Thermal Aging Time Research Articles

Mechanical properties of thermal aged HTPB composite solid propellant under confining pressure

With the purpose of investigating the effects of confining pressure and aging on the mechanical properties of Hydroxyl-terminated polybutadiene (HTPB) based composite solid propellant, tensile tests of thermal accelerated aged propellant samples under room temperature and different confining pressure conditions were performed through the use of a self-made confining pressure device and conventional testing machine. Afterwards, the maximum tensile stress σm and the corresponding strain εm for the propellant under different test conditions were obtained and analyzed. The results indicate that confining pressure and aging can significantly affect the mechanical properties of HTPB propellant, and the coupled effects are very complex. On the one hand, the stress σm increases as a whole when confining pressure becomes higher or thermal aging time rises. Besides, this stress is more sensitive to aging with increasing confining pressure. There are almost three regions in the stress increments σmP−σm0/σm0 and thermal aging time curves for HTPB propellant. The maximum value of the stress increment σmP−σm0/σm0 for the propellant is about 98% at 7.0 MPa and 170 d. On the other hand, the strain εm decreases with increasing thermal aging time under the whole confining pressure conditions. However, the variation of this strain with confining pressure is more complex at various thermal aging time, which is different from that of unaged solid propellant in previous researches. In addition, this strain is slightly less sensitive to aging as the confining pressure increases. Furthermore, there is also a critical confining pressure in this investigation, whose value is between 0.15 MPa and 4.0 MPa. Beyond this critical pressure, the trends of the stress σm and the corresponding strain εm all change. Moreover, there are some critical thermal aging time for the stress increment (σmP−σm0)/σm0 and strain increment (εmP−εm0)/εm0 of HTPB propellant in this investigation, which are about at 35, 50 and 170 d. Finally, based on the twin-shear strength theory, a new modified nonlinear strength criterion of thermal aged HTPB propellant under confining pressure was proposed. And the whole errors of fitted results are lower than 6%. Therefore, the proposed strength criterion can be selected as a failure criterion for the analysis the failure properties of aged HTPB propellant under different confining pressures, the structural integrity of solid propellant grain and the safety of solid rocket motor during ignition operation after long periods of storage.

Filtration and structure of bentonite-β-cyclodextrin polymer microspheres suspensions: Effect of thermal aging time

Effective filtration loss control is a challenge for water-based drilling fluid when drilling high temperature and high pressure (HTHP) formations. After thermal aging at 200 °C for varying times, the static filtration loss controlling capabilities of β-cyclodextrin polymer microspheres (β-CDPs) and synthetic polymer Driscal-D in bentonite suspensions were compared in this study. A comprehensive physicochemical characterization was presented using Fourier transform infrared (FT-IR), X-ray diffraction (XRD), particle size distribution, zeta potential, electrical conductivity and scanning electron microscope (SEM). The results indicated that when exposure to thermal aging of 200 °C, the filtration loss of bentonite suspension increased with the initial 6 h of aging, after that it increased slightly. While for synthetic polymer Driscal-D, the filtration loss increased gradually with thermal aging time until to 36 h. In contrast for β-CDPs, the filtration loss increased firstly and then decreased and maintained relatively stable after thermal treatment for 6 h. High temperature destroyed the hydrated structure of bentonite particles and induced the increase of filtration loss volume. In viewing of Driscal-D, degradation of the polymer in combination with dehydration of bentonite particles led to the gradual losing of suspensions stability. For β-CDPs, after aging of 6 h, the hydrothermal carbonization reaction generated numerous nano and micro carbon spheres and bentonite-carbon sphere composited structures, which fill in the gaps between the larger particles and contribute to reducing the filter cake permeability. This result identified that β-CDPs are superior to synthetic polymer Driscal-D in filtration control when subjected to long term high temperature environments.

Coexistence of two distinct fatigue failure mechanisms in Super304H welded joint at elevated temperatures

Super304H steel is a promising candidate for boiler tube materials in thermal power plants operating under ultra-supercritical conditions, and the fatigue properties of its welded joint play a key role in the structural reliability of the thermal power plants. Herein, we report that there is a transition in the fatigue failure location (i.e., fatigue failure mechanism) from the base metal at a high strain amplitude (≥~0.4%) to the weld metal at a low strain amplitude (≤~0.3%) at a constant temperature in the operating range of 500–700 °C, and this leads to a significant reduction in the fatigue resistance of the welded joint as compared to that of the base metal. We found that a longer fatigue test time at a low strain amplitude induces a thermal aging effect that promotes different microstructural evolutions in the base metal and weld metal, deepening material inhomogeneity in the welded joint, and thereby triggering strain localization in the weld metal. With increasing fatigue test time (i.e., thermal aging time), Cr23C6 carbides precipitated in the interdendritic region of the weld metal and at the austenitic grain boundary of the base metal, and Nb (C, N) phases precipitated in the interior of austenitic grain in the base metal, which increased local hardness, whereas there was no significant microstructural change in the dendrite core of the weld metal, retaining its initial hardness. The intensified local hardness inhomogeneity caused the softest zone in the welded joint, wherein strain localization occurred, serving as a crack nucleation site and propagation path, to shift from the base metal to the weld metal (dendrite core). This induced a shift in the fatigue failure location from the base metal to the weld metal with decreasing strain amplitude.

Fracture toughness of thermal aged 16MND5 bainitic forging steel under varying 3D constraint conditions: An experimental study using SENT specimens

This paper presents the experimental investigations of the effects of thermal aging on the embrittlement failure mechanism and fracture toughness of 16MND5 reactor pressure vessel (RPV) steel using clamped single edge notched tension (SENT) specimens with different sizes. In order to study the embrittlement induced by thermal aging, the 16MND5 steel was subjected to accelerated thermal aging for 1000 h, 3000 h and 5000 h at 425 °C, and then the Vickers hardness and tensile properties were determined. To investigate the combined effects of thermal aging and specimen geometric sizes, the fracture toughness of various thermal aged materials was studied using SENT specimens with different specimen thickness ratios, B/W, and crack depths, a/W. A total of 59 SENT specimens were tested covering a wide range of thermal aging time and geometric size combinations. Results showed that as the aging time increases, the material’s micro hardness and tensile properties change slightly, but the R-curves (J-R and CTOD-R) decrease significantly. The ductile fracture properties of thermal aged RPV steels depend markedly on the geometric dimensions (crack depth, a/W and specimen thickness, B/W) of tested specimens. Furthermore, the effects of geometric sizes and thermal aging on fracture behavior are shown to have a strong interaction. Specifically, as the aging time increases, the influence of the geometric sizes on the fracture toughness decreases, especially with the variations of a/W. Also, as the geometric dimensions increase, the effect of thermal aging on fracture toughness decreases. Finally, combined with fractographic observations, the influence of thermal aging on the embrittlement mechanism and ductile fracture properties of 16MND5 steel was discussed.

Mechanical and acoustical properties of Eichhornia crassipes (water hyacinth) fiber‐reinforced styrene butadiene rubber

AbstractThe essential target of this investigation is to valorize an Eichhornia crassipes fiber (ECF) and maleates of Eichhornia crassipes fiber (MoECF) as reinforcing fillers in styrene‐butadiene rubber (SBR) composites in terms of the mechanical, acoustical, thermal, and morphological properties. Fourier transforms infrared spectroscopy characterizes the esterification of ECF with maleic anhydride (MA) and the graft of MoECF onto SBR. SBR composites manufactured with different loadings of ECF and MoECF (1, 2.5, 5, 10, and 20 phr). Scanning electron microscope illustrates the MoECF upgrade interfacial adhesion with the SBR matrix. It is observed that 5 and 20 phr of MoECF enhance the tensile strength and elongation at break of SBR composites by 8% and 310%, respectively. 10 phr of MoECF maintains the tensile strength of SBR composite during the accelerated thermal aging course. While 20 phr of MoECF improves the tensile strength of SBR composite by 19% with increment‐accelerated thermal aging time to 7 days. The higher loading (10 and 20 phr) of MoECF ameliorates the thermal stability of SBR composites. SBR composite containing 10 phr of MoECF has sound absorption amplitude equal to 0.9 at the frequency of 400 Hz. The sound absorption performance improved within low‐frequency regions below 500 Hz with increasing thickness to 2.3 mm.

Effect of thermal ageing on the scratch resistance of natural-fibre-reinforced epoxy composites

In this study, the effects of thermal ageing on the scratch-resistance properties of natural-fibre-reinforced epoxy composites were investigated. Jute, kenaf and hemp-fibre-reinforced epoxy composites were fabricated using the vacuum infusion method. The natural-fibre-reinforced composites were exposed to thermal ageing for 7, 15, and 30 d at 90 °C temperature. Scratch tests were carried out using a CSM Revetest Xpress machine. The scratch resistance properties, such as the critical normal load, coefficient of friction, penetration depth, fracture toughness, scratch hardness, and scratch tearing were determined after thermal ageing. The results demonstrate that the scratch resistance of the composites decreased as the thermal ageing time increased. However, the jute-fibre-reinforced epoxy composites had the highest critical normal load, fracture toughness and scratch hardness compared to those of kenaf and hemp-fibre-reinforced epoxy composites after 30 d of thermal ageing.

Efficient Thermo-Stability and Smoke-Suppression Properties of La Doping Mg-Al LDHs on PVC Nanocomposites

Despite the advantages of the non-flammable, good performance and low price, poly (vinyl chloride) (PVC) still suffer from poor thermal stability, restricting its melting process and applications. Although addition of some heat stabilizers can be used to improve the low thermal stability, so far, they normally compromise the environmental issues and smoke density of PVC during combustion. In this work, a series of La doping Mg-Al layered double hydroxides (LaLDHs) with different molar ratio of La3+ / Al3+ were successfully synthesized by coprecipitation-hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR), Scanning Electron Microscopy (SEM) and Transmission electron microscopy (TEM). The results showed that the as-prepared LaLDHs exhibit plate-like morphology with a lateral size around 100-180 nm. The different as-prepared LaLDHs were introduced into PVC as heat stabilizer to prepare PVC nanocomposites. The thermal stability and smoke suppression of PVC nanocomposites were investigated by TGA, thermal aging, Congo red and smoke density rating test (SDR), respectively. All the results demonstrated that PVC-LaLDHs2 nanocomposites containing 2% LaLDHs2 (the molar ratio of La3+ / Al3+ is 1 / 3) were optimized, which achieved the maximal T50% value of 337.2 oC, minimal SDR value of 45.6%, and prolonged the thermal aging time from less than 10mins to 90mins, respectively thermal stability time from 1242s to 2751s. In addition, the tensile strength and elastic modulus of PVC-LaLDHs2 respectively increased by 84.4% (56.6 MPa) and 75.5% (1019.4 MPa) with little affecting elongation at break of PVC. LDHs (layered double hydroxides); rare earth ions; thermal stability; smoke suppression; poly (vinyl chloride)

Influence of Different Tougheners on the Properties of PC/PBT Alloy

This paper focuses on the influence of glycidyl methacrylate functionalized polyolefin elastomer (SOG-03) on the properties of PC/PBT alloys, and also made a contrastive analysis with ethylene-methyl acrylate-glycidyl methacrylate terpolymer (EMA-co-GMA) and methyl methacrylate-butadiene-styrene terpolymer (MBS), the common toughener in PC/PBT alloy. The impact performance test results of PC/PBT alloys with different addition of SOG-03 showed that the brittle-ductile transition began when SOG-03 content reached 3 wt%. The microstructure, differential scanning calorimeter (DSC) and multi-extrusion process results of PC/PBT alloys all showed that SOG-03 tends to be dispersed in PBT phase and the dispersed SOG-03 presents typical rubber-toughened polymer morphology. The toughening efficiency of MBS on PC/PBT alloy was much lower than EMA-co-GMA and SOG-03, and showed a worse processing stability after multi-extrusion process and long-term thermal ageing properties. The EMA-co-GMA and SOG-03 toughened PC/PBT alloys showed an equivalent toughness, while the PC/PBT alloy with SOG-03 showed a better processing stability during the multi-extrusion process and long-term thermal ageing property when the thermal aging time is more than 600 h.

Research on Short-term Aging of Lignin Modified Asphalt

In order to study the micro-aging mechanism of lignin-modified asphalt, different asphalt samples were prepared with lignin content of 0% and 12% by mass. The conventional asphalt index test and high-temperature dynamic shear rheological test were used to analyze the different asphalt samples. The change law of the performance of modified lignin modified asphalt with thermal oxygen aging time. The results show that adding appropriate amount of lignin to each asphalt will increase the penetration and ductility of the base asphalt, reduce the softening point, and significantly improve the high-temperature stability; comparative analysis before and after DSR aging, the complex shear modulus G* increases and The decrease of phase angle δ indicates that lignin can improve the high-temperature rheological properties of asphalt.

Development of probabilistic primary water stress corrosion cracking initiation model for alloy 182 welds considering thermal aging and cold work effects

We experimentally investigated the effects of thermal aging and cold work on the microstructure, mechanical properties, and primary water stress corrosion cracking (PWSCC) initiation time for Alloy 182 welds. The effects of thermal aging and cold work on the PWSCC initiation time of Alloy 182 were modeled based on the plastic energy concept and the PWSCC initiation data of this study and previous reports by considering censored data. Based on the results, it is estimated that the PWSCC resistance of the Alloy 182 weld firstly increases and then decreases with thermal aging time when the applied stress is kept constant.

Microstructural Evolution and Mechanical Behavior of Thermally Aged Cast Duplex Stainless Steel.

The microstructural evolution and mechanical behavior of cast duplex stainless steels (CDSSs) at 400 °C for different thermal aging times were investigated by transmission electron microscope (TEM) and small punch test (SPT). The results showed that the spinodal decomposition in ferrite was the main reason for the decrease in toughness, and G-phase did not play an important role in the embrittlement process. The change of membrane stretching zone (Wm) played an important role in the SPT load-displacement curve before and after thermal aging. During the deformation process of Wm in the SPT, for thermal aging for 10,000 h, some completely curved slip bands were generated inside the ferrite phase, which had no contact with the δ/γ phase interface and belonged to the slip bands produced by the independent deformation of ferrite. The combined effect of the curved slip bands and stress concentration led to the initiation of obvious micro-cracks at the δ/γ phase interface. The micro-cracks propagated along the ferrite phase curved slip bands, and eventually penetrated the entire hardened ferrite phase.

Development of Energy-Efficient Superhydrophobic Polypropylene Fabric by Oxygen Plasma Etching and Thermal Aging.

This study developed a human-friendly energy-efficient superhydrophobic polypropylene (PP) fabric by oxygen plasma etching and short-term thermal aging without additional chemicals. The effect of the microroughness on the superhydrophobicity was examined by adjusting the weave density. After the PP fabric was treated with oxygen plasma etching for 15 min and thermal aging at 120 °C for 1 h (E15H120 1 h), the static contact and shedding angles were 162.7° ± 2.4° and 5.2° ± 0.7° and the energy consumption was 136.4 ± 7.0 Wh. Oxygen plasma etching for 15 min and thermal aging at 120 °C for 24 h (E15H120 24 h) resulted in a static contact and shedding angle of 180.0° ± 0.0° and 1.8° ± 0.2° and energy consumption of 3628.5 ± 82.6 Wh. E15H120 1 h showed a lower shedding angle but had a higher sliding angle of 90°. E15H120 24 h exhibited shedding and sliding angles of less than 10°. Regardless of the thermal aging time, superhydrophobicity was higher in high-density fabrics than in low-density fabrics. The superhydrophobic PP fabric had a similar water vapor transmission rate and air permeability with the untreated PP fabric, and it showed a self-heading property after washing followed by tumble drying and hot pressing.

Growth of intermetallic compounds and brittle fracture behavior of Sn-Ag-Cu/ENIG joint with columnar Ni-P layer

A growth of intermetallic compounds (IMCs) at the interface between electroless Ni/immersion Au (ENIG) and Sn-3.0Ag-0.5Cu (SAC305) solder and related brittle fracture behavior of solder joint with microstructure changes of electroless Ni-P was investigated. An amorphous columnar Ni-P and an amorphous structureless Ni-P surface finishes were used for the comparison. The (Cu,Ni)6Sn5 IMC layers with a similar thickness were formed between solder and ENIG interface at the as-reflowed stage for both Ni-P surface finishes. During thermal aging, the interfacial IMC thicknesses for columnar Ni-P and structureless Ni-P increased up to 400 and 800 h, respectively, and then decreased with thermal aging time. This decrease of IMC thickness with thermal aging was due to spalling. The spalling rate of the columnar Ni-P sample was higher than that of the structureless sample. The shear strength decrease with aging time was lower for the columnar Ni-P sample than the structureless Ni-P sample. Brittle fracture for the columnar Ni-P sample was lower than for the structureless Ni-P sample.

Long term ageing effect on fracture toughness of the GTAW welded joints for nuclear power main pipelines

Gas tungsten arc welding (GTAW) joints of nuclear power main pipelines were subjected on accelerated thermal ageing tests until 30000 h to investigate long-term ageing effect on fracture property. The results indicate that fracture toughness values JIC and JSZW of both base metals and welded metals decreases clearly and fracture mode transits subsequently from typical ductile to brittle fracture with increasing ageing time, but J-resistance (J-R) curves still lie above the recommend low-bound lines given by NUREG/CR after 30000 h thermal ageing. Moreover, base metals have better fracture toughness than welded metals regardless of thermal ageing time, and thus welded metals are still the weakest part for the whole welded joints. In addition, thermal ageing has no significantly influence on the location of crack initiation at the crack tip and crack growth mode because the thermal ageing could enhance the hardness and strength of ferrite phases and thus the austenite phases still have lower strength.

Study on the Structure and Dielectric Properties of Zeolite/LDPE Nanocomposite under Thermal Aging.

Nanodoping is an effective way to improve the dielectric properties and the aging resistance of polyethylene. Nano-zeolite has a nano-level porous structure and larger specific surface area than ordinary nano-inorganic oxide, which can be used to improve dielectric properties of low-density polyethylene (LDPE) nanocomposite. The zeolite/LDPE nanocomposites were prepared and subjected to thermal aging treatment to obtain samples with different aging time. Using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and the differential scanning calorimetry (DSC) test to study the microscopic and structure characteristics, it was found that nano-zeolite doping can effectively reduce the thermal aging damage to the internal structure of the nanocomposite; carbonyl and hydroxyl decreased significantly during the thermal aging time, and the crystallinity effectively improved. Nano-zeolite doping significantly improved the morphology and strengthened the aging resistance of the nanocomposite. In the dielectric strength test, it was found that nanodoping can effectively improve the direct current (DC) and alternating current (AC) breakdown field strength and the stability after the thermal aging. The dielectric constant of nanocomposite can be reduced, and the dielectric loss had no obvious change during the aging process. Moreover, the zeolite/LDPE nanocomposite with the doping concentration of 1 wt % had the best performance, for the nano-zeolite was better dispersed.

Interfacial transfer and phase evolution between Cu and Sn solder doped with minor Cu, Ag and Ni: experimental and theoretical investigations

The effects of minor Cu (0.7%), Ag (3.5%) and Ni (0.1%) additions in Sn solder on the interfacial transfer and phase evolution were clarified in Sn/Cu solder joints. (Cu,M)6Sn5 and (Cu,M)3Sn (M = Cu, Ni) layers were observed at the Sn/Cu interface during thermal aging at 150 °C. The additions of Cu, Ag and Ni alloy elements were found to change the growth of the Cn–Sn intermetallic compounds (IMCs). The results showed that Cu, Ag and Ni additions were able to suppress the growth of (Cu,M)3Sn (M = Cu, Ni) layers, but promoted the (Cu,M)6Sn5 growth. This was because the reactions on the (Cu,M)6Sn5 side were promoted by the refined IMCs grain size, whose growth was at the expense of (Cu,M)3Sn. In addition, the interfacial transfer rate (thickness ratio, y) of (Cu,M)3Sn/(Cu,M)6Sn5 versus the thermal aging time (t) was found to be close to parabola relationship, the y increased gradually with the alloy elements from Ni, Cu to Ag. The phase evolution (η′-Cu6Sn5, η-Cu6Sn5, e-Cu3Sn, Ag3Sn, η′-(Cu,Ni)6Sn5, η-(Cu,Ni)6Sn5 and e-(Cu,Ni)3Sn) at the interface between Cu and Sn solder doped with minor Cu, Ag and Ni was also elucidated.

Experimental Study on Thermal Aging Performance of an RDX Based Booster

YC-1 booster (96.5% RDX, 3% F2641 and 0.5% graphite) was selected as the research object. Accelerated aging of YC-1 was carried out by 71°C thermal aging tests. The size changes of samples before and after thermal aging were measured. The initiation ability and shock sensitivity of the samples before and after accelerated aging were tested by steel compression test and SSGT small gap test. Through the establishment of equal proportion impact dynamics calculation model, the propagation law of shock wave generated by the explosion of A-= 9 \* ROMANIX-= 1 \* ROMANI was calculated. Test and calculation results show that: The diameter and height of samples increase with the increase of aging time. There was no significant change in the initiation ability of the samples, and the shock wave sensitivity increases first and then decreases with the increase of thermal aging time. The critical thickness of the gap corresponding to the samples aged 0 day, 4days, 19 days, 38 days and 76 days was 6.9mm, 7.0mm, 7.5mm, 8.1mm and 7.8mm, respectively. The corresponding shock wave pressure (without considering the interface reflection) in the gap was 2.91GPa, 2.85 GPa, 2.60 GPa, 2.40 GPa and 2.50 GPa respectively.

Study on Thermal Aging Characteristics of HTV Silicone Rubber Sheds of Composite Insulators

The composite insulator is affected by many aging factors during its operation. Thermal aging is the most common aging effect on the high temperature vulcanized (HTV) silicone rubber sheds of composite insulators. In order to study the thermal aging characteristics of HTV silicone rubber sheds of composite insulators, the artificial thermal aging test of HTV silicone rubber sheds samples was carried out, and then the aged samples were analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TG), and HTV silicone rubber was studied. The results show that the pyrolysis process of HTV silicone rubber can be divided into two stages: dehydration and decomposition of Al(OH)3 flame retardant in the first stage and polydimethylsiloxane (PDMS) molecule in the second stage. With the increase of thermal aging time, the crack width and number on the surface of the sample increase gradually, and the infrared absorption peaks of Si-C side chain, Si-O-Si main chain and O-H group decrease gradually, which indicates that thermal aging can greatly accelerate the aging process of HTV silicone rubber of composite insulators.

Thermoelectric performance of thermally aged nanostructured bulk materials—a case study of lead chalcogenides

The performance of nanostructured bulk thermoelectric material is shown to be enhanced because of phonon scattering. However, it is well-known that microstructures evolve by thermal aging, and their stability at the operating temperature is of vital importance to the success of the device. Here, we investigate the effect of long-term annealing (up to 8 weeks) on thermoelectric performance and microstructure of a high-efficiency Na-doped multiphase quaternary Pb chalcogenides.A zT of ~1.7–1.9 in the 700–850 K temperature range is achieved at all stages of thermal aging. A redistribution of Na-dopant between phases above ~630 K is believed to explain an increasing electrical resistivity in aged samples. This is balanced by a reduced total thermal conductivity as thermal aging time increases, possibly because of an increased concentration of nanoscale precipitates in samples aged for 8 weeks. Intermediate annealing of nanostructured lead chalcogenides has been shown to be an effective means of preparing thermally stable, high-performance thermoelectric materials. We have also determined the precipitates coarsening rate constant, k, to be 0.0206 μm/h1/3.

Influence of Thermal Aging on Space Charge Accumulation and Dissipation of Cross-Linked Polyethylene With Different Cross-Linking Agent Contents

This article presents the influence of thermal aging on space charge accumulation and dissipation of cross-linked polyethylene (XLPE) with different cross-linking agent contents. In this article, dicumyl peroxide (DCP) and low-density polyethylene (LDPE) respectively are used as cross-linking agent and basic experimental material. Additionally, an improved PEA method is used to measure space charge densities during voltage application and removal when thermal aging time respectively is 0, 5, 15, 30 and 80 days due to higher accuracy. And experimental results indicate thermal aging can introduce negative charge traps into pure LDPE and make space charges easier to inject and accumulate in pure LDPE and XLPE, but space charge densities in XLPE increase more rapidly as thermal aging time increases, moreover, thermal aging can simultaneously increase densities of deep and shallow charge traps in pure LDPE and XLPE, etc. And we have also explained the change of space charge accumulation and dissipation of pure LDPE and XLPE after thermal aging from the perspective of micro-structure change of pure LDPE and XLPE through the quantitative analysis of average charge densities. Besides, theoretical model of the average charge decay has also been summed up, and this model can perfectly reflect the dissipation of space charges in pure LDPE and XLPE of different thermal aging time.