Ultimate Strength Tensile Tester
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, floor above Dale's hat)
... Dale, "I need more clamps" ... note white pipe clamps
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
Test Results for Aluminum Materials for Hummel UltraCruiser
(Note: Different sources report different Tensile Strength ... I used the lower value of the different sources)| Specimen | Tensile
Strength
(psi) | Predicted Break
(psi) | Actual Break
(psi) |
| Spar carry through ... 2024-T3 plate | 64,000 | 1530 | 1500+ |
| Fittings ... 2024-T4 bar | 63,000 | 523 | 750 |
| Spar cap angle ... 2024-T4 | 63,000 | 720 | 900 |
| Angle ... 6061-T6 | 42,000 | | not tested |
| Cheap Hardware store angle | 11,000 ? | 583 | 550 |
3/32nd rivet (two in pattern used on rib)
3/32nd rivet (one in shear) | 38,000 | 384
186 | 500
200 |
| Avex 1682-0412 High Shear 1/8inch Pull Rivet (Alum with Steel mandrel) | 210 shear | 165 | 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.
... Gauge is fitted to bottle jack via tapped, drilled hole
END
... Gauge is fitted to bottle jack via tapped, drilled hole