Stress Gradient Research Articles

The impact of species phylogenetic relatedness on invasion varies distinctly along resource versus non‐resource environmental gradients

Abstract Understanding why some, but not other, plant communities are vulnerable to alien invasive species is essential for predicting and managing biological invasions. Darwin proposed two seemingly contradictory hypotheses on how native‐invader relatedness influences invasion success, emphasizing, respectively, the importance of environmental filtering and competition between natives and invaders. Despite much recent empirical research on this topic, reconciling these two hypotheses, known as Darwin's naturalization conundrum, remains a challenge. Using plot‐level data from natural forests along elevational transects covering strong environmental gradients, we examined whether the invasion of the globally invasive species crofton weed (Ageratina adenophora) can be explained by environmental filtering and/or competition from closely related species linked to environmental gradients. Abundant precipitation, warm temperatures, open canopies and postfire environments facilitated A. adenophora invasion, whereas resident taxonomic richness suppressed its invasion. Importantly, we found that invader‐resident relatedness had a strong negative effect on invader cover under resource scarcity conditions (e.g. low water availability), but not under non‐resource environmental stress gradients (e.g. low temperature). Synthesis and applications. Our results suggest that the impact of species phylogenetic relatedness on invasion success varies distinctly along resource versus non‐resource environmental gradients. These results help to reconcile Darwin's naturalization conundrum, thereby improving the ability to predict the success of alien plant invasions in a changing world. Our study stresses the need to consider adjusting forest species composition to strengthen their resistance to invasion, while taking into account resource and non‐resource environmental gradients, particularly after wildfires.

The combined effects of a heterogeneous porosity distribution and stress gradient on the high cycle fatigue behavior of high pressure die cast AlSi9Cu3

In order to expand the number of possible applications of High Pressure Die Cast (HPDC) components using the AlSi9Cu3 alloy, the automotive industry is giving greater attention to the fatigue design of components produce by this process-material combination. The process is well known for introducing a heterogeneous distribution of casting defects. This study explores the impact of the heterogeneous porosity distribution and stress gradients on the high cycle fatigue behavior of HPDC AlSi9Cu3 components. Fatigue tests under fully-reversed tension–compression and fully-reversed bending loads are used to investigate the influence of porosity and stress gradients on the fatigue behavior, by machining different specimen thicknesses. The results demonstrate that stress gradients and the heterogeneous porosity distribution determine the material’s fatigue response, including both the fatigue strength and crack initiation mechanisms. The Murakami parameter and local stress level at the critical defect make it possible to interpret the gradient effects. The nature of the entrapped gas in internal defects may influence the in-bulk crack propagation behavior. The comparison of fracture mechanics models (El Haddad, Murakami, Kitagawa-Takahashi) make it possible to highlight the link between the El Haddad’s parameter to parameters that are more microstructurally based.

Low cycle fatigue prediction for cylinder heads considering stress gradient and local yielding

Due to the complex geometry and thermo-mechanical loading, cylinder heads is the most challenging parts among all parts engines. It must endure cyclic thermo-mechanical stresses throughout their lifetime. In this paper, the low cycle fatigue (LCF) life analysis of the cylinder heads considering the stress gradient and local yielding, is performed using the finite element method and ANSYS software to predict the temperature and stresses, and then, LCF life using Morrow theory and nCode Design Life software. Elastic and plastic properties of cylinder heads obtained by LCF tests at different temperatures. The results of finite element analysis (FEA) showed that the maximum temperature and stress values in the cylinder heads are 220.29°C and 89.868 MPa and the position is at the valve bridge between exhaust valves. The numerical results showed that bridge between exhaust valves and valve seats are areas susceptible to fatigue cracks. The LCF life results showed that 1073 cycles is the minimum fatigue life and occurs between exhaust valves. 400-h durability test showed no rupture in different parts of the cylinder heads.

Numerical Simulation Study on the Mechanics and Pore Characteristics of Tectonically Deformed Coal under Multi-Level and Multi-Cycle Loading and Unloading Conditions

Horizontal well cavern completion and stress release is considered a potential technique for efficient development of coalbed methane in tectonically deformed coal (TDC). Pulsating loading and unloading is a key technique for the controlled expansion of caverns and broader stress release within the reservoir. However, current understanding of the mechanical characteristics and pore network structure evolution of TDC under cyclic loading and unloading conditions is still limited. This paper employs numerical simulation methods to study the mechanical behavior and damage characteristics of TDC under cyclic loading and unloading. After obtaining a set of micromechanical parameters reflecting the behavior of TDC samples under triaxial compression in high-stress states, the effects of different stress gradients and cyclic amplitudes on the stress–strain curve, porosity changes, and crack propagation in TDC samples were analyzed. The study results indicate that under various cyclic loading and unloading conditions, the mechanical response characteristics of TDC samples are broadly similar, primarily divided into compression, slow expansion, and accelerated expansion phases. Under low unloading level conditions, the volume expansion of TDC samples is minimal. Also, at the same unloading level, the strain increment decreases with an increasing number of cycles. Correspondingly, under these conditions, the porosity and microcrack expansion in TDC are less than in high-stress gradient scenarios. Under the same unloading level but different amplitudes, the volume expansion rate at 50% unloading amplitude is higher than at 1 MPa unloading amplitude for TDC, with an increased number of crack expansions. Therefore, under cyclic loading conditions, the sensitivity of crack propagation within TDC samples to amplitude is greater than that to unloading level. Under actual pulsating excitation conditions, a low-amplitude, low-stress gradient pulsation method should be used to maintain the stability of horizontal well caverns, and gradually increase the cyclic amplitude to achieve the efficient extraction of coalbed methane in TDC reservoirs. The findings of this study can serve as an important reference for optimizing process parameters in cyclic pulsating stress release engineering for TDC.

Compressive stress gradients direct mechanoregulation of anisotropic growth in the zebrafish jaw joint.

Mechanical stimuli arising from fetal movements are critical factors underlying joint growth. Abnormal fetal movements negatively affect joint shape features with important implications for joint health, but the mechanisms by which mechanical forces from fetal movements influence joint growth are still unclear. In this research, we quantify zebrafish jaw joint growth in 3D in free-to-move and immobilised fish larvae between four and five days post fertilisation. We found that the main changes in size and shape in normally moving fish were in the ventrodorsal axis, while growth anisotropy was lost in the immobilised larvae. We next sought to determine the cell level activities underlying mechanoregulated growth anisotropy by tracking individual cells in the presence or absence of jaw movements, finding that the most dramatic changes in growth rates due to jaw immobility were in the ventrodorsal axis. Finally, we implemented mechanobiological simulations of joint growth with which we tested hypotheses relating specific mechanical stimuli to mechanoregulated growth anisotropy. Different types of mechanical stimulation were incorporated into the simulation to provide the mechanoregulated component of growth, in addition to the baseline (non-mechanoregulated) growth which occurs in the immobilised animals. We found that when average tissue stress over the opening and closing cycle of the joint was used as the stimulus for mechanoregulated growth, joint morphogenesis was not accurately predicted. Predictions were improved when using the stress gradients along the rudiment axes (i.e., the variation in magnitude of compression to magnitude of tension between local regions). However, the most accurate predictions were obtained when using the compressive stress gradients (i.e., the variation in compressive stress magnitude) along the rudiment axes. We conclude therefore that the dominant biophysical stimulus contributing to growth anisotropy during early joint development is the gradient of compressive stress experienced along the growth axes under cyclical loading.

Notch structural stress theory: Part II predicting total fatigue lives of notched structures

Notch fatigue problems are among the most common topics for the reliability analysis and structural design of mechanical engineering components. In previous work, we proposed the Notch Structural Stress (NSS) method based on fracture mechanics for reflecting the notch effect on fatigue crack propagation lives. However, like other regular fatigue assessing procedures, NSS also cannot accurately predict the fatigue life of notched structures with slower stress gradients due to the inconsistent slope of their S-N curves. This paper presents the Theory of Notch Structural Stress (TNSS) for predicting total fatigue lives of notched structures, which overcomes the life prediction limitations caused by slope differences in S-N curves and constructs a life-predicting frame suitable for different notch types even including plain and sharp notches. Compared with the often-used stress-concentration-assessing procedures, TNSS provides a more effective consolidation of the S-N data from different notch types, mostly since it takes both Initiation and Propagation state resistance into account at the same time.

Microbial community and network responses across strong environmental gradients: How do they compare with macroorganisms?

The way strong environmental gradients shape multispecific assemblages has allowed us to examine a suite of ecological and evolutionary hypotheses about structure, regulation and community responses to fluctuating environments. But whether the highly diverse co-occurring microorganisms are shaped in similar ways as macroscopic organisms across the same gradients has yet to be addressed in most ecosystems. Here, we characterize intertidal biofilm bacteria communities, comparing zonation at both the "species" and community levels, as well as network attributes, with co-occurring macroalgae and invertebrates in the same rocky shore system. The results revealed that the desiccation gradient has a more significant impact on smaller communities, while both desiccation and submersion gradients (surge) affect the larger, macroscopic communities. At the community level, we also confirmed the existence of distinct communities within each intertidal zone for microorganisms, similar to what has been previously described for macroorganisms. But our results indicated that dominant microbial organisms along the same environmental gradient exhibited less differentiation across tidal levels than their macroscopic counterparts. However, despite the substantial differences in richness, size and attributes of co-occurrence networks, both macro- and micro-communities respond to stress gradients, leading to the formation of similar zonation patterns in the intertidal rocky shore.

Effect of laser shock peening on fretting wear behaviour of AISI 304 stainless alloy

Unlike many other surface treatment techniques, such as shot peening (SP), low plasticity burnishing (LPB), ultrasonic surface rolling process (USRP), ultrasonic nanocrystal surface modification (UNSM), laser shock peening (LSP) is a non-contact process. This means that LSP achieves surface modification without direct physical contact, thereby eliminating the risk of media contamination and undesired wear on the peening equipment. As a surface hardening technology, LSP tends to produce a hardened layer with a high gradient of residual stress (RS) in the near-surface, significantly enhancing wear resistance. The well-known wear theories applied for numerical prediction, whether Archard model or dissipated frictional-energy model, contain a multiplier for contact pressure and relative slip amplitude p·ds or shear stress and relative slip amplitude q·ds, respectively. These multipliers are sensitive to the mechanical properties near the surface. Additionally, the wear profile is governed not only by the wear effect but also by the deformation of materials. Therefore, to accurately evaluate the wear performance for surface-hardened materials, the influence of RS is hard to be overlooked. In this work, the finite element method (FEM) is utilised to construct a 3D numerical model, which ignores and considers RS, for both as-received and LSP-treated samples. Two distinct RS distributions induced by different scanning strategies (one- and five-time), are introduced in the finite element (FE) model. The numerical simulation results are then compared with the experimental data to verify the accuracy of the fretting wear model built in this study. On this basis, the impact of different RS distributions on fretting wear characteristics, including wear profile and volume, is extensively investigated. The findings reveal that higher amplitude RS can reduce the degree of fretting wear to a greater extent, highlighting the beneficial role of RS in enhancing wear resistance. Furthermore, the numerical models incorporating RS demonstrate superior predictive accuracy compared to those that omit RS. This naturally underscores the importance of integrating RS in numerical simulations of LSP and other surface-hardened materials.

Roles of chain stretch and concentration gradients in capillary thinning of polymer solutions

Abstract Polymers inhibit the breakup of a liquid filament thinning under surface tension. The coupling of elasticity, capillarity and inertia leads to the well-known beads-on-a-string (BOAS) formation. Additionally, under different conditions, smaller satellite drops can form along the liquid bridge between the main beads. The development of BOAS and satellite drops is controlled by the rheology of the polymer solution. In this study, we consider the roles played by finite extensibility and anisotropic drag on the formation of satellite beads. In particular, we show that the more stretching a polymer chain can undergo, satellite beads are suppressed. The latter stages of capillary thinning has been shown to result in a phase separation resulting in what is referred to as a blistering pattern. We thus also conduct simulations of an inhomogeneous dilute polymer model that considers the competing effects of diffusion and stress gradients. We show that polymer is pulled axially towards the region connecting string and bead. This simple model does not predict a phase separation, but does reveal that pinchoff could be inhibited by the buildup of polymer concentration.

Linking physico‐chemical parameters and macroinvertebrates for water quality assessment of Kakamega and the East Usambara montane ecosystems in Kenya

AbstractSub‐Saharan freshwater biodiversity is impacted by changes in land use and climate change. To evaluate the relationship between physico‐chemical variables and macroinvertebrate community assemblages in Kakamega and the East Usambara Mountains forests, we explored benthic macroinvertebrate community structure in relation to physico‐chemical conditions of the water along an anthropogenic stress gradient. Sampling was done in six selected streams during the rainy and dry seasons between April 2017 and November 2019. At Kakamega, 367 macroinvertebrate individuals were identified belonging to 22 families, while at Usambara, 456 individuals belonging to 25 families were identified. Spearman's correlation revealed significant interactions in species diversity, evenness, and richness between macroinvertebrates and several physico‐chemical parameters (pH, conductivity, total dissolved substances, salinity and temperature). At Usambara, species richness, evenness and diversity was significantly negatively correlated with conductivity, tds, salinity, temperature and pH. We demonstrated that macroinvertebrate indices can be used as a quick evaluation tool of water quality in response to stream systems in the region as well as help in pointing out early warnings to help mitigate and reduce threats to stream biodiversity from anthropogenic activities.

Advanced testing and characterization of low-temperature cracking in bitumen and mastic

Low-temperature cracking is one of the most common failures in asphalt pavements, especially in cold regions. Accordingly, considerable amount of research has been performed in order to understand the low-temperature cracking mechanisms and to propose test methods for characterizing and determining cracking performance of bitumen and asphalt mixtures under freezing conditions. The existing test methods, however, require expensive equipment and skilled technicians; they are thus not well suited for routine tests. As a contribution to mitigate this situation, this study intends to investigate experimentally and characterize numerically the low-temperature cracking behavior of bitumen and mastic materials using a refined thermal cracking test method. The proposed method, the annular restrained cold temperature induced cracking (ARCTIC) test, allows to determine the low-temperature cracking properties of the mastic and bitumen with a relatively simple setup. In this paper, finite element (FE) modeling is used for evaluating the effect of test parameters on the temperature, stress and strain gradients induced in the specimen during the test. The ARCTIC test is employed to measure cracking temperatures of two bitumen and two mastic materials. The measurements repeatability is examined and the effect of bitumen type on the thermal cracking potential of bitumen and mastic is evaluated. FE modeling is employed to examine the effect of thermomechanical parameters on thermal cracking performance of the materials and to back-calculate fracture stress and strain from measurements. The results highlight the potential of the proposed test and analysis method for evaluation of low-temperature cracking in bitumen and asphalt mastic.

Facilitation and competition deconstructed: a mechanistic modelling approach to the stress gradient hypothesis applied to drylands

Facilitative interactions among species are key in plant communities. While experimental tests support the Stress Gradient Hypothesis (SGH) as an association between facilitation and stress, whether the shape of net effects along stress gradients can be predicted is controversial, with no available mathematical modelling approaches. We proposed a novel test, using a modification of the R* model to study how negative and positive partial effects of plant interactions in drylands combine along two common stress gradients. We modelled different interactions: competition for water and light, amelioration of soil infiltration and/or grazing protection, obtaining that intensity and importance of facilitation did not generally increase along stress gradients, being dependent on the interaction type. While along the water stress gradient net interactions became more positive, reaching a maximum and then waning again, various outcomes were observed along the grazing gradient. Shape variety was mainly driven by the various shapes of the partial positive effects. Under resource stress, additive interaction effects can be expected, whereas when including grazing, the effects were non-additive. In the context of the SGH, deconstructing the effect of positive and negative interaction in a pairwise mechanistic models of drylands does not show a unique shape along stress gradients.

Variance reflects resilience to disturbance along a stress gradient: Experimental evidence from coastal marshes.

Quantifying ecosystem resilience to disturbance is important for understanding the effects of disturbances on ecosystems, especially in an era of rapid global change. However, there are few studies that have used standardized experimental disturbances to compare resilience patterns across abiotic gradients in real-world ecosystems. Theoretical studies have suggested that increased return times are associated with increasing variance during recovery from disturbance. However, this notion has rarely been explicitly tested in field, in part due to the challenges involved in obtaining long-term experimental data. In this study, we examined resilience to disturbance of 12 coastal marsh sites (five low-salinity and seven polyhaline [=salt] marshes) along a salinity gradient in Georgia, USA. We found that recovery times after experimental disturbance ranged from 7 to >127 months, and differed among response variables (vegetation height, cover and composition). Recovery rates decreased along the stress gradient of increasing salinity, presumably due to stress reducing plant vigor, but only when low-salinity and polyhaline sites were analyzed separately, indicating a strong role for traits of dominant plant species. The coefficient of variation of vegetation cover and height in control plots did not vary with salinity. In disturbed plots, however, the coefficient of variation (CV) was consistently elevated during the recovery period and increased with salinity. Moreover, higher CV values during recovery were correlated with slower recovery rates. Our results deepen our understanding of resilience to disturbance in natural ecosystems, and point to novel ways that variance can be used either to infer recent disturbance, or, if measured in areas with a known disturbance history, to predict recovery patterns.

Unified Failure Criterion Based on Stress and Stress Gradient Conditions.

Specimens made of various materials with different geometric features were investigated to predict the failure loads using the recently proposed criterion comprised of both stress and stress gradient conditions. The notch types were cracks and holes, and the materials were brittle, ductile, isotropic, orthotropic, or fibrous composites. The predicted failure stresses or loads were compared to experimental results, and both experimental and theoretically predicted results agreed well for all the different cases. This suggests that the stress and stress-gradient-based failure criterion is both versatile and accurate in predicting the failure of various materials and geometric features.

A discussion of fatigue properties of notched specimens subjected to prestressing treatment machined from FGH4097

Experimental and numerical investigations were performed to reveal the effect of prestress on the fatigue properties of FGH4097 so as to quantitatively evaluate the overspeed benefit of the disc of aeroengine. A corresponding experiment was performed to examine the effect of prestress on the fatigue life of FGH97. A complete model describing the fatigue properties of the prestressed notch specimen was proposed. After the modification of mean stress, stress gradient and prestressing treatment, the model is finally verified by fatigue test of presstressed notch specimens machined from FGH4097 with satisfactory accuracy and good engineering applicability. The new model highlights that for the variable fatigue loads, the introduction of appropriate prestress is expected to benefit to the notch specimens and certainly to the disc of aeroengine regards of their fatigue properties. Both the calculation method of fatigue life parameters and the prestress correction model are proved to have good prediction accuracy.

Strengthening aluminum matrix composite with additively manufactured 316L stainless steel lattice reinforcement: Processing methodology, mechanical performance and deformation mechanism

This study investigates various processing approaches, interface characteristics, and mechanical properties of an aluminum (Al) matrix composite reinforced with additively manufactured (AM) 316L stainless-steel (SS) lattice. The AM-316L-SS lattice, boasting a 30 % infill density, was fabricated using the laser powder bed fusion technique. The Al-matrix was integrated with the AM-316L-SS reinforcement through experimentation involving pre-stirring of the molten Al under pressurized and unpressurized die-casting conditions. A well-bonded interface, with cohesive regions, was obtained in the pre-stirred structures. In contrast, regions of decohesion were displayed in cast structures without pre-stirring, leading to the presence of pores and the manifestation of imperfect bond interfaces. Compression tests conducted on the sound composite demonstrated enhanced mechanical properties, with a compressive strength of approximately 163 MPa, significantly higher than that of the pure Al-matrix (approximately 80 MPa). Localized interface cracking was initiated by stress gradients and plastic deformation, leading to microcrack propagation into the Al-matrix.

Responsive Alternative Splicing Events of Opisthopappus Species against Salt Stress.

Salt stress profoundly affects plant growth, prompting intricate molecular responses, such as alternative splicing (AS), for environmental adaptation. However, the response of AS events to salt stress in Opisthopappus (Opisthopappus taihangensis and Opisthopappus longilobus) remains unclear, which is a Taihang Mountain cliff-dwelling species. Using RNA-seq data, differentially expressed genes (DEGs) were identified under time and concentration gradients of salt stress. Two types of AS, skipped exon (SE) and mutually exclusive exons (MXE), were found. Differentially alternative splicing (DAS) genes in both species were significantly enriched in "protein phosphorylation", "starch and sucrose metabolism", and "plant hormone signal transduction" pathways. Meanwhile, distinct GO terms and KEGG pathways of DAS occurred between two species. Only a small subset of DAS genes overlapped with DEGs under salt stress. Although both species likely adopted protein phosphorylation to enhance salt stress tolerance, they exhibited distinct responses. The results indicated that the salt stress mechanisms of both Opisthopappus species exhibited similarities and differences in response to salt stress, which suggested that adaptive divergence might have occurred between them. This study initially provides a comprehensive description of salt responsive AS events in Opisthopappus and conveys some insights into the molecular mechanisms behind species tolerance on the Taihang Mountains.

Numerical simulation on high-speed and heavy-load wear behavior of CoCrNi and CoCrFeMnNi alloys

The steady operation and extended service lives of rocket sled test systems are impacted by the tribological performances of the rocket sled sliders. In this work, CoCrNi and CoCrFeMnNi alloys were considered candidate materials for rocket sled sliders serving under high-speed and heavy-load conditions. The simulation analysis was carried out by ANSYS. The results showed that the stress on the contact surface was uneven, and there were stress gradient changed and stress concentration. Significant wear occurred on the front-end of the slider. The results of this work can serve as a reference for practical engineering applications involving rocket sled sliders.

Characterization of electromigration-induced short-range stress development in Al(0.25 at. % Cu) conductor line

Scanning x-ray microbeam topography and fluorescence experiments were conducted in situ to study the electromigration behavior of a 0.5 μm thick, 10 μm wide, and 200 μm long Al(0.25 at. % Cu) conductor line with 1.5 μm-thick SiO2 passivation on a single crystal Si substrate. The strain sensitivity of x-ray topography measurement allowed detailed examination of the electromigration-induced stress distribution and evolution in the conductor line in response to the depletion of Cu solute early in the electromigration process. Upon electromigration at 0.4 MA/cm2 and 303 °C, a short-range stress gradient was quickly induced by Al migration in the Cu-depleted cathode region to counteract further Al flow. The stress gradient was fully developed during the 5.3 h incubation time, extending over the critical Blech length of about 66 μm from the cathode end. Plastic deformation then occurred at the downstream end of the Cu-depleted region. The preferential electromigration of Cu did not cause detectable stress change outside the Cu-depleted region, except for the significant stress development from the Al2Cu precipitation at the anode end which appeared to initiate the fracture in the passivation. Preliminary finite difference modeling was undertaken to simulate the experimental observations, from which important parameters dictating electromigration in Al(Cu) line were extracted: an apparent effective valence of −5.6 and −1.9 for Cu and Al in Al(Cu), respectively, and a critical Cu concentration of 0.16 at. % above which Al grain boundary diffusion is effectively blocked.

Micromechanical properties of micro- and nanocrystalline CVD diamond thin films with gradient microstructures and stresses

Chemical vapor deposition produced diamonds attract considerable scientific and industrial interest due to their exceptional mechanical and functional properties. Here, hot filament (HF) chemical vapor deposition was used to synthesize two diamond thin films with different cross-sectional microstructure and residual stresses. Structural characterization by scanning electron microscopy, Raman spectroscopy, and cross-sectional synchrotron x-ray nanodiffraction revealed different diamond morphologies. While the microcrystalline diamond film exhibits pronounced microstructural gradients expressed by gradually increasing (i) intensities of the 111 Debye-Scherrer rings, (ii) ⟨110⟩ fiber texture sharpness, (iii) grain size, and (iv) slightly oscillating residual stress at the level of −0.5 GPa, the nanocrystalline diamond film showed no pronounced cross-sectional variation of microstructure above the nucleation zone of ∼0.5 μm and a steady stress level of 0.25 GPa. In situ micromechanical cantilever bending tests revealed highly different mechanical properties of the two films. In detail, Young's modulus of 830 ± 53 and 459 ± 53 GPa, fracture stress of 12.4 ± 0.8 and 7.8 ± 1.0 GPa, and fracture toughness values of 6.9 ± 0.4 and 3.6 ± 0.3 MPa⋅m½ were evaluated for the micro- and nanocrystalline diamond films, respectively. In summary, this study provides valuable insights into the microstructure-residual stress correlation in micro- and nanocrystalline diamond films, especially illuminating their influence on micromechanical properties.