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Shear-stress sensors can be found in useful applications of building, environmental, oil & gas, and many other industries. Micro-scale shear-stress sensors are now being developed to be used in the fields of medical and environmental sciences. In medicine, for example, the micro-scale sensor could measure variations in shear stress along the wall of an artery and help researchers determine the effect those fluctuations have on the development of arteriosclerosis, commonly known as hardening of the arteries. In this report, we have investigated on the background of Shear Stress, the types of Shear Stress Sensors currently being used and which have been developed. In particular many Micro Shear Stress applications and devices have been discussed. Following these, we have some discussions on the practical and improved applications of these sensors and the research on-going behind them.
BACKGROUND THEORY OF SHEAR STRESS
Stress is defined as a measure of the average force per unit area of a surface within a deformable body on which internal forces act upon. The measure of the intensity of the internal forces acting between the particles of a deformable body across imaginary internal surfaces is defined as stress (CHEN, Wai-Fah and Han, Da-Jian, 2007). These internal forces results from the reaction to external forces applied on the body where the external forces are either surface forces or body forces. Since the loaded deformable body is assumed to have continuum behavior, these internal forces are distributed constantly within the volume of the material body. (CHEN, Wai-Fah and Han, Da-Jian, 2007)
Normal Stress is known to be the most common type of stress in field of mechanics. A stress in general is defined as the load divided by the area at which the load is on. In the case of a normal stress, it is the loads that are perpendicular to the area. This can be formulated as follows:
s = P/A
where the variables are as shown in the following diagram:
We can also express this concept in a way to define the total perpendicular load P as the following equation (AMEEN, Mohammed, 2005):
Apart from normal stress, another important concept is shear stress that has to be considered and calculated. All materials need to be designed to cater to both normal and shear stress confines.
The average shear stress is given as the shear load divided by the area of implication of the load. This is given by the following formula:
t = V/A
General Shear Stress
From the above, re-writing the formula, we can define general shear stress as the following:
Where t is the general shear stress, F is the force applied and A is the cross sectional area.
Other forms of shear stress
Other forms of shear stress include the following (TIMOSHENKO, Stephen P., 1983):
ï¿½ Beam Shear - Beam shear is defined as the internal shear stress of a beam caused by the shear force applied to the beam.
ï¿½ Semi-monocoque shear- Shear stresses within a semi-monocoque structure.
ï¿½ Impact shear- The maximum shear stress created in a solid round bar subject to impact
Shear stress in fluids
Any fluids including liquids and gases moving along solid limit will acquire a shear stress on that boundary. The no-slip condition which is explained in the work by (DAY, Michael A., 2004) explains that the speed of the fluid at the limits (relative to the boundary) is 0, but at some elevation distance from the limit the flow speed must be equal to that of the fluid. The area between these two points is rightly called the boundary layer. The shear stress is proportional to the strain rate in the fluid for all Newtonian fluids in laminar flow where the viscosity is the constant of proportionality. However for Non Newtonian fluids, this does not stand true as its not constant. The shear stress is inflicted onto the boundary as a result of the loss of velocity. In the case of wind, the shear stress at the limit is called wind stress. (TIMOSHENKO, Stephen P., 1983)
TYPES OF SHEAR STRESS SENSORS
Wall shear sensors
A large number of conventional techniques exists for determining wall shear stress as illustrated in the work by (WINTER, K., 1977) (CAMBELL, T. Hanratty and J., 1983) (M. SHEPLAK, E. Spina and C. McGinley, 1994). But generally we do not use these sensors for this type of turbulent application as they exhibit poor spatial and temporal resolution (A. PADMANABHAN, H. Goldberg, K. Breuer and M. Schmidt, 1995). Nevertheless, some non-silicon measurement techniques have proved to show some good progress as illustrated in the work by (JOHANSSON, H. Alfredsson and A., 1988). The inadequacy of the traditional measurement techniques can be surmounted by the use of silicon micromachined sensors. Silicon wall shear sensors are often classified in 2 categories namely floating element sensors and thermal sensors. The floating element sensor is an example of a sensor that can do direct force measurement. When the flush mounted element is moved sideways due to the shear stress there is a restoring force set up by the springs. The floating element dislocation can then be trans-duced and measured quantitatively using some techniques as explained by (M. SCHMIDT, R. Howe, S. Senturia and J. Haritonidis, 1988) or by optical (A. PADMANABHAN, H. Goldberg, K. Breuer and M. Schmidt, 1995) detection principles. Various thermal wall shear stress sensor principles are founded on the phenomena of gas cooling of an electrically heated mass which is also known as the hot-wire/film principle and on the phenomena of the flow-induced temperature gradient on a heated chip (HUIJSING, B. Oudheusden and J., 1988).
MEMS based sensors
Micro-Electro-Mechanical Systems, or MEMS, is a form of technology can be generally defined as miniaturized mechanical and electro-mechanical elements including devices and structures) that are produced using the methodologies of microfabrication. The significant physical dimensions of MEMS devices can fluctuate from below one micron on one limit of the dimensional spectrum, and up to several millimeters. Similarly, the kinds of MEMS devices can also be diverse from considerably simple structures, to extremely complex ones with multiple moving components under the control of integrated microelectronics. (MNX, 2010)
Using recent developments in silicon carbide (SiC) MEMS fabrication techniques enabled the conceptualization of a new series of sensors that influences the high temperature potential and capabilities of silicon carbide. The shear stress sensor is one of these concept that can operate over a wide and dynamic range, and at very high temperatures.
Utilizing the advantage exhibited by MEMS fabrication techniques, problems that we had with spatial resolution, frequency response, acceleration, vibration and pressure gradient, can be diminished since we are now able to develop a really very small sensor super-light element that can respond fast and efficiently to changes in the flow (MCCARTHY, M., Frechette, L.G., Modi, V., and Tiliakos, N., 2003). A picture of the first generation sensor element made out of a multi-layered Silicon Carbide structure is shown in the figure below:
Thermal micro shear stress sensor
The principles of micro thermal shear stress sensor as explained by (JIANG, F. et al., 1997) (JIANG, F. et al., 1996) are briefly explained below.
This sensor comprises of a thermal sensor part located on the surface of a diaphragm. The thermal sensor element exists with a velocity boundary layer where the velocity increments from 0 to the value of the mean-stress flow. The shear stress is defined as per the following equation:
where ï¿½ represents the viscosity of the fluid and Uy gives the flow velocity at a distance y from the wall. The flow shear stress decides the rate of heat transfer from the heated part to the surrounding flow field. The change in temperature of the resistor is measured by the change in its resistance. The resistance R of a semiconductor sensing part at an eminent temperature T is given by the following equation:
Flexible shear stress sensor skin
Sometimes we need to have the realtime 2D physical factors of a 3D body e.g. temperature, force, pressure or shear stress for some kind of important applications.
For objects with flat surface, the use of a monolithic MEMS device with a large amount of sensors (JIANG, F. et al., 1996) is required to achieve this. But this proves to be difficult for non-planar surfaces of objects.
As an example to this we see that the most popular research objects used in aerodynamics study have non-planar or even high-curvature surfaces. Previously, if real-time distribution measurement was required, encompassing all the discrete sensors on a surface was found to be the only way to make this possible.
However, there have been lots of limitations to good measurements of parameters due to large sensor size and difficulty in packaging.
A new microfabrication technology that allows the incorporation of MEMS devices on a flexible polyimide skin has been developed as explained in (JIANG, F. et al., 1997).
In mechanical terms, the flexible skin comprises of many individual Silicon islands (which is a compulsory necessity for silicon MEMS devices) that are joined together by a thin/thick polyimide film (usually around 1-100 mm of thickness).
Silicone diaphragms are initially made with a wanted thickness (usually in the range 10-500 mm) by Silicone wet engraving and then patterns are made from the back side by reactive ion engraving (RIE) to create the islands. (JIANG, F. et al., 1997)
The figure below shows the picture of a biscuit-sized flexible skin.
APPLICATIONS OF SHEAR STRESS SENSORS
Shear Stress Sensors have been used widely in aerodynamics. Below are some discussions about their applications in the aerodynamics field:
Flexible shear stress sensor skin for aerodynamics applications
The use flexible skin technology solves many problems that were faced in the challenging topic of packaging for a large distributed sensing system.
Sensor packaging is now even much easier by completely discarding the fragile bonding wires and instead combined them with the newly developed backside contact technique.
The work by (XU, Y. et al., 2003) describes this improved flexible MEMS technology and its application to the fabrication and packaging of pragmatic shear stress sensor skins.
Successful testing and implementations in wind-tunnel have been made using an airflow separation detection system which included these skins, MOSIS bias circuits and along with a data acquisition unit. It is now being used for the aerodynamic study of a MEMS controlled super-maneuverable low-altitude unmanned aerial vehicle (UAV) .
It is important to do direct measurement of aerodynamic on the surface of an aerial vehicle but it is not that straight-forward.
This main setback is the unavailability of suitable sensors and the packaging issues as described above. Therefore, MEMS sensors look as the perfect potential for this application all due to their small sizes.
Since the surfaces of an aerial vehicle are not always flat, packaging still remains a big challenge. As explained in earlier sections, we have seen a possible solution by (JIANG, F. et al., 1997) that makes use of flexible sensor skin concept that ranges of MEMS sensors can be made into a flexible skin so they can be applied to a 3D surface.
But now the problem is the packaging of skins itself for this wide application. And today the major means of connecting sensors out is still by the use of traditional fine wire bonding and electrical wire soldering. This is not straight-forward and can get quite messy and also unreliable. This is shown in the picture below:
(JIANG, F. et al., 2002) has researched on these limitations and developed a new skin technology with many highly improved features.
One example of the improvement is that the new skin technology uses DRIE to better yield and allows backside solder bonding to reduce front side roughness.
Most importantly now bonding wires can be totally discarded as the new skins can be array-bonded to flexible PCBs.
These new improved developments are illustrated in the production and packaging of new shear stress sensor skins. These new stress sensor skins have been designed for airflow separation detection on the foremost edges of a delta-wing aircraft.
Another application in this area was developed by (XU, Y. et al., 2003) for UAVs. They developed a flexible shear-stress sensor skin. Its application was the detection of leading edge flow separation for delta platform unmanned aerial vehicles (UAVs). The sensor skin contains a one dimensional array of 36 shear-stress sensors. These sensors cover the 180 degree-surface of the 1.27 cm diameter semi-cylindrical UAV leading edge with 5 degree resolution.
A packaging scheme was used to simplify the implementation process of the sensor skin. This scheme was based on solder bonding and flexible printed circuit board (PCB). Successful testing was achieved by (XU, Y. et al., 2003) in both wind tunnel and real flight tests detected the flow separation along the leading edge.
INNOVATIVE MEDICAL APPLICATIONS
Researchers from imecï¿½s associated lab at Ghent University have created an ground-breaking way to fabricate shear sensors to be used on flexible surfaces, such as a human skin. These new sensors are made by the principles on an optical technology embedded in thin flexible substrates. This is illustrated in the figure below:
Shear sensors measure shear stress. Shear stress as defined in earlier sections are stress that are applied in parallel to the surface of the material and not perpendicular. Most shear sensors are based on micro-electromechanical systems (MEMS) which were explained in earlier sections of this report, and also are fabricated on rigid Silicon substrates.
Though these sensors can have a high density and sensitivity they are comparatively thick and does not have that much flexibility. Moreover they are more sensitive to electromagnetic interference as their activity is based on electrical measurements.
There is quite a high demand for sensors which can be used to measure modestly the shear stresses. To be able to be used on moving body parts and/or curved surfaces they need to be compact and flexible. There is a high demand of these sensors from the medical industry, which may use these for example to measure skin friction between a prosthesis and the stump. But these can also be used in robotics to create sensitive artificial skin.
The researchers at IMEC have recently come up with a way to create such flexible shear sensors by the use of optical technology. Since optiocal sensors have a high sensitivity, a large dynamic range, and are not susceptible to electromagnetic interference noise and can also be embedded in flexible subs, this makes them the most appropriate potential candidate for a very compact, robust and flexible use.
These innovative sensors were developed using a process that embeds optical components into very thin and flexible substrates (down to 50ï¿½m). An example of one sensor like this is a a stack consisting of a vertical-cavity surface-emitting laser (VCSEL) source and a photodiode separated by a transparent deformable layer made of silicone. The VCSEL and the photodiode are opposite in the setup so that most of the laser light can get captured where there is no shear stress. An increase in shear stress fluctuates the intensity of the laser light which is then relative to the measure of the shear stress.
The prototype sensors were created by using a layer of the silicone material namely the Sylgard 184. The deformation of the silicone material exhibited a linear response to the applied shear stress. Going forward, the researchers at IMEC are working to become accustomed to the sensor design so that it can also indicate the direction of the shear stress. (IMEC, 2010)
As shown in Figure above, the basic structure of the sensor is a polysilicon resistor mounted on a nitride diaphragm. It has a vacuum cavity below. This provides superb thermal isolation that can help to reduce the heat loss to substrate.
In this design setup, the input power of the resistor will vary with the wall shear stress from the ambient flow field. This variation can be easily detected electronically and also measured.
This design is conceived based on the aerial shear-stress sensor described in (LIU, C., 1994) (JIANG, F., 1996).
However, it is not straight forward to modify the aerial designs for underwater applications. There are majorly 2 challenges that arise when we try to do this. First is the development of a compatible waterproof coating so that the sensor is able to operate under water for quite a long time. And, the second challenge is reduce the cross-talk as much as possible from the sensorï¿½s pressure sensitivity. This does not pose as a problem for aerial applications but become crucial when it comes to underwater applications. And also, the water pressure exerted can change considerably. These 2 challenges were taken into great consideration by (XU, Y. et al., 2002)while developing this application.
Successful development of Micromachined thermal shear-stress sensors for underwater applications has been tested by (XU, Y. et al., 2002). A waterproof material such as parylene was used as the waterproof material and sensors were also coated. This setup could sustain for at least 1 month underwater. The pressure sensitivity could be minimized by adjusting the diaphragm width or the resistorï¿½s length. Although 0 pressure sensitivity is preferred using a large and thin diaphragm, small and thick diaphragm was used to achieve better yield and larger operating range. (XU, Y. et al., 2002)
High temperature applications
Micro-electro-mechanical systems (MEMS) are an enabling technology that has help to develop many miniature sensor concepts. Utilizing recent innovations in silicon carbide (SiC) MEMS fabrication techniques has helped to develop a new series of sensors that takes advantage of the high temperature capabilities of SiC. One such sensor concept was illustrated in the earlier sections of this report.
(TILIAKOS, N. et al., 2002)has developed a MEMS-based shear stress sensor that can be used to directly measure the shear stress on ground based articles tested in hypersonic aeropropulsion tunnels, and eventually on flight-test articles.
As introduced earlier, shear stress sensors use either direct or indirect methods for detecting shear stress. In general, massive, mass-spring-dashpot type skin friction gages are used within the inside of a test article at a single distinct location. These ï¿½macroï¿½ skin friction gages require active cooling so that the operation remains below a temperature of 250ï¿½C.
Moreover by making use of these, we need make a tradeoff between sensor special resolutions with the small forces measurement ability. Also, the measurement need be prone to increased indecision due to wrong-alignment, pressure gradients, acceleration, thermal expansion or vibration.
In terms of indirect shear stress measurement methods, the shear stress is obtained from measurements that are relative to the shear stress through some physical principles for example heat transfer. There is a common problem in flow sensors based on-heat transfer methods. The problem is that the heat losses to the substrates are usually unknown and therefore affects the sensors performance.
Their soundness in complex flow fields and/or with high heat transfer rates can be negotiated.
Moreover, due to limitations in material capabilities, most available sensors are limited in their operating temperature.
Making the most of the advantages of MEMS fabrication techniques, problems that were related to spatial resolution, frequency response, acceleration, vibration and pressure gradient can be alleviated. This is possible since the small-mass sensors by MEMS can react fast or instantly to changes in the flow. (TILIAKOS, N. et al., 2002)has considered these limitations and developed a MEMS-based shear stress sensor for high temperature applications that is very efficient.
Figure below shows a complete assembly of the sensor for installation into an aero-propulsion tunnel.
In this report, we have introduced the concept of shear stress and have also introduced the shear stress sensors along with its different types of sensors. We have also discussed about the current advancement in that sector of the industry. The various applications of shear stress have also been evaluated and investigated. Looking forward, we can see that the shear stress sensors have great potential and further research will be performed to come up with even better devices and applications. In the near future, we might see more task or time critical applications out of shear stress sensor based devices.