Synthetic aperture focusing technique (SAFT) is one of the most-used ultrasonic imaging algorithms. However, by considering only the direct sound path (i.e., pulse-echo signals) for image reconstruction, this technique is unable to show steeply inclined interfaces and bottom boundaries of objects in concrete structures. To address this problem, the current study considered various sound paths when applying SAFT for concrete elements. The proposed hypothesis is that different sound paths will provide access and imaging opportunities to previously inaccessible areas of the inspected volume/region. As a proof of concept, the proposed method, namely the ray-based SAFT, was tested with ultrasonic shear wave data for two concrete slab specimens. The results indicated a significant improvement compared to conventional SAFT imaging for simulated rebar debonding, vertical boundaries of slabs, and bottoms/sides of tendon ducts.
Overlap shear splices are common in civil FRP strengthening applications when the overall length of a saturated ply
becomes prohibitive or access to the surface being strengthened is restricted. The objective of this research was to
develop a standard test method for evaluating overlap splices in wet lay-up FRP composite samples. Single-shear
specimens were constructed from carbon fiber FRP with variable overlap splice lengths ranging from 1 in to 4 in. Each
specimen was subjected to cyclic loading at a rate of 1 Hz and an IR camera was used to monitor the temperature
variations resulting from the cyclic stress. A sinusoidal curve fit was applied to the temperature response for each pixel
and the resulting amplitude image was used to evaluate the severity of stress concentrations at the ends of the overlap
splice region as well as where the top ply of saturated composite formed a kink during lay-up.
Open steel grids are typically used on bridges to minimize the weight of the bridge deck and wearing surface. These
grids, however, require frequent maintenance and exhibit other durability concerns related to fatigue cracking and
corrosion. Bridge decks constructed from composite materials, such as a Fiber-reinforced Polymer (FRP), are strong
and lightweight; they also offer improved rideability, reduced noise levels, less maintenance, and are relatively easy to
install compared to steel grids. This research is aimed at developing an inspection protocol for FRP bridge decks using
Infrared thermography. The finite element method was used to simulate the heat transfer process and determine optimal
heating and data acquisition parameters that will be used to inspect FRP bridge decks in the field. It was demonstrated
that thermal imaging could successfully identify features of the FRP bridge deck to depths of 1.7 cm using a phase
analysis process.
Fiber-reinforced polymer (FRP) composites are widely used to increase the flexural and shear capacity of reinforced
concrete (RC) elements. One potential disadvantage is that strengthened surfaces are no longer visible and cracks or
delaminations that result from excessive loading or fatigue may go undetected. This research investigated thermal
imaging techniques for monitoring and evaluating load-induced delamination of FRP composites applied to small scale
RC beams. Two beams (3.5 in x 4.5 in x 58 in) were loaded monotonically to failure. Infrared thermography (IRT)
inspections were performed at various load levels through failure using a composite phase imaging technique. Two
similar beams were tested in fatigue and periodic IRT inspections were performed at 50,000-cycle intervals. Individual
phase values for each pixel were designated as "well-bonded", "suspect" or "unbonded" to indicate the quality of FRP
bond. Suspect areas included regions of excess thickened-epoxy tack-coat and smaller installation defects in the
unloaded specimens. The long-term objective of this research is to develop a practical framework for conducting
quantitative IRT inspections of FRP composites applied to RC and incorporating these results into acceptance criteria
for new installations and predictions for the remaining service life of in-service FRP systems. This method may also
offer insight into the necessity for repairs to in-service systems.
Reinforced concrete beams are designed to allow minor concrete cracking in the tension zone. The severity of cracking
in a beam element is a good indicator of how well a structure is performing and whether or not repairs are needed to
prevent structural failure. FRP composites are commonly used to increase the flexural and shear capacity of RC beam
elements, but one potential disadvantage of this method is that strengthened surfaces are no longer visible and cracks or
delaminations that result from excessive loading or fatigue may go undetected.
This research investigated thermal imaging techniques for detecting load induced cracking in the concrete substrate and
delamination of FRP strengthening systems applied to reinforced concrete (RC). One small-scale RC beam (5 in. x 6 in.
x 60 in.) was strengthened with FRP and loaded to failure monotonically. An infrared thermography inspection was
performed after failure. A second strengthened beam was loaded cyclically for 1,750,000 cycles to investigate how
fatigue might affect substrate cracking and delamination growth throughout the service-life of a repaired element. No
changes were observed in the FRP bond during/after the cyclic loading. The thermal imaging component of this
research included pixel normalization to enhance detectability and characterization of this specific type of damage.
This research project investigated heat transfer mechanisms that occur during radiant heating of glass/epoxy composites bonded to concrete. The ultimate goal is to develop a field procedure for estimating the thickness of fiber-reinforced polymer (FRP) composites used to strengthen existing reinforced concrete structures. Thickness is an important parameter in the design and implementation of nondestructive testing procedures that evaluate bond in FRP systems. Four concrete samples (15 cm x 30 cm x 5 cm) were constructed with glass/epoxy composite bonded to the surface. The thickness of the composite varied from 1mm to 4mm and thermocouples were placed at 1mm intervals through the depth of the composite. Experimental data was compared with a simple theoretical model that predicts the surface temperature response of a layered system subjected to a uniform heat flux. Two factors were shown to significantly influence the heat transfer mechanism: surface absorptivity of the FRP composite and convective cooling. Additional analytical modeling using the finite element method was performed to account for these affects in an effort to obtain a better estimate of FRP thickness based on experimental data.
KEYWORDS: Composites, Thermography, Fiber reinforced polymers, Nondestructive evaluation, Inspection, Data processing, Phase shifts, Eye, Interference (communication), Signal to noise ratio
This research investigated low velocity impact damage in fiber-reinforced polymer (FRP) composites. Small-scale
glass/epoxy laminates (approximately 210mm x 210mm x 2mm) were subjected to varying degrees of dynamic impact
energies ranging from 5 to 20 J and infrared thermography inspections were performed on the damaged specimens.
Three distinct damage modes were observed: penetration resulting in highly localized fiber rupture through the
thickness of the composite; penetration/delamination in which localized fiber rupture was observed on the impacted
surface and additional delamination occurred around the point of impact; and delamination/reverse side fiber rupture in
which no visible damage occurred on the impacted surface but fiber rupture and delamination occurred beneath the
surface. A modified lock-in thermography procedure was used in the nondestructive evaluation (NDE). Phase images
were constructed by applying a least-squares sinusoidal curve fit to a series of thermal images collected over one cycle
of sinusoidal heating. This method was shown to increase contrast for subsurface delaminations compared to raw
thermal data. Finally, thermography results for FRP composite samples containing simulated damage (back-drilled
holes) were compared with thermography results from impact-damaged samples.
Infrared thermography is a non-destructive evaluation technique that can be used to identify debonded areas in FRP strengthening systems applied to concrete. This research provides a summary of IR thermography experiments that were conducted on full-scale AASHTO girders strengthened with four different FRP composite systems. Significant findings were that the thickness of the FRP system as well as the material composition strongly influences the ability to
detect defects at the FRP/concrete interface. Additional experiments were conducted on small-scale specimens with implanted defects. Results from these experiments indicate that IR thermography is capable of detecting defects under multi-layer FRP composite systems; however the defect signal strength and time to maximum signal vary significantly from single-layer systems.
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