Compression Test

The compression test is used to determine the flow stress data (true-stress/true-strain relation- ships) for metals at various temperatures and strain rates. In this test, the flat platens and the cylindrical sample are maintained at the same temperature so that die chilling, with its influ- ence on metal flow, is prevented. To be appli- cable without corrections or errors, the cylin- drical sample must be upset without any barreling; i.e., the state of uniform stress in the sample must be maintained as shown in Fig. 4.4. Barreling is prevented by using adequate lubrication, e.g., Teflon or machine oil at room temperature and at hot working temperatures, graphite in oil for aluminum alloys, and glass for steel, titanium, and high-temperature alloys. The load and displacement, or sample height, are measured during the test. From this infor- mation the flow stress is calculated at each stage of deformation, or for increasing strain. Figure 4.5 shows the tooling used for compres- sion tests conducted at the Engineering Re- search Center for Net Shape Manufacturing (ERC/NSM) of the Ohio State University [Dixit et al., 2002].

Similar to the uniform elongation portion of the tensile test, the following relationships are valid for the uniform compression test:

ho A ¯ e ⳱ ln ⳱ ln (Eq 4.12) h Ao L ¯ r ⳱ (Eq 4.13) A ¯ e A⳱ A (e)o (Eq 4.14) d¯e dh V ˙¯ e ⳱ ⳱ ⳱ (Eq 4.15) dt hdt h

where V is instantaneous deformation velocity; ho and h are initial and instantaneous heights,

respectively, and Ao and A are initial and in-

stantaneous surface areas, respectively.

As discussed earlier the flow stress values de- termined at high strains in the tensile test require a correction because of necking. Therefore, the compression test, which can be conducted with- out barreling up to about 50% reduction in height(¯e ⳱ 0.693 or more), is widely used to

obtain flow stress data for metal forming appli- cations.

At room temperature, the flow stresses of most metals (except that of lead) are only slightly strain-rate dependent. Therefore, any testing machine or press can be used for the compression test, regardless of its ram speed. Adequate lubrication of the platens is usually accomplished by (a) using lubricants such as Tef- lon, molybdenum disulfide, or high-viscosity oil and (b) by using Rastegaev specimens (Fig. 4.6) or specimens with spiral grooves machined on both the flat surfaces of the specimen to hold the lubricant (Fig. 4.6). A typical load-displacement curve obtained in the uniform compression test of aluminum alloy (Al 1100, annealed) at room temperature in a testing machine is shown in Fig. 4.7. Ther-¯e¯ data obtained from this curve are shown in Fig. 4.8.

At hot working temperatures, i.e., above the recrystallization temperature, the flow stresses of nearly all metals are very much strain-rate dependent. Therefore, whenever possible, hot compression tests are conducted on a machine that provides a velocity-displacement profile such that the condition e˙¯ ⳱ velocity/sample

Fig. 4.9 Press setup and fixture used in heating and com- pression of cylinders and rings

Fig. 4.10 Uniform compression samples before and after deformation (left to right: AISI 1018 steel, INCO 718, Ti-6Al-4V)

height can be maintained throughout the test. Mechanical cam-activated presses called plas- tometer or hydraulic programmable testing ma- chines (MTS, for example) are used for this pur- pose. In order to maintain nearly isothermal and uniform compression conditions, the test is con- ducted in a furnace or a fixture such as that shown in Fig. 4.9. The specimens are lubricated with appropriate lubricants—for example, oil graphite for temperatures up to 800⬚F (425 ⬚C) and glass for temperatures up to 2300⬚F (1260 ⬚C). The fixture and the specimens are heated to the test temperature and then the test is initiated. Examples of hot-formed compression samples are shown in Fig. 4.10. Examples of high-tem- peraturer-¯e¯ data are given in Fig. 4.11 and 4.12.

4.3.1 Specimen Preparation

There are two machining techniques that can be used for preparing the specimens for the com-

pression test, viz. the spiral specimen (Fig. 4.6a) and the Rastegaev specimen (Fig. 4.6b). The specimens shown are of standard dimensions used for the compression test. The spiral grooves and the recesses of the Rastegaev specimen serve the purpose of retaining the lubricant at the tool/workpiece interface during compression thus preventing barreling. It has been deter- mined through tests conducted at the ERC/NSM that Rastegaev specimens provide better lubri- cation and hold their form better during testing compared to the spiral grooved specimens. The specifications for the specimens and the test con- ditions are [Dahl et al., 1999]:

Specimen with spiral grooves (Fig. 4.6a):

● Solid cylinder (diameter ⳱ 0.5Ⳳ0.001 in., length⳱ 0.75Ⳳ0.005in.).

● Ends should be flat and parallel within 0.0005 in./in.

● Surface should be free of grooves, nicks and burrs.

● Spiral grooves machined at the flat ends of the specimen with approximately 0.01 in. depth.

Rastegaev specimen (Fig. 4.6b):

● Flat recesses at the ends should be filled with lubricant.

● Dimensions t0⳱ 0.008Ⳳ0.0005in. and uo⳱

0.02Ⳳ0.0005 in. at the end faces have a sig- nificant effect on the lubrication conditions.

● Rastegaev specimen ensures good lubrica- tion up to high strains of about 0.8 to 1, so that the specimen remains cylindrical (due to radial pressure that the lubricant exerts on the ring).

● to/uo ⳱ 0.4 (Fig. 4.6b) for steels (optimum

Fig. 4.11 Flow stress versus strain and strain rate versus strain, for type 403 stainless steel at 1800, 1950, and 2050⬚F (980, 1065, and 1120⬚C) (tests were conducted in a mechanical press where strain rate was not constant). [Douglas et al., 1975]

drical shape up to maximum strain before bulging occurs).

4.3.2 Parallelism of the Press (or Testing Machine) Slides

In a compression test, load is applied on the billet using flat dies. In order to ensure that a uniaxial state of stress exists during the experi- ment, the load applied should be perpendicular to the axis of the cylindrical specimen. This calls for measurement of the parallelism of the platens of the press. A commonly used technique for parallelism measurement involves compressing lead billets of the same height. The difference in the heights of the lead billets is an indication of the parallelism of the platens. Lead is used since it is soft and deforms easily at room temperature. The procedure followed for determining the par- allelism for recent tests conducted at the ERC/ NSM is described below [Dixit et al., 2002]:

1. Lead bar of 1 in. diameter was cut into ap- proximately 1 in. length. The height of each specimen was noted and an average value was calculated (Table 4.1).

2. The specimen were numbered and positioned on the compression test die (Fig. 4.13 and 4.14). The distance between them was mea- sured.

3. The samples were compressed in the tooling (Fig. 4.14). The final heights of the lead blocks were determined using a digital cali- per. They are tabulated in Table 4.1. 4. From the difference in the height of two spec-

imens and the distance between their loca- tions, the parallelism was determined as shown in Table 4.2. For example, for speci- mens 1 and 2, the difference in final height was 0.386 mm. This value divided by the dis- tance between their locations (60.2 mm) gave the ratio 0.0064 mm/mm (Table 4.2). From the data summarized in Table 4.2 and the ex-

Fig. 4.12 Flow stress versus strain and strain rate versus strain, for Waspaloy at 1950, 2050, and 2100⬚F (1065, 1120, and 1150 ⬚C) (tests were conducted in a mechanical press where strain rate was not constant). [Douglas et al., 1975]

periments, it was concluded that a parallelism less than 0.01 was acceptable for conducting reliable compression tests.

4.3.3 Errors in the Compression Test

Errors in the determination of flow stress by the compression test can be classified in three categories [Dahl et al., 1999]:

● Errors in the displacement readings, which result in errors in the calculated strain

● Errors in the load readings, which result in errors in the calculated stress

● Errors in the processing of the data due to barreling of the test specimens

The first and second type errors may be reduced or eliminated by careful calibration of the trans- ducers and data acquisition equipment. How- ever, barreling of the test specimens during com- pression cannot be entirely eliminated because

there is always friction between the specimen and the tools.

4.3.4 Determination of Error in Flow Stress Due to Barreling

The maximum error in determining flow stress may be the result of friction. In order to correct the flow stress curve and to determine the percentage error in flow stress, finite element (FE) analysis is used. The amount of barreling (Fig. 4.15 and 4.16) of different specimens ex- pressed by (H2ⳮ H1) for the given height re-

ductions during a particular compression test is given in Table 4.3. Figure 4.16(a) shows the ef- fect of friction on the end face of the billet.

Figure 4.17 shows the load stroke curves ob- tained from FE simulations for different values of shear friction factors (m) and from experi- ment for one specimen. When the load stroke curves are compared it can be seen that simu-

lations slightly overpredict the load. It should be noted that the difference in the load remains the same throughout the stroke.

The stress obtained from finite element sim- ulations with shear friction factor, m, greater than zero is called apparent flow stress. Appar- ent flow stress curves can be used to determine the error in flow stress obtained in experiments due to barreling at higher strains (e.g., strain⳱ 1.0). An “apparent flow stress” curve can be cal-

culated for a given value of shear friction factor m, as follows [Dixit et al., 2002]. At several re- ductions in height:

● The value of load and the associated diam- eters (H1 and H2 in Fig. 4.15) are noted. A

mean diameter is calculated as: (H1Ⳮ H2)/

2.

● The cross-sectional area is calculated using the mean diameter.

Fig. 4.14 Lead samples on the compression test die. [Dixit et al., 2002]

Table 4.2 Parallelism between different points that are shown in Fig. 4.13

Parallelism between points 1 and 2 (mm/mm) 0.0064

Parallelism between points 2 and 4 (mm/mm) 0.0080

Parallelism between points 3 and 4 (mm/mm) 0.0019

Parallelism between points 3 and 1 (mm/mm) 0.0145

Parallelism between points 1 and 4 (mm/mm) 0.0096

Parallelism between points 2 and 3 (mm/mm) 0.0035

Source: [Dixit et al., 2002]

Table 4.1 Height of the lead specimens used in tests conducted at the ERC/NSM

Specimen No. 1 Specimen No. 2 Specimen No. 3 Specimen No. 4

Initial height, in. (mm)

0.9933 (25.23) 1.0071 (25.58) 1.0181 (25.86) 1.0185 (25.87) 0.9894 (25.13) 1.0075 (25.59) 1.0157 (25.8) 1.0197 (25.9) 0.9929 (25.22) 1.0071 (25.58) 1.0169 (25.83) 1.0236 (26) 0.9913 (25.18) 1.0075 (25.59) 1.0177 (25.85) 1.0232 (25.99) 0.9937 (25.24) 1.0067 (25.57) 1.0169 (25.83) 1.0217 (25.95) Average 0.992 (25.2) 1.0072 (25.582) 1.0171 (25.834) 1.0213 (25.942) Final height, in. (mm)

0.5594 (14.21) 0.5594 (14.21) 0.5591 (14.2) 0.5591 (14.2)

0.5610 (14.22) 0.5610 (14.22) 0.5587 (14.19) 0.5587 (14.19)

0.5610 (14.22) 0.5594 (14.21) 0.5594 (14.21) 0.5594 (14.21)

Average 0.5597 (14.217) 0.5596 (14.213) 0.5591 (14.2) 0.5591 (14.2) Difference in height 0.4324 (10.983) 0.4476 (11.369) 0.4580 (11.634) 0.4623 (11.742)

Source: [Dixit et al., 2002]

● The “apparent stress” is calculated using the load at that height reduction and cross-sec- tional area (⳱ Load/Area).

● The value of strain is calculated as loge

(original height/instantaneous height).

● A particular value of shear friction factor, m, results in an “apparent flow stress” that is higher in magnitude than the value obtained with zero friction. A graph of stress versus strain plotted for different values of shear friction factor m can be drawn as shown in Fig. 4.18. As the value of m increases, the “apparent flow stress” increases.

● The barreling of the specimen at a strain of 1.0 is noted down.

The “apparent flow stress” curves obtained above can be used to calculate the error in flow stress obtained due to nonhomogenous defor- mation (barreling) during the tests at a strain of 1.0 as follows:

1. Conduct cylinder compression tests until a strain of 1.0.

2. Determine the amount of barreling in the specimen at a strain of 1.0.

3. By comparing the barreling of the actual specimen with the “apparent flow stress” curves given in Fig. 4.18, the value of stress in the experiments can be noted.

4. Thus, the error in flow stress obtained from an experiment can be calculated with respect to the stress in the curve with m ⳱ 0 (Fig. 4.18) as a percentage value at a strain of 1.0. If needed, the “apparent flow stress” curves can be generated for higher strains and the procedure can be repeated for estimating stress at that par- ticular higher strain.