Clear to slightly translucent. Opaque. Acoustic Birefringence. N/A (isotropic) ... using the method described as Adhesion of rubber (vulcanized) to rigid substrate ...
A new Elastomeric Wedge or Delayline Material 1
E.A.Ginzel 1 , S. Zhu 2 Materials Research Institute-Waterloo, Ontario Canada 2 University of Waterloo-Waterloo, Ontario Canada
Abstract A new elastomeric material suitable for ultrasonic applications has been developed. Mechanical and acoustic properties have been investigated. Moulding and adhesion techniques have been investigated to provide suitable methods for custom shaping of components. Initial experimentation as a refracting material for low velocity polymer examinations and a low reverberation delayline has been successfully carried out. Keywords: Elastomer, low attenuation, couplant
Background In 1994, Ginzel and Ginzel [1] reported on a dry coupling elastomer that they referred to as Aqualene®. The material had unique acoustic properties that offered novel applications. Aqualene had acoustic velocity and acoustic impedance very near that of water (Vl=1590m/s Z=1.46MRayls). The material was found useful for several NDT applications but proved more popular with medical applications as its properties closely matched the acoustic properties of tissue. Another significant advantage of the material was its low attenuation (approximately 0.28dB/mm at 5MHz). In 2001 an internal report by J. Calvar and J. Méneur [2]at Ecole Nationale Supérieure d’Arts et Métiers, Paris compared several polymers identified as dry couplant materials in which Aqualene was identified as the best option by a factor of eight over the next material in the study. Users of the original formulation of Aqualene experienced problems with poor fracture toughness and low tear resistance. Modifications to the formulation were made that improved these slightly. The improvements increased the “tackiness” that improved the dry coupling effect. However, in spite of the improvements made the mechanical properties remained the most significant limitation of the material. In late 2007 it was decided to re-investigate options for a low attenuation material that could provide suitable acoustic coupling and significantly improved mechanical properties. Materials Research Institute in collaboration with the Polymer Research Institute at the University of Waterloo worked jointly on the research and development of the new elastomer. Results of the development are reported here.
Properties of the new Elastomer The material developed by the authors is based on a blend of block co-polymers and additives. Unlike Aqualene which was a clear thermosetting elastomer, the new material is a black opaque thermoplastic elastomer. A summary of the mechanical and acoustic properties is given in Table 1 where it is compared to the modified Aqualene. Table 1: Mechanical and Acoustic Properties
Property Density (g/cm3) Longitudinal Velocity (m/s) Transverse Velocity (m/s) Attenuation (long.) (dB/mm @ 5 MHz) Characteristic Impedance (MRayls) Poisson's Ratio Young’s modulus (Pa) Modulus of Shear (Pa) Transmission Coefficient of a Longitudinal wave from Water Colour Opacity
Aqualene
New Material
0.920 1615 800 0.28 1.486 0.337 1.58 x 109 0.6 x 109 0.999
0.937 1590 900* 0.65 1.490 0.264 1.92 x 109 0.8 x 109 1.000 Black Opaque
Acoustic Birefringence
Light brown cast Clear to slightly translucent N/A (isotropic)
N/A (isotropic)
Hardness Shore A Tensile Strength (MPa) Elongation at Break (%) Tear Strength (kN/m)
53 3.3 515 14.9
52.5 5.6 1199 21.5 *approx
Since the new elastomer material is not thermosetting with concerns for cross-linking, it has superior resistance to ultraviolet effects compared to Aqualene. It is chemically resistant to attack by most olefinic organic liquids and aqueous solutions of moderately concentrated acids and alkalis at room temperature. It is however, susceptible to solution in aromatic organic liquids (such as toluene and benzene).
A separate set of tests was run on the bonding options. It was found during the development of the previous material (Aqualene) that users were often required to bond the elastomer to a test surface or they wanted to bond a probe to the elastomer. It was therefore considered useful to analyse the options for adhesion. Analysis of all possible adhesives is of course not a feasible project. However, a rough grouping of classes of adhesives was carried out. A data sheet review of the components in various adhesives was used to eliminate any bonding agents containing aromatic hydrocarbons, since these would attack the elastomer structure.
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Two representative materials from each of two classes of adhesives were selected. Tests were run using the method described as Adhesion of rubber (vulcanized) to rigid substrate ASTM 429-03 Method D. The rigid substrate used was stainless steel. Four commercially available adhesives were selected from the one part or two part adhesive categories. Two were of the cyanoacrylate category and two were of a two part epoxy category. (Cyanoacrylates include the material often known by its common name “super glue”). Results of the adhesion tests are tabulated in Table 2. Table 2: Adhesion Strengths (MPa)
Material
380 (cyanoacrylate)
New Material Aqualene
480 (cyanoacrylate)
1.68 1.09
9430 (epoxy)
1.78 0.96
M-31CL (epoxy)
2.49 1.71
1.87 1.11
As a comparison of degree of strength the M-31CL epoxy for the bonding of the stainless directly to the stainless substrates was 4.18MPa. In all cases for each individual bonding agent the New Material provided a superior bonding surface compared to the Aqualene and the 2 part epoxy identified as 9430 proved to be the most effective adhesive agent. The ability to sustain relatively high frequency content is illustrated in the FFT plots made in Figure 1. FFT results were determined using a through transmission in water for a 2mm sample of the New Material and Aqualene. These standard 2mm samples were made using the ASTM D3182 “standard laboratory mould”. Signals were compared to the same path distance in water without intervening polymers. Each of the three conditions is normalised to the peak frequency transmitted and the trend to shift the original (water) frequency content lower is seen. For the configuration used the peak frequency in water was 10.9MHz, in Aqualene it shifted to 10.7MHz and for the New Material it shifted to 10.6MHz. Figure 1: FFT Plots
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FFT with 10MHz Probe 100
Water
90
New Material
80
Aqualene
70 60 50 40 30 20 10 0 0
5
10
15
20
25
Applications In the development of the material, two simple moulds were fabricated to assess suitability of the prototype material for ultrasonic NDT applications. The two mould forms included the standard 0° delayline and a custom wedge that was fitted to a phased array probe. These are imaged in Figure 2. For the phased array probe, the material was formed with a pre-calculated incident angle to fit a cavity in a holder for mechanised scanning. Initial investigation of the material as a low velocity wedge material suitable for High Density PolyEthylene (HDPE) suggests it has advantages over the presently recommended poly-tetrafluoroethylene (PTFE) (VL=1300m/s Attenuation 0.83dB/mm).
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Figure 2: Moulded delaylines
Figure 2 illustrating standard 0° delayline (lower left) and the custom phased-array wedge insert (upper left and right). Acknowledgments The authors would like to thank the Institute for Polymer Research at the University of Waterloo (Canada) for the use of facilities and assistance in the development of the formulation for this material. References 1. Ginzel. E.A., Ginzel, R.K. , Ultrasonic Properties of a New Low Attenuation Dry Couplant Elastomer, http://www.ndt.net/abstract/ut96/ginzel.htm, 1996. 2. Couplage sec: innovation industrielle dans le contrôle non destructif par ultrasons, J. Calvar, J. Méneur, Ecole Nationale Supérieure d’Arts et Métiers, Paris, France, 2001.
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