Fatigue Tests on Buried or Repaired Dented Steel Pipeline Specimens

This paper presents the results of fatigue tests performed on dented steel pipeline specimens that were tested under different environmental conditions and subjected to cyclic internal pressure. Thirty-three pipe specimens were divided into three groups and tested under three different conditions. A first set of nine dented specimens was tested in air without any restrictions. A second set of eight specimens was tested while buried in the soil. A third set of sixteen specimens was tested in air, after the dents had been repaired by composite material sleeves. Hot-spot cyclic strain amplitudes were measured using two experimental techniques: Digital Image Correlation (DIC) and Fiber Optic Bragg Strain Gauges (FBSG). At first, all thirty-three specimens were tested in air along five full cycles in order to carry out full-field measurements using DIC to identify and quantify strain concentration at sites that were potential locations for fatigue cracks to initiate. Close to these point-locations, measurements of strains using FBSG were also made, and the results were then compared with the DIC results. FBSG were also used during the cyclic pressure loading process while the specimens were being tested, in such a way as to monitor the influence of the environment in the dented areas. The test results demonstrated that a simple uniaxial Manson-Coffin fatigue equation that uses the universal exponents proposed by Manson, together with the circumferential strain amplitude measured at the hot spots can be used to predict the fatigue life of the dented specimens. Moreover, it was determined that the measured strains at the hot-spot locations were not influenced by the soil coverage, although showing a considerable and beneficial decrease in their amplitudes caused by the composite repair reinforcements.

Mechanical and Mesoscale Analyses of Cement Stabilized Macadam Prepared by Vibratory and Nonvibratory Mixing Techniques

The objective of this study is to analyze the effects of mixing techniques on the mechanical behavior and meso-structure of cement-treated aggregate. Different specimens were prepared by vibratory and nonvibratory mixing techniques. X-ray CT scans were performed to illustrate the distribution of cement mortar on aggregate. The strength, modulus, and fatigue tests under different stress states were tested to reveal the impacts of mixing techniques. Then, the relationships between strengths and loading rates and that between moduli and stress levels were established. Hereafter, the S-N fatigue equation that modified with stress ratio related to loading rates was used to describe the fatigue performance. The results indicate that the cement mortar of specimens prepared by vibratory mixing was well-distributed on aggregates. The strength, modulus, and fatigue life of the specimens prepared by vibratory mixing were higher under the test condition threshold. Moreover, the growth rate of strength and modulus with loading for specimens prepared by vibratory mixing was slightly larger than that for specimens prepared by nonvibratory mixing. Compared with the cement-treated aggregates specimens prepared by nonvibratory mixing, the fatigue life of cement-treated aggregates specimens prepared by vibratory mixing had more stable stress sensitivity.

Investigation of Strength and Fatigue Life of Rubber Asphalt Mixture

Strength and fatigue life are essential parameters of pavement structure design. To accurately determine the pavement structure resistance of rubber asphalt mixture, the strength tests at various temperatures, loading rate, and fatigue tests at different stress levels were conducted in this research. Based on the proposed experiments, the change law of rubber asphalt mixture strength with different temperatures and loading rates was revealed. The phenomenological fatigue equation of rubber asphalt mixture was established. The genetic algorithm optimized backpropagation neural network (GA-BPNN) is highly reliable for optimizing production processes in civil engineering, and it has a remarkable application effect. A GA-BPNN strength and fatigue life prediction model was created in this study. The reliability of the prediction model was verified through experiments. The results showed that the rubber asphalt mixture strength decreases and increases with the increase of temperature and loading rate, respectively. The goodness of fit of the rubber asphalt mixture strength and fatigue life prediction model based on the GA-BPNN could reach 0.989 and 0.998, respectively. The indicators of the fatigue life prediction model are superior to the conventional phenomenological fatigue equation model. The GA-BPNN provides an effective method for predicting the rubber asphalt mixture strength and fatigue life, which significantly improves the accuracy of the resistance design of the rubber asphalt pavement structure.

Estimation of total fatigue life for in-service asphalt mixture based on accelerated pavement testing and four-point bending beam fatigue tests

The purpose of this study was to estimate the total fatigue life for in-service asphalt mixture fatigue with in situ accelerated pavement testing (APT) and laboratory four-point bending beam tests. On a selected expressway in Shanghai, China, a series of full-scale APT tests were conducted. During the APT tests, a portable seismic property analyzer was used to monitor the pavement deterioration through modulus reduction. An equivalent factor between the APT loading and equivalent single axle loads (ESALs) was estimated. Additionally, asphalt concrete slabs were cut from the APT test sections and then transferred to the laboratory for four-point bending beam fatigue tests. A new fatigue equation was proposed for the in-service asphalt mixtures. Furthermore, a shift factor between the laboratory fatigue life and field ESAL was recommended. Finally, the field total fatigue life of the in-service asphalt mixture was estimated based on all the work in this study.

An Airfield Soil Pavement Design Method Based on Rut Depth and Cumulative Fatigue

The structure and damage modes of soil pavement, as well as existing problems in current design methods, were comprehensively analyzed, and a new design method for airfield soil pavement was proposed. The proposed method avoids the use of the “designed aircraft” concept and instead adopts the cumulative fatigue theory widely used in permanent airfield design at present. Moreover, in view of the lack of aircraft wheel trajectory distribution data, an approximate method for calculating the wheel trajectory distribution considering the side slip distance of the aircraft was proposed and the equivalent width of the wheel tread was calculated by introducing the modulus ratio. Finally, the pass-to-coverage ratio was obtained. According to the characteristics and damage modes of airfield soil pavement, rut depth was determined to be the unique factor affecting soil pavement damage, and resilient modulus was used as the control variable to improve the adverse impact of the empirical method. Furthermore, according to the rut prediction formula for airfield soil pavement put forward by the US Army Engineer Research and Development Center, a fatigue equation based on the resilient modulus was proposed to calculate the allowable number of repetitions. To verify the reliability of the design method, a test section was constructed at a test center in Jining, China, and the theoretical maximum allowable repetitions on the soil runway were calculated by the currently used California bearing ratio test, the β-fatigue equation, and the proposed method. Aircraft traffic tests were carried out on the test section. Finally, the theoretical and test results were compared and the values calculated via the proposed method were found to be consistent with experimental values, thereby validating the reliability of the method.

Energy-Based Approach to Predict Fatigue Life of Asphalt Mixture Using Three-Point Bending Fatigue Test

The three-point bending fatigue tests were carried out in order to accurately predict the fatigue life of an asphalt mixture based on the plateau value (PV) of the dissipated strain energy ratio (DSER). The relations of the dissipated strain energy (DSE) to the stress-strength ratio, temperature and loading rate were studied, and the constructions of the mathematical models of DSE and DSER were completed based on the change laws of the DSE. The relation of the fatigue life to the PV was determined based on the analysis of damage evolution, based on which the fatigue equation was established and used to predict the fatigue life. The results show that the change laws of DSE and DSER can be well described by the proposed mathematical models. The PV is defined as the average value of the DSER in the second stage and the fatigue life decreases in power function with the increase of PV, based on which the fatigue equation of Nf = A(PV)B was established, and the established fatigue equation is very close to that is used in the MEPDG. The fatigue equation can well predict the fatigue life asphalt mixture.