Comparison of Ring-Core Method and Hole-drilling Method Used for Determining Residual Stresses

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Figure 2. System SINT-MTS 3000

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3. Ring-Core Method

The Ring-Core method is undoubtedly less common than the hole-drilling method. In addition of being more “destructive”, the fact that actual performance of the test is more complex has limited evolution of the method. It has considerable similarities to the hole-drilling method, in fact, the two methods are complementary as they are based on the same mathematical equations for calculating residual stresses from measured strains, although the methods clearly have totally different calibration constants and curves.

Ring-Core method can be used for determining uniform and non-uniform residual stresses. The principles of this method are based on the hole-drilling method, but instead of drilling a hole through the middle of strain gage rosette, a notch is milled around the rosette ^{[2, 3]}, which has dimensions about: D= 18mm, d= 14mm, h= 4 to 5mm Figure 3. This implies that Ring-Core method is more destructive than a hole-drilling method but is still a semi-destructive one.

Small annular groove in the surface of the specimen is milled concentrically around a strain gage rosette, leaving the upper part of the core separated from the surrounding material. This separation causes the relaxation of the residual stresses presented in core, which is represented by deformation of the core. These deformations can be measured by strain gages designed for measuring relieved strains by the Ring-Core method. The annular groove can be machined with suitable cutter, like a ring saw.

This method is less sensitive to errors involved in positioning of the cutting tool relative to the strain gage and stress can be measured more accurate up to the yield stress of material.

Figure 3. Ring-Core method

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Milling is made for selected steps and at each step measured values are recorded. The process is repeated, usually to a depth of 4 mm. The obtained data are processed in a suitable evaluation procedure (eg Integral method, Incremental method etc.). Ring-core method allows to measure in greater depths by removing the milled ring. Subsequently new strain gage rosette is bonded to the surface and new ring is milled. This may continue up to the depth of 25 mm.

Assumptions, limitations and applicability of Ring-Core methods:

Ÿ Residual stresses in the direction perpendicular to the surface of the material are neglected.

Ÿ Residual stresses are constant throughout the depth or milling steps.

Ÿ In milling process it is not allowed to bring further residual stresses to the investigated material.

Ÿ The material has a homogeneous structure and it is linear elastic.

Ÿ Unloaded material surface (the surface tension of planar arises).

Ÿ The surface of the examine material is planar.

Influencing factors:

Ÿ Dimensions of the specimen are large enough that the thickness of the material allows the creation of a blind groove.

Ÿ Measuring point shall be located at a sufficient distance from the geometric characteristics (adjacent holes, radius).

Ÿ Center location of the strain gage rosette has to be the geometric center of the groove.

Ÿ Cylindricity and perpendicularity of the core with respect to the plane of the surface

Ÿ Type of the strain gage rosette - geometry and size of the strain gage measuring grids.

Ÿ The type of method, which is used for evaluating the residual stresses.

Basic steps for the determining residual stresses by the Ring-Core method:

Ÿ Apply a strain gage rosette with three overlapped grids in the test area (prepare surface, bond and solder leads).

Ÿ Protect the strain gage rosette and leads.

Ÿ Place the mechanical section of the instrument on the desired area.

Ÿ Center the crown milling cutter and strain gage rosette.

Ÿ Set a zero depth.

Ÿ An annular groove is machined around the strain gage rosette in depth increments, and at each step the strain values are measured by the three grids.

Ÿ The previous step is repeated to reach a minimum depth of 4 mm.

Ÿ The acquired strains are processed by the most appropriate method ^[4].

An example of the system used to determining residual stresses by the Ring-Core method is System MTS3000 Ring-Core Figure 4.

Figure 4. System SINT MTS 3000 Ring-Core

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4. Comparison Between the Ring-Core Method and the Hole-drilling Method

The Ring-Core method is younger, but in terms of theoretical and practical application is similar to the hole-drilling method. The basic difference between these two methods is shown in Figure 5. In first case, we drill hole through the center of the strain gage rosette and in the second case we made annual groove around the bonded strain gage rosette.

Figure 5. Difference between semi-destructive methods

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The Ring-Core method is more sensitive in comparison to the hole-drilling method because it involves almost complete relief of the surface strains. However, the size of annular groove is relatively large, causing much more damage than the hole-drilling method, which allows more localized residual stress measurement. Despite its potential disadvantages, the hole-drilling method is the most common choice. Method is well established, experimentally and theoretically.

Difference between these two methods is also in used strain gage rosette. These methods are usually used to measure the residual stresses in metals, but they are also useful for other materials, for example ceramics, polymers, etc ^[7].

The following tables (Table 1, Table 2, Table 3, Table 4) summarized the fundamental differences between the Ring-Core method and the hole-drilling method.

Table 1. Comparison of the Ring-Core method and the hole drilling method (general characteristic)

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Table 2. For uniform residual stresses

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Table 3. For non-uniform residual stresses

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Table 4. Comparison of sensitivity, measurability and influence of temperature

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Advantages of the Ring-Core method:

Ÿ Possibility to measure residual stress in greater depths.

Ÿ It is suitable for measuring coarse-grained materials.

Ÿ Less sensitive to errors caused by inaccurate location of the milling tool with regard to the middle of strain gage rosette.

Ÿ Measurement is possible up to the size of the yield stress of the material.

Ÿ In the process of milling introduced additional residual stresses have no such a great influence on the measurement of residual stresses than in hole-drilling method.

Ÿ The measured signal is at least larger than in hole-drilling method.

Disadvantages of the Ring-Core method:

Ÿ Need of determination of the calibration coefficients depending on the mechanical properties of the material, geometry of milled core, total depth of the ring, type of strain gage rosette.

Ÿ Ring-Core method is not standardized unlike the hole-drilling method.

Ÿ Limitations on applicability of the method and the need to respect the factors affecting accuracy.

Ÿ Long process of measurement.

4.1. Strain Gage Rosettes Used for Residual Stresses Measurements

In hole-drilling and Ring-Core method different types of strain gage rosettes are used. Comparing these two methods it is obvious that in hole-drilling method we drill to the middle of strain gage rosette while in Ring-Core method we mille around the strain gage rosette. Company producing strain gage rosettes is for example Hottinger Baldwin Messtechnik GmbH (HBM), which produces strain gage rosettes for both methods. Different types used in these methods from the company HBM are shown and described in the next section ^[9].

Strain gage rosettes used for Ring-Core method Figure 6:

XY 51:

Ÿ We use this strain gage rosette when we know direction of principal stresses.

Ÿ Dimensions: a = 5mm, b = 2.5 mm, c = 12 mm, resistance 350 Ω

RY 51:

Ÿ We use this strain gage rosette when we do not know direction of principal stresses.

Ÿ Dimensions: a = 5mm, b = 2.5 mm, c = 12 mm, resistance 350 Ω

Figure 6. Strain gage rosettes used for Ring-Core method

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Strain gage rosettes used for hole-drilling method:

For hole-drilling method we can use three basic types of strain gage rosettes defined by ASTM E 837 ^[9].

Type A:

Ÿ Strain gage rosette for general using, for example. RY61 made by HBM (Figure 7 a)

Ÿ Dimensions: a = 1.5 mm, b = 0.8 mm, d = 12 mm, resistance 120 Ω

Figure 7. Strain gages for the hole-drilling method

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