Strain Gage Testing
Response Dynamics performs strain gage installation, measurement, and analysis services 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 2 decades of experience in the art and science of strain measurement.
Links below provide more information and examples by industry:
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.
Our strain gage services include:
Experimental design, discussion of problem scope, strain gage selection and installation methods
Strain gage installation
Multi-variable measurement and analysis of acceleration, velocity, displacement, force, torque, temperature, or any other system parameter
Printed Circuit Board (PCBs)/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
FEA of stress concentrations and strain gradients
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.
Prototype Fan Blade Strain Testing,
Graph of strain gage data showing AC strain spectra at 894 RPM
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.
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.
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.
Multi-Channel Strain Gage Amplifier and Signal Conditioner
Printed Circuit Board Strain Testing - Components are attached to printed circuit boards, PCBs, by solder connections of various kinds. During assembly, installation, shipping, and/or use the PCBs 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 mounted electronic components on printed circuit boards.
The standard to which 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 to weakening, or failing, a solder joint depends not only on the strain levels, but on the rate of change of the strain levels, as the elasticity of the solder joint is, apparently, frequency dependent. This particular test uses the following standards:
Test procedure follows standard IPC/JEDEC-9704A, 2012 – February, titled Printed Circuit Assembly Strain Gage Test Guideline
The allowable strain levels are provided in IPC-WP-011, titled Guidance for Strain Gage Limits for Printed Circuit Assemblies” (Rate Limited)
Our client may have their own allowable limits as well that they have derived through testing of their product
Strain gage rosettes on PCB for testing solder ball joint strains as per IPC-WP-011
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
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.
Strain testing of a PC board to check component survivability
Strain gage testing to estimate force though steam pipe support