It’s Been Done Don Niles Outboard Marine Milwaukee , Wisc. Stress analysts at Outboard Marine were faced with the Problem of mounting a full torque bridge (4 gages at 45”) inside a 1/4” diameter hole in a crankshaft. This was a Problem because n o o n e had fingers small enough to fit.

A technician, Lloyd Christensen, solved the problem with a piece of 1/8” copper tube, rubber tubing, two faced cellophane tape, a n d a n air compressor. First, h e mounted a n air hose fitting to o n e e n d of the copper tube, pinched the other end shut, a n d drilled a fine hole through the wall. A piece of 3/16” rubber surgical tubing about 6” long was slipped over the copper tube so as to cover the hole, a n d sealed with rubber bands wound around the ends. Two faced sticky tape was wound around the rubber tubing. Standard 1/8” 120 OHM foil gages were to be used for the torque bridge. Four gages were prewired and

stuck face down on the sticky tape. Each was positioned at the proper 4 5 O angle, with the bonding face outward. The gages were bonded with low temperature epoxy adhesive. The exposed faces were coated with the epoxy, then the tube was inserted in the crankshaft hole a n d aligned. Compressed air applied to the copper tube forced the rubber tubing to expand a n d clamp the gages while the epoxy cured. After bonding, the deflated tubing, sticky tape, and all, was removed-very, very carefully. The completed bridge was then used to measure the torque transmitted through the crankpin under a variety of operating conditions. The tests showed that not all of the flywheel torque was transmitted to the output end as torque in the crankpin. A significant portion was transmitted by a couple between crankpin a n d main bearing.

Gaging of Rock Specimens V. William Dellorfano United States Department of the Interior Minneapolis, Minn. An attempt to determine crack propagation on a granite rock cylinder 1 2 inches long, 6 inches in diameter with a l-inch hole in the center. Applying the strain gage at the center section of the l-inch hole presented a problem. I resolved this by using a tapered rod with a pressure pad around it. Using Duco cement, I attached the four strain gages to the pressure pad. I then applied epoxy adhesive to the other surface of the strain gages. Wires were attached to the strain gages before installation (see sketch). Tapered rod, gages, a n d all were then inserted into the hole and allowed to cure. T h e rod a n d pressure pad were removed by applying acetone which dissolved the Duco cement leaving the strain gages bonded to the surface of the l-inch hole. The first series of the test after applying the strain gage bridge in the borehole generated a feedback signal proportional to the circumferential displacement. T o insure that a crack would initiate immediately beneath the

8 EXPERIMENTAL TECHNIQUES

strain gage, a load was applied diametrically to t h e specimen. This induced a n additional tensile stress field o n the diameter subjected to external loading a n d forced the fracture to initiate in this region since it must propagate perpendicularly to the least compressive stress. It was found that this method allowed complete control of the hydraulic fracturing process. However, the gages had to be replaced after each test a n d practical difficulty arose as soon as the hole was too d e e p a n d too narrow. W e then decided to develop a permanent strain gage device which we called the “miniature straddle packer”. See 1972-27th Conference a n d Exhibit Instrument of Society of America.

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Conversion of Static Strain Gage Calibrations T o Dynamic Signals Of Known Magnitudes Dale Austin Boeing Vertol Company Philadelphia, Pa. Frequently, laboratory test engineers are required to perform static strain surveys on coupons and product components prior to fatigue testing. Conversion of the static strain data to a form that is useful in measuring dynamic strains can be a simple job or a baffling requirement. T o obtain a simple conversion constant without the use of formulas, the following procedure, adapted from resistance calibration techniques, can be useful. Calibrate your component or coupon specimen in increments of load, bending moment, torque, etc. versus strain output from a conventional strain indicator. Unload the specimen and place a shunt resistor across o n e gage element of the strain gage bridge or absolute gage. The value of the shunt resistor should be selected to produce a n equivalent strain in the range of dynamic strains anticipated. This new “equivalent strain” can now be used to establish range constants for the available dynamic measuring equipment. Example Shaft Bending Fatigue Test (350 ohm gages, 2.00 gage factor) Using a strain indicator: 1. Generate a static bending vs. strain gage of bridge output curve. From the best fit straight line obtain 1 4 7 0 micro inch per inch for 240,000 in-lbs (the desired dynamic bending moment) static moment. 2. Select a 200K o h m resistor a n d place across o n e leg of the bridge. This produces a n equivalent strain of 8 7 5 micro inch per inch which is rather low compared to the strain obtained for the desired bending moment. (See note 2.) 3. Select a lOOK ohm resistor. This produces 1750 micro inch per inch equivalent strain which is reasonably close. (A decade resistor or non-inductive wire wound precision resistors in the range of lo4to lo’ ohms should be available.) Using a bridge amplifier and oscillograph: 4. Place the lOOK ohm resistor across the same bridge leg. 5. Adjust the amplifier gain to obtain the desired trace ,deflection for the required dynamic test moment. If desired trace deflection is two inches for 240,000 in-lbs moment: Actual trace deflection = desired trace deflection x Equivalent Strain Calibration Strain = 2 x 1750 = 2 . 3 8 inches 1470

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