Don’t Duct (Duck) the Problem……….Use DADA
Some say you can fix any structure with WD-40, duct tape and bailing wire. Around the house this may be true, if you don’t care about the longevity of the repair. In the real world, the presence of duct tape (except on HVAC duct work) is a pretty good indicator of an underlying problem that can benefit from proper stress analysis and a structural redesign effort. Using duct tape for a structural repair is like using a Band-Aid to repair a broken bone ………..bad decision.
Structures can, and do, break, and this may be important and sometimes dramatic. You actually need to determine whether a particular object made from particular material can carry a particular load. If we ignore a true stress, the cost of not acting is usually far higher than the cost of dealing with the problem earlier.
What is DADA in Stress Analysis? At first glance, this all seems easy and common sense. Moving through the Data-Analysis-Decision- Action Strain Measurement chain wisely is a key to defining stress vs. strain. If you’re still collecting or analyzing data while your object is active, you are in trouble since there is always some risk of poor material being built into a structure.
Stress analysts are charged with the responsibility of finding practical solutions to practical problems. That’s a tough assignment because the solutions must satisfy not only the legal, financial, marketing, manufacturing, and other related demands but the technical aspects of the problem as well. And in many cases these technical aspects are difficult enough all by themselves.
Stress analyst must be half scientist and half artist to be successful. Pablo Picasso said that every child is born an artist; the problem is how to remain one once we grow up. But they seldom have the scientist’s luxury of reducing the number of variables in a system until some cause-and-effect relationship among a few variable of choice becomes apparent. The Stress analyst must deal with every variable (Data) that is part of the problem when seeking (Analysis) a solution (Decision).
A good example is in ductile materials the crack moves slowly and is accompanied by a large amount of plastic deformation around the crack tip. The crack will usually not extend unless an increased stress is applied. Geometrical irregularities, such as cracks, sharp corners and holes, which many times are being ignored may raise the local stress.
And while intuition is an important element in both art and stress analysis, the stress analyst never has the artist’s freedom to use tools (Action) of the trade to paint a picture of reality that is unsupported by the physical universe. For example, if traditional mechanics is not applicable to a new composite material, the stress analyst must use another paint brush to create solution that will work in the real world when it leaves the drawing board or blueprint.
When a problem’s variables are many and when theory and fact have gone their separate ways, the successful stress analyst never despairs. Rather, the realities of the problem are grasped with a firm hand and all the available tools – both analytical and experimental – are engaged to find a workable solution. A real stress analyst pushed forward, knowing full well that there is no perfect solution to the problem. But the “compleat stress analyst” also knows that a workable, economically viable solution will be found.
As a manufacturer of foil strain gages, installation accessories, and instrumentation , Micro-Measurements can supply the necessary hardware for making good strain measurements. But how good the actual measurements are depends upon the stress analyst’s overall understanding of the complete strain measurement system. As shown in the following diagram, the various components of that system overlap and interact.
What are the major components of the strain measurement DADA process?
The Strain Gage
The strain gage itself is the starting point of our DADA measurement system. This device is essentially a simple electrical foil resistor, but it has been specially designed to be easily bonded to the surface of a solid object and to undergo a change in resistance when a strain is encountered in the direction of its sensing grids. The stress analyst’s task, of course, is to bond the gage in such a manner that the strains are fully transmitted from the surface to the grids and to measure the resulting microhm resistance changes.
The Sensing Circuit
For most stress analysis applications, the Wheatstone bridge is the sensing circuit utilized to measure the resistance changes produced what active gages are exposed to strain. The bridge, in effect a dual voltage divider is normally powered by a constant voltage source of a few Vdc. It converts the very small resistance changes in the foil strain gage into millivolt electrical signals. The task here is to minimize the non-strain resistance changes in the gages and all other conductors within the bridge circuit and to fully understand the cause-and-effect relationships between resistance changes and bridge out-put.
Strain Instrumentation is complex equipment that can perform any or all of the following functions:
Supply bridge power
Provide bridge completion for quarter –ad half bridge installation
Zero balance the bridge
Shunt calibrate the system of strain gages
Condition and amplify the signal from the bridge
Set gage factor for direct readout of strain, or gain for scaling output signal from the instrument.
Provide analog-to-digital conversion of the signal
These instruments are usually designed either for static strain measurements or for dynamic measurements. Instruments used for static measurements are usually stand alone “ strain indicators” and may operate with null-balanced or unbalanced bridges (Micro-Measurements P3 Strain Indicator and Recorder).
For dynamic measurements, signal conditioning amplifiers operate with either unbalanced bridges or potentiometers circuits. Data Acquisition Systems which can sample at high rates can capture dynamic events in the time and/or frequency domains (Micro-Measurements System 8000 and System 9000). The stress analyst’s task is to select the correct instrument for each specific application.
The Measurement Data
When properly selected, installed, and instrumented, the electrical resistance strain gage can easily provide strain data having combined errors and uncertainties of less than one percent. One percent is more than adequate for most stress analysis applications. But it falls to the stress analyst to understand the sources of these errors and uncertainties and to ensure that the level of these inaccuracies is appropriate for the application at hand. And when greater accuracy is necessary, the task is to eliminate the errors by correction and the uncertainties by calibration.
How the strain data is used in stress analysis work is beyond our domain… but not our imagination.