Viscoelastically Prestressed Polymeric Matrix Composites

K.S. Fancey, Department of Engineering, University of Hull

Although the concept of prestressed concrete is well known, a prestressed polymeric matrix composite is a more recent idea.  Consider a crack propagating through the matrix of a fibre-reinforced composite:

Crack blunting by local debonding can increase the fracture toughness of a fibre-reinforced composite but repeated action leads to deterioration.  Residual compressive stress can reduce frequency of local debonding, the prestressing being achieved by stretching fibres (e.g. glass fibres) as the matrix cures.  Although this elastic prestressing can significantly improve impact, tensile and flexural properties, there are potential disadvantages.  First, there is a need to keep fibres stretched as the matrix cures: this may restrict fibre orientation, distribution and ultimately, product geometry; thus commercial viability could be limited.  Second, the prestress effect could deteriorate with time because of localised matrix creep effects near the fibre-matrix interface.

These disadvantages can be overcome by the use of a novel technique, viscoelastic prestressing.  Here, viscoelastic (polymeric) fibres are stretched under a high load, but the load is released before the fibres are moulded into a matrix.  Once the matrix has solidified, compressive stresses are imparted by the viscoelastically strained fibres as they attempt to recover.  The principle of viscoelastic prestressing is demonstrated by this cross-polarised light image of nylon 6,6 monofilament embedded in polyester resin samples (15 x 3 x 0.2 cm).  The ‘test’ sample clearly shows a stress pattern, in contrast with the (unstressed) control sample.

Early work [1-3] focused on investigating the feasibility of viscoelastically prestressed composites (VPPMCs) by producing simple open-cast samples for evaluation by impact testing.  Charpy impact tests showed that nylon fibre VPPMC samples could absorb up to 50% more impact energy than otherwise identical, but unstressed control counterparts.  Compressive stresses increase the impact energy needed to make cracks propagate through the matrix.  Also fibre-matrix shear stresses (which create matrix compression) encourage fibre debonding under impact conditions, increasing opportunities to absorb impact energy.  The observed effect in test samples is shown here.

Through funding by The Leverhulme Trust, significant advances have been made in the understanding of VPPMCs.  Key findings from this work are:

• Using accelerated ageing techniques, both in strain recovery experiments and impact tests on composite samples, viscoelastic prestressing remains active (at normal room temperature) for at least 100 years [4].
• The force generated from viscoelastically stored energy in polymeric fibres can be substantial [5].
• Relative to control (unstressed) counterparts, increases in tensile properties, i.e. strength, modulus and toughness, have exceeded 15%, 30% and 40% respectively from viscoelastic prestressing [6,7].
• Viscoelastic prestressing increases flexural stiffness by 30-100% [6,8].

Since improvements in mechanical properties can be achieved without the need to increase component mass or section size, VPPMCs may provide benefits for transportation (reduced fuel consumption and/or improved crashworthiness) and for sports or safety applications (lightweight impact/blast protection).  Future work will concentrate on alternative fibre materials and process optimisation.

• [1] K.S. Fancey.  “Investigation into the feasibility of viscoelastically generated pre-stress in polymeric matrix composites.”  Mater. Sci. Eng. A279 (2000) 36.
• [2] K.S. Fancey.  “Prestressed polymeric composites produced by viscoelastically strained Nylon 6,6 fibre reinforcement.”  J. Reinf. Plast. Comp. 19 (2000) 1251.
• [3] K.S. Fancey.  “Fiber-reinforced polymeric composites with viscoelastically induced prestress.”  J. Adv. Mater. 37 (2005) 21.
• [4] J.W.C. Pang, K.S. Fancey.  “An investigation into the long-term viscoelastic recovery of Nylon 6,6 fibres through accelerated ageing.”  Mater. Sci. Eng. A431 (2006) 100.
• [5] J.W.C. Pang, B.M. Lamin, K.S. Fancey.  “Force measurement from viscoelastically recovering Nylon 6,6 fibres.”  Materials Lett. (2007) in press.
• [6] J.W.C. Pang, K.S. Fancey.  “An Evaluation of Viscoelastically Prestressed Polymeric Matrix Composite Materials.”  28th SAMPE Europe International Conference, Paris, France, 2-4 April 2007, p. 664-669.
• [7] J.W.C. Pang, K.S. Fancey.  “Analysis of the tensile behaviour of viscoelastically prestressed polymeric matrix composites.”  Under review.
• [8] J.W.C. Pang, K.S. Fancey.  “Improved long-term flexural and impact properties through viscoelastically induced prestress.”  To be submitted 2008.