Increase In Internal Friction Research Articles

Inelastic relaxation in tin oxide thin films with an amorphous structure

Inelastic relaxation in amorphous tin oxide thin films obtained by ion-beam sputtering in an argon atmosphere were studied. The films retain an amorphous structure after annealing at temperatures below 623 K for 30 min and crystallization begins after annealing at 673 K with the formation of two phases, where the SnO2 phase predominates over the SnO phase. Annealing at 723 K for 30 min leads to a partial transition of the SnO crystalline phase to the SnO2 phase.The temperature dependence of internal friction revealed maxima at 585 K and 603 K, identified as β - relaxation maxima, as well as at 690 K, identified as α - relaxation maximum. It is assumed that the β - relaxation maxima at 585 K and 603 K are associated with local hopps of oxygen atoms within the defect structure of SnO2 and with local hopps of tin atoms within the defect structure of SnO, respectively. The exponential increase in internal friction up to a temperature of 690 K in the α - relaxation region is associated with the diffusion of nonequilibrium vacancy-like defects of the amorphous structure below the glass transition temperature and equilibrium ones above the glass transition temperature. Estimates of the migration energy and formation energy of vacancy-like defects in amorphous tin oxide were made.

PPR/EPDM/POE-g-MA/OMMT nanocomposites with superior low-temperature toughness based on embedded OMMT within compound rubber dispersion phase

A new approach to produce polypropylene random copolymer/ethylene propylene diene monomer/polyolefin elastomers modified with maleic anhydride/organic modified nanomontmorillonite (PPR/EPDM/POE-g-MA/OMMT) nanocomposites with superior low-temperature toughness was conducted. The brittle-ductile transition temperature (Tbd) of PPR/EPDM/POE-g-MA/OMMT nanocomposites reduced from −20 °C to −40 °C. The incorporation of POE-g-MA results in the distribution of OMMT changing from the interfacial distribution into dispersed inside rubber phase and a compound rubber dispersed phase in PPR/EPDM/POE-g-MA/OMMT nanocomposites was comprised of EPDM, POE-g-MA and embedded OMMT. The synergistic effect of POE-g-MA and OMMT was considered to result from the special compound rubber dispersed phase consisting of EPDM and POE-g-MA as well embedded OMMT, which led to the reduction in space between rubber domains and the increase in internal friction and ultimately brought about the remarkable enhancement in mechanical properties.

Evaluation of rheological properties of sustainable self-compacting recycled aggregate concrete produced by two-stage mixing approach

The rheological properties such as static yield stress (τ0s), dynamic yield stress (τ0d), and plastic viscosity (μ) play an important role in controlling the fresh state activities of self-compacting concrete (SCC). In the present investigation, these rheological parameters of self-compacting recycled aggregate concrete (SCRAC) prepared by two-stage mixing approach (TSMA) are determined as a function of recycled coarse aggregate (RCA) percentage. Five types of samples are prepared for this purpose: M1 (0% RCA), M2 (25% RCA), M3 (50% RCA), M4 (75% RCA), and M5 (100% RCA). The static yield stress is determined by a well-accepted static yield stress formula; whereas, the dynamic yield stress and plastic viscosity are determined by Reiner-Riwlin theory in conjunction with the Bingham and modified Bingham models. The values of τ0s, τ0d, and μ are observed to increase with the increase in RCA%. This is due to the increase in internal friction between the particles. However, the influence is less in the proposed TSMA. Various conventional fresh properties of SCC of the same mixes are evaluated and the correlations between the conventional fresh properties and rheological properties have been established. Finally, the compressive strength of the samples is reported and a plausible model is developed to elucidate the mechanism based on the various characterizations including X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and field emission scanning electron microscopy (FESEM). The study found that the proposed TSMA is an effective approach for improving the rheological and mechanical properties of SCRAC.

Internal Heat Transfer Measurement on Metal-Based Additively Manufactured Channels Using a Transient Technique

Abstract The advancements in additive manufacturing (AM) of metals open new possibilities in the design of gas turbine parts. Especially the cooling efficiency of internal channels can be improved with more complex geometries. Naturally, AM channels have a higher surface roughness than conventionally manufactured parts, which influences the cooling air pressure loss as well as the heat transfer. Implementing novel cooling designs using AM can be possible only if the effect of increased surface roughness on the flow and on the heat transfer can be predicted with an appropriate accuracy. The objective of the current study was to measure these parameters experimentally in simple AM channels to build a database for designing complex and efficient cooling designs using the AM technique. A test rig and postprocessing method was elaborated to derive the local internal heat transfer distribution of metal-based AM channels. Six circular single channel coupons made by selective laser melting (SLM) were tested for Reynolds numbers ranging from 20,000 to 50,000. The coupon with the lowest relative roughness shows good agreement with the Dittus–Boelter correlation. All the other coupons show a consistent increase of internal heat transfer and flow friction with the increase of the internal surface roughness.

Experimental Study on the Effect of Thermal Shock on Physical and Mechanical Properties of Limestone

Thermal shock is the physical process of rapid cooling of a hot object. The physical and mechanical properties of rocks that have undergone thermal shock will change. This variation can be applied to the development of geothermal in limestone reservoirs and serve as an effective means of enhancing heat exchange capacity. The essence of this is rock thermal shock rupture. The reason for the change in rock properties due to thermal shock is distinguished from other factors and is a process of instantaneous impact thermal stress caused by non-constant heat transfer, resulting in rock damage from microscopic damage to macroscopic damage. In order to study the variation of physical and mechanical properties of limestone with heating and cooling temperatures under the effect of thermal shock, a self-developed thermal shock test device was used to test the physical and mechanical properties of limestone heated to 100 °C~600 °C after thermal shock. The results strongly suggest that the heating temperature under the effect of thermal shock is the main factor affecting the evolution of basic physical properties of limestone; with the increase in heating temperature, the color of the specimen changes from off-white to white, the mass decreases and the volume nonlinearly increases, with a maximum reduction of 1.39% in mass and a maximum expansion of 2.79% in volume at 600 °C. Indeed, 500 °C is the temperature of abrupt mass loss. The heating temperature and the cooling medium temperature act together to deteriorate the mechanical properties of limestone. With increasing heating temperature (decreasing cooling temperature), the uniaxial compressive strength of limestone decreased by 39.5% (19.3%), modulus of elasticity by 59.5% (22.9%), tensile strength by 42.9% (7.6%), and cohesion by 43.2% (22.5%). The peak strain increases by 74.2% and the angle of internal friction increases by 27% (25.9%). The above data are average values. The empirical equations of the compressive strength, modulus of elasticity, tensile strength of limestone under the action of thermal shock versus heating temperature were obtained. The differences in the physical and mechanical properties of limestone after two heat treatments (thermal shock and high-temperature heating) were compared and analyzed, and the results showed that the physical and mechanical properties of limestone deteriorated more severely after thermal shock compared to high-temperature heating. The research results can provide technical support for the drilling of geothermal development and wellbore stability assessment in limestone reservoirs, and enrich the theory of high-temperature rock mechanics.

Study of the influence of gamma irradiation on internal friction in dislocation silicon

The effect of radiation defects formed during gamma irradiation of single-crystal silicon on the internal friction in dislocations was studied. It was found that in dislocation silicon, after irradiation with gamma rays of a source of ionizing radiation 60Co, at first the internal friction (Q-1) increases to a maximum value and then Q-1 gradually decreases to the initial values. It is shown that, as a result of gamma irradiation of silicon, the resulting change in Q-1 is associated with the process of relaxation of vacancies with the formation of vacancy defect complexes near dislocation lines. The data of the acoustic emission method (obtained AE spectra) confirm that AE signals in gamma-irradiated silicon arise due to moving dislocations, and the intensity of AE signals initially increases during the first 1.5–2 h, which is also observed with an increase in internal friction depending on Q-1(t), and then the AE signals decrease in the form of pulses of discrete AE signals.

Triaxial Experimental Study of Zinc Contaminated Red Clay under Different Temperature Conditions

Temperature is one of the important factors affecting the mechanical properties of geotechnical soils, and its role in engineering construction in China cannot be underestimated. In order to study the effects of temperature and zinc contamination concentration on the mechanical properties of Guilin local red clay, a temperature-controlled triaxial shear test was conducted on Guilin red clay under three variables of temperature, zinc contamination concentration and surrounding pressure. The test findings revealed that there are significant differences in the effects of temperature, zinc contamination concentration and surrounding pressure on the mechanical properties of Guilin red clay. The stress–strain curves of the red clay at various temperatures, contamination concentrations and envelope pressures are of the strain-hardening type, and the deformation modulus showed a tendency to increase rapidly with increasing strain, then decrease rapidly, and finally, decrease slowly. With the increase of temperature, the cohesion of Zn-contaminated red clay increases, while the angle of internal friction increases and then decreases, both of which increase the shear strength of red clay. As the concentration of Zn contamination grows, the shear strength of the red clay increases, while the internal friction angle increases and then decreases, and the shear strength of the soil increases and then decreases. The shear strength of the Zn-contaminated red clay improved as the surrounding pressure increased.

Tuning ultra-high damping capacity of Mn–Cu alloy to fit wide-range service temperature

Mn–Cu-based damping alloy prepared by directional solidification is very promising for vibration and noise control. Aging treatment with and without application of magnetic field on the microstructure and damping performance of directionally solidified Mn–Cu alloy was studied by means of X-ray diffraction, differential scanning calorimetry, dynamic thermal mechanical analysis and scanning electron microscope. It is found that aging treatment makes the orientation shift from (200) to (111), and aging treatment under magnetic field helps Mn–Cu alloy keep high damping in the wide temperature range. The polymorphic microstructure characterised by fine twin bands with low-angle misorientation boundaries leads to a substantial increase of twin boundary internal friction up to several times higher than that of polycrystalline state.

Thermal stability of SPD-processed aluminum alloys - Internal friction as an indication for recovery, recrystallization and abnormal grain growth

In this study, we investigate the effect of different microstructures generated by severe plastic deformation on the thermal stability of an age-hardenable Al–Mg–Si alloy (AA6082). To analyze recovery and recrystallization we investigate the internal friction at various excitation frequencies and the microstructural evolution of the material in a wide range of temperatures. Severely deformed conditions of the alloy show a distinct increase of internal friction and a pronounced reduction of the materials hardness at elevated temperatures. Microstructural investigations reveal that different stages of recovery and recrystallization take place during the heating process. In a certain case, a remarkable correlation between frequency-dependent internal friction and abnormal grain growth in the microstructure is found. It is derived that internal friction measurements help to characterize the thermal stability of SPD processed materials. Since thermally-induced microstructural changes are directly related to changes of internal friction, these measurements help to identify characteristic temperatures of recovery, recrystallization and abnormal grain growth.

The Effect of Subsequent Stress-Induced Martensite Aging on the Viscoelastic Properties of Aged NiTiHf Polycrystals

This study investigated the effect of stress-induced martensite aging under tensile and compressive stresses on the functional and viscoelastic properties in Ni50.3Ti32.2Hf17.5 polycrystals containing dispersed H-phase particles up to 70 nm in size obtained by preliminary austenite aging at 873 K for 3 h. It was found that stress-induced martensite aging at 428 K for 12 h results in the appearance of a two-way shape memory effect of −0.5% in compression and +1.8% in tension. Moreover, a significant change in viscoelastic properties can be observed: an increase in internal friction (by 25%) and a change in elastic modulus in tensile samples. The increase in internal friction during martensitic transformation after stress-induced martensite aging is associated with the oriented growth of thermal-induced martensite. After stress-induced martensite aging, the elastic modulus of martensite (EM) increased by 8 GPa, and the elastic modulus of austenite (EA) decreased by 8 GPa. It was shown that stress-induced martensite aging strongly affects the functional and viscoelastic properties of material and can be used to control them.

Effect of warm-up and muscle fatiguing exercise on knee joint sounds in motion by vibroarthrography: A randomized crossover trial.

Vibroarthrography measures joint sounds caused by sliding of the joint surfaces over each other. and can be affected by joint health, load and type of movement. Since both warm-up and muscle fatigue lead to local changes in the knee joint (e.g., temperature increase, lubrication of the joint, muscle activation), these may impact knee joint sounds. Therefore, this study investigates the effects of warm-up and muscle fatiguing exercise on knee joint sounds during an activity of daily living. Seventeen healthy, physically active volunteers (25.7 ± 2 years, 7 males) performed a control and an intervention session with a wash-out phase of one week. The control session consisted of sitting on a chair, while the intervention session contained a warm-up (walking on a treadmill) followed by a fatiguing exercise (modified sit-to-stand) protocol. Knee sounds were recorded by vibroarthrography (at the medial tibia plateau and at the patella) at three time points in each session during a sit-to-stand movement. The primary outcome was the mean signal amplitude (MSA, dB). Differences between sessions were determined by repeated measures ANOVA with intra-individual pre-post differences for the warm-up and for the muscle fatigue effect. We found a significant difference for MSA at the medial tibia plateau (intervention: mean 1.51 dB, standard deviation 2.51 dB; control: mean -1.28 dB, SD 2.61 dB; F = 9.5; p = .007; η2 = .37) during extension (from sit to stand) after the warm-up. There was no significant difference for any parameter after the muscle fatiguing exercise (p > .05). The increase in MSA may mostly be explained by an increase in internal knee load and joint friction. However, neuromuscular changes may also have played a role. It appears that the muscle fatiguing exercise has no impact on knee joint sounds in young, active, symptom-free participants during sit to stand.

Fe-27Mn-4Co-2Al 합금의 감쇠능에 미치는 Si의 영향

This study investigated the effect of 3%Si on the damping capacity of Fe-27Mn-4Co-2Al alloy. The internal friction was measured with a horizontal internal friction equipment in a vacuum of 10<SUP>-4</SUP> torr using a specimen of (120×10×2) mm obtained from cold-rolled material. The degree of cold rolling was made into four types. Martensite was formed with according to the specific direction and surface relief. The amount of α′-martensite increased sharply as the degree of cold rolling increased. However, the amount of ϵ-martensite decreased after reaching the maximum value in a certain range of cold rolling. internal friction was increased, and then decreased with an increasing of degree of cold rolling. Internal friction increased, and then decreased with an increasing of degree of cold rolling. By adding Si, the formation of ε-martensite was promoted, but the α′-martensite was not affected. Internal friction increased by adding Si. The increase of internal friction was strongly influenced by the amount of ϵ-martensite by cold rolling. Internal friction of cold rolled alloys were dominated by the α′-martensite.

BEHAVIOR OF SALINE SOIL STABILIZED WITH POLYPROPYLENE FIBER AND CEMENT

This research deals with a problematic saline soil, which is widespread in many cities of the world, and try to obtain a solution for the reduction in its strength due to soaking by using more than one additives. In order to improve saline soil and to explain its effect, more than one additive were used in the research; polypropylene fiber is the major additive, which was used with cement in different percentages. Soil strength parameters were assessed by conducting direct shear test for both the natural and treated soil, which were tested under soaked condition after, submerged in water for 24 hour. The result of direct shear test showed that, for natural soil, there are large lose in the strength of soaked soil compared to that of dry soil due to the solubility of salt in soil. This decay in soil strength about (48.6%) for cohesion and (67.8%) from angel of internal friction. Meanwhile, with adding PPF to soil and under soaked state, the soil strength parameters grew significantly with the increase in the rate of fiber, where both the soil cohesion and angel of internal friction increase. Soils, treated with (1.5%) polypropylene fibers as well as cement, their strength parameters improved significantly as cement amount increases, where the soil cohesion and angel of internal friction increase. From experimental model which is built up to study and examine the laboratory result under complete submerged state, has a result for natural and treated soil showed improvement in bearing capacity of treated soil about 10 times compared to natural soil bearing capacity when tested under soaked condition.

State dependence of magnetized fluidized bed reactor on operation mode

There exist four typical modes to operate the magnetized fluidized bed (MFB) reactor: magnetization-FIRST, de-fluidization, magnetization-LAST, and de-magnetization. This work aimed to compare the bed states obtained under these operation modes for both the MFB with purely magnetizable particles and the MFB with the binary admixture of magnetizable and nonmagnetizable particles. It was found that the covered operating range could be divided into four zones. Particularly in zones I to III, the bed state depended not only on the magnetic field intensity (H) and superficial gas velocity (Ug) but also on the operation mode, i.e., the bed state was a path function. Such a path-dependence feature resulted from that the MFB therein could have different equilibrium states at the same H and Ug. From our perspective, such a polymorphic characteristic was caused by the internal friction inside the bed. Furthermore, many of these bed states were demonstrated to be metastable. The metastable states in zones II and III behaved like the metastates of water. They evolved directly and quickly into the corresponding stable states after relatively large perturbations. The metastable states in zone I behaved like the amorphous/glass state. They resided at another metastable state during the evolution towards the stable state after relatively large perturbations. In zone I, the metastable states formed from the sharp increase of internal friction with decreasing Ug. The particles lost the ability to move before reaching the stable positions and were entrapped at the intermediate and metastable positions.

Farming Influence on Physical-Mechanical Properties and Microstructural Characteristics of Backfilled Loess Farmland in Yan’an, China

A gigantic project named Gully Land Consolidation (GLC) was launched in the hill-gully region of the Chinese Loess Plateau in 2011 to cope with land degradation and create new farmlands for cultivation. However, as a particular kind of remolded loess, the newly created and backfilled farmland may bring new engineering and environmental problems because the soil structure was disturbed and destroyed. In this study, current situations and characteristics of GLC are introduced. Test results show that physical-mechanical properties and microstructural characteristics of backfilled loess of one-year and five-year farmland are significantly affected by the Gully Land Consolidation project. Compared to natural loess, the moisture content, density, and internal friction angle of backfilled loess increase. On the contrary, the porosity, plasticity index, particle size index, and cohesion index decrease. Through SEM tests, it is observed that the particles of backfilled loess are rounded, with large pores filled with crushed fine particles, which results in skeleton strength weakness among particles and pores. The pore size distribution (PSD) of the four types of loess (Q3 loess, Q2 loess, one-year farmland, and five-year farmland) was measured using mercury intrusion porosimetry (MIP) tests, showing that the pore size of Q3 loess is mainly mesopores 4000–20,000 nm in size, accounting for 67.5%. The Q2, five-year, and one-year farmland loess have mainly small pores 100–4000 nm in size, accounting for 52.5%, 51.7%, and 71.7%, respectively. The microscopic analysis shows that backfill action degrades the macropores and mesopores into small pores and micropores, leading to weak connection strength among soil particles, which further affects the physical-mechanical properties of loess. The disturbance of backfilled loess leads to an obvious decrease in cohesion and a slight increase in internal friction compared to natural loess. The farming effect becomes prominent with increased backfill time, while the loess soil moisture content increases gradually. Both the cohesion and internal friction of the backfilled loess soil decrease to different degrees. This study is helpful to investigate sustainable land use in the Chinese Loess Plateau and similar areas.

An ultrasonic study of relaxation processes in pure and mechanically alloyed tungsten

High-temperature internal friction in a pure tungsten single crystal, polycrystals with different grain sizes, and mechanically alloyed tungsten polycrystals was studied. Positron annihilation spectroscopy was used to prove that all studied materials contain a detectable amount of dislocations. Then, resonant ultrasound spectroscopy was applied to determine the internal friction evolution with temperature from room temperature up to 740 °C. For the pure tungsten samples, a sharp increase of internal friction was observed for temperatures above 470 °C (for the single crystal) or above 400 °C (for the polycrystals); the activation energy corresponding to this increase was the same as the activation energy for the ductile-to-brittle transition in tungsten reported in the literature. For the alloyed materials, direct observation of the onset of this relaxation mechanism was impossible due to additional effects resulting from the secondary phases in the material.

Influence of Thermomagnetic Treatment on the Damping Properties and Structure of Iron-Based Alloys

The paper describes the influence of thermomagnetic treatment on the parameters of the amplitude dependence of internal friction, the crystal and magneto-crystalline structure of Fe alloys with ferrite structure: Fe – 8%Cr, Fe – 16%Cr, Fe – 19%Cr, Fe – 4%Cr – 2%V, Fe – 3%Cr – 3%Al, and Fe–1.2%C–2% Si–2%Al graphitic steel with graphite and ferrite structure. The thermomagnetic treatment was carried out at a temperature of 450ºС. The structure was studied by means of metallography, X-ray crystal structure analysis, ferromagnetic resonance, and nuclear gamma resonance spectroscopy. It is shown that, the larger the ferrite grain in alloys with ferrite structure is, the greater an increase in internal friction is, in the amplitude range up to approximately the amplitude of its maximum owing to thermomagnetic treatment. The change of the damping properties of graphitic steel caused by thermomagnetic treatment is relatively small, due to a large number of diamagnetic inclusions of graphite and quite fine-grained ferrite matrix. The results of the X-ray crystal structure analysis, the ferromagnetic resonance and nuclear gamma resonance spectroscopy have shown that, in the case of thermomagnetic treatment, diffusion redistribution of atoms of alloying elements, combined with the change of structural anisotropy, takes place in Fe-Cr alloys.

A study on the viscoelastic behaviors of tire cords using dynamic mechanical analysis

Design of experiments was adopted to evaluate the effect of selected test parameters on the viscoelastic behaviors of polyester tire cords through dynamic mechanical analysis systematically. Design of experiments results showed that temperature, static load, and dynamic amplitude had significant effects on the responses of complex modulus (E*) and Tan δ. Furthermore, temperature had significant interactions with static load and dynamic amplitude factors on the responses. None of the test parameters had any significant effect on the response of glass transition temperature (Tg). Below Tg, thermoplastic tire cords exhibited a high and constant dynamic modulus, whereas, beyond this point, the modulus decreased dramatically. The thermosetting tire cords exhibited a constant performance due to an ordered and tight molecular chain arrangement. The magnitude of Tan δ reached its peak value at Tg due to the increase in internal friction as a result of increasing temperature. The dynamic modulus increased and Tan δ decreased with the increasing static load as a result of restricting the mobility of chain segments. The reverse was true when the dynamic amplitude increased, most probably because of higher chain segment mobility and early stage of polymer chain slippage. Activation energy (Ea), derived from Arrhenius equation, can be used to predict its long-term performance. Tg shifted to a higher temperature as the frequency increased. In addition, by increasing the twist level of the polyester tire cord, the dynamic modulus decreased and Tan δ increased. Tg was evaluated as the upper limit working temperature, Tan δ was related to energy dissipation, and E* determined the overall performance of the tire cord. By displacing Tg to a higher temperature, reducing the magnitude of Tan δ and increasing the dynamic modulus are of great importance to a tire cord’s performance.

Exergy-based ecological optimization of an irreversible quantum Carnot heat pump with harmonic oscillators

This paper studies ecological performance by taking a quantum Carnot heat pump as object. The working substance of the heat pump is harmonic oscillator. Besides heat resistance, this heat pump also has heat leakage and internal friction. This paper derives heating load, coefficient of performance and ecological function and analyse these important parameters with numerical illustrations. At high temperature limit, this paper simplifies these important parameters and deduces the optimal performance by taking ecological function as objective. Also, this paper illustrates effects of irreversibilities with help of numerical calculation. The results reveal that ecological function of the pump has maximum value. Results at high temperature limit reveal that ecological function versus COP characteristic curves are parabolic-like as when heat leakage is absent and ecological function has maximum value. Heat leakage makes characteristic curves become loop-shaped curves and leads to smaller ecological function and COP. With a constant heat leakage, maximum attained ecological function decreases as internal friction increases. Comparison between maximum points of ecological function and COP at high temperature limit reveals that ecological optimization makes COP decrease 16.8%, exergy loss rate increases about 7.20 times, but heating load and exergy output rate greatly increase about 3.18 times and 3.23 times, respectively. Comparison maximum point of ecological function and point where the value of exergy output rate is A∕τ=6.09×10−4 reveals that the second point makes COP decrease 5.21%, exergy output rate increase only 20.9% and heating load increase only 21.1%, but the exergy lose rate increases 40.7%. Taking maximum ecological function as objective makes larger improvement in COP and reduction in exergy loss rate, and the cost is smaller heating load drop.

Microstructure, thermophysical property and ablation behavior of high thermal conductivity carbon/carbon composites after heat-treatment

Uni-directional carbon/carbon composites with high thermal conductivity are suitable to supply continuous thermal protection for future reentry vehicles since they could reduce surface temperature and ablation rates simultaneously in harsh environments. In this work, the high thermal conductivity carbon/carbon composites were prepared by chemical vapor infiltration. After heat-treatment, both their open porosity and internal friction increase due to the fiber/matrix thermal expansion mismatch; while their thermal conductive performance become better due to more complete carbon structure. With raising heat-treatment temperature from 1800 °C to 2450 °C, the mass and linear ablation rates of C/C composites with fibers vertical to the oxyacetylene torch for 60 s decrease from 0.66 mg/s and 2.95 μm/s to 0.51 mg/s and 2.05 μm/s respectively. The improved ablation resistance is resulted from the increased thermal conductivity from 282 to 508 W/(m·K) and more carbon fibers exposed to the flame during ablation, which have better oxidation resistance than those of carbon matrix. While such ablation rates become larger for composites with fibers parallel to the flame, from 1.02 mg/s and 3.73 μm/s to 1.28 mg/s and 5.01 μm/s respectively since the ablation occurred more easily through gaps at the fiber/matrix interfaces, which become larger and are always exposed to the flame for this case.