Damage Evolution in High-Temperature-Treated Granite: Combined DIC and AE Experimental Study

Abstract

As mineral resource extraction progresses to greater depths, it has become imperative for geomechanical applications to understand the thermomechanical degradation mechanisms of rocks under thermal loading. To investigate the thermomechanical characteristics of granite subjected to thermal treatments ranging from ambient to 1000 °C, we conducted uniaxial compression tests integrating P-wave velocity measurements, digital image correlation (DIC), and acoustic emission (AE) monitoring. The key findings reveal the following: (1) the specimen volume exhibits thermal expansion while the mass loss and P-wave velocity reduction demonstrate a temperature dependence; (2) the uniaxial compressive strength (UCS) and elastic modulus display progressive thermal degradation, while the peak strain shows an inverse relationship with temperature; (3) acoustic emission signals exhibit a strong correlation with failure–time curves, progressing through three distinct phases: quiescent, progressive accumulation, and accelerated failure, and fracture mechanisms transition progressively from tensile-dominated brittle failure to shear-induced ductile failure with increasing thermal loading; and (4) the damage evolution parameter exhibits exponential growth beyond 600 °C, reaching 98.85% at 1000 °C, where specimens demonstrate a complete loss of load-bearing capacity. These findings provide critical insights for designing deep geological engineering systems involving thermomechanical rock interactions.

1. Introduction

In recent years, high-temperature rock mechanics has garnered increasing scientific attention, particularly in resource extraction engineering [1,2], subsurface energy storage [3,4], geothermal energy exploitation [5,6], and radioactive waste containment [7,8]. The evolutionary characteristics and underlying mechanisms governing the thermomechanical behavior of rocks under thermal loading now represent critical research frontiers in geomechanics.

Given the heterogeneous mineral composition of granite and the differential thermal expansion coefficients among constituent minerals, thermal loading induces intergranular thermal stresses. These stresses initiate a microcrack propagation that compromises structural integrity while significantly degrading the geomechanical performance. Distinct thermochemical alterations across mineral phases under thermal exposure exhibit phase-dependent variations [9], causing the irreversible degradation of the primary mineralogical architecture. Consequently, understanding the evolutionary patterns of thermomechanical behavior in thermally treated granite and evaluating the long-term stability of geological engineering systems constitute fundamental challenges in geotechnical engineering. Experimental investigations conducted by international researchers on thermally treated granite have revealed significant alterations in thermomechanical characteristics compared to intact specimens [10,11,12]. Standard laboratory techniques, including uniaxial compression, Brazilian splitting, triaxial compression [13], and shear testing, enable the quantification of key mechanical parameters: compressive strength [14], tensile strength [15], shear strength [16], and the elastic modulus [17,18,19]. Hu et al. [17], who categorized the thermal response into four regimes (20–200 °C, 200–400 °C, 400–600 °C, and 600–1000 °C), observed that the tensile strength and fracture toughness exhibited a marked degradation beyond 200 °C, while the compressive strength and elastic modulus demonstrated a progressive deterioration when exceeding 400 °C. Yang et al. [18] revealed that the uniaxial compressive strength, crack threshold strength, and static elastic modulus followed a non-monotonic pattern characterized by an initial enhancement followed by a reduction with increasing temperature. The dynamic elastic modulus displayed progressive thermal degradation, while the static Poisson’s ratio displayed a progressive reduction up to 640 °C followed by a significant escalation. International research efforts have extensively investigated high-temperature rock mechanics to elucidate temperature-dependent mechanical behavior in geological engineering applications.

Research on the numerical simulation of high-temperature granites has yielded substantial advancements in both thermal rupture behavior and thermodynamic formation mechanisms [19]. Regarding thermal rupture behavior, particle flow numerical software (e.g., DEM) based on the discrete element method has been extensively implemented to simulate the mechanical response of granite under elevated temperatures and confining pressure conditions [19,20]. Through numerical simulations of uniaxial compression, Brazilian splitting, and triaxial testing protocols, this investigation elucidates the correlation between the mesoscopic crack evolution and macroscopic mechanical parameters (e.g., elastic modulus and Poisson’s ratio) [21]. The numerical results demonstrate remarkable consistency with stress–strain curves obtained from laboratory-based physical experiments, conclusively demonstrating the efficacy of the numerical framework in characterizing the progressive thermal rupture behavior of granite and associated damage modalities [22,23].

Acoustic emission (AE) refers to transient elastic waves generated through the rapid energy release during material deformation processes. These signals encode critical information about fracture initiation and propagation characteristics [24,25,26,27]. The quantitative analysis of AE parameters provides mechanistic insights into damage evolution, enabling the real-time detection of structural integrity changes within the material’s internal structure. The digital image correlation (DIC) method enables the quantitative determination of displacement fields through the computational analysis of speckle pattern images acquired during deformation sequences [28,29]. This technique enables the spatial identification of strain localization zones preceding fracture evolution mechanisms. The synergistic application of AE monitoring and DIC analysis enables the comprehensive characterization of damage progression in geomaterials [28].

The current research on thermal effects in granite predominantly utilizes 200 °C temperature increments with isothermal durations generally limited to 2–4 h, while strain measurements primarily rely on conventional strain gauge techniques [29,30]. This methodological framework necessitates systematic validation through controlled experimentation to establish precise thresholds for physico-mechanical property degradation. To address these limitations, this study implements refined thermal protocols with incremental stages at 25 °C (baseline), 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, and 1000 °C, maintaining 24 h isothermal conditioning. The strain evolution during uniaxial compression was quantified through Carl Zeiss AG’s digital image correlation (DIC) system. The comprehensive characterization included the mass loss, volumetric changes, P-wave velocity, compressive strength, elastic modulus, peak strain, and Poisson’s ratio. Concurrent AE monitoring captured signal patterns during failure processes, enabling the comprehensive analysis of the temperature-dependent property evolution in granitic formations.

2. Materials and Methods

2.1. Sample Preparation

The granite specimens employed in this investigation were sourced from Jining City, Shandong Province, China. The samples were extracted from a homogeneous rock mass to minimize structural and compositional variations. These specimens exhibit a sesame gray coloration, with an average density of 2.63 g/cm³ and demonstrating an average P-wave velocity of 4870 m/s. X-ray diffraction analysis (Figure 1) reveals that the mineralogical composition comprises microplagioclase feldspar (70.6%), quartz (18.3%), sodium feldspar (8.9%), and calcite (2.2%). Cylindrical specimens measuring 100 mm in height and 50 mm in diameter were fabricated in strict compliance with International Society for Rock Mechanics (ISRM) specifications [31,32,33,34,35]. Prior to testing, all samples underwent comprehensive physical characterization, including P-wave velocity measurements, to account for inherent material heterogeneity. To ensure measurement reliability, triplicate specimens per test condition were prepared to achieve methodological consistency through majority agreement principle. Prior to testing, a stochastic speckle pattern was created through controlled application of black and white aerosol paints on the specimen surface, to facilitate displacement field quantification through digital image correlation (DIC) analysis.

2.2. Experimental Procedure

Thermally induced damage assessment in granite specimens employed triaxial quantification using precision instruments: a 0.01 g-resolution electronic balance, digital calipers (±0.01 mm), and ZBL-U5100 (Produced by Beijing Zhibolian, Beijing, China) non-metallic ultrasonic tester (Figure 2d). For ultrasonic measurements, piezoelectric transducers were mounted on opposite specimen ends, with a uniform coupling layer achieved through application of acoustic-grade petroleum jelly to optimize ultrasonic signal transmission. Subsequent thermal conditioning was conducted in a programmable muffle furnace (Figure 2c), employing a temperature gradient spanning 200–1000 °C in 100 °C increments. To mitigate thermal shock effects [36,37], a controlled heating rate of 5 °C/min was maintained. Specimens underwent 24 h isothermal conditioning at target temperatures before passive cooling to ambient temperature within the furnace chamber prior to post-treatment characterization. The thermal treatment protocol resulted in distinct physical alterations, as documented in Figure 2b.

The mechanical testing system comprised a servo-controlled testing system (National Key Laboratory of Coal Mine Hazard Dynamics and Control, Chongqing University) operating under quasi-static loading regime (0.1 mm/min displacement rate). This configuration enabled acquisition of stress–strain curves for thermally treated granite specimens. A PAC PCI-2 acoustic emission acquisition system continuously acquired acoustic emission (AE) signatures during compressive loading, implementing a 40 dB detection threshold for noise filtration. All experimental data were synchronously recorded through computerized data acquisition from initial loading to specimen failure. Full-field deformation monitoring was performed using a Zeiss two-dimensional digital image correlation (2D-DIC) system equipped with a 33.76-megapixel CCD sensor operating at 1 Hz sampling frequency to capture speckled surface displacements. The integrated experimental configuration and monitoring systems are schematically presented in Figure 3.

To ensure the reliability of the experimental results, the number of experimental rock specimens examined under the same experimental conditions was set to 3. Test procedures applied to each rock specimen were as follows:

1. Before thermal processing, all specimens were numerically labeled and characterized for key physical parameters (mass, diameter, height, and P-wave velocity), enabling elimination of specimens exhibiting significant heterogeneity.

2. Granite specimens were subjected to thermal treatment in a muffle furnace at predetermined temperature levels, employing a controlled heating rate of 5 °C/min to mitigate thermal shock effects. Following attainment of target temperatures, the specimens underwent a 24 h isothermal holding period. The furnace was subsequently deactivated, allowing specimens to undergo natural cooling to ambient temperature within the chamber. Post-treatment protocol included extraction of specimens from the furnace, re-evaluation of physical properties, and systematic screening to eliminate samples demonstrating significant property deviations.

3. Prior to mechanical testing, black and white aerosol coatings were applied to specimen surfaces to generate stochastic speckle patterns essential for digital image correlation analysis. Speckle pattern quality was quantitatively assessed using Zeiss GOM Correlate software, with specimens exhibiting suboptimal pattern characteristics being systematically excluded from subsequent testing protocols.

4. Uniaxial compression testing was conducted while synchronously acquiring acoustic emission (AE) signals, digital image correlation (DIC) datasets, and axial stress measurements through a unified data acquisition system. This integrated approach ensured temporal synchronization of all recorded parameters throughout the loading protocol.

Figure 3. Loading device combining AE and DIC.

Figure 3. Loading device combining AE and DIC.

3. Results and Discussion

During the thermal treatment, the granite specimens exhibited a progressive chromatic alteration to yellowish-brown hues. The thermal expansion anisotropy among the constituent minerals induced a heterogeneous deformation at elevated temperatures, resulting in intergranular fissures initiating at 800 °C. Macro-fractures developed at 1000 °C and exhibited a 47% greater density and a 2.3-fold width increase compared to the 800 °C specimens, establishing 800 °C as the critical temperature for incipient fracture nucleation. Furthermore, the impact testing revealed a distinct acoustic evolution: specimens treated below 500 °C produced a metallic resonance, while those above 600 °C showed a dampened acoustic response. This damping effect intensified progressively, culminating in complete sonic attenuation at 1000 °C.

3.1. Changes in Quality, Volume, Density, and Longitudinal Wave Velocity

Figure 4 presents the physical parameters of the granite specimens, including the mass, geometric dimensions (diameter/height), and compressional wave velocity measured pre- and post-thermal treatment. Three dimensionless parameters were formulated to quantify the thermal degradation: the mass loss ratio (${\mathsf{\eta }}_m$), volumetric expansion coefficient (${\mathsf{\eta }}_V$), and P-wave velocity attenuation factor (${\mathsf{\eta }}_p$), defined mathematically as [38]:

$${\mathsf{\eta }}_m={\displaystyle \frac{m_1-m_2}{m_1}}\times 100\mathrm{\%}$$

(1)

$${\mathsf{\eta }}_V={\displaystyle \frac{V_2-V_1}{V_1}}\times 100\mathrm{\%}$$

(2)

$${\mathsf{\eta }}_p={\displaystyle \frac{v_2-v_1}{v_1}}\times 100\mathrm{\%}$$

(3)

where $m_1$, $V_1$, and $v_1$ denote the mass, volume, and P-wave velocity of the original granite, and $m_2$, $V_2$, and $v_2$ denote the mass, volume, and P-wave velocity of the high-temperature-treated granite, respectively.

Figure 4a presents the temperature-dependent mass loss profile of the heat-treated granite, indicating a progressive decrease in sample mass with rising temperatures. Below 500 °C, the mass loss rate exhibited a gradual increase, while reaching 600 °C triggered a sharp decline in mass. This rate accelerated further at 1000 °C, culminating in a total mass loss of 1.02%, primarily attributed to the combined evaporation of free and bound water. Notably, at 1000 °C significant data variability emerged among triplicate samples, which was caused by minor surface detachment phenomena observed during thermal treatment.

Figure 4b demonstrates the thermal expansion characteristics of granite, showing a progressive volume increase with temperature elevation. The material exhibits comparable expansion rates between 25 °C and 300 °C, peaking at 0.11% volumetric expansion at 300 °C. Subsequently, the expansion rate showed a moderate acceleration from 300 °C to 400 °C, reaching 0.28% expansion. This thermal expansion primarily originates from mineral lattice dilatation. The rate stabilized within the 400–500 °C range, displaying negligible growth. Beyond 500 °C, a resurgence in the expansion rate occurred, driven by the intensified mineral expansion and intergranular boundary fracturing. The most significant expansion (2.29% at 600 °C) coincided with quartz’s α-β phase transition, manifesting a rapid volumetric increase approaching its 573 °C transition point, accompanied by accelerated periclase cracking. Stabilization resumed between 600 °C and 800 °C, before the expansion rate escalated dramatically to 8.68% at 1000 °C, corresponding with advanced intergranular fracturing.

Due to the substantial velocity discrepancy between solid and air phases, the longitudinal wave velocity is typically employed to assess rock damage. Figure 4c,d illustrate the impact of the thermal treatment on granite’s P-wave velocity. The figures demonstrate that the longitudinal wave velocities exhibited a significant reduction following the high-temperature exposure, with an approximately linear reduction pattern observed below 500 °C. The baseline wave velocity measured 4870 m/s in untreated specimens, decreasing by 53.85% at 500 °C. A precipitous decline of 76.56% occurred at 600 °C, suggesting substantial physico-chemical alterations within the rock matrix at this threshold temperature. This observation aligns with the previous mass and volume measurements recorded at 600 °C. Within the 600–800 °C range, the longitudinal wave velocity remained relatively stable, ultimately decreasing by 82.68% at 800 °C. At the maximum tested temperature of 1000 °C, the velocity decreased by 94.48%. This phenomenon can be attributed to the extensive macroscopic crack formation induced by thermal stresses, which substantially impedes acoustic wave propagation through the material.

3.2. Thermally Induced Mechanical Property Alterations in Granite

Triplicate specimens were subjected to uniaxial compression testing at each target temperature, with concurrent AE monitoring and DIC analysis. Figure 5 presents the obtained stress–strain relationships.

The stress–strain profiles exhibit pronounced nonlinear deformation characteristics during the initial loading (Figure 5), which is attributable to the closure of pre-existing microcracks within the granite matrix. Notably, this nonlinear phase demonstrates progressive intensification with elevated temperatures, indicative of enhanced thermally induced microcrack closure effects. The subsequent linear elastic response transitions to progressive nonlinearity near the peak stress attainment.

A critical transition occurs at 600 °C and is characterized by the following: (1) a marked reduction in peak stress (62.3% decrease relative to baseline), (2) diminished brittle characteristics (brittleness index reduction from 2.41 to 0.87), and (3) enhanced ductile behavior (plastic strain increment of 187%). Within the 600–800 °C range, specimens demonstrate substantial plastic deformation with attenuated stress accumulation rates, exhibiting a distinct post-yield ductile flow. At 1000 °C, the granite lost most of its elastic properties. These observations collectively suggest that the brittle–ductile transition threshold for the studied granite occurs between 500 and 600 °C.

Figure 6a delineates the evolutionary profile of the uniaxial compressive strength (UCS) with thermal exposure. The granite specimens exhibited progressive strength degradation from 25 to 200 °C, retaining 68.42% residual strength relative to the baseline (25 °C). Notably, a marginal recovery phase occurred between 200 and 300 °C (89.19% strength retention), likely attributable to thermal hardening effects. Subsequent abrupt strength deterioration commenced at 500 °C, culminating in a 59.92% UCS reduction at 600 °C. Although a temporary strength recovery was recorded at 700 °C, the progressive strength degradation resumed at 800 °C (35.5% retention) and reached catastrophic levels (6.78% retention) at 1000 °C.

The strain evolution diagram (Figure 6b) reveals the deformation behavior. Below 300 °C, the critical strain thresholds fluctuated within ±8.6% of baseline values, and were primarily governed by hydroxyl group dehydration and differential mineral expansion. Above 400 °C, the accelerated strain development manifested due to following: (1) the intergranular crack propagation through quartz transition zones and (2) feldspar network softening. A critical inflection occurred at 600 °C with a 214.29% strain amplification, progressing to 258.35% (3× baseline) at 700 °C through viscous flow mechanisms. The ultimate strain capacity reached 507.93% at 1000 °C, indicative of complete microstructure collapse and pseudo-viscous behavior.

The elastic modulus evolution (Figure 6c) reveals critical thermal thresholds. Specimens at 200 °C showed a marginal stiffness reduction (2.92%), followed by an anomalous 7.81% modulus recovery at 300 °C—potentially indicating thermal compaction effects. A critical transition occurred at 600 °C with a catastrophic modulus degradation (79.04% reduction). The subsequent thermal loading (700–1000 °C) demonstrated a quasi-linear stiffness loss, culminating in a 98.85% modulus depletion at 1000 °C, where specimens exhibited a complete loss of structural integrity.

Figure 6d delineates the Poisson’s ratio evolutionary pattern. Below 400 °C, the ratio displayed a gradual decline (0.22→0.13), reflecting constrained lateral deformation. Notably, an abrupt augmentation (0.13→0.26) occurred at 600 °C, coinciding with quartz’s α-β phase transition (573 °C). This 100% ratio increase signifies fundamental alterations in the deformation mechanisms, transitioning from brittle fracture to crystal plasticity-dominated behavior.

Figure 6. Mechanical parameters of heat-treated granite: (a) change curve of peak stress with temperature; (b) change curve of peak strain with temperature; (c) change curve of Young’s modulus; and (d) change curve of Poisson’s ratio.

Figure 6. Mechanical parameters of heat-treated granite: (a) change curve of peak stress with temperature; (b) change curve of peak strain with temperature; (c) change curve of Young’s modulus; and (d) change curve of Poisson’s ratio.

Through comprehensive parameter analysis, we propose tripartite thermomechanical response stages:

Stage I (25–400 °C): the mechanical response of granite exhibited minimal variation with the increasing temperature.

Stage II (400–600 °C): deformation parameters, such as Young’s modulus and peak strain, were influenced by the rising temperature, while the uniaxial compressive strength (UCS) experienced a further decline.

Stage III (600–1000 °C): Both the deformation and strength parameters were profoundly impacted by the rising temperature. The physical properties of granite underwent severe deterioration.

3.3. Damage Mechanism Characterization

3.3.1. Thermo-Acoustic Damage Evolution

Acoustic emission (AE) monitoring has become an essential technique for characterizing the progressive damage evolution and evaluating the failure intensity in rock mechanics [34,35]. As demonstrated in Figure 7, thermally treated granites exhibit distinct AE response patterns during uniaxial compression across different temperature regimes. In the 25–300 °C range (Figure 7a–c), three characteristic AE evolution phases were identified: (I) initial quiescent phase (pre-A1), with negligible AE activity corresponding to the crack closure; (II) linear growth phase (A1–A2), indicative of stable crack propagation; and (III) accelerated growth phase (A2–A3), marking an unstable crack development. The critical transition at A2 corresponds to the crack damage stress threshold, serving as a crucial precursor for engineering failure warnings.

Specimens treated at 400–500 °C (Figure 7d,e) revealed modified AE characteristics, particularly the nonlinear progression in the Stage II AE evolution, contrasting with the linear pattern observed in low-temperature counterparts. This behavioral shift arises from the activation and propagation of thermally induced microcracks under compressive loading. When subjected to 600–800 °C treatments (Figure 7f–h), specimens exhibited a premature AE activation during the initial loading stages, which is attributed to the closure and extension of extensive thermal crack networks. Notably, the substantial reduction in the cumulative AE energy for specimens above 600 °C (Figure 7f–h) reflects a compromised intergranular energy storage capacity due to thermal degradation, which is consistent with the observed strength deterioration. Specimens exposed to extreme 1000 °C conditions (Figure 7i) demonstrated persistent high-intensity AE signals, providing direct evidence of catastrophic thermal damage within the material microstructure.

The evolution of cumulative AE energy demonstrates distinct behavioral patterns across temperature regimes: specimens below 600 °C exhibit a characteristic biphasic growth pattern corresponding to the crack initiation (Stage I) and damage threshold attainment (Stage II), whereas high-temperature specimens (≥600 °C) display a progressive monotonic increase. The comparative analysis demonstrates that 600 °C constitutes the transition threshold for AE energy release mechanisms, where pre-existing thermal crack networks fundamentally modify energy accumulation dynamics. This thermal-induced alteration in the energy transfer mechanisms establishes a critical evaluation criterion for assessing the stability of thermally compromised rock masses.

3.3.2. Deformation Evolution Mechanism

To elucidate the detailed progression of the deformation evolution during loading, the maximum principal strains obtained through the digital image correlation (DIC) at selected temporal reference points are presented in Figure 8. These points are indicated on the stress–strain curves in Figure 7. This approach enables the correlation between acoustic emission signatures and full-field deformation patterns to investigate the damage evolution and fracture behavior in thermally treated granite specimens. The surface deformation fields of granite specimens subjected to cyclic thermal shock were monitored during the uniaxial compression test. Capturing the evolution of the deformation field at the post-peak stage is challenging, due to the speckle degradation on the rock surface. Consequently, this study focused exclusively on the evolution of the deformation field on the granite surface during the pre-peak stage. Figure 8 presents the contour plots of the principal strain distribution on the surface of the granite specimen during the pre-peak and peak stages at various temperatures. Figure 8a displays the maximum principal strain distribution for the room temperature specimen. The strain localization initiates at Point A1, coinciding with the onset of the sustained acoustic emission (AE) detection. Subsequently, high-strain bands coalesce at Point A2 within the specimen’s central region, resulting from the unstable propagation of multiple microcracks. At the peak axial stress (Point A3), the microcrack coalescence generates intense acoustic emission activity, forming macroscopic fractures. It is noteworthy that the progressive fracturing mechanism remains consistent for specimens subjected to temperatures below 500 °C (Figure 7a–e). The strain localization initially develops in the central region when axial stress attains the crack initiation threshold. The localized strain intensifies with increasing axial stress, culminating in a longitudinal high-strain zone formation at the damage stress threshold. During the unstable crack propagation, high-strain zones propagate radially while a secondary crack formation occurs. These processes ultimately result in macroscopic fracture networks that precipitate catastrophic failure.

Thermally treated specimens below 500 °C (Figure 8a–e) maintain consistent fracture progression: (1) the stress-dependent expansion of strain localization zones during crack initiation; (2) the development of axial high-strain bands with secondary cracking at the damage threshold; and (3) the macroscopic fracture surface formation through coalescence. Conversely, specimens treated above 600 °C (Figure 8f–i) exhibit ductile failure characteristics: (1) an early-stage distributed strain field development under low stress; (2) a reticulated conjugate shear band formation with stress escalation; and (3) a multi-crack coordinated propagation culminating in failure. The 1000 °C specimens demonstrate a complete loss of deformation resistance, exhibiting excessive AE signals and principal strain magnitudes during initial loading phases.

The strain field evolution analysis reveals a temperature-dependent failure mode transition: specimens below 500 °C predominantly fail through tensile splitting mechanisms, while those above 600 °C transition to shear-dominated ductile failure. This mechanistic shift originates from the quartz phase transformation-induced microcrack networks that redistribute internal stresses, fundamentally altering macroscopic mechanical responses.

3.3.3. Analysis of Damage Variables

Numerous researchers [24,25,26,27] have defined rock damage variables based on the modulus of elasticity and investigated the mechanical damage models of granite, utilizing the modulus of elasticity as the damage variable and evaluation index, which can be defined as follows:

$$D_E=1-{\displaystyle \frac{E_T}{E_0}}$$

(4)

where $E_T$ represents the elastic modulus of granite after various temperature treatments, and $E_0$ denotes the elastic modulus of granite at room temperature (25 °C).

Based on the aforementioned equation, the thermal damage variable of high-temperature granite can be determined, and its variation is illustrated in Figure 9.

As illustrated in Figure 9, the damage variable of granite increases progressively with rising temperatures. However, when the temperature of granite increases from 200 °C to 300 °C, the damage variable decreases. This phenomenon is attributable to thermal stress, which closes the internal pores of granite and improves the overall integrity of the specimen. As the temperature continues to rise, the thermal stress induces the formation of both transgranular and intergranular cracks within the granite’s internal minerals, causing the damage variable to increase gradually and the deformation resistance of the granite specimen to weaken. Upon reaching 600 °C, the damage variable escalates rapidly due to the phase transformation of quartz within the granite, resulting in a significant increase in internal pores. At 1000 °C, the granite almost completely loses its ability to resist deformation. The evolution of thermal damage variables with temperature exhibits a triphasic pattern consistent with the stages previously described.

Stage I (25–400 °C): during the initial stage, the mechanical properties of granite exhibit minimal variation at low temperatures, while the thermal damage variable demonstrates a slower rate of increase.

Stage II (400–600 °C): Within the temperature range from 400 °C to 600 °C, the phase transition of quartz induces rapid crack propagation, resulting in the significant deterioration of the sample’s mechanical properties. Concurrently, thermal damage variables exhibit a pronounced upward trend.

Stage III (600–1000 °C): Beyond 600 °C, the crack development reaches a saturation point, resulting in the deceleration of the thermal damage variable growth. Ultimately, at 1000 °C, when the thermal damage approaches 100%, the granite essentially loses its deformation capacity.

4. Conclusions

In this study, granite specimens subjected to thermal treatment were analyzed through uniaxial compression testing; strain data acquisition using digital image correlation (DIC) techniques; and the concurrent monitoring of acoustic emission characteristics. The mechanical property variations and damage evolution mechanisms of granite under varying thermal conditions were investigated, yielding the following principal findings:

(1) The thermal exposure induced a progressive volumetric expansion in granite specimens, accompanied by a mass reduction and P-wave velocity attenuation, exhibiting a marked discontinuity at 600 °C.

(2) Both the uniaxial compressive strength (USC) and elastic modulus demonstrated a progressive thermal degradation, while the deformation magnitude exhibited a temperature-dependent amplification. A critical strength reduction occurred within the 500–600 °C range, potentially attributable to the quartz phase transformation. Conversely, the peak strain displayed continuous thermal enhancement, demonstrating enhanced plastic deformation characteristics.

(3) Acoustic emission patterns showed a strong correlation with DIC-derived strain fields, which can be distinctly categorized into three evolutionary phases: calm initiation, stable progression, and abrupt intensification. These phases became indistinct at 1000 °C, suggesting extensive macrocrack development. Damage mechanisms transitioned progressively with thermal loading: specimens treated below 500 °C predominantly failed through tensile splitting, while those above 600 °C exhibited shear-dominated ductile failure modes.

(4) Damage variables generally increased with thermal exposure, though specimens exposed to 300 °C exhibited an anomalous reduction in damage variables due to the thermal stress-induced closure of internal pores. The rapid damage accumulation initiated at 600 °C, culminating in 98.85% damage at 1000 °C, indicating a complete loss of the deformation resistance capacity.

This study elucidates the thermomechanical degradation mechanisms of granite under high-temperature treatments (25–1000 °C), identifying 600 °C as a critical threshold characterized by accelerated P-wave velocity attenuation, intergranular fracturing, and a brittle-ductile transition marked by the severe UCS degradation and amplified peak strain. These findings directly inform applications in deep geological engineering, such as geothermal energy extraction and nuclear waste containment, where temperatures exceeding 600 °C demand advanced stabilization strategies to mitigate shear-driven failure and preserve load-bearing capacity. The integration of AE and DIC techniques establishes a robust framework for real-time thermal damage monitoring, enabling early warning systems in subsurface environments. Looking ahead, future work should prioritize multiaxial stress testing to simulate in situ geostresses, assess cumulative damage from thermal cycling, and isolate mineral-specific contributions to degradation. Coupling these insights with thermo-chemical interaction studies and predictive modeling will refine risk mitigation strategies and advance the design of resilient geological systems for high-temperature applications, bridging experimental findings with practical engineering solutions.