PowerPoint presentation

Contents

• limiting strains in stressed composites: static strain
• limiting strains in stressed composites: creep/fatigue loading
• creep
• fatigue
• impact

Limiting strains in stressed composites: static strain

The elongation at break of E-glass virgin filaments is 3.37% with a strength of 3.4 GPa. [Lovell page 25].

When the matrix has a lower failure strain than the fibre, then above the matrix failure strain, the matrix starts to undergo microcracking and this corresponds with the appearance of a 'knee' in the stress-strain curve [Hull and Clyne, 1996].

Hull and Clyne give failure strains for glass fibres, epoxy matrix and polyester matrix as 2.8%, 2% and 2% respectively.  However, when a cross-plied or woven composite is considered the 'knee' in the stress-strain curve may result from fibre/matrix debonding and/or transverse cracking in the plies normal to the applied load.  Hull and Clyne (Figure 8.28) indicate a knee-point at ~0.3% strain for a typical cross-ply laminate in uniaxial tension.

In the design codes for reinforced plastic pressure vessels [BS4994:1987], the following values are given:

"Debonding occurs at a strain of approximately 0.3% for the resin-glass fibre composites at present in general use" [BS4994 page 3].

"The maximum allowable strain shall not exceed 0.1ε_R or 0.2%, whichever is the smaller" where ε_R represents the "extension to failure (fracture strain) of unreinforced resin determined in accordance with Appendix B"
[BS4994 page 14 Section 9.2.4a].

"It shall be ensured that the strain transverse to the fibre direction is less than 0.1%" [BS4994 page 14 Section 9.2.7].

"... during hydraulic pressure or static head tests  the laminate is at no point strained beyond the limiting values  specified in clause 39, i.e. 0.26% or 1.3 ε_D, whichever is the smaller, where ε_D is the least strain, determined from allowable loadings and resin properties" [BS4994 page 16 Section 12.2].

Liao et al [1998] state that "creep of glass fibres is considered insignificant" citing 13 references but giving no indication of stress levels!  Further, "0° laminates exhibit a minimal amount of creep at low stress 6.2 MPa (900 psi) and moisture content (0.5-0.94% by mass) at room temperature".  Also "... the major cause of creep of FRP comes from creep of the polymer matrix, creep of glass fibers is considered insignificant" [Liao page 6 centre column].

For Scotchply 1002 glass/epoxy unidirectional laminates "the 0° laminates exhibit a minimal amount of creep at low stress 6.2 MPa (900 psi) and moisture content (0.5 - 0.94% by mass) at room temperature compared to the 90° and 45° [loading axis] laminates, a significant increase in creep deformation results when the temperature was increased to 102 °C.  Creep of the 45° and 90° laminates was strongly influenced by moisture and temperature, even at low stress levels". [Liao page 10 left column].

For filament wound glass/polyester resin composites in strong acid environments, "No failure was recorded at 0.2% applied strain in the strain corrosion tests" [Collins, 1978]

"Thomas has used a Weibull statistical analysis to predict the long-term stress-rupture behaviour of unidirectional fiber/epoxy systems based on short term results.  Their calculations indicates that under a static load of 50% ultimate stress, the probability of survival for carbon/epoxy, Kevlar/epoxy and glass/epoxy over a 30 year period are 99.99%, 99.8% and 22% respectively.  Under a load of 40% ultimate stress, the survival probability for glass/epoxy is 97%" [Liao page 17 left column].

"... cyclically loaded at 20-30% quasi-static strength, unidirectional glass/epoxy can last for about a million cycles" [Liao page 19 centre column].

"Dharan also suggested that loading below the matrix micro crack initiation stress (which is equivalent to about 0.75% strain level) for glass/epoxy will not lead to fatigue failure" [Liao page 21 left column].

Mottram [2002] has said "Creep is a tricky topic which no-one (my observation) has properly sorted out. It is clear that it is governed by the matrix (polymer) response to long-term loading and so its affect along the length of a UD (60% Vf) is likely to be low. Transverse and shear creep deformations for such a UD composite will be much higher. Woven composites will exhibit a tensile creep response between the limits of the longitudinal and transverse situation; tending slight to the transverse as there is now more matrix involved. Shear creep will also be less than for the pure UD case. We can expect the creep response to change with composition of the matrix while keeping the same reinforcement".

And also "Creep is stress dependent and from the limited knowledge I possess there is no lower limit on the stress. The simplest model is that by Findley. Its treatment by Liao et al is okay. Typical values for the n creep parameter (shape) under tension loading for GRPs is 0.16 to 22 ('Structural plastics design manual, ASCE manuals and reports on engineering practice,' No. 63, ASCE NY, 1984). Parameter m is stress dependent and is therefore quoted with the constant stress level. If you can get hold of the ASCE report No.63 the Table with constants for Findley's model is on Page 207".

To minimise creep in a polymeric matrix composite

1. align the fibres with the stress directions
2. use long or preferably continuous fibres
3. use higher modulus fibres (carbon>aramid>glass) or hybridise with stiffer fibres
4. use a less creep-prone matrix (thermoset rather than thermoplastic)
5. reduce the resin content (increase the fibre volume fraction)
6. limit the operating temperature range (stay below the glass transition temperature)
7. redesign to reduce stress levels (eg greater cross section area)

Frequency effects in fatigue

Matthews and Rawlings [1999] suggest that as a general consideration in the fatigue testing of polymeric composite materials, the test frequency should be chosen to minimise heating of the material.  For axially oriented unidirectional continuous fibres, where strains are low and hysteresis heating is limited, test frequencies around 10Hz are considered suitable.  For resin dominated configurations, eg off-axis fibres and/or resin dominated laminates, where strains are higher and hysteresis heating is significant, test frequencies of 5Hz or less are recommended.

Impact

There are several different methods which can be used to establish the impact strength of material test coupons:

• pendulum methods, such as Charpy/Izod - low velocity methods which may not give a true indication of the material response unless the crack runs parallel to the striker motion.
• drop weight - low velocity method in which energy imparted to the sample can be varied by changing the weight of the striker or the height from which it is dropped
• hydraulic tests machines - low velocity methods in which the machine is run at high strain rates
• Hopkinson-bar techniques
• ballistic - high-rate impact technique normally conducted in the horizontal plane [Bhatnagar, 2006]

A common approach to determine the residual properties of a composite after impact is by measuring the compression strength.  A standard procedure exists in CRAG method 403 [Curtis, 1988] in which (a) the laminate is impacted over a range of energy levels, (b) the type and size of damage produced is monitored (ultrasound scanning is recommended), and (c) coupons are cut and tested to determine the residual compression strength.

Wambua et al [2007] have evaluated the ballistic performance of natural fibre composites both as monolithic composites and having a mild steel sheet attached to one or both faces.  The hybrid structures absorbed more energy than either mild steel alone or the monolithic composites.  The critical absorbed kinetic energy of monolithic materials was ranked flax composites > mild steel > hemp/jute composites.

For composite structures, there are various facilities for crash testing of road vehicles (e.g. MIRA) or bird-strike testing of aerospace components

Videos

1. Composite bicycle wheel impacting a kerbstone (TAG on YouTube)
2. Comtek Advanced Structures worked with Bombardier in 1996 to develop improved repair methods for the Dash-8 leading edge. The "bullet time" video (2.98 MB .WMV Windows Mediafile) demonstrates its ability to withstand bird-strikes.

Click here for some review papers on the impact properties of composite materials.

References

1. DR Lovell, Chapter 2A: Reinforcements in NL Hancox, Fibre Composite Hybrid Materials
   Elsevier Applied Science, Barking, 1981, page 25.  UOP Library
2. D Hull and TW Clyne, An Introduction to Composite Materials - 2nd edition, Cambridge UP, 1996.  UOP Library
3. British Standard Specification for Design and Construction of Vessels and Tanks in Reinforced Plastics
   BS 4994 : 1987 (British Standards Institution, London) This standard "covers part of the field, namely, the use of polyester, epoxy and furane resins in wet lay-up systems".  BS4994 is generally accepted to provide a safe, but often over-engineered solution!
4. Kin Liao, C R Schultheisz, D L Hunston and L C Brinson, Long-Term Durability of Fiber-Reinforced Polymer-Matrix Composite Materials for Infrastructure Applications: A Review, Journal of Advanced Materials, 1998, 30(4), 3-40.
5. HH Collins, Strain-corrosion cracking of GRP laminates, Plastics and Rubber: Materials and Applications, February 1978, 3(1), 6-10.
6. Toby Mottram (University of Warwick), E-mail of 22/08/02 14:15.
7. FL Matthews and RD Rawlings, Composite Materials: Engineering and Science, Woodhead Publishing, Cambridge, 1999, page 382.
   ISBN 1-85573-473-7. UOP Library
8. A Bhatnagar, Lightweight ballistic composites: Military and law-enforcement applications, Woodhead Publishing, Cambridge, 2006, ISBN 1-85573-941-0.
9. CRAG test 403: Method of test for residual compression strength after impact of multi-directional fibre reinforced plastics.  In PT Curtis, CRAG Test Methods for the Measurement of the Engineering Properties of Fibre Reinforced Plastics, Royal Aerospace Establishment Technical Report RAE-TR-88-012, February 1988.
10. P Wambua, B Vangrimde, S Lomov and I Verpoest, The response of natural fibre composites to ballistic impact by fragment simulating projectiles, Composite Structures, 2007, 77(2), 232-240.

Further reading

1. M McLean, Creep of composites, In A Kelly "Concise Encylopaedia of Composite Materials", Pergamon Press, Oxford, 1989, pp 67-69.
   ISBN 0-08-034718-5. UOP Library
2. R Talreja, Fatigue of composites, In A Kelly "Concise Encylopaedia of Composite Materials", Pergamon Press, Oxford, 1989, pp 77-81.
   ISBN 0-08-034718-5.  UOP Library
3. SB Batdorf, Strength of composites: statistical theories, In A Kelly "Concise Encylopaedia of Composite Materials", Pergamon Press, Oxford, 1989, pp 254-261. ISBN 0-08-034718-5.  UOP Library
4. Serge Abrate - Impact on composite structures, Cambridge University Press, Cambridge, 1998. ISBN 0-521-47389-6.
5. Bryan Harris - Fatigue in composites : science and technology of the fatigue response of fibre-reinforced plastics, Woodhead, Cambridge, 2003. ISBN 1-85573-608-x.
6. RM Mayer - Design of composite structures against fatigue: applications to wind turbine blades, MEP, Bury St Edmunds, 1996. ISBN 0-85298-957-1.
7. SR Reid and G Zhou - Impact behaviour of fibre-reinforced composite materials and structures, Woodhead, Cambridge, 2000. ISBN 1-85573-423-0.
8. Ramesh Talreja - Fatigue of composite materials, Technomic, Lancaster PA, 1987. ISBN 0-87762-516-6.
9. MOW Richardson and MJ Wisheart, Review of low-velocity impact properties of composite materials, Composites Part A: Applied Science and Manufacturing, 1996, 27(12), 1123-1131.
    

Advanced Composites Manufacturing Centre School of Engineering Faculty of Technology University of Plymouth Plymouth  PL4 8AA  United Kingdom