A numerical study on the thermal expansion coefficients of fiber
A numerical study on the thermal expansion coefficients of fiber A numerical study on the thermal expansion coefficients of fiber
35 need to be optically flat and reflective to act as their own mirrors. If this is impossible, as is usually the case with composite materials, mirrors need to be placed on the end surfaces, which poses questions of mechanical stability and correction for the expansion of the mirrors. Figure 3.4 illustrates the basic features of a Michelson laser interferometer. Figure 3.4 Basic illustration of the Michelson laser (http://www.pmiclab.com/index.html). Other factors requiring consideration are alignment of the mirrors, temperature homogeneity of the test-piece and the temperature stability needed to obtain stable fringes. In addition, the measurement normally has to be conducted in vacuum to avoid the distorting effects of convection in air along the beam path. The technique is best suited to accurate measurements near room temperature, particularly on low thermal expansion materials. Separate mirrors have to be used because composites cannot be adequately polished to give reflective surfaces. Resting the test-piece stably on a mirror and placing a second mirror on the top surface is the simplest configuration to use. The test-piece surfaces need to be accurately flat and parallel, and moulded surfaces may have to be machined to achieve the alignment conditions required for the mirrors. Care is needed not to unbalance systems in such a machining process. Typically, testpieces might be quite small, but for larger items made from low thermal expansion materials, carefully aligned mirrors can be glued to a face of the composite in order to achieve representative lengths to measure expansion.
36 3.3.3 Strain Gauges This relatively little-used technique has been evaluated on composite materials and found to be quite promising provided care is taken over calibration routines (Morrell, R. 1997). The technique used employs a thermal expansion reference material with an identical strain gauge attached to it and wired so that the differential expansion is recorded. There are considerable advantages in this technique for examining the spatial homogeneity of expansion or dimensional stability of large pieces inappropriate for dilatometry or interferometry. The main drawback is the temperature limitation of the gauge and the means of securing it to the material. Conventional foil gauges can be used typically to 200°C, and although highertemperature versions and appropriate adhesives can be obtained, they are relatively expensive and specialized, so that 200°C should be taken as the upper temperature limit. It is essential that identical installation techniques are employed on the test-piece and on the reference, and that the reference is well characterized over the required temperature range. In the tests they must also be at identical temperatures, and thus this technique should ideally to be performed at a series of temperature holds with stabilization periods, unless the test-pieces have sufficiently small thermal masses that the test can be done reliably during temperature ramping. The technique had been validated using copper and aluminum with ULE silica (Corning Glass Works, near zero expansion coefficient glass) as the zero reference (Morrell, R. 1997). Questions of the lateral sensitivity of strain gauges do not apply to isotropic materials as the test-pieces are not mechanically strained, and any secondary effects are eliminated in the difference calculations. Comparisons between mechanical dilatometry and the strain gauge technique had been carried out on metal matrix composites by Morrell (1997), and a good match was found to 200°C, above which the properties of the gauge and/or its adhesive affected the results. With anisotropic materials, lateral sensitivity is more important, and increases the potential errors in the results.
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- Page 21 and 22: 12 Metals are strong and tough. The
- Page 23 and 24: 14 Table 2.1 Properties of reinforc
- Page 25 and 26: 16 2.2.2.2 Carbon Fibers Carbon is
- Page 27 and 28: 18 use is in aircraft industry foll
- Page 29 and 30: 20 strength and a reasonable Young
- Page 31 and 32: 22 1. Processing the conventional f
- Page 33 and 34: 24 (orthorhombic) of polyethylene h
- Page 35 and 36: 26 Whiskers are monocrystalline sho
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- Page 39 and 40: 30 3.2.4 Thermal Cycling The primar
- Page 41 and 42: 32 3.3.1 Mechanical Dilatometry Thi
- Page 43: 34 absolute accuracy of about ± 0.
- Page 47 and 48: 38 • The composite is macroscopic
- Page 49 and 50: 40 3.4.1.3 Equation of Van Fo Fy In
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36<br />
3.3.3 Strain Gauges<br />
This relatively little-used technique has been evaluated <strong>on</strong> composite materials<br />
and found to be quite promising provided care is taken over calibrati<strong>on</strong> routines<br />
(Morrell, R. 1997). The technique used employs a <strong>the</strong>rmal expansi<strong>on</strong> reference<br />
material with an identical strain gauge attached to it and wired so that <strong>the</strong> differential<br />
expansi<strong>on</strong> is recorded. There are c<strong>on</strong>siderable advantages in this technique for<br />
examining <strong>the</strong> spatial homogeneity <strong>of</strong> expansi<strong>on</strong> or dimensi<strong>on</strong>al stability <strong>of</strong> large<br />
pieces inappropriate for dilatometry or interferometry. The main drawback is <strong>the</strong><br />
temperature limitati<strong>on</strong> <strong>of</strong> <strong>the</strong> gauge and <strong>the</strong> means <strong>of</strong> securing it to <strong>the</strong> material.<br />
C<strong>on</strong>venti<strong>on</strong>al foil gauges can be used typically to 200°C, and although highertemperature<br />
versi<strong>on</strong>s and appropriate adhesives can be obtained, <strong>the</strong>y are relatively<br />
expensive and specialized, so that 200°C should be taken as <strong>the</strong> upper temperature<br />
limit.<br />
It is essential that identical installati<strong>on</strong> techniques are employed <strong>on</strong> <strong>the</strong> test-piece<br />
and <strong>on</strong> <strong>the</strong> reference, and that <strong>the</strong> reference is well characterized over <strong>the</strong> required<br />
temperature range. In <strong>the</strong> tests <strong>the</strong>y must also be at identical temperatures, and thus<br />
this technique should ideally to be performed at a series <strong>of</strong> temperature holds with<br />
stabilizati<strong>on</strong> periods, unless <strong>the</strong> test-pieces have sufficiently small <strong>the</strong>rmal masses<br />
that <strong>the</strong> test can be d<strong>on</strong>e reliably during temperature ramping.<br />
The technique had been validated using copper and aluminum with ULE silica<br />
(Corning Glass Works, near zero expansi<strong>on</strong> coefficient glass) as <strong>the</strong> zero reference<br />
(Morrell, R. 1997). Questi<strong>on</strong>s <strong>of</strong> <strong>the</strong> lateral sensitivity <strong>of</strong> strain gauges do not apply<br />
to isotropic materials as <strong>the</strong> test-pieces are not mechanically strained, and any<br />
sec<strong>on</strong>dary effects are eliminated in <strong>the</strong> difference calculati<strong>on</strong>s. Comparis<strong>on</strong>s between<br />
mechanical dilatometry and <strong>the</strong> strain gauge technique had been carried out <strong>on</strong> metal<br />
matrix composites by Morrell (1997), and a good match was found to 200°C, above<br />
which <strong>the</strong> properties <strong>of</strong> <strong>the</strong> gauge and/or its adhesive affected <strong>the</strong> results. With<br />
anisotropic materials, lateral sensitivity is more important, and increases <strong>the</strong> potential<br />
errors in <strong>the</strong> results.
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