STRAIN GAGE INSTALLATION HOLE DRILLING

Transcription

1 Via Delle Calandre Calenzano (Florence) Italy Tel: Fax: STRAIN GAGE INSTALLATION HOLE DRILLING How to install strain gages for residual stress measurements using the hole drilling method

2 Main steps for the strain gage installation The main steps of the strain gage installation for hole drilling residual stress measurement are: Selection of the strain gage Surface preparation Marking layout lines (if applicable) Cleaning of the surface Strain gage positioning Bonding of the rosette Soldering of the wires (if applicable) Blocking of the cables Coating of the strain gage rosette (if applicable) Test of the strain gage installation 2

3 Strain gage selection The selection of the strain gage depends on the test positioning and on the type of test. Particularly: Positioning of the measurement point Base material Sizes of the strain gage rosette Correction algorithms Soldering availability Cost Time of the strain gage installation Pre-wired strain gage allows to obtain less errors (in soldering) with less time compared with standard strain gage. Otherwise, the cost are about 30% higher. 3

4 Standard HBM rosettes for hole drilling 1-RY61-1.5/120S Type A rosette For general use 1-RY61-1.5/120R Type B rosette For general use and installation near an edge 1-RY61-1.5/120K Type B rosette For general use and installation near an edge or a corner 4

5 Special HBM rosettes for hole drilling K-RY61-1.5/120R Type B rosette For general use and installation near an edge Also used inside pipes and on polymeric material 1-VY61-1.5/120S Type A rosette For general use Also used for local plasticity correction 1-RY61-1.5/120M For general use Also used for eccentricity correction 5

6 Surface preparation It is always necessary to prevent forms of surface contamination: the contamination (e.g. oxides) can introduced instability in the measurement of strains. The surface preparation has to remove: Oxides Metallic coating (based on Cr, Cd, Ni) Anodizing and burnishing Paints The surface preparation have to be made using sand papers. Surface preparation must introduce the correct bonding roughness (2 4 μm). For steels, the preparation must be done using sand paper 220 and 320; for alloys using 320 and 400. Surface preparation must be done by hand or using air compressed grinding machine. Never use electrical tools 6

7 Marking layout lines (if applicable) Residual stress rosettes, in some application, need a higher precision during the positioning on the workpiece. The reference lines on the strain gage rosette have to be placed and aligned in the same direction of the reference lines made by a pen without ink on the testing workpiece. The reference lines on the workpiece should not be made under the strain gage bonding area but just outside 7

8 Cleaning of the surface Before bonding the strain gage rosettes, the surface of the workpiece must be cleaned from all the contaminants and dust. For the surface cleaning, the HBM RMS1 (liquid or spray) with its cleaning pads has to be used. During this operation the hands of the operator and all the tools must be kept clean. The cleaning of the surface must be done using the cleaning pads in the same direction since the pads are not able to remove additional dust. 8

9 Strain gage positioning The strain gage has to be placed on a glass (clean) and, using a piece of strain gage tape, it has to be positioned on the testing point. Using the tape it s possible to avoid touching the strain gage rosette during this operation, and also to move the rosette whenever it s necessary. The tape needs a good strength because it has to be removed after bonding. HBM Klebeband Polyimide tape can be used. 9

10 Hole drilling strain gage method to ASTM E The HBM Z70 glue allows to bond the strain gage rosette on the surface of the workpiece. The HBM Z70 can be used with/without its rapid activator HBM BCY-01. HBM Z70 glue HBM BCY-01 activator must be used when temperature is lower than 10 C. BCY 01 activator For bonding the strain gage, put the glue on the back of the strain gage (and the activator on the base material), apply the strain gage on the workpiece and, using the teflon foil, make a constant pressure on the rosettes for some minutes. 10

11 Strain gage bonding 11

12 Soldering of the wires (if applicable) Using a standard strain gage (not pre-wired) it s necessary to solder the connection cables for acquiring the data from the strain gage rosettes. Particularly, it s necessary to connect 3 wires (min. 2) for each strain gage grid: in some strain gage rosettes you can directly solder the wires in specified areas on the rosettes, in other rosettes you have to install on the workpiece some pads for the connection. Correct solder must be rounded and brilliant. A solder with edge is fragile and brakes during the tests. 12

13 Blocking of the cables Before starting the test, the cable must be blocked to be sure that during the measurement there are no forces in the area of the strain gage (weldings, grids). Particularly, the following material should be used: Paper tapes HBM X60 glue (suggested): the rapid glue ensures a fixed connection of the cable during the time, and it has also a higher mechanical strength. 13

14 Coating of the strain gage (if applicable) For hole drilling measurements in hard surfaces (e.g. quenched, special steels), it s suggested to lubricate the drilling area with some drops of cutting oil for making the hole in the center of the strain gage rosette in the correct way Oil introduces problems of insulation of the strain gage: it s necessary to protect the grids using a special coating. HBM NG150 has to be placed on the surface of the strain gage with exception of the center (it can introduce uncertainty in the determination of the zero-depth) 14

15 Test of the strain gage installation At the end of the strain gage installation, the correct connection of the strain gage rosette has to be verified. Particularly, some tests have to carried out: Visual check: solder, cables and grids have to be checked Check of the nominal resistance: for each grid, using a digital tester, the values of resistance must be checked. The values have to be close to the nominal value specified in the SG datasheet (generally, 120 Ohm) Check of the insulation: the insulation between the strain gage and the workpiece must be more than 2000 MΩ for tests inside laboratory and 500 MΩ for tests outside Rubber test: After a pressure with a rubber on the strain gage, the reading values have to return in zero 15

16 Strain gage datasheet Type Nominal resistance Gage factor (K) Thermal Compensation Lot and batch 16

17 Resuming 17

18 Via Delle Calandre Calenzano (Florence) Italy Tel: Fax: