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Product category: Loadcells, Force Sensors and Torque Sensors
News Release from: Sensor Products | Subject: Pressurex film
Edited by the Engineeringtalk Editorial Team on 03 July 2003

Novel technique shows surface stress
distribution

Until relatively recently, obtaining data on normal surface stress distribution and predicting stress distribution uniformity was, at best, a process of multiple iterations of trial and error.

Until relatively recently, obtaining data on normal surface stress distribution and predicting stress distribution uniformity was, at best, a process of multiple iterations of trial and error These iterations involved the use of carbon paper or conventional pressure measurement tools such as load cells and strain gauges (often too invasive or time/cost intensive to implement)

Engineers and scientists have now begun examining the use of thin pressure indicating films to study exactly how force is disbursed between any two mating or impacting surfaces.

Tactile force representation, or surface pressure measurement, can be facilitated through use of two distinct technologies.

One approach, which is the focus of this piece, involves static analysis only.

The other, which uses magnetic resistive inks, is a real-time system primarily intended for dynamic analysis.

Advantages of the static system are accuracy, costs, ease and efficiency of use, environmental immunity and spatial resolution.

The static system, known as tactile pressure sensor film technology (TPSF), is inherently nondestructive, relatively accurate, extremely efficient and easily calibrated.

TPSF (such as Pressurex film from Sensor Products) can be used in both laboratory environments simulating actual manufacturing conditions and also during the live process itself.

Furthermore, as the film actually "deforms" in the areas where contact or impact occurs, it captures a snapshot image that is quantifiable, yielding fairly precise values of force magnitude of any point across the contact surface area.

One might even consider application of TPSF as an extension of finite element analysis (FEA), with one substantive distinction: the TPSF is a post-process interpretive tool that actually collects stress and force data.

With TPSF, no longer must engineers sustain the iterative process of trial and error to achieve product optimisation.

Generally speaking, any situation where two components either contact or impact in a normal (perpendicular) fashion is a viable candidate application for using the TPSF technique.

A large fastener designer and manufacturer located in the Midwestern states of the USA needed assistance in redesigning a conical spring washer assembly.

The fastener manufacturer's customer theorised that it could reduce the size of the assembly; however, the company was uncertain as to how to quantify the change in load distribution and how the installation torque would need to be adjusted to maintain adequate pre-load.

The joint consisted of two 6.35mm-thick copper plates fastened together by a M12 x 1.75 bolt and nut with a conical spring washer.

The manufacturer knew ultrasonic measurements could assess the overall load on the bolt, but spoke nothing of distribution of that force.

The company investigated using load washers and thin-film transducers, but found that these instruments again only provided total load information and not distribution.

Furthermore, it wanted to determine the aggregate surface area under compressive load, which could not be determined by any of the aforementioned techniques.

Finally, using these techniques tended to be fairly invasive to the application and adversely affected the results by virtue or their presence.

The most cost effective and suitable option available was TPSF.

TPSF comes in the form of thin sheets, physically similar in appearance to a typical magazine page or sheet of paper.

These sheets range in thickness from 0.1016 to 0.2032mm and are structurally affixed to a Mylar substrate for stiffness.

While ensuring stiffness, the Mylar substrate is pliable enough to allow the film to fit between intricate and curvaceous surfaces.

The films come in various pressure sensitivities ranging from 2 to 1300kg/cm2.

An engineer first needs to calculate the approximate amount of pressure loading at the interfacial surface before selecting the applicable film range to use.

The film ranges are purposefully kept within precise limits to maximise accuracy of pressure mapping information.

If the ranges were wider, the levels of graduation of colour intensity would be so tightly spaced along the colour spectrum that pressure variation might be indistinguishable.

The company chose the highest pressure range available after performing some simple calculations that indicated average pressure over 492.146kg/cm2.

Pieces of the TPSF were cut to the dimension of the test fixture with a hole inserted in the centre for the bolt to pass through.

TPSF is thin and pliable and can be easily die cut to the dimensions of the interface being investigated.

To conceptualise the placement of the TPSF, consider this example: you are inserting an additional, very low gauge "gasket" in between two mating surfaces.

This "gasket", which represents a mirror image of the actual gasket in use, will be subsequently removed and examined after compressive load has been applied.

After the two contacting surfaces are bolted together with the TPSF in place, the interface must then be disassembled to remove the sensor film.

Development is instantaneous, so the user need not leave the interface bolted for any specified period of time.

A series of controlled experiments were performed after placing the TPSF in between the copper plates.

Various combinations of installation torque and washer size were used, and for each test, a sheet of TPSF was employed to capture the total force and force distribution.

The customer's desire was to obtain a combination of high contact pressure, uniform force distribution, and minimal bolt/washer widths.

Additionally, TPSF, when analysed with an optical image analysis system, is able to threshold an image to determine precisely how much contact area resides above and below certain parameters.

The manufacturer ultimately was able to make important decisions, in a timely and inexpensive fashion, regarding fastener specifications that became employed in their customers' product.

Now many organisations' ISO9000 programmes incorporate the use of this metrology technique as a means of quantifying and visually comparing normal surface stress distribution and magnitude.

TPSF is a single-use system, like photo film, capturing a snapshot of maximal pressure loading at a specific moment in time.

What the TPSF reveals is a very high resolution image of both pressure distribution and pressure magnitude.

The way the film works is quite simple.

Microcapsules embedded in the film rupture at precise pressure levels causing them to release chemical contents, which then interacts with a developer component, and thus produces a visible colour change.

This quantifiable colour change is directly proportional to the amount of pressure applied.

The user is able to visually inspect the film for significant variations aberration and general uniformity.

Spatial resolution of the TPSF system is 0.005 to 0.015 mm, yielding ultra-high-definition imagery of force profile.

The colour change of the film is both instantaneous and permanent, allowing for the film to be both immediately analysed and then saved for archival purposes.

Once a compressive load has been applied, the TPSF can now be analysed in several different ways.

The quickest and most economical technique is by visual comparison to a colour calibration chart (conceptually similar to using litmus paper).

The engineer can match the film's resultant colour to this calibration table to obtain an approximate indication of pressure magnitude.

Voids, pits, micro-cracks, warping, and surface aberrations are immediately apparent on visual inspection.

Typically the visual interpretation method yields accuracy of +/-15% full-scale pressure.

For applications or situations where shearing or tangential stresses are occurring between interfacial surfaces, a little ingenuity is required.

Small pieces of the sensor film can be applied selectively at the points of greatest concern, preventing the smudging and crinkling (noise) of the TPSF that might occur from sideways shifting.

A more sophisticated and accurate technique of analysis and interpretation is by using an optical imaging tool (such as Topaq from Sensor Products).

The pressure analysis system consists of Windows based software and a specially calibrated scanner that reads and interprets the film.

Images rendered by the image analysis are accurate to +/-2 to 4% full-scale, and are accompanied by a wealth of statistical data about the interface.

Key features of the image analysis system include the ability to produce histogram data of the image being analysed.

The system provides not only for analysing the entire interfacial surface, but also for the study of small and problematic areas (which can be enlarged and enhanced and carefully scrutinised).

Where implemented, TPSF as a measurement tool has generally resulted in a quite positive return on investment.

With relative ease, the two fundamental attributes of seal efficacy, namely total force and average pressure, can be visually assessed.

A technician can be trained in the TPSF technique in a matter of hours.

As a result of using the TPSF technique, manufacturers are able to assess optimal surface stress distribution required for a given interface.

The TPSF technique takes manufacturers and users a step closer to full-process standardisation.

The TPSF and the optical analysis system have become powerful tools in the arsenals of manufacturers' production and R and D facilities.

Using TPSF in conjunction with the image analysis system lends a high degree of statistical validation to interfacial analysis.

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