30 June 2001
Additional outlets for the electrical leads from these measuring devices are required. Cell pressures
of 80 psi are generally sufficient to duplicate the maximum confining pressures under aircraft
loadings. Compressed air is generally used as the confining fluid to avoid detrimental effects of
water on the internally mounted electronic measuring equipment.
b. End Platens. End platens should be "frictionless," as "barreling" caused by end restraint
jeopardizes resilient Poisson's ratio values by causing lateral deformations to be concentrated in the
middle of the specimen. Furthermore, nonuniform displacements can create problems with axial
strain measurements due to realignment of the LVDT clamps. Whereas "frictionless" platens
(Figure N-2) may not be entirely frictionless under short-term repetitive loadings, they constitute an
improvement over conventional end platens. The essential features of "frictionless" end platens
are hard polished end plates, coated by high-vacuum silicone grease, and covered by a thin rubber
sheet. If externally mounted axial deformation measuring devices such as an LVDT or
potentiometer mounted on the loading piston, or devices measuring the total specimen
displacements are used, the use of frictionless caps and bases with grease invalidates any
measurements. In this case, the deformation due to the grease and rubber sheet or Teflon probably
exceeds the actual deformation of the specimen. Hence, frictionless caps and bases are restricted
to use with internally mounted deformation sensors.
c. Repetitive Loading Equipment. The external loading source may be any device capable of
providing a variable load of fixed cycle and load duration, ranging from simple switch control of
static weights or air pistons to a close-loop electrohydraulic system. A load duration of 0.1 to
0.2 second and a cycle duration of 3 seconds have been found satisfactory for most applications.
A haversine wave form is recommended; however, a rectangular wave form can be used.
d. Deformation and Load Measuring Equipment. The deformation measuring equipment
consists of four LVDTs attached to the soil specimen with a pair of clamps, as shown in
Figure N-1. Two LVDTs are used to measure axial deformations, and two are used to measure
lateral deformations. Figures N-3 and N-4 show the details of the clamps for attaching the LVDTs
to the soil specimens. Only alternating current transducers that have a minimum sensitivity of
0.2 millivolt per 0.025 millimeter (0.001 inch) per volt should be used. Load is measured with an
internally mounted load cell that is sufficiently lightweight so as not to provide any significant
inertia forces. It should have a capacity no greater than two to three times that of the maximum
applied load and a minimum sensitivity of 2 millivolts per volt.
e. Additional Equipment. In addition to the equipment described above, the following items
are also used:
(2) Rubber membranes (0.03 to 0.06 millimeter (0.012 to 0.025 inch) thick) and a
(3) Rubber O-rings.
(4) Guide rods for positioning LVDT clamps.
(5) Epoxy for cementing clamps to membrane.