Degradation Of Mechanical Properties Research Articles

Ultrastrong and ductile steel welds achieved by fine interlocking microstructures with film-like retained austenite

The degradation of mechanical properties caused by grain coarsening or the formation of brittle phases during welding reduces the longevity of products. Here, we report advances in the weld quality of ultra-high strength steels by utilizing Nb and Cr instead of Ni. Sole addition of Cr, as an alternative to Ni, has limitations in developing fine weld microstructure, while it is revealed that the coupling effects of Nb and Cr additions make a finer interlocking weld microstructures with a higher fraction of retained austenite due to the decrease in austenite to acicular ferrite and bainite transformation temperature and carbon activity. As a result, an alloying design with Nb and Cr creates ultrastrong and ductile steel welds with enhanced tensile properties, impact toughness, and fatigue strength, at 45% lower material costs and lower environmental impact by removing Ni.

Effects of Oxidized Metal Powders on Pore Defects in Powder-Fed Direct Energy Deposition.

Laser-based additive manufacturing processes, particularly direct energy deposition (DED), have gained prominence for fabricating complex, functionally graded, or customized parts. DED employs a high-powered heat source to melt metallic powder or wire, enabling precise control of grain structures and the production of high-strength objects. However, common defects, such as a lack of fusion and pores between layers or beads, can compromise the mechanical properties of the printed components. This study focuses on investigating the recurrent causes of pore defects in the powder-fed DED process, with a specific emphasis on the influence of oxidized metal powders. This research explores the impact of intentionally oxidizing metal powders of hot work tool steel H13 by exposing them to regulated humidity and temperature conditions. Scanning electron microscopy images and energy-dispersive X-ray spectroscopy results demonstrate the clumping of powders and the deposition of iron oxides in the oxidized powders at elevated temperatures (70 °C for 72 h). Multi-layered depositions of the oxidized H13 powders on STD61 substrate do not show significant differences in cross sections among specimens, suggesting that oxidation does not visibly form large pores. However, fine pores, detected through CT scanning, are observed in depositions of oxidized powders at higher temperatures. These fine pores, typically less than 250 µm in diameter, are irregularly distributed throughout the deposition, indicating a potential degradation in mechanical properties. The findings highlight the need for careful consideration of oxidation effects in optimizing process parameters for enhanced additive manufacturing quality.

Strength and Microstructural Evolution of Magnesium Phosphate Cement Mortar in Plateau Environment

Climatic conditions in plateau areas can enormously affect the properties and microstructure of cement-based materials. This research investigates the strength development and microstructural changes in magnesium phosphate cement (MPC) mortars in a plateau environment. Experiments were conducted in parallel in a plateau area (Lhasa) and a plain area (Chengdu) to evaluate the effects of the water-to-binder ratio (w/b = 0.12, 0.14 and 0.16) and sand-to-binder ratio (s/b = 0.5, 0.75 and 1) on the compressive and flexural strength of MPC mortars. At the same time, hydration products were characterized via XRD, TGA, and SEM/EDX micro-analyses, and the porosity of the materials was also analyzed via MIP. The results demonstrated that curing in a plateau environment resulted in a decrease in workability and yielded higher strength at an early age (before 1 day) but degraded the long-term (180-day) strength of MPC mortars when compared with curing in a plain environment, irrespective of w/b and s/b ratios. Unlike the plain group, the plateau group revealed the deterioration of microstructures over time, including the decrease in struvite content, the morphology change in struvite crystals, and the increase in porosity, which resulted in the degradation of mechanical properties between 1 and 180 days. The strength loss can be effectively alleviated at lower w/b and s/b ratios.

Unraveling the hydrogen sulfide aging mechanism on electrical-thermal–mechanical property degradation of sintered nanocopper interconnects used in power electronics packaging

During operation in environments containing hydrogen sulfide (H2S), such as in offshore and coastal environments, sintered nanoCu in power electronics is susceptible to degradation caused by corrosion. In this study, experimental and molecular dynamics (MD) simulation analyses were conducted to investigate the evolution and mechanism of H2S-induced corrosion of sintered nanoCu, and bulk Cu was used as the reference. The following results are obtained: (1) Both sintered nanoCu and bulk Cu reacted with O2 prior to reacting with H2S, forming Cu2O, Cu2S, CuO, and CuS. In addition, sintered nanoCu exhibited more severe corrosion. (2) For both sintered nanoCu and bulk Cu, H2S-induced corrosion resulted in the deterioration of electrical, thermal, and mechanical properties, and sintered nanoCu experienced a greater extent of deterioration. (3) As was ascertained through Reactive Force Field (ReaxFF) MD simulations, the penetration of H2S and O2 combined with the upward migration of Cu resulted in the formation of a corrosion film. In addition, compared to bulk Cu, the H2S and O2 penetration in the sintered nanoCu structure was observed to occur to a greater depth, accounting for the more pronounced performance degradation.

Mitigation of Desulfovibrio ferrophilus IS5 degradation of X80 carbon steel mechanical properties using a green biocide.

Most microbiologically influenced corrosion (MIC) studies focus on the threat of pinhole leaks caused by MIC pitting. However, microbes can also lead to structural failures. Tetrakis hydroxymethyl phosphonium sulfate (THPS) biocide mitigated the microbial degradation of mechanical properties of X80 steel pipeline by Desulfovibrio ferrophilus (IS5 strain), a very corrosive sulfate reducing bacterium. It was found that 100 ppm (w/w) THPS added to the enriched artificial seawater (EASW) culture medium before incubation resulted in 2.8-log reduction in sessile cell count after a 7-d incubation at 28 °C under anaerobic conditions, leading to 94% uniform corrosion rate reduction (from 1.3to 0.07mm/a), and 84% pitting corrosion rate reduction (from 0.70to 0.11mm/a). The X80 dogbone coupon incubated with 100 ppm THPS for 7 d suffered 3% loss in ultimate tensile strain and 0% loss in ultimate tensile strength compared with the abiotic control in EASW. In comparison, the no-treatment X80 dogbone coupon suffered losses of 13% in ultimate tensile strain and 6% in ultimate tensile stress, demonstrating very good THPS efficacy.

Multiscale simulation of hygrothermal stress and mechanical properties degradation of carbon fiber reinforced polymers laminates

AbstractA mesoscopic representative volume element (RVE) model of carbon fiber reinforced polymers (CFRP) laminate with 15 plies was established based on the principle of fiber random disturbance. The RVE model was validated by comparing the elastic parameters calculated by the RVE model, Chamis model, and Bridge model. Wet stress and thermal stress were coupled using a sequential method. Macroscopic and microscopic subroutines were developed to calculate the stress–strain field of CFRP laminate and RVE model. A multiscale numerical simulation of three‐point bending of moisture‐saturated CFRP laminate was conducted. Failure process and failure modes of CFRP laminate were discussed based on the multiscale simulation and SEM observations. The results show that the RVE model and the multiscale simulation strategy presented in this work can predict the coupled hygrothermal stress and the strength degradation due to moisture absorption very well. The maximum error between the simulated and experimental averaged force of the moisture‐saturated CFRP laminate subjected to three‐point bending is about 3.4%. Multiscale simulation demonstrated that uneven distribution of fibers can result in the fiber/matrix interfaces bearing a larger transverse stress in the fiber's dense distribution area. It is the main reason that results in fiber/matrix interface failure of CFRP laminates working in hygrothermal environments.Highlights A RVE model of CFRP laminate was presented using fibers random disturbance. Coupled hygrothermal stress of single fiber and surrounded matrix was obtained. A multiscale simulation of the strength change of the CFRP plate due to wet‐thermal stress was conducted. Failure modes of CFRP laminate are discussed based on the multiscale simulation.

Influence of Cu/Mg ratio and content on heat-resistance of Al–Cu–Mg alloys

Heat-resistant Al alloys used in such as aerospace, transportation fields are attracting more and more attention in recent years. Within Al alloy families, Al–Cu–Mg alloys have shown promising heat resistance properties. This work aims to investigate the influence of Cu/Mg ratio and content on the heat resistance of Al–Cu–Mg alloys, based on alloys of Al–4.5Cu–2.5 Mg (referred to as alloy A), Al–4.0Cu–2.2 Mg (alloy B) and Al–4.5Cu–1.6 Mg (alloy C). The alloys A and B possessed approximate Cu/Mg ratio, and they also exhibited nearly identical hardness retention rate during exposure at 200 °C. After 200 h, the rate is ∼75 %, though alloy A showed higher hardness (105 vs. 102 HBW) due to higher Cu, Mg content. In contrast, alloy C with a higher Cu/Mg ratio was less heat-resistant, with hardness retention rate of ∼70.5 % after 200 h exposure. Nano-sized S′(Al2CuMg) precipitate was main strengthening phase for the three alloys. Also, micron and submicron Al2CuMg particles could be formed with increase of Cu and Mg contents, which contributed a lot to yield strength for T6 heat-treated alloys, but slight contribution after exposure at 200 °C for 200 h. The degradation of mechanical properties during heat exposure can be attributed to the transformation and coarsening of S′ precipitates. In alloys with lower Cu/Mg ratio, there was excess Mg dissolved in Al matrix, which reduced Cu solubility in α-Al, and then slowed diffusion flux of Cu element, thus inhibited coarsening of Al2CuMg phase.

Review of Density and Ultrasonic Velocity of Various Complexes and Its Applications

This review paper delves into the fundamental principles, measurement techniques, and diverse applications of ultrasonic velocity and density. Ultrasonic velocity serves as a crucial parameter in various fields such as materials science, acoustics, and medical diagnostics whereas density is used for identifying substances, quality control, analyzing mixtures, assessing purity, explaining buoyancy, determining concentrations, calibrating instruments, monitoring pollution, and understanding reactivity. This paper provides an in-depth exploration of the underlying concepts, different measurement methods, and the wide-ranging applications of ultrasonic velocity and density. The ultrasonic velocity and density data have also been used to evaluate various acoustic parameters such as isentropic compressibility, apparent isentropic molar compressibility, solvation number of the solute and relative association. Ultrasonic velocity measurements are found to be useful for on-line assessment of the extent of degradation of mechanical properties associated with precipitation of intermetallics. Keywords: Density, Ultrasonic velocityThis review paper delves into the fundamental principles, measurement techniques, and diverse applications of ultrasonic velocity and density. Ultrasonic velocity serves as a crucial parameter in various fields such as materials science, acoustics, and medical diagnostics whereas density is used for identifying substances, quality control, analyzing mixtures, assessing purity, explaining buoyancy, determining concentrations, calibrating instruments, monitoring pollution, and understanding reactivity. This paper provides an in-depth exploration of the underlying concepts, different measurement methods, and the wide-ranging applications of ultrasonic velocity and density. The ultrasonic velocity and density data have also been used to evaluate various acoustic parameters such as isentropic compressibility, apparent isentropic molar compressibility, solvation number of the solute and relative association. Ultrasonic velocity measurements are found to be useful for on-line assessment of the extent of degradation of mechanical properties associated with precipitation of intermetallics. Keywords: Density, Ultrasonic velocity

Heat affected zone evolution in fine grained aluminum alloys during laser welding: Phase-field simulation and analytical investigation

One of the biggest challenges in the welding of strengthened alloys by grain refinement is the grain growth phenomenon in the heat-affected zone (HAZ), which would result in the degradation of mechanical properties. The goal of this study is to comprehend the HAZ evolution in the fine-grained aluminum alloy during laser welding as a low heat input process. Spot laser welding of fined grain 1050 aluminum alloy revealed that grain growth and HAZ formation occurred in samples with 4 µm grain size, but no grain growth occurred outside of the fusion line (FL) in larger grain samples. The microstructure of HAZ was simulated using the numerical phase-field technique. The numerical model's findings demonstrated that a critical grain size (Dcr) for the base metal can be established, above which grain growth would not occur outside FL. To predict grain size distribution outside FL, an analytical model was also used. According to both models, as grain size increases, the HAZ width decreases, and above a critical grain size, no HAZ will form outside of FL. In spot welding, Dcr was independent of power density but for continuous welding processes, Dcr has a direct relationship with heat input per unit length of weld. Moreover, HAZ grain growth mainly occurred in heating step for both types of processes.

Elastomer-Based Sealing O-Rings and Their Compatibility with Methanol, Ethanol, and Hydrotreated Vegetable Oil for Fueling Internal Combustion Engines.

Green methanol, ethanol, and diesel-based hydrotreated vegetable oils are some of the renewable liquid fuels that show satisfactory performance in diesel engines. A notable advantage of these fuels is that they are renewable and do not require significant modifications in the existing engines for successful operation. Suitable fuel systems, especially their material compatibility, remain unresolved, and therefore, it is a weak link in their large-scale adaptation. Elastomer-based sealing O-rings lose their mechanical properties after a short exposure time to these fuels, adversely impacting their functionality. This research study evaluated the long-term material compatibility of different elastomer-based sealing materials by immersing the O-rings in these test fuels (hydrotreated vegetable oil, methanol, ethanol, and diesel) for different time intervals (i.e., up to 15 months). The material compatibility was assessed mainly by investigating these changes in various mechanical properties of these O-rings, namely tensile strength (ΔTs), elongation at break (ΔEb), Shore A hardness (ΔH), and mass (ΔM). The degradation of mechanical properties was studied and analyzed during the immersion interval from 0.9 to 15.2 months and compared with O-rings kept in a normal atmosphere. It was noted that individual fuels affect various mechanical properties significantly. In a short interval of 0.9 months (28 days), significant changes in the mechanical properties of the sealing O-rings were observed.

Effect of a calcium silicate cement and experimental glass ionomer cements containing calcium orthophosphate particles on demineralized dentin.

The study aims to evaluate the effect of a glass ionomer cement (GIC; Fuji 9 Gold Label, GC) with added calcium orthophosphate particles and a calcium silicate cement (CSC; Biodentine, Septodont) regarding ion release, degradation in water, mineral content, and mechanical properties of demineralized dentin samples. GIC, GIC + 5% DCPD (dicalcium phosphate dihydrate), GIC + 15% DCPD, GIC + 5% β-TCP (tricalcium phosphate), GIC + 15% β-TCP (by mass), and CSC were evaluated for Ca2+/Sr2+/F- release in water for 56days. Cement mass loss was evaluated after 7-day immersion in water. Partially demineralized dentin disks were kept in contact with materials while immersed in simulated body fluid (SBF) at 37°C for 56days. The "mineral-to-matrix ratio" (MMR) was determined by ATR-FTIR spectroscopy. Dentin hardness and elastic modulus were obtained by nanoindentation. Samples were observed under scanning and transmission electron microscopy. Data were analyzed by ANOVA/Tukey test (α = 0.05). Ca2+ release from CSC and GIC (μg/cm2) were 4737.0 ± 735.9 and 13.6 ± 1.6, respectively. In relation to the unmodified GIC, the addition of DCPD or β-TCP increased ion release (p < 0.001). Only the dentin disks in contact with CSC presented higher MMR (p < 0.05) and mechanical properties than those restored with a resin composite used as control (p < 0.05). Mass loss was similar for GIC and CSC; however, the addition of DCPD or β-TCP increased GIC degradation (p < 0.05). Despite the increase in ion release, the additional Ca2+ sources did not impart remineralizing capability to GIC. Both unmodified GIC and CSC showed similar degradation in water. CSC was able to promote dentin remineralization.

Systematic Investigation of the Degradation Properties of Nitrile-Butadiene Rubber/Polyamide Elastomer/Single-Walled Carbon Nanotube Composites in Thermo-Oxidative and Hot Oil Environments.

The physical blending method was used in order to prepare nitrile-butadiene rubber/polyamide elastomer/single-walled carbon nanotube (NBR/PAE/SWCNT) composites with better thermal-oxidative aging resistance. The interactions between SWCNTs and NBR/PAE were characterized using the Moving Die Rheometer 2000 (MDR 2000), rheological behavior tests, the equilibrium swelling method, and mechanical property tests. The 100% constant tensile stress and hardness of NBR/PAE/SWCNT composites increased from 2.59 MPa to 4.14 MPa and from 62 Shore A to 69 Shore A, respectively, and the elongation decreased from 421% to 355% with increasing SWCNT content. NBR/PAE/SWCNT composites had improved thermal-oxidative aging resistance due to better interactions between SWCNTs and NBR/PAE. During the aging process, the tensile strength and elongation at break decreased with the increase in aging time compared to the unaged samples, and the constant tensile stress gradually increased. There was a more significant difference in the degradation of mechanical properties when aged in a variety of oils. The 100% constant tensile stress of NBR/PAE/SWCNT composites aged in IRM 903 gradually increased with aging time while it gradually decreased in biodiesel. The swelling index gradually increased with increasing SWCNT content. Interestingly, the swelling index of the composites in cyclohexanone decreased with the increase in SWCNT content. The reasons leading to different swelling behaviors when immersed in different kinds of liquids were investigated using the Hansen solubility parameter (HSP) method, which provides an excellent guide for the application of some oil-resistant products.

Durability of GFRP Composites Produced by Pultrusion under Thermal Environments

Fibre-reinforced polymer (FRP) composites have gained popularity in civil structural applications due to their advantages over traditional materials. Despite their proven better performance in harsh conditions, FRP composites are susceptible to degradation due to various environmental agents; moreover, the lack of durability data for relatively thick laminates produced by pultrusion is a concern. To address this, in the scope of the Durable-FRP project, an extensive experimental program was conducted to assess the durability of thick FRP laminates produced by pultrusion, consisting of glass fibres and unsaturated polyester and vinyl ester resin matrices, subjected to accelerated laboratory ageing and in-situ natural weathering. This paper presents preliminary results from durability tests on such glass-FRP (GFRP) laminates under particular thermal ageing environments, including constant temperatures of -15 °C, 20 °C, 40 °C, and 60 °C for a period over 12 months, as well as 100 and 200 thermal cycles between -15 °C and 60 °C. Following environmental exposure, the degradation of mechanical properties was evaluated using various tests, including tension (in both longitudinal and transverse directions), compression, flexure, in-plane shear, and interlaminar shear. In general, minor mechanical degradation was observed, except in terms of transverse tensile strength (resin-dominated) following constant exposure to extreme temperatures, with minimum property retentions of 74% and 83% at -15 °C, for vinyl ester and unsaturated polyester matrices, respectively.

Acoustic Emission monitoring of OPC mortars at elevated temperatures in fracture tests

The action of moderate and high temperatures produces the degradation of main physical and mechanical properties of Ordinary Portland Cement (OPC) composites. As an example, from an application point of view and at a structural level, the case of concrete radiation shielding containers in nuclear facilities can be mentioned, which can be severely affected by thermal cracking. In this work, the results of Acoustic Emission (AE) monitoring of mortar prismatic specimens under three-point bending tests are analyzed. Samples are firstly submitted to a heating treatment in an electrical furnace, with maximum target temperatures ranging between 100 to 600 ºC. After the heating process, the prisms were cooled down and finally, mechanically tested, i.e. under residual conditions. The AE signals were continuously recorded during each bending test and its characteristics at different loading stages were explored, scrutinizing variations in AE parameters, such as event rise time, amplitude and energy. Additionally, average frequency (AF) and RA value parameters were employed to assess fracture modes aiming to highlight situations where tension or shear/mixed modes predominate through the test. A comparative analysis of the AE characteristics during loading path and progressive damage is presented.

High deformation and delamination mechanisms explained through mesoscale and nanoscale phenomena in pearlitic steel wires

Pearlitic steel wires subjected to severe cold drawing deformation exhibit a unique combination of high strength and excellent ductility, making them suitable for critical applications such as hanging bridge cables, crane cables and tire cord. Torsion tests up to rupture are commonly used in the industry for quality control purposes. A flat fracture surface is indicative of wire aptitude, while a delaminated surface suggests degradation of mechanical properties. This paper aims to expand the understanding of the structural evolution and deformation mechanisms that occur during wire drawing, as well as the origin of delamination problems. Different deformation and strengthening mechanisms are identified and discussed, applying different microscopy techniques (optical microscopy, scanning electron microscopy, electron backscatter diffraction), synchrotron diffraction and atom probe tomography. The information is correlated with the cementite stability on the structure studied at meso, nano, and atomic scale, reaching an understanding of the compositional evolution of cementite and ferrite even at the very lowest level. All the obtained information, including thermal analysis and thermodynamic simulation results, allow to identify the deformation mechanisms present during cold drawing and at the delamination process. In addition, the compositional evolution of cementite and ferrite is also clarified by atom probe tomography.

Review on Corrosion Mechanism and Mechanical Properties Degradation of Building Structural Steel

Steel structure is favored by many designers because of its light weight, high strength, short construction period and good seismic performance, and is widely used in high-rise, super high-rise and long-span structures, but the shortcomings of steel structure corrosion are gradually highlighted. In the past 20 years, literature in relevant domestic and foreign journals, conference proceedings, and electronic databases were searched. The mechanical properties of corroded components were studied and summarized from the corrosion mechanism, corrosion influencing factors and rust pit morphology. It is found that the dangerous area of stress concentration of corroded steel is in the middle of the pit, and the maximum stress value is located at the lower edge of the pit mouth. The bearing capacity and ductility of the corroded steel structure are deteriorated, which seriously affects the structural safety under dynamic load. Due to the complexity of correlation analysis of atmospheric corrosion test of metal materials, it is still necessary to improve the corrosion data of existing steel grades and newly developed steel grades in natural environment. At present, there are few researches on corrosion of high strength steel, molded steel plate and prefabricated buildings. With the increasing application of high-strength steel, further research is needed to obtain the latest research results and apply them in practical engineering.

Chemical Modification of Iron – Rich Beta Intermetallic Phase in Aluminium-Silicon Alloys: A Short Review

Iron-rich intermetallic phases normally form during solidification of aluminium – silicon alloys with low level of iron. This type of phases is known for creating a mechanical properties degradation, especially the beta intermetallic phase (β-Al9Fe2Si2). Various studies and researches have led to mitigate the harmful effect of these phases through physical separation, thermal treatment or microstructure alteration by chemical neutralization such as with Mn addition which changes their structure into cubic alpha phase (α-Al15(Fe,Mn)3Si2). In this paper, the effect of chemical addition was reviewed and discussed in terms of effectiveness of modifying the crystal structure or morphology of beta intermetallic phase

Impact of metakaolin on hydration properties of magnesium phosphate cement

The degradation of mechanical properties in magnesium phosphate cement (MPC) is commonly observed under conditions of rapid heat release and high ambient temperature. To mitigate this phenomenon, the incorporation of metakaolin has been investigated as a means to reduce the hydration heat of MPC and enhance its mechanical properties at elevated temperatures. Experimental findings reveal a pronounced influence of metakaolin on the heat flow behavior of MPC. Specifically, the addition of a small fraction of metakaolin (not exceeding 20 wt%) leads to an accelerated heat release rate during the initial 0.13 h, followed by a substantial reduction in the heat release rate with further increments of metakaolin. Furthermore, the inclusion of metakaolin significantly improves the mechanical properties of MPC. For instance, a composition comprising 30 % metakaolin yields a notable 79.2 % increase in strength after 7 days. It is worth noting that temperatures below 400 ℃ are found to have an adverse impact on the mechanical properties of MPC, whereas a temperature range encompassing 600 ℃>T > 400 ℃ enhances these properties due to vitrification at high temperatures. Importantly, MPC incorporating metakaolin exhibits superior mechanical performance compared to its metakaolin-free counterpart across all tested temperatures, which can be attributed to the attainment of a denser microstructure facilitated by metakaolin incorporation.

Fatigue and fracture of soft collagenous tissues mineralized in vitro

Biomineralization occurs in natural organisms, such as bones, teeth, shells and tendons. The formation mechanism and mechanical properties have been extensively studied for mineralized hard tissues, especially for bone and teeth. However, the research on the evolution of mechanical properties for mineralized soft tissues remains challenging due to the long period of mineralization. Here we use polymer-induced liquid-precursor (PILP) method to achieve accelerated in vitro mineralization of bovine pericardium with SrCO3. The modulus, tensile strength, toughness, and suture retention force of mineralized tissues have been characterized. After 30 days of mineralization, both the tensile strength and toughness decrease by two orders of magnitude. We carry out cyclic suture retention test to characterize the fatigue properties of mineralized tissues, and find that the cycle to failure of mineralized tissues is 10-100 times lower than that of unmineralized tissues. The degradation of mechanical properties results from the microstructure change of mineralized tissues, where collagen fibers change from curled to straight configuration. After mineralization, the interfibrous slippage is limited, and the fracture mode changes from fiber pullout to fiber break. Finite element analysis has been carried out to simulate the fracture and suture retention properties, and the simulation can well reproduce experimental results. The study may provide insights for tissue replacement and repairing.

Investigation of high-temperature effects on the strengthening and degradation of mechanical property in sandstone

The underground coal gasification (UCG) mining technology constitutes an innovative form of clean energy technology pertaining to coal resources. The sandstone specimens from the roof and floor of coal seam were collected and heat treated at 25 °C–800 °C in laboratory. According to the test results of physical parameters, 400 °C is the temperature turning point for the physical parameters such as mass, longitudinal P-wave velocity, porosity, and water absorption of sandstone specimens to change from “slow” to “drastic”. The uniaxial compression test showed that the uniaxial compressive strength (UCS) of sandstone specimens decreases by 3.22% when the temperature rises from 25 °C to 200 °C, and by 5.70% when the temperature rises from 600 °C to 800 °C, both of which show temperature degradation effect. However, in the range of 200 °C–600 °C, the average UCS of sandstone specimens increases from 122.80 MPa to 162.54 MPa, showing obvious strength strengthening effect. In order to fully elucidate the mechanism of mechanical properties strengthening and degradation of tunnel surrounding rock under high temperature, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were combined in the experiment. We concluded that these two intensity effects can be explained by physical and chemical reactions, including mineral composition thermal expansion, thermally induced microcrack and healing effects, phase transformation of the rock-forming mineral quartz, dehydration and thermal decomposition of mica, as well as dehydration and dehydroxylation of illite. The research results can provide theoretical basis for UCG roof and floor disaster prevention evaluation and tunnel support system design.