temperature 77.0 F   
Thermal Conductivity    0.0400 W/m-K   0.278 BTU-in/hr-ft-F   
Maximum Service Temperature, Air    149 - 177 C   300 - 351 F   For long-term use
Shrinkage    <= 0.10 %   <= 0.10 %   In water at 212F (100C) and in air at 351F (177C)

Red highlighted is one of the key items here.  

Since this is the yarn, we will need to adjust for the fact that half the yarn is running in the wrong direction for our test.  So our tensile strength will be 262,500psi in a single direction for the fabric in question in our planned test.  Steel is in the 80,000 range.  Fabric total thickness is 0.0095 inches, so if we end up building something that is one foot wide, the tensile strength of one side will be 0.0095*12= 0.114 square inches multiplied by the ultimate tensile strength of the fibers that are running in the direction of interest gives us a ultimate tensile strength of 29,925 pounds per side.  Various people report that Kevlar is equally good in compression and tension so for our purposes I will assume it is equal. 

« Last Edit: April 08, 2017, 11:29:28 AM by jeffrey.miller » Logged
Posts: 82

« Reply #1 on: April 09, 2017, 09:32:15 AM »

Rob Schwartz and I built the two test pieces for the upcoming meeting last night.  At the very least Rob learned how much of a pain Kevlar is to cut, even with the right scissors. 

The foam we used is on the low end of strength compared to the ones that we have used in our builds.  It was donated by Bruce as the leftovers from his efforts building simulated lithium batteries for the design phase of his HupMobile, (he wasn't concerned about density for his purposes).  The fabric and epoxy were leftovers from both the Porsche and Dakota project.  Thanks everyone for the materials, and Rob for putting in the effort. 

It will be a destructive presentation! 
I don't expect things to fly in that direction, the force will be down towards the floor, but unplanned stuff happens when you break things.   
Posts: 82

« Reply #2 on: April 11, 2017, 09:09:00 AM »

The panels are fully cured.
Weight of the thin one is 1 pound 1.35 ounces
Thick one is 2 pounds 9.06 ounces

They both contain roughly the same amount of Kevlar and epoxy, so the main difference in weight is the additional foam core material. 
Posts: 778

'92 S-10 EV

« Reply #3 on: April 11, 2017, 02:12:45 PM »

  ... but unplanned stuff happens when you break things.   

 Cheesy Wink Smiley
Posts: 132

1930 Hupmobile PHEV

« Reply #4 on: April 11, 2017, 07:31:05 PM »

Maybe we should do this outside?  Do you need any weights?
Posts: 82

« Reply #5 on: April 11, 2017, 08:08:14 PM »

I plan to use human volunteers and four bags of lead I have laying around.  If it isn't enough we will revisit the still standing panel another time.  I ran some initial estimates today, and the numbers are pretty big, but the real world isn't that perfect.

I think I am also going to start with some smaller scale tests built out of lower strength materials, to help show the different failure modes before we go for it with the high strength fabric parts.  

It would be good to have a bathroom scale, to weigh people on, can I get a volunteer to bring one?  
Bruce, if you have some weights in the 5 to 10 pound range, that could be useful for the small scale tests. 

If we are concerned about flying debris, I can bring a bag to put the test pieces in for the actual test, I was planning to lay one below the test pieces to ease any cleanup.  
« Last Edit: April 11, 2017, 08:25:00 PM by jeffrey.miller » Logged
Posts: 82

« Reply #6 on: April 15, 2017, 06:41:59 PM »

Core material is generally polystyrene
ASTM C578 Type X, 15 psi, 1.3 pcf density
They claim 15 psi, and I believe them. 

The panels are roughly 30x16 = 480 square inches x 15 pounds per square inch =  7200 pounds. 
Of course we aren't loading it up evenly like that.  It gets tough to calculate, but I estimate the core will fail, at the highest when 1/3rd of the area hits it's max loading, which is 2400 pounds.  This is because the stresses will be concentrated by the loading, and the overall design.  I don't have any actual math on core failure beyond guesswork at this point. 

The math I understand for the Kevlar is showing that the kevlar will fail higher than 2400 though.  On the three inch thick panel, it is much much higher.  I need to redo my math and post that up at some point, but not tonight. 

Also, we have traditionally used the heavier 5 ounce fabric, they make a lighter fabric, but the cost per yard isn't proportional to it's strength so I just go for the heavy stuff and that way I am pretty confident that the fabric won't be the failure mode without doing a bunch of math. 
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