Stress measurement on any bolted object

Hi,

it depends what you want to measure. If only the force between bolt and object you can also use special ring transducers between the bolt head and the body you clamp.

The problem with those transducers is that they usually do not accept torques and when the bolt is tightened a friction torque appears. This is the reason they should be mounted not directly under the head.

If you want to define forces generated in the body by clamping with several bolts then you can apply strain gages on the body and measure them. Combining such measurements with a finite elements analysis you can get a full image of the generated stresses.

It is more difficult to obtain values if the bolt body is loaded in shear (calibrated bolts in calibrated holes). In such situations only the bolt with strain gages on it can be the "transducer".

To give a correct answer to your last question i should know if it is a single bolt or a group of fasteners you want to analyse, which are the dimensions of the shear section and how big you estimate the shear load on the shear section. It would be of help to have a sketch of the assembly to understand its nature.

Thanks a lot,

really it is a basic thing still complecated for me.I really want to measure the forces acting on the body from two perpendicular direction.The body is bolted on an object.Initially my plan was to use force washer transducer on these two perpendicular direction.Please refer the attached file for reference.But i think using a strain gauge will be the best option.Pl refer the attached file which shows all these cases.

i am not aware of using a strain gauge in my case.

The use of the washers would have lead to wrong measurements.

I still do not under stand how the "body" is fastened to the "parent". you you can introduce an interface it could be possible to obtain much better results with special transducers able to accept without distortions loads from several directions. I am myself active in the design of such specific sensors equipped with several bridges and able to characterize all the components of the tensor acting on a body. I have now on my desk a request for a special sensor for high velocity aerodynamic forces measurements.

If you want to have a more precise answer i am sorry but i should have more details, if you do not want to put them so that they are available to all you can mail them via direct post.

The washers would not have worked since the friction of the one due to the clamping would have perturbed the measurements of the other.

To measure a complex combination of forces and torques is i dare say an "art" based of course on science but it asks for a very thorough and in the depth understanding of interactions at all possible levels.

Hi,

i am attaching a ppt for better visualization of the load condition.I guess it will serve to understand my situation better.

The force measurement is a so called "series measurement" which means that the force can be measured ONLY if it fully goes through the "sensor". This is the difference with pressure or temperature measurements which are "parallel" measurements.

In your case you need a sensor able to CORRECTLY quantify two forces and its behaviour should be insensitive to possible parasitic effects or even to influences called cross-talk.

Sorry but it is still not enough although it is already better. Dimensions of the body? if it is fastened with only one bolt or more ? could it turn around the bolt axis? Estimated forces magnitudes? Could be an element inserted between "body" and"parent(the sensor)? Are any restrictions ? Environment? It seems that you need a "special" !

The body is not rotating along the bolt.size of the body is 120X70X90 mm.It is a hollow body of thickness 4 to 5 mm.Can i use strain gauge on the inner surface of the body to measure the force?

You could measure the strain (at the inner surface) that results from the hollow body being compressed by the bolt. The more you tighten the bolt, the more the wall of the hollow body (around the bolt head) will deform, and that deformation could be measured by a strain gauge. If the strain gauge is mounted inside the body (on the surface opposite the bolt head, for example), then if the body as a whole is in compression, the inner surface will be in tension. In this case, different locations of the strain gauge will produce much different measurements. If you tightened the bolt enough to see permanent deformation, you'd be able to see which areas stretch most: the expectation would be that the area directly under the bolt head would stretch most.

If you are instead trying to measure a force applied over the top surface of the hollow body by something external, then you would need to know the distribution of that force. Depending upon that distribution, there may be very little deformation of the material near the bolt head, with the load being accepted mainly by compression of the vertical walls of the hollow body.

Incidentally, if you are trying to retain this hollow body with a single bolt, to enable the bolt to be fully tightened, you would need to provide a compression tube in the hollow body.

The shear force could be measured (indirectly) by a strain gauge applied to the vertical wall of the hollow body. If this were mounted at about 45 degrees from horizontal, the strain could be expected to be larger (and the measurement more sensitive) than if it were mounted horizontally. Again, the distribution of force applied would affect the strain at the gauge. Also, the way in which the joint is bolted together would affect strain at the strain gauge: If, for example, the bolt has more clearance around the hole closest to the parent, then initially, the resistance to movement would be from parent/hollow body friction, followed by bending of the bolt, followed by eventually stopping the motion by the bolt as it emerges from the parent and local deformation around the bolt at that area.

For all the above, you would need to know the material properties and the details of the part geometry, to make any sense of the strain gauge readings. FEA would certainly give you a good indication of where to put strain gauges.

can i have your email id so that i can explain my concern in a better way.Once again thanks for your help.

Hi,

if you use 6 bolts, 2 in each of the x,y,z-directions oriented along a line for any direction, and provide a laterally compliant mounting that will allow only tensile forces on the bolts,

then the forces will be resolved to the coordinates and a measurement will give good results.

The "Intellifast"-company is manufacturing bolts with integrated piezo-sensors good for this purpose and used in safety critical equipment. (Not suited for high temperatures ).

RHABE

Stress is a derived quantity. You don't measure stress, you measure strain!!!