US2543429A - Extensometer - Google Patents

Description

Feb. 27, 1951 D. s. woon 2,543,429

EXTENSOMETER Filed June 20, 1949 INVENTOR.

` ATTORNEYS.

Patented Feb. 27, 1951 UNHTED STATES PATENT OFFICE EXTENSOMETER David S. Wood, Sierra Madre, Calif.

Application June 20, 1949, Serial No. 100,189

Claims. (Cl. 33-149) My invention relates to extensometers; that is, to devices intended to measure the extension or contraction of a test specimen mounted between the relatively movable jaws of a testing machine, and among the objects of my invention are:

First, to provide an extensometer wherein a cemented wire strain gauge is incorporated with the extensometer and forms the sensitive element thereof.

Second, to provide an extensometer of this class which is capable of measuring displacements greater than is possible by use of a cemented wire strain gauge mounted directly on the test specimen.

Third, to provide an extensometer which may be readily and quickly mounted in place and which is not likely to be damaged in the event of failure of the test specimen.

With the above and other objects in View as may appear hereinafter, reference is directed to the accompanying drawings in which:

Figure 1 is a partial sectional, partial top view of my extensometer shown in place on the specimen; the View being taken along the line of I-l of Figure 2.

Figure 2 is a partial elevational, partial sectional view thereof taken substantially along the line 2 2 of Figure 1.

Figure 3 is an end view thereof.

Figure 4 is a sectional view thereof taken through l-l of Figure 2.

My extensometer comprises two substantially identical units A and B adapted to be mounted on opposite sides of a test specimen S.

Each unit includes a bodyl on two sides of Which are mounted plates 2. The plates project past the inner or specimen side of the body l to form therewith a channel 3.

One end of the body l, the upper end as shown in the drawings, is provided with a slot midway between the plates 2. The Slot is traversed by a pin ll which pivots a lever member 5 which is substantially L-shaped. One leg 5a of the lever 5 extends in this slot toward the main portion of the body l, and receives a-set screw B. The set screw bears against a shoulder Ia forming a part of the body I and projecting into the slot under the leg 5e of the lever. A leaf spring 1 is secured to the body member between the plates 2 and extends upwardly to bear against the lever 5, in a direction tending to urge the leg 5a and set screw away from the shoulder la.

The remaining leg 5b of the lever 5 extends toward the specimen and terminates in a knife edge extremity 8 which bears in a groove C provided in the specimen S. This leg of the lever is also 2 provided on opposite sides with a pair of knife edges 9 pointing in opposite direction from the knife edge 8.

A beam member Hl is centered in the channel 3. The beam is in the form of a flat strip capable of substantial ilexure. At one end the beam is connected integrally to a head l i. The extremity of the beam member remote from the head I l is bifurcated as indicated by Ille,r straddles the leg 5b of the lever 5 and bears against the knife edges 9.

The head is provided with opposed knife edges 2 and i3 adapted to bear respectively in a second groove D in the specimen and in a seat lb provided in the body member near its end remote from the lever 5.

Secured to one or both sides of the beam l@ is a cemented wire strain gauge l 5 which is indicated diagrammatically in Figures 2 and 4. The strain gauge per se is conventional and comprises a resistance wire arranged in grid form, with parallel portions either transversely or longitudinally disposed relative to the beam I0. The wire is cemented to the beam lil in such a manner that flexure of the beam causes change in the resistance of the wire. Measurement of this change in resistance is made to determine the extent of flexure of the beam. The ends of the wire element are connected to suitable terminals l5 provided in the body l.

The two extensometer units A and B are provided with laterally directed lugs Il located approximately midway between the upper and lower extremities of the units.

A U-strap or yoke I8 straddles the two units. Near the cross portion of the U-strap, the legs thereof are provided with slots i9 which receive the lugs Il' of one of the units. The extremities of the U-strap legs are provided with notches 2E) which t over the lugs il of the other unit so that the two units may be held in parallel relation. A set screw 2| is provided in the cross portion of the U-strap i8 so as to bear against the adjacent extensometer unit.

Operation of my device is as follows:

The specimen S is prepared in the usual man-- ner; that is, the extremities (not shown) are constructed so as to fitin the jaws of a testing machine, the central portion of the specimen is reduced in section. Just beyond the reduced portion of the specimen the notches C and D are cut. The spacing of these notches is such that the knife edges 8 of the two units may nt into the upper notch C and the knife edges $2 may lit into the lower notch D.

The set screw S is employed to pivot the lever so as to move the corresponding knife edge 8 to or from the knife edge I2. The yoke or U- strap i8 is employed to hold the extensometer units in place and the set screw 2| is tightened so that the knife edges bear with appropriate pressure in their respective grooves.

The several strain gauges are connected to an electronic device (not shown) capable of measuring changes in resistance of the sensitive elements contained in the strain gauges. The strain gauges may be so arranged that their changes in resistances are cumulative. In the pre-adjustment of the extensometer the beams ID may be free of strain or may have a pre-determined initial strain; this being accomplished by adjustment of the set screws 6. Y

Upon attachment of the extensometers, the specimen is subjected to whatever loads it is intended to impose, and the resulting elongation results in pivotal movement of the head mem.- bers H, and consequent flexure of the beams I which fiexure is measured by the strain gauges l5. The resulting changes in resistances may be correlated with the change in dimension of the specimen. It should be pointed out that the cemented wire type of strain gauge shown is instantly responsive to changes in dimension of the specimen so that even transient loads imposed on the specimen may be detected and recorded or measured.

If the specimen is to be subjected to compression loads, the knife edges are initially adjusted so that the beams l0 are flexed before compression is applied. Then when the specimen is subjected to compression, the beams lil straighten.

Having thus described certain embodiments and applications of my invention, I do not desire to be limited, but intend to claim all novelty inherent in the appended claims.

I claim:

1. An extensometer unit, involving: a beamv member including a flexible blade and a head at one end, said head having opposed knife edge bearings, one bearing adapted to engage a specimen; a body structure pivotally engaged by the other knife edge of said head; a lever pivotally mounted in said body member and including a leg terminating in a first knife edge bearing engageable with said specimen at a point spaced from the knife edge bearing of said head; said lever having a second knife edge bearing facing in the opposite direction therefrom the extremity of said flexible blade remote from said head and overlying said second knife edge bearing; and cemented wire strain gauges mounted on said blade.

2. An extensometer, involving: a pair of hinged members one arranged to bear against a specimen; a beam structure including a flexible blade bearing against said one member, and a head interposed between the other of said members and said specimen and pivotally connected with said other member whereby change in longitudinal dimension of said specimen causes fiexure of said blades; and cemented wire strain gauges mounted on said blades to reflect the degree of flexure thereof.

3. An extensometer unit, involving: a beam including a flexible blade and a head member having opposed bearing elements disposed in a plane normal to said blade; a lever disposed adjacent the extremity of said blade and having opposed bearing elements, one of said bearing elements being engageable by the extended portion of said blade; a body structure journalling said lever and engaging one of the bearing elements of said head member; the other bearing elements of said lever and head adapted to engage a test specimen at spaced points and move longitudinally therewith causing relative pivotal movement of said lever and body and flexure of said blade; and cemented wire strain gauges mounted on said blade.

4. An extensometer unit, involving: a body member, a lever pivotally connected to said body member and having an arm terminating in a bearing engageable with a test specimen; a beam member including a head disposed in approximate parallelism with said arm and having bearings engageable with said specimen and said body, and a flexible blade rigidly connected to said head and extending therefrom to said arm in approximate parallelism with said specimen; said arm having a second bearing supporting the extremity ofV said beam; and strain gauges secured to said beam, said head adapted to tilt on change in the longitudinal dimension of said specimen to cause flexure of said beam.

5. An extensometer, involving: a body member; a lever pivotally connected to said body member and including an arm extending at approximately right angles thereto and terminating in a knife edge bearing facing away from said body member, said arm also havingY a second knife edge bearing facing toward said body member; a beam member including a head having a irstV knife edge bearing facing away from said body member anda second knife edge bearing engagingsaid body member, and a flexible blade, extending from said head in substantial parallelism withv said body member with its extremity resting against the second bearing of said arm; and strain gauge means on said blade, s aid first knife edge bearings of said arm and said head adapted to engage aspeciman and on change in length of said specimen to cause exure of said blade. c,

DAVID S. WOOD.

REFERENCES C'I'ED v The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,416,664 Ruge Feb. 25, 1947 2,484,164 Hathaway Oct. 11, 1949 FOREIGN PATENTS- Number., Country Date 147,987 Great Britain July 23, 1920 OTHER REFERENCES Publ.: Metal Progress, Nov. 1948, pagesf

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