ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

Подождите немного. Документ загружается.

solutions (such as weep holes) included in the design. If

there is potential for liquid to get into a space, provide

exit relief rather than trying to seal the liquid out.

Oxygen-rich (flammable) condensation can form

on chilled surfaces (surfaces that are chilled to

temperatures below 90 K and then exposed to air;

common on uninsulated liquid nitrogen transfer

lines and inside open-mouth dewars containing

liquid nitrogen or helium residue).

Inform personnel of potential fire hazard, and take

appropriate precautions.

Low temperatures may cause embrittlement of the

material, causing it to fail at lower-than-expected

loads.

Test personnel must be aware of the potential and

prepare for brittle fracture of structural components.

Reference cited in this section

10. J.H. Lieb and R.E. Mowers, Testing of Polymers at Cryogenic Temperatures, Testing of Polymers,

J.V.Schmitz, Ed., Vol 2, John Wiley & Sons, 1965, p 84–108

Tension and Compression Testing at Low Temperatures

Robert P. Walsh, National High Magnetic Field Laboratory, Florida State University

References

1. E.R. Parker, Brittle Behavior of Engineering Structures, John Wiley & Sons, 1957

2. R.P. Reed and A.F. Clark, Ed., Materials at Low Temperatures, ASM, 1983

3. L.E. Neilsen and R.F. Landel, Mechanical Properties of Polymers and Composites, Marcel Dekker, NY,

1994, p 249–263

4. M.B. Kasen et al., Mechanical, Electrical, and Thermal Characterization of G-10CR and G-11CR Glass-

Cloth/Epoxy Laminates Between Room Temperature and 4 K, Advances in Cryogenic Engineering, Vol

28, 1980, p 235–244

5. R.P. Reed and M. Golda, Cryogenic Properties of Unidirectional Composites, Cryogenics, Vol 34 (No.

11), 1994, p 909–928

6. E.P. Popov, Mechanics of Materials, 2nd ed., Prentice-Hall, NJ, 1976

7. J.P Hirth and M. Cohen, Metall. Trans., Vol 1, Jan 1970, p 3

8. G. Hartwig and F. Wuchner, Low Temperature Mechanical Testing Machine, Rev. Sci. Instrum., Vol 46,

1975, p 481–485

9. R.P. Reed, A Cryostat for Tensile Test in the Temperature Range 300 to 4 K, Advances in Cryogenic

Engineering, Vol 7, Plenum Press, NY, 1961, p 448–454

10. J.H. Lieb and R.E. Mowers, Testing of Polymers at Cryogenic Temperatures, Testing of Polymers,

J.V.Schmitz, Ed., Vol 2, John Wiley & Sons, 1965, p 84–108

11. R.P. Reed and R.L. Durcholz, Cryostat and Strain Measurement for Tensile Tests to 1.5 K, Advances in

Cryogenic Engineering, Vol 15, Plenum Press, NY, 1970, p 109–116

12. C. Ferrero, Stress Analysis Down to Liquid Helium Temperature, Cryogenics, Vol 30, March 1990, p

249–254

13. R.P. Reed and R.P. Walsh, Tensile Strain Effects in Liquid Helium, Advances in Cryogenic

Engineering, Vol 34, Plenum Press, 1988, p 199–208

14. C.C. Perry, Strain Gage Reinforcement Effects on Low Modulus Materials, Manual on Experimental

Methods for Mechanical Testing of Composites, R.L. Pendelton and M.E. Tuttle, Ed., Society for

Experimental Mechanics, 1989, p 35–38

15. R.P. Reed and R.P. Walsh, Tensile Properties of Resins at Low Temperatures, Advances in Cryogenic

Engineering, Vol 40, Plenum Press, NY, 1994, p 1129–1136

16. R.P. Walsh, J.D. McColskey, and R.P. Reed, Low Temperature Properties of a Unidirectionally

Reinforced Epoxy Fibreglass Composite, Cryogenics, Vol 35 (No. 11), 1995, p 723–725

17. N.R. Adsit, Compression Testing of Graphite Epoxy, Compression Testing of Homogeneous Materials

and Composites, STP 808, R. Chait and R. Papirno, Ed., ASTM, 1983, p 175–186

18. V.J. Toplosky, R.P. Walsh, S.W. Tozer, and F. Motamedi, Mechanical and Thermal Properties of

Unreinforced and Reinforced Polyphenylenes at Cryogenic Temperatures, Advances in Cryogenic

Engineering, Vol 46, Plenum Press, NY, 1999

Bend Testing

Eugene Shapiro, Olin Corporation

Introduction

BEND TESTS are conducted to determine the ductility or strength of a material. The tests typically used are

discussed in this article with details on test methods, apparatuses, procedures, specimen preparation, and

interpretation and reporting of results. The section “Bending Ductility Tests” also includes representative test

data for many metals.

Bend tests for ductility differ fundamentally from other mechanical tests in that most mechanical tests are

designed to give a quantitative result and have an objective endpoint. In contrast, bending ductility tests give a

pass/fail result with a subjective endpoint; the test operator judges whether a surface has undergone cracking.

The bending ductility test developed as a shop-floor material inspection test because of its pass/fail qualities

and the simplicity and low cost of the required tooling. As a consequence, the development of bending ductility

test methods and apparatuses has been carried out by users rather than by mechanical-test equipment

manufacturers.

Test procedures and specimen preparation methods have evolved without close attention to detail. Therefore,

despite the value of the test and its long history of use, there has been minimal standardization. There are,

however, two ASTM standards—ASTM E 190, “Standard Test Method for Guided Bend Test for Ductility of

Welds” (Ref 1), and ASTM E 290, “Standard Test Method for Semi-Guided Bend Test for Ductility of Metallic

Materials” (Ref 2)—which provide guidelines for testing strip, sheet, plate, and weldments.

Tests for determining the bending strength of metals have not been used widely, although the information from

such tests is clearly useful. ASTM E 855, “Standard Methods of Bend Testing of Metallic Flat Materials for

Spring Applications” (Ref 3), discusses the techniques for determining the bending strength of metal. These

testing methodologies are described extensively in this article with greater emphasis on interpretation and

analysis. Also included are descriptions of methods used in Europe and Japan. Finally, a section on

computerized testing is included.

Bending tests that have been developed for brittle materials, coatings, construction (girder and beam) sections,

and other specific product forms are not covered in this article. However, descriptions of these test methods can

be found in the ASTM standards.

References cited in this section

1. “Standard Test Method for Guided Bend Test for Ductility of Welds,” ASTM E 190, Annual Book of

ASTM Standards, Vol 03.01, ASTM, 1984, p 336–339

2. “Standard Test Method for Semi-Guided Bend Test for Ductility of Metallic Materials,” ASTM E 290,

Annual Book of ASTM Standards, Vol 03.01, ASTM, 1984, p 430–434

3. “Standard Test Methods of Bend Testing of Metallic Flat Materials for Spring Applications Involving

Static Loading,” ASTM E 855, Annual Book of ASTM Standards, Vol 03.01, ASTM, 1998, p 640–647

Bend Testing

Eugene Shapiro, Olin Corporation

Bending Ductility Tests

Frank N. Mandigo, Olin Corporation

Bending ductility tests determine the smallest radius around which a specimen can be bent without cracks being

observed in the outer fiber (tension) surface. This forming limit commonly is called the minimum bend radius

and is expressed in multiples of specimen thickness, t. A material with a minimum bend radius of 3t can be bent

without cracking through a radius equal to three times the specimen thickness. It thus follows that a material

with a minimum bend radius of 1t has greater ductility than a material with minimum bend radius of 5t.

Alternatively, the bend radius can be fixed, and the angle of bend at which fracture occurs noted. Figure 1

illustrates bend radius, angle of bend, and other concepts associated with bending tests.

Fig. 1 Terms used in bend testing

This article describes apparatuses, specimen, preparation, and test procedures used in bending ductility testing.

Bending tests are usually performed on strip, sheet, or plate; however, the same methods and apparatuses can be

used to evaluate the bending ductility of other product forms (drawn or extruded rounds, squares, or polygonal

shapes) and of weldments.

Evaluating and reporting results of bending tests are also discussed. This discussion includes a description of

failure criteria, strain distribution, directionality, and factors that affect bend ductility. Finally, a compilation of

bend data for select engineering materials is presented.

Bend Testing

Eugene Shapiro, Olin Corporation

Apparatuses

Bending test apparatuses include wrap, wipe, V-block, and soft tooling devices that may have interchangeable

die radii and are able to bend test specimens to several preset angles. The pins, mandrels, rollers, radiused flats,

and clamping devices must be longer than the specimen width, and they must be strong and rigid enough to

resist deformation and wear. Descriptions of basic types of bending devices follow.

Wrap bending devices (Fig. 2) grip the test specimen at one end; a mandrel, reaction pin, or block with the

desired bend radius is positioned at midlength. A roller that sweeps concentrically around the bend radius

applies the bending force. The distance from the mandrel to the loading roller generally is equal to the thickness

or diameter of the test piece, plus clearance. The clearance is adjusted to allow the test specimen to bend to the

desired radius or angle without scuffing, smearing, or galling of strip and die surfaces.

Fig. 2 Wrap bending device. Bending force is applied by a roller that sweeps

concentrically around the bend radius.

Wipe bending devices (Fig. 3) are similar to wrap bending devices, except that the bending force is applied by a

mandrel or roller that moves perpendicular to the clamped specimen.

Fig. 3 Wipe bending devices. (a) Mandrel type with force applied near free end. (b) Die

type with force applied near free end

V-block bending devices consist of a mandrel and a bottom block (Fig. 4a) or specimen supports (Fig. 4b). The

sample rests on supports or on the bottom block and is not clamped during the test. The distance between

supports is selected to force the specimen to conform to the mandrel radius without excessive interference. This

clearance is often the mandrel diameter, d, plus three times the specimen thickness, t. Bending force is applied

at the center of the specimen. The bottom block normally is a V or U shape. Bends made with conforming

bottom block radii are bottoming or closed-die bends (Fig. 4a); those without conforming bottom block radii

are air or free bends (Fig. 4b).

Fig. 4 V-block bending devices. (a) Closed V-block. (b) Open V-block

Soft tool bending, or rubber pad bending, is similar to V-block bending, except that the bottom block is

replaced by highly compressible materials such as polyurethane. Figure 5 illustrates various setups for soft tool

bending.

Fig. 5 Setups for soft tool (rubber pad) bending of sheet metal. (a) Simple 90° V-bend. Air

space below die pad permits deep penetration. (b) Simple U-bend or channel. Spacers

enable U-bends of varying widths to be formed in the same die-pad retainer. Deflector

bars help provide uniform distribution of punch pressure. (c) Modified U-bend with

partial air bending. (d) Acute-angle U-bend. High side pressures are obtained by using a

conforming rubber die pad and deflector bars.

Bend Testing

Eugene Shapiro, Olin Corporation

Specimen Preparation

Sheet and plate specimens normally are full thickness and are prepared by shearing or sawing. Narrow strip and

bar sections can be tested with specimens cut to length. For polygonal shapes, it is sometimes necessary to

machine or grind a flat, with the unmachined surface to be placed on the tension side of the bend. The rough

edges of cut samples can be removed by processes such as belt sanding and filing. The edges of rectangular-

section test pieces can be rounded to a radius up to one tenth of the sample thickness. Edge preparation is not

required for specimens with width-to-thickness ratios (w/t) greater than 8 to 1, unless cracking initiates from the

edge during bending.

A specimen can be machined to fit if it is too thick for the bending device or if the required bending forces

exceed the capacity of the bending device. The unmachined surface is placed on the tension side. If reduced

thickness samples are used, the specimen dimensions must be held constant during testing and noted in the test

report.

Specimens must be long enough to be clamped securely to prevent slippage in wrap and wipe tests. Specimens

for V-block and soft tool bending can be of any length greater than the distance between specimen supports

(Fig. 4b).

Test specimens usually are cut parallel or perpendicular to the direction of rolling, drawing, or extrusion.

However, any orientation in the plane of the product width and length can be tested. Specimen dimensions

should be the same for any test orientation. Figure 6 shows the orientation of a bending test specimen with the

rolling direction for a longitudinal orientation and a transverse orientation. The transverse orientation generally

produces lower ductility because the tensile bending stresses are oriented perpendicular to the fiber structure

developed by the rolling procedure used to produce sheet and plate products.

Fig. 6 Relative orientation of longitudinal and transverse bending tests

Cold rolling oils or other protective coatings usually are left on specimens to serve as a lubricating film during

bending tests. Other lubricants can be used if they reflect field conditions.

The principal stress and strain directions developed during bending are defined in Fig. 7(a). The specimen

width-to-thickness ratio (w/t) determines the respective stress and strain states. At w/t > 8, bending occurs under

plane-strain conditions (ε

= 0 and σ

/σ

= 0.5) and bend ductility is independent of the exact width-to-thickness

ratio (Fig. 7b). At w/t < 8, bending occurs under plane-stress conditions (σ

/σ

< 0.5) with plastic deformation

in all principal strain directions, and the measured bend ductility is strongly dependent on the width-to-

thickness ratio (Fig. 7b). Therefore, bending tests are conducted at width-to-thickness ratios greater than 8 to 1

whenever possible to eliminate geometric effects on the test results. Although specimens with width-to-

thickness ratios less than 8 to 1 can be tested, the entire test lot must have the same width-to-thickness ratio.

Fig. 7 Stress and strain in bending. (a) Schematic of the bend region defining the

direction of principal stresses and strains. (b) Outer fiber strain at fracture versus width-

to-thickness ratio

Bend Testing

Eugene Shapiro, Olin Corporation

Test Method

Specimens and apparatuses should be carefully inspected. Specimens with scuff marks, scratches, excessive

curvature, twisting, or other surface defects should be discarded. Bend radii, mandrels, and support blocks must

be free of scuff marks or other visible damage.

In bending tests, specimens are bent around progressively tighter radii, or to large bend angles, until failure or

cracking occurs on the convex surface. If wipe or wrap bending apparatus is used, the clearance must be

adjusted for each radius. Any method can be used to force the specimen to obtain the desired radius or angle;

however, it must be applied slowly and steadily without significant lateral motion. Bend angles of 180° are

obtained by pressing bent specimens between platens (Fig. 8), maintaining the bend radius with a spacer block

twice as thick as the radius between the legs of the specimen.

Fig. 8 Methods used to develop 180° bend angles. (a) Bend sample from wipe or V-block

placed between platens. (b) Sharp (180°) bend. (c) Bend with radius equal to one-half the

spacer-block thickness

During bending, the specimen can be removed to inspect the convex surface for cracks. The test is complete

when product specifications have been achieved.

Bend Testing

Eugene Shapiro, Olin Corporation

Interpretation

Specimens are examined for cracking at the apex of the bend with magnifications of up to 20×. A specimen is

acceptable if there are no visible cracks on its outside surface. Surface rumples and orange peeling are not

considered fracture sites. If cracking occurs at the edges of the bent sample when the specimen width-to-

thickness ratio is 8 to 1 or greater, the edges of the sample should be polished or ground and the specimen

retested. At width-to-thickness ratios less than 8 to 1, edge preparation may be required to obtain reproducible

measurements of minimum bend radii.

The bending method can influence the strain distribution on the surface of the specimen (Fig. 9). V-block

bending (Fig. 9b) develops a nonuniform strain distribution, while wrap or wipe bending produces strain that

increases progressively with the bend angle until saturation. Circumferential strain becomes uniform only after

the bend angle exceeds certain minimum values (Fig. 10 and 11). For wipe or wrap bending, these values are a

90° bend angle and a 1t radius.

Fig. 9 Comparison of strain distrubutions produced by different bending methods. (a)

Application of a pure bending moment (not achievable in commercial bending devices).

(b) V-block bending. (c) Wipe bending

Fig. 10 Effect of bend angle on strain distribution along the circumference of bends for

tempered aluminum alloy 2024 sheet. (a) 3.2 mm (0.125 in.) thick sheet, R/t = 0.7. (b) 6.4

mm (0.25 in.) thick sheet, R/t = 2.5

Fig. 11 Dependence of measured circumferential strain on bend angle for tempered

aluminum alloy 2024 sheet. Radius expressed in terms of thickness, t

Minimum bend radii reported are subjective measurements, not intrinsic material properties. This subjectivity is

due to visual assessment of pass/fail criteria and the incremental steps of the available bend radii. The problems

associated with visual assessment include reproducibility by one tester over time and the difficulty of two or

more testers agreeing on the definition of a visible crack. The problems associated with the incremental steps of

the bend radii can be minimized by using closely spaced bend radii.

A number of characteristics can be expected when performing bending tests on metallic materials. The

minimum bend radius is dependent on alloy composition. The minimum bend radius increases as strip or bar

temper increases. Annealed strips generally have isotropic bend characteristics in the plane of the sheet (the

minimum bend radii are similar both parallel or perpendicular to the process direction). Strips in highly cold-

rolled tempers usually have better bend properties when bends are made perpendicular to the rolling direction.

For a given alloy and temper, the minimum bend radius usually is directly proportional to strip thickness, as

indicated in the following equation:

Because of these characteristics, caution should be used in choosing a bending test method or using tabulated

data. The best practice is to use the same test method, specimen dimensions, bend angle, and bend radii that are

used during part fabrication. Representative data for aluminum and ferrous sheet are provided in Tables 1, 2, 3,

and 4.

Table 1 Recommended minimum bend radii for 90° cold forming of aluminum alloy

sheet and plate

Radii for various thicknesses expressed in terms of thickness, t Alloy Temper

0.4 mm

( in.)

0.8 mm

( in.)

1.6 mm

( in.)

3.2 mm

( in.)

4.8 mm

( in.)

6.4 mm

( in.)

9.5 mm

( in.)

12.7 mm

( in.)

O 0 0 0 0

1t 1t

1 t

H12 0 0 0

1t 1t

1 t

H14 0 0 0 1t 1t

1 t

1100

H16 0

1 t 1t 2t

3t 4t