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Tech Note TN-

Micro-MeasureMeNTs

Strain Gage Rosettes:

Selection, Application and Data Reduction

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Strain Gages and Instruments

1.0 Introduction

A strain gage rosette is, by definition, an arrangement of two or more closely positioned gage grids, separately oriented to measure the normal strains along different directions in the underlying surface of the test part. Rosettes are designed to perform a very practical and important function in experimental stress analysis. It can be shown that for the not-uncommon case of the general biaxial stress state, with the principal directions unknown, three independent strain measurements (in different directions) are required to determine the principal strains and stresses. And even when the principal directions are known in advance, two independent strain measurements are needed to obtain the principal strains and stresses.

To meet the foregoing requirements, the Micro- Measurements manufactures three basic types of strain gage rosettes (each in a variety of forms):

• Tee : two mutually perpendicular grids.
• 45°-Rectangular : three grids, with the second and third grids angularly displaced from the first grid by 45° and 90°, respectively.
• 60°-Delta : three grids, with the second and third grids 60° and 120° away, respectively, from the first grid.

Representative gage patterns for the three rosette types are reproduced in Figure 1.

In common with single-element strain gages, rosettes are manufactured from different combinations of grid alloy and backing material to meet varying application

requirements. They are also offered in a number of gage lengths, noting that the gage length specified for a rosette refers to the active length of each individual grid within the rosette. As illustrated in Figure 2, rectangular and delta rosettes may appear in any of several geometrically different, but functionally equivalent, forms. Guidance in choosing the most suitable rosette for a particular application is provided in Section 2.0 , where selection considerations are reviewed.

Figure 1 – Basic rosette types, classified by grid orientation: (a) tee; (b) 45º-rectangular; (c) 60º delta.

5X (a)

5X (b)

5X (c)

rectangular

Delta

Figure 2 – Geometrically different, but functionally equivalent configurations of rectangular and delta rosettes.

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Strain Gage Rosettes: Selection, Application and Data Reduction

Since biaxial stress states occur very commonly in machine parts and structural members, it might be presumed that half or so of the strain gages used in experimental stress analysis would be rosettes. This does not seem to be the case, however, and ten percent (or less) rosette usage may be more nearly representative. To what degree this pattern of usage reflects an inclination for on-site makeup of rosettes from single-element gages, or simply an undue tendency to assume uniaxiality of the stress state, is an open question. At any rate, neither practice can generally be recommended for the accurate determination of principal strains.

It must be appreciated that while the use of a strain gage rosette is, in many cases, a necessary condition for obtaining the principal strains, it is not a sufficient condition for doing so accurately. Knowledgeability in the selection and application of rosettes is critical to their successful use in experimental stress analysis; and the information contained in this Tech Note is intended to help the user obtain reliably accurate principal strain data.

2.0 Rosette Selection Considerations

A comprehensive guide for use in selecting Micro- Measurements strain gages is provided in Reference 1. This publication should first be consulted for recommendations on the strain-sensitive alloy, backing material, self- temperature-compensation number, gage length, and other strain gage characteristics suitable to the expected application. In addition to basic parameters such as the foregoing, which must be considered in the selection of any strain gage, two other parameters are important in rosette selection. These are: (1) the rosette type — tee, rectangular, or delta; and (2) the rosette construction — planar (single- plane) or stacked (layered).

The tee rosette should be used only when the principal strain directions are known in advance from other considerations. Cylindrical pressure vessels and shafts in torsion are two classical examples of the latter condition. However, care must be exercised in all such cases that extraneous stresses (bending, axial stress, etc.) are not present, since these will affect the directions of the principal axes. Attention must also be given to nearby geometric irregularities, such as holes, ribs, or shoulders, which can locally alter the principal directions. The error magnitudes due to misalignment of a tee rosette from the principal axes are given in Reference 2. As a rule, if there is uncertainty about the principal directions, a three-element rectangular or delta rosette is preferable. When necessary (and, using the proper data-reduction relationships ), the tee rosette can be installed with its axes at any precisely known angle from the principal axes; but greatest accuracy will be achieved by alignment along the principal directions. In the latter case, except for the readily corrected error due to transverse sensitivity, the two gage elements in the rosette indicate the corresponding principal strains directly.

Where the directions of the principal strains are unknown, a three-element rectangular or delta rosette is always required; and the rosette can be installed without regard to orientation. The data-reduction relationships given in Section 4.0 yield not only the principal strains, but also the directions for the principal axes relative to the reference grid (Grid 1) of the rosette. Functionally, there is little choice between the rectangular and delta rosettes. Because the gage axes in the delta rosette have the maximum possible uniform angular separation (effectively 120°), this rosette is presumed to produce the optimum sampling of the underlying strain distribution. Rectangular rosettes have historically been the more popular of the two, primarily because the data-reduction relationships are somewhat simpler. Currently, however, with the widespread access to computers and programmable calculators, the computational advantage of the rectangular rosette is of little consequence. As a result of the foregoing, the choice between rectangular and delta rosettes is more apt to be based on practical application considerations such as availability from stock, compatibility with the space available for installation, convenience of solder tab arrangement, etc.

All three types of rosettes (tee, rectangular, and delta) are manufactured in both planar and stacked versions. As indicated (for the rectangular rosette) in Figure 3, the

Figure 3 – rectangular rosettes (of the same gage length) in planar and stacked construction.

stacked

Planar

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Micro-Measurements stacked rosettes are not numbered, as a matter of manufacturing economics. The user should number the gages in stacked rosettes according to the rules given here and illustrated in Figures 2 and 4.

4.0 Principal Strains and Directions

from Rosette Measurements

The equations for calculating principal strains from three rosette strain measurements are derived from what is known as a “strain-transformation” relationship. As employed here in its simplest form, such a relationship expresses the normal strain in any direction on a test surface in terms of the two principal strains and the angle from the principal axis to the direction of the specified strain. This situation can be envisioned most readily with the aid of the well-known Mohr’s circle for strain, as shown in Figure 5**. It can be seen from Figure 5a (noting that the angles in Mohr’s circle are double the physical angles on the test surface) that the normal strain at any angle θ from the major principal axis is simply expressed by:

ε

ε ε ε ε θ =^ θ

P Q + P Q

cos (1)

Figure 4 – counterclockwise numbering of grids in Micro-Measurements general-purpose strain gage rosettes (see text).

** The Mohr’s circle in Figure 5 is constructed with positive shear strain plotted downward. This is done so that the positive rotational direction in Mohr’s circle is the same (CCW) as for the rosette, while maintaining the usual sign convention for shear (i.e., positive shear corresonds to a reduction in the initial right angle at the origin of the X-Y axes as labeled in Figure 5b).

Figure 5 – strain transformation from the principal strains to the strain in any direction: (a) εθ in terms of principal strains ε P , and ε Q , as shown by Mohr’s circle for strain; (b) rectangular rosette installed on a test surface, with Grid 1 at the arbitrary angle θ from the major principal axis; (c) axes of the rectangular rosette superimposed on Mohr’s circle for strain.

(a)

(b)

(c)

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Strain Gage Rosettes: Selection, Application and Data Reduction

Figure 5b represents a small area of the test surface, with a rectangular rosette installed, and with the reference grid (#1) oriented at θ degrees from ε p. Mohr’s circle, with the axes of the rosette superimposed, is shown in Figure 5c.

By successively substituting into Equation (1) the angles for the three grid directions, the strain sensed by each grid can be expressed as follows:

ε

ε ε ε ε θ

ε

ε ε ε ε θ

1

2

P Q P Q

P Q P Q

cos

cos 445

o

o

( )

ε + (^) ( + )

ε ε ε ε P Q P Q θ

cos

(2a)

(2b)

(2c)

When the rosette is installed on a test part subjected to an arbitrary strain state, the variables on the right-hand side of Equations (2) are unknown. But the strains ε 1 , ε 2 and ε 3 can be measured. Thus, by solving Equations (2) simultaneously for the unknown quantities ε P , ε Q , and θ, the principal strains and angle can be expressed in terms of the three measured strains. Following is the result of this procedure:

ε

ε ε ε ε ε ε

θ

ε

P Q , –^ –

± ( ) + ( )

1 3 1 2

2 2 3

2

1

tan –^

1 3

ε ε ε ε

If the rosette is properly numbered, the principal strains can be calculated from Equation (3) by substituting the measured strains for ε 1 , ε 2 and ε 3. The plus and minus alternatives in Equation (3) yield the algebraically maximum and minimum principal strains, respectively. Unambiguous determination of the principal angle from Equation (4) requires, however, some interpretation, as described in the following. To begin with, the angle θ represents the acute angle from the principal axis to the reference grid of the rosette, as indicated in Figure

5. In the practice of experimental stress analysis, it is somewhat more convenient, and easier to visualize, if this is reexpressed as the angle from Grid 1 to the principal axis. To change the sense of the angle requires only reversing the sign of Equation (4). Thus:

φ θ

ε ε ε P Q , (^) ε ε

1 3

tan –1^ (5)

The physical direction of the acute angle given by either Equation (4) or Equation (5) is always counterclockwise if positive, and clockwise if negative. The only difference is that θ is measured from the principal axis to Grid 1, while φ is measured from Grid 1 to the principal axis. Unfortunately, since tan 2 φ ≡ tan 2( φ + 90°), the calculated angle can refer to either principal axis; and hence the identification in Equation (5) as φ P,Q. This ambiguity can readily be resolved (for the rectangular rosette) by application of the following simple rules:

(a) if ε 1 > ε 3 , then φ P,Q = φ P (b) if ε 1 < ε 3 , then φ P,Q = φ Q (c) if ε 1 = ε 3 and ε 2 < ε 1 , then φ P,Q = φ P = –45º (d) if ε 1 = ε 3 and ε 2 > ε 1 , then φ P,Q = φ P = +45º (e) if ε 1 = ε 2 = ε 3 , then φ P,Q is indeterminate (equal biaxial strain). The reasoning which underlies the preceding rules becomes obvious when a sketch is made of the corresponding Mohr’s circle for strain, and the rosette axes are superimposed as in Figure 5c. A similar technique for assuring correct interpretation of the angle is given in the form of a step-by- step algorithm in Reference 3.

The preceding development has been applied to the rectangular rosette, but the same procedure can be used to derive corresponding data-reduction equations for the delta rosette shown in Figure 6. When grid angles θ, θ + 60°, and θ + 120° are successively substituted into Equation (1), the resulting three equations can again be solved simultaneously for the principal strains and angle. Thus, for the delta rosette:

ε

ε ε ε P Q , =^ ε^ –^ ε^ ε^ –^ ε^ ε^ – ε

(^1 2 3) ± ( 1 2 ) 2 + ( 2 3 ) + ( ) 2 3 1

2 3

θ

ε ε ε ε ε

 ( )



3 2 1 2 3

tan –^

As in the case of Equation (4), the angle θ calculated from Equation (7) refers to the angular displacement of Grid 1 from the principal axis. The sense of the angle can again be changed by reversing its sign to give the angle from Grid 1 to a principal axis:

φ θ

ε ε P Q , (^) ε ε ε

 ( )



2 3 1 2 3

tan –1^ (8)

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our System 6000, calculate both the principal strains and the principal stresses, the following material is provided as background information.

If the test material is homogeneous in composition, and is isotropic in its mechanical properties (i.e., the properties are the same in every direction), and if the stress/strain relationship is linear, with stress proportional to strain, then the biaxial form of Hooke’s law can be used to convert the principal strains into principal stresses. This procedure requires, of course, that the elastic modulus ( E ) and Poisson’s ratio ( ν ) of the material be known. Hooke’s law for the biaxial stress state can be expressed as follows:

σ ν P ε^ P νε Q

E

= (^) ( + ) 1 – 2

(10a)

σ ν Q ε^ Q νε P

E

= (^) ( + ) 1 – 2

(10b)

The numerical values of the principal strains calculated form Equation (3) or Equation (6) can be substituted into equations (10), along with the elastic properties, to obtain the principal stresses. As an alternative, Equation (3) or Equation (6) depending on the rosette type) can be substituted algebraically into Equations (10) to express the principal stresses directly in terms of the three measured strains and the material properties. The results are as follows:

Rectangular:

σ

ε ε ν ν P Q ε^ ε^ ε^ ε

E

, =^ – –^ –

( ) + ( )

1 3 1 2

2 2 3

Delta:

σ

ε ε ε ν

ν

ε ε ε ε ε

P Q

E ,

• • –

=

• • (^) ±

( ) + ( ) +

3

1 2 1

1 2 3

1 2

2 2 3

2 3 – –^ ε 1

2 ( )





   





   

When the test material is isotropic and linear-elastic in its mechanical properties (as required for the validity of the preceding strain-to-stress conversion), the principal stress axes coincide in direction with the principal strains. Because of this, the angle from Grid 1 of the rosette to

the principal stress direction is given by Equation (5) for rectangular rosettes, and by Equation (8) for delta rosettes.

6.0 Errors, Corrections, and Limitations

The obvious aim of experimental stress analysis is to determine the significant stresses in a test object as accurately as necessary to assure product reliability under expected service conditions. As demonstrated in the preceding sections of this Tech Note, the process of obtaining the principal stresses involves three basic, and sequential, steps: (1) measurement of surface strains with a strain gage rosette; (2) transformation of measured strains to principal strains; and (3) conversion of principal strains to principal stresses. Each step in this procedure has its own characteristic error sources and limits of applicability; and the stress analyst must carefully consider these to avoid potentially serious errors in the resulting principal stresses.

Of first importance is that the measured strains be as free as possible of error. Strain measurements with rosettes are subject, of course, to the same errors (thermal output, transverse sensitivity, leadwire resistance effects, etc.) as those with single-element strain gages. Thus, the same controlling and/or corrective measures are required to obtain accurate data. For instance, signal attenuation due to leadwire resistance should be eliminated by shunt calibration 4 , or by numerically correcting the strain data for the calculated attenuation, based on the known resistances of the leadwires and strain gages.

Because at least one of the gage grids in any rosette will in every case be subjected to a transverse strain which is equal to or greater than the strain along the grid axis, consideration should always be given to the transverse-sensitivity error when performing rosette data reduction. The magnitude of the error in any particular case depends on the transverse- sensitivity coefficient ( K (^) t ) of the gage grid, and on the ratio of the principal strains (ε P / ε Q ). In general, when K (^) t ≤ 1%, the transverse-sensitivity error is small enough to be ignored. However, at larger values of K (^) t , depending on the required measurement accuracy, correction for transverse sensitivity may be necessary. Detailed procedures, as well as correction equations for all cases and all rosette types, are given in Reference 5.

When strain measurements must be made in a variable thermal environment, the thermal output of the strain gage can produce rather large errors, unless the instrumentation can be zero-balanced at the testing temperature, under strain-free conditions. In addition, the gage factor of the strain gage changes slightly with temperature. Reference 6 provides a thorough treatment of errors due to thermal effects in strain gages, including specific compensation and correction techniques for minimizing these errors.

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After making certain that strain measurement errors such as the foregoing have been eliminated or controlled to the degree feasible, attention can next be given to possible errors in the strain-transformation procedure for obtaining the principal strains. A potentially serious source of error can be created when the user attempts to make up a rosette on the specimen from three conventional single-element gages. The error is caused by misalignment of the individual gages within the rosette. If, for example, the second and third gages in a rectangular rosette configuration are not accurately oriented at 45° and 90°, respectively, from the first gage, the calculated principal strains will be in error.

The magnitude of the error depends, of course, on the magnitude (and direction) of the misalignment; but it also depends on the principal strain ratio, ε P / ε Q , and on the overall orientation of the rosette with respect to the principal axes. For certain combinations of principal strain ratio and rosette orientation, 5-degree alignment errors in gages 2 and 3 relative to Gage 1 can produce an error of 20 percent or more in one of the principal strains.

Since it is very difficult for most persons to install a small strain gage with the required precision in alignment, the user is well-advised to employ commercially available rosettes. The manufacturing procedures for Micro-Measurements strain gage rosettes are such that errors due to grid alignment within the rosette need never be considered. For those cases in which it is necessary, for whatever reason, to assemble a rosette from single-element gages, extreme care should be exercised to obtain accurate gage alignment. And when the principal strain directions can be predicted in advance, even approximately, alignment of Gage 1 or 3 in a rectangular rosette, or alignment of any gage in a delta rosette, with a principal axis, will minimize the error in that principal strain caused by inter-gage misalignment.

The strain-transformation relationships and data-reduction equations given in Section 4.0 assume a uniform state of strain at the site of the rosette installation. Since the rosette necessarily covers a finite area of the test surface, severe variations in the strain field over this area can produce significant errors in the principal strains — particularly with planar rosettes.^7 For this type of application, the stacked rosette is distinctly superior; both because it covers a much smaller area (for the same gage length), and because the centroids of all three grids lie over the same point on the test surface.

The requirements for a homogeneous material and uniform strain state can be (and are) relaxed under certain circumstances. A case in point is the use of strain gage rosettes on fiber-reinforced composite materials. If the distance between inhomogeneities in the material (i.e., fiber-to-fiber spacing) is small compared to the gage length of the rosette, each grid will indicate the “macroscopic” or average strain in the direction of its axis. These measured

strains (after the usual error corrections) can be substituted into Equation (3) or Equation (6) to obtain the macroscopic principal strains for use in the stress analysis of test objects made from composite materials. 8 As noted later in this section, however, Equations (10)-(12) cannot be used for this purpose.

There is an additional limitation to the strain-transformat- ion relationship in Equation (1) which, although not frequently encountered in routine experimental stress analysis, should be noted. The subject of the strain distribution about a point, as universally treated in handbooks and in mechanics of materials textbooks, is developed from what is known as “infinitesimal-strain” theory. That is, in the process of deriving relatively simple relationships such as Equation (1), the strain magnitudes are assumed to be small enough so that normal- and shear-strain approximations of the following types can be employed without introducing excessive error:

ε + ε 2 ≈ ε (13)

sin γ ≈ tan γ ≈ γ (14)

Although often unrecognized, these approximations are embodied in the equations used throughout the contemporary practice of theoretical and experimental stress analysis (where strain transformation is involved). This includes the concept of Mohr’s circle for strain, and thus all of the equations in Section 4.0 , which are consistent with the strain circle. Infinitesimal-strain theory has proven highly satisfactory for most stress analysis applications with conventional structural materials, since the strains, if not truly “infinitesimal”, are normally very small compared to unity. Thus, for a not-untypical working strain level of 0.002 (2000 μ ε), the error in ignoring ε^2 compared to ε is only about 0.2 percent.

However, strain gage rosettes are sometimes used in the measurement of much larger strains, as in applications on plastics and elastomers, and in post-yield studies of metal behavior. Strain magnitudes greater than about 0. (10 000 μ ε) are commonly referred to as “large” or “finite”, and, for these, the strain-transformation relationship in Equation (1) may not adequately represent the actual variation in strain about a point. Depending on the strain magnitudes involved in a particular application, and on the required accuracy for the principal strains, it may be necessary to employ large-strain analysis methods for rosette data reduction.^9

The final step in obtaining the principal stresses is the introduction of Hooke’s law [Equations (10)] for the biaxial stress state. To convert principal strains to principal stresses with Hooke’s law requires, of course, that the elastic modulus and Poisson’s ratio of the test material

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Appendix

Derivation of Strain-Transformation Relationship [Equation (1) in text] from Deformation Geometry

Consider a small area of a test surface, as sketched in Figure A-1. The line O-P, of length L (^) O , and at the angle θ from the X axis, is scribed on the surface in the unstrained state. When uniform principal strains ε P and ε Q are applied in the directions of the X and Y axes, respectively, the point P moves to P ′ as a result of the displacements ΔX and ΔY (greatly exaggerated in the sketch).

It is evident from the figure that:

ΔX = ε P ( L (^) O cos θ) (A-1)

ΔY = ε Q ( L (^) O sin θ) (A-2)

It can also be seen (Figure A-2), by enlarging the detail in the vicinity of points P and P ′, that for small strains:

Δ L ≈ ΔX cos θ + ΔY sin θ (A-3)

Substituting from Equations (A-1) and (A-2),

Δ L ≈ L (^) O ( ε P cos 2 θ + ε Q sin 2 θ) (A-3)

Or,

ε (^) θ = ε θ ε θ

L

Lo P^ Q

cos 2 sin^2 (A-5)

But,

cos cos

sin – cos

2

2

θ θ

θ θ

After substituting the above identities,

ε

ε ε ε ε θ =^ θ

P Q + P Q

cos (A-6)

Alternate Data Reduction Equations

In the extensive technical literature dealing with strain gage rosettes, the user will often encounter data-reduction relationships which are noticeably different from one another, and from those in the body of this Tech Note. As a rule, these published equations yield the same results, and differ only in algebraic format — although proving so in any given case may be rather time consuming. Since certain forms of the equations may be preferred for mnemonic reasons, or for computational convenience, several alternative expressions are given here. All of the following are equally correct when the gage elements in the rosette are numbered as described in this Tech Note.

Figure a-1 Figure a-

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Rectangular Rosette:

where:

ε

ε ε ε ε ε ε ε

ε

P Q

P Q

,

,

(^1 3) ± ( 1 3 ) +  ( + ) 2 2 1 3

2 2

± ( ) + ( ) 

ε ε ε ε ε ε

ε ε

1 3 1 2

2 2 3

2

1

P Q , C^ (( C^ – )^ + (^ )

2 2

2

1 3 2

C

C

• ε

ε ε

(A-7)

(A-8)

(A-9)

(A-9)

Delta Rosette:

(A-10)

(A-11)

(A-12)

where:

ε

ε ε ε ε ε ε P Q , ε^ ε

 ( + )



(^1 2 3)  + ( 1 2 3

2

3 2 3

))

± ( ) + ( ) + (

2

1 2 3 1 2

2 2 3

2 ε 3 2 3 1

ε ε ε P Q ,  ε^ –^ ε^ ε^ –^ ε^ ε^ –^ ε))

= ± ( ) + ( ) + ( )

2

1

2 2

2 3

2

ε P Q , C C – ε C – ε C – ε 

C^ ε^1 ε^2 ε^3

Cartesian Strain Components from Rosette Measurements

It is sometimes desired to obtain the Cartesian components of strain ( ε X , ε Y , and γ XY ) relative to a specified set of X-Y coordinate axes. This need can arise, for example, when making strain measurements on orthotropic composite materials. The Cartesian strain components are also useful when calculating principal strains from rosette data using matrix transformation methods.*

When the X axis of the coordinate system coincides with the axis of the reference grid (Grid 1) of the rosette, the Cartesian components of strain are as follows:

Rectangular Rosette:

ε X = ε 1

ε Y = ε 3

γ XY = 2 ε 2 – ( ε 1 + ε 3 )

Y

3 2

1 X

• Milner, R.R., “A Modern Approach to Principal Stresses and Strains”, Strain , November, 1989, pp.135-138.