The fatigue and fracture toughness of laminated composites based on 7075-T6 aluminium alloy

The fatigue and fracture toughness of laminated composites based on 7075-T6 aluminium alloy L. G. TAYLOR and D. A. RYDER

Laminated composites of 7075 AI-Zn-Mg-Cu alloy interleaved with 5-15% of either (a) commercial purity aluminium or (b) sintered aluminium powder have been produced by a hot-rolled-bonding technique. The total number of layers was varied in the range 3 - 1 5 but the thickness was kept nominally constant at 6.5 mm. The tensile, fatigue and fracture toughness properties of these composites have been measured and compared with those of the monolithic 7075-T6 alloy of the same thickness. All the tensile properties were within or very close to the 7075-T6 specifications. The low-mean-stress fatigue crack growth rates of all the composites were similar to that of the monolithic alloy within the range of crack lengths measured, but superior properties are predicted at high-mean-stress and long crack lengths. Valid plane strain fracture toughness values were measured for the 7075-T6 alloy, but could not be determined for any of the composites. The results of fracture toughness tests were expressed in terms of crack growth resistance curves from which values of 'Kc' were determined. All the composites showed a much higher resistance to crack extension than the monolithic alloy. Fractured specimens have been examined using metallographic and fractographic techniques. Particular attention has been paid to the regions adjacent to the interfaces between the various layers.

The idea of developing ultra-high-strength low-weight composites, using whiskers or fibres of near theoretical strength as the strong component, has been discussed at length over the last decade. 1 A number of disadvantages, including rather poor fracture characteristics, loss of composite strength with increase of isotropy, together with the high cost of the sophisticated technology required to .produce such composites, have made this approach less attractive than it once appeared. In many cases it may be more desirable to obtain a relatively modest improvement in a particular property without the use of complex and expensive manufacturing methods. Such a technique is especially applicable to the A1-Zn-Mg-Cu alloys which offer the highest strength/weight ratio of any group of aluminium alloys. Their relatively poor fatigue and fracture toughness properties have resulted, especially in the UK, in their replacement in certain aircraft applications by the lower strength alloys based on the A1-Cu system with a consequent weight penalty. There is evidence that lamination can be an effective method of improving the fracture toughness and fatigue crack growth resistance of a steel. 2-6 However, only limited work has been carried out on laminated composites of aluminium alloys, and the results are contradictory. 7-11 The object of the work reported here was to determine how the mechanical properties of A1-Zn-Mg-Cu alloy sheet laminDr Taylor is with the Department of Mechanical Engineering, University of Southampton and Dr Ryder with the Department of Metallurgy, University of Manchester Institute of Science and Technology

COMPOSITES . JANUARY 1976

ated with commercial aluminium or sintered aluminium powder (SAP) compared with the properties of the alloy in monolithic form. Notched specimens were tested in the 'crack-divider' orientation (Fig.l). An attempt has been made to take advantage of the well-known fact that the fracture toughness parameters K c and Gc vary with thickness. 12 K c max occurs at a critical thickness Bo at which fully slant fracture is just developed. Thicknesses below Bo give a decreased value o f K c and thicknesses greater than Bo give lower fracture toughness values reaching a plateau when essentially plain strain conditions obtain and Kk is measured. Since K c max is ~ 3 KIc it seemed sensible to ensure that the alloy elements of the laminate were near the critical thickness B0 which for 7075-T6 has been found 12,13 to be ~ 1.2mm. Additionally there is evidence that da/dN for Stage II fatigue crack growth decreases with specimen thickness. 14,Is For enhanced toughness and improved fatigue properties each strong sheet in the composite must deform independently of its neighbours at the advancing crack front. It was considered that intermediate layers of pure aluminium might permit this to occur by plastic deformation while intermediate layers of the stronger sintered aluminium powder (SAP) would favour delamination at interfaces. It was not known what effect the constraint due to other layers would have on Bo; thus, 7075 elements in the range 0.7 mm to 3.1 mm were used. The total thickness was kept nominally constant at 6.5 mm, but the total number of layers (3-15) and the total percentage of intermediate material (5-15%) were altered. Thus thickness effects in the individual components could be investigated.

27

layers of A1-Zn-Mg-Cu alloy so that a composite described as, for example, '7 ply-15% commercial Al' consisted of four layers of alloy of equal thickness interleaved with three similar sheets of aluminium such that the total volume fraction of commercial purity aluminium was 15%.

Fig.1

Tensile specimens and compact tension fracture toughness specimens (62.5 mm x 50 mm x 6.5 mm) were machined from each laminate with their major tensile axes parallel to the rolling direction. Fracture toughness specimens were fatigue cracked and tested in accordance with the currnt draft ASTM 16 and BS117 standards. Because the fatigue cracked specimens were to be used for fracture toughness tests, the fatigue tests were limited to low-mean-stress tests with maximum stress intensity factor < 16 MNm'3/2 and short crack lengths of up to ~ 5 mm (a/w ~- 0.5). The growth rate of each fatigue crack was monitored by optical microscopy to permit computation of da/dN x AK curves. While valid Kzc values could be obtained for the monolithic alloy, the composites did not meet the appropriate validity criteria. Their toughness, and that of the alloy, was therefore assessed in terms of fracture resistance. This involved the construction of crack growth resistance curves using a compliance technique to determine the instantaneous crack length and the calculation of the corresponding stress intensity K from standard formulae. 16,17 Each fracture resistance curve was superimposed on a family of stress intensity/ crack length plots for specimens of similar geometry and the fracture toughness Kc determined from the point of cotangency of the curves. ~s

A schematic diagram of a 7-ply 'crack-divider' laminate

EXPERIMENTAL PROCEDURE

The materials used were: (1) Two batches of aluminiumzinc-magnesium-copper alloy each of which conformed to both DTD 5060A and the equivalent American Specification 7075 and which had the following composition by weight: Batch I, Zn-6.0%, Mg-2.9%, Cu-1.3%, Mn-0.25%, Cr-0.2%, Si, Fe, T i - Traces; and Batch 2, Zn-6.0%, Mg-2.7%, Cu-1.5%, Mn-0.2%, Cr-0.17%, Si, Fe, T i Traces. (Batch 1 material was only used for the initial experiments aimed at relating toughness to thickness and these results are reported elsewhere.1 a All of the work on laminates used batch 2 material). (2) Commercial purity aluminium sheet. (3) SAP sheet containing 10% Al203 by volume and aluminium clad 5% on each side. These materials were hot-rolled to the appropriate thicknesses required for the manufacture of a particular composite, cut to size, degreased, stacked in the required manner and wrapped in aluminium foil to prevent contamination by oil. The packs were then bonded by hot rolling at 420°C for laminates containing aluminium sheet or 500°C for those containing SAP. The resulting composites were solution treated at 465°C, quenched into water at 20°C, then stretched to 2% permanent extension and finally aged at 135°C for 18 h. This treatment conformed with the procedure required to produce the 'T6' condition in 7075 alloy and also with the requirements of the DTD 5060A specification. The composites always had outer

Fractured specimens were examined using standard optical and electron-optical metallographic and fractographic techniques. EXPERIMENTAL RESUL TS Mechanical testing

The tensile and fracture toughness test results are summar. ised in Table 1. Each figure quoted is an average of at least

T a b l e 1. Tensile and f r a c t u r e toughness results

Material

0.2% Proof stress (MN/m 2 )

Tensile strength ( M N / m 2)

True fracture stress (MN/m')

Elongation (5)

e0uc tion in area (5)

Klc (MN m "3/2)

Kc (MN m "3/2)

Kd (MN m "3/2)

7075-T6 Commercial purity AI SAP

581.5 39.7 315.8

615.5 66.9 318.6

774.8 * 368.9

9.6 37.2 6.0

22.4 * 21.7

26.5 ** **

38.8 ** **

26.5

5,2

3 p l y - 5 % comm AI 7-5 11-5 15-5

517.7 504.1 478,9 526.5

540.0 562.3 540.6 565.1

716.7 722.3 695.7 687.1

11.0 9.6 9.2 8.8

27.3 26.8 25.2 17.7

*** *** *** ***

71.3 74.1 73.2 72.8

32.2 30.2 25.7 21.2

13.1 15.9 14.6 10.8

3 p l y - 1 0 % comm AI 7-10 11-10 15-10

524.0 498.5 482.3 479.9

571.6 555.4 536.8 545.1

753.3 712.6 691.2 684.7

10.5 10.0 9.4 8.8

27.1 25.0 25.2 20.6

*** *** *** ***

71.3 74.4 72.4 72.0

25.4 24.8 23.1 21.1

10.8 12.3 12.0 10.8

3 ply--15% comm AI 7-15 11-15 16-15

532.0 498.1 485.5 475.4

596.4 548.9 543.0 539.2

797.1 696,4 658.5 663.7

10.0 9.7 11.0 9.9

29.0 23.8 25.5 20.4

*** *** *** ***

74.8 74.6 72.1 70.9

18.8 19.4 2O.5 21.0

7.9 8.9 9.6 10.3

3 ply--5% SAP 7--5

556.8 537.5

596.8 556.8

768.8 723.3

10.6 9.6

22.4 20.0

** * ***

78.5 76.3

30.9 29.4

10.8 12.5

3 p l y - 1 5 % SAP 7--15

511.3 502.0

542.7 519.2

670.2 627.5

9.5 9.0

18.9 17.3

*** * **

70.2 74.0

25.8 29.9

11.3 14.2

Note:

28

* **

2.5 (ram)

True fracture stress indeterminate since load fell to zero as reduction in area approached 1OO%. Fracture toughness tests were not carried out on these materials. * * * Kic test not valid (see text).

COMPOSITES . JANUARY

1976

two results in the case of tensile tests and at least three results for fracture toughness tests. All specimens were 6.5 mm thick except for the commercial purity aluminium and SAP specimens which were 1.5 mm thick. All the specimens exceeded the minimum specified proof stress and elongation and only one composite (7-ply-- 15% SAP) failed to meet the minimum specified tensile strength for 7075-T6 of 538 MN/m 2 by about 4% and even this composite exceeded the slightly less rigorous DTD 5060A specification (Table 2). The strengths of the various composites can be compared with those predicted by a simple rule of mixtures (Figs 2 - 4). The 3-ply composites tended to be stronger than predicted, but as the number of layers increased their strengths fell to, or even slightly below, the predicted values. The true fracture stress also showed a gradual decrease as the number of layers increased. The reduction of area for those composites containing aluminium layers passed through a maximum when the total number of layers was 3, but only fell below this value for the monolithic material when the number of layers reached 15 (Fig.5). No such maximum was observed for those composites containing SAP which had a lower reduction in area than the monolithic material except for the 3-ply-5% SAP laminate which had the same value as the 7075-T6 alloy. The fracture toughness tests produced three different types of load versus clip gauge displacement record and these are illustrated in Fig.6:

~\ 7oo Z 0

600 u~ Pred,cted tens,le strength 500

Type 3 behaviour occurred in all composites containing SAP and in the 3-ply-5% Al material. Deviation from linearity occurred at quite high loads and small sharp discontinuities in the record curve were then observed. From each load/displacement curve a candidate fracture toughness vah, e Kq was determined by standard procedures. 16Av Kq values so obtained were used to compute the minimum thickness required for the test to meet the plane strain validity requirement of the relevant specification. This is given by

~ Predmted proof stress I

,8>_-25

/

where B is specimen thickness and oy the yield stress of the material in uniaxial tension. Only the monolithic material met this criterion and it is only for this material that a K,c Table 2. Specified mechanical properties

Property

7075-T6

DTD 5060A

0.2% Proof stress

462 MN/m 2

402 MN/m 2

Tensile strength

538 MN/m 2

479 MN/m 2

Elongation

8%

6%

COMPOSITES . JANUARY

1976

7 Totol number of layers

F 8col o

II

15

t

o True frocture stress o

O Tensde strength

~

~ Proof stress

i *~ 600

o Predicted tensile strength cted proof stress & ?

500 ___

,,

,

Fig.3 alloy

,5

Totol number of Ioyers Tensile properties of composites containing 90% 7075

800! o e

oo

~

o True fracture stress Tensile strength Proof stress

O

~ 7oo

Z

"~ ~, 6oo

0 [3

Predicted~tensile strength ~

500 I

(Kq~2

3

Fig.2 Tensile properties of composites containing 95% 7075 alloy: solid symbols indicate composites containing SAP: estimates shown are for composites containing commercial aluminium: curves are for all specimens

Type 1 was characteristic of the monolithic 7075 alloy and showed initial linear elastic behaviour followed by pronounced 'pop-in'. Type 2 records were obtained from nearly all the laminates containing aluminium and showed gradual deviation from linear behaviour at quite low loads and only slight 'pop-in' at high loads.

o True fracture stress o Tensde strength a Proof stress

o 8 ~ e

800

t

~Predicted

3

7 Total number of layers

pro_of stress II

,~1 15

Fig.4 Tensile properties of composites containing 85% 7075 alloy: solid symbols indicate composites containing SAP: estimates shown are for composites containing commercial aluminium: curves are for all specimens

value can be reported. Fracture resistance curves were therefor plotted for each specimen and K c yalues determined from these graphs. The results obtained are listed in Table 1 together with Kd, the stress intensity at which deviation from linear elastic behaviour was observed. The fracture resistance curves are summarised in Fig.7: the curves for individual specimens are given elsewhere. 19 The values of Kd for the monolithic alloy and for all the composites were similar, but Kc for the composites was, in all cases, ~ 2 Kc and "~ 2.5 to 3 K/c for the 7075-T6 alloy.

29

50 ¸

o a a •

• n

g

25

),

"6

:

x

5% I0% 15% 5%

The fatigue results for all of the specimens tested are shown in Fig.8 in terms of a plot of fatigue crack growth rate (da/dN) versus the stress intensity factor range (AK). All of the results are quite well represented by a single straight line, but a least-squares fit has been made for each specimen using the equation da/dN = C (AK)m and the results are given in Table 3.

commercial AI commercial AI commercial AI SAP

,

o

o

x

:

20

16

J I

J 3

i

i 7

1 II

l 15

Total number of layers Fig.5 Reduction of area: curve showh is f o r composites containing commercial a l u m i n i u m

Monolithic 7 0 7 5 alloy

800

Typical of laminates containing AI sheet

Typical of laminates containing SAP

600

200

0(~

C .

0

0.5

I

0

0.5

I

Metallographic examination

The tensile specimens of 7075-T6 alloy and of SAP failed by 45 ° slant fracture whereas the commercial purity aluminium specimens necked down to almost 100% reduction in area. Most composites containing commercial purity aluminium layers failed without delamination, but the majority of those containing 15 layers showed gross delamination, visible to the naked eye, during necking and immediately before fracture. All of the composites containing SAP exhibited some delamination at fracture (Fig.9). The fatigue fractures were macroscopically flat and normal to the axis of principal tensile stress. On a microscopic scale, the fatigue regions of the various aUoy layers were seen to consist of numerous 'plateaux' approximately normal to the tensile axis linked by 'cliffs' approximately parallel to the tensile axis. In the laminates containing commercial purity aluminium, the aluminium layers tended to separate into two plateau regions each being an extension of the adjacent plateau in the alloy layer and these were again linked by a cliff (Fig. 10). Very small delaminations (~ 30/am) were sometimes observed immediately adjacent

.

Clip gouge displacement (ram)

Fig.6 Types of load v displacement records determined experimentally

Material

C

m

Correlation coefficient

Monolithic alloy

5.6 x 10q2

4.18

.95

3-ply 5% AI 7-ply 5% AI 11-ply 5%AI 15-ply 5% AI

8.09 8.57 1.19 1.35

10"12 10 "12 10 11 1011

4.07 4.06 3.88 3.79

.974 .972 .992 .992

3-ply 10% AI 7-ply 10% AI 11-ply10%AI 15-ply 10% AI

1.64 x 10 11 7.4 x 10"12 1 . 1 6 x 1 0 "]l 2.83 x 10"11

3.81 4.1 4.0 2.7

.962 .991 .964 .970

3-ply 15%AI 7-ply 15% AI 11-ply 15% AI 15-ply 15% AI

4.21 x 10 12 2.76 x 10 "12 3.0 x 10"12 7.0 x 10 12

4.36 4.48 4.49 4.17

.986 .98 .929 .936

3-ply 5% SAP 7-ply 5%SAP

2.19 x 10 "11 3.88 x 10"1]

3.66 3.35

.952 .969

50

3-ply 15% SAP 7-ply 15% SAP

5.87 x 10"13 2.45 x 10"Is

5.2 7.43

.91 .83

20;

A verage values 8.92 x 10"12 1.56 x 10 "11 3.48 x 10"]2 3.0 x 10 "11

4.02 3.84 4.43 3.49

.975 .956 .954 .955

5.72 x 10"14

6.15

.867

I00

90

8O I

7o i 6O Z

50

/

a:

I0;-

Z Fig.7

30

Table 3. Least-squares analysis of fatigue results

3 ZXo (mm)

5

6

All All All All All

5% AI composites 10% AI composites 15% AI composites 5% SAP composites 15% SAP composites

x x x x

S u m m a r y o f f r a c t u r e resistance c u r v e s f o r all specimens

COMPOSITES. JANUARY 1976

5xlO

-7

-

x

Monolithic7075 T6

o

5% - AI composite

z~

I 0 % - AI composite

a

15% - AI composite

•

5 % - SAP composite

•

15%- SAP composite

X

~,oE?. [3

E Z10

oO®x • x~x 15--

10-7

_

9

--

8

--

3'

--

6

--

Fig.9 Fractured tensile specimens: from left: monolithic 7075-T6; commercial purity aluminium; SAP; 3-ply--15% commercial AI; 15-ply-5% commercial AI; 3-ply--5% SAP

5 ×10-8 --

o&p A

l

l

I

5

6

7

~Cl

I

8 9 I0 Z~K (MNm- 3/2)

1 15

_

I 20

Fig.8 Plot of fatigue crack g r o w t h rate (da/dN) versus stress intensity factor range ( A K ) f o r all specimens

to the fracture surface in these composites, but in those containing SAP larger delaminations of up to about 1 mm were often seen at either the alloy/cladding or the cladding/ SAP interface (Fig.l 1). Such delaminations did not occur in all specimens containing SAP and, in some cases, were observed at some interfaces and not at others in the same specimen. Where delamination did not occur, the SAP layers tended to form a single low angle slant fracture linking the adjacent layers of the alloy at each interface. However, when delamination was observed, the SAP layers formed either single 45 ° slant fractures or, in a few cases, a small region of flat fracture bounded by 45 ° slant lips.

Fig.10 Fatigue fracture of 3 - p l y - 1 5 % commercial AI composite: section, mechanically polished and etched in Wasserman's reagent

The overload fracture surfaces of the 7075 alloy specimens were fiat in the centre with small slant lips at the sheet surfaces. Those specimens containing relatively thick elements of commercial aluminium, notably 3-ply-15% commercial A1, 7-ply- 15% commercial A1 and 3-ply- 10% commercial A1, showed a single 'tongue' of flat fracture extending from the fatigue crack. The aluminium layers failed by coalescence of very large voids (Fig. 12). In the other composites, each element of the 7075 alloy had its own separate fiat fracture tongue (Fig.13). In each case, fully slant fractures developed in the alloy layers and SAP layers. Some of the aluminium layers also behaved in this way but others failed by necking to 100% reduction of area. Again, delaminations were observed in some cases, but not in others. In order to obtain information on the mode of slow crack growth in laminates where delamination was not observed a fracture toughness specimen of 3-ply-15% A1 was loaded to the maximum value determined from the appropriate load-displacement curve of a nominally identical specimen.

COMPOSITES.

JANUARY

1976

Fig.11 Fatigue fracture of 3-ply - 15% SAP composite: section, mechanically polished and etched in Wasserman's reagent

31

After unloading the specimen was sectioned in the plane of the specimen and normal to the fracture plane to determine the extent of slow crack growth. The crack length was measured after removing known increments of materials from the section. The results obtained are plotted in Fig.14 and show that both a free surface and an aluminium layer inhibit metastable crack extention to a similar extent.

50

u

DISCUSS~ON

The experimental results suggest that roll-bonded A1-Zn-MgCu alloy interleaved with pure aluminium or SAP has substantially higher toughness than the monolithic material while retaining the specified proof stress, tensile strength and elongation of the alloy aged to peak strength. This would seem to have considerable practical advantages over obtaining a relatively modest increase in toughness by overaging which decreases both proof and tensile strength, 2° or even the modern thermomechanical treatments which can give up to ~ 30% improvement in fracture toughness while retaining the tensile properties. 21 A recent paper 22 suggests that the practice of reporting only K c values from a fracture toughness test is analogous to carrying out tensile tests and only reporting the tensile strength. If fracture toughness results are presented in the form of fracture resistance curves, then the appropriate value of Kc for any specimen or component design may be determined providing an appropriate K- calibration is known. 23 This is because fracture resistance curves, unlike

20 I o Y e

I0

t

1.0

ml

30

20

Depth below surfoce ( m m

)

Fig.14 Increase in crack length (ZXa) as a f u n c t i o n o f depth below the surface for the specimen loaded to m a x i m u m load and then released

values of Kc, represent material properties dependent only on thickness and condition of the material and environment, but not on specimen design.22, 24 The fact that Kc is geometry dependent 22,24 does not invalidate the present results since the parameter may be regarded as a satisfactory 'ranking test' if geometrically identical specimens are used, as was the case in this work. It is also necessary to make allowance for the plastic zone site which a compliance technique does automatically. 24 Thus, the Kc values reported are valid measures of toughness for this particular specimen geometry. Moreover, while the overload fracture in the monolithic alloy and the composites initiated at similar values Of Kd, the much higher resistance to crack extension of the composites can be clearly seen from the fracture resistance curves.

Fig.12 Macrograph of fatigue -- overload transition region o f 3-ply--15% commercial AI composite

Fig.13 Macrograph of fatigue -- overload transition region of 3-ply--15% SAP composite

32

The toughness of all the composites approached the value K c max ~ 3 Kk reported for 7075-T6 alloy. 12 This similarity is a little surprising since a maximum value had been anticipated for those composites containing alloy elements near to the critical thickness Bo ", 1.2 mm. This condition was most nearly achieved in the 7-ply (alloy elements 1.5 ram) and 11-ply ( ~ 1.0 mm) composites, but the 3-ply composites had alloy layers ~ 3 mm thick, a thickness which had previously been found to give essentially plane strain fracture toughness in the monolithic aUoy. 13 Delaminations were observed in several of the composite test specimens. Presumably delamination allowed the interfaces to act as free surfaces where plane stress plastic zones could be formed which would restrain any pop-in extension of the fiat fracture tongues. It also seems reasonable to assume that energy would be absorbed in causing delamination and that this must be added to the energy required to overcome crack resistance in the individual layers. If this energy term were constant for each delamination, then the largest effect would have appeared in the curve for the 15-ply composite. However, both this and the 11-ply laminate had alloy

COMPOSITES. JANUARY 1976

layers of thickness less than Bo for the monolithic alloy. Thus the benefit of the additional delamination energy may have been offset by the lower fracture resistance of these thinner "alloy layers. Fig. 14 suggests that the restraint of crack propagation in the alloy by the aluminium layer was similar to that developed by the free surface. Thus plastic deformation and failure o f the aluminium layer may be considered as an 'effective delamination' which could have allowed each alloy layer to act essentially independently. Since the fracture resistance curves and Kc values for those specimens which showed delamination at the interface are very similar to those which did not, the energy for effective delamination must be of similar magnitude to that for true delamination. It is important that the low-mean-stress fatigue crack proposition rates for all the composites are similar to those for the 7075-T6 alloy. The improved fracture toughness of the composites should give a longer critical crack length and hence an improved fatigue life. Also, in 'fail-safe' design, the longer crack would be more easily detected by nondestructive testing techniques. There are also indications 19 that these composites could have substantially better highmean-stress fatigue properties than the alloy similar to those achieved by other laminates 9 because bursts o f tensile failure may be eliminated. The significant result of this work has been to strengthen the evidence that lamination provides a relatively simple (and, therefore, cheap) method of modifying the mechanical properties of materials. We have looked at fatigue and toughness, but other properties could be considered. One example is to attempt to use the good creep and high temperature tensile properties of SAP in composites of the type produced in the present work. Finally, it must be emphasised that, before such composites could be used in engineering applications, it would be essential to test composites of the thickness required for service and also necessary to examine a wide range o f number and thicknesses of the individual component layers under the predicted service conditions o f load and environment.

REFERENCES 1 2 3 4 5 6 7 8

9

10 11

12 13

14

15

CONCL USI ONS Laminated composites of 7075-T6 alloy interleaved with either pure aluminium or SAP have been produced which, with one slight exception, exceeded the minimum specified tensile properties of the monolithic 7075-T6 alloy. The toughness of such 6 5 m m thick crack divider laminates is substantially greater than the monolithic alloy. Kc for the composites "~ 2K c and ~ 2.5 to 3 Kic for 7075-T6 alloy. The fatigue crack _pro_pag_ationrates of all the composites v~ere similar to that of the monolithic 7075-T6 alloy, but a longer critical crack length is predicted. It is emphasised that the stress intensity range studied was small. An improvement in fatigue crack growth behaviour of the composites over the monolithic alloy can reasonably be predicted in high-mean-stress fatigue. The number of elements in the composites (3, 7, 11 or 15) and the volume fraction of the aluminium or the SAP layers (5%, 10% or 15%) did not greatly influence the toughness or fatigue properties. A CKNOWL EDGEMEN TS The authors are grateful to the Ministry of Defence (Procurement Executive) for financing this research and to Professor K. M. Entwistle, UMIST, for the provision of laboratory facilities.

COMPOSITES. JANUARY 1976

16 17 18 19 20 21 22 23

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