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Preliminary studies of creep at room temperature of laminates composed of aluminium and SAP sheets
l esearch report Preliminary studies of creep at room temperature of laminates composed of aluminium and SAP sheets w. MOORE, T. J. DA VIES and D. A. RYDER
Results are presented of a preliminary study of tensile and creep behaviour at room temperature of a laminate composed of aluminium and SAP (sintered aluminium powder sheet). This is considered as a 'model' system only in an attempt to utilise the creep resistance of the SAP. A decrease in creep rate and an extended time to rupture was obtained by laminating. It is proposed that an apparent increase in ductility of the SAP is due to restraint of catastrophic crack growth.
The development of improved mechanical properties in conventional monolithic materials is very near its ultimate and in general such improvements that are likely to be made in these materials will be relatively minor. The field of composite materials, however, is relatively unexplored in a technological sense and there is great scope for production of materials for specific purposes by combining the properties of various proven materials to give the optimum properties for a given application. In the case of sheet materials, fibre reinforced composites suffer the disadvantage of providing useful reinforcement in one direction only. For many applications planar as opposed to linear reinforcement is necessary; this requirement is fulfilled by the laminate. The development of precipitation hardened alloys, particularly with respect to the light alloys used in the aircraft industry has reached the above stage of minor gains in mechanical properties. These alloys may be used only up to certain temperatures above which over-ageing occurs with a consequent deterioration in mechanical properties. With increased air speeds (and consequently skin temperatures) strength at temperature is becoming increasingly important of which the most important property is probably creep resistance. It is well known that the dispersion hardened alloy SAP has good creep properties which do not vary substantially with temperature and would therefore form a good basis for a creep resistant laminate for aircraft applications. The choice of material with which to laminate this is arbitrary as the creep properties of a laminate are not as yet known as a function of the creep behaviour of the individual materials. Consequently, for a model system superpure aluminium was chosen as the other laminate component, primarily because it has well documented and reproducible properties, whereas a practical system would combine the high temperature creep properties of SAP with the higher low temperature strength and creep resistance of a commercially developed alloy. Department of Metallurgy, University of Manchester Institute of Science and Technology, Manchester, England
COMPOSITES. MARCH 1975
EXPERIMENTAL PROCEDURES Materials and manufacture of specimens
Three ply (AI/SAP/A1) laminates were prepared by diffusion bonding. The aluminium was cold rolled high purity (99.995%) sheet supplied by the British Aluminium Company Limited and the SAP, supplied by High Duty Alloys Limited under the trade name of Hiduminium 100, contained a specified 10% oxide and was clad with commercial purity aluminium. The diffusion bonding procedure consisted of cleaning the sheets; encasing these in a vacuum envelope and pressing at an elevated temperature. The 152.4 mm (6 in) square sheets were cleaned by degreasing in acetone followed by treatment in baths of boiling 10% NaOH, concentrated HNO3, and water. The stacked laminate was then wrapped in A1 foil and inserted into a mild steel vacuum envelope. After attaining a suitable vacuum the envelope was placed between two heated platens at a temperature of 623 K and a pressure of approximately 60 MN/m 2. After six hours at temperature the compact was allowed to cool under slowly reducing pressure and was subsequently removed from the envelope. Some edge cracking of the SAP was evident after this procedure and therefore the sheets where inspected using Xradiography and dye penetration and the cracked portions removed. The 127 mm (5 in) long specimens (conforming to BS 3500 (part 3)) were prepared by drilling blanks clamped between hardened tool steel templates corresponding to the specification of the test piece. The previously drilled blanks were held in the templates by locating pins and excess material was removed from the blanks by hand filing. All specimens were polished to 400 grade emery prior to testing. Tensile tests were carried out in air at room temperature in a Hounsfield Type 'E' tensometer at a constant cross-head speed of I 1.6 mm/min (0.4 in/min). Load/extension curves were plotted automatically on a chart recorder attached to the tensometer.
79
Specimens were tested in creep at constant load in air at room temperature. The creep rig consisted of six beams, the load being applied by means of weights attached to a single lever of ratio 20:1. The load was applied slowly by releasing the pressure in a hydraulic jack. Creep strain was measured by a fuil-bridge circuit of resistance strain gauges and monitored by a Bach-Simpson digital strain indicator to give a direct readout in microstrain. The output of the strain indicator was transferred to a Telsec 700 fiat-bed chart recorder to avoid constant supervision in the more rapid tests.
0 • .
I
3OC ]/°--~°" E
2oe
/ I
b
/
/
I
I/
I00
II
Tensile testing The aluminium specimens failed in a typically ductile manner; the values of proof stress (30 MN/m 2) and uts (53 MN/m 2) were in accordance with published values. The SAP failed in brittle manner type with fracture at 45 ° to the tensile axis; the values of proof stress (257 MN/m 2) and uts (310 MN/m 2) were in this case also in accordance with published values. A typical stress/strain curve of SAP aluminium and a laminate is shown in Fig. 1. The stress/strain curve of the laminate may be considered to consist of stages as shown in Fig.1. Stage t of the curve may be termed elastic as it exhibits the shape of the elastic portion of a stress/strain curve of a monolithic material. As may be seen the 'elastic' strain of the composite is greater than that of the aluminium or the SAP (5% as compared to 3% and 2% respectively). This apparent increased ductility of SAP when laminated is also evident in Stage II as plastic deformation continues for a further 8% strain in the composite whereas brittle failure of the SAP occurred well before this value of strain when the SAP was tested in isolation. Stage II is terminated by failure of the SAP. Stage III consists of rapid plastic deformation in the aluminium in the vicinity of the SAP failure. The experimental curves shown in Fig. 1 are similar in form to the schematic diagrams showing the behaviour of fibre and matrix tested separately and in composite forms as reported by Mileiko. t In the idealised form described by Mileiko fibre failure entails simultaneous failure of the matrix at the fracture strain of the fibre which may be increased by constraint effects. In the present case the inflection between Stage II and Stage III probably corresponds to fracture of the SAP and delamination. It is difficult to quantify constraint effects since they not only depend on the mechanical properties of the individual components of the composite but also on the strength and geometry of the interfaces separating the components. 2 The fracture characteristics of the laminates are, to a great extent, the same as those of the individual components. The SAP still failed in a brittle manner at 45 ° to the tensile axis. The aluminium fractures were all ductile and the aluminium elements necked to a point in all cases. Delamination occurred to a greater or lesser extent in most of the composite specimens, ranging from a very small area in the immediate vicinity of the fracture in some cases to total delamination over the whole gauge length in others. The various types of fracture, shown schematically in Fig.2, could not in any way be related to the method of manufacture, defects produced during manufacture or to the method of testing. The average experimental value of the proof stress of the laminate is 129.6 MN/m z and of the uts 143.6 MN/m z. A
80
]I
I
o-
I
• I
,I 0
RESULTS
Aluminium SAP Laminar.
t
0
l
'"
0 ~ 0
II
~
I0
l
20
I
I ~
J
30
40
Strain, E (O/o) Fig.1 Tensile stress/strain curves of aluminium, SAP and 3-ply laminates at room temperature
At
a
SAP
AI.
b
~C
C
~C )< Fig.2 T y p e s o f fractures and delamination observed at room temperature: (a) no delamination; (b) partial delamination near to a fracture; (c) complete delamination of one element; (d) complete delamination
rigorous treatment of the factors which influence the ultimate stress of the composite and which refines the analyses of Kelly and Tyson 3 is given by Mileiko 1 and by Garmond and Thompson. 4 An approximate assessment of the mechanical properties of the composite, which conforms approximately to the relationships of Mileiko and Garmong for I/e> Vcfit (Vf ~ 1/3 and single-valued in this case), may be made by inserting the average values of uts of the individual components obtained from tensile tests into the Jech mixture rule (derived for continuous fibre composites): Oc = O f V f + O m
(1
-
V0
where of = 311.07 MN/mZ; (7m
=
53.63 MN/m 2 and Vf = 1/3.
COMPOSITES. MARCH 1975
part of the gauge length of the specimen and the probability of failure occurring at the same location as the strain gauge is therefore low. Generally the tertiary stage is attributed to time dependent intergranular cracking which leads to failure and also to necking. Tertiary creep strain will be concentrated around the point of fracture and consequently the strain gauges are unlikely to measure it.
5 4
v w
3
d O t_
2 96
I
+ 65 I
-I
I
40
O
I
I
80 120 Time (hours)
160
Fig.3 Creep strain/time curves for 3-ply laminates at various stress levels at room temperature (the figures on the curves refer to the stress level in MN/m 2)
--
5~-/135 I~8/
The mode of failure of the SAP was basically the same as the mode of failure in the tensile situation. That is, a typical brittle fracture consisting of a single or double 45 ° fracture. The aluminium did not fail but was plastically deformed.
//-
2[//
O
2.oor,-
I
The creep curves of aluminium are shown in Fig.5 together with the curves obtained from laminates at the same average stress. The aluminium curve is truncated again due to the strain measurement technique. The strains produced in the individual aluminium specimens (without the restraining effect of the SAP) were so large as to result in the failure of the strain gauges at approximately 7% strain. After this the creep rate of the aluminium gradually decreased to a very low value and failure did not occur within the test time of 100 h. This is in accordance with the established creep behaviour of super-pure aluminium at room temperature. The SAP specimens failed in a relatively short time at a stress level of 324.3 MN/m 2.
2
3 4 Time (hours)
5
6
Fig.4 Detail o f s h o r t - t i m e b e h a v i o u r in creep at r o o m t e m p e r a t u r e (region near to t i m e origin in Fig.3): the figures on the curves refer to the creep stress level in M N / m 2
The mode of failure of the laminates was similar to that observed in the tensile specimens. This is illustrated in Figs. 6 and 7. In Fig.6 a small amount of delamination is evident in the immediate vicinity of fracture and necking of the aluminium may be seen to be confined also to this area. In Fig.7, however, gross delamination has occurred and excessive deformation of the aluminium is shown, necking occurring not only in the short transverse but also in the long transverse directions. The SAP is clearly visible, beneath the aluminium, having suffered little reduction in area. Discussion
This gives a value of 139.5 MN/m 2 for oc which compares reasonably with an average experimental uts value of 143.6 MN/m 2. Creep
Creep tests were performed in air at room temperature on the laminates at several stress levels and upon the SAP and the aluminium separately at stress levels approximately equal to the calculated stress supported by each when incorporated in a laminate. As the tests performed on composites were at stresses approaching its yield strength the load sharing at a given strain is assumed to be proportional to the yield stresses of the individual components. The creep strain/time curves for the laminates at various stress levels are shown in Fig.3. The left hand portion of the diagram is enlarged in Fig.4. As may be seen from these two diagrams various stages, which may be associated with conventional 'primary' and 'secondary' creep, are evident. The duration of the primary stage, as would be expected, varies with stress level, decreasing with increasing stress. A well established stage of steady state creep is visible at the lower stress levels but as the stress increases this becomes shorter and at very high stress levels is considerably reduced. The total lack of any evidence of a tertiary stage of creep is due to the method of strain measurement. Strain gauges measure strain over a very small
COMPOSITES. M A R C H 1975
A useful gain in ductility appears to occur as a result of lamination. As previously stated the 'elastic' portion of the stress/strain diagram of the laminate is approximately twice that of both the SAP and the a]uminium. It is unlikely, however, that all the strain produced in Stage [ is in fact elastic. It is probable that the elastic strain range is extended by selective load sharing although this would not be expected to
7 6
o Alurninium • SAP • Lominatc
/
/
o///~6
5 o 4 oa
324.re ~ " - /
e" - ' ' ~
~....~._~---12S
e'-'"IS hours ~
d 0 t_
125 ~ e'-''-'--'-'--e
O
92hOurs~
©____.__.-..------- • 125
-
I
I
I
2
144 h o u r s ~ t
1,
3 4 Time (hours)
5
I
.-
I
6
Fig.5 Creep s t r a i n / t i m e curves f o r a l u m i n i u m SAP and l a m i n a t e (at various stress levels) at r o o m t e m p e r a t u r e (the figures on the curves refer t o the stress level in M N / m 2)
81
Fig.6 Cross-section of a fractured laminate illustrating delamination near to the fracture surface (magnification X 5)
Fig.7 Cross-section of a partially failed laminate (see Fig.2, type C) showing reduction of area in the aluminium in the short transverse and long transverse direction (magnification X 5)
lead to a doubling of the elastic strain. This could be accounted for in two ways,
between failure of the first fibre and total failure of a fibre composite depends partially on fibre content and this time gap decreases with increasing fibre content. The basic difference between the fibre and the laminate situation is in the transition from Stage II to Stage III.
(a)
(b)
one of the components (aluminium) deforms plastically in this range but not sufficiently to detract from the essential straight line characteristics; on this basis the end of Stage I may be considered as the composite yield point. slip and shear occurring at the interface is linear with stress which gives the straight line characteristic of Stage I.
In Stage II the presence of the aluminium increases the apparent ductility of the SAP as plastic deformation continues for a further 8% strain in the composite whereas brittle failure occurred in single element SAP at 2% strain. This is probably due to the prevention of catastrophic propagation of microcracks in the SAP by load and strain sharing with the aluminium, as previously observed and reported by Lee et al 2 for aluminium transversely reinforced with stainless steel wires, and discussed by Mileiko i and Garmong. 4 In Stage III the aluminium extends, necks and fails in a ductile manner resulting in a gradual decrease in the nominal stress although the true stress increases. The extent of Stage III varies in a similar way to continuous fully oriented fibres in ductile matrices. A dissimilarity exists because whereas a sharp drop in load coincident with element failure occurs in the laminate a series of stepped load decrements occurs in fibre composites when fibres fail sequentially. The time
82
It is the extent of delamination which determines the onset and range of Stage III. Because Stage III consists primarily of plastic extension of the aluminium alone, which is related to the amount and location of delamination, it may be inferred that the extent of Stage III is dependent on (a)
fracture of SAP;
(b)
the number of fractures in the SAP in the gauge length;
(c)
the incidence and number of delaminations;
(d)
the bond strength between the SAP and the aluminium, ie, the shear strength of the bond.
The amount of delamination does not appear to affect any other portion of the stress/strain characteristics such as uts, yield strength or strain to failure of the SAP. It would appear therefore, that the bond strength is relatively unimportant as far as the tensile properties are concerned as the effect of delamination is limited to a part of the stress/strain diagram which is essentially after the material has failed (in a load bearing sense). Comparing the creep curves of the laminate and the aluminium (Fig.5) it is apparent that laminating significantly re-
COMPOSITES. MARCH 1975
duces the initial strain rate and the total strain. The main contribution to this is probably the strain behaviour of the SAP ie the aluminium is constrained to creep at rates compatible with the strain rate and total strain of the SAP. The effect of laminating upon the creep of SAP is somewhat more difficult to explain. As may be seen from Fig.5 the single element SAP creeps at a much higher rate than the laminate and fails in a much shorter time. The effect of laminating on the strain to fracture of the SAP appears to be very small. There is some evidence of an increase in strain to failure but this is too small to be assessed conclusively. In SAP at room temperature stress is relieved by formation of cracks rather than by plastic deformation. Cracks on individual specimens of SAP propagate catastrophically at small strains whereas in the laminate situation crack growth is restricted by the presence of the ductile aluminium. The probability of any one crack becoming predominant will be reduced thus increasing the time to failure and possibly slightly increasing the strain to fracture. From observations during creep tests it would appear that delamination before fracture of the SAP is insignificant. No meaningful correlation between strain to failure, strain rate, or time to failure and extent of delamination could be elucidated from these preliminary experiments. It would be expected that delamination before catastrophic propagation of cracks in the SAP would result in an increase in creep rate because the restraining effect of the aluminium on the microcracks in the SAP would be reduced. CONCLUSIONS The effect of laminating on the tensile properties of the individual components was to
COMPOSITES. MARCH 1975
(a)
almost double the apparent elasticity of the components;
(b)
increase the apparent ductility of the SAP, probably by preventing catastrophic propagation of microcracks. The mode of failure of the composites in tension was failure of the SAP in the first instance followed by plastic deformation and failure of the aluminium. The amount of deformation of the aluminium was related in some unspecified way to delamination and was, therefore, a function of bond strength. The tensile strength of the laminate was in good agreement with that predicted from the Jech mixture rule;
Primary and secondary creep stages were seen in the laminates. SAP constrains the A1 to creep at a much lower initial rate and also to exhibit a greatly reduced total strain compared to the unlaminated condition at an equivalent stress. The steady state creep rate of the SAP at 324.3 MN/m 2 was 1.56%/h whereas the composite steady state creep rate (when the SAP was carrying a similar load) was between 0.023 and 0.026% per hour. A decrease in steady state creep rate is obtained by laminating. It is proposed that these improvements are due to restraint of crack propagation in the SAP. The mode of fracture of the laminates in creep was similar to that in tension.
REFERENCES 1 Mileiko,S. T. J Mat Sci 4 (1969) 974 2 Lee, H., Ryder, D. A. and Davies, T. J. lnt J Mech Sci 12 (1970) p 739 3 Kelly, A. and Tyson, W. R. JMech Phys Solids 13 (1965) p 329 4 Gatmong,G. and Thompson, R. B. Metall Trans 4 (March 1973) p 863
83
Results are presented of a preliminary study of tensile and creep behaviour at room temperature of a laminate composed of aluminium and SAP (sintered aluminium powder sheet). This is considered as a 'model' system only in an attempt to utilise the creep resistance of the SAP. A decrease in creep rate and an extended time to rupture was obtained by laminating. It is proposed that an apparent increase in ductility of the SAP is due to restraint of catastrophic crack growth.
The development of improved mechanical properties in conventional monolithic materials is very near its ultimate and in general such improvements that are likely to be made in these materials will be relatively minor. The field of composite materials, however, is relatively unexplored in a technological sense and there is great scope for production of materials for specific purposes by combining the properties of various proven materials to give the optimum properties for a given application. In the case of sheet materials, fibre reinforced composites suffer the disadvantage of providing useful reinforcement in one direction only. For many applications planar as opposed to linear reinforcement is necessary; this requirement is fulfilled by the laminate. The development of precipitation hardened alloys, particularly with respect to the light alloys used in the aircraft industry has reached the above stage of minor gains in mechanical properties. These alloys may be used only up to certain temperatures above which over-ageing occurs with a consequent deterioration in mechanical properties. With increased air speeds (and consequently skin temperatures) strength at temperature is becoming increasingly important of which the most important property is probably creep resistance. It is well known that the dispersion hardened alloy SAP has good creep properties which do not vary substantially with temperature and would therefore form a good basis for a creep resistant laminate for aircraft applications. The choice of material with which to laminate this is arbitrary as the creep properties of a laminate are not as yet known as a function of the creep behaviour of the individual materials. Consequently, for a model system superpure aluminium was chosen as the other laminate component, primarily because it has well documented and reproducible properties, whereas a practical system would combine the high temperature creep properties of SAP with the higher low temperature strength and creep resistance of a commercially developed alloy. Department of Metallurgy, University of Manchester Institute of Science and Technology, Manchester, England
COMPOSITES. MARCH 1975
EXPERIMENTAL PROCEDURES Materials and manufacture of specimens
Three ply (AI/SAP/A1) laminates were prepared by diffusion bonding. The aluminium was cold rolled high purity (99.995%) sheet supplied by the British Aluminium Company Limited and the SAP, supplied by High Duty Alloys Limited under the trade name of Hiduminium 100, contained a specified 10% oxide and was clad with commercial purity aluminium. The diffusion bonding procedure consisted of cleaning the sheets; encasing these in a vacuum envelope and pressing at an elevated temperature. The 152.4 mm (6 in) square sheets were cleaned by degreasing in acetone followed by treatment in baths of boiling 10% NaOH, concentrated HNO3, and water. The stacked laminate was then wrapped in A1 foil and inserted into a mild steel vacuum envelope. After attaining a suitable vacuum the envelope was placed between two heated platens at a temperature of 623 K and a pressure of approximately 60 MN/m 2. After six hours at temperature the compact was allowed to cool under slowly reducing pressure and was subsequently removed from the envelope. Some edge cracking of the SAP was evident after this procedure and therefore the sheets where inspected using Xradiography and dye penetration and the cracked portions removed. The 127 mm (5 in) long specimens (conforming to BS 3500 (part 3)) were prepared by drilling blanks clamped between hardened tool steel templates corresponding to the specification of the test piece. The previously drilled blanks were held in the templates by locating pins and excess material was removed from the blanks by hand filing. All specimens were polished to 400 grade emery prior to testing. Tensile tests were carried out in air at room temperature in a Hounsfield Type 'E' tensometer at a constant cross-head speed of I 1.6 mm/min (0.4 in/min). Load/extension curves were plotted automatically on a chart recorder attached to the tensometer.
79
Specimens were tested in creep at constant load in air at room temperature. The creep rig consisted of six beams, the load being applied by means of weights attached to a single lever of ratio 20:1. The load was applied slowly by releasing the pressure in a hydraulic jack. Creep strain was measured by a fuil-bridge circuit of resistance strain gauges and monitored by a Bach-Simpson digital strain indicator to give a direct readout in microstrain. The output of the strain indicator was transferred to a Telsec 700 fiat-bed chart recorder to avoid constant supervision in the more rapid tests.
0 • .
I
3OC ]/°--~°" E
2oe
/ I
b
/
/
I
I/
I00
II
Tensile testing The aluminium specimens failed in a typically ductile manner; the values of proof stress (30 MN/m 2) and uts (53 MN/m 2) were in accordance with published values. The SAP failed in brittle manner type with fracture at 45 ° to the tensile axis; the values of proof stress (257 MN/m 2) and uts (310 MN/m 2) were in this case also in accordance with published values. A typical stress/strain curve of SAP aluminium and a laminate is shown in Fig. 1. The stress/strain curve of the laminate may be considered to consist of stages as shown in Fig.1. Stage t of the curve may be termed elastic as it exhibits the shape of the elastic portion of a stress/strain curve of a monolithic material. As may be seen the 'elastic' strain of the composite is greater than that of the aluminium or the SAP (5% as compared to 3% and 2% respectively). This apparent increased ductility of SAP when laminated is also evident in Stage II as plastic deformation continues for a further 8% strain in the composite whereas brittle failure of the SAP occurred well before this value of strain when the SAP was tested in isolation. Stage II is terminated by failure of the SAP. Stage III consists of rapid plastic deformation in the aluminium in the vicinity of the SAP failure. The experimental curves shown in Fig. 1 are similar in form to the schematic diagrams showing the behaviour of fibre and matrix tested separately and in composite forms as reported by Mileiko. t In the idealised form described by Mileiko fibre failure entails simultaneous failure of the matrix at the fracture strain of the fibre which may be increased by constraint effects. In the present case the inflection between Stage II and Stage III probably corresponds to fracture of the SAP and delamination. It is difficult to quantify constraint effects since they not only depend on the mechanical properties of the individual components of the composite but also on the strength and geometry of the interfaces separating the components. 2 The fracture characteristics of the laminates are, to a great extent, the same as those of the individual components. The SAP still failed in a brittle manner at 45 ° to the tensile axis. The aluminium fractures were all ductile and the aluminium elements necked to a point in all cases. Delamination occurred to a greater or lesser extent in most of the composite specimens, ranging from a very small area in the immediate vicinity of the fracture in some cases to total delamination over the whole gauge length in others. The various types of fracture, shown schematically in Fig.2, could not in any way be related to the method of manufacture, defects produced during manufacture or to the method of testing. The average experimental value of the proof stress of the laminate is 129.6 MN/m z and of the uts 143.6 MN/m z. A
80
]I
I
o-
I
• I
,I 0
RESULTS
Aluminium SAP Laminar.
t
0
l
'"
0 ~ 0
II
~
I0
l
20
I
I ~
J
30
40
Strain, E (O/o) Fig.1 Tensile stress/strain curves of aluminium, SAP and 3-ply laminates at room temperature
At
a
SAP
AI.
b
~C
C
~C )< Fig.2 T y p e s o f fractures and delamination observed at room temperature: (a) no delamination; (b) partial delamination near to a fracture; (c) complete delamination of one element; (d) complete delamination
rigorous treatment of the factors which influence the ultimate stress of the composite and which refines the analyses of Kelly and Tyson 3 is given by Mileiko 1 and by Garmond and Thompson. 4 An approximate assessment of the mechanical properties of the composite, which conforms approximately to the relationships of Mileiko and Garmong for I/e> Vcfit (Vf ~ 1/3 and single-valued in this case), may be made by inserting the average values of uts of the individual components obtained from tensile tests into the Jech mixture rule (derived for continuous fibre composites): Oc = O f V f + O m
(1
-
V0
where of = 311.07 MN/mZ; (7m
=
53.63 MN/m 2 and Vf = 1/3.
COMPOSITES. MARCH 1975
part of the gauge length of the specimen and the probability of failure occurring at the same location as the strain gauge is therefore low. Generally the tertiary stage is attributed to time dependent intergranular cracking which leads to failure and also to necking. Tertiary creep strain will be concentrated around the point of fracture and consequently the strain gauges are unlikely to measure it.
5 4
v w
3
d O t_
2 96
I
+ 65 I
-I
I
40
O
I
I
80 120 Time (hours)
160
Fig.3 Creep strain/time curves for 3-ply laminates at various stress levels at room temperature (the figures on the curves refer to the stress level in MN/m 2)
--
5~-/135 I~8/
The mode of failure of the SAP was basically the same as the mode of failure in the tensile situation. That is, a typical brittle fracture consisting of a single or double 45 ° fracture. The aluminium did not fail but was plastically deformed.
//-
2[//
O
2.oor,-
I
The creep curves of aluminium are shown in Fig.5 together with the curves obtained from laminates at the same average stress. The aluminium curve is truncated again due to the strain measurement technique. The strains produced in the individual aluminium specimens (without the restraining effect of the SAP) were so large as to result in the failure of the strain gauges at approximately 7% strain. After this the creep rate of the aluminium gradually decreased to a very low value and failure did not occur within the test time of 100 h. This is in accordance with the established creep behaviour of super-pure aluminium at room temperature. The SAP specimens failed in a relatively short time at a stress level of 324.3 MN/m 2.
2
3 4 Time (hours)
5
6
Fig.4 Detail o f s h o r t - t i m e b e h a v i o u r in creep at r o o m t e m p e r a t u r e (region near to t i m e origin in Fig.3): the figures on the curves refer to the creep stress level in M N / m 2
The mode of failure of the laminates was similar to that observed in the tensile specimens. This is illustrated in Figs. 6 and 7. In Fig.6 a small amount of delamination is evident in the immediate vicinity of fracture and necking of the aluminium may be seen to be confined also to this area. In Fig.7, however, gross delamination has occurred and excessive deformation of the aluminium is shown, necking occurring not only in the short transverse but also in the long transverse directions. The SAP is clearly visible, beneath the aluminium, having suffered little reduction in area. Discussion
This gives a value of 139.5 MN/m 2 for oc which compares reasonably with an average experimental uts value of 143.6 MN/m 2. Creep
Creep tests were performed in air at room temperature on the laminates at several stress levels and upon the SAP and the aluminium separately at stress levels approximately equal to the calculated stress supported by each when incorporated in a laminate. As the tests performed on composites were at stresses approaching its yield strength the load sharing at a given strain is assumed to be proportional to the yield stresses of the individual components. The creep strain/time curves for the laminates at various stress levels are shown in Fig.3. The left hand portion of the diagram is enlarged in Fig.4. As may be seen from these two diagrams various stages, which may be associated with conventional 'primary' and 'secondary' creep, are evident. The duration of the primary stage, as would be expected, varies with stress level, decreasing with increasing stress. A well established stage of steady state creep is visible at the lower stress levels but as the stress increases this becomes shorter and at very high stress levels is considerably reduced. The total lack of any evidence of a tertiary stage of creep is due to the method of strain measurement. Strain gauges measure strain over a very small
COMPOSITES. M A R C H 1975
A useful gain in ductility appears to occur as a result of lamination. As previously stated the 'elastic' portion of the stress/strain diagram of the laminate is approximately twice that of both the SAP and the a]uminium. It is unlikely, however, that all the strain produced in Stage [ is in fact elastic. It is probable that the elastic strain range is extended by selective load sharing although this would not be expected to
7 6
o Alurninium • SAP • Lominatc
/
/
o///~6
5 o 4 oa
324.re ~ " - /
e" - ' ' ~
~....~._~---12S
e'-'"IS hours ~
d 0 t_
125 ~ e'-''-'--'-'--e
O
92hOurs~
©____.__.-..------- • 125
-
I
I
I
2
144 h o u r s ~ t
1,
3 4 Time (hours)
5
I
.-
I
6
Fig.5 Creep s t r a i n / t i m e curves f o r a l u m i n i u m SAP and l a m i n a t e (at various stress levels) at r o o m t e m p e r a t u r e (the figures on the curves refer t o the stress level in M N / m 2)
81
Fig.6 Cross-section of a fractured laminate illustrating delamination near to the fracture surface (magnification X 5)
Fig.7 Cross-section of a partially failed laminate (see Fig.2, type C) showing reduction of area in the aluminium in the short transverse and long transverse direction (magnification X 5)
lead to a doubling of the elastic strain. This could be accounted for in two ways,
between failure of the first fibre and total failure of a fibre composite depends partially on fibre content and this time gap decreases with increasing fibre content. The basic difference between the fibre and the laminate situation is in the transition from Stage II to Stage III.
(a)
(b)
one of the components (aluminium) deforms plastically in this range but not sufficiently to detract from the essential straight line characteristics; on this basis the end of Stage I may be considered as the composite yield point. slip and shear occurring at the interface is linear with stress which gives the straight line characteristic of Stage I.
In Stage II the presence of the aluminium increases the apparent ductility of the SAP as plastic deformation continues for a further 8% strain in the composite whereas brittle failure occurred in single element SAP at 2% strain. This is probably due to the prevention of catastrophic propagation of microcracks in the SAP by load and strain sharing with the aluminium, as previously observed and reported by Lee et al 2 for aluminium transversely reinforced with stainless steel wires, and discussed by Mileiko i and Garmong. 4 In Stage III the aluminium extends, necks and fails in a ductile manner resulting in a gradual decrease in the nominal stress although the true stress increases. The extent of Stage III varies in a similar way to continuous fully oriented fibres in ductile matrices. A dissimilarity exists because whereas a sharp drop in load coincident with element failure occurs in the laminate a series of stepped load decrements occurs in fibre composites when fibres fail sequentially. The time
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It is the extent of delamination which determines the onset and range of Stage III. Because Stage III consists primarily of plastic extension of the aluminium alone, which is related to the amount and location of delamination, it may be inferred that the extent of Stage III is dependent on (a)
fracture of SAP;
(b)
the number of fractures in the SAP in the gauge length;
(c)
the incidence and number of delaminations;
(d)
the bond strength between the SAP and the aluminium, ie, the shear strength of the bond.
The amount of delamination does not appear to affect any other portion of the stress/strain characteristics such as uts, yield strength or strain to failure of the SAP. It would appear therefore, that the bond strength is relatively unimportant as far as the tensile properties are concerned as the effect of delamination is limited to a part of the stress/strain diagram which is essentially after the material has failed (in a load bearing sense). Comparing the creep curves of the laminate and the aluminium (Fig.5) it is apparent that laminating significantly re-
COMPOSITES. MARCH 1975
duces the initial strain rate and the total strain. The main contribution to this is probably the strain behaviour of the SAP ie the aluminium is constrained to creep at rates compatible with the strain rate and total strain of the SAP. The effect of laminating upon the creep of SAP is somewhat more difficult to explain. As may be seen from Fig.5 the single element SAP creeps at a much higher rate than the laminate and fails in a much shorter time. The effect of laminating on the strain to fracture of the SAP appears to be very small. There is some evidence of an increase in strain to failure but this is too small to be assessed conclusively. In SAP at room temperature stress is relieved by formation of cracks rather than by plastic deformation. Cracks on individual specimens of SAP propagate catastrophically at small strains whereas in the laminate situation crack growth is restricted by the presence of the ductile aluminium. The probability of any one crack becoming predominant will be reduced thus increasing the time to failure and possibly slightly increasing the strain to fracture. From observations during creep tests it would appear that delamination before fracture of the SAP is insignificant. No meaningful correlation between strain to failure, strain rate, or time to failure and extent of delamination could be elucidated from these preliminary experiments. It would be expected that delamination before catastrophic propagation of cracks in the SAP would result in an increase in creep rate because the restraining effect of the aluminium on the microcracks in the SAP would be reduced. CONCLUSIONS The effect of laminating on the tensile properties of the individual components was to
COMPOSITES. MARCH 1975
(a)
almost double the apparent elasticity of the components;
(b)
increase the apparent ductility of the SAP, probably by preventing catastrophic propagation of microcracks. The mode of failure of the composites in tension was failure of the SAP in the first instance followed by plastic deformation and failure of the aluminium. The amount of deformation of the aluminium was related in some unspecified way to delamination and was, therefore, a function of bond strength. The tensile strength of the laminate was in good agreement with that predicted from the Jech mixture rule;
Primary and secondary creep stages were seen in the laminates. SAP constrains the A1 to creep at a much lower initial rate and also to exhibit a greatly reduced total strain compared to the unlaminated condition at an equivalent stress. The steady state creep rate of the SAP at 324.3 MN/m 2 was 1.56%/h whereas the composite steady state creep rate (when the SAP was carrying a similar load) was between 0.023 and 0.026% per hour. A decrease in steady state creep rate is obtained by laminating. It is proposed that these improvements are due to restraint of crack propagation in the SAP. The mode of fracture of the laminates in creep was similar to that in tension.
REFERENCES 1 Mileiko,S. T. J Mat Sci 4 (1969) 974 2 Lee, H., Ryder, D. A. and Davies, T. J. lnt J Mech Sci 12 (1970) p 739 3 Kelly, A. and Tyson, W. R. JMech Phys Solids 13 (1965) p 329 4 Gatmong,G. and Thompson, R. B. Metall Trans 4 (March 1973) p 863
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