Cracking Stage Research Articles

An insight into shale's time-dependent brittleness based on the dynamic mechanical characteristics induced by fracturing fluid invasion

Traditionally, the shale's brittleness is defined as a constant, actually brittleness is a variable during the fracturing fluid invasion, which is crucial to decide the optimum fracturing time. In this paper, the brittleness evolution under fluid-shale interaction based on the shale's multi-scale mechanical behaviors were investigated. The results indicate that the stress-strain curve of the original samples shows obvious linear elastic characteristics, while the performance of the elastic stage and the unsteady fracture development stage of the samples are distinctive as interaction proceeding. The treated samples failures manifest as the radial tensile failure and shear slip failure along bedding plane compared with the blasting failure of original samples. Moreover, the energy and damage accumulations show a staged increase at pre-peak stage due to the duration of elastic growth stage of crack is shortened, while that of unstable development stage is extended. Finally, a time-dependent brittleness model with comprehensive multi mechanical factors is established through the analytic hierarchy process (AHP), which is verified by five independent brittleness indexes. The results show the new brittleness indicator can accurately describe the time-dependent characteristics of shale's brittleness. This investigation has proposed a new insight into evaluate the time-dependent brittleness induced by fluid-shale interaction, further, to determine the fracturing's optimum time points.

Chloride-induced corrosion in reinforced concrete and fiber reinforced concrete elements under tensile service loads

Corrosion of rebars is one of the main issues affecting the service life of Reinforced Concrete (RC) structures. It is well-known that the use of fibers enhances the mechanical behavior of RC structures at both Serviceability Limit States (SLS) and Ultimate Limit States (ULS). In the former, fibers mainly influence the cracking pattern, leading to narrower and more closely spaced cracks. The crack width is a key parameter for controlling the durability of RC structures along with concrete quality and the thickness of the concrete cover. Even though many research studies have been carried out on this topic, the behavior of RC elements, with and without fibers, in both cracked stage and aggressive environments is still not well understood. This article describes an innovative test procedure specifically developed to evaluate the chloride-induced corrosion in RC elements with and without fibers in service condition (under tensile stress and in the cracked stage). Prismatic specimens 90 × 90 × 830 mm with Ø12 rebar (called hereafter tension ties) were subjected for 280 days both to a tensile load and to wet-dry cycles in a water solution containing 5% of NaCl. Experimental results indicate that this test procedure was suitable to assess the chloride-induced corrosion in tensile elements in the cracked stage. Furthermore, a positive influence of fibers was observed, while no relationship between crack width and pitting depth was found.

Residual fatigue lives assessment of riveted lap joints based on a crack growth model

Many riveted joints bear the action of out-of-plane bending moments in steel truss bridges. The fatigue resistance of these joints isn’t stipulated in the current design criteria. Ensuring the safety of structures necessitates analysing the fatigue process of such riveted joints. This paper attempts to evaluate the fatigue life of riveted lap joints from the perspective of engineering applications. The study first found through fatigue tests with two types of fatigue details in riveted lap joints: plate surface cracking and rivet hole edge cracking. Then, the fatigue progression is divided into three stages: the microstructure short crack (MSC) stage, the physical short crack (PSC) stage and the long crack stage. The crack propagation model in the short crack stage is established by combining a scanning electron microscope test of the material and an empirical formula. The crack propagation model of the long crack stage is established by the crack propagation test and considering the crack closure effect. Results show that the fatigue life corresponding to the hole edge crack fatigue detail category is shorter than that to the plate surface crack. In each crack propagation stage, the crack propagation life decreases with increasing load and decreasing load ratio RL. For the two fatigue detail categories, the proportion of the long crack stage to the whole fatigue life is small, and the effect of the crack closure on the overall fatigue life is limited; therefore the effect of the load ratio RL on the fatigue life can be ignored.

Experimental study on blast damage of RC T-beam bridge

Purpose The purpose of this paper is to investigate the explosive performance and explosion damage mechanism of T-beam bridge structure. Design/methodology/approach On the basis of the existing specification, two T-beam bridge models were designed and fabricated. Test specimens of different explosive dosage and different blast height were carried out. The mechanical process, failure mode, blast damage model, damage identification mechanism and blast evolution law and quantitative evaluation were taken into account. Findings The results revealed that the web plate fracture failure is the key to the unstable failure of the whole T-beam bridge. The explosion failure phenomenon and blast damage evaluation criterion of RC T-beam bridge was divided into five stages: the original cracks stage of concrete material (D = 0 ∼ 0.1), the fractures initiation stage of concrete material (D = 0.1 ∼ 0.3), the stable expansion stage of cracks in concrete material (D = 0.3 ∼ 0.55), the unstable expansion stage of cracks in concrete material (D = 0.55 ∼ 0.8), the explosion fracture of steel bars and the overall instability and damage of the bridge (D = 0.8 ∼ 1.0), which can also be described as basically intact, slight damage, moderate damage, severe damage and collapsed. Social implications The research result will provide basis for the antiknock evaluation and damage repair technical specifications of the RC T-beam bridge. Originality/value The research results of damage evaluation serve as a basis for damage repair and reinforcement of bridge structures after explosion.

Development of crack induced impulse-based condition indicators for early tooth crack severity assessment

Diagnosis of gearbox tooth cracks at an early stage is important to prevent catastrophic failures. The time synchronous average (TSA) of vibration signals of gearboxes with a tooth crack mainly consists of the gear meshing frequency (GMF) and its harmonics, the crack-related amplitude modulation and frequency modulation (AM-FM) and the crack induced impulse (CII). To conduct tooth crack diagnosis, many diagnostic techniques have been developed based on the TSA signal. However, most of them achieved poor performance in terms of assessing tooth crack severity in the early crack stage. The main problem is that they are not developed using the signal components that can well indicate early tooth crack severity progression. To overcome this problem, this paper proposes a method to conduct a thorough analysis on the CII since it provides more valuable information on tooth cracks than other components. In the proposed method, the GMF and its harmonics and the crack-related AM-FM are first removed from the TSA signal using comb notch filters. The notch filtered signal mainly contains components related to the CII. A modal model is adopted to fit the dominant resonances in the notch filtered signal and modal parameters are obtained using the matrix pencil method. With the modal parameter estimates, the dominant resonances, which are single degree-of-freedom impulse responses (SDOF IRs), are identified and used to reconstruct the CII. Besides, the SDOF IRs are grouped according to their carrier frequencies. The energy of the reconstructed CII and the sum of the energy of the SDOF IRs with carrier frequencies in a specific frequency band are proposed as two new condition indicators (CIs) for tooth crack diagnosis. The effectiveness of the proposed method and the new CIs for early tooth crack severity assessment are demonstrated using simulated gearbox vibration signals and experimental gearbox vibration datasets.

RETRACTED ARTICLE: Rainfall soil erosion based on color image super resolution and rural revitalization design

Based on the super-resolution color image, this paper studies the design of rainfall soil erosion and rural revitalization. The steep slope system of the Loess Plateau is not only an important cause of soil erosion by rain, but also an important area for landslides, debris flows, and other natural disasters. In this study, high and steep slope system was used as the research object. Through indoor artificial rainfall experiments, under the conditions of intermittent rainfall, outflow and sediment yield characteristics, soil moisture content changes, and rainfall erosion dynamic parameters, the appearance of steep slope system under various conditions was studied from the beginning of rainfall to slope crack stage. At the same time, the development of agriculture and rural areas is not only an important foundation and support for the realization of national modernization, but also an important factor affecting economic and social development. At present, the development level of China’s agriculture and rural areas is still relatively backward, which is a significant defect in the development of China’s modernization. According to the relevant research results of domestic and foreign researchers, this study takes the main content of Rural Revitalization Strategy as the starting point, and, based on the explanation of development background and implication, deeply analyzes the development background of rural complex model and local economic revitalization strategy. At the same time, taking the rural complex as the main carrier to implement the rural revitalization strategy, this paper studies the theoretical source of the rural complex model in detail, and briefly summarizes the significance and characteristics of the rural complex model. This paper expounds the important role of the development of rural complex facilities in the implementation of China’s rural revitalization strategy.

Self-localization of the flexural cracks of fiber reinforced concrete beams

In order to localize the crack and to sense the crack widening of concrete, an alternative method is proposed in this study. The basic element of this method consists of six electrodes. To verify the validity of this method, four-point bending test is carried out on six types of fiber reinforced concrete beams with different fiber types. The voltage between two inner electrodes is used as the indicator for crack localization and widening. Both the voltage–time curve and the load-time curve can be divided into three stages by two turning points corresponding to the uncracked stage, the cracking stage and the crack propagation stage, respectively. In addition, the relationship between crack opening displacement (COD) and voltage may be established by an exponential function. The method is experimentally proved to be highly effective given that the voltage change and crack occur simultaneously. By increasing the number of electrodes, the method proposed in this paper can be applied to sense the crack appearance and widening in concrete structure. The method can be used to localize the cracks in fiber reinforced concrete beams.

Bond behavior of FRP bars in CR concrete

This paper evaluated the bond behavior between recycled crumb rubber (CR) concrete and fiber reinforced polymer (FRP) bar. An experimental study consisting of fifty-seven pull-out specimens with fine recycled crumb rubber aggregate content of 0%, 5%,10% and 15% was described. The results showed that recycled crumb rubber concrete pull-out specimens exhibited similar bond stress-slip curves, which had four stages: the micro-slip, concrete internal cracking, pullout and residual stages. The use of crumb rubber in concrete reduced the bond strength. The bond strength decreased with the increase of the compressive strength of recycled crumb rubber concrete. The type and content of recycled crumb rubber also affected the bond strength. The glass fiber reinforced polymer (GFRP) bars tended to have stronger bonds than that of basalt fiber reinforced polymer (BFRP) bars.

EXPERIMENTAL AND ANALYTICAL STUDY ON THE IMPACT BEHAVIOR OF STEEL–CONCRETE COMPOSITE SLAB

Concrete structures have been widely used for many years to resist impact loads. Steel–concrete composite structures may be considered efficient structures in the emerging modern construction field. Hence, the main objective of this research was to study the impact behavior of steel concrete composite slabs with different shear connectors and compare them with conventional slabs. Seven specimens of dimensions 500 × 500 × 50 mm were cast, which included plain cement concrete slabs, two reinforced cement concrete slabs with steel mesh and steel rebars as reinforcement, and four steel-concrete composite slabs with four different shear connectors: stud, tee, angle, and channel connectors. The composite action was achieved using a steel decking sheet welded with connectors on which the concrete layer was poured at the top. The test setup was fabricated with slots provided for specimens with simply supported end conditions and a mild steel drop weight. The specimens were impacted at the center of the span by dropping a steel mass from a free fall height of 1 m. The number of blows corresponding to the initial cracking and ultimate failure stages was recorded. The parameters that were used to compare the specimens were the impact energy absorbed, crack pattern, crack width, and increase in impact energy from the first blow to the last blow. The experimental results were very close to the analytical results obtained using ANSYS. The experimental and analytical results showed that the composite slabs with channel connectors performed better than the others, and it was proven that the composite slabs performed better under impact loading than conventional slabs.

Experimental research and finite element analysis on anti-wind bearing capacity of ECC large-hollow rate wallboard

This paper investigated the wind resistance of high-toughness cement-based composite (ECC) wall panels with large hollow rates. Two large hollow wall panels were prepared for the test, one was a ribbed large hollow rate wall panel, and the other was an integral cavity large hollow rate wall panel. The test variable was whether there were ribs inside the wall panel. The test results showed that the flexural performance and crack resistance of ribbed wall panel were better than non-ribbed wall panel, which wan increased by 70 % and 47 %, respectively. In terms of resistance to deformation, ribbed wall panel also performed better, which was particularly evident in the cracking stage. Perform finite element analysis on the test process of two wall panels. There was a good correlation between the test results and the finite element results. Models of wall panel with different parameters included the height of rid and the thickness of wallboard were also established. Through experiments and finite element analysis, it was concluded that the two large hollow rate wall panels have reliable wind resistance and can be used as the enclosure structure of 200 m high-rise buildings. The ribbed large hollow rate wall panels can be applied to higher load requirements in the engineering environment.

Evaluation of stiffness loss of reinforced concrete beams using the diffuse ultrasound method

Flexural cracks are common in reinforced concrete (RC) beams. At service loads, the tensile stresses induced by the bending moments cause beam sections to crack, leading to loss of stiffness and a consequent increase in beam deflections. Serviceability limit states in RC beam design include maximum deflection and maximum crack widths. Cracks affect the propagation of ultrasound by disrupting its travel path, which leads to a strongly scattering of the ultrasonic waves. As a result, there is a delay in the arrival of the ultrasonic energy flux, which can be observed by the increasing formation of coda waves. This resultant incoherent wavefield can be approximated by the diffuse ultrasound method. The diffuse ultrasound method can better describe the cracking effects over a larger region of the RC element compared to the ultrasonic pulse velocity, the most used ultrasound parameter in concrete applications. Changes in the diffuse ultrasound parameters (diffusivity, dissipation and ATME) can be related to the extent of cracking in a RC element. The objective of this research was to apply the diffuse ultrasound method to evaluate the stiffness loss due to flexural cracking of RC beams. Beams with different longitudinal flexural reinforcement ratios were cast and submitted to a bending test. The deflection at mid-span, and thus beam stiffness, was monitored during the test. Ultrasound transducers were installed in the central region of the beams with ultrasound readings performed during the tests in order to acquire the waveforms at various loading stages. For each waveform, the diffuse ultrasound parameters were recovered using a time–frequency analysis. The behavior of the diffuse parameters with increasing progressive damage caused by flexural cracking was analyzed and correlated to the stiffness loss of the beams. As a result, it was observed that diffusivity and ATME were the most sensitive parameters to identify the onset of cracking and also were seen to be related to beam stiffness variation at early cracking stage. When correlated with stiffness loss values up to 70%, diffusivity and ATME presented high mean correlation coefficients, allowing to conclude that it is possible to estimate the stiffness loss through the diffuse ultrasound parameters in the interval following the beginning of the cracking process.

Neural network-aided prediction of post-cracking tensile strength of fibre-reinforced concrete

Structural fibres are an effective method to improve concrete post-cracking tensile strength (fctR). Currently, the characterization of this property is mainly performed experimentally. This is a source of uncertainties at design stages, which hinders the development of new fibre type and/or optimization of those currently existing. This paper presents a multilayer perceptron neural network to predict fctR of fibre-reinforced concrete (FRC) subjected to the Barcelona Test. The optimal architecture of the predictor is obtained by evaluating 9216 configurations of input dimension and number of hidden layers and neurons. The generalization performance is assessed using repeated random sub-sampling validation with 50 iterations. The final model can predict with high accuracy the fctR of FRC for different cracking stages. A parametric analysis is performed to prove coherence between the results predicted by the model and the established understanding of the FRC behaviour. Finally, numerical expressions are provided as an alternative tool to traditional testing to predict the residual strength of the Barcelona Test for pre-design and quality control purposes based on fibre dosage, concrete strength, specimen type and height and fibre geometric characteristics. These type of approaches are found to be necessary for boosting the development of the FRC technology.

Fluorescence lifetime evolution of crude oils during thermal cracking: Implications from pyrolysis experiments in a closed system

In this study, pyrolysis experiments were performed on two crude oils with different origins in a closed system, and the pyrolyzed oils were then analyzed for spectral parameters, including fluorescence lifetime, fluorescence spectra and infrared spectra, as well as for gross chemical compositions. The results show that the evolution of the fluorescence lifetime of crude oil (τoil) can be divided approximately into two stages during oil cracking. With the increasing extent of oil cracking (PC), the τoil first increases at the early cracking stage of PC < 40%, and then it decreases at the late cracking stage of PC > 40%. The type and concentration of aromatics in crude oils, which can be respectively characterized by the fluorescence lifetime of aromatics (τaro) and the saturates/aromatics ratio (Rsat/aro) of the oils, largely control the fluorescence quantum yield and fluorescence quenching degree of the oils, respectively, and thus the changes in the two factors during oil cracking determine the fluorescence lifetime evolution of crude oils. The differences in fluorescence lifetime of the crude oils that are sourced from diverse types of organic matter cannot be masked by the oil cracking process when the PC is smaller than 80%. Combined with fluorescence or infrared spectral data, the fluorescence lifetime potentially can be applied to discriminate oils of various origins in petroleum reservoirs, especially in deep, light oil/condensate reservoirs where biomarker concentration is extremely low.

The whole fatigue crack propagation life prediction of Ni-based single crystal super-alloy specimen with single hole based on EIFS and ‘Fish-eye’ theory

In this work, based on the ‘Fish-eye’ theory and EIFS method, a prediction model of the whole fatigue crack propagation life of the predictable structure under multi-axial stress fatigue is proposed. Then, the fatigue crack propagation life of single-hole nickel-based single crystal superalloy samples under nine working conditions was predicted and verified by finite element simulation. Finally, the effects of three loads and three initial crack lengths on the fatigue crack propagation life of single-hole nickel-based single crystal superalloy were discussed by finite element simulation. The results show that the analysis results of the theoretical model and the finite element model proposed in this work are basically consistent under certain conditions. It is found that the fatigue crack propagation life of the initial crack stage, short crack stage and long crack stage accounts for 2%, 6%–10% and 90% of the whole crack propagation fatigue crack propagation life, respectively. Under the same loading stress condition, the higher the proportion of short crack stage in crack propagation is, the faster the fracture speed of the sample is. At the same time, the progressive crack propagation process of single hole specimen under fatigue load can be seen through the calculation results of finite element model. The above conclusions provide a good theoretical reference for the design and fatigue crack propagation life prediction of nickel-based single crystal superalloy turbine blades with cooling film holes.

Метод расчета усталостного повреждения регулярных зон крыла самолета при случайном нагружении на этапах типового полета

In accordance with the airworthiness standards, the aircraft structure must be operationally survivable, it means that structure must be able to remain efficiency in the presence of admissible damage. But, accumulating above than a certain level, damages cause fatigue failure of the structure, in the form of micro- and submicrocracks, thus reducing its strength characteristics. Currently, several approaches have been formed to ensure the safety of an aircraft structures in terms of strength. One of them is ensuring a safe resource (safe durability). This principle implies that during the specified service life of the product, no damage will occur in it, reducing the strength below the permissible level. The aircraft resource is limited “from above” by the durability of the regular zones of airframe. Therefore, predicting the durability of an aircraft wing structure at the design stage is a fundamental engineering problem to ensure its safety and economic efficiency. At the same time, the first step in dealing with aircraft fatigue damage at the design stage is the collection and assessment of the operational loads of the analog aircraft. However, at the design stage of a new aircraft model, obtaining such data is not always possible. Therefore, the purpose of this article is to develop a method for calculating fatigue damage at the stage of cracking and assessing the durability of regular zones of a transport aircraft wing, taking into account the conditions of its operation. The tasks to be solved are: to isolate the factors that determine the durability of the aircraft at flying in turbulent air; to take into account the asymmetry of loads and accumulated damage that occurs at each stage during the entire flight of the aircraft; to determine the aircraft's resource depending on the profile of a typical flight. The method is based on a standardized atmospheric turbulence model, typical flight profiles, fatigue characteristics of materials, the hypothesis of linear summation of damages and calculation based on nominal stresses. As result, comparison between the calculated integral repeatability of overloads and equivalent bending moments with the results of processing flight test data showed good agreements. Conclusions. The scientific novelty of the work lies in the fact that a method for calculating the fatigue damage of the regular wing zones, taking into account the expected flight profile of the aircraft was developed. This means that the proposed method makes it possible to carry out a preliminary assessment of the resource when designing an airplane without using data on the operational loads of an analogue airplane, and also estimate the residual resource of the airplane during its operation.

Crack propagation behavior of ultra-high-performance concrete (UHPC) reinforced with hybrid steel fibers under flexural loading

This study investigates the crack propagation behavior of UHPC blended with straight steel fibers of different lengths (6, 13, and 20 mm), at varying replacement ratios (0.5%, 1.0%, and 1.5%) and a constant total fiber volume fraction (Vf = 2.0%), under flexural loading. The results indicate that blending with 0.5% medium length fibers and 1.5% long fibers yields the best flexural behavior compared to other investigated specimens. Using digital image correlation (DIC) technique, the evolution of surface deformation and strain field was analyzed. Based on the recorded images and corresponding analyses, the full debonding strain between the fibers and the matrix was determined to be about 0.32%. Moreover, the maximum strain for generating cracks was approximately 3.25%, irrespective of the fiber length. When longer fibers were replaced with shorter fibers at 0.5% or 1.0% replacement ratios, Crack opening displacement (COD) variations along the beam presented a linear shape. However, at higher replacement ratios (1.5% and 2.0%), the profile showed an abrupt change, and COD variations were non-constant. The samples comprising hybrid fibers presented a faster decline in the crack initiation strain rate at higher replacement ratios of the longer fibers. In the initial stage of cracking, the crack propagated rapidly; however, the crack propagation speed dropped dramatically during crack evolution before it stabilizes.

Calculation of maximum crack width for practical design of reinforced concrete

AbstractCracks are an essential characteristic of reinforced concrete (RC) construction. Serviceability and durability, however, require a reasonable limitation of the maximum crack width. The prediction of the maximum crack width of RC, however, is not trivial and has been much debated over the past decades. This article starts with a fundamental comparison of basic procedures for determining the crack width, namely using a mechanical or calibrated model. Following, the behavior of reinforced concrete during crack formation is thoroughly discussed with a focus on the bond stress‐slip relation at the reinforcement‐concrete interface and with regard to the particular crack stage. Based on these fundamentals, a mechanical calculation model is then proposed for practical calculation of maximum crack width in RC members. The suitability of the proposed model is demonstrated by the comparison of predicted and experimentally obtained maximum crack widths for a database including 460 crack width measurements. It is shown that the current state of knowledge enables a mechanically plausible calculation of the maximum crack width. Although the formulation based on the bond law is not trivial, the calculation model can be prepared with appropriate simplifications in a practical way.

Characteristics and mechanism of a landslide at Anqian iron mine, China

Owing to the heavy rainfall, a landslide occurred at the Anqian Iron mine, at 18:00(UTC + 8) on November 24, 2019, in China. The landslide was about 3.0 × 104 m3 and caused damage to the road of transporting waste materials. Failure characteristics and the mechanism of this landslide were analyzed in this study. The landslide area was divided into three parts: the rear tension cracking area, the middle sliding deformation area, and the front colluvium area. A contact-free measuring technique using the new ShapeMetrix3D system was applied and 204 joints were analyzed based on equal-angle stereographic projection. Thus, a conceptual model of the mechanism of the landslide was constructed and the formation process of the landslide was divided into three stages: the first shearing and dislocation stage; the second sliding, front bulging, and rear tractive cracking stage; and the third local rock mass collapse and colluvium depositing stage. Numerical modeling was performed to discover the landslide mechanism by progressively reducing the shear strength of rock mass. The results showed that the original slope was stable, whereas heavy rainfall triggered the landslide, and the predicted failure surface matched closely the field investigations. The factor of safety obtained by real three-dimensional analyses was slightly higher than that obtained by plane problem analyses, and the difference was attributed to the three-dimensional effect of the landslide. This paper also presents the results obtained from the parametric analysis in order to understand the impact of shear strength parameters on the overall stability of the slope.

Crack initiation and propagation from geometric microdefects: Experiment and transition fatigue behavior

AbstractThe present work aims to study short fatigue crack initiation and propagation induced by geometric microdefects. An empirical approach is used to evaluate the microstructural influences on crack growth behavior. Small fatigue crack test on titanium alloy Ti‐6Al‐4V is performed to investigate the crack induced from a geometric microdefect. Analyses on crack initiation and fracture are respectively performed to study the microstructural influences on the stages of initiation and fracture. Subsequently, an empirical multiscale model is employed to predict the short fatigue crack propagation of alloys underlining the transition features. Grain size variation is adopted to reflect the main microstructural contribution to the short crack propagation. An alpha factor is defined to demonstrate the transition interim from microstructurally short crack to physically short crack stages. Furthermore, a relationship between transition factor and grain size is correlated. The results indicate that the value of transition factor tends to decrease with the augment of average grain size. The empirical approach is validated through a variety of fatigue experimental data of alloys including Ti‐6Al‐4V, 2024‐T3, and GH4169.

Hyperbranched poly(amidoamine) as an efficient macroinitiator for steam cracking of naphtha

The water-soluble and oil-soluble amidoamine-structured hyperbranched polymers (PAMAM & PPAMAM) were synthesized as initiators for steam cracking of naphtha to promote the production of light olefins. Within 740–840 °C, both PPAMAM and PAMAM significantly improved the gas yield and yields of light olefins, and the PPAMAM showed better performance. The effect of PPAMAM increased with the molecular weight. At 800 °C, when adding 570 ppm PPAMAM with the molecular weight of 12,500 , yields of ethylene and propylene increased from 29.81 wt% and 17.47 wt% to 33.06 wt% and 17.70 wt% respectively, which were even higher than those obtained by cracking pure naphtha at 840 °C (32.72 wt% and 16.08 wt%). Combined with the simulated pyrolysis reaction network and measured thermal decomposition products of PAMAM and PPAMAM, it was found that three active free radicals (•H, •NH2, •CH3) generated by the additives played important roles in the initial cracking stage of hydrocarbons.