ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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occurs under tensile load. However, for the same polymer/environment, crazing may occur readily under

torsion (Ref 161) or tension with superimposed hydrostatic compression (Ref 162). Thus, evaluation of a

polymer for a given application requires testing under loading conditions corresponding to those expected

during service. An apparatus for tension or compression superimposed with the hydrostatic pressure and an

environmental chamber may be obtained from Ref 163 and 164.

Environmentally assisted crack/craze propagation can be evaluated with double-edge (Ref 149) or single-edge

notched (Ref 153) specimens. Razor-blade cutting is a common precracking technique. A procedure for

obtaining sharp reproducible cracks of controlled length is detailed in Ref 165. Good results may also be

accomplished with fatigue (Ref 166) or controlled impact (Ref 167). With a sustained load apparatus and an

environmental chamber, as described in Ref 168, environmentally assisted craze and crack growth kinetics may

be evaluated under constant load conditions (Ref 149, 153, and 169).

To summarize this section on environmentally induced degradation of polymeric systems, a number of the

important chemomechanical factors are given in Table 4, and few examples of environmental interactions are

summarized in Table 5.

Table 4 Summary of chemomechanical factors influencing environmental cracking and crazing in

polymers

Factor

Considerations

Degree of crystallinity

Molecular weight

Glass transition temperature

Coefficient of hydroelasticity

Polymer

Solubility

Viscosity

Spreading coefficient on a polymer

Type: active or inert

Medium

For active media: solubility as compared to that for a polymer; frequently, crack growth

rates increase with the decreasing difference between solubility of a polymer and that of

a liquid (δ

- δ

) (Ref 149). Also, how much does it change T

of a polymer; Q/R, a ratio of

ionic charge to the ionic radius, is a critical parameter for several salt solutions (Ref 149).

Temperature

Testing temperature and a local crack-tip temperature (as increased locally at a tip of a

rapidly growing crack of a) with respect to T

(as decreased by an environment)

Dilatational component (necessary for crazing)

Stress state

Hydrostatic compression (suppresses crazing; facilitates liquid flow towards a tip of a

moving crack) (Ref 150)

Strain rate

Affects craze and crack growth rate

Influences craze growth

Crack

growth rate

For high crack-growth rates: local temperature elevation at the crack tip (Ref 148),

liquid transport (Ref 152), or absorption-controlled mechanisms (Ref 153)

Table 5 Examples of environmental interactions for selected systems

Material Environment Sample

geometry

Loading Environmental

effects

measured in

terms of:

Suggested

mechanism

Critical

parameters

Low-density

polyethylene

(Ref 153)

Methanol,

ethanol,

ethyleneglycol,

diethylene

triamine

DETA-NaOH

Single

notched

(sharp

crack)

Sustained load

0.13 MPa

applied stress

(yield stress

11.3 MPa)

Crack velocity

versus applied

stress intensity

Micelles

absorption on

the fibril

surface with

the

subsequent

Highest

crack-growth

rates with the

lowest (δ

) crack-

growth rates

(detergent) fibril fracture

increasing

with the

increasing

detergent

concentration

Nylon (Ref

149)

Aqueous salt

solutions:

CaCl, LiCl,

MgCl

Precracked

tensile bars

Constant

applied stress

intensity

Craze growth

velocity

Environment-

induced

decrease in

Relaxation-

controlled

craze growth

The higher

Q/R ratio, the

significant T

reduction,

the more

enhanced

craze growth

Polystyrene

(Ref 150)

Silicon oil Smooth

cylindrical

Tension or

tension with

superimposed

hydrostatic

compression

Change in

crazing

behavior

Liquid flow

controlled as

affected by a

stress-state

hydrostatic

compression

facilitates

liquid flow

toward a

crack tip

Type of a

stress state

References cited in this section

142. G.H. Michler, Correlation Between Craze Formation and Mechanical Behaviour of Amorphous

Polymers, J. Mater. Sci., Vol 25, 1990, p 2321–2334

143. A.S. Argon, Sources of Toughness in Polymers, Advances in Fracture Research, Proc. Seventh

Int. Conf. on Fracture (ICF), Vol 4, Pergamon, 1989, p 2661–2681

144. A. Carre and J. Shultz, Polymer-Aluminum Adhesion III. Effect of a Liquid Environment, J.

Adhes., Vol 15, 1983 p 151–162

145. G. Marom and D. Cohn, Deformations of Polymers and Composite under Swelling Conditions,

Plastics & Rubber: Materials & Applications, Vol 5, 1980, p 165–168

146. S.V. Hoa, Relative Influence of the Mobility and the Solubility Parameters of Fluids on the

Mechanical Behavior of High Impact Polystyrene, Polym. Eng. Sci., Vol 20, 1980, p 1157–1160

147. A.G. Atkins and Y.W. Mai, Elastic and Plastic Fracture: Metals, Polymers, Ceramics,

Composites, Biological Materials, Halsted Press, 1985, p 817

148. K.N.G. Fuller and P.G. Fox, The Temperature Rise at the Tip of Fast-Moving Cracks in Glassy

Polymers. Proc. R. Soc. (London) A, Vol 341, 1974, p 537–557

149. M.G. Wyzgoski and G.E. Novak, Stress Cracking of Nylon in Aqueous Salt Solutions. Part 3

Craze-Growth Kinetics, J. Mater. Sci., Vol 22, 1987, p 2615–2623

150. A. Moet and E. Baer, On the Mechanism of Pressure Induced Environmental Stress Cracking in

Polystyrene, J. Mater. Sci., Vol 15, 1980, p 31–41

151. M.E.R. Shanahan, M Debski, F. Bomo, and J. Shultz, J. Polym. Sci., Polymer Physics Edition,

Vol 21, 1983, p 1103–1109

152. J.G. Williams and G.P. Marshall, Environmental Crack and Craze Growth Phenomena in

Polymers, Proc. R. Soc. (London) A, Vol 342, 1975, p 55–77

153. K. Tonyali and C.E. Rogers, Stress-Cracking of Polyethylene in Organic Liquids, Polymer, Vol

28, 1987, p 1472–1477

154. M.E.R. Shanahan and J. Shultz, Environmental Stress Cracking of Polyethylene: Analysis of the

Three Zones of Behavior and Determination of Crack-Front Dimensions, J. Polym. Sci., Polymer

Physics Edition, Vol 18, 1980, p 1747–1752

155. Y.W. Mai and A.G. Atkins, On the Velocity-Dependent Fracture Toughness of Epoxy Resins, J.

Mater. Sci., Vol 10, 1975, p 2000–2003

156. J.C. Arnold, The Influence of Liquid Uptake on Environmental Stress Cracking of Glassy

Polymers, Mater. Sci. Eng. A, Vol 197, 1995, p 119–124

157. H.M. El-Hakeem and L.E. Culver, Environmental Dynamic Fatigue Crack Propagation in High

Density Polyethylene: An Empirical Modelling Approach, Int. J. Fat., Vol 3, 1981, p 3–8

158. W.-Ch.V. Wang, E.J. Kramer, and W.H. Sachse, Effects of High-Pressure CO

on the Glass

Transition Temperature and Mechanical Properties of Polystyrene, J. Polym. Sci., Vol 20, 1982, p

1371–1384

159. A. Peterlin and H.G. Olf, Crazing and Fracture in Crystalline, Isotactic Polypropylene and the

Effect of Morphology, Gaseous Environments, and Temperature, J. Polym. Sci., 1975, p 243–264

160. N. Brown and S. Fischer, Nucleation and Growth of Crazes in Amorphous

Polychlorotrifluoroethylene in Liquid Nitrogen, J. Polym. Sci., Polymer Physics Edition, Vol 13, 1975,

p 1979–1982

161. R.A. Duckett, B.C. Goswami, L. Stewart, I.M. Ward, and A.M. Zihlif, The Yielding and Crazing

Behaviour of Polycarbonate in Torsion under Superposed Hydrostatic Pressure, Br. Polym. J., Vol 10,

1978, p 11–16

162. K. Matsushige, S. Radcliffe, and E. Baer, The Environmental Stress-Crazing and Cracking in

Polystyrene under High Pressure, J. Macromol. Sci. Phys., Vol B11, 1975, p 565–592

163. H.Li. Pugh, Ed., High Pressure Engineering, Mechanical Engineering Publications, 1975, p 41

164. D.R. Mears, K.D. Pae, and J.A. Sauer, Effects of Hydrostatic Pressure on the Mechanical

Behavior of Polyethylene and Polypropylene, J. Appl. Phys., Vol 40, 1969, p 4229–4237

165. E.H. Andrews and L. Bevan, Mechanics and Mechanism of Environmental Crazing in a

Polymeric Glass, Polymer, Vol 13, 1972, p 337–347

166. J.R. Atkinson and P.G. Faulker, J. Appl. Polym. Sci., Vol 15, 1971, p 209

167. G.P. Marshall, G.P. Culver, and J.G. Williams, Craze Growth in Polymethylmethacrylate: A

Fracture Mechanics Approach, Proc. R. Soc. (London) A, Vol 319, 1970, p 165–187

168. M.G. Wyzgoski and G.E. Novak, Stress Cracking of Nylon Polymers in Aqueous Salt Solutions.

I. Stress-Rupture Behaviour, J. Mater. Sci., Vol 22, 1987, p 1707–1714

169. M.C. Kenney, J.F. Mandell, and F.J. McGarry, The Effects of Sea Water and Concentrated Salt

Solutions on the Fatigue of Nylon 6,6 Fibres, J. Mater. Sci., Vol 20, 1985, p 2060–2070

Evaluation of Environmentally Assisted Crack Growth

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

Nanomechanical Testing

Nanomechanical techniques such as nanoindentation and nanoscratch methods have emerged from

conventional hardness testing as described in more detail in the article “Instrumented Indentation Testing” in

this Volume. Nanoscaled depth and load resolution available with the commercial nanoindenters allows probing

of material properties at very shallow penetration depth. During a typical indentation test, an indenter is driven

in and out of a surface under investigation. Resulting load-displacement curves serve as a basis for the

mechanical behavior evaluation (Ref 170). Indentation curve discontinuities may be indicative of plasticity

onset or various fracture events. Additional insight can be gained by examining a residual indentation cavity

(Ref 171, 172, and 173). Here, features of plastic deformation and fracture would be of primary interest. One

instrument, the Hysitron Triboscope, attached to an atomic force microscope (AFM), combines nanoindentation

with the imaging of an indented area. This becomes especially advantageous for the evaluation of time-

dependent phenomena. With nanoindentation, basic elastic-plastic properties such as Young's modulus (Ref

170) and yield stress (Ref 173) can be determined. Indentation-induced cracks allow evaluation of fracture

behavior for bulk materials and thin films (Ref 174, 175, and 176).

During a continuous microscratch test, controlled vertical displacement alongside with the normal and

tangential loads are recorded while an indenter is being moved across a surface under applied load.

Examination of a residual scratch groove provides insight into evolution of plastic deformation and fracture

with increasing load.

Note that for the systems where controlling flaws have a contact origin, nanoindentation and nanoscratch

appear to be the most appropriate evaluation techniques. That is, they account for a specific stress-strain state

around cracks initiated with normal or sliding contact such as environmentally enhanced fretting fatigue.

References cited in this section

170. W.C. Oliver and G.M. Pharr, An Improved Technique for Determining Hardness and Elastic

Modulus Using Load and Displacement Sensing Indentation Experiments, J. Mater. Res., Vol 7, 1992, p

1564–1583

171. T.F. Page, L. Riester, and S.V. Hainsworth, The Plasticity Response of 6H-SiC and Related

Isostructural Materials to Nanoindentation, Mater. Res. Soc. Symp. Proc., Vol 522, 1998, p 113–118

172. W. Zielinski, H. Huang, S. Venkataraman, and W.W. Gerberich, Dislocation Distribution under

a Microindentation into an Iron-Silicon Single Crystal, Philos. Mag. A, Vol 72, 1995, p 1221–1237

173. D. Kramer, H. Huang, M. Kriese, J. Robach, J. Nelson, J.N.A Wright, D. Bahr, and W.W.

Gerberich, Yield Strength Predictions from the Plastic Zone around Nanocontacts, Acta Mater., Vol 47,

1998, p 333–343

174. D.S. Harding, W.C. Oliver, and G.M. Pharr, Cracking During Nanoindentation and Its Use in the

Measurement of Fracture Toughness, Thin Films: Stresses and Mechanical Properties V, Symposium,

Materials Research Society, 1995, p 663–668

175. X. Li and B. Bhushan, Micro/Nanomechanical Characterization of Ceramic Films for

Microdevices, Thin Solid Films, Vol 340, 1999, p 210–217

176. M.R. Begley, A.G. Evans, and J.W. Hutchinson, Spherical Impression of Thin Elastic Films on

Elastic-Plastic Substrates, Int. J. Solid. Struct., Vol 36, 1999, p 2773–2788

Evaluation of Environmentally Assisted Crack Growth

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

Evaluation of Nanoscale Environmental Effects

Here, indentation curve analysis may be combined with the imaging of a residual indentation cavity or a scratch

groove. While both AFM and scanning electron microscopy (SEM) may provide high resolution images, AFM

gives the advantage of quantitative evaluation of nanoscale fracture and plasticity features as discussed in detail

in the section “Scanning Probe Microscopy.”

Environmentally Affected Plastic Deformation. For indentation into dislocation-free single-crystal materials, a

characteristic discontinuity of a load-displacement curve may be indicative of plasticity initiation. Similar

effects may be observed for coarse-grained polycrystalline materials where indentation size is small compared

with the grain size.

With the ex situ testing of sapphire surfaces aged in different environments, clear evidence of chemomechanical

effects was obtained by Hainsworth et al. (Ref 177). Here, presence of a damaged surface layer was suggested

to explain decreased plasticity initiation loads after environmental exposure.

Several researchers have investigated the role of chemical or electrochemical oxide film removal on crystal

plasticity (Ref 178, 179, and 180). With the potentiostatic control (Ref 178 and 180), it is possible to promote

either oxide dissolution or growth by application of appropriate potential steps. It was discovered that

electrochemical oxide film removal resulted in a dramatic decrease of the materials strength. With the

electrochemical repassivation, an ability to support near theoretical strength was returned to the material. An

example is shown in Fig. 33 (Ref 181), where indentation curves correspond to tests under different surface

conditions controlled via applied potential. The yield point is influenced strongly by the presence of a passive

film on titanium (Ref 181). A grade II titanium was vacuum annealed producing alpha grains of approximately

50 μm in diameter. Using the Hysitron Triboscope in conjunction with a Park Scientific Autoprobe CP

scanning probe microscope, individual grains were imaged in a 0.01 N sulfuric acid solution, and a grain large

enough to perform multiple indentations with no overlap was selected for indentation. This sample was then

electrochemically polarized using a three-electrode system, with the titanium acting as the working electrode, a

Ag/AgCl reference electrode, and a platinum counterelectrode. After identifying an appropriate grain,

indentations were made using a Berkovich indenter tip after stepping the potential to 0.68 V above open circuit,

showing reproducible yield points. The tip was then withdrawn 100 μm from the surface, and the passive film

was removed by holding the specimen at 0.03 V above open circuit until the current began to increase

(signifying the removal of the passive film). Indentations at this point show no significant yield point. However,

after stepping the voltage back to the passive regime (0.77 V above open circuit) the yield point is recovered,

and after 5 min the initial and newly passivated surface show almost identical indentation curves.

Fig. 33 Four indentations into grade II titanium in 0.01 N sulfuric acid during electrochemical

polarization. The first indentation was made in the passive regime, at 0.63 V above open circuit (OC) by

stepping to that potential from open circuit. The second was after removing the passive film by holding

at potential 0.03 V above open circuit for about 10 min. The potential was then stepped to 0.77 V above

open circuit, and indentations were made immediately after stepping and after 5 min at that potential.

Source: Ref 181

To investigate hydrogen effects on yield initiation in 316 stainless steel, hydrogen charged and noncharged

samples have been evaluated under indentation load (Ref 182). Reproducible load excursions at an average load

of 200 μN were observed for noncharged samples as shown in Fig. 34(a) (Ref 182). These were attributed to the

plasticity initiation as unloading just prior to an excursion load yielded no residual deformation. For

comparison, indentation testing was carried immediately after hydrogen charging. Here, two samples charged at

different conditions have been evaluated. Sample C1 was charged at 500 mA/m

for 6 h, and sample C2 was

exposed to charging at 10 mA for 6 h. Figure 34(b) (Ref 182) shows indentation testing results for sample C1.

As shown in this figure, yield initiation occurred at 650 to 700 μN immediately after charging. With increasing

time, the load for plasticity initiation decreased down to 300 μN. For sample C2, yield initiation was observed

at 350 μN instantly after charging. Then, the excursion load decreased approaching 200 μN. It became evident

that the yield initiation load increases with increasing initial hydrogen concentrations. Hydrogen outgasses with

time, causing the yield points to decrease back to its initial value. The above would agree with the notion that

hydrogen suppresses dislocation nucleation and/or kinetics.

Fig. 34 Hydrogen effects on yield initiation in 316 stainless steel. (a) Noncharged sample. Curves 1 to 4

represent a sequence of indents over a 35 min time frame. (b) Indentation immediately after charging.

Sample C1 at high hydrogen concentration. Time increases for curves 1 to 5 over a 35 min time frame.

Source: Ref 182

Hydrogen/plasticity interactions were further explored with continuous scratch testing (Ref 181 and 183).

Evaluation in a low-load regime allowed probing of near-surface hydrogen-saturated regions. A marked

difference in both localized plastic deformation and microfracture has been observed as shown in Fig. 35 and

36. Here, both scratch grooves correspond to the same level of applied load. Atomic force microscopy enabled

a quantitative analysis of environmental effects on plasticity and fracture. A comparison of plastic deformation

characteristics for hydrogen-charged and noncharged surfaces as obtained from AFM measurements is shown

in Table 6 (Ref 182 and 183).

Table 6 Hydrogen-charging effects on plastic deformation characteristics

Surface features Noncharged

Charged

Along the pileup

Slip-step spacing (s), nm

100

290

Slip-step height (h), nm 15

Perpendicular to the pileup

Slip-step spacing (s), nm

105

260

Slip-step height (h), nm

13 16

Fig. 35 Plastic deformation features typical for a noncharged sample (see also Fig. 36.) (a) SEM

micrograph. (b) AFM deflection image corresponding to a square area in (a). (c) Cross sections

corresponding to line traces in (b). (d) AFM micrograph showing a magnified view of slip steps. (e) Cross

section corresponding to (d). Source: Ref 182

Fig. 36 Plastic deformation and fracture features typical for a hydrogen charged sample. (a) SEM

micrograph. (b) AFM deflection image corresponding to a square area in (a). (c) Cross sections

corresponding to line traces in (b). (d) AFM micrograph showing a magnified view of slip steps. (e) Cross

section corresponding to (d). Arrows point toward cracks. Source: Ref 182

Crack Initiation. Examples where nanomechanical testing in conjunction with the imaging techniques has been

applied for crack initiation studies in different types of materials are provided in this section.

Metals. With the continuous microscratch under increasing load, crack initiation onset can be determined by

analyzing a cracking pattern along a scratch groove. Location of the first crack is correlated to the magnitude of

the applied load as determined based on the known loading and horizontal indenter motion rates. For the same

load level, microcracking was evident for a hydrogen-charged sample, while there no cracks were seen for a

noncharged sample, as shown in Fig. 35 and 36 (Ref 182). This suggests decreased stress for crack initiation in

hydrogen-charged material.

Ceramics. The initiation of radial cracking in Vickers indentation of soda-lime glass was found to be strongly

rate and environment dependent (Ref 184). With sufficiently long contact times, radial cracking initiated during

unloading. For short contacts, crack initiation occurred after a complete load removal with significant scatter in

delay times. These decreased with increasing peak load and increasing water content in an environment.

Observed phenomena were interpreted in terms of incubation times necessary to develop a critical nucleus for

crack initiation. With SEM, constrained shear fault were identified as possible crack precursors. A proposed

theoretical model considers a two-step process with precursor faulting followed by crack growth to an

instability. Moisture may influence both of these steps: first by enhancing the initiation of decohesion and

second by slow crack growth. A stress-intensity factor analysis for the microcrack extension in residual-contact

and applied-stress fields was used in conjunction with appropriate fracture conditions to determine critical

instability configurations (Ref 185).

Polymers. Indentation-induced cracking in polystyrene (Ref 186) originated at the tip of a yielded zone where a

characteristic shear band pattern has been observed. Based on a slip-line field analysis for the stress distribution

within a plastic zone and elastic analysis for a surrounding material, the principal stresses corresponding to

crack initiation were estimated. These were found to be similar to crazing initiation stresses for this material.

An approach may possibly be extended to evaluate environmental effects on cracking and crazing initiation in

polymers.

References cited in this section

177. S.V. Hainsworth and T.F. Page, Nanoindentation Studies of the Chemomechanical Effect in

Sapphire, J. Mater. Sci., Vol 29, 1994, p 5529–5540

178. S.K. Venkataraman, D.L. Kohlstedt, and W.W. Gerberich, Continuous Microindentation of

Passivating Surfaces, J. Mater. Res., Vol 8, 1993, p 685–688

179. D.F. Bahr, J.C. Nelson, N.I. Tymiak, and W.W. Gerberich, The Mechanical Behaviour of a

Passivating Surface under Potentiostatic Control, J. Mater. Res., Vol 12, 1997, p 3345–3353

180. S.G. Corcoran, S.R. Brankovic, N. Dimitrov, and K. Sieradzki, Nanoindentation of Atomically

Modified Surfaces, Thin-Films—Stresses and Mechanical Properties VII, R.C. Cammarata, M. Nastasi,

E.P. Busso, and W.C. Oliver, Ed., Symposium Materials Research Society, 1998, p 77–84

181. M. Pang, D.E. Wilson, and D.F. Bahr, The Strength and Fracture of Passive Oxide Films on

Metals, submitted to Proc. Materials Research Society, Thin Films: Stresses and Mechanical Properties

VIII, 1999

182. Y. Katz, N.I. Tymiak, and W.W. Gerberich, Nano-mechanical Probes as New Approaches to

Hydrogen/Deformation Interaction Studies, An invited paper for Recent Advances in the Engineering

Aspects of Hydrogen Embrittlement of the Engineering Fracture Mechanics, 1999

183. N.I. Tymiak, S. Gabric-Miskulin, Y. Katz, and W.W. Gerberich, Nanoindentation Evaluation of

Hydrogen Affected Yield and Fracture in 316 Stainless Steel, Int. Conf. Environmental Degradation of

Engineering Materials, Gdansk, Poland, 1999

184. B.R. Lawn, T.P. Dabbs, and C.J. Fairbanks, Kinetics of Shear-Activated Indentation Crack

Initiation in Soda-Lime Glass, J. Mater. Sci., Vol 18, 1983, p 2785–2797

185. S. Lathabai, J. Rodel, T. Dabbs, and B.R. Lawn, Fracture Mechanics Model for Subthreshold

Indentation Flaws. I. Equilibrium Fracture, J. Mater. Sci., Vol 26, 1991, p 2157–2168

186. D. Kells and N.J. Mills, The Initiation of Indentation Fracture in Polystyrene, Philos. Mag. A,

Vol 44, 1981, p 1149–1163

Evaluation of Environmentally Assisted Crack Growth

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

Methods for Small-Volume Testing

Indentation and Continuous Scratch-Induced Cracking. Both cohesive and adhesive fracture are considered

here.

Indentation-Induced Cohesive Cracking. Different systems of cracks may form during loading and unloading

parts of an indentation process. These systems include cone, median, radial, half-penny, and lateral as discussed

in a recent review (Ref 187). Sequential occurrence of different cracking patterns is determined by combination