The experimental stress investigations in bars of the draw installation of pipes.

Atanasiu, Costica ; Vlasceanu, Daniel ; Baciu, Florin 等

1. INTRODUCTION

The draw is the processing method by plastic deformation (Dieter, 1991), which consists in forced going the material through one aperture draw die with section smaller than material section under the traction force action (Fig. 1).

The principal tool in draw procedure is the drawing-die, by this depend the product quality and the drawing machine capacity.

The drawing-die is made from alloy steel for tools, hard alloys.

2. CHOOSING THE METHOD OF MEASUREMENT AND CALIBRATION OF THE BARS

Contemplate the way working of bars of some draw installations of pipes it has come to conclusion as stresses and tensile strain determinations in the bars samples it can be realize using the strain gauge techniques (Avril, 1984, Atanasiu et al., 2008).

The entire assemble are shapely by four bars.

The bars numbered 1 and 2 have a circular transversal section with diameter equal to 19.7 mm and are fabricated by steel having [sigma]r = 700 MPa and [sigma]c = 300 MPa. The bars numbered 3 and 4 with annular transversal section, the internal diameter is equal to 6 mm and the external diameter equal to 20 mm, are made by steel having [sigma]r = (850 ... 1050) MPa and [sigma]c = 650 MPa.

On the each bar was applied four transducers, two was orientated in the bar axis directions and the other two are perpendicularly on the bar axis, figures 2a.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

If, the four transducers are connecting in full bridge (fig. 2, b) the intensity variation measured are:

[DELTA]R = [DELTA][R.sub.1]-[DELTA][R.sub.2] + [DELTA]R.sub.3]--[DELTA][R.sub.4] (1)

where (Iliescu & Atanasiu, 2006, Theocaris et al., 1976):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII],

In these relations:

[[epsilon].sub.1] = [[epsilon].sub.3], represent the tensile strain, indicated by the 1 and 3 transducers,

[[epsilon].sub.i1] = [[epsilon].sub.3], represent the bending strain,

[[epsilon].sub.2] = -v([[epsilon].sub.1] + [[epsilon].sub.1]) represent the unit strain indicated by the transducer no. 3.

[[epsilon].sub.4] = -v([[epsilon].sub.3] + [[epsilon].sub.3]) = -v([[epsilon].sub.1]-[[epsilon].sub.2]) represent the unit strain indicated by the transducer no. 4.

[K.sub.1], [K.sub.2], [K.sub.3], [K.sub.4] represent the transducers constant. [R.sub.1], [R.sub.2], [R.sub.3], [R.sub.4] represent the electrical resistance of the transducers.

[DELTA][R.sub.t1], [DELTA][R.sub.t2], [DELTA][R.sub.t3], [DELTA][R.sub.t4], represent the variation of electrical resistance produced by the temperature variation, which can be appearing in the measurement process.

Choosing the transducers so that [R.sub.1] = [R.sub.2] = [R.sub.3] = [R.sub.4] and [K.sub.1] = [K.sub.2] = [K.sub.3] = [K.sub.4] result [DELTA][R.sub.t1] = [DELTA] [R.SUB.t2] = [DELTA][R.sub.t3] = [DELTA][R.sub.t4] and then using relation (1) we obtained:

[DELTA]R = 2[[K.sub.1]R[[epsilon].sub.1] + [vK.sub.2]R[[epsilon].sub.1]] (2)

In this way the temperature variation effect and the bending effect are excluded. and,

[DELTA]R/R = [[epsilon].sub.cit] K = 2[[K.sub.1] [[epsilon].sub.1] + [K.sub.2] v[[epsilon].sub.1] (3)

where: [[epsilon].sub.cit] represent the experimental unit strain and K = 2 are the transducers constant.

The measured unit strain is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

On the bars numbered 1 and 2 it was applied transducers having the follow constants [K.sub.1] = 2.07 and [K.sub.2] = 2.03 so that

[[epsilon].sub.1]= K[[epsilon].sub.cit]/2x2,04(1+0,3) = 0,378 [[epsilon].sub.cit] (5)

The stress value in the bars numbered 1 and 2 for a 1kN force are:

[[sigma].sub.1] = P/A = 1000/304 = 32,8 MPa (6)

In addition, the corresponding unit strain is:

[[epsilon].sub.1] = [[sigma].sub.1]/E = 32,8/21X[10.sup.4] = 1.56 x [10.sup.-6] (7)

The measured strains values in experimental process are:

[[epsilon].sub.cit] = 1/0,378 [[epsilon].sub.1] = 417 x [10.sup.-6] (8)

Because, the bars Young modulus are different toward the calculus value, E = 21 x [10.sup.4] MPa, considered, it was necessarily a calibration of each type of bars for establish the effective relation between P (load) and [epsilon] (strain).

For this end was manufactured two standard bars made from the same material as good as the real one, one having circular section with d=19.8 mm diameter and the other having a annular transversal section, the internal diameter is equal to 6 mm and the external diameter equal to 20 mm.

On the each bar, it was applied strain gauges (figure 2a) in a full bridge circuit (figure 2b).

Then, the bars were loaded with known forces using a 300 kN universal testing machine and the strain were read with a Hottinger bridge.

Using the experimental values for bar no. 1 it makes some graphics [epsilon] = f(P) presented in figures 3.

It was observe, from the graphics, so the relationship between P (load) and [epsilon] (strain) it is linearly, resulting [epsilon]/1kN = 420 x [10.sup.-3] for standard bar no.1.

In the calculus process will be used the medium values [[epsilon].sub.med]/1kN = 420 x [10.sup.-3] because the diameter of the machine bars are different toward the diameter of the standard bar no.1.

The some differences appear between the values determined by calculus toward the experimental for [[epsilon].sub.med]/1kN what conduce to conclusion the value of Young modulus is not just equal to the value which are take in calculus (E = 21 x [10.sup.4] MPa).

Using the results obtained in calibration bars process it can be determined the Young modulus values for each bar.

So, for bar no.1:

[E.sub.1] = [P.sub.1]/[A.sub.1][[epsilon].sub.1] 30700X[10.sup.3]/304X0,483 = 21 x [10.sup.4] Mpa (9)

[FIGURE 3 OMITTED]

3. MEASUREMENTS FOR DETERMINE THE TENSIONS AND THE EXTENSIONS IN BARS

The four bars, which were, applied strain gauges transducers were mount in the draw installations of pipes. For measuring, it was use strain gauge bridges and dynamic recording systems. It was made many draws concomitantly with 1 and 2 numbered bars as well as concomitantly with 3 and 4 numbered bars and then was collected the tensile strains.

In case of testing VI it was obtain, in time of draw effectuated on bar no. 1 (table 1), the following values:

[P.sub.max] = [[epsilon].sub.max]/[[epsilon].sub.min] = 1290/42 = 30.7 kN, (10)

[P.sub.min] = 260/42 = -6.2 kN (11)

Corresponsive, the maximum extension in time of this draw was:

[DELTA]l = [[epsilon].sub.cit]/2,679 = 1290X[10.sup.3]/2,679 = 0,483 mm (12)

4. RESULTS AND CONCLUSIONS

In table, one the results obtained after data processing for bars no. 1

In case of apparition of some vibrations in draw procedure time can be appear higher loading variations.

5. REFERENCES

Atanasiu, C., Iliescu, N., Pastrama, St., (2008). Design and testing of a transducer for measuring the loads on the rolles of crane carriage, The 19th International DAAM Symposium, Trnava, Slovakia, Vol. 19, pg. 0037

Avril, J., (1984). Encyclopaedia d'analyse des contraintes, Micromesures, Malakoff

Dieter, G., (1991). Mechanical Metallurgy, McGraw-Hill Book, New-York

Iliescu, N., Atanasiu, C., (2006). Strain gauge techniques in engineering, Ed. Agir, Bucharest

Theocaris, P., Atanasiu, C., Iliescu, N., Pastrav, I., (1976). Experimental stress analysis, Vol. 1, Ed. Tehnica, Bucharest

Tab. 1. The loads and strains values in time of starting machine
for bar no. 1

 Start

 [[epsilon]. [epsilon]min [P.sub.min]
 sub.t]
 [micro] [micro]
Case [epsilon]/mm mm [epsilon] N

I -13 -422 -10000
II -8 -260 -6180
III 32.3 -15 -487 -11600
IV -15 -487 -11600
V -16 -520 12400
VI -12 -390 -9260

 Start

 [[epsilon]. [[epsilon].sub.max] [P.sub.max]
 sub.t]
 [micro] [micro]
Case [epsilon]/mm mm [epsilon] N

I 4 130 3080
II 4 130 3080
III 32.3 5 163 3880
IV 5 163 3880
V 6 195 4640
VI 4 130 3080

Tab. 2. The loads, strains and extensions values in time of draw
procedure for bar no. 1

Case In time of draw process

 [[epsilon].sub.min] [P.sub.min]
 mm [micro][epsilon] kN

I 8 260 6.2
II 11 365 8.5
III 12 390 9.3
IV 8 260 6.2
V 10 323 7.7
VI -8 -260 -6.2

Case In time of draw process In time of draw process

 [[epsilon].sub.min] [P.sub.max] [DELTA]L
 mm [micro][epsilon] kN mm/m

I 18 583 13.5 0.217
II 18 583 13.5 0.217
III 20 648 15.4 0.241
IV 16 520 12.4 0.194
V 22 712 17 0.266
VI 40 1290 30.7 0.483