Damage significantly influences response of a strain sensor only if it occurs in the proximity of the sensor. Thus, two-dimensional (2D) sensing sheets covering large areas offer reliable early-stage damage detection for structural health monitoring (SHM) applications. This paper presents a scalable sensing sheet design consisting of a dense array of thin-film resistive strain sensors. The sensing sheet is fabricated using flexible printed circuit board (Flex-PCB) manufacturing process which enables low-cost and high-volume sensors that can cover large areas. The lab tests on an aluminum beam showed the sheet has a gauge factor of 2.1 and has a low drift of 1.5 μ ϵ / d a y . The field test on a pedestrian bridge showed the sheet is sensitive enough to track strain induced by the bridge’s temperature variations. The strain measured by the sheet had a root-mean-square (RMS) error of 7 μ ϵ r m s compared to a reference strain on the surface, extrapolated from fiber-optic sensors embedded within the bridge structure. The field tests on an existing crack showed that the sensing sheet can track the early-stage damage growth, where it sensed 600 μ ϵ peak strain, whereas the nearby sensors on a damage-free surface did not observe significant strain change.

Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are novel sensors designed to enable reliable early-stage detection of local unusual structural behaviors. Such a device consists of a dense array of strain sensors, patterned onto a flexible polyimide substrate along with associated electronics. Previous tests performed on steel specimens equipped with sensing sheet prototypes and subjected to fatigue cracking pointed to a potential issue: individual sensors that were on or near a crack would immediately be damaged by the crack, thereby rendering them useless in assessing the size of the crack opening or to monitor future crack growth. In these tests, a stiff adhesive was used to bond the sensing sheet prototype to the steel specimen. Such an adhesive provided excellent strain transfer, but it also caused premature failure of individual sensors within the sheet. Therefore, the aim of this paper is to identify an alternative adhesive that survives minor damage, yet provides strain transfer that is sufficient for reliable early-stage crack detection. A sensor sheet prototype is then calibrated for use with the selected adhesive.

The corrosion behavior of commercial tantalum in high-purity flowing sodium from 700 to 1200 F and the effect of sodium exposure on the creep strength of tantalum at 1200 F have been investigated. The tests were conducted in forced-convection flow loops constructed of Type 316 stainless steel. With continuously gettered sodium systems (probably containing less than 10 ppm oxygen) operating for periods up to 50 days, weight losses sustained by 1200 F specimens corresponded to only about 0.1 mil of metal removed per year. Maximum weight losses, encountered with a continuously cold-trapped (about 40 ppm oxygen) sodium system, were equivalent to about 3 mils per year for 1200 F specimens. For the most part, metallographie examinations revealed no deleterious corrosion effects; however, one particular group of arc-cast tubing specimens suffered severe intergranular attack in sodium with about 80 ppm oxygen. In addition to the corrosion findings, the corrosion tests yielded information about the partition of interstitials, particularly oxygen, in the sodium-tantalum system. Oxygen, for example, showed a tendency for migration from tantalum to high-purity sodium considerably stronger than would be predicted from limited thermodynamic data available. In the creep experiments, the oxygen content of the sodium was maintained at the <10 ppm level by continuous gettering. In general, the results demonstrated that sodium exposure at 1200 F had very little, if any, effect on tantalum creep strength. Baseline data necessary to establish this were obtained in supplementary helium-atmosphere creep tests.

The feasibility of two types of gages for measuring the plate separation in flat fuel-plate subassemblies was investigated with experimental model gages. One gage is based on a four-arm strain-gage bridge mounted on a flexible berylliumcopper beam. The deflection of the sensing beam as the gage traverses the channel is continuously recorded by a strain-gage analyzer. The second gage is a liquid-filled system consisting of a small flat pluager-and-cylinder arrangement. .4 rubber diaphragm between the piston and the cylinder serves as the seal. .4s the element is inserted in the space hettveen the plates, the piston is forced into the cylinder. The liquid, distilled water, thus displaced expands an external bellows. The variable expansion during a scan is measured by a differential transformer and recorded. The recorded output from either gage is directly proportional to the actual channel spacing and is reproducible to within 0.001 in. The general performances of the tno gages are equivalent except that an error of about 0.001 in. is introduced into the calibration of the hydraulic gage for each 3 F change in ambient temperature. The strain-gage-type gage is temperature compensated.

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