Striation Counting (JFAP)

TECHNICAL ARTICLE—PEER-REVIEWED

Counting on Fatigue: Striations and Their Measure

P. H. DeVries • K. T. Ruth • D. P. Dennies

Submitted: 8 September 2009 / Published online: 23 February 2010

� ASM International 2010

Abstract Fatigue has been the subject of innumerable

books, papers, and studies since the late 1800s and con-

tinues to be a major factor in component failure with an

estimated 90% of all mechanical failures being attributed to

fatigue (Dieter, Mechanical metallurgy, 1976). The classic

fatigue ‘‘thumbnail’’ often visible to the naked eye may

allege fatigue, but microscopic striations—tiny ridges that

bear immutable witness to cyclical loading—pronounce the

verdict. As such, counting striations should provide the

analyst with valuable information regarding the loading

regimen, the time to failure, and insight into events that

gave birth to fracture. This paper presents both pros and

cons of striation counting and discusses striation count

accuracy. It also presents practical techniques for use in

fatigue striation counting based on laboratory experience

and observation.

Keywords Crack growth rate � Fatigue analysis �Fatigue crack growth � Fatigue cracking � Fatigue failure �Fatigue striations

Introduction

Fatigue is the term given to the time-dependant fracture

growth resulting from repeated or cyclical tensile stress.

Since fatigue is a function of local applied stress levels,

crack initiation and growth can often occur at loads con-

sidered safe for the equipment or component. Local stress

levels can be readily influenced by the presence of notches

or ‘‘stress risers’’ at sharp geometric contours, as the result

of poor machining practices, or due to service-induced

damage. Fatigue can also initiate from crack precursors

such as grinding quench cracks, corrosion pits, or stress

corrosion cracking [1].

Taking a Crack at Fatigue

Most failure analysts are familiar with the classic fatigue

‘‘thumbnail’’ (Fig. 1) often evident on a fatigue fracture

face. Other fatigue features (e.g., flat fracture morphology

or multiple initiation sites with stepped fracture features)

may be so prominent that the analyst may make a confident

failure mode diagnosis without further inspection. How-

ever, absolute confirmation of fatigue comes with positive

identification of striations—those tiny ridges that bear

immutable witness to cyclical loading (Fig. 2).

Down for the Count

From an engineering standpoint, fatigue exhibits no notable

bulk ductility (plastic deformation), thus it can often

propagate with little notice until catastrophic failure

occurs. Detecting, assessing, and preventing fatigue failure

are outside the scope of this paper; however, once failure

has occurred, significant knowledge about the failure may

be gained from fracture surface evaluation.

Since fatigue is brought about by cyclical loading, it

follows that counting striations may provide valuable

information that could be used to assist in design life cal-

culations or risk mitigation. Evaluating striations can help

determine the type of loading regime experienced by the

P. H. DeVries (&) � K. T. Ruth

The Boeing Company, 5301 Bolsa Ave., MC H021-F116,

Huntington Beach, CA 92647-2099, USA

e-mail: [email protected]; [email protected]

D. P. Dennies

The Boeing Company, 5301 Bolsa Ave., MC H019-F781,

Huntington Beach, CA 92647-2099, USA

123

J Fail. Anal. and Preven. (2010) 10:120–137

DOI 10.1007/s11668-009-9320-4

failed component, provide insight into the time frame or

point in process at which failure initiated, or assist in

verifying the accuracy of warranty or injury claims. Having

touted the usefulness of striation counting, just how is this

information gathered and how accurate is it?

Bumps Along the Way: Understanding Striations

Setting the Stage

Before discussing striation counting, it is useful to review

the various stages of fatigue. Fatigue is caused by localized

cyclic plastic deformation [2, p. 242], and the resulting

failure is typically described as occurring in three stages

[2, p. 250].

Stage I: Initiation

Stage I fatigue can comprise over 90% of the total number

of cycles applied. During Stage I fatigue, localized plastic

slip produces intruding and extruding slip steps (Fig. 3) up

to several grains deep. As the number of cycles increases,

the number of slip steps increases forming stress risers

from which microcracking can initiate. These cracks are

essentially featureless and oriented 45� to the primary

tensile load. Multiple slip steps regions can form simulta-

neously, thus fatigue cracks will often nucleate at a number

of locations at the same time. Not all fatigue, however,

initiates from the sample’s surface. Subsurface stress risers

may also result in fatigue initiation (Fig. 4). Resolvable

striations are not typically associated with Stage I Fatigue

[3, p. 15, 4].

Stage II: Crack Propagation

By definition, Stage II fatigue begins when shear planes

give way to fracture oriented normal to the direction of

maximum tensile stress [4] (Fig. 5). As cyclical loading

Fig. 1 Failure in 7075-T73

aluminum forging initiated from

drilled hole (L), fracture

exhibits classic fatigue

‘‘thumbnails’’ (R)

Fig. 2 Fatigue striations in a

magnesium casting (L) and

carbon steel wire (R)

Surface Surface Surface

Fig. 3 Formation of extrusion and intrusion by Stage I Fatigue

loading

J Fail. Anal. and Preven. (2010) 10:120–137 121

123

continues, one or more of the microcracks will reach crit-

ical flaw size and begin to propagate under cyclical

loading. Eventually, multiple initiation sites will begin to

join together producing plateaus or ridges (sometimes

called ratchet marks) to form a common fracture (Figs. 6

and 7). Crack propagation continues as a successive series

of crack fronts until the remaining uncracked cross section

becomes too weak to support the imposed loads and final

rapid fracture occurs.

While Stage I may account for the majority of fatigue

life, striations (and therefore striation counting) are limited

to Stage II crack propagation. Since Stage I is typically

small and most fracture details within approximately

0.010 in. of the origin are often too rubbed, oxidized, and/or

corroded to discern clearly (Fig. 8), the analyst will usually

extrapolate from the closest known area of discernable

striations back to the origin assuming constant striation

spacing. Since striation spacing typically gets smaller closer

the origin, this assumption can lead to an undercount in the

number of striations estimated in crack propagation.

Stage III: Fracture

Stage III occurs during the last fatigue cycle when tensile

stress exceeds the load carrying capacity of the remaining

material section and overload fracture ensues (Figs. 9, 10).

Overload fracture will be ductile, brittle, or a combination

of both [4].

Fig. 4 Fatigue rupture

initiating at a subsurface void,

300 M

Stage 1 Stage 2 Stage 3

Initiation Propagation Overload

Load

Load

Fig. 5 The stages of fatigue

FractureSurface

MaterialSurface

Fig. 6 Plateaus or ridges produced between multiple fatigue fracture

initiation sites

Fig. 7 Micrograph of multiple

fatigue initiation sites in

3Al–2.5V titanium. Right hand

edge represents thin-wall

fatigue fracture

122 J Fail. Anal. and Preven. (2010) 10:120–137

123

Reading the Signs

Equipment or structural loads such as those created by

engine thrust or cabin pressurization may be predictable,

whereas loads produced by gusting winds, turning, braking,

or towing occur randomly. Cracks propagated under each

of these load regimes leave characteristic ‘‘fingerprints’’ to

document their passing.

Constant Amplitude

Constant amplitude loading represents the simplest load

scenario. Loads of this type might occur in rotating or

reciprocating machinery and can be characterized by an

alternating maximum and minimum stress. Minimum

and maximum loading can consist of tensile-tensile, zero-

tensile, or compression-tension regimes. The highly repeat-

able nature of constant amplitude loading creates fatigue

striations which exhibit consistent, even spacing (Fig. 11).

Variable/Spectrum/Random Fatigue Loads

Random, variable, or spectrum loads have no readily dis-

cernable pattern. Correspondingly, the striations left by

such loads will be of randomly varying widths (Fig. 12).

The variable nature of these loads means some cycles will

produce enough stress to propagate the crack with an

associated striation while others may not. Thus striations

may represent only a portion of the actual load history.

Well-Defined Block Spectrum A subset of variable/spec-

trum loading, block loading consists of a repeating load

sequence in which each ‘‘block’’ typically contains a spe-

cific number of constant amplitude or frequency cycles

(Fig. 13) that occur in a particular pattern. Block loading

might occur on rotating machinery or parts subject to a

repeating process such that striation size/frequency docu-

ments load differences at various points in that process.

This approach has also been used to advantage during load

experiments where a load change (block) can be inserted

into the test to document the passing of a predetermined

number of cycles (i.e., marker bands).

Undefined Mixed Spectrum Mixed loading is also a sub-

set of variable/spectrum loading in which one variable load

is interspersed with another (Fig. 14). This type of loading

might occur where two processes are interspersed such as

might occur during machinery start and shut down, or in

aircraft ground loads versus airborne loads.

Striation Counting Ups and Downs

The Ups

Striations remain the best evidence for identifying and

confirming the presence of fatigue. In addition to the

benefits mentioned earlier, striation counting can:

• Be used to measure crack growth rates

• Help estimate the number of initiation cycles if the total

number of cycles is known [5].

• Help determine when failure initiated (e.g., in produc-

tion or in service).

• Can be used to generate crack growth curves for

estimating inspection intervals and service life esti-

mates [6, p. 70].

Fig. 8 Fracture origin exhibiting heavily rubbed striations, 2014-T6

aluminum forging

Fig. 9 Fatigue to dimpled rupture transition in 7075-T73


123

Fig. 10 Ductile overload

failure (dimple rupture) in 7050-

T7451 Plate (L) and 321 CRES

(R)

Constant Amplitude Loading

Load

Time

10um

Fig. 11 Constant amplitude

load graph and fatigue striations

in 7075-T73 aluminum

Variable/Spectrum Loading

Load

Time 3 um

Fig. 12 Striations resulting

from spectrum loading

Block Load Spectrum

Load

Time 30um

Fig. 13 Block loading graph

and striations in titanium


123

• Define the loading mode encountered and determine

when and where load changes occurred.

• Correlate the load history with respect to stated

operational history

• Can be used to estimate stress levels if da/dN data is

available for the material

The Downs

There are many circumstances in which striations are not

generated or which lead to inaccurate striations counting.

These circumstances include:

• Stage I (fatigue initiation) does not typically produce

fatigue striations, thus striation counting alone cannot

determine Stage I cycles.

• Where striations have been damaged (rubbed, oxidized,

corroded, or fretted) counting may not be possible.

• There is not always a one-to-one correlation between

load cycle and striation formation. Spectrum load

cycles below the stress level critical for crack propa-

gation will not produce a striation and will not be

included in the count.

• Extreme high-cycle fatigue can produce high-striation

densities which cannot be clearly resolved making

accurate counting difficult.

• Fatigue fracture may include regions of unstable

propagation—regions in which fatigue striations are

mixed with dimple rupture or cleavage. The analyst

must subjectively determine to evaluate dimple rupture

portions as striations, or evaluate striation portions as

dimple rupture.

• As crack depth increases and local stress levels rise,

striation spacing tends to increase, thus striation count

is affected by location. Striation counting methodology

must address this trend to maximize count accuracy.

• Ideally, striation counting is done across a flat surface.

Since most fracture surfaces are not entirely flat,

striations counted on an angled surface will be

foreshortened thereby artificially reducing striation

spacing.

• As the crack front grows stresses may reorient or

redistribute to accommodate geometrical, load, or

microstructure variations. Thus, fracture may grow

faster or slower in one region than in another in order to

maintain a constant stress level at the propagating crack

tip. This can produce variations in striation density at a

given distance from the origin depending on the

direction of counting.

• The analyst may encounter striation-like features (false

striations) which are not fatigue related.

Counting Math or Magic

Given the many factors which impact count accuracy,

striation counting may seem more magic than math. A

recent experiment which tested a titanium tee section in

fatigue (Fig. 15) offers some insight into the correlation

between striation counting and instrumental data. The test

evaluated titanium tee samples subjected to two different

loading regimes each consisting of a combined tensile and

bending load. The total number of cycles was known, and

acoustic emission (AE) was used to determine the number

of cycles required to initiate fatigue (Stage I fatigue). By

counting fatigue striations (Stage II fatigue) and sub-

tracting this number from the total number of cycles

applied, a comparison could be made between crack ini-

tiation as determined by AE and as calculated by striation

counting. Conversely, by knowing the total number of

applied cycles and subtracting the number of cycles

required to initiate fatigue as determined by AE, the

number of propagation cycles could be determined and

compared with those obtained by striation counting

(Fig. 16).

AE techniques can measure transient elastic waves

produced by fatigue when energy is suddenly released

through particle fracture and slip or dislocation move-

ments. Fatigue crack initiation is identified by observing

changes in such things as acoustic wave amplitude, form,

and rate. While AE can detect crack initiation, precisely

identifying/defining the Stage I–Stage II transition (i.e., the

precise point at which a crack front actually begins to

Load

Time Mixed Load Spectrum

AA

B

A

BFig. 14 Mixed load spectrum.

A Small bands and B large

bands


123

propagate) may be difficult. This subjectivity can result in

possible over-counting or under-counting of actual initia-

tion cycles. In addition, the AE equipment was not directly

synchronized with the load frame load controller. This

independence may have resulted in some ‘‘drift’’ between

load control and AE cycle counts. Despite these factors,

reasonable correlation between the two methods was

observed. Variation between initiation (Stage I) cycles as a

percentage of total cycles as counted by AE and calculated

by striation counting ranged from\1% to as much as 22%

with striation counting typically predicting fewer crack

initiation cycles and more crack propagation cycles than

AE. Stage I fatigue represented approximately 67–96% of

all applied cycles while Stage II fatigue represented

approximately 3–33% of all applied cycles. The striation

counting method used during this analysis is illustrated in

Case Study #1.

A Practical Approach to Striation Counting

Fracture Face Fundamentals

Many variables such as material hardness, changing load

distribution, metallurgical or microstructural changes, and

environmental conditions can affect crack propagation

through a material. While no two fractures are identical,

crack fronts tend to fall into one of two broad categories:

single crack fronts and integrated1 crack fronts. Single

crack fronts arise from a localized area consisting of one or

a few closely spaced initiation sites which join to propagate

a crack in a more or less continuous semi-circular or semi-

elliptical format through the material. Several assumptions

are made when evaluating striations in a single crack front:

(a) That secondary cracks within the localized area

initiated concurrently.

(b) The largest (or primary) single crack represents the

location of highest localized stress.

(c) That the area of highest localizes stress contains the

most complete crack propagation/striation record and

is therefore the best location to perform striation

counting.

These fracture fronts may be entirely contained in the

material cross section, may penetrate the cross section, may

have nearby secondary fracture fronts, or may exhibit rapid

lateral expansion when compared to fracture depth

(Fig. 17). In these cases, striation counting should take

place along the longest, straightest uninterrupted fracture

dimension—which is most often into the material thickness

Fig. 15 Test setup

0

10

20

30

40Smpl 1

Smpl 2

Smpl 3

Smpl 4

Smpl 5Smpl 6

Smpl 7

Smpl 8

Smpl 9

Propagation Cycles by AE Propagation Cycles by SEM Count

Fig. 16 Propagation cycles as predicted by AE versus propagation

cycles as counted by SEM as a percentage of total cycles

1 The term ‘‘integrated’’ is used here to describe a large number of

crack origins spaced closely together.


123

(Fig. 17 dimension ‘B’).2 Striation counting along the edge

of the primary fracture (Fig. 17 dimension ‘A’) can be

done; however, striations along the surface may be dam-

aged or poorly formed making counting difficult. Where

fracture penetrates the material thickness, striation count-

ing takes place in the ‘B’ direction through the material

thickness and then recommences along the center of the

material in the ‘A*’ direction as shown in Fig. 17(b).

A special case arises in fractures which exhibit a single

‘‘Thumbnail’’, but which contain independent fracture

fronts with no straight uninterrupted fracture path

(Fig. 17e). Thus counting along the ‘A’ dimension may be

the only location in this situation where meaningful stria-

tion counting can be done.

Integrated crack fronts arise from a surface containing

numerous initiation sites (ratchet marks) which quickly

join and propagate as a continuous band (Fig. 18). Striation

counting must be done into the material thickness as the

proximity of the initiation sites to each other prevents any

meaningful striation counting along the surface. Where

reverse bending occurs, a secondary crack front may ini-

tiate from the opposite side of the material. A line of

dimple rupture will typically be found separating the two

crack fronts. Like single crack fronts, several assumptions

are made for integrated crack fronts.

(a) That all cracks along the integrated crack front

initiated concurrently.

(b) Where opposing crack fronts exist, the deeper crack

front initiated first and is considered the primary

crack front.

(c) Striation counting is done through thickness as shown

by the ‘B’ dimension in Fig. 18.

Simply Sampling

Sample preparation for striation counting is similar to that

used for routine SEM imaging. Sample cleaning by repli-

cation, material compatible stripping compounds, or

ultrasonic cleaning is recommended as is demagnetization

of ferritic samples. Sputter coating the sample surface will

help reduce charging from tenacious debris and oxides. An

acceleration voltage of 5–10 keV is recommended for

lower atomic weight materials such as aluminum or mag-

nesium [6, p. 65] while 20 keV is suggested for higher

atomic weight materials such as iron or nickel base alloys.

A working distance of 15 mm is suggested to help maxi-

mize image sharpness.

Preparations for counting will vary depending on SEM

requirements but typically follow the routine outlined

below:

• Strip loose debris from the fracture surface by

replication.

• Identify the fracture origins by using fatigue ‘‘thumb-

nails,’’ fracture traces, and/or noting the fracture

‘‘steps’’ along the surface of the component.

A

B

(a) (b)

(c) (d)

(e)

A

B

A*

A

B

Primary Secondary A

B

A

Fig. 17 Typical single crack

front regions. (a) Contained

single crack front; (b) through-

wall crack front; (c) primary

crack front with adjacent

secondary crack front(s);

(d) Laterally expanding crack

front; and (e) discontinuous

crack front

Primary Fracture

Secondary Fracture

Dimple Rupture

B B

Fig. 18 Reverse bending fatigue with multiple initiation sites

producing an ‘‘integral’’ crack front

2 The angle of ‘‘B’’ dimension need not be 90� to the material

surface.


123

• Identify the primary initiation site. Be aware that

fracture origins in plated surfaces or heavily rubbed/

corroded components may not be readily identifiable.

• Photograph the primary initiation site at 20–1009. Note

any scratches, pits, burns, imbedded particles, or other

surface damage associated with the origin (example:

grit blast particles, arc strikes, or inclusions).

• Perform energy dispersive spectroscopy of the origin to

determine if foreign materials are present.

Counting on Striations

Unless one is dealing with very low cycle fatigue, counting

every striation will not be practical. Striations near the

origin are likely to be smeared, oxidized, and finely spaced

while those near the end of the fatigue region may be better

preserved but also may be intermixed with dimple rupture.

Striation counting is done on photographs taken at conve-

nient intervals along a line running from the initiation point

to the furthest reaches of the fracture zone. Magnifications

of 2,0009 to 7,0009 are typical, although high-cycle

fatigue may require magnifications of up to 20,0009 to

resolve. Resolution at higher magnifications may be limited

by either equipment capabilities or poor striation quality.

One may find counting easier if one begins at the extreme

region of the fracture zone and works back toward the

initiation point.

The counting process generally progresses along the

following sequence:

• Survey the primary fatigue fracture area to determine

where fatigue ends and overload begins. Examine the

striations as close to the initiation point and as far from

the initiation point as possible. If the spacing is

significantly different, multiple areas will need to be

counted to ensure accuracy. If unstable regions of

fatigue mixed with dimple rupture exist, the analyst

must decide where counting will stop.

• Orient the sample in the SEM such that the straightest,

longest uninterrupted fracture front is parallel to the X- or

Y-stage axis to facilitate easy distance measurements.

• Measure the distance from the origin to the point where

fatigue stops and divide it into convenient intervals.

Striation spacing at the beginning of the interval should

not differ significantly from those at the end of the

interval.

• Begin by locating and measuring the distance from

the origin to the nearest stable growth region using

the SEM stage micrometer. Mark the location on the

overall photograph.

• Photograph a typical striation region within the interval

at a magnification high enough to resolve individual

striations. Be sure to include a micrometer bar on the

photo. Six to ten striations should be clearly discern-

able. Actual striation counting may be done directly on

the photograph, or if the striations are still difficult to

discern, the photograph can be enlarged on the

computer screen using image processing software and

the striations marked on the digital image. The photo

can then be printed out in a convenient size and the

marked striations measured using the micron bar

attached to the photo.

• Obtain striation counts from several areas in the

interval. The striation counts will be averaged and the

average reported as the striation count for the interval.

• Move to next location and repeat until representative

photos have been obtained from each interval.

• Using the micron bar on the photos, measure the

distance between six to fifteen striations. Divide

striations counted by the distance the number of and

Sample 3 Fracture Zone Sketch

Origin 1 Showing Striation Counting Direction

2.6”

Fatigue Fatigue O/L O/L

Origin 1 Origin 2 1.0”

0.19-1.5” Origin 1

O/L Fatigue

0.01-0.16”

Fig. 19 Fatigue sample 3


123

report the count for that photo in terms of striations per

unit length.

• A plot of the striations per inch as a function of crack

depth can be made. The area under the curve is an

estimate of the total number of cycles the crack has

propagated [5].

• One can estimate the number cycles to initiation if the

number of propagation cycles and total number of

cycles are known.

Document all assumptions.

Case Study #1: Striation Counting in a Titanium Test

Sample Under Constant Amplitude Loading

Sample 3 (Fig. 19) from the fatigue testing described in

Striation Counting Math or Magic was forwarded for stri-

ation count following complete through-wall failure. As

shown in the fracture zone sketch, cracking proceeded

through the web thickness first, followed by cracking along

the web. In this case, to capture the number of cycles

required to produce through-wall penetration, striation

counting proceeded through thickness first as represented

by the counts recorded for the distance of 0.01–0.16 in. in

Table 1. Counting then proceeded along the length of the

web fracture from 0.19 to 1.5 in. at which point dimple

Table 1 Striation counting sample 3

Distance (in.)

Ave. striation

size (in./striation)

Number

of striations

Through thickness

0.00–0.01 N/A (subsurface origin) N/A

0.01–0.03 2.8 9 10�6 7,140

0.03–0.05 3.7 9 10�6 5,410

0.05–0.07 5.9 9 10�6 3,390

0.07–0.09 4.0 9 10�6 5,000

0.09–0.11 5.6 9 10�6 3,570

0.11–0.13 6.9 9 10�6 2,900

0.13–0.16 5.0 9 10�6 6,000

Along web

0.19–0.30 2.5 9 10�5 4,400

0.30–0.40 4.8 9 10�5 2,080a

0.40–0.60 5.9 9 10�5 3,390a

0.60–0.80 1.2 9 10�4 –b

0.80–1.00 2.0 9 10�4 –b

1.00–1.50 4.7 9 10�4 –b

Total 43,280

a A small amount of dimple rupture contributed to crack extension;

however, these small regions were ignored for this countb Fatigue was present; however, there was too much dimple rupture

to produce a meaningful count

....

....

...

..

....

...

....

...

...

...

....

....

...

..

0.01”–0.03” : 2.8x10-6 inch/cycle 0.60”–0.80” : 1.2x10-4 inch/cycle

1.00”–1.50” : 4.7x10-4 inch/cycle

100 um

40 um2 um

Fig. 20 Striation counts at

selected locations, fatigue

sample 3


123

rupture became predominant. The fatigue portions of the

fracture were divided into sections from which represen-

tative photographs (Fig. 20) were taken. One or more

striation counts were made on each photograph and an

average striation count calculated for the region (see

Table 1). The region 0.00–0.01 in. was not counted as

crack initiation had occurred subsurface.

A Rough Count

While striation counts may be desired, fractures are

often damaged in some fashion limiting counting oppor-

tunities. Examples of some of these damage mechanisms

are summarized below.

Spectrum Loading/Load Changes

Spectrum loading is readily identifiable by random striation

spacing. Generally, no attempt is made to determine indi-

vidual striation spacing as the count is divided over the

length of the count area and reported as an average

(Fig. 21). When counting mixed load or block spectrum,

the counting approach is determined on a case by case basis

depending on the width of the band(s) and how often the

bands appear. A single, narrow band can be noted and

documented, but need not be counted separately. Where the

bands are wide or occur repeatedly, they will need to be

counted separately and their contribution to the interval

determined (Fig. 22).

Poorly Defined Striations

Rubbing or oxidation of the fracture faces may obscure

fatigue striations such that they cannot be readily counted

(Fig. 23). Where rubbing and oxidation are limited to the

origin, the analyst can move to the closest area where

striations can be resolved and count there, extrapolating

this count back to the origin. Despite poor striation defi-

nition in the 3Al–2.5V titanium alloy seen in Fig. 24, bands

of adjacent, parallel secondary cracking identify this frac-

ture as fatigue.

Fig. 21 Striation counting of spectrum fatigue striations

Fig. 22 Counting block or mixed load striations

Fig. 23 Rubbed, oxidized, and fine-spaced striations in 17-4 PH that

exhibited evidence of spectrum l loading


123

Obliterated Fractures

Some fractures are so badly damaged that most of the

striations are smeared, obliterated, or otherwise indiscern-

ible. In such cases, small regions of undamaged striations

may remain at the furthest extreme ends of the fracture.

Low spots or areas adjacent to sharp changes in topography

may also provide sufficient protection to preserve striation

‘‘islands’’ where counting can be performed (Fig. 25).

Where the fracture surface has been destroyed, such as

by grit blasting (Fig. 26), striation counting cannot be done

and the failure mode may have to be determined purely

from macrofeatures alone. In this case, the presence of an

apparent ‘‘thumbnail’’ morphology (Fig. 26, photo insert)

and fracture traces emanating from the corner of the web

suggest possible crack initiation by fatigue. The remainder

of the crack appears to have propagated by fast fracture.

Mixed Mode and Unstable Regions

Mixed mode regions, also known as partially unstable

regions, contain striations mixed with dimple rupture

indicative of momentary overload conditions. These

regions often appear at the end of fatigue as failure nears

Stage III Fracture and typically contain poor defined stri-

ations characterized by secondary cracking (Fig. 27). The

analyst must determine the point at which overload domi-

nates the failure mode (or the point at which striations

become too poorly defined) and end fatigue counting at that

location. Striation counting can proceed normally up to that

point with dimple rupture zoned being considered as fati-

gue. Fatigue zones occurring beyond that point are

Fig. 24 Poorly defined striations in a 3Al-2.5V titanium fatigue is

recognizable by bands of secondary cracking

Fig. 25 Small discernable striation ‘‘island’’ found on a badly

smeared fracture

“Thumbnail”

Shear Lip

FastFracture

Fig. 26 Fracture obliterated by grit blast is evaluated from the overall

fracture topography

Fig. 27 Mixed fatigue and dimple rupture in 300 M high strength

steel (encircled areas indicate dimple rupture)


123

considered dimple rupture under the assumption that this

method effectively evens out the count. The total crack

length in which fatigue is actually observed should be

noted and reported that the customer is aware that fatigue

continued even though striation counting stopped.

Case Study #2: Counting Through Dimple Zones

As noted earlier, fatigue fractures can exhibit striations

mixed with increasing amounts of dimple rupture as the

fracture nears Stage III Failure. In cases of random loading,

0.39”

0.13”

0.46”0.61”

1.11”1.22

0.12”0.32” 0.36”

4 Intervals (Stable)

2 Intervals

1 Interval

PrimaryFracture

SecondaryFracture

Fatigue Interspersed with Dimple Rupture

Dimple Rupture

Fig. 28 Counting through well-

defined dimple rupture bands

...........

.....

....

......

......

..........

....

..........

.......

....

.

0.00 – 0.03 inch; 6.5 x 10-6 inch/cycle 0.13 – 0.26 inch ; 2.5 x 10-5 inch/cycle

1.11 – 1.22 inches ; 2.5 x 10-5 inch/cycle

10 um4 um

10 um

Fig. 29 Striation counting at

various locations


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fatigue may be interspersed with bands of intermittent

dimple rupture (Fig. 28). Where bands are fairly well

defined, striation counting can continue through these

bands by simply noting the width of the band and where

they occur. Figure 29 shows typical striations found on this

fracture at selected locations listed in Table 2.

Case Study #3: Unexpected Mixed Load in a 321 CRES

Flex Hose

Type 321 CRES flex hoses (Fig. 30) installed in an aero-

space application were suffering high rates of failure in

various fluid systems after system maintenance. As a result,

a number of flex hoses were tested to failure under a variety

of mechanical impulse pressurization and flexing cycle

combinations to determine if failure was due to operational

loads or maintenance procedures.

Flex hose sample 4E underwent flex testing only (no

pressurization loads) and was subsequently submitted for

striation counting to characterize flextural fatigue damage.

Primary fracture had initiated from the flex hose exterior

exhibiting the characteristic ‘‘integrated crack front’’ mir-

rored by secondary cracking that initiated from the hose

interior. Both crack fronts progressed through-wall until

they met, at a final fracture ridge consisting of overload

(Figs. 31 and 32). Confusion arose when a band of fine-

spaced striations typical of those created by impulse

loading were found at the end of the wide-spaced striations

generated by flexing (Figs. 33, 34, and Table 3). These fine

striations were attributed to vibration generated when the

flex hose was sectioned for inspection. The investigation

demonstrated that striation spacing could be used to dif-

ferentiate between flex induced fatigue and that generated

by impulse loading and also showed that fatigue data can

be misleading if part loading history is unknown. Alter-

natively, the band of fine-spaced striations may well have

been missed entirely had the analyst failed to examine the

striations at high magnification.

Castings

The dendritic microstructure found in castings tends to

produce a fracture morphology that is more difficult to

Table 2 Striation count through dimple rupture bands

Upper section striation count

Dist. from hole (in.)

Ave. striation size

(in./striation)

Number

of striations

0.00–0.03 6.5 9 10�6 4,615

0.03–0.06 6.7 9 10�6 4,480

0.06–0.09 7.6 9 10�6 3,950

0.09–0.13 1.2 9 10�5 3,335

0.13–0.26 2.5 9 10�5a 5,200

0.26–0.39 2.7 9 10�5a 4,815

0.39–0.46 Dimple rupture –

0.46–0.61 2.2 9 10�5a 6,820

0.61–1.11 Dimple rupture –

1.11–1.22 2.5 9 10�5a 4,400

Total 37,615

a Small bands of dimple rupture were interspersed within these

fatigue regions; however, the small dimple regions were ignored for

this count Fig. 30 Typical flex hose assembly

Fig. 31 Flex testing fracture

axial section illustrates the tube

convolutes (L) and transverse

fracture through the convolute

root (R)


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evaluate when compared with wrought materials. Macro-

scopic features such as ‘‘thumb nails,’’ fracture traces (i.e.,

chevron marks or herring bones), and beach marks com-

monly seen in wrought material fracture are often missing

from cast material fractures making location and identifi-

cation of fracture origins and crack propagation direction

difficult. Fracture features may be interspersed with

porosity, shrinkage, and/or second-phase particles that

complicate analysis further. By modifying the analysis

Wall thickness = 0.007”

Tube ID

Tube OD

500 um 100 um

0.0012”(Secondary)

0.0045”(Primary)

Overload

Fig. 32 Close-up of convolute

root fracture showing primary

and secondary cracking

Fig. 33 Unexpected fatigue

band at the lateral end of the

primary crack the band was

approximately 0.000700 thick

Fig. 34 The fatigue band contained very fine, poorly resolved

striations

Table 3 Crack front striation counting

Load

Distance from

origin (in.)

Ave. striation size

(in./cycle)

Number of

striations

Flextural cycling 0.0000–0.0006 2.2 9 10�5 27

0.0006–0.0019 3.1 9 10�5 42

0.0019–0.0032 6.3 9 10�5 21

0.0032–0.0045 1.3 9 10�4 10

Total cycles 100

Band 0.0045–0.0052 5.7 9 10�6

Total cycles 123


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approach, these challenges can often be overcome per-

mitting striation counting to be performed on fractured cast

material.

Sample cleaning and preparation will be similar to

those used for wrought materials and an overall photo of

the fracture will once again prove useful. As there may be

no fracture traces leading back to the origin, the analyst

may have to ‘‘spot check’’ various locations around the

fracture face to identify the boundary between dimpled

rupture and fatigue (Figs. 35–37). Careful SEM exami-

nation at higher magnifications should be able to resolve

fatigue striations and their orientation can be then used to

identify the fracture propagation direction. Once the

fracture and its origins have been ‘‘mapped’’ on the

overall fracture photo, the analyst can chose the best line

of inquiry along which striation counting will be made.

Counting can proceed in the same manner described for

wrought materials. Small voids, porosity, and secondary

particles can be ignored while larger voids, particles or

porosity will need to be documented in the manner pre-

viously discussed when counting areas interspersed with

dimpled rupture bands.

Fig. 35 The fracture origin exhibits a dendritic structure typical of

shrinkage porosity, K01-T7 aluminum sand casting

Fig. 36 Slow growth fatigue rupture in a K01-T7 aluminum sand

casting

Fig. 37 Dimple rupture interspersed with second-phase particles in a

K01-T7 aluminum sand casting

Fig. 38 Striation-like features created by machining


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False Striations

It has been said that ‘‘if one looks hard enough, striations can

be found anywhere.’’ False striations can be generated by

such things as machining (Fig. 38) or weld solidification

(Fig. 39). Smearing, chatter (Fig. 40), and slip/stretch marks

(Fig. 41) seen on fracture surfaces can often look like fati-

gue. Interaction between elastic pulses and running crack

fronts can produce striation-like Wallner lines on the cleaved

surfaces of brittle materials. Alternatively, striation-like

artifacts can appear on SEM generated images due to the

influence of vibration or magnetic fields (Figs. 42 and 43).

False striations can be separated from true fatigue by

inspecting the surrounding grains. Fatigue should exhibit

continuity with and between adjacent grains since fatigue

rarely occurs as isolated ‘‘islands.’’ Striation marks must

be consistent with the general direction of fracture propa-

gation. Knowledge of the manufacturing processes

and service history help ensure proper identification of

striation-like features.

Fig. 39 Striation-like solidification marks on a 321 CRES weld

Fig. 40 Chatter marks/tire

tracks in a tin–lead solder joint

failed by creep

Fig. 41 Striation-like slip/stretch marks in lab-induced overload


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Case Closed

From a practical perspective, striations remain the best

evidence for identifying and confirming the presence of

fatigue. By measuring and counting striations, valuable

information regarding crack growth rates and load levels

can be obtained and inspection intervals and service life

estimates can be made. Striation counting can provide an

accurate accounting of fatigue propagation cycles, however

that accuracy is not guaranteed as results are typically

estimates based on averages and limited by the fracture

condition, topography and imaging capabilities. Variations

upward of 22%, as shown in Case Study #1, may exist

between actual fatigue propagation cycles and those pre-

dicted by striation counting. Thus, it is important to discuss

with the customer the accuracy and limitations of the

counting process and that striation counting data should be

used with proper caution.

References

1. Jacoby, G.: Fractographic methods in fatigue research. Exp. Mech.

65 (1965)

2. Barsom, J.M., Rolfe, S.T.: Fracture and Fatigue Control in

Structures, 2nd edn. Prentice-Hall, NJ (1987)

3. LeMay, I.: Failure mechanisms and metallography. In: McCall,

J.L., French, P.M. (eds.) Metallography in Failure Analysis, p. 16.

Plenum Press, Houston, TX (1977)

4. Laird, C.: Fatigue Failures. In: Boyer, H.E., Gall, T.L., (eds.)

American Society for Metals Handbook Desk Edition, p. 32-16.

ASM, Metals Park (1985)

5. Cherolis, N.E.: Fatigue in the aerospace industry: striations, J. Fail.

Anal. Preven. 8, 255–258 (2008)

6. Russo, M.: Analysis of fractures utilizing the SEM. In: Metallog-

raphy in Failure Analysis, p. 70. Houston, TX (1977)

Fig. 42 Striation-like image artifacts caused by vibration (Reference/

Source Pittinato, G.F., Kerlins, V., Phillips, A., Russo, M.A., SEM/

TEM Fractography Handbook, AFML-TR-75-159, p. 38, Dec 1975)

Fig. 43 Striation-like image artifacts caused by magnetic

interference


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Striation Counting (JFAP)

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