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CHAPTER 3

MECHANICAL TESTING AND ANALYSIS

Before applying a tubular material to hydroforming, the material

composition and its mechanical properties must be analyzed carefully to the

extent of their influences in the deformation process.

3.1 TUBULAR MATERIALS BEFORE AND AFTER

ANNEALING

Hardness, surface roughness, and tensile values were determined

for tubular materials before and after annealing. Conventional alloys (copper

and brass) when subjected to high temperatures (annealing) for long periods

tend to soften, a well-understood metallurgical effect. On annealing, metal

atoms in the metal lattice rearrange through solid-state diffusion, effectively

removing deformations that would otherwise strengthen the alloy. The

resulting decrease in yield strength is particularly steep for metals previously

strengthened by rolling or other hardening processes.

Because annealing is based on solid-state diffusion, metals and

alloys can significantly lose strength well below the melting point; however,

annealing is much more pronounced at temperatures close to the melting

point. Annealing temperatures used for the materials under study were:

aluminum 413 C, copper 525 C, and brass 550 C. The soaking time taken as

2 hours.

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3.1.1 Surface Hardness Before and After Annealing

Hardness of materials before annealing was measured using a micro

Vickers hardness testing equipment as per IS1501–2002 standard. The test

data are shown in Table 3.1 and Figure 3.1 plots hardness values measured at

different points. After annealing, hardness was evaluated for aluminum,

copper and brass materials (Table 3.2, Figure 3.2).

Table 3.1 Surface hardness before annealing

MaterialApplied

load(kg)

Hardness at various points (Hv) Averagehardness

(Hv)point 1 point 2 point 3 point 4Aluminum 10 90 88 85.9 87 88

Copper 5 107 110 114 109 110

Brass 5 169 162 172 177 170

Figure 3.1 Surface hardness of materials before annealing

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Table 3.2 Surface hardness after annealing

MaterialApplied

load(kg)

Measured at various points (Hv) Averagehardness

(Hv)point 1 point 2 point 3 point 4Aluminum 10 40.9 42.3 42.6 42.7 42

Copper 5 74.4 73.6 72 77.9 74

Brass 5 102 102.4 99.7 101 101

Figure 3.2 Surface hardness of materials after annealing

3.1.2 Surface Roughness Before and After Annealing

Surface roughness was measured before and after annealing of

materials as per ASTM D7127–2005 standard by using a surface roughness

tester at different locations with a probe cutoff length of 0.8 mm (Tables 3.3

and 3.4). The respective graphical plots are shown in Figures 3.3 and 3.4.

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Table 3.3 Surface roughness before annealing

MaterialRoughness at different points ( m) Average

roughness, Ra

( m)1 2 3 4Aluminum 0.24 0.29 0.23 0.27 0.26

Copper 0.26 0.29 0.26 0.28 0.27

Brass 0.24 0.27 0.28 0.26 0.26

Table 3.4 Surface roughness after annealing

MaterialsRoughness at different points ( m) Average

roughness, Ra( m)1 2 3 4

Aluminum 0.28 0.31 0.32 0.29 0.30

Copper 0.34 0.44 0.4 0.42 0.40

Brass 0.9 0.85 0.88 0.87 0.88

Figure 3.3 Surface roughness before annealing

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Figure 3.4 Surface roughness after annealing

3.2 TENSILE TEST

Tensile test was performed for different materials by using a

universal testing machine (UTM) (Figure 3.5). The stress–strain curve and the

mechanical properties (yield stress, tensile stress, elongation) obtained from a

tensile test are useful to describe the formability of tubular materials. The

machining of specimens for tensile test was as per the American Society for

Testing Materials (ASTM) standard B557M.

In this test, a prepared specimen of material was axially loaded in

tension, and it was pulled until it fractured. The applied axial load and

corresponding deformation of the sample were measured. Stresses and strains

were then calculated from these values.

Stress strain values were obtained from tensile test results. The

specimens for tubular materials were prepared and tested as per ASTM

standard B 557M for curved specimens.

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Figure 3.5 The universal testing machine

For tensile test, the specimens of tube wall of aluminum, copper,

and brass materials were prepared as per the ASTM standard B557M

(Figure 3.6). The specimens before and after tensile test are shown in Figure

3.7. The results of tensile tests are shown in Table 3.5. The elongation of

aluminum, copper and brass materials after tensile test for specimens before

and after annealing is illustrated in Figure 3.8.

Figure 3.6 Specimen preparation for tensile test

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(a) (b)

Figure 3.7 Specimens (a) Before tensile test (b) After tensile test

Table 3.5 Results of tensile tests for specimens before and after annealing

Material

Before annealing After annealingTensile

strength(MPa)

Yieldstrength(MPa)

Elongation(%)

Tensilestrength

Yieldstrength Elongation

Aluminum 243 172 22 100 80 28Copper 387 352 42 252 120 46Brass 453 407 17 368 215 54

Figure 3.8 Elongation of materials after tensile test for specimens

before and after annealing

Before and after annealing

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3.2.1 Stress–Strain Graph

The stress–strain graphs obtained from the results of tensile tests for

the test specimens before and after annealing are presented here (aluminium

Figure 3.9 (a,b); copper Figure 3.10(a,b) and brass Figure 3.11(a,b).

Figure 3.9(a) Stress–strain curve for aluminum before annealing

Figure 3.9(b) Stress–strain curve for aluminum after annealing

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Figure 3.10(a) Stress–strain curve for copper before annealing

Figure 3.10(b) Stress–strain curve for copper after annealing

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Figure 3.11(a) Stress–strain curve for brass before annealing

Figure 3.11(b) Stress–strain curve for brass after annealing

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It is clear from tensile test results that annealing enhanced the

properties of aluminum, copper, and brass tubular materials. After annealing,

the material is soften and formability characteristics has improved.

3.3 ANALYSIS OF THE MICROSTRUCTURE

For microstructural analysis of the grain size before and after

annealing of test materials, an inverted microscope was used along with

Biovis software. Table 3.6 shows the preparation of etchant.

Table 3.6 Microetching of aluminum

Etchant Composition Conditions

Kellers etchant

190 ml distilled water,

5 ml nitric acid, 3 ml

hydrochloric acid, 2 ml

hydrofluoric acid

10-30 second

immersion; use

only fresh etchant

3.3.1 Microstructure of Aluminum Before and After Annealing

The tubular specimen of aluminum was polished to mirror-like

finish and etched using kellers etchant (1% con. HF, 2.5% NHO3, 1.5% HCl,

and 95% distilled water) for 15 to 20 seconds. The microscopic structure

obtained of the specimen before and after annealing is shown in Figure 3.12.

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(a) (b)

Figure 3.12 (a) Aluminum before annealing (b) aluminium after annealing

3.3.2 Microstructure of Copper Before and After Annealing

The tubular specimen of copper was polished to mirror-like finish

and etched using an etchant (20 ml NH4OH, 10 ml H2O, and 20 ml H2O2) for

15 to 20 seconds. The microscopic structure obtained of the specimen before

and after annealing is shown in Figure 3.13.

(a) (b)

Figure 3.13 (a) Copper before annealing (b) copper after annealing

3.3.3 Microstructure of Brass Before and After Annealing

The tubular specimen of brass was polished to mirror-like finish and

etched using an etchant (K2Cr2O7) for 20 seconds. The microscopic structure

obtained of the specimen before and after annealing is shown in Figure 3.14.

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(a) (b)

Figure 3.14 (a) Brass before annealing (b) brass after annealing

3.3 4 Microstructure analysis on the failed tubes

Aluminum

The tubular specimen of failed portion of aluminum was polished

to mirror-like finish and etched using kellers etchant 1ml HF, 200ml H2O.

The test methed of IS 7739 part III : 1976 R (2007), ASM Handbook Vol. 9

was followed. The microstructure consist of Magnesium Silicide particles

present in matrix of aluminum solid solution. The microstructure obtained is

shown in Figure 3.15.

100X 200X

Figure 3.15 Microstructure falied tube-Aluminum

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Brass

The tubular specimen of failed portion of brass was polished to

mirror-like finish and etched using etchant 5ml HNO3, 5ml Acetic acid 2ml

H3P04. The test methed of IS 7739 part V : 1976 R (2007), ASM handbook

Vol 9 was followed. The microstructure consist of beta phase in alpha solid

solution. The microstructure obtained is shown in Figure 3.16.

Figure 3.16 Microstructure of failed tube - Brass

3.4 SUMMARY

Mechanical and micro structural tests for tubular materials of

aluminum, copper, and brass were performed to evaluate the following.

Surface hardness of materials before and after annealing

Surface roughness of materials before and after annealing

Tensile test of materials before and after annealing

Microstructure of materials before and after annealing

It is concluded that surface hardness, roughness, tensile strength,

yield strength, elongation and grain size improved after the annealing the

materials.