Strain and Stress and Stress Analyses Driven Non-destructive Testing

Measurement of strain and upstream stress analyses can be used before non-destructive testing to identify areas of abnormal stress levels for targeting non-destructive testing.  Similarly, strain gaging and stress calculations can be used downstream of non-destructive testing to assess stress levels of assets.  This article explores the how strain gaging and stress analyses can be used in conjunction with ultrasonic and acoustic emission testing. 

Figure 1: Uniaxial strain gage installed on anchor bolt to confirm ultrasonic bolt tensioning results.

Introduction to Strain Gaging and Stress Analysis

Strain gage installation is a time consuming multi-step process that required multiple surface preparation steps, specialty strain gage materials and mechanical and/or soldering joint connections.  Shown below is a uniaxial strain gage installed on a bolt to use to measure axial strain or stress.    Uniaxial strain gages used to support non-destructive testing are relatively small ranging from an 0.25” to 1” gage length.  Strain gage installation requires that the surface be cleaned thoroughly to remove any dirt, grease, or contaminants.  The cleaned surface must be abraded to create a smooth, clean, and flat area where the strain gage will be installed.  A solvent is then used to remove the residue created.   The strain gage must be positioned accurately to align with the direction axial strain.  Once aligned,  a thin layer of adhesive is applied to the prepared surface and to the back of the strain gage.  The strain gage is placed carefully onto the adhesive-covered surface, ensuring it is positioned accurately.  Even pressure is applied to the strain gage and allowed to cure.  Strain gages come with or without pre-soldered leads and due to the complexity of soldering small diameter wire onto very small dimension strain gage solder pads it is recommended to use gages with pre-soldered leads.   Finally, the strain gage factor and resistance of the strain gage must be properly selected on the data acquisition system to assure correct data values.  The gage factor (GF) or strain factor of a strain gauge is the ratio of relative change in electrical resistance R, to the mechanical strain ε. 

Ultrasonic Bolt Tension Monitoring

Ultrasonic bolt tension monitoring instruments are used to measure the elongation, load, stress, and % strain of a tensioned fastener.  Non-destructive measurement of the above parameters required transmitting a longitudinal ultrasonic wave through an unloaded and tensioned fastener.  The time-of-flight change between the two states is used to assess the bolt elongation [3].   This NDT measurement technique uses a single wave and as the bolt is tightened, the bolt length increases and the longitudinal wave velocity decreases.   For redundancy purposes and safety factors, it is recommended to confirm ultrasonic bolt measurements with uniaxial strain gages for applications that require precise tension measurement or fracture critical installations.  Example uniaxial bolt strain measurement results for a tension bolt that was previously

Figure 2: Uniaxial strain showing gradual increase during fastening, temporary hold, and then load release. Max strain values were compared to NDT ultrasonic bolt tensioning measurements.

Overhead Sign and Lighting Pole Non-destructive and Strain Gage Assessment

Overhead sign and lighting poles due to their cylindrical structure lend themselves to cost-effective inspection with ultrasonic phased array, guided wave ultrasonic testing (GWUT),  eddy current, alternating current field measurement (ACFM), magnetic particle testing,  liquid penetrant testing and acoustic emission techniques.   In a simulated test bed,  it has been shown that the entire light pole structure may be screened for flaws from a single sensor position using GWUT technology.  Earlier work by TKS explored the feasibility of using permanently mounted sensors to track damage over the lifetime of the pole.   The light pole was fatigued for over  1,200,000 cycles.  During this fatigue test, guided wave data were acquired and waveform features were extracted to determine if the technology was sensitive to fatigue cracks in the bottom weld of the structure.  The research showed that the technology was sensitive to cracks as small as 0.25”.  It was also shown that waveform features may be used to track the damage in the pole over its lifetime.   The permanently installed sensor performance was validated over the fatigue lifetime of the pole via high quality and reproducible from known physical features on the light pole.    Strain gage data was used to correlate the increases in pole axial strain with GWUT damage metrics.

Figure 3: Uniaxial strain position on overhead signage for axial strain measurement.

Fiberglass Reinforced Plastic Tank Stress Analysis

As FRP tank age, non-destructive testing is required to assess the asset for structural flaws and the tank’s load bearing capacity of the tank.  Fiberglass tank non-destructive testing techniques include ultrasonic thickness testing, thermographic imaging, visual inspection, and acoustic emission testing.  Strain gage evaluation is used to assess axial and hoop strain for localized bulging due to FRP tank flaws which may include corrosion barrier thinning, interior/exterior physical damage, structural layer fiber breaks and resin matric cracking.  Axial and hoop strain gages are applied to a properly prepared FRP tank shell surface.  Compared to a metal tank surface, an FRP tank shell surface is often rougher and requires extra surface preparation and cleaning. The strain gages are installed at targeted areas based on the NDT inspection. For a general strain assessment, the gages are applied at 3 or more elevations around the tank circumference.   After installation on an empty tank, the tank is filled to capacity preferably with product with comparable specific gravity,   The hoop and axial strains are measured and compared to ASME -RTP-1 code for compliance. 

Figure 4: Hoop and axial strain gage measurement on FRP tank.

Summary of Strain Gage Testing and Non-destructive Testing

Strain gage assessment can be used upstream or downstream of non-destructive testing.  Fracture critical areas can be monitored for spikes in strain to prompt follow-up nondestructive testing, maintenance, or repairs,  Similarly, thinning areas, fatigue crack areas, and other structural flaws detected with NDT can be monitored for compliant stress levels with strain gages. 

References

1. Instruction Bulletin B-130 Micro-Measurements Strain Gage Installations with M-Bond 43-B, 600, and 610 Adhesive Systems

2. Vishay Precision Group General Purpose Strain Gages—Linear Pattern linear strain gage for NDT spec

3. Miao R, Shen R, Zhang S, Xue S. A Review of Bolt Tightening Force Measurement and Loosening Detection. Sensors (Basel). 2020 Jun 2;20(11):3165. doi: 10.3390/s20113165. PMID: 32498473; PMCID: PMC7309045.