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Analysis of a Strain Gage Rosette

Analysis of a Strain Gage Rosette - Calculating Principal...

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Ronald B. Bucinell, Ph.D., P.E. Union College 1 Department of Mechanical Engineering Schenectady, NY 12308 Calculating Principal Strains using a Rectangular Strain Gage Rosette Strain gage rosettes are used often in engineering practice to determine strain states at specific points on a structure. Figure 1 illustrates three commonly used strain gage rosette configurations. Each of these configurations is designed with a specific task in mind. The far right rosette in Figure 1 is referred to as a tee rosette. Tee rosettes are two element rosettes and should only be used when the principal strain directions are known in advance from other considerations. The middle rosette in Figure 1 is referred to as a rectangular rosette and the far left as a delta rosette. These three element rosettes are used in applications where the principal strains are unknown . There are many other parameters that need to be taken into consideration when choosing a strain gage. Literature from manufacturers of strain gages provides some guidance in choosing the proper strain gage for a given application. The primary focus of this treatise is the rectangular rosette. Figure 2 illustrates the numbering sequence and geometry that will be used in the following discussion. Figure 3 shows an actual rectangular strain gage rosette installed on a structure with the lead wires soldered onto the terminal tabs. In the remainder of this document the procedures for calculating principal strains from the strains that are read from a rectangular strain gage rosette are presented. First the correction of the raw strain readings for the loads transverse to the principal axis of the individual gages is presented. This is followed by a summary of the calculations used to compute principal strains directly from the corrected strain gage data. Finally a method used to calculate the principal strains from the corrected strain values using Mohr’s Circle is discussed. Figure 1: Examples of common Strain gage Rosettes encountered in engineering practice. Figure 3: Rectangular strain gage rosette installed on a structure with lead wires attached. Figure 2: Illustration of the orientation of the rectangular strain gage rosette being evaluated in this example. 45 ° 45 ° a b c loop

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Ronald B. Bucinell, Ph.D., P.E. Union College 2 Department of Mechanical Engineering Schenectady, NY 12308 Correcting Strain Gage Data Each of the strain gages in a rosette is attached to separate bridge circuits through the lead wires. The type of bridge circuit used is a function of the application being considered. Under load, three strain values are recorded at each load increment. These raw strain readings are designated as the normal strains ˆ a ! , ˆ b , and ˆ c . The “ ^ ” designates that the strain is a raw or uncorrected strain.
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Analysis of a Strain Gage Rosette - Calculating Principal...

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