During the flight-test stress analysis of a titanium aircraft wing tip section, with, and without, a missle module attached, the operating temperature range for strain measurements is from -65 deg to +450 deg F (-55 deg to +230 deg C), and will be a dominant factor in the gage selection.

Gage Length     Preliminary design studies using the PhotoStress® photoelastic coating technique indicate that a gage length of 0.062 in (1.6 mm) represents the best compromise in view of the strain gradients, areas of peak strain, and space for gage installation.

Gage Pattern     With information about the stress state and directions of principal axes gained from the photoelastic coating studies, there are some areas of the wing tip where single-grid gages and two-element -"Tee" rosettes can be employed. In other locations, where principal strain directions vary with the nature of the flight maneuver, 45-degree rectangular rosettes are required.

The strain gradients are sufficiently steep that stacked rosettes should be selected. From Catalog 500, Micro-Measurements Precision Strain Gages , the foregoing requirements suggest the selection of 060WT and 060WR gage patterns for the stacked rosettes, and the 062AP pattern for the single-grid gage. In making this selection, attention was given to the fact that all three patterns are available in the WK Series , which is compatible with the specified operating temperature range.

Gage Series     The maximum operating temperature, along with the requirement for static as well as dynamic strain measurement, clearly dictates use of K alloy for the grid material. Either the SK or WK Series could be selected, but the WK gages are preferred because they have integral leadwires.

Options     For ease of gage installation, Option W , with integral soldering terminals, is advantageous. This option is not applicable to stacked rosettes, however, and is therefore specified for only the single-grid gages.

Resistance     When available, as in this case, 350-ohm gages should be specified because of the benefits associated with the higher gage resistance.

S-T-C Number     The titanium alloy used in the wing tip section is the 6Al-4V type, with a thermal expansion coefficient of 4.9 x 10 ^-6 per deg F (8.8 x 10 ^-6 per deg C). K alloy of S-T-C number 05 is the appropriate choice.

Combining the results of the above selection procedure, the gages to be employed are:

WK-05-062AP-350/Option W
WK-05-060WT-350
WK-05-060WR-350

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