Features and design of elastomer / textile engineered structures D Boast, C Eng. F Mech E.
Title of meeting IOM Bla blah
Topics Materials: Environmental acids, water, oil, radiation etc. Bonding. Cost. Processing Methods. Design issues resulting from processing. Where features needed to perform function favour particular processes. Methods for effective design of important details and design criteria. Failure and selection of failure examples for discussion. Videos showing tyre manufacture and filament winding methods of manufacture.
Ways to combine fabrics and elastomers Elastomer is applied to the fabric by spreading an elastomer solution co extruding onto the fibre or textile calendering compression moulding. Products are made and cured or made and shaped and cured. Products are made from joined cured sheets by bonding or welded in the case of thermoplastic elastomers.
Textiles used in rubber composites Commonly used fabric materials, Polyamide, Aramid, Polyester, Rayon, Steel Speciality fabrics, Glass, Carbon, PEEK. Fabric constructions: Woven (square woven cords), also highly anisotropic cords as in tyre manufacture where the weft is very small compared with the warp. Knitted Braided Short fibres (tyre sidewall, conveyor belting) Oriented (by process) felted (random and overlapping) Filament winding methods (Taniq BV)) Tapes
Fabric parameters that define the mechanical performance Cord material, manmade polymer, wool, silk, glass, mineral, carbon, etc. Cord diameter. Cord strength, stiffness and stress strain curve. Cord density, Ends per m (warp), Picks per m (weft). Tex is a unit of measure for the linear mass density of fibres and is defined as the mass in grams per 1000 meters. (Dtex is 1/10 tex). Twist turns per m. Construction: S or Z denotes either left hand or right hand and of the twist, different layers can be built up.
Methods of stress analysis Often rubber / fabric structures see large strains. Rubber can undergo large strains fabrics change shape by the a change in the geometry of the cords. There often is strain concentration around the fabric / rubber interface. Finite element analysis is used by many people but is difficult to do correctly with rubbers and fabrics. Classical theory develops from the geometry of the textile fibres. It gives good results and also a visual appreciation of the construction and stress / strain issues issues involved.
Membrane theory Tension = Pressure * radius (for one radius of curvature). Pressure = Tension / Radius For 2 radii of curvature P = T 1 /R 1 +T 2 /R 2 For the case of the inside bends of hoses then r2 is ive and T1 (hoop stress) is larger.
Relationship between cord angles and deformation Cords crossing at A, X, B and Y assumed to be pin jointed Poisson's ratio is not a singular constant
Cord stress resolved in non isotropic materials
Neutral angle in hose Hoop tension = P * hose rad = Tc*n*sin 2 θ (N/m) Where Tc is cord tension and θ the angle to the axis, n is number f cords per m Axial tension = (½)* pressure * radius = Tc*n*cos 2 θ Hoop stress = 2* axial stress tan 2 θ = 2, θ = 54.7 or neutral angle. The relationship between the forces in the circumferential and meridional directions in a tyre is also is related to tan 2 α, where α is the cord angle to the circumference.
Typical shaped component (airspring)
Cord Terminations bead type Combined friction and e μγ T 1 /T 2 =e μα Friction (and bonded). Clips, jubilee and other types, swaged rings
Shear strain due to unequal strain in top and bottom cords Bead termination design
Tension link details: 6 tonne break load, 2E5 fatigue cycles at 0-4 tonnes cyclic load 15 articulation and able to alter axial length when supplied with internal pressure cm
Crawler track, product example Individual cord break load 1 tonne, fabric strength 250 tonne / m. Cord diameter 3 mm Total strength in 300 mm wide track = 50 tonnes.
Crawler track, product example
Hovercraft skirt Skirt pressure 0.1 MPa, skirt tension (static) 200 kn / m Cured fabric /.
Failure Fibres Tension fatigue Mechanical fatigue Abrasion / fretting Chemical changes Fibres should not be used in compression Elastomer, especially at the fabric / elastomer interface. But this also is a complex stress / strain field. Part failed products are important to look at to find the initiation.
General failure criteria for fibre stress in rubber composites Most commonly used is the ratio of burst to working stress so: Burst pressure / working pressure > 4. Tyres use 9 or 10. note that working stress is often difficult to get and mostly involves an unknown spectrum of loads. Even when the spectrum is known then it is not easy to combine high numbers of cycles at low loads with low numbers of cycles at higher loads. Can be built up from various factors such as: Load sharing between cords (0.5 to 0.7). Cord and rubber fatigue factors. Design factors, stress concentrations etc. Safety factors. Note that we are often working with engineers used to calculating the stress based on well known and understood fatigue levels for steals. With some well established safety factor (i.e.. Norske VERITAS factor for pipelines is 1.4). Comprehensive fatigue testing is desirable but not always possible.
Rubber technology issues Can good consolidation pressure be applied? What is the method of heat application and can the correct temperatures be applied thorough the component to get uniform heat history? Do the materials in the composite have the appropriate cure rates so that everything cures together? Bonding between layers
Strain measurement, (0 to 20%, or 50% maximum) Strain gauges (<15% static). Other resistivity or electrical property methods (Peratech). Visual methods Grids and photo s (for instance the photographic measurement of strain in the fabric of supersonic parachutes, G. Brown and J. E. Ferrier ). Also pixel analysis. Photo elastic. Video strain analysis (i.e. La Vision). Laser Stain Master. Acceleration and double integrate (but big drift). Getting the signal (wires) out (?).
End Thanks to: Cooper / Avon tyres Yokohama fenders as used by Fendercare Marine Taniq La Vision Strain Master