Embedded Stress Sensors Research Articles

Characterization of Propellant Modulus and Rocket Motor Stress-Free Temperature Using Stress Sensors

Embedded stress sensors have previously been employed as part of health monitoring systems for solid rocket motors; however, the sensor readings were used to detect anomalies such as bore cracks and bondline delaminations. This work is unique because, instead of structural defects, the sensors are used to determine changes to both the modulus and the stress-free temperature: both of which are useful and difficult to obtain. To do this, sensors were embedded into a test motor (a small motor with a metal case, insulation, and an inert composite propellant formulation) and subjected to stepwise cooling cycles: the sensor readings at each plateau in the cooling cycle were a function of the stress-free temperature and modulus. When analyzed, the data gave both the modulus and stress-free temperature shift in the motor. The sensor and accompanying analyses were used to establish baseline readings; then, the motor was aged, and the procedures were repeated. Over an equivalent age of several years (about 4.5 years of aging at ambient temperature), the modulus changed by about 20%. Shifts in stress-free temperature were harder to detect and will be investigated in future work.

Electronics Outside the Box: Building a Manufacturing Ecosystem for Flexible Hybrid Electronics

As the electronics in our daily lives proliferate, they continue to be largely limited to rigid form factors with bulky packaging dictated by traditional electronics manufacturing processes and fragile components. Yet for applications ranging from wireless, low-profile medical devices to smart food labels to aircraft with embedded stress sensors, there's a need for high-performance electronics that conform to the shape of our bodies, vehicles, and consumer goods. Flexible Hybrid Electronics (FHE), which combine additive manufacturing processes with flexible silicon will enable these capabilities. To move these concepts from the lab to the manufacturing floor in the United States, the Department of Defense established NextFlex, America's Flexible Hybrid Electronics Manufacturing Institute in 2015. Based in San Jose, CA, NextFlex is a $170M public-private partnership that is building a domestic FHE manufacturing ecosystem by developing manufacturing processes and tools with its member companies and universities, standing up an FHE manufacturing pilot line in Silicon Valley, and establishing education and workforce development programs to train tomorrow's workforce. This presentation will focus on the FHE opportunity, the NextFlex FHE manufacturing roadmaps, and NextFlex projects in areas such as device integration & packaging, modeling & design tools, and printed flexible components.

Modelling stress distribution in substructure of French conventional railway tracks

Many studies were conducted on the modelling of stress distribution of soil foundations based on the elastic theory or uniformly spread hypothesis, but few works focused on the modelling of dynamic stress distribution in railway substructures. In this study, field monitoring was performed at Vierzon, France, using two embedded stress sensors. Dynamic stress data were recorded during train passages at different speeds, from 60km/h to 200km/h. Note that the Vierzon site involves a conventional line that is characterised by the presence of an interlayer soil, sandwiched between the ballast layer and the subgrade. The recorded data was firstly used to verify the existing models currently used in practice, revealing the advantages and drawbacks of such models. Then, based on the theory of stochastic stress diffusion in particulate media, a modified load spread model was developed. Comparison with the recorded data shows the relevance of the proposed model. In particular, significant effect of modulus ratio bewteen two adjacent layers on stress distribution was evidenced.

Four-Wire Bridge Measurements of Silicon van der Pauw Stress Sensors

Under the proper orientations and excitations, the transverse output of rotationally symmetric four-contact van der Pauw (VDP) stress sensors depends upon only the in-plane shear stress or the difference of the in-plane normal stresses on (100) silicon. In bridge-mode, each sensor requires only one four-wire measurement and produces an output voltage with a sensitivity that is 3.16 times that of the equivalent resistor rosettes or bridges, just as in the normal VDP sensor mode that requires two separate measurements. Both numerical and experimental results are presented to validate the conjectured behavior of the sensor. Similar results apply to sensors on (111) silicon. The output voltage results provide a simple mathematical expression for the offset voltage in Hall effect devices or the response of pseudo Hall-effect sensors. Bridge operation facilitates use of the VDP structure in embedded stress sensors in integrated circuits.

Detectability of Delaminations in Solid Rocket Motors with Embedded Stress Sensors

A finite element model is used to investigate the effect of delaminations on the radial stress distribution along the bondline during the cooling process of a solid rocket motor composed of propellant, insulation, and casing. Under the assumption of stress sensors evenly distributed along the circumference of the interface between the propellant and insulation, a relationship is established between the debond angle, the number of sensors, and the required sensor accuracy. Two approaches are proposed to detect debonds based on the radial stress readings from these embedded sensors. In addition, a quantitative mapping is obtained between the debond size and the sensor data to inversely estimate the extent of the delamination. It is demonstrated that the proposed framework can detect delaminations in solid rocket motors.

Embedded Stress Sensors for Strained Technologies Process Control

Mechanical stress management becomes a major issue for state-of-the-art CMOS technologies. Mechanical stress induced by the process steps is often at the origin of yield losses but also opportunities for technologies performance enhancement. Usual methods for mechanical stress measurement generally require offline measurements and are not compatible with fast correction of process parameters. We propose here embedded stress piezoresistive sensors to allow fast monitoring of mechanical stress and enable real time correction of the process parameters. The test vehicle presented here is dedicated to the gate structure stress monitoring. It is especially very sensitive to the nitride contact etch stop layer stress level. It is shown, in particular, that the structure is able to monitor process-related stress change in the nitride layer. Its feasibility, sensitivity, and relevance in an advanced process control scheme are investigated.

On the Hugoniot elastic limit in polycrystalline alumina

The use of polycrystalline ceramics in engineering applications requires a better knowledge of microstructural response than exists at present. The continuum response of the alumina AD995 to shock loading has been extensively investigated. This work aims to connect microstructural response with continuum observations. Over recent years, workers have reported failure in various polycrystalline ceramics occurring behind a propagating front running behind the shock. These phenomena have been investigated using embedded stress sensors and a recovery technique that has allowed the observation of the microstructure above and below the Hugoniot elastic limit (HEL). These results are brought together here to explain the observed behavior. The failure front velocity is found to change with the applied stress, in particular, it slows markedly as the HEL is exceeded. Further, the curvature of histories recorded by sensors may be related to the observed response. The evidence in the microstructure shows that the response below HEL is dominated by the intergranular failure, while above it by plasticity in grains (including twinning), which alters the deformation and failure characteristics. These microstructural features are combined with the continuum observations to review the response of the alumina.