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In the current study, hyperspectral imaging technique was employed to map the spatial distribution of total nitrogen in pepper plant. The specific objectives were as follows: (1) to acquire hyperspectral images and measure TNCs of samples (leaves, stems, and roots) using Dumas combustion method; (2) to extract the spectral data and employ Random frog (RF) to select important wavelengths; (3) to build multivariate calibration models for predicting TNCs in organs (leaf, stem, and root) and whole-plant (leaf-stem-root) by partial least-squares regression (PLSR) based on full spectra and the selected important wavelengths; (4) to apply the optimal PLSR model to predict TNC of each pixel in samples and generate spatial distribution of TNCs in whole pepper plant.
Chemical imaging is a technique for building visual color images to display the spatial distribution of chemical components in heterogeneity . The chemical value of each pixel can be predicted by inputting its corresponding spectral reflectance values into the established quantitative model . Spatial distribution of chemical components could be generated with the established quantitative model combined with image processing. To observe the variance of TNCs in whole pepper plant, distribution maps of TNCs were required. In particular, an optimal model built using the mean spectra of important wavelengths was applied to predict the TNC in each pixel. Subsequently, the spatial position of each pixel along with its TNC was used to form the spatial distribution maps .
The linear function (2) obtained from the RF-PLSR model was employed to predict the TNC of each pixel within the leaf, stem, and root images. Pixels with similar spectral patterns in raw hyperspectral images would produce similar predicted values of the TNCs-HSI, and then would appear in similar colors in the resulting chemical images . As a result, the spatial distribution maps of the TNCs-HSI in 10 samples (6 leaves and 3 stems in upper, middle, and lower positions and a root) of pepper plant were generated in Fig. 5. The TNCs-DC covered a broad range from 0.75% (stem in lower) to 4.76% (leaf in upper). Pixels providing similar spectral information in original hyperspectral images would result in similar results of the TNCs-HSI , , thus producing similar colors in the resultant chemical images. Indeed some samples (stems in middle/lower and root) with the relatively low TNCs-DC might show similar color, which was hard to distinguish based on the pixels of high or low TNCs. Hence, in the predictive map, three color scales described the ranges of the TNCs-HSI in each spot of the leaves, stems, and root samples, respectively. Compared with the images of original samples, the difference of the TNCs-HSI coloring within a sample could be easily identified by the naked eyes.
As shown in Fig. 5, three linear color scales in different colors from red to blue represented the different TNCs-HSI from high to low, respectively. The 10 resulting images of samples revealed the changing spatial tendency of the TNCs-HSI. Nitrogen status showed a decreasing trend from upper to lower positions in leaves and stems, which was in alignment with the measured nitrogen variation shown in Table 3 and the respective TNC-DC values of those samples. This phenomenon could be explained as follows: N is a removable and active element and exists in the form of organic matter in a living plant. It can be reabsorbed from older leaves to young leaves . Meanwhile, N and other nutrients in plant are transported from roots to growth center (fresh leaves) with the xylem sap in stems under the transpiration pull , , resulting in the high TNCs in fresh leaves. In addition, TNC distribution is related to seasons, plant age, and photon flux density of leaves in different positions .
Abrasive water jet (AWJ) breaking technology is suitable for the maintenance and repair of concrete structures, generating minimal dust, low tool wear, and no vibrations or selective destruction. The failure features and mechanisms of concrete subjected to AWJ impact are fundamental issues of AWJ breaking technology, which are also related to the safety and quality of engineering construction. Based on computed tomography (CT), scanning electron microscopy (SEM), and image processing technology, this paper studied the fragmentation pattern and removal mechanism of concrete under AWJ impact. The general failure characteristics and crack propagation law of concrete subjected to AWJ impact were described through AWJ impact concrete tests. The spatial distribution of damage in concrete subjected to AWJ impact can be divided into the intensive action zone, the transition zone, and the weak action zone. The removal mechanism of AWJ was discussed by comparing the impact performance of a pure water jet (PWJ) system. The results indicate that abrasive particles can cause cliff-shaped fracture and lip-shaped distortion in the aggregate part and flat fracture surface in the matrix part. There is no obvious crack in the interfacial transition zone (ITZ) due to the weakening of the water wedge effect.
Concrete is the most widely used building material due to its good plasticity, low production cost, and high durability. Nevertheless, due to unreasonable design and construction, there are often some primitive defects in concrete materials. When facing weathering, corrosion, repeated expansion and contraction, and external impact, it is easy for cracks, holes, edge damage, arch camber, and other defects to occur as time goes by, threatening the stability and safety of the concrete structures . It is foreseeable that the repair and maintenance of infrastructures are placing an increasing burden on the government, so safe and efficient repair technologies are urgently needed.
In summary, this paper attempts to investigate the fragmentation pattern and removal mechanism of concrete subjected to AWJ impact from macroscopic and microscopic perspectives, respectively. It discusses the failure characteristics, crack extension law, and spatial distribution of damage by observing the external fragmentation pattern of concrete and CT scans. Furthermore, this paper explores the removal mechanism of AWJ by comparing the impact performance of pure water jet (PWJ) with SEM scans. The results can provide important theoretical bases for understanding the development law and derivation mechanism of microcracks in concrete under the impact of AWJ, which can help the technology be better applied in the maintenance and repair of concrete structures as well as in emergency demolition.
To evade the randomness of test results and obtain the general failure characteristics and the crack propagation law of concrete impacted by AWJ, four groups of impact experiments were carried out. Figure 2 shows the failure morphologies of four concrete samples under the same impact parameters. This paper mainly investigates the fragmentation pattern and removal mechanism of concrete subjected to AWJ impact in the maintenance and repair of damaged concrete structures, and therefore, initial defects such as holes were added to make the studied concrete more similar to real damaged concrete.
By analysing the features of the four groups of concrete impacted by AWJ, it can be found that the four crater surfaces were similar to some extent, which was reflected by their irregular shapes due to the random distribution of aggregates, initial defects, and rough crater walls as well as the exposed aggregates. In addition to the highly fractured zone near the crater, the cracks penetrating the impact surface generated near the crater, which caused the macrodamage under the combined action of the shear component and the tensile component of the shock stress wave. Some internal cracks and cracks on the impacted surface propagated to the lateral surface, forming a network of radial cracks and circumferential cracks intersecting each other.
To further explore the crack propagation law, CT scanning was used to detect the internal structure of concrete. Figure 3 shows the CT images of two cross sections of concrete after AWJ impact. The results show that the macrocracks mainly propagated along the interfacial transition zone (ITZ) during the extension process due to its weak mechanical properties. Compared to hardened cement paste, ITZ is more porous and has fewer unhydrated cement particles, a higher Ca/Si ratio, and higher Ca(OH)2 contents, and these pores are also the preferential infiltration and splitting zone of high-pressure water . For the most macrocracks propagated along ITZ, the aggregate could keep the integrity, and the undamaged zone of concrete could still retain its strength.
Though the variations in the jet parameters have certain effects on the fragmentation feature of concrete impacted by AWJ, the fragmentation pattern of concrete still exhibits many similarities. The general failure characteristics, crack extension law, and spatial distribution of the abovementioned damage can be analysed to evaluate the crack distribution feature during concrete hydrodemolition and predict the fragmentation degree and damage dimension in concrete under AWJ impact, which can improve the application level of AWJ breaking technology in concrete structure maintenance and emergency demolition in accidents.
Unlike AWJ, due to the high density and diffusion coefficient of high-pressure water in PWJ, strong water hammer compression is produced in the liquid-solid contact centre (Figure 8(a)), which generates the tension effect in concrete. When the tensile stress exceeds the dynamic tensile strength, cracks form in concrete. Besides, PWJ can cause shear failure at the contact edge of the jet and concrete. Once the abovementioned damage initiates, the high-speed fluid enters the cracks in an instant, and the strong rush and delivery action of the fluid cause the block to be peeled off and carried away, leading to the scattering of broken particles on the fracture section (Figure 7(b)). 2b1af7f3a8