Application of ultrasound for fatigue testing of lightweight alloys

H . R . M AY E R , H j . L I P OW S K Y 1, M . PA PA K Y R I AC O U, R . R O S C H 1 , A . S T I C H 1 a n d S. S TA N Z L - T S C H E G GUniversity of Agricultural Sciences, Institute of Meteorology and Physics, Turkenschanzstraße 18, A-1180 Vienna, Austria, 1AUDI AG, Department ofEG-34, D-85045 Ingolstadt, Germany

A B S T R A C T The use of aluminium and magnesium alloys offers a great potential for weightreduction in automotive applications. Load-bearing car components are subjected to108 cycles and more during service, and the high-cycle fatigue properties of constructionmaterials are therefore of great interest.

The time-saving ultrasound fatigue testing method has been used to study the fatigueproperties of a high-pressure, die-cast magnesium alloy AZ91 hp and a post-forged,cast-aluminium alloy AlSi7Mg0.3 in ambient air and saltwater (5wt% sodium chloride)spray. In ambient air, fatigue cracks in AZ91 hp emanate from voids, and it is possibleto correlate void areas with the numbers of cycles-to-failure. Post-forging of AlSi7Mg0.3reduces the numbers and size of voids. The remaining small voids (void areas smallerthan 9000 mm2) do not significantly reduce lifetimes. Saltwater deteriorates the fatigueproperties of both the lightweight alloys. With increasing numbers of cycles, theinfluence of the corrosive liquid on fatigue strength becomes more pronounced.

Keywords high-cycle fatigue; ultrasonic frequency; endurance limit; cast lightweightalloys; AlSi7Mg0.3; AZ91.

N O M E N C L A T U R E KImax=maximum mode I stress intensity factorKIth,eff=effective mode I threshold stress intensity factor

smax=maximum stress amplitude

obtained. Cyclic plastic deformation and associatedI N T R O D U C T I O N fatigue damage of technical metallic alloys in inert

environments is not (or only moderately) influenced byThe use of lightweight metallic materials with highstrength-to-weight ratios makes weight reduction and the cycling frequency, if cyclic plastic deformation is

small.4 Therefore, ultrasonic fatigue testing allows onefuel saving in automobiles possible. Structural parts ofthe engine, power transmission and car-body may be to investigate high-cycle fatigue properties of materials

within short testing times. For example, 109 cycles maymade of aluminium and magnesium alloys.1 Fatigueproperties of lightweight materials are relevant for such be imposed on a specimen in a single day. This work

describes the results of fatigue experiments on two newcomponents as cyclic loads rather than constant stresscause failure in automotive components. Car components materials which are developed for potential use in auto-

motive applications (high-pressure, die-cast magnesiumhave to survive a relatively high number of load cyclesduring service. A car wheel, e.g. is exposed to #3×108 alloy AZ91 hp), or which are used for car components

(post-forged, cast-aluminium alloy AlSi7Mg0.3) already.cycles during a lifetime of 300 000 km. The fatigueproperties of new engineering materials in the regime In practical applications, automotive components are

not cycled in inert environments but corrosive influencesof very high numbers of cycles are therefore of greatinterest. have to be considered. The fatigue properties of alu-

minium alloys become worse if the specimens are cycledTesting times and costs to investigate high-cyclefatigue properties of new materials may be saved if the in ambient air instead of a vacuum.5,6 Even at ultrasonic

frequency, cycling of a high-strength aluminium alloycycling frequency is increased up to 20 kHz. Ultrasonicequipment has achieved a high technical standard during (2024 T3) in ambient air showed material embrittling

and accelerated fatigue crack growth for crack growththe last years,2,3 so that reliable results for materialresearch and their comparative characterization can be rates below #10−9 m/cycle.7 Comparing crack growth

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and threshold behaviour in humid and dried air showed Former investigations showed that the S–N curve ofchromium steel AISI 403 which was tested in 22wt%that water vapour in ambient air which leads to hydrogen

embrittlement is responsible for the resulting higher sodium chloride solution at ultrasonic frequencies wassimilar at cycles-to-failure above 107 to that obtained atgrowth rates and the lower threshold in comparison to

vacuum. The mechanism involved is surface diffusion of conventional frequencies.14,15 Similarly, no significantdifferences in lifetimes were found when chromium steelhydrogen into the material in front of the crack tip.

Fatigue crack growth rates for conventional and ultra- AISI 630 and titanium alloy TiAl6V4 were tested atconventional and ultrasonic frequencies in saltwater.14sonic frequency testing are similar if the fatigue crack

growth is slower than the diffusion process, i.e. 10−9 In another study, endurance experiments were performedwith a low-pressure, permanent mould-casting material,m/cycle.7,8 Similarly, the formation of a layer of reaction

products on newly formed fracture surfaces is possible AZ91 hp (heat treatment T4), at low and ultrasonicfrequencies in ambient air and in a saltwater spray.16 Inunder ultrasonic cycling if high-cycle fatigue experiments

are performed and fatigue crack growth rates are in the ambient air, the S–N curves determined at 50 Hz and20 kHz coincided within the range of scatter. The S–Nthreshold regime.7,8

A technically important corrosive fluid for automotive curve determined in saltwater spray at a cyclic frequencyof 50 Hz, however, showed #10 times lower lifetimesapplications is aqueous sodium chloride solution.

Saltwater acts as a corrosive fluid for aluminium and in comparison to ultrasonic cycling test data.magnesium alloys, and fatigue lifetimes can be signifi-cantly shortened in comparison to those in ambient air.

M A T E R I A L A N D E X P E R I M E N T A L P R O C E D U R ESeveral interactions between the process of cumulativefatigue damage and an aggressive environment are poss-

Materialsible.9 Dissolution of the exposed surface and the forma-tion of corrosion pits may cause stress concentration. A Fatigue experiments were performed with the high-

pressure, die-cast magnesium alloy AZ91 hp and thecorrosive attack at slip steps leads to enhanced slipirreversibility. Microcrack formation is enhanced due to post-forged, cast-aluminium alloy AlSi7Mg0.3. The

AZ91 hp material was obtained from a bar of 7.5 mm,preferential electrochemical attack in regions of localizedplastic deformation and at places where a protective film 1.25 mm was removed from both sides of the bar to

obtain the final specimen thickness of 5 mm. Figure 1(a)is ruptured. In addition, a reduction of surface energyincreases microcrack growth rates.10 Embrittling of the shows the specimen shape used in the present fatigue

tests.material at the crack tip leads to corrosion fatigue, andcauses increased crack growth rates and a lower threshold AlSi7Mg0.3 (A356.0) was produced by the so-called

CobapreßA-technique, i.e. structural parts are producedstress intensity factor.11

Enhanced crack formation and crack growth leads to by permanent mould casting and forged afterwards toshorter lifetimes in comparison with inert environments.In order to predict lifetimes of components in a changingor aggressive environment (e.g. saltwater), it is necessaryto perform fatigue experiments under conditions that areclose to the technical application.12 Such experiments,however, are very time consuming and expensive. As a firststep, it is necessary to compare possible constructionmaterials and to find an appropriate alloy with favourablefatigue properties. It may be expected that ultrasonicfatigue tests in corrosive fluids render different results incomparison to measurements at conventional frequencies,but they may serve to compare different materials undersimilar testing conditions. The experimental procedure issimple (i.e. one specimen end vibrates freely and thesample therefore may be easily mounted in an environmentchamber13). Comparison with fatigue experiments per-formed with conventional fatigue testing equipment, how-ever, is difficult, as fatigue damage in a corrosiveenvironment involves time-dependent processes so that

a

b

the lifetimes at ultrasonic frequencies (i.e. number of cycles Fig. 1 Shape of the specimens used for fatigue experiments on(a) AZ91 hp and (b) AlSi7Mg0.3. Dimensions in mm.to fracture) are expected to be longer.

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obtain the final contour. The specimens were extracted hardened at 160 °C for 6 h. The resulting static strengthvalues are: yield stress Rp0.2=214 MPa, tensile strengthfrom an actual structural part of a car. Figure 1(b) shows

the shape of the specimen used for fatigue testing of the Rm=297 MPa and elongation A5=13%. Figure 2(b)shows a microsection of AlSi7Mg0.3. This microsectionAlSi7Mg0.3 material.

The chemical composition of AZ91 hp (wt%) is: was orientated parallel to the crack plane. Dendrite armsof primary crystallized a-aluminium and an eutecticAl8.90, Zn0.79, Mn0.21, Si0.01, Fe0.003, Cu0.001,

Ni0.001 and remainder Mg. The solidification pressure phase are visible. Mean dendrite arm spacing is#20–40 mm.of the die-casting procedure was 600 bar. After quench-

ing the material in water at 60 °C, the material was The surfaces of the specimens were polished parallelto the specimen axis prior to the fatigue investigationsnot further heat treated. Static strength values of the in-

vestigated material are:17 yield stress Rp0.2=131 MPa, with abrasive paper of grade 1000 in order to eliminatenotches and to obtain a smooth surface.tensile strength Rm=208 MPa and elongation A5=4%.

Figure 2(a) shows a microsection of AZ91 hp. Thismicrosection was cut out of a specimen parallel to the

Experimental procedureplane of the crack (normal to the applied stress). Themean grain size is #100 mm. The high-frequency resonance testing method was used

in order to perform lifetime investigations in the veryThe chemical composition (wt%) of AlSi7Mg0.3 isaccording to DIN 1725/2 standard: Si6.5–7.5, Mg0.30, high-cycle regime up to 109 cycles. Using this method,

specimens are excited to longitudinal resonanceFe0.20, Cu0.20, Mn0.10, Zn0.10, Ti0.20 and remainderAl. The aluminium alloy was tested in the age-hardened vibrations at ultrasonic frequencies (#20 kHz). This

leads to sinusoidal cyclic loading with a maximum loadcondition (T6). The heat treatment was solutionannealing at 540 °C for 8 h and quenching in water. amplitude in the centre of the specimen. Strain ampli-

tudes are measured using strain gauges, and stress ampli-After 10 h at room temperature, the material was agetudes are calculated using the measured cyclic strain andYoung’s modulus according to Hooke’s law. The fatigueexperiments were performed with constant cyclic loads,and no static preload was superimposed (fully reversedloading conditions, load ratio R=−1).

An amplitude control unit guarantees that the pre-stated and the actual displacement amplitudes agreewithin 99% during experiments. Because crack growthincreases the compliance of the specimens (and reducesthe resonance frequency), control of the loading fre-quency is necessary. Failure of specimens may bedetected by monitoring the resonance frequency, whichmakes automatic operation of the experiments possible.The specimens were cycled until they failed or up to atleast 109 cycles if they did not fail.

Fatigue experiments were performed in ambient air(temperature 20–22 °C, relative humidity 40–60%) andin a saltwater spray (5wt% iodized sodium chloridedissolved in water) in order to simulate road conditionsin winter. Prior to cycling in the corrosive environment,saltwater was sprayed for 30 min onto the specimensin order to ensure identical specimen surfaces at thestart of all the experiments. The pulse length was 500cycles (#25 ms), and the pause length in ambient air(25–250 ms) was chosen such that heating of thespecimen by inner friction was avoided. The specimentemperature was controlled using a semiconductor (atemperature-dependent resistor) glued to the specimensurface. The maximum temperature of the specimen

a

b

during the experiments does not exceed 25 °C. In ambi-Fig. 2 Microsection of (a) high-pressure, die-cast AZ91 hp and(b) post-forged, cast-aluminium AlSi7Mg0.3. ent air, a fan is used additionally for cooling purposes.

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In the saltwater spray, the specimen is effectively cooled decreases in the regime above 107 cycles, and in contrastto the ambient air environment, no endurance limit wasby the liquid, and the pulse and pause length was chosen

to be 25 ms irrespective of the stress amplitude. found. The maximum number of load cycles imposedon car components (i.e. wheels, engine components)during service is below 109. Therefore, a technically

R E S U L T Srelevant endurance limit may be defined, e.g. by aprobability of 90% of surviving 109 cycles. This ‘techni-The results of fatigue experiments on AZ91 hp are

shown in Fig. 3(a). Specimens which did not fail are cal endurance limit’ of AZ91 hp determined in saltwaterspray at a cyclic frequency of 20 kHz and a fixed pulsemarked with an arrow. Lines indicate a fracture prob-

ability of 50%. In ambient air, failures beyond 107 cycles and pause length each of 25 ms, is 28 MPa. The technicalendurance limit in practice, however, is expected to beare very rare (only one specimen failed beyond 107

cycles), and the S–N curve becomes parallel to the lower, as the material will be cycled at lower frequenciesin technical applications.abscissa in the high-cycle regime. Because no specimen

failed within 109 cycles at a stress level of 38 MPa, this Results of endurance experiments with AlSi7Mg0.3are shown in Fig. 3(b). Fatigue data in both environmentsstress amplitude is defined as the endurance limit of

this material. may be well approximated using a straight line on adouble logarithmic plot. The S–N curve is steeper forIn saltwater spray, no significant reduction of fatigue

lifetimes, in comparison to ambient air, was found for saltwater spray than for ambient air. Saltwater spraydeteriorates the cyclic fatigue properties more at thecycles-to-failure below #107, and the fatigue data

coincide within the range of scatter. For higher numbers high than low numbers of cycles-to-failure. In ambientair as well as in saltwater spray, no endurance limit wasof cycles, however, an influence of the corrosive fluid on

the fatigue properties was observed. The S–N curve found in the investigated load range. On the basis of aprobability of 90% to survive 109 cycles, a technicalendurance limit of 71 MPa in ambient air and 26 MPain saltwater spray was determined for AlSi7Mg0.3.

D I S C U S S I O N

The influence of porosity

The fracture surfaces of all AZ91 hp specimens cycledin ambient air showed voids at the crack initiation site.As an example, Fig. 4 shows a SEM micrograph of acasting void on the fracture surface of an AZ91 hpspecimen which is situated close to the surface of thespecimen. Some areas on the surface of the void showdendrite arms. Most fatigue cracks started at voids onthe surfaces or close to the surfaces of the specimens,

Fig. 3 Results of endurance fatigue tests on (a) AZ91 hp andFig. 4 A void on the fracture surface of an AZ91 hp specimen.(b) AlSi7Mg0.3 in ambient air at 20 °C and in saltwater spray.

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which means that such flaws are more detrimental than shows the frequency of voids with different cross-sectionareas at the crack initiation places of the specimens faileddefects in the inner sections of the material. Fatigue

crack growth is transcrystalline, irrespective of the load in ambient air. The areas of these voids range from 0.04to 0.45 mm2. Approximately half of the detected voidlevel.

To evaluate the porosity of AZ91 hp, polished surfaces areas are smaller and larger, respectively, than 0.1 mm2.Voids on the fracture surfaces are generally larger thanand fracture surfaces were investigated. The mean diam-

eters of voids which were found with an optical micro- areas determined on polished surfaces. Fatigue cracksemanated from one of the largest voids in the cycledscope on #50 cm2 polished surfaces were measured and

classified. All voids with a minimum diameter of 20 mm volume, which was usually not found on polishedsurfaces.were evaluated. Figure 5(a) shows the frequency of voids

with different mean diameters. The largest void visible The fatigue strength data for AZ91 hp are subjectedto a pronounced statistical scatter [Fig. 3(a)]. Both theon a polished surface had a mean diameter of #230 mm

and a cross-section area of #0.04 mm2. poor fatigue properties in the low-cycle (up to 2×106)range as well as the pronounced scatter obviously comeThe areas of the voids where fatigue cracks started to

grow were similarly evaluated using a SEM. Figure 5(b) from the varying size of the microstructural defects. Inorder to quantify their influence, the number of cycles-to-failure and area size of voids where cracks started togrow were correlated (Fig. 6). Subdividing the specimensinto two groups containing voids with areas: (i) larger;and (ii) smaller than #0.1 mm2, one may clearly seethat the number of cycles-to-failure is generally smallerfor the group with larger voids.

The cross-section of AZ91 hp specimens [Fig. 1(a)] is35 mm2 and the maximum void area is 0.45 mm2. Stressconcentrations near voids are the source of crackinitiation. The increase of the nominal stress in theremaining cross-section is only limited. Because a largerinitially broken area (i.e. void+ initial crack) causes ahigher cyclic stress intensity amplitude, the crack growthperiod at a certain cyclic load is shorter for larger flaws.It is interesting to note that most fractured specimensfailed below 1–2×107 cycles, and the S–N curve at

Fig. 5 Frequency of voids of different sizes on (a) polished surfacesFig. 6 Influence of void area on the number of cycles-to-failure for(#50 cm2 analysed) and (b) at the crack initiation place of

AZ91 hp. AZ91 hp material.

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higher numbers of cycles is approximately parallel to the it cannot be decided whether or not a limiting crackgrowth rate for long cracks in magnesium alloys existsabscissa. This result may be explained by the observed

minimum crack growth rate of cast and wrought mag- in vacuum, as fatigue crack growth curves reported inthe literature were usually determined in ambient air ornesium alloys.18 Fatigue crack growth data show a mini-

mum crack propagation rate of #5–10×10−11 m/cycle corrosive fluid.18 A limiting crack growth rate of theorder of one atomic distance per cycle was found for ain ambient air at room temperature, which is a typical

lower limit for long cracks in many metallic materials. high-strength aluminium alloy in ambient air, whereasin vacuum, fatigue crack growth rates down to 10−12Assuming minimum crack growth rates of #5×10−11

m/cycle, a crack will need a maximum 1–2×107 cycles m/cycle were measured.7

The fatigue lifetimes of cast aluminium alloy AlSi7Mgto become 0.5–1 mm long. During most of the fatiguelife of a specimen, the crack length is smaller than similarly may be strongly affected by the presence of

voids if these material defects are not avoided using an0.5–1 mm. The number of cycles to advance a fatiguecrack from 0.5 to 1 mm to final failure of the specimen appropriate casting technique.20–24 Cycles-to-failure

could be well predicted for porous AlSi7Mg cast alloysis less than 10% of its lifetime.The rather low endurance limit of AZ91 hp is attrib- calculating the crack propagation rates of a fatigue crack

starting from a void.21,23 Under in-service loading con-uted to the manufacturing process leading to relativelylarge casting voids and is not an inherent property of ditions, porosity leads to early fatigue crack initiation,

and the lifetime of a structural component is mainlythis magnesium cast alloy. AZ91 hp material which wasobtained from a car wheel, produced by low-pressure, determined by the cycles necessary to propagate a crack

until final failure.24permanent mould casting, was similarly tested using theultrasonic fatigue testing method.16 Several specimens Additional investigations were performed on

AlSi7Mg0.3 cast and post-forged alloy with very lowcut out of those wheels did not show voids at the site ofcrack initiation, and fatigue cracks started to grow at the porosity in this study. It was expected that post-forging

reduces the detrimental influence of voids on the fatiguesurface of the specimens. In Fig. 7, endurance tests withAZ91 hp produced with different casting techniques are properties. Most fatigue cracks initiated at the surface of

the specimens, and 90% of specimens, which fracturedcompared. The S–N curve determined with permanentmould-casted AZ91 hp material without voids lies above in ambient air, did not show voids at the crack initiation

site. This indicates that post-forging effectively reducesthe S–N curve of die-cast AZ91 hp, however, it decreasesin the entire investigated regime between 105 and 109 the porosity of this cast alloy. Fracture surfaces of four

(from 30) investigated specimens, however, showedcycles and does not show an endurance limit.Similar S–N curves to AZ91 hp were found for the high- fatigue cracks emanating from voids which were situated

at inner sections of the specimens. As an example, Fig. 8pressure, die-cast AS21 hp material and AM60 hp.19 Incases where specimens failed after more than 107 cycles shows a void which acted as a starting point for the

growth of a fatigue crack. The largest void which was(which rarely occurred), cracks emanated from voids inthe inner section of the material. This means that fatigue found on fracture surfaces of AlSi7Mg0.3 material had

a maximum diameter of #120 mm and a void area ofcracks grew mainly under vacuum conditions. However,#9000 mm2. No significant influence of these relativelysmall casting voids on the number of cycles-to-failure

Fig. 7 Results of endurance fatigue tests on die-cast AZ91 hp andFig. 8 A void on the fracture surface of an AlSi7Mg0.3 specimen.defect-free permanent-mould cast AZ91 hp.

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was found, and the lifetimes of the four specimens, significant influence of the corrosive fluid. The colourof the polished surfaces of specimens tested in saltwaterwhere fatigue cracks started from voids, are within the

range of scatter of the lifetimes of specimens without spray changed, but no significant increase in surfaceroughness or other material damage was observed, obvi-voids.

If casting voids are relatively large, it may be assumed ously because the testing time is relatively short. Cyclingtime in saltwater spray was 17 min at a failure period ofthat a fatigue crack emanating from a void may propa-

gate and cause final failure if the cyclic stress intensity 107 cycles, e.g.At very high numbers of cycles-to-failure (e.g. 108 orvalue is larger than the effective threshold value of

the aluminium alloy.23,25 The effective threshold above), cyclic loading in the corrosive fluid leads to theformation of corrosion pits on the surface of the material.value of AlSi7Mg0.3 for a load ratio of R=−1 is

1.4 MPaEm.23 If, however, the initial defect size is After 1.5×109 cycles (#42 h) at a stress amplitude of30 MPa, e.g. the deepest pits on the surface of AZ91 hprelatively small and situated in the inner section of the

specimen, fatigue crack initiation at the surface of specimens were 0.8–1.1 mm. Microsection studies ofspecimens cycled with the same stress amplitude inthe specimen is more probable than crack initiation at

the site of the void. saltwater spray during different time periods showedthat the depth of pits increases approximately linearlyAn estimation of the critical size of a casting defect to

cause a significant reduction in the number of cycles- with exposure time of the specimen. It may be expected,therefore, that the lifetime in saltwater spray is not onlyto-failure may be found on the basis of LEFM principles.

The greatest influence of a void on lifetime may be shortened due to accelerated crack growth, but addition-ally by the notch effect of these pits which cause localexpected if it is situated at or near the surface. The

cyclic stress intensity factor of a crack emanating from a stress concentrations and enable crack initiation at lowapplied stress amplitudes. It should be noted, however,flaw at the surface of the material may be calculated

according to Murakami26 in the following way: that AZ91 hp does not show nearly as deep corrosionpits (%0.1 mm) after 42 h corrosion attack if no cyclicloading is applied, which means that the depth of cor-KImax=0.65smax√pEarea (1)rosion pits depends on both the cyclic load amplitude

According to Ref. [26], the stress intensity factor is and the exposure time in the corrosive fluid.mainly a function of the initially broken area, and the The results of fatigue tests on AZ91 hp in saltwatershape of the flaw influences the absolute value of K by spray are different from the corrosion fatigue data ofless than 10%. If the specimen is loaded, e.g. at a cyclic this alloy as described in the literature. A significantstress amplitude of 71 MPa (the technical endurance reduction in lifetimes was found for sand-cast AZ91E-T6limit of post-forged AlSi7Mg0.3 in ambient air), the when specimens were cycled at conventional frequenciesminimum area of the flaw is 8600 mm2, and the cyclic in saltwater instead of ambient air.27 On the other hand,stress intensity amplitude is greater than the effective low-pressure, permanent mould-casting AZ91 hp (heatthreshold. treatment T4) which was tested at an ultrasonic fre-

quency showed likewise reduced lifetimes in the corros-ive fluid in the whole investigated range (106–109KImax<KIth,eff#1.4 MPaEm

smax=71 MPa[ area<8600 mm2(2)

cycles).16 It may be assumed that voids have a dominantinfluence on lifetimes of high-pressure, die-cast AZ91 hpin saltwater spray as well as in ambient air if cycles-to-This rough estimation may explain why the voids with

an area size below a maximum 9000 mm2 in the inner failure are below 107. The testing times at ultrasonicfrequencies below 107 cycles-to-failure probably are toosection of post-forged aluminium alloy AlSi7Mg0.3 did

not cause a significant reduction in the fatigue lifetime, short to lead to a strong corrosive effect in this material.Due to the relatively large scatter of fatigue data, thisand cycles-to-failure coincide with the lifetimes of speci-

mens where fatigue cracks emanated from the polished additional—not too pronounced—damage caused by thesaltwater spray is clearly visible only in the very high-surface.cycle regime.

In AlSi7Mg0.3, all fatigue cracks initiated at the surfaceInfluence of the corrosive fluidof the specimen in saltwater spray. In contrast to AZ91 hp,a pronounced influence of the corrosive environment isIf the number of cycles-to-failure is below 107, the

fracture surfaces of AZ91 hp specimens fatigued in ambi- already visible in the regime of cycles-to-failure between105 and 106 [Fig. 3(b)]. At these low numbers of cycles-ent air and saltwater spray are similar. Fatigue cracks

emanate from voids in both cases, and the fracture to-failure, fatigue cycling in the corrosive environmenttakes a relatively short time (<2 min). The colour of thesurfaces show transgranular crack propagation with no

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C O N C L U S I O N S

The fatigue properties of high-pressure, die-cast mag-nesium alloy AZ91 hp and post-forged, cast-aluminiumalloy AlSi7Mg0.3 have been investigated in ambient airand saltwater spray using an ultrasonic fatigue testingfacility. The fatigue properties in the regime between105 and 109 cycles may be summarized in the followingway.

1 The fatigue data of AZ91 hp in ambient air may beapproximated by a sloping line in a double logarithmicplot. Cycles-to-failure beyond 107 are very rare, andan endurance limit is detected. Fatigue cracks start togrow from voids which are situated at the surface or

Fig. 9 Corrosion pits at the surface of AlSi7Mg0.3 after 5×108

close to the surface of the specimens in most cases.cycles at 35 MPa in saltwater spray.

Cycles-to-failure decrease with increasing void size.2 In saltwater spray, an endurance limit for AZ91 hp

does not exist. Corrosion pits are formed on thematerial surface after high numbers of cycles, whichfacilitate crack initiation. Below 107 cycles-to-fail-

polished surface is slightly changed, but the surface rough-ure, lifetimes in saltwater spray are similar to those

ness did not increase as in AZ91 hp. Fracture surfaces, in ambient air at the high testing frequency ofhowever, are different from those of specimens which were

20 kHz.fatigued in ambient air. They show a transcrystalline and

3 The endurance data of AlSi7Mg0.3 in ambient air andmore brittle fracture mode. saltwater spray may be approximated using straight

At high numbers of cycles (i.e. �108), corrosion pitslines on a double logarithmic plot over the whole

were formed at the surface of AlSi7Mg0.3 specimen.range between 105 and 109 cycles. No endurance limit

Figure 9 shows the surface and the crack initiation area of is found in either environment. In ambient air, aa specimen which was fatigued for 5×108 cycles at 35 MPa

fatigue crack is usually initiated at the surface. Voidsin saltwater spray. Corrosion pits are visible which reached

on fracture surfaces are rare and relatively smalla maximum depth of #200 mm. Cyclic loading was found(maximum diameter #120 mm, maximum void area

to accelerate the corrosion attack and the formation of#9000 mm2). No significant reduction of cycles-to-

corrosion pits significantly. Without cyclic loading, nofailure was found with specimens containing smallcorrosion pits could be found on the specimen surfaces,voids in the inner section of the material.

even on exposing them for a whole day.4 The slope of the S–N curve of AlSi7Mg0.3 in the

At high numbers-of-cycles, deterioration of fatiguecorrosive environment is steeper than in ambient air.properties caused by the influence of saltwater spray isThe influence of saltwater spray was previously found

more pronounced for both alloys than in the low-cycleat 105 cycles-to-failure, when the lifetime was

regime. This is in accordance with former fatigue crack #2 min). At high numbers of cycles, additional mate-growth studies at ultrasonic frequency in saltwater on

rial damage is caused by the formation of corrosioncopper, chromium steel, mild steel28 and brass,29 which

pits on the surface of the material.showed that fatigue crack growth is accelerated if the crack 5 The ultrasound fatigue testing method is especiallygrowth rates are low enough (<#10−8 m/cycle for copper

appropriate to study the fatigue properties of materialsand brass, <#10−9 m/cycle for chromium steel and mild

at very high numbers of cycles. Frequency effects,steel). The time which is needed for the liquid phase to however, have to be considered if fatigue tests arereach the crack tip explains why the influence of the

performed in corrosive environments.corrosive fluid is found only for low-crack growth rates.8

Additionally, threshold values of the investigated materialsare lowered due to the corrosive fluid. Both results arelikewise responsible for the S–N results of this study.Interaction of the corrosive fluid with the crack tip is

Acknowledgementeffective and leads to accelerated crack growth, and thuslower numbers of cycles-to-failure compared with ambi- The fatigue experiments were initiated and financed by

AUDI AG, Ingolstadt, which is gratefully acknowledged.ent air.

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