Repetitive Loading Research Articles

Expression of myostatin and folistatin as a response to high-load strength training to counteract the loss of muscle mass in heart failure with preserved ejection fraction

Abstract Background and Purpose Muscle loss is a common complication in patients with heart failure (HF) associated with a significant increase in morbidity and mortality. Myokines (muscle proteins) play a crucial role in the pathophysiology of the disease; in particular, an imbalance in myostatin and follistatin has been reported. Although strength training (ST) within a cardiac rehabilitation (CR) program can modulate the expression of these myokines, its effect in patients with HF with preserved ejection fraction (HFpEF) is unknown. We hypothesize that in this population, ST with high load and low repetitions can mitigate muscle cachexia with better myiokine overexpression in this population. Our objective is to evaluate the impact of a CR program with emphasis on high-load, low-repetition ST on the regulation of these muscle proteins, muscle gain and cardiorespiratory fitness (CRFit) in patients with HFpEF. Methods Quasi-experimental study of a CR program (24 sessions of 90 minutes, 3 times a week, for 8 weeks), with the following structure: warm-up (10 minutes), 30-minute ST of high loads (>85% maximum repetition, 1MR) with low repetitions (3-5) and rest times between sets of 2 minutes; then 30 minutes of aerobic resistance training at moderate intensity (60-80% of maximum heart rate) and finally breathing exercises and cool-down (10 minutes). Myokines were determined by immunoassays. Muscular strength was determined by 1MR in biceps, triceps, lateral deltoid, quadriceps, hamstrings, gastrocnemius and soleus. For CRFit, maximum oxygen consumption (VO2) was measured and six-minute walk test protocol was also performed. Pre and post-intervention comparison was made using paired t test, Pearson's correlation was used in data analysis and a value of p<0.05 was considered significant. Results 59 patients were admitted, of which 8 of them left due to decompensation. Of the remaining 51 (62.7% male), the mean age was 66±8 years, all of them with HFpEF (M:55±2.4% vs W:54±2.1%; p=0.743). The other diagnoses in the study population were myocardial revascularization (52.94%), angioplasty (29.41%), valve replacement (11.6%), arterial hypertension (60%) and obesity (35%). After CR program it was observerd an improvement in VO2 (from 9.95±2.6 to 13.32±1.7; p=0.022) and meters traveled (243±41 to 349±14; p=0.002), and a decrease in myostatin levels (3337.1±69.2 to 2448.0±70.6; p=0.001) and elevation of follistatin (2167.4±96.9 to 3231.6±67.7; p=0.001). We found a strong positive correlation between follistatin levels and muscle percentage (r=0.8947 Pearson, graph1) and a strong negative correlation with myostatin and muscle percentage (r= -0.8034 Pearson, graph2). Conclusion A phase II CR program with a ST component based on high loads and low repetitions improved muscle strength and mass, FitCR and distance traveled in patients with HFpEF, without modify hemodynamic function. Follistatin levels increased by 49.09% and myostatin levels decreased by 26.65%.

Fatigue Behavior of H-Section Piles under Lateral Loads in Cohesive Soil

Most structures supporting solar panels are found on thin-walled metal piles partially driven into the ground, optimizing costs and construction time. These pile foundations are subjected to repetitive lateral loads from various external forces, such as wind, which can compromise the integrity of the pile-soil system. Given that the expected operational lifespan of photovoltaic solar plants is generally 20–30 years, predicting their service life under fatigue loads is crucial. This research analyzes the response of H-section piles to lateral fatigue loads in cohesive rigid soils through four field tests, subjected to load cycles of 55%, 72%, and 77% of the static failure load, corresponding to maximum loads of 25 kN, 32 kN, and 35 kN, respectively. Additionally, the effect of load cycles on the degradation of pile-soil adhesion is studied through two pull-out tests following cyclic tests. This study reveals that soil fatigue does not occur under repetitive loads and that soil stiffness remains constant once the cycles causing soil compaction have been overcome. Nevertheless, the accumulated plastic deflection of the soil increases steadily once soil compaction occurs due to cyclic loading. The implications of these results on the fatigue life of photovoltaic solar panel foundations are discussed.

Stress fracture of bone under physiological multiaxial cyclic loading: Activity-based predictive models

While it is known that excessive accumulation of fatigue damage from daily activities contributes to fracture, a model of bone failure under physiologically relevant multiaxial cyclic loading needs to be developed in order to develop effective management strategies for stress or fatigue fractures. The role of strain-induced damage from repetitive loading is a strong candidate for such a model, as cycles of mechanical loading leading to failure can be measured directly. However, this approach has been limited by the restrictions of uniaxial loading models, which often overestimates the fatigue life of bone and suggests that bone will only break well beyond the realistic limits of exercise. To address this gap and develop a physiologically relevant model, our study leverages the power of four commonly used engineering failure criteria as a model for multiaxial loading using a cohort of human tibiae from cadaveric donors (age range 21–85 years old). Four failure criteria (Von Mises, Tsai-Wu, Findley critical plane, and maximum shear strain) were found to be effective in vitro models of tibial fracture when age groups of donors were combined (r2 > 0.84) and stratified (younger: 21–52-years-old versus older: 57–85-years-old) (r2 > 0.83) (p < 0.001). Each failure criterion displayed distinctly lower fatigue curves for the older age group. The maximum shear strain model was used to determine the efficacy of this approach to predict fatigue fractures in humans using published in vivo data from human volunteers. Consistent with in vivo observations in general population, the model demonstrated failure at 5000 to 200,000 loading cycles depending on activities such as jumping, sprinting, and walking, with a 3-fold reduction of fatigue life in older donors. These findings dramatically improve estimates of fatigue life under repetitive loading and demonstrate how age-related changes in bone significantly increase its propensity for fatigue-induced fractures.

Temperature Effects on Traffic Load-Induced Accumulating Strains in Flexible Pavement Structures

AbstractRutting is a major distress mode in flexible pavements, results from the repetitive loading caused by traffic movement. Pavement deformation consists of both recoverable (elastic) and unrecoverable (plastic) components. The continuous movement of vehicles contributes to the overall deformation in the flexible pavement system, involving all pavement components. In regions with hot climates or in the hot summer season, rutting tends to be more prominent due to the substantial reduction in the viscosity of the asphalt binder. This decrease in viscosity, which is inversely linked to rutting, occurs as temperatures rise, leading to a heightened susceptibility of the Hot Mix Asphalt (HMA) blend to rut formation. However, according to studies, a significant amount of permanent deformation takes place in the unbound layers beneath the asphalt course, it is therefore essential to prioritize attention on these layers. Temperature exerts besides viscosity a substantial impact on asphalt stiffness, leading to the transfer of higher vertical deviatoric stresses to the unbound layers beneath the asphalt course (base, subbase, subgrade). This research presents a study integrating the High Cycle Accumulation (HCA) model into a laminar model to determine permanent deformations in the unbound granular layer of flexible pavements and taking into account the temperature dependent stiffness of asphalt. Rutting depths at the end of the design lifetime were computed, accounting for seasonal stiffness variations. It was shown that the softer asphalt behavior significantly increases the development of ruts in the underlaying soil layers. The findings were compared with results obtained from mean annual temperature and the typical equivalent asphalt stiffness utilized in fatigue tests. Additionally, an analysis was conducted to assess whether the timing of road implementation influences settlements throughout the design lifetime. The results suggest that the sequence of seasons is most relevant during the first year of service, showing a distinct effect at that time. However, with a higher number of axle passes, the initial differences fade away, and the curves start to merge.

Experimental insights into the seismic behaviour of flanged URM walls

Considerable attention has been given to understanding and modelling the seismic response of unreinforced masonry piers with rectangular cross-sections. However, the seismic performance of load-bearing masonry walls with flanges, commonly found in actual building configurations, remains less explored. This study investigates the behaviour of flanged walls through cyclic quasi-static tests on four full-scale C-shaped specimens comprising two primary seismic-resistant walls (webs) interconnected by a single flange. The construction of the specimens varied: two were built using standard solid calcium-silicate bricks with general-purpose mortar, while the other two utilised large-sized calcium-silicate blocks with tongue-and-groove interlocks and a thin layer of cement-based adhesive mortar. All specimens were subjected to cyclic horizontal loading parallel to the webs under consistent overburden load and double-fixed boundary conditions. The primary variable in testing the two walls of each type was the number of cyclic loading repetitions. This paper initially describes the key characteristics of the wall specimens, including geometry, construction details, and mechanical properties. Subsequently, it details the instrumentation plan and test protocol, and summarises the major observations from the tests, illustrating damage evolution and hysteretic force-displacement response. The results highlight the effect of the flange on initial stiffness and lateral force resistance, as well as the influence of block type and loading repetitions on failure mode, displacement capacity, and energy dissipation. Additionally, the study demonstrates the impact of floor slab uplift due to in-plane rocking of the webs on the top boundary conditions and the out-of-plane stability of the flange.

Alterations in magnitude and spatial distribution of erector spinae muscle activity in cyclists with a recent history of low back pain.

While cycling offers several health benefits, repetitive loading and maintenance of static postures for prolonged periods expose cyclists to low back pain (LBP). Despite high LBP prevalence in cyclists, underlying pathomechanics and specific lumbar region muscle activation patterns during cycling are unclear. Here, we compared lumbar erector spinae (ES) muscles activation and spatial distribution activity in cyclists with and without recent LBP history. Ten cyclists with recent LBP history (LBPG; Oswestry Disability Index score ~ 17.8%) and 11 healthy cyclists (CG) were recruited. After assessing the Functional Threshold Power (FTP), participants underwent an incremental cycling test with 4 × 3min steps at 70%, 80%, 90%, and 100% of their FTP. High-density surface electromyography (HDsEMG) signals were recorded from both lumbar ES using two 64-channel grids. Information about ES activation levels (root-mean-square, RMS), degree of homogeneity (entropy), and cranio-caudal displacement of muscle activity (Y-axis coordinate of the barycenter of RMS maps) was extracted from each grid separately and then grand-averaged across both grids. Repeated-measure 2-way ANOVAs showed a significant intensity by group interaction for RMS amplitude (p = 0.003), entropy (p = 0.038), and Y-bar displacement (p = 0.033). LBPG increased RMS amplitude between 70-100% (+ 19%, p = 0.010) and 80-100% FTP (+ 21%, p = 0.004) and decreased entropy between 70-100% FTP (-8.4%, p = 0.003) and 80-100% FTP (-8.5%, p = 0.002). Between-group differences emerged only at 100% FTP (+ 9.6%, p = 0.049) for RMS amplitude. Our findings suggest that cyclists with recent LBP history exhibit higher ES muscles activation and less homogeneous activity compared to healthy controls, suggesting potential inefficient muscle recruitment strategy. HEC-DSB/09-2023.

Enhanced In-situ Measurement and Evaluation Methods for Subgrade Modulus Utilizing Falling Weight Deflectometer

Falling Weight Deflectometer (FWD) tests have gained increasing popularity in evaluating the stiffness modulus of pavement subgrade. This paper presents an enhanced method for in-situ testing and evaluation of subgrade modulus using FWD apparatus. The method comprises three components: 1) guidelines for conducting field FWD tests with appropriate loading magnitudes and repetitions; 2) selection of optimal deflection positions for modulus back-calculation; and 3) determination of the suitable depth of the semi-infinite rigid layer. This method was developed based on measurements and analysis of FWD data from five highway subgrade sites, covering various subgrade soil types and climatic conditions. The enhanced method helps to promote the robustness and accuracy of modulus back-calculation results. Furthermore, to validate the proposed method, the back-calculated modulus was compared with moduli obtained from the other two commonly used tests, named the Portable Seismic Pavement Analyzer (PSPA) modulus test and the triaxial modulus test. Comparison results indicate strong correlations between the modulus values obtained from the proposed method and those from the PSPA and triaxial modulus tests, affirming the practicality of the proposed method in determining subgrade stiffness modulus.

Multiple blast behavior of steel wire mesh reinforced geopolymer based high performance concrete (G‐HPC) slab: Experiment and numerical simulation

AbstractEngineering structures face the potential of encountering repetitive or multiple blast loads stemming from accidental explosions and terrorist attacks. However, current research in this field is still relatively limited, and further investigation is needed to understand the damage mechanisms of structures under multiple explosions. Therefore, this study explores the blast resistance of G‐HPC slabs reinforced with steel wire mesh (SWM) under multiple blast loads. The failure modes of the SWM‐reinforced G‐HPC slab were experimentally studied under two consecutive explosions (with explosive equivalents of 1.6 and 3.2 kg, both at a standoff distance of 0.4 m). The results revealed that, after two consecutive explosions, the slab exhibited bulging with minimal concrete spalling, showcasing overall integrity. Subsequently, a numerical model was established, followed by a comprehensive parameter analysis. The parameter analysis investigated the effects of SWM diameters and grid size, the arrangement of SWM, and the sequence of TNT equivalents on the performance of the slab under three consecutive blast loads. The findings revealed that increasing the SWM diameter or reducing the grid size significantly enhanced the blast resistance of the slab under three consecutive explosive loads. Strategically arranging the SWM in the tensile zone reduced damage and deflection. Furthermore, the sequence of TNT equivalents had a notable impact on the damage and energy absorption of the slab.

Pemanfaatan Limbah Serbuk Kayu dengan Zat Aditif Anti Stripping Agent Terhadap Uji Durabilitas AC-WC (Asphalt Concrete–Wearing Course)

The increasing number of vehicles each year in Indonesia causes excessive repetition of loads that occur on road pavements, as well as extreme weather changes resulting in damage to the pavement layer, especially at the level of durability, especially in the AC-WC layer. The use of sawdust waste in the AC-WC mixture has met the marshall test values/characteristics, but it is necessary to test its durability; a high durability value indicates that the road is more durable and resistant to weather and water. Therefore, it is necessary to improve the quality of road pavement by adding anti-stripping agent additives at levels of 0.3%, 0.4%, and 0.5% to the AC-WC mixture of 25% wood powder waste substitution in order to improve the properties of asphalt in increasing the coating of asphalt with aggregates in a wet state to produce stronger bonds to extend the life of the road. This study aims to determine the effect of adding anti-stripping agent additives in AC-WC mixtures with sawdust waste on durability tests. Experimental tests were conducted using the research method at the Civil Engineering Laboratory of Diponegoro University Vocational School Semarang. The study results obtained a durability value based on the calculation parameters of the Residual Strength Index (IKS) based on the General Specifications of Bina Marga 2018 (Rev 2), which shows the durability of AC-WC asphalt. The results of marshall testing obtained based on averages include VIM 4.11%, VMA 13.15%, VFA 77.33%, Stability 1893.73 Kg, Flow 2.17 mm and MQ 1070.03 kg/mm from 25% wood powder waste variation with anti-stripping agent additives 0.3%; 0.4%; 0.5% in AC-WC wear layer has met the General Specifications of Bina Marga 2018 (Rev 2). The average durability values of IKS with sequential soaking time are 24 hours 92.24%, 48 hours 88.87%, and 72 hours 80.08%.

Variation of MEMS Thin Film Device Parameters under the Influence of Thermal Stresses.

With the advancement of semiconductor manufacturing technology, thin film structures were widely used in MEMS devices. These films played critical roles in providing support, reinforcement, and insulation in MEMS devices. However, due to their microscopic dimensions, the sensitivity of their parameters and performance to thermal stress increased significantly. In this study, a Pirani gauge sample with a multilayer thin film structure was designed and fabricated. Based on this sample, finite element modeling analysis and thermal stress experiments were conducted. The finite element modeling analysis employed a combination of steady-state and transient methods to simulate the deformation and stress distribution of the device at room temperature (25 °C), low temperature (-55 °C), and high temperature (125 °C). The thermal stress test involved placing the sample in a temperature cycling chamber for temperature cycling tests. After the tests, the resonant frequency and surface deformation of the device were measured to quantitatively evaluate the impact of thermal stress on the deformation and resonant frequency parameters of the device. After the experiments, it was found that the clamped-end beams made of Pt were a stress concentration area. Additionally, the repetitive thermal load caused the cantilever beam to move cyclically in the Z direction. This movement altered the deformation of the film and the resonant frequency. The suspended film exhibited concavity, and the overall trend of the resonant frequency was downward. Over time, this could even lead to the fracture of the clamped-end beams. The variation of mechanical parameters derived from finite element simulations and experiments provided an important reference value for device design improvement and played a crucial role in enhancing the reliability of thin film devices.

Experimental evaluation of bonded/unbonded prestressed precast concrete joints under repetitive loading scenarios: Seismic performance and retrofit intervention

Recently, demands for mitigating structural damage and post-earthquake repair costs have spurred considerable research on prestressed precast frames. In this study, quasi-static tests were conducted on bonded and unbonded prestressed precast beam-column joints (BPPJs and UPPJs) to investigate the influences of bonded/unbonded post-tensioned (PT) strands, evaluate the performance of BPPJs under repeated loading, and verify the effectiveness of post-earthquake retrofitting with external buckling-restrained rods (BRRs). Additionally, the difference between the bonded and unbonded joints was further revealed from the perspective of the tensile forces of the strands at the beamcolumn interface. Test results showed that compared with the UPPJ, the BPPJ exhibited superior performance in terms of lateral stiffness, lateral resistance, and energy dissipating capacity, with a 76.4 % increase in the load-bearing capacity. The compressive zone at the beam end was also deeper in the BPPJ, accompanied by a denser distribution of compressive cracks in the concrete. Furthermore, the bond between the strands and grouting material in the duct accelerated the increase in the tensile forces of the strands at the interface, resulting in considerably more prestress loss in the BPPJ after the tests, which was 120.0 % greater than that in the UPPJ. Under repetitive loading scenarios, although the yielding resistance of the BPPJ decreased by 35.0 % compared with that of the first loading owing to the reduced tensile forces in the strands, relatively minor variations in the load-bearing capacity and energy dissipation were observed. After retrofitting with BRRs, the BPPJ exhibited substantial enhancements in both lateral stiffness and resistance, validating the effectiveness of post-earthquake retrofitting with BRRs.

Enhanced understanding on permanent deformation behaviour of subgrade compacted clay under long-term cyclic loading

The long-term performance of pavement structures is heavily reliant on the sustained load-carrying capacity of the subgrade soil. Under repetitive traffic loads, permanent deformation (PD) gradually accumulates in the subgrade due to plastic yielding and soil particle rearrangement, which can compromise the serviceability and durability of overlying pavement layers. This study aimed to enhance the understanding of compacted clay response under long-term cyclic loads through a systematic repeated load triaxial (RLT) testing approach. The proposed approach considered depth-dependent static and dynamic stresses exerted on compacted clay beneath pavement structures and traffic loads. A series of RLT tests were conducted to investigate the impact of key factors, including soil properties (moisture content and compaction degree), stress conditions (confining pressure and deviator stress), and load characteristics (load duration and rest period), on the PD behaviour of compacted clay subgrade. Stress-strain hysteresis loops and damping ratios were analyzed to enhance the fundamental understanding of subgrade PD evolution. The results showed that higher moisture content and lower compaction degree significantly increased PD, with the PD response transitioning from plastic shakedown to plastic creep. Greater deviator stress also exacerbated PD accumulation. Variations in loading duration and rest period influenced the PD behaviour, demonstrating the importance of accurately simulating the stress history experienced by subgrade soil elements under traffic loading. The findings provide valuable insights to optimize subgrade design and implement performance-based management of pavements.

MCM2-7 loading-dependent ORC release ensures genome-wide origin licensing

Origin recognition complex (ORC)-dependent loading of the replicative helicase MCM2-7 onto replication origins in G1-phase forms the basis of replication fork establishment in S-phase. However, how ORC and MCM2-7 facilitate genome-wide DNA licensing is not fully understood. Mapping the molecular footprints of budding yeast ORC and MCM2-7 genome-wide, we discovered that MCM2-7 loading is associated with ORC release from origins and redistribution to non-origin sites. Our bioinformatic analysis revealed that origins are compact units, where a single MCM2-7 double hexamer blocks repetitive loading through steric ORC binding site occlusion. Analyses of A-elements and an improved B2-element consensus motif uncovered that DNA shape, DNA flexibility, and the correct, face-to-face spacing of the two DNA elements are hallmarks of ORC-binding and efficient helicase loading sites. Thus, our work identified fundamental principles for MCM2-7 helicase loading that explain how origin licensing is realised across the genome.

Post-Tensioned Hollow-Core Concrete Slabs with Unbonded Tendons for Truck Scale Platforms: Design Assumptions and Tests.

At Cracow University of Technology, attempts were made to develop national truck scale platforms with a capacity of 60 tons, made from prestressed concrete. For this work, we designed slabs partially prestressed with unbonded tendons featuring a cross-section of 1.00 × 0.28 m and a span of 5.94 m. To reduce the weight of the slabs, four channels made from commonly used ø110 × 2.2 mm PVC pipes were used. In this way, we created post-tensioned hollow-core slabs. Due to the unpredictable behavior of slabs operating in a cracked state under a repetitive load, two slabs were subjected to cyclic loads amounting to 1,000,000 cycles with different load values. This paper presents the basic design principles and design details of the slabs, as well as the methodology and results of the research conducted. Lastly, we provide appropriate conclusions directed at further optimizing the slabs.

Differences in the Size of Individual Plantar Intrinsic Foot Muscles Between Ballet Dancers and Non-Dancers.

In classic ballet, choreography often involves tiptoe standing. Tiptoe standing requires a high and stable foot arch structure, which is achieved by contraction of the plantar intrinsic foot muscles (PIFMs). Long-term repetitive loading with a specific movement can induce hypertrophic adaptation of the associated muscles. For dancers, however, limited information on the size of individual PIFMs is available from previous studies. The purpose of this study was to determine the differences in the sizes of 10 individual PIFMs between dancers and non-dancers. Muscle volumes (MVs) of 10 individual PIFMs were measured using magnetic resonance imaging in 15 female dancers and 15 female non-dancers. Muscles analyzed included abductor hallucis, flexor digitorum brevis, abductor digiti minimi, quadratus plantae, lumbricals, flexor hallucis brevis, adductor hallucis oblique head, adductor hallucis transverse head, flexor digiti minimi, plantar/dorsal interossei. In addition to absolute MVs, relative MVs normalized to body mass (rMVBM) and the percentage of individual MVs relative to the sum of 10 individual PIFM MVs (%MVWHOLE) were calculated. The absolute MVs of 6 individual PIFMs, including the flexor digitorum brevis and lumbricals, were +16% to 59% larger in dancers than in non-dancers (P ≤ .048). The rMVBM of all individual PIFMs were +35% to 95% larger in dancers than in non-dancers (P ≤ .019). The %MVWHOLE of the flexor digitorum brevis and lumbricals were +10% to 36% higher (P ≤ .014) and those of the abductor digiti minimi and adductor hallucis oblique head were +8% to 11% lower (P ≤ .037) in dancers than in non-dancers. For all 3 MV measures, only the flexor digitorum brevis and lumbricals, which are functionally specialized for flexion of the second to fifth metatarsophalangeal joints, were consistently larger in dancers than in non-dancers. This may be due to long-term repetitive loading on these PIFMs during ballet training involving tiptoe standing.

Quantifying the flexural stiffness changes in the concrete beams with externally bonded carbon fiber sheets under elevated environment temperature

Structural strengthening solutions typically employ externally bonded reinforcement (EBR) systems with carbon fiber (CF) sheets because of these materials’ lightweight, corrosion resistance, and electromagnetic immunity. However, elevated ambient temperatures can negatively impact the mechanical performance of EBR, as documented in the literature. Therefore, American bridge design specifications assume a concrete surface temperature of 60 °C (140 °F), which reflects the extreme conditions that bridge structures may experience, particularly in regions with intense sunlight and high ambient temperatures. Therefore, sustainable design solutions require a reliable quantification of the effects of temperature. This study extends a recently developed bending test layout and builds the analytical modeling procedure to quantify the stiffness degradation under repeated temperature and mechanical loads. The explicitly obtained equivalent stresses in the tensile concrete determine the stiffness measure independent of the loading condition and sequence. This test program used 12 laboratory samples. All beam samples faced the mechanical load repetitions and entire unloading. Three selected beams were additionally treated at 60 °C for 10 hours in a heating chamber between the loading repetitions. These tests identified a substantial (three times) decrease in the bonding performance of the CF sheets after eight heating rounds. Scanning electron microscopy (SEM) identified the corresponding microstructure changes.

Substructure Evolution and Mechanical Properties in Martensitic Stainless Steel During a High Cycle Number of Repetitive High Stress Loading–Unloading Process

Substructure Evolution and Mechanical Properties in Martensitic Stainless Steel During a High Cycle Number of Repetitive High Stress Loading–Unloading Process

Cyclic Mechanism Affects Lumbar Spine Creep Response.

This study aims to explore how cyclic loading influences creep response in the lumbar spine under combined flexion-compression loading. Ten porcine functional spinal units (FSUs) were mechanically tested in cyclic or static combined flexion-compression loading. Creep response between loading regimes was compared using strain-time histories and linear regression. High-resolution computed tomography (µCT) visualized damage to FSUs. Statistical methods, ANCOVA and ANOVA, assessed differences in behavior between loading regimes. Cyclic and static loading regimes exhibited distinct creep response patterns and biphasic response. ANCOVA and ANOVA analyses revealed significant differences in slopes of creep behavior in both linear phases. Cyclic tests consistently showed endplate fractures in µCT imaging. The study reveals statistically significant differences in creep response between cyclic and static loading regimes in porcine lumbar spinal units under combined flexion-compression loading. The observed biphasic behavior suggests distinct phases of tissue response, indicating potential shifts in load transfer mechanisms. Endplate fractures in cyclic tests suggest increased injury risk compared to static loading. These findings underscore the importance of considering loading conditions in computational models and designing preventive measures for occupations involving repetitive spinal loading.

Development of a cyclic semi-circular bending test protocol to characterize fatigue cracking of asphalt mixture at intermediate temperature

The Semi-Circular Bending (SCB) test has been traditionally conducted in a monotonic loading and displacement-controlled mode at intermediate temperature to assess the fatigue crack resistance of asphalt mixtures. However, fatigue damage is essentially deterioration in material integrity as a result of repeated loading. The SCB monotonic loading does not realistically reflect the traffic loading repetition. This may have limitation on mechanism of the fatigue cracking in asphalt mixture. This study aims to develop a cyclic SCB test protocol to characterize the fatigue cracking of asphalt mixtures. Digital Image Correlation (DIC) was utilized to identify crack length associated with the number of loading cycles applied. A full factorial design was conducted for the optimization of testing parameters of the cyclic SCB test, including three levels of notch depth and loading frequency. Two indicators including the number of cycles to failure and Paris’ Law coefficients were used to characterize the fatigue cracking resistance of asphalt mixture. The proposed test protocol of cyclic SCB was validated by three asphalt mixtures of good, medium, and poor cracking performance. The critical strain energy release rate Jc of three mixtures is also measured by the monotonic SCB test for studying the relationship between Jc and Paris’ Law coefficients. The results indicate that Paris’ Law coefficients have relatively low variance compared to the number of cycles to failure and are independent of notch depth and loading frequency. The optimal testing parameter combination of 25 mm notch depth and 10 Hz loading frequency was recommended in perspective of test variance and test practicability. The Paris Law coefficient n ranking was consistent with the one of fatigue cracking resistance of asphalt mixtures evaluated.

Numerical investigation of wellbore damage due to drill string lateral vibration

During drilling operations, cyclic loading is exerted on the wellbore wall by the vibrations of the drill string. This loading could lead to rock fatigue, which in turn might result in wellbore failure. In this study, a numerical model is developed to simulate the effects of repeated loading on rock fatigue and failure. The simulation is based on an elasto-plastic constitutive model coupled with a damage mechanics approach, which allows us to examine the wellbore instability due to drill string vibrations. The model is verified with the existing data in the literature related to experiments on impact of a steel ball against a curved wall. The findings indicate that cyclic loading increases the development of plastic strain around the wellbore significantly compared to static conditions, promoting rock fatigue. Furthermore, the cyclic loading expands the radius of the yielded zone substantially, a critical factor for maintaining wellbore integrity. The proposed model can be used to evaluate the wellbore stability under repetitive loading caused by the drill string action.