FATIGUE CRACK GROWTH OF THE 8090 ALLOY UNDER OVERLOADING

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FATIGUE CRACK GROWTH OF THE 8090 ALLOY UNDER OVERLOADING

C. Lespinasse, C. Bathias

To cite this version:

C. Lespinasse, C. Bathias. FATIGUE CRACK GROWTH OF THE 8090 ALLOY UN- DER OVERLOADING. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-793-C3-799.

�10.1051/jphyscol:1987393�. �jpa-00226625�

FATIGUE CRACK GROWTH OF THE 8090 ALLOY UNDER OVERLOADING

C.

and

Universite de Technologie d e Compiegne, CNRS-UA 849, Rue P. d e Roberval, B.P. 233, F-65256 Compi&gne Cedex, France

ABSTRACT

In the present paper, the fatigue crack growth retardation following a single tensile overload applied to the 8090 alloy i s studied and compared w i t h results obtained from conventional aluminium alloys. The 8090 alloy exhibits a high retardation effect to overloads; reasons for this good behaviour are given considering mechanical and fractographic aspects.

l NTRODUCT l ON

The crack growth retardation following a tenstle overload can be considered as one of the main sources of difficulties in the re diction of fatigue life. Since usual fatigue operating conditions in the aircraft industry do not result in constant amoli tude loading (C.A.L), a very important reauirement for acceptance of materials used in airframe i s a uood response to spectrum loading. To date l i t t l e i s known about the fatigue crack propagation (F.C.P.) behaviour of new AI-L1 alloy when tested w i t h overloads.

The retardation effect due to a single tensile overload i s described on f i g . { . From a practical point of view the number of delay cycles Nd i s the most important parameter.

The value of Nd increases w i t h increasing overload ratio r. or decreasing load ratio R, every other parameters being equal. From a scientific point of view the crack extension during the delay stage ad, the crack growth rate during the retardation stage (da/dN)D, the AKeff value after overload, and the plastic zone size created by the overload are characteristic parameters of the retardation process. Shih and Wei 11 I have reported that the retardation effect i s less pronounced w i t h increasing the specimen width. In plane stress state the vaiues of Nd would be greater than i n plane strain conditions. The i n i t i a l stress intensity factor AKo also influences the retardation process. For low overload ratios T, the higher retardatlon effect i s obtained w i t h low aK0 values 121. For high overload ratios, it appears that the influence of AKo depends on the aluminium alloy considered.

MATERlAL

The chemical composition of the material investigated, the 8090 alloy, i s listed in table I. The alloy was supplied i n the form of 13mm thick extruded plates. The plates had been 1 / solution treated at 535"C, 2 / quenched in cold water, 3/ stretched by 2%. Then two ageing treatments were performed : (B) 12H a t 190°C which results in a peak aged temper, (C) 24H a t 190°C.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987393

C3-794 JOURNAL DE PHYSIQUE

Element L i Cu Mg Zr Fe

Weight % 2,2-2,8 - 1 7 0 7 - 3 0,08-0,16 < 0,3

Table : Chemical composition of the material

Tensile properties for L orlentation are glven tn table \ I . The alloy i s highly textured and exhibits an unrecrystallized structure w i t h very long grains along the L orientation.

Yield Stress (MPa) U.T.5. (MPa) Elongation (A%)

8090 8 5 12 548 4,74

8090 C 430 499 9,9

Table I t : Tensile properties for L orientation Subgrains can be observed by optical microscopy.

The gram size i s not uniform through the ST and LT directions. Larger gralns (1OOprn) being located at the mld-thickness of the plate and small gralns ( 1 ~ r n j at the t w o surfaces.

EXPFRIMFNTAL PROCFDURES

Constant amplitude loading and overloading tests were conducted on 12mm thlck compact tension specimens (CT W75B12) i n laboratory air (40% < RH < 80%), w i t h a sinusoidal waveform, at a frequency of 20Hz excepted for the overload cycle (5Hz). The CT specimens were machined both i n the LT and TL orientations. Constant amplitude loading results are only given f o r the LT orientation. Crack t i p opening displacements (C.T.O.D.) were measured using a clip gage located just ahead of the crack tip. The crack length was monitored w i t h an optical method.

RESULTS a/

The "da/dN- vs -AK curves are given In fig. 3 and 4. On fig. 3, the FCP rates of 8090 alloy compares w e l l i n the medium range w i t h the 7075 alloy in peak aged condition [3].

However, the 8090 alloy exhibits higher crack growth rates for low AK values and has thus a lower AKth value. Figure 4 indicates that there i s no significant difference i n fatigue crack growth rates for the two ageing treatments. No F.C.P. result i s given for the TL orientation since a phenomenon of crack branching occurs frequently i n this orientation. The crack can follow two paths forming at the surface of the specimen an angle roughly equal to 60°, so that the usual ex~ression of AK i s unsuitable. Before branching becomes visible at the surface of the specimen crack growth rates decrease significantly. This suggests that the phenomenon i s initiated i n plane strain conditions.

Optical microscopy observations reveal that branching I s associated w i t h a transgranular mode of propagation.

Measurements of crack t i p opening displacements of 8090 alloy (R=O.O 1, AK,= 12 MPaJm) indicate that the crack closure ratio (Uo = bKeff/ AK, 1) i s equal t o 0.37. This value i s very low when compared t o other alloys ( Uo =0.5)

More information about constant amplitude F.C.P. i s given In ref. [4]

b/ Sinale tensile overload tests

increasing overload ratios I applied t o the 8090 C a1 loy for both orientations and R=O.O 1 . The basellne load amplitude AP, i s equal t o 5500N. Failure occurs i n the TL orientation for an overload ratio r = 2 and AK0=12 MPaJm. The LT specimen was tested up to an overload ratio 1=2.3 which did not induce failure.

Figure 5 displays the number of delay cycles Nd f o r the conventional alloys [2J and 8090 allov tested at t w o different overload ratios. On the same lot i s also shown the i n i t i a l crack growth rate (da/dNIo.

The comparison of the retardation effects between the various aluminium alloys, i s performed f o r the same values of AKo, overload ratio I, specimen width and orientation.

The 8090 alloy compares extremely w e l l w i t h conventional alloys particularly for the overload ratio I= 1.5. In this condition, the retardation effect i s at least 10 times higher.

I t must be noticed that the value of (da/dN)* f o r 8090 alloy i s much lower than that of other alloys. For 1=2.5, the number of delay cycles for 8090, 2 124 T351, 221 4 T65 1 alloys reaches lo6 cycles which i s the conventional l i m i t of crack arrest.

OlSCUSSlON

a/ Mechanical a s p e e

Wheeler's model has been chosen t o find out.which parameter i s responstble f o r the high retardation effect exhibited by the 8090 alloy. In this retardation model, the extension of crack during the retardation stage i s dlscussed i n relation to the plastic zone size created by the overload [51. The retardation process i s assumed t o continue u n t i l the outer l i m i t of the plastic zone under stress intensity factor (K, ) ,, reaches the outer l i m i t of that created by the overload (Kpeak). The minimum crack growth rate after overload i s given by :

(da/dNImi, = Cpi (da/dN)CA,L ( 1 )

where Cpi i s a macroscopic delay factor. For R=O, the minimum value of Cpi i s defined by:

cpm ^{= (} A K ~ / A K ~ = ( ~ 1 / 3 2 n ~ ) ~ ~ (2) Wheeler's factor n can be calculated combining equations ( 1 ) and (2) :

Cpm = (da/dN)D/(da/dN)o = ( (3)

In Wheeler's model, the effective stress intensity factor, the crack arrest and the cumulativ6 effect due to closely spaced single overloads are not taken into account.

Despite these limitations and for the present experimental conditions the value of Nu can

be well predicted lntegrating equation ( 1 ) over the crack extension ad w i t h the value of

(da/dN)C.A,L assumed t o be (da/dN)o. Hence, Nd depends on ad, n, and (da/dNIo. For a strong

overload ratio, the AI-Li alloy presents 1/ a similar retardation effect t o that of 2124

T35 1 alloy known to be ductile, 2/ a value of ad comparable to that of more b r i t t l e alloys

such as 261 8. Moreover, i n table Ill 8090 and 7050 alloys exhibit the same n value though

7050 presents a very low retardation effect so that the n value cannot here be invoked

for the dlfference In number of delay cycles. Consequently, i t i s clear that the strong

retardation effect i s due t o the low (da/dNIo values. The high number of delay cycles

does not result from the exceptional value of parameters characteristic of the overload

Process such as Cpi but results from the C.A.L. behaviour.

C3-796 JOURNAL DE PHYSIQUE

Alloy 2 1 24T35 1 26 18T85 1 7050T7365 t 8090C

Wheeler's factor n 1.8 1.3 2.2 2.2

Table I l l : Wheeler's factor value for conventional aluminium alloys [61 and 8090 alloy According t o the results obtained from crack closure measurements at ~K,=12MPa?fm, the 8090 alloy presents a low Uo value when compared to other alloys. This result i s consistent w i t h the lower F.C.P. rates for the same AK range

b/ Fractograohic asoects

Several overloads were applied to the LT specimen exhibited on fig. 6. The overload crack front i s rather straight. During overloading, abrupt crack advance was not detected by optical microscopy at the suface of the specimen. However, a fine static tear zone appears on the fracture surface for each overload, i t s size being much smaller than that obtained i n other conventional alloys.

Darkened regions can be noticed f o r high overload ratios and i n the centre of the specimen (i.e, in the large grain area) which suggests that darkening of these regions are certainly due to oxide debris created by fretting when subsequent constant amplitude loading i s resumed. Since after high overloads FCP rates become very low, mechanisms characteristic of the threshold regime (e.g. m i xed mode of propagation, roughness induced crack closure) should occur and promote the formation of oxide debris, particularly i n the regions of large grain width [71.

Another very important feature observed on the fracture specimen i s large secondary cracks perpendicular to the crack front and to the fracture surface as shown on fig. 7.

These cracks are wider and longer a t the centre of the speclmen. A L-ST section of a LT specimen made i n the static tear zone created by an overload ratio

= 2.5 i s shown on fig. 8. I t i s clear that these secondary cracks are intergranular. On the same L-ST section a second type of secondary cracks i s also visible; they form an angle w i t h the L orientation roughly equal to 30". Noting that slip bands are parallel t o this very direction suggests that oblique secondary cracks form i n ( 1 I11 planes. The creation of large secondary cracks during the overload cycle dissipates partly the mechanical energy which could have been used for static tear of the primary crack, leading thus to a smaller crack advance.

CONCLUSION

When subjected t o a single tensile overload i n the range of aK0=12MPaJm, the 8090 Al-Li alloy shows good performance i n comparison w i t h conventional aluminium alloys used i n the aircraft industry.

Wheeler's model demonstrates that the strong retardation exhibited by 8090 alloy i s mainly due to the low value of the i n i t i a l crack growth rate (da/dNIo. Crack closure effects could account f o r this low value of (da/dN),.

Large secondary cracks observed on the fracture surfaces can be also partly reponsible for the good response t o overloads, reducing the abrupt primary crack advance.

We would like to thank Mr. Ferton (Pechiney Voreppe) for his cooperation and Mr. Clerivet

(U.T.C.) for helpful discussions.

1 . 5Y!H and WE1 - J. of Test, and Eva!.. V o l . 3, no 1 , 1975, p. 46

2 ^t-?. GATEAU - SNIAS, P V 3801 2, Final, 1978

3. J. PETIT- Fatigue crack growth threshold c o n c e ~ t s edited by The Metallilrglcal Society of AIME, JanGary 1984, 2. 3

4. T. MAGNIN, P. RIEUX, C. LESPINASSE, C. BATHIAS - to be published i n Aluminium Lithium IV

5. O.E. WHEELER - Journal o f Basic Eng., March 1972, P. 181 6. J.D. BERTEL - These de docteur ingenieur, U.T.C., January 198 1 7. R.O. RITCHIE and S. SURESH

Met. Trans., V o l 13A, May 1982, p937

C I N b c y c l e s

crack length

N

FIG. 1 - Crsck retardation phenomenon after a single tensile overload performed for an overload r a t i o r

10

Number of cycles (10

FIG. 2 - Change of crack length versus number of cycles for increasing overload ratio5 applied

to the 8093 C allw in both orientations (R=0.01).

C3-798 JOURNAL DE PHYSIQUE

FIG. 3 - FCG comparison of 8090 C and

7075 1651 [31 alloys tested i n FIG. 4 - FCG resultsof 8090 allay tested LT orientations for R=0.1 i n 2 ageing conditions for R=0.01

1 0 ~ c ~ c l e s

8090 C 80908 7050 7475 7475 2214 2124 2124 2618 173651 Vh51 T7351 T651 TE51 T351 AT6

Allovs

10 9 R

7 b

Nd 5

4

'2 I 0

609dC 80906 7050 7475 7475 2214 2124 2124 2618 l73651 T7651 T7351 T651 T851 AT6

Alloys

FIG. 5 - Number af delay cycles Nd and initial crack growth rate (da/dN)o for 8090 and

conventional aluminium alloys [2] tested with CT W75B12 specimens at

aK0= 12MPaJm, for LT orientation and R=0.0 1.

H Crack growth direction

FIG. 6- Fracture surface of 8090 C alloy tested with increasing overload ratios T (from 1.2 to 2.3) for R=0.0 1 and LT orlentation ( aP0=5500N)

FIG. 7 - Fractograph of 8090 C alloy tested for LT orientstion, 0verl0ad ratio t=2.5 and R=0.01. Secondary cracks perpendicular to the overload crack front are visible.

FIG. 8 - L-ST section made in the static tear zone perpendicular to the fracture surface.