Review of Sensor-Based Subgrade Distress Identifications.

The attributes of diversity and concealment pose formidable challenges in the accurate detection and efficacious management of distresses within subgrade structures. The onset of subgrade distresses may precipitate structural degradation, thereby amplifying the frequency of traffic incidents and instigating economic ramifications. Accurate and timely detection of subgrade distresses is essential for maintaining and repairing road sections with existing distresses. This helps to prolong the service life of road infrastructure and reduce financial burden. In recent years, the advent of numerous novel technologies and methodologies has propelled significant advancements in subgrade distress detection. Therefore, this review delineates a concentrated examination of subgrade distress detection, methodically consolidating and presenting various techniques while dissecting their respective merits and constraints. By furnishing comprehensive guidance on subgrade distress detection, this review facilitates the expedient identification and targeted treatment of subgrade distresses, thereby fortifying safety and enhancing durability. The pivotal role of this review in bolstering the construction and operational facets of transportation infrastructure is underscored.

Shear deformation response of a holographic sensor based on elastic poly(MMA-co-LMA) photopolymer.

A holographic sensor based on camphorquinon doped poly (methyl methacrylate-co-lauryl methacrylate) (poly (MMA-co-LMA)) elastic photopolymer is developed for characterizing the shear deformation of material. A shear angle and its transverse displacement, which are induced by a couple of shear stresses, are analyzed using a diffraction spectrum of a transmission holographic sensor. The dependence of the peak wavelength shift on the shear deformation presents a good linear relationship which provides a quantitative characterization means. The detectable maximum of the shear angle exceeds 26.1 deg, and the peak wavelength shift closes to 4.0 nm. The available sensitivity is better than 3.33 deg/0.5 nm (shear angle/wavelength shift) using a commercial spectrometer with 0.5 nm of resolution. Finally, the reversibility response of shear deformation further confirmed the practical applicability of the elastic polymer-based shear deformation sensor. The spectrum measurement of shear deformation provides a novel measurement means for the mechanical deformation of materials and expands the application of a holographic sensor.

Characterizing spatial uniformity of tensile deformation with an elastic polymer based holographic sensor.

Tensile deformation uniformity of material has been studied with a stretchable polymer based holographic sensor. The diffraction spectrum distribution of a holographic grating with a large area as a main response parameter is scanned. A linear spatial distribution of peak wavelength provides an important foundation for exploring the tensile uniformity. The same ratio of wavelength to position confirms that the tensile deformation of the material is uniform in a small spot size. Over the entire length of the materials, gradually increasing deformation accumulation is the main uniformity feature of tensile deformation. The uniformity response is expected to apply in sensing the deformation and stress fluctuation distribution in the middle of the thin surface. The non-uniform distribution of stress can be expressed by the nonlinear distribution of the grating diffraction spectrum. The optical measurement of tensile deformation uniformity further validates the applicability of a stretchable polymer based holographic sensor.

Expansion of axial dispersion in a photopolymer-based holographic lens and its improvement for measuring displacement.

Coaxial multiple holographic lenses as high-dispersion elements are developed for a spectral confocal displacement measurement device. Wavelength and coaxial spatial multiplexing methods are used to record the holographic lens with two coaxial foci. The expansion of axial spatial dispersion in photopolymer-based multiple holographic lenses has been demonstrated and studied experimentally. The multiple holographic lenses provide a larger spatial dispersion to improve the characteristic parameters for measuring the displacement. Compared to single holographic lenses, the maximum of axial dispersion wavelength difference of the multiple lenses increases from 134.63 to 162.81 nm, and the corresponding measurable range increases from 203 to 385 mm. The axial spatial dispersion conforms to a typical exponential function. The overall spatial position sensitivity of multiple holographic lenses reaches 2.36 mm/nm. In addition, the multiple lenses also decrease the lateral dispersion compared to the single lenses. The multiple lenses can efficiently reduce the transverse measurement error. Finally, the displacement measurement result confirms the improvement of measureable spatial range. The multiple holographic lenses can accelerate the practical application of holographic lens-based optical elements.

Sensing response characterization of a micro-holographic sensor and its kinetics simulation.

A photopolymer-based micro-holographic sensor was developed to improve the sensitivity in sensing environmental factors. The micro-holographic grating can be formed by interference of two counter-propagating beams. The response rate and sensitivity of the micro-grating based sensor were measured. The response rate reached 0.171 nm/s in sensing relative humidity and 0.03 nm/s in sensing ethanol vapor. The corresponding maximum of the peak wavelength shifted up to 51.2 nm and 8.9 nm, respectively, for 5 min of sensing. The minimum detectable ethanol vapor concentration was as low as 10 ppm, while the sensitivity approached up to 0.179 nm/ppm. Theoretically, relating the absorption and modulation along the thickness direction, a diffusion model with nonlocal response was proposed to describe the micro-grating formation and its sensing characterization. The numerical simulation provides a significant foundation for understanding the grating formation and sensing the physical mechanism inside the materials. These results can accelerate the development and practicality of novel holographic sensing elements.