T h e hybrid e f f e c t - does it exist? L. N. PHILLIPS
There were the makings of a genuine, old-fashioned scientific controversy at Geneva a few months ago l concerning the behaviour of hybrids. BunseU got up and said flatly that there was nothing 'special' in them at all .... They had done the work 2 and the Mixture Law applied, right through the mechanical properties. Tsai begged to differ. He had the proverbial intuitive feeling that there must be more to it than that. If not, why were so many presumably intelligent people so interested? Excellent points both, not seriously weakened when Tsai added that he, for one, did not know how to predict the strength of a hybrid anyway. Regrettably, time was short on that occasion; so why not set the thing out at leisure and discuss at greater length a subject of great technical and commercial importance?
THE PROBLEM Let us take a hybrid comprising two reinforcing fibres of different stiffnesses in a common matrix, for example carbon and glass in a vinyl ester resin. The question is, when we apply load to failure, say in tension or flexure, Does it fail when it is supposed to
But, putting it gently, the question is still academic since the stiffer fibre will fail at its normal strain; and the strength and stiffness thereafter will be that of somewhat battered grp."
Materials scientist (obstinately): "But what about load-sharing? Would you not expect that the proportion of the load taken by the two fibres will depend not only on their absolute stiffnesses but also on the amount of each present? Surely if there is a larger area of glass in the cross-section as compared with carbon, this would tend to compensate for its lower modulus?"
Both engineers together, shaken but unrepentant: "OK, perhaps on reflection the load might be shared initially .... But the main point remains. The carbon must fail as soon as it reaches the usual breaking strain for cfrp - or the normal breaking stress if you prefer to look at it that way, though the breaking stress might be difficult to ascertain if the load is shared." (The reader will note that during this discussion there is neither expectation nor prediction that failure could occur at an abnormal stress, or strain.)
- or later?
Does it fail in the manner expected - or some other?
Above all, is the behaviour useful
Let us avoid further theoretical argument and appeal directly to experiment; but before giving the results of some recent work one should point out that there are hybrids - and hybrids.
- or isn't it?
An opportunity for many thousands of pounds worth of
new business hangs in the balance ... Now if one takes a sounding of scientific opinion, the answers which emerge cover a wide spectrum, as follows:
Orthodox, die-hard engineer: "Waste of time, old chap! The stiffer fibre takes all the load, goes bang when it should, end of composite !"
Progressive engineer: "Not so fast, my impetuous friend! It may not 'go bang' as you suppose. With a small amount of the stiffer fibre present, the shock may not be catastrophic and what remains may hang on for a bit .... Materials Department, RAE, Farnborough, Hants, England. ©Crown Copyright reserved. Permission to publish this article has been granted by the Ministry of Defence.
COMPOSITES. JANUARY 1976
In our hybrids each layer of material is identical to the others in the lay-up, since they consist of a woven (unidirectional) cloth in which the proportion of glass to carbon has been fixed and regularly distributed among the warp tows. Between the limits of all-warps-carbon and all-warps-glass the ratio of glass to carbon in the laminate can be altered at will by re-setting the loom. There are other sorts of hybrid. For example, the lay-up may consist of separate consecutive layers containing only glass or carbon. Again, hybrids are known where bars of cfrp are inserted locally within a grp moulding during manufacture. Because little is known about stress distribution in
hybrids containing high local concentrations of carbon we have not used them in the present investigation.
Normal failing strain for
EXPERIMENTAL RESULTS There are so far three pieces of evidence: stress/strain diagrams; impact; and fatigue.
Stress~strain diagram (i) In a tensile test the straight-line portion of the curve rises much beyond the point where the carbon would be expected to fail if it were taking all the load on the specimen. Therefore load is being shared. (ii) The straight-line portion of the curve rises somewhat beyond the normal breaking strain for cfrp before failure begins. This is the so-called 'hybrid effect') (iii) When failure does occur there is no sudden fall to zero of the load carried. On the contrary the load stays relatively steady or may even rise a little while it transfers progressively from carbon to glass. The area under the stress/strain curve is considerably greater than for cfrp (Fig.l). Similar load/ extension behaviour has been observed in flexure)
L-T h e o r e t ~ l u r e point if carbon fibre takes all the load I
2 tol hybrid to I hybrid
4 to I hybrid
Taking a particular stress level - say 300 MPa or about 50% of ultimate - the aU-glass control failed after 2 x 104 cycles and a 3-to-1 hybrid after 3 x 106 cycles, roughly a hundredfold improvement.
Estimates of the total fracture energy due t O puil-out and debonding of both carbon and glass were made and these corresponded roughly with the experimental results.
The reality was completely different, the all-glass control giving the lowest results, with an increasing fatigue life as the carbon fibre content increased (Fig.2).
This was an important project, since, with the conventional picture of a hybrid as a glass fibre laminate seeded with many stress-raisers at the broken ends of carbon fibre tows, the fatigue life might be worse than the control (shades of "somewhat battered grp"!).
Little difference was found in the impact strength of hybrids made with different ratios of glass to carbon and it was evident that the major contribution to the toughness of these materials came from the glass fibres.
Hybrids with different carbon/glass ratios, together with an all-glass control, were subjected to fatigue tests in fluctuating tension over a range of loadings.
The hybrid specimens had impact strengths about three times greater than the carbon, and failed with extensive compression failure and fibre pull-out without complete fracture, whereas the controls fractured right across.
Strain (%) Hybrid material stress/strain curve: weaving ratio of 3 warps glass to 1 warp carbon
The findings of this work - ie the much greater toughness and the load sharing - were rather expected after the earlier stress/strain diagrams, but no further light could be shed on the hybrid effect itself.
Tests have been carried out, in conjunction with Surrey University, on the impact strength of hybrids, using Charpy specimens both notched and unnotched, s Bars made from unidirectional carbon cloth and the same matrix were used as a control.
I06 I07 Number of cycles of stress to failure
Fatigue of glass fibre composite and glass/carbon hybrids
Taking together the three aspects of behaviour given above we can give a preliminary answer as follows. The hybrids appear to fail later than predicted, in a tough and fatigue resistant manner. It is up to the non-load-sharing school to explain this. Meanwhile there is no doubt that this behaviour is very useful; and the combination of practical utility and scientific interest should appeal to many a hard-pressed Research and Development director. It may even cause a stir in the Boardroom!
REFERENCES 1 First International Conference on Advanced Composites (Geneva, April 1975, organised by AIME) see also Composites 6 No 6 (November 1975) pp 271-276 2 Harris, B. and Bunsell, A. IL Composites'5 No 4 (July 1974) p 157 and Composites 6 No 5 (September 1975) p 197 3 Hayashi,T. et aL Fukugo Zairyo (Composite Materials) 2 No 2 p18 4 Marshall,D. A. 'The mechanical properties of glass/carbon composites manufactured by a vacuum-box moulding technique', MSc Thesis (Lancaster University, 1974) 5 Golding, C. 'The impact properties of composites manufactured from hybrid glass/carbon fibre tapes', BSc Thesis (University of Surrey, 1975)
C O M P O S I T E S . J A N U A R Y 1976