Measurement Of Dielectric Properties Of Plastic Tensile Specimens

By LAWRENCE C. LYNNWORTH*

Avco Research and Advanced Development Division Wilmington, Massachusetts

Introduction

New research and production-line applications for instruments of calibration-laboratory caliber are being motivated by reliability requirements for aerospace materials. These requirements, and the corresponding need for electrical measurements, may be explained as follows:

The tensile properties of metals, ceramics, and plastics exhibit considerable variability. Plastics, in particular, show variations attributable to composition nonuniformity, porosity, degree of cure, anisotropy, cracks or foreign inclusions, and absorbed moisture. In view of this observed variability, a nondestructive means of predicting the tensile properties of plastics is desirable. Prediction of properties becomes especially important in aerospace applications where safety factors are minimum.

One approach to the problem of predicting the tensile properties of variable

•Now with Parametrics, Inc., Waltham, Mass.

Frequency Range: DC to 9 Gc.

VSWR: Less than 1.001 + 0.001 X foc per connector. (Connectors are tested by pairs, and this figure is used as the test limit for a pair of connectors.)

Repeatability: Within 0.05%.

Leakage: Better than 130 db below signal level. Insertion Loss: Less than 0.003 \/foc db per pair.

plastics lies in the correlation of dielectric properties to the above-mentioned factors that influence mechanical properties. These correlations have been reported in the literature and occasionally

Figure 1. Standard tensile bar in dielectric sample holder. Pin in jig acts as stop for shorter specimen.

Figure 1. Standard tensile bar in dielectric sample holder. Pin in jig acts as stop for shorter specimen.

Figure 2. Hypothetical inverse relation when water absorption reduces tensile strength of a hygroscopic plastic.

provide the basis for quality-control procedures.1-8 For example, absorbed water weakens plastic, while increasing its dielectric constant K and dissipation factor D. Therefore, measurement of K or L) indicates water content, from which reduction in strength may be predicted.

In this note a fast and simple means of determining the degree of correlation between dielectric and tensile properties is described.

Discussion

In Figure 1, a tensile bar is centered in a General Radio Dielectric Sample Holder, Type 1690-A. The tensile bar conforms to the dimensions recommended in ASTM D638-61T; i.e., the gage length is two inches. This is fortunate, because the sample holder (see ASTM D150-59T) accordingly samples only the gage length.

The unorthodox removal of the side doors has not yet caused measurement difficulty, as was verified by tests on the Teflon specimens in the foreground. In an electrically noisy environment, however, shielding would be required.

Figure 3.

Closeup of probe and slabs.

Since the specimen is one-quarter inch thick, but the gage area is only about one square inch, C = ereoA/d is usually between one and ten pf. Therefore, a shunt capacitor is required, such as Variable Air Capacitor, Type 874-VC, to bring the capacity up to the range of the Capacitance-Measuring Assembly, Type 1610.

Procedure

In practice, to determine whether a useful correlation exists between dielectric and tensile properties, a tensile bar, after suitable environmental conditioning, is wiped dry, centered in the sample holder, and Cx and Dx are determined at 10 kc, for example. The bar is removed and is ready for tensile testing, with less than one minute added to the interval between pretest environment and tensile test.

When more than one factor is suspected of influencing tensile properties (for example, moisture absorption and resin-hardener ratio), it may be necessary to use two or more test frequencies.

Results

For a hypothetical case, wherein moisture absorption reduces tensile strength and is the only variable present, correlation is illustrated in Figure 2.

Figure 4. Capacitance vs thickness of air-baked dielectric slab.

C, picofarads(pf)

C, picofarads(pf)

Figure 4. Capacitance vs thickness of air-baked dielectric slab.

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