Strain Gage Testing Services

 
 

Response Dynamics provides strain gage testing sevices, including: Instalation, measurement, and analysis in conjunction with our Vibration Consulting Services on a wide variety of projects. We instrument and test structures ranging from small delicate medical devices to large industrial structures with over three decades of experience in the art and science of strain measurement.

FEA of stress concentrations and strain gradients

As strain gage consultants, our strain gage measurement services include the experimental design, that will often include other sensors. We select the proper strain gages, install them carefully with attention to surface prep. We consider the expected strain field in selecting strain gage locations, given the geometry of the structure and applied loads. We provide the cabling, signal conditioning, recording, monitoring, and time and frequency domain analysis, as part of our strain gage consulting services.

A strain gage is a thin wire pattern printed on an elastomeric backing to measure stretch in a material. From a measurement of strain, and using known cross sectional geometry and material properties, we can make estimates of the tensile and compressive forces, moments, torques, and shears that result from this measurable stretch. The units are often given in micro-strain (με), which is a tiny bit of stretch, a mm of stretch for a kilometer of structure, or a micron of stretch for a meter of length.

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Our strain gage services include:

 

  • Experimental design, discussion of problem scope, strain gage selection and installation methods

  • Strain gage installation

  • Strain measurement

  • Strain analysis

  • Stress analysis

  • Multi-variable measurement and analysis of acceleration, velocity, displacement, force, torque, temperature, or any other system parameter

  • Rotating systems

  • Printed Circuit Board (PCBAs)/Solder Joint & Component testing

  • Finite Element Analysis (FEA), Review of existing FEA models, and model validation through proper comparison with experimental test results

  • In-situ testing, On-Site testing

  • Testing in our lab

  • Analysis, troubleshooting, engineered solutions

Although strain measurement is often a key part of a root cause diagnosis, our Diagnostic Testing often also involve measurements of other parameters such as acceleration, relative displacement, pressure, temperature, flow, speed, load, dynamic stiffness, etc, to explore multiple parameters that may have a cause/effect relationship with the problem at hand. These measurements form part of a fact based hypothesis for the cause of the problem at hand. We use strain gage testing and/or strain modeling and analysis, in conjunction with our structural dynamics expertise to characterize all the important pieces of the puzzle that come into play with dynamic strain issues.

We often use Finite Element Analysis to create dynamic models of portions of a complex vibration/thermal/strain system that are time and frequency dependent, and may involve resonant amplification. We also use FEA to estimate stress concentrations on a location that is either too small to instrument, or has geometry that is too irregular to instrument with a strain gage. With our broad background in these relevant fields we are able to quickly and very efficiently characterize the problem and make recommendations.

Prototype Fan Blade Strain Testing, 
Graph of strain gage data showing AC strain spectra at 894 RPM

The Test Plan - Our strain measurement services include the experimental design, which is the key to extracting meaningful data. There are important considerations in selecting and installing strain gages, such as the type of environment (high-temperature, marine, corrosive material, destructive elements). Errors due to thermal expansion, thermal noise, non-linearities, and electrical noise must be considered. The geometry of the expected strain field under the expected loads, stress concentrations, high strain levels, the different strain fields that may result if the loads and boundary conditions are not as expected, are also a fundamental part of the experimental design that is necessary to allow the gathering of meaningful test data. (See our discussion on Vibration Induced Fatigue Cracking). The pitfalls and "gotchas" in strain gage measurement are endless. We have decades of experience identifying what is good data and what is bad data, and extracting meaningful information with meaningful error estimates.

Multi-Channel Strain Gage Amplifier and Signal Conditioner

Multi-Gage Testing - Multiple strain signals must be measured to discern the nature of the strain field. Three strain signals are necessary to determine the magnitude of maximum strain, if the axis of the strain is not known. A "rosette" of three strain gages is often used for this purpose. We often report strain in its principal axes that resolve the strain field estimate at a point into two orthogonal axial strains. The strain field at a point can also be represented by two orthogonal axial strains and a shear strain resolved in any arbitrary axes, using Mohr's Circle.

Multiple strain signals can also be combined mathematically to infer the nature of the load applied; i.e. axial force, bending moment, torque, and shear force. In the past this was done with various bridge arrangements (circuits of multiple strain gages) that will tend to give little output to the strains caused by axial force, and respond with a reinforced signal to bending, shear, or torque.

Today, multi-channel measurement allows for the digital processing of strain measurements to combine them mathematically with relative ease. For instance, our VMS1 Monitoring System will combine signals mathematically to produce a live digital signal of principal strain, bending strain, shear strain, torque, or any combination our clients may need to track, record, trigger off of, and characterize in the Time or Frequency domains. When necessary, a bridge circuit of multiple strain gages is still helpful, however, to improve signal to noise at the sensor location.

Strain gage testing to estimate force though steam pipe support

PCBAs, Electronics & Semiconductors

 

Printed Circuit Board Assembly Strain Gage Testing - Components are attached to printed circuit boards, PCBAs, by solder connections of various kinds. During assembly, installation, shipping, and/or use the PCBAs may bend and strain the solder joints leading to immediate failure, or an unreliable product down the road. We have performed strain measurements to determine the risk of failure to mount electronic components on printed circuit boards.

Strain gage rosettes on PCBA for testing solder ball grid array (BGA) strains as per IPC-WP-011 / JEDEC-9704A

The standard we often test requires measurement of both the principal strain and the time rate of change of strain, Strain Rate, characterized for various parts of the manufacture and assembly process. The risk of weakening, or failing, a solder joint depends not only on the strain levels, but on the rate of change of strain level, as the elasticity of the solder joint is, apparently, frequency dependent. One particular test procedure we use follows standard IPC/JEDEC-9704A, 2012 – February, titled Printed Circuit Assembly Strain Gage Test Guideline. The allowable strain levels, for instance, are provided in IPC-WP-011, titled Guidance for Strain Gage Limits for Printed Circuit Assemblies(Rate Limited), but our client may have their own allowable limits as well that they have derived through testing of their product.

Based on IPC/JEDEC-9704A, the highest priority components to measure are the components that utilize ball grid array (BGA) connections. Strain signals were measured from each of the three strain signals coming from each of the four rosettes mounted near the four corners of the graphics processor chip. The raw strain signals were digitally processed in real-time to compute the two principal strain signals and the strain rates.

Strain and strain rate, time domain signals

Strain vs. strain rate for IPC-WP-001

Strain gage measurement on PCBA investigating solder ball connection strains in various loading configurations

Recommended strain gage placement for BGA components

We provide strain gage services to compliment our analysis in many areas of the electronics industry. This includes R&D of the many high resolution tools and processes of the semiconductor industry, to the product level of the electronics industries.

In the semiconductor industry, at the R&D phase, we have used strain gages to help develop the cutting edge high-tech semiconductor equipment that is part of the effort to produce electrical components of ever diminishing size. As vibration consultants in the semiconductor industry for over 3 decades we have been involved in many unique projects where strain is of interest. In validating and trouble shooting a design we may use strain gage testing, in conjunction with our Vibration Testing Techniques, to deduce the force transmission paths, or to estimate vibration isolation effectiveness. In this area the strain gages have been used to determine the load acting on the vacuum chamber of a scanning electron microscope from a novel turbo-pump isolation system. Micro-vibration limits how small lines can be etched, or how well a tool can resolve an image. Design for mitigation of vibration is a major consideration.

For example, we have used strain gages to separate out the moment and shear forces acting at the bearing frequencies of the high speed vacuum turbo pump to evaluate various novel vibration isolation designs. Our approach allowed us to efficiently quantify the residual transmitted forces. Knowing the force geometry, phase, and spectral content, our team was able to optimize the isolation mount. We could also provide customers with valid estimates of resulting pump induced chamber vibration without testing the pump on the large variety of vacuum chambers sizes. These turbo pumps are used in scanning electron microscopes and other process and inspection tools.

Semiconductor cleanrooms contain some of the most sophisticated technologies, we have designed tests for many of these tools

Scanning Electron Microscope (SEM) Image, showing image disturbance

Contact Us:
Call Reuben Hale, P.E. at 510-507-1300
or click this email link:
Reuben@ResponseDynamics.com
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