Abstract
The columnar joint skeleton of 3D printed Acrylonitrile Butadiene Styrene (ABS) material, the skeleton of cement mortar and ultraviolet aging treatment are combined to pour the columnar joint rock mass (CJRM) test block. The strength, deformation, energy and failure modes of the specimens with different dip angles were analyzed by uniaxial compression test. The influence of joint skeleton on the strength of the test block was analyzed. The effect of aging time on ABS parameters was evaluated. The results show that the uniaxial compressive strength and elastic modulus are ' U ' type with the increase of dip angle, the elastic strain energy is ' V ' type, and the dip angle of 45° is the minimum value. The four failure modes are: shear failure, splitting failure, shear tensile mixed failure, shear splitting mixed failure. The anisotropy of CJRM is extremely high. The skeleton reduces the strength and elastic modulus of the solid test block, and has the greatest influence on the strength of the test block with dip angle of 75°. With the increase of aging time, the strength and deformation parameters of ABS decreased, and the yellowness index and infrared spectrum peak area increased.
Keywords: 3D printing, Columnar jointed rock mass, UV aging of ABS, Mechanical properties, Laboratory test
Subject terms: Civil engineering, Solid Earth sciences
The columnar joint is a primary tensile fracture structure formed by the cooling of basalt magma1–3. It cuts the complete rock mass into regular or irregular polygonal prisms4. The geometric anisotropy in the plane leads to the significant discontinuity and heterogeneity of the columnar jointed rock mass5,6, as shown in Fig. 1. A large area of columnar jointed rock mass (CJRM) was found in the survey of dam foundation and high slope of intake in southwest China7. The CJRM exposed by excavation directly threatens the stability of the foundation of the structure8. Studying the anisotropic mechanical characteristics and failure characteristics of CJRM has reference significance for the safety of dam construction process.
Fig. 1.
Distribution pattern of columnar jointed rock mass.
Researchers used in-situ test, numerical simulation and indoor physical model test to study the mechanical properties of CJRM9,10. Due to the large geometric scale of columnar jointed rock mass, it is difficult to obtain test blocks containing sufficient columnar joints, and because the separated rock mass is loose and easy to fall off, it is difficult to sample on site11. At the same time, a small amount of field test data is discrete, and it is difficult to determine the mechanical law of the test block with the change of dip angle12. Numerical simulation involves many parameters, and it is difficult to determine the mechanical parameters of the joint interface, so it is difficult to ensure the accuracy of the numerical model calculation results13. The indoor physical model test can repeat the same model in batches. The rock-like material is easy to obtain and the mechanical parameters are stable, which makes the physical model test widely used14,15. Zhang et al. used a template to make a cement mortar prism, and then used white cement to paste the prism to make a columnar jointed rock mass test block16. The test blocks with different dip angles were obtained by a rock coring machine. However, the bond strength of white cement used to simulate the joint surface is too large, which leads to the increase of the integrity of the test block and limits the slip of the independent cylinder inside the 45 ° test block. Xiao et al. uses gypsum instead of cement as the columnar joint interface17. The complete test block is affected by the vibration of the blade at the cutting angle. The dry gypsum has weak bonding ability, and the column collapses in advance under the influence of mechanical vibration. In the later stage, the broken column needs to be bonded again, and the splicing reinforcement affects the overall mechanical properties of the test block. The above methods all require molds to make independent columns, use binders to bond large-volume samples, and drill, cut and other secondary processing to obtain test blocks that meet the size. Too many programs affect the machining accuracy of the test block, which indirectly affects the strength and deformation characteristics of the test block.
3D printing has the characteristics of one-time forming18, which can simplify the production process of columnar jointed rock mass test block, and can control the precise size of joint surface through CAD software. Lin et al. uses photosensitive resin to make a regular hexagonal joint skeleton model19, and then pours the cement slurry into the cavity of the skeleton, so that the columnar jointed rock mass test block can be made in a short time. However, the photosensitive resin has strong toughness, and the deformation of the skeleton during the compression process can still be recovered after unloading. The cement column inside the fractured test block has been broken, and the joint skeleton has not been broken, which indicates that the deformation of the column and the joint material is not coordinated. Using a special photosensitive resin material, Xia et al. printed both the cylinder and the joint surface between the cylinders20, and produced a complete columnar joint test block using the least number of programs. However, because the volume to be printed is too large, the printing duration is too long, which seriously affects the efficiency of sample preparation. In addition, due to the high cost of 3D printing materials, there are few types of test blocks made by Xia et al., and the complete law of the influence of dip angle on the mechanical properties of columnar jointed rock mass has not been obtained21. In addition, the existing research takes the sum of the strength of the joint skeleton and the column as the strength value of the test block, and does not analyze the influence of the 3D printing skeleton on the strength of the test block, and the ductility of the commonly used 3D printing material is much larger than that of brittle materials such as cement. The large deformation of the joint skeleton amplifies the deformation of the brittle column. Therefore, it is the key to make the physical model of columnar jointed rock mass by using 3D printing technology and reasonable test materials to eliminate the adverse effects of joint skeleton as much as possible on the basis of ensuring the accuracy of test block making.
The above two test methods have certain reference significance for 3D printing columnar joint test blocks, but it is difficult to balance the test cost and the demand for rapid batch sample preparation; in addition, making more types of dip angle is helpful to understand the influence of dip angle on the strength and deformation characteristics of columnar jointed rock mass. In this paper, the low-cost ABS material is used to make the columnar joint skeleton by 3D printing. The aging degree of the skeleton is accelerated by ultraviolet light (UV), and the strength and plasticity of the skeleton are reduced. Use high-strength cement mixed mortar to fill the entire skeleton cavity. While reducing the strength of the joint skeleton, the strength of the column is significantly improved. Through one-time pouring and forming of the mold, seven types of dip angle test blocks were made, which not only reduced the influence of manual operation on the test block, but also obtained more columnar jointed rock mass test blocks with dip angle. Based on the uniaxial compression test, the strength, deformation and failure characteristics of the test block containing the skeleton were evaluated. In addition, the influence of skeleton on the strength of the test block was determined, and the aging degree of ABS was evaluated by color difference and infrared spectrum. The corresponding relationship between ABS mechanical parameters, yellowness index and infrared spectrum peak area with different aging time was established. It can provide reference for making physical model of rock mass containing joint surface.
Materials and methods
Method
Aiming at the physical model of columnar jointed rock mass required for laboratory tests, the columnar jointed rock mass test block was made by using 3D printed joint ' skeleton ' and rock-like materials. The geometric structure of the columnar jointed basalt is similar to that of the hexagon22,23. The cross-section of the regular hexagon column has the advantage of full filling24. The columns formed by condensation shrinkage under ideal conditions are regular hexagonal prisms25,26. Therefore, this paper will use hexagonal prism as the research object to carry out the experiment.
Fabrication of joint skeleton: The regular hexagon graphics in the plane are stretched to form a ' honeycomb ' three-dimensional graphics, and the three-dimensional graphics with different dip angles are formed by ' cutting ' Boolean operation. After the 3D printer reads the graphic information, the production of the joint skeleton in Fig. 2a is completed, and the side length of the cube skeleton is 110 mm.
Fig. 2.
Composite material test block production and loading test process.
Because the material of the skeleton is ABS plastic, in order to eliminate the high ductility of ABS plastic and reduce the influence on the brittleness characteristics of cement mortar after solidification, UVB ultraviolet light irradiation (there are two 40 W-313 lamp tubes in the photoaging box, single lamp tube UV = 140µW/cm2) is used. High-energy ultraviolet light destroys its internal organic compound bonds, which increases the brittleness of ABS plastic and reduces the influence of ABS material on the plastic platform during compression27,28. The irradiation time of a single skeleton is 816 h.
Composite test block: Fig. 2b, the skeleton after photoaging treatment is placed in a transparent acrylic mold, and the cement mortar is poured into the mold. The composite test block composed of cement mortar and ABS skeleton. The mass ratio of cement, sand and water is 1 : 0.5 : 0.3. After the cement mortar solidified for 24 h, the test block was transferred to a curing box with a temperature of 20 ºC and a humidity of 95% for 28 days.
Experiment
The RMT-150 B rock mechanics test system was used to carry out unconfined uniaxial loading on the test block. The displacement loading mode was adopted, and the loading rate was 0.005 mm/s. Figure 2c loading process automatically reads the load information of the test block. During the test, Basler industrial camera was used to record the crack propagation during the loading process of the test block. The digital speckle technique is used to monitor the propagation and failure of micro-cracks during the fracturing process of the test block.
In addition, the same loading mode was used to test the independent bearing performance of the skeleton after photoaging.
Data
The stress-strain curves of the specimens with different dip angles are shown in Fig. 3. The curve type and peak intensity obtained by different dip angles (β) are different, showing strong anisotropic characteristics. When β = 0° and 90°, the peak stress is larger, which is much larger than that of the test block with residual dip angle. The peak strength (σci) of the test block without skeleton is much larger than that of the test block with skeleton.
Fig. 3.
Stress-strain curve of test block.
In Fig. 3b, the complete stress-strain curve can be divided into five obvious sections by four characteristic points. The characteristic points of stress value (σx) are: crack stable propagation stress threshold (σcs), crack unstable propagation stress threshold (σcu), peak stress value (σp), residual stress value (σr) ; stage 1-crack closure stage, stage 2-crack stable propagation stage, stage 3-crack unstable propagation stage, stage 4-failure stage, stage 5 after σr is not considered. Stage 1 is reflected by the physical phenomenon of the gradual compaction of the test block. Stage 2 is reflected by the linear growth of the stress of the test block. Stage 3 shows that there are significant extended cracks on the surface of the test block. Stage 4 shows that the stress is reduced and accompanied by the significant cracking sound of the test block.
Result
Strength characteristics of test block
With the increase of the dip angle, the characteristic points of the stress value of the test block show a ' U ' type distribution that decreases first and then increases, and the minimum value is obtained when β = 45º, As shown in Fig. 4a. The order of peak stress is: 
. The order of stress value feature points is: 
. Compared with the peak stress value of the test block without skeleton (σp−c = 48.21 MPa), the peak stress of the test block with skeleton is obviously reduced, and the peak stress of β = 45° is 2.52 MPa, which is reduced by 94.75%. The peak stress of β = 90° is 30.63 MPa, and the decrease is 33.30%.
Fig. 4.
The influence of dip angle on the strength of the test block. (a) Stress value. (b) Comparison of stress values.
When the dip angle changes, the ratio of the characteristic point of the stress value to the peak stress value of the test block is not constant. In Fig. 4b, the value range of σcs/σp is 20 -40%. The value range of σcu/σp is 75–90% ; the fluctuation range of σr/σp is large, and the value range of different dip angles is 40–90%. Although the proportion of each eigenvalue varies greatly, it can be found that the curve at β = 15 ° and 75 ° has obvious inflection point.
Different dip angles have a significant effect on the stress value feature points. When the joint dip angle is perpendicular to the loading direction, the strength of the test block is the largest, and the stress value of the dip angle of 15 º and 75 º changes abruptly. With the increase of dip angle, the strength of jointed rock mass increases first and then decreases. Alshkane et al. also obtained similar test results in the study of interlocking block jointed rock mass29.
Deformation characteristics of test block
Modulus of deformation
According to the stress-strain curve in Fig. 3, two kinds of deformation modulus can be calculated: Eline, E0.2−0.8. According to the characteristic points of stress value and strain value, the deformation modulus (Eline) Eq. (1) is calculated and compared with the existing deformation modulus Eq. (2) ( E0.2−0.8 ) calculation Equation :
 |
1 |
 |
2 |
In the Equation: εcu, εcs, εp are the strain values corresponding to the characteristic point stresses σcu, σcu,σp.
Although there are some differences in the calculation results of the two types of deformation modulus, the variation of the deformation modulus with the dip angle is consistent, and the change trend of the curve in Fig. 5 can be approximated as a ′U′ type distribution. indicating that the reliability of the feature point information based on the stress value is better. When the dip angle increases, the deformation modulus decreases first and then increases. When β = 45º, the minimum value is obtained, and the deformation modulus is only 0.1 GPa. The order of deformation modulus values is :
. The deformation modulus value (Eci) of the test block without skeleton is much larger than that of the test block containing skeleton (Ecj). The value of Eci is 5.67 GPa, and the value range of Ecj is 0.1 GPa–2.83 GPa. The deformation modulus of CJRM test block decreases by (46.92%–98.1%), and the maximum deformation modulus value of β = 90° test block is 2.83 GPa. The skeleton can effectively weaken the ability of the test block to resist deformation; the influence of different dip angles on the deformation modulus (Ecj) of the test block is different. There are significant differences in the influence of dip angle on the deformation modulus of the test block. β.
Fig. 5.
Effect of dip angle on deformation modulus of test block.
The decrease of the deformation modulus of the test block is directly related to the dip angle. With the increase of the dip angle ( 
), the relative sliding trend between the column and the joint skeleton increases, the lateral deformation of the test block gradually increases, and the deformation modulus decreases. When the dip angle is greater than 45°, the deformation and failure of the test block changes from lateral slip failure to axial compression failure, and the ability of the test block to resist deformation gradually increases, so that the calculated value of the deformation modulus gradually increases.
Peak strain
The peak strain value (εp) of the test block with skeleton is larger than that of the test block without skeleton (εci), and the maximum increase of peak strain corresponding to the test block with β = 75º is 150.60%. It shows that the implantation of the skeleton increases the ability of the test block to resist deformation; there are differences in the deformation effect of the test blocks with different dip angles during the loading process ( in order to eliminate the influence of excessive strain in the compaction stage of Fig. 3a, β = 0º, the initial strain of the 90º test block takes the strain value corresponding to the 1% peak stress ). When the dip angle of the skeleton is not parallel to the loading direction (β < 90º), it will bear the axial load together with the cement mortar column, and the loading process will produce a certain lateral deformation. It is worth noting that the peak strain is the smallest at β = 0º. At this time, the column and skeleton in the test block are only compressed, and the lateral deformation is basically not generated, so the relative increase of εci is the smallest. The peak strain value with the change of dip angle is shown in Table 1.
Table 1.
Peak strain of test blocks with different dip angles.
| Dip angle | ||||||||
|---|---|---|---|---|---|---|---|---|
| 0 º | 15 º | 30 º | 45 º | 60 º | 75 º | 90 º | Without skeleton εci | |
| Peak strain /% | 2.32 | 3.13 | 4.17 | 3.14 | 2.99 | 4.44 | 2.40 | 1.77 |
| Increase amplification /% | 31.11 | 77.03 | 135.52 | 77.64 | 68.71 | 150.60 | 35.45 | – |
Energy characteristic
The accumulation and release of energy during the loading of the test block to the peak stress point. The sum of the elastic strain energy Pe stored before the peak stress and the crack dissipation energy Pd is the total energy P, Eq. (3). Figure 3a The area enclosed by the stress-strain curve has a physical meaning, and the product of the two is the total energy per unit volume of the test block, Eq. (4).
 |
3 |
 |
4 |
Figure 6 shows the influence of dip angle on the energy of the test block. With the increase of dip angle, the elastic strain energy (Pe) and total energy (P) of the test block decrease first and then increase, and the dissipation energy (Pd) decreases first and then increases. β = 90º, P and Pe reached the maximum value. β = 45º, and each energy characteristic value has a minimum value. The elastic strain energy rate γ and the dissipated energy rate η have a sudden change at the dip angle of 60º; β = 60º, 75º, and the change of energy ratio is directly related to the slip cracking of the test block. The total energy (P-ci) of the test block without skeleton is 26.48 × 104J/m− 3, which is close to the total energy of the test block with dip angle of 0º and 90°. The elastic strain energy (Pe-ci) is greater than that of the test block containing the skeleton, indicating that the brittleness of the test block without the skeleton is stronger. According to the parameters Pd, γ, η, the failure modes of the test blocks with different dip angles can be preliminarily determined, which can be divided into three categories, namely ( 0º, 90º ), ( 15º, 30º, 45º ), ( 60º, 75º ).
Fig. 6.
Effect of dip angle on energy.
Failure characteristics of test block
Figure 7 is the main strain field cloud diagram corresponding to the stress characteristic point σcu of the test block and the post-fracturing failure diagram. The strain concentration zone in the main strain field cloud map originates from the top surface of the test block. The loading process is guided by the joint dip angle, and the strain concentration zone gradually evolves into a fracture. There are four main characteristics of the failure of the test block.
Fig. 7.
Test block failure diagram.
Shear-tensile mixed failure (15º, 30º): During the loading process, the column slips along the contact surface of the skeleton, the strain on the left and right sides of the column increases and a large number of micro-cracks appear, and the column moves to the left and right sides.
Shear failure (45º): The test block produces shear failure along the skeleton and gradually penetrates into the main slip surface, and the skeleton slip failure surface is gradually formed inside the test block.
Shear splitting mixed failure (60º, 75º): Shear sliding failure and main cracks occur along the contact surface of the skeleton. Due to the limitation of the top and bottom bearing plates, the sliding speed is gradually less than the vertical axial force loading speed, resulting in tensile cracks consistent with the loading direction.
Splitting failure (0º, 90º): During the compression process of the test block, the contact surface between the skeleton and the column gradually cracked, and the cracks were mainly concentrated at the interface between the skeleton and the column. The strength of the column material can not bear the excessive axial force, resulting in cracks in the vertical column, and finally scattered into multiple independent columns.
Discussion
Analysis of mechanical index characteristics
The coefficient of variation (CV) can eliminate the influence of different mechanical parameter units and numerical fluctuations30. The variation coefficient of peak strain is the smallest, which is 0.25. The coefficient of variation of the deformation modulus is the largest, which is 1.23. The results show that the change of the dip angle of the test block has a significant influence on the deformation modulus, the characteristic points of the stress value and the dissipated energy. Because the deformation modulus has a direct correlation with σcs and σcu, it can be determined that the stress value feature points can effectively characterize the mechanical properties of the test block. The data are shown in Table 2.
Table 2.
Coefficient of variation (CV) of different mechanical parameters (ratio of standard deviation to mean value).
| σcs | σcu | σp | ɛ | E | P e | P d | |
|---|---|---|---|---|---|---|---|
| CV | 1.02 | 0.90 | 0.86 | 0.25 | 1.23 | 0.57 | 0.88 |
Through correlation analysis, the correlation between parameters can be quantified31. In Fig. 8, it can be seen that there is a significant correlation between the characteristic points of stress value and deformation modulus and elastic strain energy, and the correlation coefficient is not less than 0.78. There is a significant negative correlation between peak strain and stress value feature points and deformation modulus.
Fig. 8.
Correlation coefficient of mechanical parameters of test block.
The variance contribution rate of the principal component is further counted in Fig. 9. The seven parameters discussed in this paper can be summarized into six orthogonal independent parameters. The contribution rate of the independent parameter PC-1 is 77.24%, and the contribution rate of the independent parameter PC-2 is 17.58%. The cumulative contribution rate of the first two principal components is 94.82%. There are five key parameters determined in the principal component PC-1, which are σcs, σcu, σp, E, Pe ; the principal component PC-2 contains the remaining two key parameters ; the calculated load values of each parameter are positive, indicating that the parameters in each principal component are positively correlated.
Fig. 9.
Contribution rate of principal components.
Taking the index value (score) of PC-1 as the abscissa and the index value of PC-2 as the ordinate, a plane coordinate system for comprehensively characterizing the mechanical parameters of the test block is established in Fig. 10a. Principal component data analysis divides the test blocks with different dip angles into three categories. The Euclidean distance between a certain dip angle and other dip angle coordinate points can measure the comprehensive difference of the dip angle on the mechanical properties of the test block. The calculated values obtained from Fig. 10b, β = 45º, 90º are far more than the average, indicating that these two angles have the most significant effect on the mechanical properties of the test block. The mechanical characteristic coordinate system in Fig. 10a can clearly distinguish the failure modes of test blocks with different dip angles, which is the same as the classification of test blocks in Fig. 7. The failure mode of 45º test block only has shear failure, and the failure mode of 90º test block only has splitting failure.
Fig. 10.
Mechanical properties coordinate system and comprehensive influence measure of test block.
The influence of aging skeleton on the strength of test block
Figure 11a tests the strength of the aged skeleton at four angles. The maximum stress of the uncast empty skeleton is 41.70 MPa (β = 90º), and the minimum stress is 3.03 MPa when β = 45º. The contact area S1 between the bottom of the skeleton and the bearing plate is small, and the bottom area S2 of the skeleton and the test block poured with cement mortar is 1.21 × 10− 2m2. The actual strength after the strength reduction of the empty skeleton is shown in Table 3. In Fig. 11b, the highest ratio of β = 75º skeleton to the strength of the test block is 16.5%, and the lowest ratio of β = 45º skeleton to the strength of the test block is 6.4%. The ratio of skeleton strength to the strength of the test block is not constant. Although the skeleton has undergone light aging, it has not completely eliminated the strength of the skeleton itself. The skeleton does not cut the rock mass as a joint surface alone, and also plays a role in transmitting force and deformation to a certain extent. There is still a certain impact on the strength of cement mortar 3D printing test blocks. Eliminating the impact of skeleton strength is still a challenge for future work.
Fig. 11.
Strength evaluation of aging skeleton.
Table 3.
Skeleton strength of different cross-sectional areas.
| Dip angle | ||||
|---|---|---|---|---|
| 45 º | 60 º | 75 º | 90 º | |
| Cross-sectional area of skeleton (S1) | 0.64 × 10− 3m2 | 0.91 × 10− 3m2 | 0.90 × 10− 3m2 | 0.86 × 10− 3m2 |
| The stress value of empty skeleton | 3.03 MPa | 7.30 MPa | 28.08 MPa | 41.70 MPa |
| Cross-sectional area of test block(S2) | 1.21 × 10− 2m2 | 1.21 × 10− 2m2 | 1.21 × 10− 2m2 | 1.21 × 10− 2m2 |
| The actual stress after area conversion | 0.16 MPa | 0.55 MPa | 1.86 MPa | 2.96 MPa |
The mechanical parameters of the 3D printed columnar jointed rock mass are shown in Table 4, and the strength and deformation modulus of the test blocks with different dip angles are listed19,33,34. Except that the result of β = 0º is slightly larger, the mechanical parameters of the test block designed in this paper are similar to the existing research results, which ensures the reliability of the test results in this paper. Based on the strength of the 90º test block, the normalized values of the strength of the test block with different dip angles are shown in Table 3. The peak strength ratio of a group of test blocks with different dip angles represents the mechanical anisotropy. The anisotropy ratio grade Rc is shown in Eq. (5). Ramamurthy stipulates that the test block obtained by Rc > 6.0 is extremely high degree of anisotropy34. The Rc value of the test block in this paper is 12.5, and the Rc values of the relevant literature are 7.14, 20, 6.67, respectively, indicating that the mechanical anisotropy of the cement mortar test block is extremely high.
 |
5 |
Table 4.
Comparison of strength and deformation modulus of CJRM test blocks in different literatures.
| Parameters | Different author | Dip angle | ||||||
|---|---|---|---|---|---|---|---|---|
| 0 º | 15 º | 30 º | 45 º | 60 º | 75 º | 90 º | ||
| E min/ E max | Xu-this article | 0.95 | 0.15 | 0.12 | 0.04 | 0.06 | 0.13 | 1.00 |
| Xia32 | 0.67 | – | 0.79 | 0.69 | 0.57 | – | 1.00 | |
| Lin19 | 0.22 | – | 0.08 | 0.05 | 0.13 | 0.15 | 1.00 | |
| Zhao33 | 0.52 | – | 0.22 | – | 0.21 | – | 1.00 | |
| σmin/σmax | Xu | 0.97 | 0.32 | 0.28 | 0.08 | 0.12 | 0.34 | 1.00 |
| Xia32 | 0.50 | – | 0.33 | 0.26 | 0.14 | 1.00 | ||
| Lin19 | 0.25 | – | 0.05 | 0.16 | 0.19 | 0.21 | 1.00 | |
| Zhao33 | 0.23 | – | 0.15 | – | 0.40 | – | 1.00 | |
In the Equation, σmax and σmin are the maximum and minimum values of the test block, respectively.
The correlation between the physical and chemical parameters of ABS and the aging time was studied by analyzing the mechanical properties, color difference and infrared spectrum of ABS materials during artificial accelerated aging.
Aging time and ABS parameter evaluation
Aging time and mechanical parameters
With the increase of light aging time, the tensile strength (σt) of ′dog bone′ in Fig. 12a showed a decreasing trend. Within 288 h of UV irradiation, the decrease of σt was not obvious. In Fig. 12b, the strength of dog bone was only 51.43% of the initial strength after 816 h. The peak strain (ε) increases first and then decreases with the aging time, and the maximum decrease of ε is 72% after the end of photoaging. The deformation modulus Et is approximately ′W′ type with the increase of length, which is 44.37% higher than the initial value Et. Detailed data are shown in Table 5.
Fig. 12.
Dog bone size and mechanical parameters (measuring the deterioration degree of ABS mechanical parameters with the increase of aging time by pulling instead of pressing).
Table 5.
Corresponding relationship between detection index and aging time.
| Parameter | Digestion time (unit: h) | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| 0 (Initial) | 16 | 40 | 64 | 80 | 288 | 480 | 648 | 816 | |
| σp (MPa) | 21.29 | 22.55 | 19.73 | 20.94 | 21.51 | 22.67 | 14.82 | 13.52 | 10.95 |
| ε (%) | 1.75 | 1.20 | 2.09 | 1.99 | 0.93 | 1.63 | 1.09 | 1.10 | 0.49 |
| Et (GPa) | 1.51 | 1.98 | 1.03 | 1.10 | 2.42 | 1.49 | 1.36 | 1.32 | 2.18 |
| YI | 30.87 | 69.62 | 78.85 | 83.37 | 84.75 | 90.50 | 90.12 | 94.02 | – |
| A1 | 852.38 | 856.67 | 866.03 | 878.91 | 889.46 | 1169.17 | 1662.37 | 2279.0 | 3068.37 |
| A2 | 110.13 | 117.08 | 127.50 | 137.93 | 144.87 | 235.20 | 318.58 | 391.53 | 464.13 |
Color difference and infrared spectrum detection of aging degree
With the increase of aging time, the yellowness of dog bones in Fig. 13a increases. The yellowness index YI and yellowing rate change greatly at the initial stage of aging. When the aging time exceeds 240 h, the value will tend to be stable. YI Equation is Eq. (6) :
 |
6 |
Fig. 13.
Physical and chemical detection of aging degree (yellowness, infrared spectrum analysis).
X, Y and Z are tristimulus values, and the values of Cx and Cz are 1.31 and 1.15, respectively.
The increase of aging time will significantly affect the curve vibration of the hydroxyl and carbonyl functional groups in the infrared spectrum of Fig. 13b35. The peak areas A1 and A2 of the O-H base region 3150–3700 cm− 1 and the C = O double bond stretching vibration region 1690–1780 cm− 1 were calculated respectively. Figure 13c shows that A1 and A2 increase significantly with the increase of aging time, and the peak area after 816 h is 3.60 times and 4.21 times that of 0 h, respectively.
The material properties of ABS will deteriorate after photoaging. Table 5 establishes a one-to-one correspondence between aging time and mechanical parameters, yellowness index, and functional group absorption peak area.
Conclusion
The test block of columnar jointed rock mass was poured with UV-aged ABS skeleton and cement mortar. The mechanical test of the test block under uniaxial compression test conditions was carried out, and the mechanical properties and aging degree of the aged joint skeleton were evaluated. The main conclusions are as follows:
The 3D printed CJRM is a highly anisotropic rock mass. The uniaxial compressive strength and deformation modulus show a symmetrical distribution of approximate ' U ' type with the increase of dip angle, and the elastic strain energy shows a ' V ' type distribution. The strength and elastic modulus of the test block containing the skeleton are smaller than those of the solid test block without the skeleton.
The principal strain field cloud diagram and the principal component analysis of the feature points can divide the failure modes of the test block into four categories: shear tensile mixed failure, shear failure, shear splitting mixed failure, and splitting failure.
The stress value and strain value of ABS material decreased significantly with the increase of light aging time. After 816 h light aging, the strength value was only 51.43% of the initial strength, and the strain value was only 28% of the initial strain. The peak area of hydroxyl and carbonyl functional groups showed a linear increase trend, and the yellowness value did not change significantly.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 41831278 and 51579081).
Author contributions
Z.X. conceived the structure of the article and completed the writing of the paper. Z.X. designed and carried out the experiment. Z.X. measured the experimental data and data processing ; Z.Z. provided a testing ground ; C.J. auxiliary test and drawing.
Data availability
All data generated or analysed during this study are included in this published article. Data are however available from the corresponding authors upon reasonable request.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
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Data Availability Statement
All data generated or analysed during this study are included in this published article. Data are however available from the corresponding authors upon reasonable request.