Patent GB-415520-A (GB)
Title: Improvements in surface extensometers
IPC: G01B11/16
Filed: 1933-04-07
Abstract: 415,520. Extensometers. CLARKE, T. W. K., Lindley, Farnborough Park, Hampshire. April 7, 1933, No. 10430. [Class 106 (ii).] Comprises a pair of relatively displaceable gauge legs for contacting with the surface the extension of which is to be measured, and a pair of mirrors mounted at 90‹ and so connected to the gauge legs that their relative inclination varies in proportion to the relative displacement of the gauge legs. The device is specially applicable to the investigation of strain in thin plates. A body piece 20, Fig. 1, is extended at 21 to form a handle and carries fixed to it one gauge leg 22. The other and pivoted gauge leg 23 is carried by a horizontal plate 25 which is attached by a pair of fine vertical bronze springs 26 to the part 20. These springs are disposed one at each side of the leg 23 and are connected to horizontal cantilever springs 27. The upper pointed end of the leg 23 bears against the flat end of a plug 36 screwed into the part 20. The plate 25 carries a plane mirror 29 and is provided with an aperture through which the fixed leg 22 passes with clearance. The rear of the plate 25 has attached to it a fine screw 31 upon which is a nut by which it may be clamped. The forward end of the part 20 carries a spring arm 33 on which is a vertical plane mirror 34. A screw 35 serves to adjust the angle between the mirrors. Instead of a single pivot and two springs, two spaced transverse pivots and one spring may be used, or crossed springs may effect the hinging. In a modification, Fig. 3, the leg 23 has a single gauge point but at its upper end carries two gauge points resting against the lower ends of two plugs. The lower end is located by passing through a hole in the end of a thin metal plate 37 attached to the fixed leg 22. An upper thin plate 41 attached to the front of part 20 similarly locates the upper end of the leg 23. Between the spring plate 41 and the body is a lever 38 pivoted to the body and extended to form a handle 40. In this construction the movable and fixed mirrors 29, 34 are inclined at 45‹ to the vertical when the device is horizontal. To hold the device to the work a spring clip 43, Fig. 5, is engaged with wire loops 42 cemented to the plate and is then sprung over a cup 44 engaging with a vertical needle 45 on the body. Two further forms of spring clip are described for use with bars. The telescope is of the collimating type with an object glass of about 36" focal length. The eyepiece, of thin lens type contains a micrometer of hair-line type operated by a screw 50. The source of light is a slit illuminated by a lamp in a side tube 53. Opposite the lamp is a reflecting mirror so placed that the light emanates from a virtual source at the focus of the objective. The mirror practically obstructs the left half of the field, but two line images of the slit are seen in the right-hand half. The angular separators of the images will be 4a where "alpha" is the difference between 90‹ and the angle of between the extensometer mirrors ; so that if the distance between the images is divided by four times the focal length of the objective "alpha" may be obtained. The telescope must be directed towards the centre of the mirrors and its axis must be at right angles to the intersection of the mirrors. The first condition is satisfied by removing the eyepiece when the mirrors can be seen and the telescope directed towards them. To obtain the second condition, the telescope is adjusted by a linkage. The telescope is pivoted to a bracket 63 pivoted to a link 65. A cap 57 fitting on a tripod 58 carries a spindle 59. which may be clamped in any desired position relative to the cap. To the spindle are pivoted arms 60, 61. The bracket 63 carries a pin 64 which rests on the cap. Equal links 66, 67 are pivoted to the opposite ends of the link 65 and at their ends away therefrom are pivoted to arms 60, 61. An oppositely threaded screw 68 interconnects the arms 60, 61. The spindle 59 carries a nut 69 which can grip the arms 60, 16 together and lock the links 66, 67. At the lower end of spindle 59, a looking arm 70 is provided having an upstanding portion 71 projecting through a cut away portion on the arm 60 and gripped between a screw 72 and a co-operating plunger. The inner end of the arm 70 is bifurcated and a wing nut 74 serves to clamp the arm to the spindle 59. One link 66 is first adjusted to point towards line of intersection of mirrors 29, 34. The screw 68 is then adjusted until link 67 points towards the line and then nut 74 is tightened up. By turning screw 72 the linkage 66, 65, 67 and screw 68 turns as a whole about spindle 59.