ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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Fig. 13 General types of tests related to stress-corrosion cracking (SCC), hydrogen embrittlement, and

corrosion fatigue. (a) Smooth specimen SCC testing for determination of a stress threshold, σ

. (b) Slow-

strain-rate (SSR) testing for strain-rate controlled evaluation of σ

and time to failure, t

. (c) Fracture

mechanics evaluation with a notched or precracked specimen for determination of crack growth rate

(da/dt) or crack growth threshold (K

ISCC

). (d) Corrosion fatigue life (S-N) testing. (e) Corrosion fatigue

crack growth testing for determination of crack growth rates of fatigue cracking threshold (ΔK

)

If the objective of testing is to predict the service behavior or to screen alloys for service in a specific

environment, it is often necessary to obtain SCC information in a relatively short time. This requires

acceleration of testing by increasing the severity of the environment or the critical test parameters. The former

can be accomplished by increasing the test temperature or the concentration of corrosive species in the test

solution and by electrochemical stimulation. Test parameters that can be changed to reduce the testing time

include the application of higher stresses, continuous straining, and precracking, which allows bypassing the

crack nucleation phase of the SCC process.

Stress-corrosion specimens can be divided into two main categories, namely smooth, and precracked or notched

specimens. Further distinction can be made in the loading mode, such as constant deflection, constant load, and

constant extension or strain rate. These different loading modes are briefly reviewed in the following sections.

More detailed information is in Ref 76.

Important parameters during SCC testing include the orientation grain flow direction relative to stressing

direction, surface condition, and residual stress. The nucleation of stress-corrosion cracks strongly depends on

initial surface reactions, and thus the surface condition of the test specimens, particularly smooth specimens,

has a significant effect on the test results. Smooth test specimens are often tested with a mechanically

(machined or abraded) or (electro) chemically treated surface. It is very important to avoid or to remove

machining marks or scratches perpendicular to the loading direction (per ASTM G 1, “Standard Practice for

Preparing, Cleaning, and Evaluating Corrosion Test Specimens”).

Reference cited in this section

76. D. Sprowls, Evaluation of Stress Corrosion Cracking, Stress Corrosion Cracking, Materials

Performance and Evaluation, ASM International, 1992, p 363–416

Evaluation of Environmentally Assisted Crack Growth

Y. Katz, N. Tymiak, and W.W. Gerberich, University of Minnesota

Smooth Specimen SCC Testing

Smooth SCC specimens allow for the evaluation of the total SCC life, which includes crack nucleation and

propagation. Testing can be conducted under constant extension or strain, constant load, and constant extension

or strain rate. The selection of a specific test method for SCC strongly depends on the particular service

application and the time allowed for testing.

Constant Extension Testing

Constant extension or constant strain tests on smooth specimens are widely used and do not require elaborate

testing fixtures. Depending on the specific configuration of the test articles, different types of constant

extension tests are being used, the most common being bent-beam, U-bend, C-ring, and tensile type specimens.

The simplest method of providing a constant load consists of a deadweight hung on one end of the specimen.

This method is particularly useful for wire specimens. For specimens of larger cross section, however, lever

systems such as those used in creep-testing machines are more practical. The primary advantage of any

deadweight loading device is the constancy of the applied load.

Constant-strain SCC tests are performed in low-compliance tension-testing machines. The specimen is loaded

to the required stress level, and the moving beam is then locked in position. Other laboratory stressing frames

have been used, generally for testing specimens of smaller cross section.

Bent-Beam Specimens. The different types of bent-beam specimens are illustrated in Fig. 14. These specimens

may be used to test sheet plate and flat extruded material, or wires and extrusions with a circular cross section.

The figure shows that bending can be accomplished in several ways depending on the dimensions of the

specimen. Stressing of the specimen is accomplished by bending the specimen in a stressing device, while

restraining the ends. During stress-corrosion testing, both specimen and stressing device are exposed to the test

environment. The most simple loading arrangement is the two-point loaded bent beam, which can only be used

on relatively thin sheet or wire material. The elastic stress at the mid-point of the specimen can be estimated

from the following equation:

L = (ktE/σ) sin

-1

(H/HktE)

where Lis the specimen length, σ is the maximum stress, E is the elastic modulus, H is the length of holder, t is

the specimen thickness, and k is the empirical constant (1.280).

Fig. 14 Schematic specimen and holder configurations for bent-beam specimens. (a) Two-point loaded

specimen. (b) Three-point loaded specimen. (c) Four-point loaded specimen. (d) Welded double-beam

specimen. Source: ASTM G 9

Three-Point Bend Specimens. Three-point bend tests are commonly used because of the ease of load

application and the ability to use the same loading rigs for different stresses. The load is applied by turning a

bolt in the rig, deflecting the specimen. The elastic stress at the mid-point of the specimen is calculated from the

following equation:

σ = 6Ety/H

where σ is the maximum tensile stress, E is the elastic modulus, t is the specimen thickness, y is the maximum

deflection, and H is the length of holder.

This test has a number of disadvantages. First, dissimilar metal corrosion and/or crevice corrosion can occur

under the bolt. Secondly, once the crack has formed, the stress condition changes such that the outer layer of the

specimen is not subject to a tensile stress only, but to a complex combination at tensile and bending stresses.

The propagating crack will then deviate from the centerline. Thus, the three-point bend test can only be used as

a qualitative test to assess the susceptibility to stress-corrosion cracking. With the four-point bend test,

described in the next paragraph, tensile stresses can be maintained during the growth of the crack.

Four-Point Bend Specimens. Four-point bend testing provides a uniform tensile stress over a relatively large

area of the specimen. The elastic stress in the outer layer of the specimen between the two inner supports can be

calculated from the following equation:

σ = 12Ety/(3H

- 4A

)

where σ is the maximum tensile stress, E is the elastic modulus, t is the specimen thickness, y is the maximum

deflection, H is the distance between outer supports, and A is the distance between outer and inner supports.

U-bend specimens are prepared by bending a strip 180° around a mandrel with a predetermined radius (Fig.

15). The figure shows that bends less than 180° degrees are also used. Standardized test methods are described

in ASTM G 30. Because of the ease of fabrication, a large amount of specimens can be fabricated, and this test

is therefore widely used to qualitatively evaluate the susceptibility of alloy and heat treatment to stress-

corrosion cracking.

Fig. 15 Schematic two-stage stressing of a U-bend specimen. Source: ASTM G 30

A good approximation of the strain at the apex of the U-bend is:

ε = t/2R, when t < R

where t is the specimen thickness and R is the radius of the bend. Then, an appropriate value for the maximum

stress can be obtained from the stress-strain curve of the test material.

C-ring specimens are commonly used to determine the susceptibility to stress-corrosion cracking of alloys in

different product forms. This test is particularly useful for testing of tubing, rod, and bar in the short-transverse

direction, as illustrated in Fig. 16. The specimens are typically bolt loaded to a constant strain or constant load

per ASTM G 338. If the stresses in the outer layers of the apex of the C-rings are in the elastic region, the

stresses can be accurately calculated using the following equations:

= D - Δ

where

Δ = σd

/4EtZ

and where D is the outer diameter of the C-ring before stressing, D

is the outer diameter of stressed C-ring, σ is

the elastic stress, Δ is the change of D at the desired stress, d is the mean diameter (D - t), t is the wall thickness,

E is the elastic modulus, and Z is the correction factor for curved beam.

Fig. 16 Sampling procedure for testing various products with C-rings. (a) Tube. (b) Rod and bar. (c)

Plate. Source: ASTM G 338

The stress on C-ring specimens can be more accurately determined by attaching circumferential and transverse

strain gages to the stressed surface. The circumferential (σ

) and transverse (σ

) elastic stresses can be

calculated with (Ref 82 and 83):

= E/(1 - μ

) × (ε

+ με

)

= E/(1 - μ

) × (ε

+ με

)

where E is the elastic modulus, μ is the Poisson's ratio, ε

is the circumferential strain, and ε

is the transverse

strain.

Tensile Specimens. For specific purposes, such as alloy development, a large number of stress-corrosion

specimens need to be evaluated. Tensile specimens have been used for this purpose where specimens used to

determine tensile properties in air are adapted to SCC, as discussed in ASTM G 49. When uniaxially loaded in

tension, the stress pattern is simple and uniform, and the magnitude of the applied stress can be accurately

determined. Specimens can be quantitatively stressed by using equipment for application of either a constant

load, a constant strain, or an increasing load or strain.

This type of test is one of the most versatile methods of SCC testing because of the flexibility permitted in the

type and size of the test specimen, the stressing procedures, and the range of stress level. It allows the

simultaneous exposure of unstressed specimens (no applied load) with stressed specimens and subsequent

tension testing to distinguish between the effects of true SCC and mechanical overload.

A wide range of test specimen sizes can be used, depending primarily on the dimensions of the product to be

tested. Stress-corrosion test results can be significantly influenced by the cross section of the test specimen.

Although large specimens may be more representative of most structures, they often cannot be prepared from

the available product forms being evaluated. They also present more difficulties in stressing and handling in

laboratory testing.

Smaller cross-sectional specimens are widely used. They have a greater sensitivity to SCC initiation, usually

yield test results rapidly, and permit greater convenience in testing. However, the smaller specimens are more

difficult to machine, and test results are more likely to be influenced by extraneous stress concentrations

resulting from nonaxial loading, corrosion pits, and so on. Therefore, use of specimens less than about 10 mm

(0.4 in.) in gage length and 3 mm (0.12 in.) in diameter is not recommended, except when testing wire

specimens.

Tension specimens containing machined notches can be used to study SCC and hydrogen embrittlement. The

presence of a notch induces a triaxial stress state at the root of the notch, in which the actual stress will be

greater by a concentration factor that is dependent on the notch geometry. The advantages of such specimens

include the localization of cracking to the notch region and acceleration of failure. However, unless directly

related to practical service conditions, the results may not be relevant.

Tension specimens can be subjected to a wide range of stress levels associated with either elastic or plastic

strain. Because the stress system is intended to be essentially uniaxial (except in the case of notched

specimens), great care must be exercised in the construction of stressing frames to prevent or minimize bending

or torsional stresses.

Constant Load Testing

Although the constant extension tests are widely used for evaluating the susceptibility of alloys to stress-

corrosion cracking because of the ease of specimen preparation and the ability to test a large number of

specimens at one time, there is one major drawback. Once stress-corrosion cracks have formed, the gross cross

section stress decreases, which will eventually cause the crack to stop. Application of a constant or a static load

provides an alternative test method that represents some actual field conditions that can provide threshold

values. It should be cautioned, however, that such threshold values are strongly dependent on the method of

loading (i.e., deadweight or spring) and the specimen size and cannot be considered a material property.

Moreover, Fig. 17 and (Ref 84) shows that as a crack develops, the stress at the crack tip increases, possibly

decreasing the time to failure.

Fig. 17 Schematic comparison of determination of threshold stress integrity factor (K

Iscc

or K

). (a)

Constant-load (K-increasing) test. (b) Constant crack opening displacement (K-decreasing) test. Source:

Ref 84

Constant Strain Rate Testing

Constant or slow-strain-rate testing is a very useful technique to evaluate the susceptibility of materials to SCC

in a relatively short period of time (Ref 84 and 85). Typical strain rates range between 10

-5

-1

and 10

-7

-1

, but

for most materials the typical strain rate is at 10

-6

-1

. The strain sensitivity to SCC can change for different

alloys, even of the same metal. Figure 18 (Ref 85) shows that for the 2000 series aluminum alloys, the critical

strain rate for the highest susceptibility to cracking is 10

-6

-1

, whereas no such critical strain rate exists for the

7000 series aluminum alloys. This difference in slow-strain-rate behavior of the two alloys may indicate

different mechanisms for SCC. The slow-strain-rate behavior indicates that the principal mechanism for

cracking of the 2000 series alloys is film rupture—anodic dissolution model, while the predominant mechanism

for cracking of the 7000 series alloys is hydrogen embrittlement.

Fig. 18 Strain rate regimes for SCC of 2000, 5000, and 7000 series aluminum alloys in a 3% aqueous

NaCl solution plus 0.3% H

. Source: Ref 85

The parameters that are typically measured in slow-strain-rate testing to determine the susceptibility to SCC

are:

• Time to failure

• Percent elongation

• Percent reduction in cross-sectional area at the fracture surface

• Reduction in ultimate and yield tensile stress

• Presence of secondary cracking on the specimen gage section

• Appearance of the fracture surface

In order to assess the susceptibility of a material to SCC, the results of the slow-strain-rate test in a particular

environment must be compared with those in an inert environment, such as dry nitrogen gas.

References cited in this section

82. Stress Corrosion Testing, STP 425, ASTM, 1967, p 3

83. H.L. Craig, D.O. Sprowls, and D.E. Piper, Handbook on Corrosion Testing and Evaluation, W.H. Ails,

Ed., John Wiley and Sons, 1976, p 213

84. G. Vogt, Werkstoff. Korros., Vol 29, 1978, p 721

85. N.J.H. Holroyd and G.M. Scamans, Slow-Strain Rate Stress Corrosion Testing of Aluminum Alloys,

Environment-Sensitive Fracture, STP 821, S.V. Dean, E.N. Pugh, and G.M. Ugianski, Ed., ASTM,

1984, p 202

Evaluation of Environmentally Assisted Crack Growth

Y. Katz, N. Tymiak, and W.W. Gerberich, University of Minnesota

Precracked Specimen Testing

The use of precracked specimens in the evaluation of SCC is based on the engineering concept that all

structures contain cracklike flaws. Moreover, precracking can contribute to the susceptibility to SCC of alloys

such as titanium alloys, and this susceptibility may not always be evident from smooth specimens.

Precracking eliminates the uncertainties that are associated with crack nucleation and can provide a flaw

geometry for which a stress analysis is available through fracture mechanics. Expressing stress-corrosion

characteristics in terms of fracture mechanics provides a relationship between applied stress, crack length, and

crack growth in a corrosive environment. When the plasticity can be ignored, or in other words, when the

plastic zone ahead of the propagating crack is below a certain value and a triaxial or plane strain stress state

exists at the crack tip, linear elastic fracture mechanics (LEFM) can be applied to describe the relationship

between crack length (a) and the applied stress (σ) by the stress-intensity factor K:

K = Fσ ·

where F is a polynomial factor that accounts for the specimen geometry. Linear elastic fracture mechanics, and

thus the K factor, cannot be used to describe the relationship between applied stress and the crack length when

there is significant plasticity or when the stress state at the crack tip is plane stress. Then, a more fundamental

parameter, the J integral, is used.

Almost all standard plane strain fracture mechanics test specimens can be adapted to SCC testing. Several

examples are illustrated schematically in Fig. 19 (Ref 76). ASTM E 399 describes the allowable specimen

dimensions and test procedures for precracked specimens.

Fig. 19 Classification of precracked specimens for SCC testing. Source: Ref 76

Specimen Preparation. When using precracked fracture mechanics specimens, specific dimensional

requirements need to be considered, as well as crack configuration and orientation. The basic dimensional

requirement for application of LEFM is that dimensions are such that plane strain condition can be maintained.

In general, for a valid K measurement, neither that crack length nor the specimen thickness should be less than

2.5 (K

/σ

)

Several designs of initial crack configuration are available. ASTM E 399 recommends that the notch root radius

is not greater than 0.127 mm (0.005 in.), unless a chevron notch is used, in which case it may be 0.25 mm (0.01

in.). In order to start out with a crack as sharp as possible, ASTM E 399 describes procedures for precracking.