Ultimate Strength Tensile Tester
Updated 8/30/2015
Designer: Dale Johnson

[Note: Click on any image for larger image]

When I walked into Dale' workshop.  I couldn't find him.  I scanned the workshop several times but thought he must be back at the house.  Finally my hearing and eye-sight coordinated:


Nose of WWII CG-4A Waco Troop Glider (upside down, the floor is above Dale's hat)
... Dale, "I need more clamps" ... note white "ring" clamps fashioned from plastic pipe
Note: This photo was the Cover photo on "Bungee Cord" magazine, 2010
         



Below is an article written for EAA Chapter 25

Junk Bin Tester
Designer: Dale Johnson
Article by: S. Steve Adkins


While serving aboard the Polaris Submarine for one year, I leaned the value of testing.  But recently, I became more personally involved with testing ... the strength of the materials used to build the spar caps for my Hummel UltraCruiser.  As you look at a 3/16 inch aluminum plate that will serve as the "spar carry through" or you look at the 1/16 inch aluminum angle that will serve as part of the  "spar cap", you begin to think, "My life depends on this part!" and wonder, "Have I been delivered true aircraft quality material or is it bogus material from some foreign country."

Fortunately, Dale Johnson has designed and built a parts and material tester.  No money was spent to build this tester.  Dale used the tester to test my Hummel material including:  the spar cap angle, spar carry through plate, hardware store angle, aircraft aluminum bar stock, standard aircraft rivet and blind or pull rivet.  I predicted the failure point for each test specimen.  It was very exciting to see each part fail close to my prediction.  All  rupture points were at or slightly beyond my prediction.

The tester was built to test the uncountable hand-made turnbuckles which Dale built for the Pietenpol. Also, he tested the drag and anti-drag wires.  At the opposite end of  testing a/c parts, Dale has used the tester to proof-test the tow ropes used by our glider club.  Note: his testing of rope found that a knot in the rope would fail at less than one-half the strength of the rope which indicates one should be sure to remove any knots from a rope.  The Dale Johnson Tensile Tester has become well known and used in Britain.  When Chris Bobka learned that builders in that country are required to proof-test their parts, he immediately pointed them towards Dale.

Dale built the tester from the multitude of parts and materials stored in his backyard workshop.  A neighbor provided a heavy-duty, 1/4-inch by 7-foot construction steel which forms the frame for the test fixture.  It has plenty of holes for adjusting the rig for different length parts.  A 4-ton bottle hydraulic jack serves as the source of tension.  A 3000 pound pressure gauge from an oxy/acetylene torch serves to measure the tension.  A few pieces of large steel angle coupled with 3/8ths-inch threaded rod and two aircraft clevis tie rod terminals complete the fixture.  Two welds were required to create a sliding fixture but clever use of additional angles and bolts could accomplish the same purpose without welding.  The anchoring parts are generally in the 65,000 pounds tensile strength range, so their cross-section must be great than the cross-section of your test specimen.

The first test specimen was made from a scrap from my 3/8-inch aluminum spar carry through plate.  I spent considerable time cutting, grinding and polishing to have the precise cross-section and no abrupt changes in dimension. (to avoid necking).  This took too much time.  Eventually, I stumbled on the idea to drill two holes close enough to create the desired cross-section.  Then one merely uses a band saw to make cuts from the edge of the material to the holes.  While testing, the specimun may begin to stretch or "yield".  This stretching will result in a drop in indicated pressure while one is pump the pump handle.  I made no attempt to determine the yield point

  tension_test_graph1.gif

Test Results for Aluminum Materials for Hummel UltraCruiser
(Note: Different sources report different Tensile Strength ... I used the lower value of the different sources)
SpecimenTensile
Strength
(psi)
Predicted Break
Pounds
Actual Break
Pounds
Spar carry through ... 2024-T3 plate64,00015301500+
Fittings ... 2024-T4 bar63,000523750
Spar cap angle ... 2024-T463,000720900
Angle ... 6061-T642,000
not tested
Cheap Hardware store angle11,000 ?583550
3/32nd rivet (two in pattern used on rib)
3/32nd rivet (one in shear)
38,000384
186
500
200
Avex 1682-0412 High Shear 1/8inch Pull Rivet (Alum with Steel mandrel)210 shear165
200
edge tear-out

Note: Accuracy of readings are limited by the gauge which shows instantaneous readings only; thus, when the sample breaks

the pressure drops to zero immediately.  Also, the bottle jack and gauge sometimes jumps upon specimen break. 

The Predicted Break column was based on the cross-sectional area of the part under test mulitiplied by the tensile strength.  The cross sectional area was

made small enough so that the failure would be less than the limit of the guage below.  The best method to create a small cross section,

was to drill two holes close together ... then cut the connection material from each hole to the outside edge (see belwo).


... Gauge is fitted to bottle jack via tapped, drilled hole

END