Stress Relaxation Research Articles

Schiff base crosslinked hyaluronic acid hydrogels with tunable and cell instructive time-dependent mechanical properties

The dynamic interplay between cells and their native extracellular matrix (ECM) influences cellular behavior, imposing a challenge in biomaterial design. Dynamic covalent hydrogels are viscoelastic and show self-healing ability, making them a potential scaffold for recapitulating native ECM properties. We aimed to implement kinetically and thermodynamically distinct crosslinkers to prepare self-healing dynamic hydrogels to explore the arising properties and their effects on cellular behavior. To do so, aldehyde-substituted hyaluronic acid (HA) was synthesized to generate imine, hydrazone, and oxime crosslinked dynamic covalent hydrogels. Differences in equilibrium constants of these bonds yielded distinct properties including stiffness, stress relaxation, and self-healing ability. The effects of degree of substitution (DS), polymer concentration, crosslinker to aldehyde ratio, and crosslinker functionality on hydrogel properties were evaluated. The self-healing ability of hydrogels was investigated on samples of the same and different crosslinkers and DS to obtain hydrogels with gradient properties. Subsequently, human dermal fibroblasts were cultured in 2D and 3D to assess the cellular response considering the dynamic properties of the hydrogels. Moreover, assessing cell spreading and morphology on hydrogels having similar modulus but different stress relaxation rates showed the effects of matrix viscoelasticity with higher cell spreading in slower relaxing hydrogels.

Evaluation of a manualised neurofeedback training in psychosomatic-psychotherapeutic outpatient treatment (Neuro-pp-out): study protocol for a clinical mixed-methods pilot study

IntroductionElectroencephalographic neurofeedback (NFB), as a non-invasive form of brainwave training, has been shown to be effective in the treatment of various mental health disorders. However, only few results regarding manualised...

Inverse primitive path analysis

The primitive-path analysis (PPA) [R. Everaers et al. Science 303, 823, (2004)] is an algorithm that transforms a model polymer melt into its topologically equivalent mesh by removing excess contour length stored in thermal fluctuations. Here we present an inverse PPA algorithm (iPPA) that gradually reintroduces contour length in a PPA mesh. We validate the method preserves topology by performing 100 cycles of PPA-iPPA transformations on a ring polymer melt. We show how a parametric curve can be used as a synthetic mesh to generate model materials of rings with entanglements forming a regular 2D cubic lattice. This shows how iPPA can be used to generate model polymer materials with well controlled topology. Finally we show how PPA-iPPA can be used to accelerate stress relaxation approximately by an order of magnitude in simulation time. This reduces the computational cost of computational studies of structure-property relations for polymer materials.

Experimental study of the mechanical properties and microscopic mechanism of carbonate rock subjected to high-temperature acid stimulation

To clarify the effect of acidification on the stress distribution around wells in deep carbonate reservoirs, it is necessary to study the changes in mechanical properties of carbonate rocks under different acid stimulation conditions. In this paper, a novel acid stimulation experiment is designed to investigate the mechanical properties of the carbonate rock through a self-built acid-rock reaction experimental system and triaxial rock mechanics apparatus. The changes in the core mechanical parameters with the acid stimulation time at different acid stimulation temperatures are obtained. Based on stress relaxation, the rheological properties of the core after acid stimulation are explored. The internal structural damage mechanism of the core after high-temperature acid stimulation is as analysed via by SEM. This research finds that the elastic modulus and compressive strength of the core exponentially decrease with increasing of acid stimulation time. The higher the acid stimulation temperature, the faster the core damage rate changes, and the earlier the damage amount tends to stabilize. The core has an obvious rheological mechanical property after acid stimulation. The stress relaxation amplitude exponentially increases with increasing strain during loading. Interestingly, when the acid stimulation temperature reaches 120 °C, a sudden stress drop occurs during stress relaxation. It is also found that the stress relaxation amplitude increases exponentially with increasing acid stimulation time under the same strain level. Moreover, a higher acid stimulation temperature results in greater stress relaxation under a constant acid stimulation time and strain. In addition, the core skeleton is destroyed through acid stimulation, and many complex damaged structures appear inside the core, which leads to a decrease in the core strength, which is the reason for the enhanced rheological characteristics of the rock after acid stimulation. These results can provide insights into the mechanical response mechanism of wellbore acidification.

Controlling associative transcarbamoylation reactions by light in dynamic polyurethane networks using reversible spiropyran photoswitches

Polyurethanes represent a versatile class of polymers and are one of the most employed in the thermoset market. However, due to their thermodynamically stable carbamate bond, they suffer from a lack of reprocessability, recyclability, and degradability, and are therefore usually discarded after use. Recently, transcarbamoylation in polyurethane thermosets has been demonstrated via associative exchange in the presence of a strong organic acid. Intending to introduce spatiotemporal control in the processability of polyurethane materials, in this work we investigate dynamic transcarbamoylation by the addition of a latent acidic catalyst into an aromatic and an aliphatic polyurethane network. We introduce three photoswitches based on merocyanine/spiropyran compounds which, upon exposure to visible light, undergo a reversible cyclization reaction, resulting in the formation of strong acids. The use of such photoswitches prevents the presence of a permanent acidic species in the network, which would eventually lead to its degradation over time. Importantly, due to the reversible nature of the cyclization reaction, the release of the acidic catalyst is not permanent, as evidenced by stress relaxation measurements. Overall, the evidence of the light-induced, acid-catalyzed transcarbamoylation is supported by stress relaxation, optical microscopy, and self-healing measurements.

Study on mechanical properties of dual-channel cryogenic 3D printing scaffold for mandibular defect repair.

Mandibular defect repair has always been a clinical challenge, facing technical bottleneck. The new materials directly affect technological breakthroughs in mandibular defect repair field. Our aim is to fabricate a scaffold of advanced biomaterials for repairing of small mandibular defect. Therefore, a novel dual-channel scaffold consisting of silk fibroin/collagen type-I/hydroxyapatite (SCH) and polycaprolactone/hydroxyapatite (PCL/HA) was fabricated by cryogenic 3D printing technology with double nozzles. The mechanical properties and behaviors of the dual-channel scaffold were investigated by performing uniaxial compression, creep, stress relaxation, and ratcheting experiments respectively. The experiments indicated that the dual-channel scaffold was typical non-linear viscoelastic consistent with cancellous tissue; the Young's modulus of this scaffold was 60.1kPa. Finite element analysis (FEA) was employed performing a numerical simulation to evaluate the implantation effect in mandible. The stress distribution of the contact area between scaffold and defect was uniform, the maximum Mises stress of cortical bone and cancellous bone in defect area were 54.520MPa and 3.196MPa, and the maximum displacement of cortical bone and cancellous bone in defect area were 0.1575mm and 0.1555mm respectively, which distributed in the incisor region. The peak maximum Mises stress experienced by the implanted scaffold was 3.128 × 10-3MPa, and the maximum displacement was 6.453 × 10-2mm distributed near incisor area. The displacement distribution of the scaffold was consistent with that of cortical and cancellous bone. The scaffold recovered well when the force applied on it disappeared. Above all, the dual-channel scaffold had excellent bio-mechanical properties in implanting mandible, which provides a new idea for the reconstruction of irregular bone defects in the mandible and has good clinical development prospects.

Surface Chemistry Study of Normal and Diseased Human Meibum Films Prior to and after Supplementation with Tear Mimetic Eyedrop Formulation

Ophthalmic nanoemulsions that can treat the deficiencies of meibum (MGS) in Meibomian gland disease and restore its functionality in the tear film are greatly sought. The Rohto Dry Aid (RDA) formulation employs TEARSHIELD TECHNOLOGYTM, which uses a multicomponent oil phase of polar and non-polar lipid-like molecules selected to mimic the profiles of healthy meibum. Thus, the interactions of RDA with “diseased” Meibomian (dMGS) films merit deeper analysis, as these interactions might offer important clues for both the development of new ocular formulations and the processes behind the therapeutic action of the nanoemulsions. Pseudobinary dMGS/RDA films were spread at the air–water surface of the Langmuir trough. Surface pressure-area isocycles and stress relaxations were used to access the layer’s response to blink-like cycling and dilatational viscoelasticity, respectively, while film morphology was recorded via Brewster angle microscopy. It was found that RDA is able to reverse the brittleness and to restore the stability of “diseased” MGS films and thus to revert the layer’s properties to the functionality of healthy Meibomian lipids. Therefore, in order to effectively treat dry eyes with MGS-oriented therapy, ophthalmic nanoemulsions warrant more research.

Effects of P2O5 content on the structure, stress distribution and vickers hardness of Na2O-MgO-Al2O3-SiO2 glasses with chemical strengthening treatments

In this work, effects of P2O5 content on the structure, stress distribution and vickers hardness of alkali aluminosilicate glasses with different chemical strengthening treatments were investigated. The results showed that the addition of P2O5 increased the degree of polymerization for glass network. The coefficient of thermal expansion, glass transition temperature and density of the parent glasses were all decreased. Meanwhile, the molar volume of the samples increased. The chemical strengthening experiments revealed that increasing P2O5 content promoted the diffusion of K+ ions, which resulted in an increase in the depth of stress layer. However, surface compressive stress of the glasses decreased with P2O5 content increasing, mainly attributed to stress relaxation of the glass structure. After chemical strengthening treatments at 370 °C and 390 °C respectively, the vickers hardness of the glasses decreased with the increase of P2O5, while an opposite trend was observed at 410 °C.

Local ratcheting and stress relaxation at notch root of CP-Ti specimen under cyclic loading: Experiment and simulation

Damage and crack always origin from notch, and it is necessary to understand the local ratcheting and stress relaxation at notch root for commercially pure titanium (CP-Ti). Symmetric strain-controlled cyclic test for smooth CP-Ti specimen is conducted to determine the cyclic constitutive relationship based on the combined hardening model. Asymmetric force-controlled cyclic test combined with Digital Image Correlation (DIC) technology is conducted to reveal local ratcheting strain distribution at notch root of CP-Ti specimen, while the distributions of stress, local stress ratio, and local hysteresis curve at notch root are obtained by finite element (FE) simulation. It is found that plastic zone, local ratcheting and stress relaxation at notch root area change significantly with cyclic load levels. In the low stress amplitude region with the mean nominal stress lower than 87 MPa, the small-scale yield occurs, and the notch root is in a state of plastic shakedown. With the mean nominal stress increasing to 96 MPa in the transition region, the notch root is in a state that exceeds the plastic shakedown limit but the continuous ratcheting progress is not found. In the high stress amplitude region with the mean nominal stress higher than 104 MPa, the plastic zone significantly expends, and the notch root is in a continuous ratcheting progress state, accompanied by stress relaxation. For a specific structural configuration, the local ratcheting at notch root is affected by both the load level and the cyclic deformation characteristics of the material. In the high stress amplitude region, the reverse yield occurs at the notch root during unloading, which promotes continuous ratcheting progress and stress redistribution, resulting in stress relaxation. The higher the load level, the larger the ratcheting strain rate, and the lower the stress relaxation rate.

Self-Healing Transparent Poly(dimethyl)siloxane with Tunable Mechanical Properties: Toward Enhanced Aging Materials for Space Applications.

When exposed to the geostationary orbit, polymeric materials tend to degrade on their surface due to the appearance of cracks. Implementing the self-healing concept in polymers going to space is a new approach to enhancing the lifespan of materials that cannot be replaced once launched. In this study, the elaboration of autonomous self-healing transparent poly(dimethylsiloxane) (PDMS) materials resistant to proton particles is presented. The PDMS materials are functionalized with various compositions of urea and imine moieties, forming dynamic covalent and/or supramolecular networks. The hydrogen bonds induced by the urea ensure the formation of a supramolecular network, while the dynamic covalent imine bonds are capable of undergoing exchange reactions. Materials with a broad range of mechanical properties were obtained depending on the composition and structure of the PDMS networks. As coating applications in a spatial environment were mainly targeted, the surface properties of the polymer are essential. Thus, percentages of scratch recovery were determined by AFM. From these data, self-healing kinetics were extracted and rationalized based on the polymer structures. The obtained data were in good agreement with the relaxation times determined by rheology in stress relaxation experiments. Moreover, the accelerated aging of materials under proton irradiation, simulating a major part of the geostationary environment, revealed a strong limitation or disappearance of cracks while keeping the transparency of the PDMS. These promising results open routes to prepare new flexible autonomous polymeric materials for space applications.

The mechanical properties of different cross-veins in the hind wing of locust Locusta migratoria under uniaxial tensile and stress relaxation tests.

Locust Locusta migratoria exhibits remarkable aerial performances, relying predominantly on its hind wings that generate most of lift and thrust for flight. The mechanical properties of the cross-veins determine the deformation of the hind wing, which greatly affect the aerodynamic performance of flapping flight. However, whether the mechanical behaviours of the locust cross-veins change with loading rate is still unknown. In this study, cross-veins in four physiological regions (anterior-medial, anterior-lateral, posterior-medial and posterior-lateral) of the hind wing from adult locusts were investigated using uniaxial tensile test, stress relaxation test and fluorescence microscopy. It was found that the cross-veins were a type of viscoelastic material (including rate-independent elastic modulus and obvious stress relaxation). The cross-veins in the two anterior regions of the hind wing had significantly higher elastic moduli and higher ultimate tensile stress than those of its two posterior regions. This difference might be attributed to different resilin distribution patterns in the cross-veins. These findings furnish new insights into the mechanical characteristics of the locust cross-veins, which might deepen our understanding of the aerodynamic mechanisms of locust flapping flight.

PWHT influence on welding residual stress in 45 mm bridge steel butt-welded joint

To investigate the spatial distribution of welding residual stress (WRS) and effect of post welding heat treatment (PWHT) on WRS relaxation in the welded components of steel bridges, a 45 mm-thick Q345qD butt-welded specimen was manufactured. Experimental measurement and numerical simulation methods were employed to examine specimen surface and internal WRSs. The simulated results exhibited good agreement with the measured ones, confirming the rationality of the numerical simulation process. Subsequently, through the PWHT simulation in ABAQUS, the evolution of WRS during PWHT was comprehensively analyzed and the effect of PWHT parameters on the stress relaxation was studied. The measured and simulated results show that the longitudinal welding residual stresses (LWRSs) are tensile in the weld region and compressive away from the weld. High-value stresses are mainly concentrated on specimen surfaces and the mid-thickness. Transverse welding residual stresses (TWRSs) present multi-layered distribution, alternating between tension and compression within the weld region. WRS reduction mainly occurs during the heating stage of PWHT, with slight decrease during the holding stage and an increase during the cooling stage. Optimizing PWHT parameters, including temperature, and holding time, heating/cooling rates, proved effective in achieving substantial WRS relaxation.

Continuous heating aging characteristics of Al-Cu-Mg-Ag alloy based on stress relaxation method

Clarifying the aging characteristics of aluminum alloys during continuous heating is crucial for formulating isothermal and non-isothermal aging (continuous heating aging) processes. This paper investigates the stress relaxation characteristics of an Al-Cu-Mg-Ag alloy under three-point bending during continuous heating. Taking into account the mechanical properties and precipitate analysis, it demonstrates the feasibility of using the stress relaxation method to determine the peak aging temperature during continuous heating. This approach is efficient, requires minimal sample, and yields continuous data. The peak aging temperature of the sample increases from 260 °C to 305 °C when the heating rate increases from 1 to 3 °C/min with the same initial load (3 N). The peak aging temperature is around 280 ℃, when the initial load increases from 1 N to 9 N with the same heating rate (2 °C/min). Under low temperatures (25 ℃∼ 200 ℃), the sample displays the presence of GP zones, and a few Ω precipitates during the continuous heating process of 2 ℃/min. As the temperature increases, the GP zones dissolve and transform into the Ω precipitates. A large number of Ω precipitates appear in the matrix in a continuous precipitation manner when the temperature reaches 235 °C. The precipitation strengthening effect is strongest at 280 °C, and the Ω precipitates begin to coarsen as the temperature exceeds 280 °C.

Design engineering of non-linear graded InAlAs metamorphic buffer layers for efficient reduction of misfit dislocation density

Metamorphic InxAl1-xAs (xmax > 0.75) buffer layer (MBL) grown on GaAs with an optimized non-linear graded composition profile along the growth direction is proposed for enhanced reduction of misfit dislocation (MD) density in highly mismatched III-V/GaAs metamorphic heterostructures. The equilibrium distributions of MD density throughout such InxAl1-xAs/GaAs MBLs are calculated. The influence of the initial composition (xmin) of MBL and the elastically strained thin GaAs layer embedded into the InxAl1-xAs MBL on the MD density distribution was studied. An optimum value of the inverse step (Δ), representing the difference between the top In content of the InxAl1-xAs MBL and that of a In0.75Al0.25As virtual substrate (VS) grown atop is determined. It was theoretically shown that Δ above 0.04 results in relaxation of the elastic stresses in VS via formation of the MDs, while the lower Δ values allow growing the VS completely free of MDs. Finally, the metamorphic In0.75Al0.25As/graded-InAlAs/GaAs heterostructures differing from each other by only the composition profile of the graded-InAlAs MBL were grown by MBE on GaAs(001) and studied by atomic force microscopy. The structure with proposed optimized non-linear graded (x = 0.05–0.77) MBL demonstrated the ∼ 1.5 times less MD density as compared to that with the convex-graded (x = 0.05–0.81) MBL, which was estimated to be at the level of ∼ 2·107 cm−2.

Virtual Reality Relaxation for Mental Health Service Users with Complex Care Needs: a Pilot Study in Residential Care Homes and Supported Accommodation

AbstractVirtual reality (VR) relaxation is a promising, novel intervention for service users with a range of mental health conditions and has potential wellbeing benefits such as promoting relaxation and reducing stress and anxiety. Less is known about the implementation of VR relaxation for service users in complex care mental health services. This study aimed to investigate implementation of a single session of VR relaxation for service users in residential care homes and supported accommodation, who were supported by a South London community team. Participants could explore relaxing, natural environments in VR. Pre- and post-VR visual analogue scales of mood, immersiveness, helpfulness, narrative feedback and clinician reflections were recorded. Participants (N = 20) had psychotic disorders and other serious mental health conditions. Post-VR, there were significant increases in relaxation (large effect), happiness and connectedness to nature (both medium effects), and decreases in anxiety and sadness (both medium effects). Stress was low at baseline and there was no significant effect post-VR. Helpfulness and immersion scores were high (both > 8). Participants reported the intervention was very relaxing, exciting, uplifting and were enthusiastic about engaging in more VR relaxation and further psychological interventions, including therapy and other digital interventions. Participants reported finding a session facilitator helpful. Technical issues were minor and resolved by the facilitator. Findings indicate scope to widen access to VR relaxation within complex care services. More facilitators could offer multiple sessions to larger, more diverse groups of service users. Future research with more robust methodologies is needed to test effectiveness and longer-term impact.

Viscoelastic hydrogels regulate adipose-derived mesenchymal stem cells for nucleus pulposus regeneration

Low back pain is a leading cause of disability worldwide, often attributed to intervertebral disc (IVD) degeneration with loss of the functional nucleus pulposus (NP). Regenerative strategies utilizing biomaterials and stem cells are promising for NP repair. Human NP tissue is highly viscoelastic, relaxing stress rapidly under deformation. However, the impact of tissue-specific viscoelasticity on the activities of adipose-derived stem cells (ASC) remains largely unexplored. Here, we investigated the role of matrix viscoelasticity in regulating ASC differentiation for IVD regeneration. Viscoelastic alginate hydrogels with stress relaxation time scales ranging from 100 s to 1000s were developed and used to culture human ASCs for 21 days. Our results demonstrated that the fast-relaxing hydrogel significantly enhanced ASCs long-term cell survival and NP-like extracellular matrix secretion of aggrecan and type-II collagen. Moreover, gene expression analysis revealed a substantial upregulation of the mechanosensitive ion channel marker TRPV4 and NP-specific markers such as SOX9, HIF-1α, KRT18, CDH2 and CD24 in ASCs cultured within the fast-relaxing hydrogel, compared to slower-relaxing hydrogels. These findings highlight the critical role of matrix viscoelasticity in regulating ASC behavior and suggest that viscoelasticity is a key parameter for novel biomaterials design to improve the efficacy of stem cell therapy for IVD regeneration. Statement of significanceSystematically characterized the influence of tissue-mimetic viscoelasticity on ASC. NP-mimetic hydrogels with tunable viscoelasticity and tissue-matched stiffness. Long-term survival and metabolic activity of ASCs are substantially improved in the fast-relaxing hydrogel. The fast-relaxing hydrogel allows higher rate of cell protrusions formation and matrix remodeling. ASC differentiation towards an NP-like cell phenotype is promoted in the fast-relaxing hydrogel, with more CD24 positive expression indicating NP committed cell fate. The expression of TRPV4, a molecular sensor of matrix viscoelasticity, is significantly enhanced in the fast-relaxing hydrogel, indicating ASC sensing matrix viscoelasticity during cell development. The NP-specific ECM secretion of ASC is considerably influenced by matrix viscoelasticity, where the deposition of aggrecan and type-II collagen are significantly enhanced in the fast-relaxing hydrogel.

Optimizing TPU performance: The role of mold temperature on injection molding of TPU

AbstractThis study explores the influence of mold temperatures below 60°C on thermoplastic polyurethane (TPU) properties during injection molding, focusing on phase separation and its impact on mechanical, thermal, and viscoelastic properties. Using a combination of micro‐indentation, temperature scanning stress relaxation, and conventional characterization methods, the research highlights how increased mold temperatures promote more distinct phase separation, enhancing mechanical stability and physical properties. The novel use of micro‐indentation revealed a gradient in material stiffness from the surface to the core of injection‐molded samples, attributed to differential cooling rates and shear forces, which affect phase separation and crystallinity of the hard domains. These insights are critical for applications requiring specific surface properties and underscore the importance of understanding the interplay between chemical composition and processing conditions for optimizing TPU properties. Furthermore, the paper shows that tensile testing, differential scanning calorimetry, and Shore hardness cannot quantify the effects of mold temperatures below 60°C. The research highlights the influence and importance of chemical composition, rheological history, and thermal history on the properties of TPU.

Creep Fatigue Analysis of the 316SS Monolith in the MegaPower Heat Pipe Reactor

The heat pipe reactor represents a promising high-temperature microreactor design comprising heat pipes, fuel rods, and monoliths. Prolonged operation at elevated temperatures leads to an obvious thermal creep and thermal stress within the monolith. The monolith may have structural failure due to creep damage and fatigue damage caused by temperature fatigue load. This paper presents an analysis of the creep fatigue damage in the monolith of the MegaPower heat pipe reactor using the American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code Section III, Division 5 (BPVC Sec. III, Div. 5) inelastic design-by-analysis rules. The research findings demonstrate pronounced stress relaxation in the monolith caused by thermal creep, resulting in a redistribution of thermal stress. The region experiencing peak thermal stress within the monolith transitions from the thinnest web between the fuel rods to the edge of the monolith after 50 000 h of operation at full power. Thermal creep results in a 40.5% decrease in peak thermal stress and a 0.023% increase in the displacement amplitude of the monolith. The creep fatigue damage in the monolith at full power for 50 cycles, each lasting 1000 h, adheres to the design rule limitation of the ASME BPVC. The damage is primarily concentrated in the thinnest web region at the edge of the monolith, predominantly attributed to creep damage. The creep fatigue damage check in the monolith should carefully consider the effect of stress relaxation.

Toward static and transient stress-constrained topology optimization for shell-infill structures

This paper contributes to a stress-constrained topology optimization approach for the design of shell-infill structures. Based on the density method, we utilize the two-step filtering and projection scheme and local volume constraint for generating the shell and non-uniform infills, respectively. The topology optimization formulation is defined as a Robust Minimum Compliance problem with Volume and Stress constraints (RMCVS), where the robust method is used to eliminate undesired topological characteristics. We globalize the static and transient stress constraints through the p-norm function, and tailor material interpolation and stress relaxation schemes for both cases, respectively, to avoid numerical difficulties. Sensitivity analysis is provided, and the derivatives of the objective function and constraints with respect to the design variable fields are derived. We then validate the suggested method through several 2D and 3D benchmarks. The results illustrate that the method is robust to generate various shell-infill structures while limiting the maximum static and transient stress.

Comparative Investigation of Nano-Sized Silica and Micrometer-Sized Calcium Carbonate on Structure and Properties of Natural Rubber Composites.

Fillers have been widely used in natural rubber (NR) products. They are introduced to serve as a strategy for modifying the final properties of NR vulcanizates. Silica and calcium carbonate (CaCO3) are among the fillers of choice when the color of the products is concerned. In this case, a special focus was to compare the vulcanizing efficiency of NR filled with two different filler types, namely nano-sized silica and micrometer-sized CaCO3. This study focused on the effects of the loading level (10-50 parts per hundred parts of rubber, phr) on the final properties and structural changes of NR composites. The results indicated that increased filler loading led to higher curing torques and stiffness of the rubber composites irrespective of the type of filler used. The better filler dispersion was achieved in composites filled with CaCO3 which is responsible for less polarity of CaCO3 compared to silica. Good filler distribution enhanced filler-matrix interactions, improving swelling resistance and total crosslink density, and delaying stress relaxation. The modulus and tensile strength of both composites also improved over the content of fillers. The CaCO3-filled composites reached their maximum tensile strength at 40 phr, exceeding, by roughly 88%, the strength of an unfilled sample. Conversely, the maximum tensile strength of silica-filled NR was at 20 phr and was only slightly higher than that of its unfilled counterpart. This discrepancy was ascribed to the stronger rubber-filler interactions in cases with CaCO3 filler. Effective rubber-filler interactions improved strain-induced crystallization, increasing crystallinity during stretching and reducing the strain at which crystallization begins. In contrast, large silica aggregates with poor dispersion reduced the overall crosslink density, and degraded the thermomechanical properties, tensile properties, and strain-induced crystallization ability of the NR. The results clearly indicate that CaCO3 should be favored over silica as a filler in the production of some rubber products where high performance was not the main characteristic.