Composites Design and Manufacture (Plymouth University teaching support materials)
Creep.  Damping.  Fatigue.  Impact.		 Lecture
PowerPoint		 Review
papers		 Subject
Index

PowerPoint presentation

It would be useful to consider limiting strains in stressed composites before reading this page.

Contents

CREEP (review papers)

Creep is the situation where a material progressively changes dimensions when subjected to a sustained load.  Creep behaviour can normally be modelled using  ε(t) = ε0 + εc  where ε(t) is the total strain at some time t after the application of a stress, εc is the the time-dependent (creep) component of strain at some time t after the application of a stress, and εo is the immediate (elastic) strain upon the application of a stress.  Creep deformation excludes the instantaneous elastic deformation on loading.  To minimise (or eliminate) 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, or crystalline rather than amorphous)
  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)

For composites, creep can usually be modelled using the Abbot creep law [1] such that:

ε(t) = a + ln tb
although Findlay [2] has suggested that a better correlation to experimental data for higher applied stresses can be obtained using:
ε(t) = ε0 + m (t/t0)nF
where a is a constant, b is the creep kinetic coefficient

, m is a dimensionless parameter, nF is the dimensionless Findley material parameter, and t0 is one hour.  As t0 is unity, the term t/t0 simplifies to t.

Liao et al [3] 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".

For Scotchply 1002 glass/epoxy unidirectional laminates [3], "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".

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" [4]

"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%" [3].

Mottram [5] 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".

Scott et al [6] reviewed the creep behaviour of fibre-reinforced polymer matrix composites and found that Findley's power law model was used extensively with good agreement.  Alternative models include the Time-Temperature-Stress Superposition Principle (TTSSP), Boltzmann Superposition Principle or the Schapery Single Integral Procedure.

DAMPING (review papers)

Damping is an energy dissipation property of vibrating structures.  In composites, it normally has a complex physical dynamic nature.  The dimensionless measure of this characteristic is the damping ratio or logarithmic decrement.  A reduction in fibre volume fraction enhances energy dissipation by increasing the loss associated with the resin matrix.  The normal damped resonant mode of vibration may be studied using modal analysis. 

FATIGUE (review papers)

The long-term performance of composites may be limited by fatigue (cyclic stress loading) considerations.  Fatigue data is normally acquired by subjecting material samples to an alternating (sinusoidal as default or square, triangular, sawtooth or pre-recorded) waveform between a maximum absolute stress, Smax, and a minimum absolute stress, Smin.  When loading varies between the same magnitude in tension and compression, the stress ratio is -1 .  If loads are restricted to just tension then 0 < R < 1, while if loads are solely compressive then R>1.

ISO 13003:2003 requires five specimens to be tested to establish the static/monotonic strength, before at least five specimens are tested in fatigue at a minimum of four levels of imposed stress/strain.  Data are normally plotted with a linear y-axis showing peak stresses or strains (S) against a log x-axis showing number of cycles (N).  This plot is is known as a Wohler diagram.

Basquin’s relationship (1910) [7] states that:

σa = σf' (2Nf)b
where σa is the constant stress amplitude, σf' is the fatigue strength coefficient, Nf is the number of cycles to failure and b is the fatigue strength exponent (the slope of the S-N plot, which is usually negative).

Matthews and Rawlings [8] 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.

Stinchcomb and Reifsnider [9], Post et al [10], Passipoularidis and Philippidis [11] and Vassilopoulos et al [12] have all reviewed aspects of the fatigue performance of composites.  Liao et al [3] stated that "... cyclically loaded at 20-30% quasi-static strength, unidirectional glass/epoxy can last for about a million cycles" and that "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".

IMPACT (review papers)

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

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 [14] 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 [15] 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

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

References

  1. RF Wolfseher, Die Superposition von Zeit, Temperatur und Spannung dargestellt am Kriechverhalten eines glasfaserverstärkten Kunststoffes, Diss. Techn. Wiss. 8306, ETH Zürich, 1987.
  2. WN Findley, Mechanism and mechanics of creep of plastics, SPE Journal, 1960, 16(1), 57-65.
  3. K Liao, CR Schultheisz, DL Hunston and LC 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.
  4. HH Collins, Strain-corrosion cracking of GRP laminates, Plastics and Rubber: Materials and Applications, February 1978, 3(1), 6-10.
  5. T Mottram (University of Warwick), E-mail of 22/08/02 14:15.
  6. DW Scott, JS Lai and A-H Zureick, Creep behavior of fiber-reinforced polymeric composites: a review of the technical literature, Journal of Reinforced Plastics and Composites, June 1995, 14(6), 588-617.
  7. OH Basquin, The exponential law of endurance tests, Proceedings of the American Society for Testing and Materials, 1910, 10(II), 625-630.
  8. FL Matthews and RD Rawlings, Composite Materials: Engineering and Science, Woodhead Publishing, Cambridge, 1999, page 382.
    ISBN 1-85573-473-7.
  9. WW Stinchcomb and KL Reifsnider, Fatigue damage mechanisms in composite materials: a review, ASTM STP675 Fatigue Mechanisms, American Society for Testing and Materials, 1979, 762–787.
  10. NL Post, SW Case and JJ Lesko, Modeling the variable amplitude fatigue of composite materials: a review and evaluation of the state of the art for spectrum loading, International Journal of Fatigue, 2008, 30(12), 2064–2086.
  11. VA Passipoularidis and TP Philippidis, A study of factors affecting life prediction of composites under spectrum loading. International Journal of Fatigue, 2009, 31(3), 408–417.
  12. AP Vassilopoulos, BD Manshadi and T Keller, Influence of the constant life diagram formulation on the fatigue life prediction of composite materials, International Journal of Fatigue, 2010, 32(4), 659–669.
  13. A Bhatnagar, Lightweight ballistic composites: Military and law-enforcement applications, Woodhead Publishing, Cambridge, 2006, ISBN 1-85573-941-0.
  14. 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.  MooDLE.
  15. 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 Encyclopaedia of Composite Materials, Pergamon Press, Oxford, 1989, pp 67-69. ISBN 0-08-034718-5.  PU CSH Library
  2. R Talreja, Fatigue of composites, In A Kelly, Concise Encyclopaedia of Composite Materials, Pergamon Press, Oxford, 1989, pp 77-81. ISBN 0-08-034718-5.  PU CSH Library
  3. SB Batdorf, Strength of composites: statistical theories, In A Kelly, Concise Encyclopaedia of Composite Materials, Pergamon Press, Oxford, 1989, pp 254-261. ISBN 0-08-034718-5.  PU CSH Library
  4. Serge Abrate, Impact on composite structures, Cambridge University Press, Cambridge, 1998. ISBN 0-521-47389-6.  PU CSH Library.
  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.  PU CSH Library.
  6. RM Mayer - Design of composite structures against fatigue: applications to wind turbine blades, MEP, Bury St Edmunds, 1996. ISBN 0-85298-957-1.  PU CSH Library.
  7. SR Reid and G Zhou, Impact behaviour of fibre-reinforced composite materials and structures, Woodhead, Cambridge, 2000. ISBN 1-85573-423-0.  PU CSH Library.
  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.
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Updated by John Summerscales on 06-Oct-2023 16:18. Terms and conditions. Errors and omissions. Corrections.