Low Strain Levels Research Articles

Impact of prolonged environmental exposure on stress transfer efficiency in poly(p‐phenylene terephthalamide)/epoxy composites

AbstractThe mechanical performance of fiber‐reinforced composites is seriously affected by their exposure to the environment. The combined action of elevated temperatures, humidity, and UV radiation (aging) on aramid fibers/epoxy resin composites is examined by employing Raman spectroscopy in tandem with mechanical loading. The imposed conditions drastically affect the interface of the composites altering the performance of the material under tensile loads. Although the maximum values of interfacial shear strength for the aged material are similar to that of the pristine material; however, they appear at lower strain levels, which is attributed to the significant decrease of the shear modulus of the epoxy matrix combined with the failure of aramid fibers to sustain the external loading due to the aging process. At high strains, the interface fails at lower strain levels (compared to the pristine sample) due to significant increase of micro‐cracking and micro‐voids on the surface of the aged composites. The formation of such flaws is the primary reason for the reduced mechanical performance and early failure of the composite material.

Job strain in German novice physical therapists / Psychische Beanspruchung am Arbeitsplatz von Berufsanfängern in der Physiotherapie

Abstract Introduction International research shows high levels of job demands and psychological resources for physical therapists, and job strain is considered to be moderate. So far, publications are based on therapists with varying length of service. Little is known about job strain during the first year in the profession. Aim To investigate the level of job strain of German novice physical therapists and to identify common workplace stressors. Methods We conducted a web-based survey among physical therapists who had been working for 12 month or less. Using a self-administrated questionnaire, job strain was measured by its effects on work commitment, general health, job satisfaction, burnout and turnover intention. Subgroup analyses based on age, sex and rating of professional training were performed. Stressors at work were evaluated by priority and frequency of appearance. Results Data of 153 physical therapists was analysed. Low levels of job strain were reported. Novice physical therapists showed high levels of workplace commitment and general health, and low levels of turnover. Moderate ratings were recorded for job satisfaction and symptoms of burnout. Inadequate compensation for work, high caseload, time pressure and physical stress were the most common stressors mentioned. Therapists who rated their professional training as positive reported lower levels of job strain. No significant differences between women and men or between therapists of different age were found. Conclusion Although many workplace stressors are reported, novice physical therapists show low levels of job strain. Vocational training may be one important protective factor with regard to job strain.

Study on Permeability of Deep‐Buried Sandstone under Triaxial Cyclic Loads

The triaxial cyclic loading and unloading test was carried out on a TAW‐2000 rock mechanics to study the permeability characteristics of deep‐buried sandstone. This paper analyzed the evolution laws of permeability, elastic modulus, rock damage, dissipated energy, and acoustic emission events of sandstone under different confining pressures. It also introduced the concept of relative strain and further discussed the relationship between relative strain and permeability. The test results showed that the permeability of sandstone under cyclic loading and unloading obviously experienced three stages. At a low strain level, the damage degree of sandstone was low. As a result, both the number of acoustic emission events and the proportion of the dissipated energy density were small. In this stage, with increasing the stress, the permeability decreased. With the increase of the relative strain, the propagation of fissure increased through rock interior and the damage of rock was accumulated. Consequently, the number of acoustic emission events grew slowly, and the proportion of dissipated energy density and the damage variable (D) increased gradually. In this stage, the permeability increases. As the axial strain reached the peak strain, the fissures developed into cracks and the rock failure happened. The number of acoustic emission events increased rapidly; both the proportion of the dissipated energy density and the damage variable (D) obtain the maximum value. In this stage, the permeability increased greatly. In this study, the point of fissure propagation of rock specimens was used as the point of demarcation. Before the fissures propagated, the permeability increased slowly and it was in accordance with a linear function. After the fissures propagated, the degree of rock damage increased, and the permeability increased in the form of an exponential function. The larger the confining pressure was, the smaller the relative strain corresponding to the point of fissure propagation was.

Stress relaxation of porcine tendon under simulated biological environment: experiment and modeling

The aim of the study was to investigate the viscoelastic response in the low and high physiological strain with the use of experimental and modeling approach. Viscoelastic response in the low, transition and high physiologic strain (3, 6 and 9%) with consideration of simulated biological environment (0.9% saline solution, 37 °C) was measured in relaxation tests. Preconditioning of tendons was considered in the testing protocol and the applied range of load was obtained from tensile testing. The quasi-linear viscoelasticity theory was used to fit experimental data to obtain constants (moduli and times of relaxation), which can be used for description of the viscoelastic behavior of tendons. The exponential non-linear elastic representation of the stress response in ramp strain was also estimated. Differences between stress relaxation process can be seen between tendons stretched to the physiological strain range (3%) and exceeding this range (6 and 9%). The strains of 6% and 9% showed a similar stress relaxation trend displaying relatively rapid relaxation for the first 70 seconds, whereas the lowest strain of 3% displayed relatively slow relaxation. Results of the model fitting showed that the quasi-linear viscoelastic model gives the best fit in the range of low physiological strain level.

Mechanical reliability of flexible encapsulation of III-V compound thin film solar cells

Flexible III-V compound thin film solar cells are promising candidates in the applications of Internet of Thing, electronics, civil, automotive and aerospace. In this study, theoretical, numerical and experimental methods are employed to investigate the mechanical reliability of the GaAs thin film solar cells with polymer film encapsulation. Experimental investigation by uniaxial tensile testing has revealed the stress failure of cell can be divided into the two different stages. For lower applied strain (<2.0%), the stress concentration caused the local plasticity of encapsulation material along the polymer-semiconductor interface (e.g. EVA-GaAs). For larger applied strain (>2.0%), semiconductor materials (e.g. junction and window) have shown brittle cracking failures. The observed brittle and ductile failures of the sublayer can be well-explained by analytical model based on theory of solid and fracture mechanics. It is also found that the interface stiffness mismatch plays an important role of encapsulation failure at lower strain level. The degree of the stiffness mismatch can be characterised by the proposed strain intensity factor. To overcome the mechanical failure, new encapsulation material such as ultra-thin glass can be promising in the future application.

A detailed investigation on the grain structure evolution of AA7005 aluminum alloy during hot deformation

Obtaining a fine grain structure in Al-Zn-Mg-Cu aluminum alloys with a relatively low alloying content, such as AA7003 and AA7005, is required when they are used for vehicle bodies. However, comprehensive studies on these materials are still needed. In this study, the grain refinement capability of the commercially used AA7005 aluminum alloy through a hot deformation process with relatively a low strain level was investigated. Hot compression tests were performed over a wide range of temperatures, strain rates, and strains (300–500 °C, 0.005–50 s−1, and 0.3–1.1, respectively). Diverse types of grain structure evolution, including dynamic recovery (DRV) and dynamic recrystallization (DRX), occurred during the thermomechanical process, and the related mechanisms were analyzed. The results show that DRV occurred regardless of the deformation parameters, while the occurrence of DRX was highly dependent on the parameters. DRX was more favorable at high temperatures, low strain rates, and large strains (e.g., 500 °C, 0.005 s-1, and 1.1, respectively). It formed new high-angle grain boundaries and resulted in fine grains through low-angle grain boundary rotation, high-angle grain boundary migration, and serration mechanisms, which played the primary role in grain refinement. By carefully controlling the deformation parameters, the original coarse grains were successfully refined down from hundreds of micrometers to 8–25 μm. The results indicate that the AA7005 aluminum alloy has a great potential for grain refinement through hot deformation, but special attention should be paid to the parameters.

Low velocity impact on pre-loaded composite plates: A novel standard-based experimental apparatus

Despite composite materials presenting a viable design solution for structural weight savings, their low resistance to impact damage portraits a potential drawback. This work presents a study on the compressive pre-loading effects on Low Velocity Impact, LVI, for composite laminated plates. Despite the absence of a standard testing procedure accounting for pre-load effects, a test rig was designed specifically for LVI on pre-loaded plates while preserving ASTM D7136M-15 guidelines for the stress-free specimens. Woven fabric composite laminated plates were manufactured using the Resin Transfer Moulding (RTM) process and subjected to impact on pre-loaded conditions. Three pre-load levels were prescribed, representing conditions ranging from low strain levels until post buckling regime. FE numerical models were implemented in ABAQUS FE code, comparing different Continuum Damage Mechanics (CDM) constitutive model formulations with experimental results. Analytical and numerical predictions showed good correlation with experimental results, allowing for further application of the test apparatus and methodology.

Analyzing the effect of waste jarosite as an alternative filler on the engineering properties of asphalt mixes

Zinc extraction engenders massive production of non-biodegradable Jarosite (JS) as an industrial waste that faces difficulty concerning its disposal. This study presents a novel approach to utilize JS as a filler in asphalt mixes. The asphalt mixes containing JS was designed as per Marshall mix design, and their engineering properties were compared with the mixes containing two conventional fillers (stone dust (SD) and hydrated lime (HL)) and one widely used waste filler (coal fly ash (FA)) at same mass proportion. The performance of asphalt mixes was compared using methods like indirect tensile strength test, four-point beam bending test, wheel rut test, Cantabro test, resilient modulus and modified Lottman tests. The use of JS in place of conventional SD improved the Marshall stability, indirect tensile strength, and rutting resistance of the asphalt mixes by 17, 25, and 27% respectively. This was majorly attributed to the high porosity, finer nature, higher specific surface area and high asphalt-filler interaction of JS. The moisture resistance of JS mixes was also found to be 3.5 and 10% better than FA and SD mixes respectively. Although, JS mixes showed relatively lower fatigue life than other mixes due to their high stiffness; however, fatigue life was found to be comparable with conventional HL mixes tested at lower strain levels. The overall conclusions of this study ascertain the utility of JS as an alternative filler and also paved its way for its safe disposal.

Influence of microstructure on cyclic deformation response and micromechanics of Ti–55531 alloy

Abstract Cyclic deformation response and micromechanics in strain controlled low cycle fatigue of Ti–5Al–5Mo–5V–3Cr–1Zr (Ti–55531) alloy with bimodal microstructure (BM) and lamellar microstructure (LM) were comparatively investigated at ambient temperature. Results showed that cyclic hardening/softening behavior of Ti–55531 alloy was found to be significantly dependent on both the strain amplitude and microstructure. The cyclic softening rate of LM was always larger than that of BM at strain level below 1.0%; however, the opposite result was produced for strain amplitude above 1.0%. Dislocation features revealed that cyclic deformation of the alloy primarily relied on primary α (αp) and secondary α (αs) phases. At low strain levels, lasting hardening of BM was due to the interaction of multiple prismatic and basal slips, and pyramidal slip in αp particles, while slight softening of LM was owing to the activation of a few twins in lamellar αs. However, at high strain levels, dislocation annihilation and numerous twins were detected in both αp and αs phases in BM, which resulted in the larger cyclic softening rate of BM than that of LM. Evidently, besides dislocation annihilation, twinning was found to be another important softening mechanism, which was not revealed by other studies on Ti alloys.

Evolution of shear wave velocity during triaxial compression

Accurate design of geotechnical structures requires precise estimation of the shear wave velocity (Vs) and the small-strain shear modulus. However, the interpretation of Vs data measured in deformed/sheared soil has not been extensively considered. This study used a triaxial apparatus equipped with planar piezoelectric transducers to monitor the evolution of Vs during triaxial compression of cohesionless soils. Recognizing that the grain shape and surface characteristics affect the overall mechanical response of granular materials, various natural sands and glass bead samples were considered. Discrete element method (DEM) simulations using spherical particles were carried out to compute particle-scale responses that cannot be measured in the laboratory. The experimental results revealed that the Vs values for samples with different initial densities tend to approach one another and have similar values (merge) at large axial strains. This merging occurs at a lower strain level for spherical particles in comparison with non-spherical particles. The linear Vs-void ratio relationship, which is often developed and used for homogeneous and isotropic stress states, is no longer applicable during shearing. It is the mean coordination number that dictates the evolution of Vs during triaxial compression. Furthermore, the axial strain at which the peak Vs is achieved is found to be comparable to the axial strain at which specimen dilation takes place.

The nonlinear stress relaxation behavior after a step shear of star‐shaped styrene‐butadiene rubber filled with precipitated silica: Experiment and simulation

AbstractThe nonlinear stress relaxation behavior after a step shear strain of star‐shaped SSBR/silica compounds containing 21 vol% filler of various surface areas was measured and simulated using constitutive equations. A styrene‐butadiene rubber (SBR) gum and SBR filled with silica having BET surface areas of 55, 135, 160, and 195 m2/g were used. Relaxation modulus behavior of the filled compounds was found to be dependent on surface area. Specifically, stress relaxation tests indicated that an increase in surface area led to increase in values of relaxation moduli in both the linear and nonlinear regimes. The time‐dependent relaxation modulus exhibited a plateau at long times of relaxation in compounds containing silica of high surface area. Additionally, good time‐strain superpositions were achieved for all samples at intermediate times of relaxation, and the strain‐dependent damping function decreased with filler surface area. The constitutive equations proposed by Leonov and Simhambhatla and Leonov, modified to include multimodal relaxation of the particle network, were used to predict the time evolution of the relaxation modulus in the nonlinear regime for all samples. The simulations provided good results for the SBR gum for all tested strain levels. Also, in the compounds filled with silica, both models satisfactorily described the experimental observation in the nonlinear regime at low strain levels. However, at higher strain levels, due to a possible slip effect, the simulations overpredicted measured values of the relaxation moduli, thus leading to only qualitative predictions of the observed behavior. It is also possible that neither model accurately captured the floc rupture kinetics of these complex rubber compounds.

Association for combined exposure to job strain, shift work on mental health among Chinese railway workers: a cross-sectional study

ObjectivesThe aim of this study was to assess the effects of coexposure to job strain and shift work on mental health in railway workers.DesignCross-sectional study.SettingOne Railway Bureau Group in China.ParticipantsA...

PATTERNS OF FRIENDSHIP SUPPORT AND STRAIN OVER TIME AMONG MARRIED OLDER ADULTS: THE ROLE OF DEMOGRAPHIC, SOCIAL, AND WELL-BEING FACTORS

The present study examines older adult married couples’ friendship support and strain trajectories. Friendship is essential across the life course. However, most studies examining friendship within the context of long-term marriages have not examined how friendship quality develops over time and have not treated the dyad as the unit of analysis. Growth mixture modeling and 3,608 married couples across three waves of the Health and Retirement Study (HRS) were used. Support and strain trajectories were assessed separately and spouses were assessed together in each analysis. Analyses of Variance (ANOVAs) were used to examine whether well-being and demographic factors distinguished classes. Three classes of friendship support trajectories and two classes of friendship strain trajectories were identified. The support classes were characterized by husbands’ friendship support stability, with the Spouses’ High, Stable Support serving as the optimal support trajectory class. The strain classes were characterized by relatively low levels of strain, with the Spouses’ Low, Decreasing Strain class serving as the optimal strain trajectory class. Both support and strain trajectories were distinguished by demographic, social, and well-being factors. The results provide evidence for the gendered nature of friendship support and strain patterns, and contributes to knowledge regarding factors associated with distinct longitudinal friendship patterns among married older adults. The findings also have implications for programmatic efforts and interventions aimed to strengthen friendships.

Integration of piezoelectric transducers (PZT and PVDF) within polymer-matrix composites for structural health monitoring applications: new success and challenges

ABSTRACT This article investigates the interest of using in-situ piezoelectric (PZT and PVDF) disks to perform real-time Structural Health Monitoring (SHM) of glass fiber-reinforced polymer composites submitted to various tensile loadings. The goal is to evaluate the working range and SHM potential of such embedded transducers for relatively simple mechanical loadings, with the long-term aim of using them to monitor complete 3D structures submitted to more complex loadings. SHM is performed acquiring the electrical capacitance variation of the embedded transducers. To study the potential links between the in-situ capacitance signal and the global response of the loaded host specimens, a multi-instrumentation composed of external Nondestructive Testing techniques was implemented on the surfaces of the specimens to search for multi-physical couplings between these external measurements and the capacitance curves. Results confirmed the non-intrusiveness of the embedded transducers, and allowed estimating their working domain. PZT capacitance signal follows well the mechanical loadings, but the piezoceramic transducer gets damaged after a determined relatively low strain level due to its brittleness. The limits of this working domain are extended by using a stretchable PolyVinylidene Fluoride (PVDF) polymer transducer, allowing this one to perform in-situ and real-time SHM of its host tensile specimens until failure.

Effect of wax-based warm mix additives on fatigue and rutting performance of crumb rubber modified asphalt

Nowadays, the use of crumb rubber modified asphalt (CRMA) has gained popularity among the contractors and construction agencies due to the addition of crumb rubber in bitumen, which can increase the life span of asphalt pavement. The experimental studies have also shown that CRMA binder can improve some mechanical properties of asphalt mixture such as rutting and fatigue resistance. However, as a result of higher viscosity in the CRMA binders, higher mixing and compaction temperatures are required. So, to overcome this problem, the use of warm mix asphalt (WMA) additives accompanied with rubberized binders has been proposed. Since it is believed that crumb rubber modifier (CRM) can improve the rheological characteristics of bitumen; therefore, there is a need to more investigate the properties of CRMA binder modified with WMA additives to ascertain its proper usage in pavement construction. In this regard, the main objective of this study was to investigate the effects of two wax-based additives, as Slack wax and Polypropylene wax, on fatigue and rutting resistance of the rubberized binders modified with 15% CRM. Accordingly, the obtained results indicate that both wax-based additives have the ability of reducing the viscosity of the rubberized binders at high temperatures. Also, 2% of Polypropylene wax increased the high temperature performance of rubberized binder up to one grade. According to the results of multiple stress creep recovery (MSCR) test, the lower Jnr3.2 and higher R% values represent that Polypropylene wax can improve the rutting resistance of the rubberized binder, so that the samples containing 2% of Polypropylene wax have a higher rutting resistance than the 4% and 6% samples. Moreover, the Linear Amplitude Sweep (LAS) test’s results indicated that CRMA binders containing 6% of Slack wax have considerably higher fatigue life compared to the control rubberized binder, particularly in low strain levels.

Laboratory evaluation of rheological behavior of binder and performance of stone matrix asphalt (SMA) mixtures containing zycotherm nanotechnology, sasobit, and rheofalt warm mixture additives

In recent years, using Stone matrix asphalt (SMA) has gained a lot of attention due to its high performance in rutting and low cost of pavement maintenance, however it consumes high energy like other hot mixtures during production. Also, as Warm mix asphalt (WMA) has several advantages like environmental concerns, many countriesshowed interest in using this new technology. One of the shortcomings of WMA is moisture sensitivity, which is investigated by several researchers. However, the interaction between various factors on moisture resistance of SMA mixture have not been investigated. Also, the addition of warm additives may affect other properties of mixtures. So in current work, the effect of WMA additives on rheological properties of binder and performance of mixtures were investigated. The present research aimed to evaluate the effect of three different gradations (9.5 mm, 12.5 mm, 19 mm), three warm additives (Sasobit, Rheofalt, Zycotherm) with various contents on water damage resistance and performance of warm SMA mixtures. The rheological behavior of original and warm modified bitumens was investigated by performing viscosity, dynamic shear rheometer (DSR), Linear Amplitude Sweep (LAS), Multiple Stress Creep Recovery (MSCR) tests. Performance properties of mixtures were investigated through four-point beam fatigue (FPB), resilient modulus, Indirect tensile strength (ITS), dynamic creep, and wheel track tests. The moisture sensitivity of modified mixtures was assessed through four different methods, such as: Resilient Modulus Ratio, Fracture Energy Ratio, Tensile Strength Ratio, and Texas boiling test. The LAS test results indicated that bitumen containing 1% Sasobit has the highest fatigue life in low strain levels, while in high strain levels, the Zycotherm modified binder has the highest fatigue life. The MSCR test outcomes revealed that binder containing 3% Sasobit has the highest rutting resistance. The outcomes revealed that the addition of warm additives enhanced moisture resistance of samples. Among warm modified mixtures, Zycotherm modified specimens had the highest resistance to water susceptibility. The results indicated that the addition of warm additives enhanced performance properties of mixtures. Among warm additives, 3% Sasobit has the highest effect on enhancing the rutting and fatigue behavior of mixtures.

On the crushing response of the functionally graded metallic foams based on 3D Voronoi model

This paper presents the study on crushing response of functionally stepwise graded foam under quasi-static and dynamic compression using three-dimensional (3D) Voronoi model. A series of simulations is conducted to acquire proper computing parameters and to validate the predictability of numerical model by comparing with experiment results. Two different strategies are proposed to achieve the gradient change in relative density by changing equivalent radius and cell-wall thickness. The main focus of present paper is placed on the effect of gradation configuration on the stress-strain response, deformation propagation and energy absorption. The results show that the gradation configuration has little effect on the crushing behavior of graded foams under quasi-static compression. However, the performance of graded foam under dynamic compression could be promoted by tailoring its gradation configuration. Adopting a negative gradation is beneficial for reducing the stress at the stationary side and maximizing the energy absorption under low strain level. A positive gradation would be preferred as an energy absorber under high strain level.

Perceived job strain among Swedish occupational therapists with less than 10 years of work experience

Background Occupational therapists with less than 10 years of work experience have been shown to be at higher risk of work-related stress. There is a need for further research on the level of job strain they perceive. Aims To investigate the perceived level of job strain among Swedish occupational therapists with less than 10 years of work experience and differences between groups who have worked for 0–5 years and 6–10 years. Material and methods In this cross-sectional study, a web-based survey was sent to 345 occupational therapists with less than 10 years of work experience, with 252 responding. Data was collected using the Swedish Demand-Control-Support Questionnaire and questions on job strain and sociodemographics. Results The participants scored 19.68 points on the control scale and 20.01 points on the support scale, indicating high levels of control and support, and 13.33 points on the demand scale (high demand ≤ 14 points). Twelve percent worked in high strain jobs. Most (75%) worked in low strain and active jobs. Conclusions and significance The participants reported low levels of job strain. However, to prevent mental health problems, employers need to identify those in active jobs to ensure that they have time to exercise control over their work situation.

Sub-trabecular strain evolution in human trabecular bone

To comprehend the most detrimental characteristics behind bone fractures, it is key to understand the material and tissue level strain limits and their relation to failure sites. The aim of this study was to investigate the three-dimensional strain distribution and its evolution during loading at the sub-trabecular level in trabecular bone tissue. Human cadaver trabecular bone samples were compressed in situ until failure, while imaging with high-resolution synchrotron radiation X-ray tomography. Digital volume correlation was used to determine the strains inside the trabeculae. Regions without emerging damage were compared to those about to crack. Local strains in close vicinity of developing cracks were higher than previously reported for a whole trabecular structure and similar to those reported for single isolated trabeculae. Early literature on bone fracture strain thresholds at the tissue level seem to underestimate the maximum strain magnitudes in trabecular bone. Furthermore, we found lower strain levels and a reduced ability to capture detailed crack-paths with increased image voxel size. This highlights the dependence between the observed strain levels and the voxel size and that high-resolution is needed to investigate behavior of individual trabeculae. Furthermore, low trabecular thickness appears to be one predictor of developing cracks. In summary, this study investigated the local strains in whole trabecular structure at sub-trabecular resolution in human bone and confirmed the high strain magnitudes reported for single trabeculae under loading and, importantly extends its translation to the whole trabecular structure.

Dynamic Behaviour of Sand–Crumbed Rubber Mixture at Low Strain Level

In the present study, the dynamic behaviour of Barak river sand and sand crumbed rubber mixture is studied under different conditions. The resonant column test is used to study the dynamic behaviour of sand and sand mixed with varying percentages (1, 2, 3, 4, 5 and 6%) of crumbed rubber at the low strain level. The tests are conducted for different relative densities (40, 50, and 60%) under different confining pressures (50, 100 and 200 kPa). The effects of saturation, relative density (Dr), confining pressure (CP) and crumbed rubber content on the dynamic behaviour of local Barak river sand is investigated. Experimental results show that the difference of shear modulus for saturated and dry conditions is not much different for small strain level. Similar behaviour is observed for damping ratio as well. However, the effect of crumbed rubber content is significant on dynamic behaviour of sand. Further, an optimum percentage of crumbed rubber content is obtained from the present study.