The experimental investigations of residual stresses in tube with thick walls.

Atanasiu, Costica ; Vlasceanu, Daniel ; Tripa, Pavel 等

1. INTRODUCTION

The analyzed tube was obtained from band steel revolved welded longitudinally. As long as tack weld correspond of the local heat treatment, the higher temperature level leading to some transformations in the material structures associate with elongations which can varying from one point to other point in the heated zone. By cooling the tube are contracted and in the welded seams appear the residual stresses.

The experimental determination methods of the residual stresses are diverse (Ajovalisit & Petrucci, 1990; Alman & Black, 1993; Kockelmann, 1990).

Using this methods, it was applied the strain gauge rosette with transducers in three directions on the specimen. The next step was to make some holes using a drilling machine, with different depths, in the point of measure. Along with hole-drilling process it was measured the unit deformations, on the three directions, and then the residual stresses were determinate.

The Hole-Drilling Strain-Gage method covers the procedure for determining residual stresses near the surface of isotropic linearly-elastic materials. Although the concept is quite general, the test method described here is applicable in those cases where the stresses do not vary significantly with depth and do not exceed one half of the yield strength. The test method is often described as "semi-destructive" because the damage that it causes is much localized and in many cases does not significantly affect the usefulness of the specimen. In contrast, most other mechanical methods for measuring residual stress substantially destroy the specimen. Since the test method described here does cause some damage, it should be applied only in those cases either where the specimen is expendable or where the introduction of small shallow hole will not significantly affect the usefulness of the specimen.

The experimental structures, the tube, are presented in figure 1.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

From tube it was extracted few specimens, like in figure 2, used for residual stress determinations using the strain gauge method.

The choice method for measuring of residual stress in two specimens dependent on circumstance as at the borders in interior and exterior of the tube, on radius direction, the principal stress [[sigma].sub.2] is zero. This thing is showed in fig. 2 because on this direction does not exist load.

The normal stress on circumference direction, [[sigma].sub.2], can be determinate by partial and progressive sectioning in vicinity of zone where was applied the transducers (TER).

These stresses are calculated in function of unit deformations [[epsilon].sub.ext] and [[epsilon].sub.int] which are unbound by sectioning, using the generalized Hooke's law for case where [[sigma].sub.2]=0, resulting:

[[sigma].sub.ext]=-E[[epsilon].sub.ext], [[sigma].sub.int]=-E[[epsilon].sub.int] (1)

The unit deformations [[epsilon].sub.ext] and [[epsilon].sub.int] was measured with applied transducers and E represents the Young modulus of the tube material.

2. THE EXPERIMENTAL DETERMINATIONS

For applied the hole-drilling strain-gage method it was select and it was marking off the zones near the welded seams where applied the transducers. These zones was cleaned mechanically and chemically and then was applied, on the specimen, two transducers LY11 type with measurement length of 3 mm, electrical resistance R= 120 [OMEGA] and transducer constant were k=2. For adhesion for these transducers was used adhesive type Z70 and their protection was realized by coating to protective mastic. After mounting and verification of these transducers it was realized connection with the acquisition data device. In the fig. 3 are presented the position of the transducers on specimen.

For each transducer, connected in bridge, it was realized the zero registered intensity, [i.sub.0], and then was isolated the zones with transducers making the holes with different depths by drilling method. On the each depth it was registered the intensity noted with [i.sub.1].

[FIGURE 3 OMITTED]

A strain gage rosette with three elements is placed in the area under consideration.

A hole is drilled at the geometric center of the strain gage rosette to a depth of about 0.4 of the mean diameter of the strain gage circle.

The residual stresses in the area surrounding the drilled hole relax. The relieved strains are measured with a suitable strain-recording instrument. Within the close vicinity of the hole, the relief is nearly complete when the depth of the drilled hole approaches 0.4 of the mean diameter of the strain gage circle.

The difference between [i.sub.1] and [i.sub.0] representing the unit deformation in point where was applied the transducer.

3. RESULTS

The measured values for unit deformations dependency by the depth holes and residual stresses calculated with relations (1) are showed in table 1.

It is determined that the values of residual stresses increase with the drilled depth till to a value where to make steady.

In the second step of investigations the specimens was subject to stress relieving by vibration method.

After stress relieving process, on reverse side of specimen, was applied, in accordance to procedure presented before, transducers in the correspondent point toward to initially points.

After mounting and verification of these transducers it was realized connection with the acquisition data device.

The measured values for unit deformations (strain) dependency by the depth holes and residual stresses calculated with relations (1) are showed in table 2.

4. CONCLUSION

From the study of results presented in table 1 and 2 it is carry on to next conclusions:

* the residual stresses values decrease from -63.6 N/[mm.sup.2] to -15.6 N/[mm.sup.2].

* keep on top of increase bearing to residual stresses along to drilling depth.

* keep on top of stability bearing of residual stress at certain depth.

Since any residual stress created by the selected drilling method will adversely affect the accuracy of results, a verification of the selected process is recommended when no prior experience is available. Such verification could consist of applying a strain gage rosette, identical to the rosette used in the test, to a stress-free specimen of the same nominal composition, and then drilling a hole.

The experimental determinations it had as scope the establishment, in parallel, the level of residual stress in the tubes with thick walls welded, before and after stress relieving.

The conclusion resulted from these investigations is that the residual stress was reduced at an important percentage.

5. REFERENCES

Ajovalisit, A. & Petrucci, G. (1990). Experimantal evolution of hole eccentricity of the measurement of residual stresses by the hole drilling method, the 9th International conference on experimental mechanical, vol. 3, pp 1132-1141 Copenhagen

Alman, J.O. & Black, P.H. (1993). Residual stresses and fatigue in metals, Mec. Graw-Hill Book, London

Iliescu, N.; Atanasiu, C. & Sandu, M. (1999). Investigations of the residual stresses in the welding seam of cover, the 16th symposium Danubia-Adria, Sibiu, Romania

Kockelmann, H. (1990). Mechanical methods of determining residual stresses, AWT Tagung Eigenspannungen, Darmstadt

Theocaris, P.; Atanasiu, C. & Iliescu, N. (1976). Experimental stress analysis, vol. 1, Ed. Technical, Bucharest, Romania

Weng, C., Lin, Y.C. & Chou, P.C. (2008). A new approach for determining the induced drilling stresses in the hole drilling method of residual stresses measurement, Experimental Technique, vol. 16

*** (2001) Standard test method for determining residual stresses by the hole-drilling strain gage method, E 837-01

Tab. 1. The measured values of strain in first step investigation

 Depth, Strain, Maximum stress,
Specimen TER mm [epsilon], [micro]m/m [sigma], MPa

1 1' 0.5 -50 -10.5
 0.6 -100 -21
 0.8 -115 -24
 1.0 -125 -26
 1.1 -130 -27
 1.4 -175 -37
 1.8 -200 -42
 2.0 -205 -43
 2.6 -220 -46
 2.8 -235 -49

 2' 0.5 -150 -31.5
 0.6 -155 -32.5
 0.8 -168 -35
 1.0 -178 -37
 1.1 -215 -45
 1.35 -235 -49
 1.8 -250 -52.5
 2.2 -297 -62.4
 2.4 -298 -62.6
 2.7 -303 -63.6

Tab. 2. The values of strain in second step investigation

Specimen TER Depth, Strain, Maximum stress,
 mm [epsilon], [micro]m/m [sigma], MPa

1 1' 0.5 -1.44 -0.3
 0.6 -2.1 -0.4
 0.8 -3.8 -0.8
 1.2 -4.3 -0.9
 1.6 -6.24 -1.3
 2.4 -9.36 -2
 2.7 -11.26 -2.4
 2.9 -15.12 -3.2
 3.1 -15.84 -3.3

 2' 0.5 -7.8 -1.6
 0.7 -13.68 -2.9
 1.1 -21 -4.4
 1.4 -28.56 -6.0
 1.7 -37.44 -7.9
 2.4 -52.08 -10.9
 2.7 -64.08 -13.5
 2.9 -72.48 -15.2
 3.0 -74.28 -15.6