Pure Stress Research Articles

A novel semi-analytical approach for predicting the buckling and postbuckling behaviour of Glare fibre metal laminates

A semi-analytical model based on the variational principle, the Rayleigh–Ritz method and the Ramberg–Osgood formula is developed to analyse the buckling and postbuckling analysis of Glare® fibre-metal laminates (FMLs) under axial compression. The model is applied to examine the failure behaviour of standard Glare® 4B specimens. It allows computational efficient modelling of composite plates under pure or mixed stress boundary conditions with geometric imperfections, which have been shown to cause a considerable effect on their buckling and postbuckling behaviour. It is implemented in MATLAB. Results are compared with those obtained from a 3D Finite Element (FE) explicit dynamic nonlinear analysis implemented in the Abaqus/Explicit solver. The FE model incorporates progressive damage and failure using a cohesive zone model for inter-laminar layers and continuum material damage models for constituents, considering both geometric imperfections and load eccentricity. Finally, a series of specimens under compression loading are tested for the purpose of validation. Tests are monitored using Digital Image Correlation (DIC) for the determination and visualisation of principal strains, and Acoustic Emission (AE) for detection and location of damage initiation and evolution. Excellent correlation is observed between the analytical predictions, and both FEA and experimental results.

Bending Stresses and Deformations in Prismatic Profiled Shafts with Noncircular Contours Based on Higher Hybrid Trochoids

This paper presents an analytical method for determining the bending stresses and deformations in prismatic, noncircular profile shafts with trochoidal cross sections. The so-called higher trochoids can be used as form-fit shaft-hub connections. Hybrid (mixed) higher trochoids (M-profiles) were developed for the special application as a profile contour for the form-fit shaft and hub connections in an earlier work by the author. M-profiles combine the advantages of the two standardised polygonal and spline contours, which are used as shaft-hub connections for the transmission of high torques. In this study, the geometric and mechanical properties of the higher hybrid trochoids were investigated using complex functions to simplify the calculations. The pure bending stress and shaft deflection were determined for M-profiles using bending theory based on the theory of mathematical elasticity. The loading cases consisted of static and rotating bends. Analytical, numerical, and experimental results agreed well. The calculation formulas developed in this work enable reliable and low-cost dimensioning with regard to the stresses and elastic deformations of profile shafts subjected to bending loads.

The undrained behaviour of over-consolidated clay subjected to pure rotation of principal stress directions

Foundation soil of offshore structures may be subjected to various non-proportional cyclic loadings, such as wave loading, tide loading and earthquake loading. The consolidation state may affect the undrained behaviour of clay under pure rotation of principal stress directions. A series of cyclic undrained torsional tests were performed on normally and over consolidated clay subjected to pure rotation of principal stress directions at constant deviatoric stress conditions. The effects of over consolidation ratio (OCR) on the excess pore water pressure, stress-strain loop, stiffness and non-coaxial behaviour of the clay were analyzed under different deviatoric stresses and intermediate principal stress coefficients (b values). The results indicate that owing to the pure principal stress rotation alone, strain and excess pore water pressure accumulated in samples and the accumulation is affected by OCRs, deviatoric stress amplitudes and b values. Negative excess pore water pressure and stress dilatancy can be observed in over consolidated clay. Compared with shear stiffness, the normal differential stiffness experiences greater degradation under the pure rotation of principal stress. The shear stiffness of over consolidated clay may show an increase with loading cycles, while the normal differential stiffness degrades at low deviatoric stress value. Over consolidated clay shows non-coaxial behaviour and the greater the OCR values or the smaller the deviatoric stress, the larger the non-coaxial angle. The effects of b values on the non-coaxial behaviour of over consolidated clay are also observed. HIGHLIGHTS Over-consolidated clay subjected to pure rotation of principal stress. The effects of OCR, deviatoric stress and b value were analyzed. Negative EPWP and stress dilatancy were observed in over consolidated clay. The greater the OCR, the larger the non-coaxial angle.

Modeling cyclic behavior of clay subjected to principal stress rotation

Experimental evidence indicates that principal stress rotation yields complex cyclic behavior in clay, which is significant in many cases but not adequately described by constitutive modeling. To address this issue, a constitutive model was developed to describe the cyclic behavior of clay within the framework of bounding surface elastoplasticity. To describe the plastic mechanism induced during reversal loading, the mapping rules for the relocatable projection center were formulated in the deviatoric stress space, which are also applicable to pure principal stress rotation. The effects of small strain stiffness and anisotropic elasticity were incorporated into the model to describe cyclic degradation and plastic accumulation behavior under principal stress rotation. To describe the non-coaxiality, a three-dimensional non-coaxial flow rule was incorporated into the model. It was assumed to be colinear with a non-coaxial stress rate orthogonal to a reference stress tensor and dependent on the stress ratio. The developed model was validated against the undrained hollow cylindrical torsional shear tests subjected to cyclic pure principal stress rotation for Shanghai and Hangzhou clay with various intermediate principal stress coefficients. The comparison between simulations and experimental results demonstrates that the proposed model can model the cyclic behavior of clay subjected to principal stress rotation.

A Study on the In-Plane Shear-after-Impact Properties of CFRP Composite Laminates.

Impact loading on carbon fiber reinforced polymer matrix (CFRP) composite laminates can result in a significant reduction in their residual properties, and the (ShAI) properties of the composite material are essential to obtain the material allowable values of the shear dominated composite structures. In order to obtain the ShAI properties of the composite material in pure shear stress at a coupon level, this study presents theoretical, experimental, and numerical methods and analysis work on the in-plane shear and ShAI properties of the composite laminates. Theoretically, a method of sizing the composite specimen loading in shear is developed through comparing the load values due to buckling and the material failure. Following this, both impact tests using the drop-weight method and ShAI tests using the picture frame test method are conducted, and the influences of the impact energies on the impact damage and the residual ShAI values are evaluated. Moreover, a progressive failure finite element model based on the Hashin’s failure criterion and the cohesive zone model is developed, and a two-step dynamic analysis method is performed to simulate the failure process of the composite laminates under impact loading and ShAI loading. It is found that the impact damage with the cut-off energy, 50 J, causes a 26.8% reduction in the residual strength and the residual effective shear failure strain is about 0.0132. The primary reason of the shear failure is the propagation of both the matrix tensile failure and interlaminar delamination. It can be concluded that the proposed theoretical, experimental, and numerical methods are promising factors to study the ShAI properties of the composite materials.

Stress formulation of acoustoelasticity

Acoustoelasticity describes the relationship between elastic wave velocities and the initial stress present in a material. Traditional theories consider successive deformations involving small amplitude wave motion superimposed on an initially deformed material. Then, a constitutive relationship must be applied to relate the initial static deformation to the desired relationships involving initial stress, resulting in expressions of wave velocities involving a mix of elastic stiffness and compliance constants. In this article, a pure stress formulation is developed for acoustoelasticity. In this setting, the problem involves the superposition of a dynamic stress wave on an initially stressed material configuration, rather than the superposition of kinematic variables. The phase velocity of the stress wave is naturally related to the initial stress through only the compliance constants. Thus, compliances are the fundamental constants of acoustoelasticity.

Numerical study on the hot compression test for bulk metal forming application

In this study, a comparison between the well-established Lagrangian approach and the Arbitrary Lagrangian–Eulerian (ALE) approach is presented. This comparison aims to verify the ALE's approach suitability for modeling thermomechanical processes. After that, a study on the material's stress state evolution inside the specimen is provided. The stress state is evaluated through the triaxiality factor and Lode parameter. Ideally, under pure compression, these parameters' values are − 1/3 and − 1, respectively. However, it is not possible to achieve ideal conditions in actual experiments. The Lagrangian model was done in QForm, and the ALE model was done in LS-Dyna. The results from both models are in good agreement between them and agree with the force vs. stroke measured during the experiments. Two paths were defined to study the stress state inside the sample, in the radial direction (equator line) and axial direction (axial line). It was concluded that some areas in both paths might be considered as approximately under pure compression stress state. In addition, the ALE approach accuracy for thermomechanical modeling was verified.

The Effects of Some Priming Applications on Seed Quality Parameters in Melon (Cucumis melo L.) Seeds Under Different Doses of NaCl Stress

Salinity of irrigation water and soil-based salinity is an important problem. Priming is an application that increases the germination rate by keeping the seeds in distilled water or different aqueous solutions for a certain period of time. Ipsala variety melon (Cucumis melo L.) seeds obtained from Manisa Kırkağaç region were used in the study to determine the effects of different NaCl stress and priming applications on seed quality. Melon seeds subjected to different priming applications were germinated in pure water (0 mM NaCl) and salt stress (140 mM NaCl) conditions. In the study, herbal origin thyme (Origanum vulgare L. subsp. hirtum), sage (Salvia officinalis L.) applications as well as KNO3 (50 mM) and pure water applications were included as priming applications. In the study, it was determined that increase in the germination rate (%) and germination time (day) parameters which are important quality parameters in seeds, were more pronounced with priming applications. Priming with using sage and thyme plants were more effective respectively in increasing seed quality parameters.

Urinary incontinence during pregnancy – of predictive value?

Objective: Primary objective was to find the prevalence of urinary incontinence during pregnancy by its type and see how predicative it was of urinary incontinence in the long term. Associated risk factors were identified as secondary objectives. Method: Antenatal clinics provide opportunistic screening for diseases of the breast, the heart or even the mind. Preventive gynaecological issues could also be addressed likewise. It was felt worth investigating the prevalence of urinary incontinence during pregnancy without undue harassment to the patients by way of a few routinely directed questions about urinary incontinence.This information is rarely volunteered due to embarrassment. 867 women attending antenatal clinics led by the author at teaching hospital Peradeniya, in 2007 and 2008 were interviewed during pregnancy and those incontinent were followed up post-partum at 6 weeks,one and two years till 2010. Patients were inquired regarding the predominant type of incontinence, when the symptoms were mixed(stress and urge both). Results: Of the 867 women analysed, 209 (24%) had some form of urinary incontinence. 110 (52%) of those affected had pure or predominant stress, and 15 (7%) pure or predominantly urge incontinence. The rest 41% had a mixed picture. 154 were still incontinent at 6 weeks post-partum. The proportion of the type of incontinence had changed. 97 (63%) had pure or predominantly stress incontinence, 6 (3.9%) had pure or predominantly urgency and the 33.1% had a mixed picture. 47 of them still complained of some degree of incontinence at the end of one year despite interventions of pelvic floor exercise (PFE),for urodynamic stress incontinence or mixed symptoms. The patterns were even more stress predominant. 33(70%). 2 patients (4.2%) had pure urgency and the rest 12 (25.5%) had a mixed pattern. End of 2 years only 43 patients of those incontinent at 1year were traceable at least over the telephone. Only 23 complained of persistence of the problems. Of the 23 with symptoms at 2 years, 2 were pregnant again. 18 (78%) had pure SI and the rest 5 (22%) had a mixed picture. This indicated that pure stress incontinence tended to persist more than urge incontinence probably indicating some form of permanent damage to the pelvic floor. Older and women of higher parity were found to be more affected again indicating possibility of such injury. Conclusions: The finding that of the initial 209 women who leaked urine during pregnancy 23 remained incontinent at the end of 2 years (11%), is evidence that incontinence during pregnancy is of some predictive value in finding those likely to have long term incontinence.The symptoms of stress incontinence persisted despite PFE. This places doubts on benign physiological and hormonal change theories which is discussed alongside these findings. Incontinence during pregnancy maybe a useful indicator of injury sustained to the pelvic floor providing early warning for trouble ahead with the sphincter mechanism. Questions about urinary incontinence as a routine during antenatal consultations may be a worthwhile exercise. This would take only a very short time and should be encouraged as a higher percentage would have remained incontinent had PFE not been initiated as an ethical intervention.

Ultrafast Modulation of the Molten Metal Surface Tension under Femtosecond Laser Irradiation

We predict ultrafast modulation of the pure molten metal surface stress fields under the irradiation of the single femtosecond laser pulse through the two-temperature model molecular-dynamics simulations. High-resolution and precision calculations are used to resolve the ultrafast laser-induced anisotropic relaxations of the pressure components on the time-scale comparable to the intrinsic liquid density relaxation time. The magnitudes of the dynamic surface tensions are found being modulated sharply within picoseconds after the irradiation, due to the development of the nanometer scale non-hydrostatic regime behind the exterior atomic layer of the liquid surfaces. The reported novel regulation mechanism of the liquid surface stress field and the dynamic surface tension hints at levitating the manipulation of liquid surfaces, such as ultrafast steering the surface directional transport and patterning.

STRESSED HEART MORPHOLOGY AND NEUROLOGIC STRESS SCORE EFFECT BEYOND HEMODYNAMIC STRESS ON FOCAL GEOMETRY

Objective: Recent work have emphasized the importance of adaptive cardiac emodelling (Plos One, IJCCRP, Am J Cardiovasc Dis., 2021). In addition to hemodynamic stress in hypertension, novel cardiac imaging have shown that stressed heart morphology (SHM: basal septal hypertrophy, the early imaging biomarker in hypertension, J Applied Physiol 2019) could be affected by emotional stress and aortic stenosis (B Press Monitor 2020, ATCS 2021). Design and method: The aim of the study was to explore the potential effect of chronic emotional and mental overdrive in addition to chronic hemodynamic stress due to chronic hypertension. Therefore, we planned to determine imaging data as well as neurologic stress scores in SHM and compare them in terms of geometric details and severity of stress scores. Results: SHM was diagnosed in 50 chronic patients with systemic hypertension using the universal criteria (thicker than 1.4 cm septal base on the parasternal long axis). Then, we decided to detect quantitative scores of neurologic stress for all patients of the study group. The patients with SHM with irregular septal basal emodelling with septal bulging were associated with high neurologic stress score, while the other SHM patients with regular emodelling had lower score (p < 0.05). Conclusions: SHM seems like a quite complex phenomenon which possibly affected by emotional and mental overdrive beyond pure hemodynamic stress due to hypertension. Complex imaging findings also are consistent with adaptive phase SHM (J Clinic Med 2022, J of Hypertension 2022) showing irregular nature of human SHM which is likely to depend on cognitive function compared to regular emodelling in animals.

Anisotropic and Noncoaxial Behavior of Soft Marine Clay under Stress Path Considering the Variation of Principal Stress Direction

The influence of the variation in the principal stress direction on the anisotropic and noncoaxial behavior was investigated through several hollow cylinder apparatus (HCA) tests that were performed on reconstituted soft clay specimens under a stress path with a fixed or continuous rotation of the major principal stress axis. In particular, the varying amplitude of the deviator stress (q) and intermediate principal stress coefficients (b) were focused upon. The results indicated that the shear stiffness and strength were strongly dependent on the fixed major principal stress angle. Furthermore, apparent noncoaxiality was observed between the orientations of the major principal strain increment and the major principal stress. Consequently, the noncoaxial behavior of the reconstituted soft clay was sufficiently and effectively described using rose and polar diagrams. Furthermore, the data fluctuation indicated that the noncoaxiality of soft clay was strongly dependent on both q and b during the pure principal stress rotation (PSR). The results obtained for undisturbed clay and sand are not expected to be applicable to reconstituted clay, because its behavior is in contrast to the former two. Regarding reconstituted soft clay, the degree of noncoaxiality was observed to increase during the second rotation (N = 2) compared with that during the first rotation (N = 1). However, this trend gradually weakened with an increase in q or b values. Moreover, regardless of whether N = 1 or 2, the degrees of noncoaxiality decreased with the increase in q for b = 0.5 whereas it increased with the increase in b values for N = 1. However, those for b = 1 were the smallest for N = 2. The results are expected to provide an experimental basis for the constitutive modeling of soft clay with fixed and continuous rotations of the major principal stress axes in ocean engineering.

Investigation of Tensile Lap-Splice Hybrid Steel Bars Used in RC Beams under Flexure

During the last two decades, fiber reinforced polymer (FRP) reinforcing bars for concrete structure has been extensively investigated and a number of FRP bars are commercially available. Using glass fibers and unsaturated polyester resins, steel fiber reinforced polymer composite bars (SFCB) were manufactured and introduced to the commercial market. Lap splice of such bars must be thoroughly investigated. Limitations of steel reinforcement lengths available from the plant (generally 12 m), causing the construction actors to splice the reinforcement. The simplest and easiest way is to make a lap splice, which made overlap by a certain length is called the developing length of lap splice bar. The main objective of this research using the concept of material hybridization, is to investigate the effect of bar diameter after adding fibers & code equation coefficients for best efficient developing length for tensile lap splice in beams under flexure. Nine RC beams were casted with dimensions of (25*40*420) cms and concrete strength of 300 kg/cm2. Beams were divided into three groups according to the fiber thickness added to 10 mms’ steel bars. First group contained three beams reinforced with hybrid bars of diameter 12 mms, 10mms steel coated with 2mms fiber. Beam one casted using bars with lap splice of 1.3*50φ, beam two casted using bars with lap splice of 1.3*55 φ & beam three casted using bars with lap splice of 1.3*60 φ. Second group contained three beams reinforced with hybrid bars of diameter 14 mms, 10mms steel coated with 4 mms fiber, beam one casted using bars with lap splice of 50φ, beam two casted using bars with lap splice of 55 φ & beam three casted using bars with lap splice of 60 φ. Third group contained three beams reinforced with hybrid bars of diameter 16 mms, 10mms steel coated with 6 mms fiber , beam one casted using bars with lap splice of1.3* 50φ , beam two casted using bars with lap splice of 1.3*55 φ & beam three casted using bars with lap splice of 1.3*60 φ . Beams were tested under four-point load to create a region of pure flexure stress. Load –deflection curves are presented along with percentage differences. Finally, conclusions were drawn and presented in this paper.

Efficacy of 3 Tesla Functional Magnetic Stimulation for the Treatment of Female Urinary Incontinence.

Functional magnetic stimulation (FMS) is a new technique for the conservative treatment of Urinary incontinence (UI), based on magnetic induction. It induces controlled depolarization of the nerves, resulting in pelvic muscle contraction and sacral S2-S4 roots neuromodulation. The aim of this study was to assess the efficacy of the new 3 Tesla FMS chair, both in patients with pure stress urinary incontinence (SUI) and in women with pure overactive bladder (OAB) symptoms. A prospective observational study was conducted in our urogynaecologic unit. All the patients involved were consecutive women with pure SUI or pure OAB symptoms treated by a 3 Tesla electromagnetic chair. The primary outcome was a subjective outcome evaluation by the PGI-I Scale and a patient-satisfaction scale. The secondary outcome was the change score of the UDI-6, IIQ-7, ICIQ-SF and OAB-q SF questionnaires from baseline to final visit. At 2 months follow-up, 28 out of 60 patients (47%) with SUI symptoms and 20 out of 40 patients (50%) with OAB symptoms declared themselves cured. Considering cured and improved patients, the subjective cure rates were 68.3% (41/60) and 70% (28/40) for patients with SUI and OAB symptoms, respectively. The results of this study showed that the 3 Tesla electromagnetic chair may be an effective option for the treatment of UI.

Constitutive modeling for cyclic responses of saturated soft clay under principal stress rotation induced by wave loads

A bounding surface model is proposed to reproduce the cyclic responses of saturated soft clay under principal stress rotation induced by wave loads. While extensively reported in experimental studies, this particular type of clay response has not been adequately addressed by constitutive modeling. By incorporating the mapping rules of relocatable projection center into the bounding surface, the plastic effects under pure principal stress rotation can be established. To represent the non-coaxiality during pure principal stress rotation, the presented model employs a non-coaxial flow rule sharing the same directions with the non-coaxial stress rate. However, it has been modified to related to the coaxial strain rate and the current stress ratio. Besides, the anisotropic elasticity is introduced for simulating the effects on plastic accumulation behavior during principal stress rotation. The developed model is validated through pure principal stress rotation tests of Shanghai clay and Wenzhou clay. The comparisons show that the proposed model can reasonably reproduce cyclic responses of saturated soft clay under principal stress rotation induced by wave loads.

Evolution of yield behavior for AA6016-T4 and DP490—Towards a systematic evaluation strategy for material models

Uniaxial and cruciform biaxial tensile tests are performed on a face-centered cubic material AA6016-T4 and a body-centered cubic material DP490 under 19 different loading paths, i.e., uniaxial tension in seven directions, simple shear along the rolling direction, and biaxial tension in rolling/transverse and 45°/135° directions, with seven and four stress ratios, respectively. The ability of several yield criteria to describe the plastic anisotropy for the tested materials, under the associated and non-associated flow rules, is evaluated systematically. A new evaluation strategy to check the validity and applicability of the material models is proposed in this study. This strategy is different from the traditional evaluation strategy in that the biaxial tensile mechanical properties measured in the 45°/135° sampling direction are included in the investigation scope, and it is not limited to the rolling/transverse sampling direction, i.e., the yield stress functions are evaluated by comparing the experimental and predicted uniaxial yield stresses and plastic work contours on the normal and diagonal planes. Further, the plastic potential functions are confirmed by the description accuracy of uniaxial rα-values and the directions of plastic strain rate in the rolling/transverse and 45°/135° directions. A new analytical calibration program is developed for the material parameters of the Poly6 yield criterion related to normal (a1,a2,...,a7) and shear stress components (a8,a9,...,a16), which can further introduce the yield stresses under the near plane strain states (σPS0, σPS45, and σPS90) and pure shear yield stress along the rolling direction (τ0). The results indicate that the new calibration strategy of Analytical Poly6-II&2 can accurately describe the anisotropic yield behavior of the tested materials compared with other advanced yield criteria, especially for plastic work contours on the diagonal plane. Significant anisotropic yield behavior was observed under biaxial tensile stress states with the same loading ratio in different sampling directions. Therefore, investigating the prediction accuracy of the biaxial tensile mechanical properties in only the rolling/transverse sampling direction is insufficient for evaluating the effectiveness of the material models; the mechanical properties at the initial yield point cannot fully reflect the plastic anisotropy of the materials. The material parameters of the Analytical Poly6-II&2 yield criterion are expressed as functions of equivalent plastic strain, which can continuously capture the evolution of the anisotropic yield behavior for AA6016-T4 and DP490. Three different calibration strategies of the material parameter a16 are evaluated in this study. These strategies do not affect the description ability of the uniaxial yield stresses, rα-values, and plastic work contours on the normal plane; however, they influence the calculation results of plastic work contours on the diagonal plane. For attaining a balanced prediction accuracy, it is recommended to provide the values of both τ0 and σPS45.

Torsional Vibration Tests of Extruded Polystyrene with Improved Accuracy in Determining the Shear Modulus.

Recently, extruded polystyrene (XPS) foam has been used as a component of construction materials; therefore, it is important to characterize its mechanical properties, including shear modulus. Despite the importance, it is often difficult to determine the shear modulus accurately by using many of the conventional methods; therefore, it is desirable to establish another method to measure the shear modulus with a high accuracy. Among various methods, torsional vibration test is advantageous because it can be performed easily under the pure shear stress condition in the test sample and both the in-plane and out-of-plane shear moduli can be obtained. However, it is difficult to find any examples performing the torsional vibration tests. In this study, the in-plane and out-of-plane shear moduli of XPS were determined through torsional vibration tests using samples of various widths. In addition, the shear moduli were also determined through flexural vibration tests and compared with those obtained from the torsional vibration tests. In the torsional vibration tests, the anisotropy in these shear moduli became an obstacle, and the in-plane shear modulus determined using a single sample was often dependent on the width/thickness ratio of the sample. In this condition, the coefficient of variation of the in-plane shear modulus value was often close to 10%. However, when using data obtained from the samples with various width/thickness ratios, both the in-plane and out-of-plane shear moduli could be obtained while reducing the abovementioned dependence. Additionally, the coefficients of variation were restricted to those below 2% and 7% for the in-plane and out-of-plane shear moduli, respectively, and these values were obviously lower than those obtained from the flexural vibration tests (approximately 20%). In the proposed method, both the in-plane and out-of-plane shear moduli can be obtained accurately without using any numerical analyses, which are often required in the standardized methods to improve the accuracy. Thus, for accurate measurement of both types of shear moduli of XPS, we recommend performing torsional vibration tests using a range of samples of various width/thickness ratios.

Time Evolution of Partial Discharges in a Dielectric Subjected to the DC Periodic Voltage

Partial discharge (PD) detection can be considered one of the most useful tools for assessing the insulation conditions of the power apparatus in high-voltage systems. Under AC conditions, this analysis is widely employed in online and offline tests, such as type tests or commissioning, and can be carried out by applying the phase-resolved PD (PRPD) method, since the patterns can give information about the defect classification. Under DC voltages, the classic pattern recognition method cannot be performed, and the measurements show complexities related to the nature of the phenomena. For this reason, to date, a standard for PD measurements under DC does not exist. In previous papers, a new method for PD detection under DC stress voltages has been proposed by the authors. It is based on the application of a direct current periodic (DCP) waveform useful in obtaining PRPD patterns. The dependence of partial discharge inception voltage (PDIV) and PD repetition rate (PDRR) on the δ shape parameter of the DCP for different materials, as well as the capability to recognize different discharge phenomena, provided valid indications on the behavior of PD in the transition from AC to DC. The aim of this paper is to evaluate the time dependence of PD occurring in a dielectric by applying the DCP waveform. In our previous studies, the investigations were focused on the PD behavior under different values of the DC voltage periodic part. In another work, the DCP waveform with both positive and negative polarity was applied to several dielectric materials. In the proposed work, instead, the DCP waveform is applied for a long time in order to observe its effect on the PD behavior for 72 h. In this way, due to the space charge accumulation phenomenon, the aging effect, also due to the space charge accumulation phenomenon, is evaluated. The methodological approach was to acquire PRPD patterns over time and evaluate their trends in comparison with the sinusoidal case. The experimental results show that, with a DCP waveform having δ = 0.6, the aging effect similar to that provided by pure DC stress is observed, while the acquired PRPD patterns are easily interpretated, as in the AC case.

A modified three rail shear test procedure for rigid foamed materials

In this study, a modified three rail shear test (or double shear test) procedure is described for the determination of the shear moduli and the shear strength values of rigid foamed materials and test results are presented for light extruded polystyrene, which is a typical core material used in sandwich construction. The main idea is to obtain a nearly uniform shear stress distribution in the shear test specimens by strengthening them against bending. First, using a finite element model, it is numerically shown that a nearly pure shear stress state may be obtained in the strengthened three rail shear test specimens. Then, it is shown that the finite element numerical results obtained for the strengthened specimens are in excellent agreement with the analytical results. Next, three rail shear tests are performed on bare and strengthened specimens and the experimental results obtained are presented in comparison with the analytical results. Also, clamped beam vibration and three-point bending tests are conducted to validate the shear modulus values obtained. The results obtained show that, the strengthened three rail shear test specimens yield more accurate results for both shear modulus, and strength comparing with the bare specimens.

Effect of temperature on the evolution and distribution for particle size of loose broken coal during the uniaxial confined compression process

Explorations into the breakage properties of coal-rock mass in goaf is significant for stratum movement control, solid waste backfilling, and thermal-dynamic disaster prevention. In this work, a self-developed multi-field coupling experimental system for broken coal was adopted to probe both the compaction behaviors and breakage characteristics of loose coal samples as temperature increased between 30 °C and 120 °C at an increment of 10 °C, the results were compared the with those obtained under different stresses at room temperature (30 °C). Results indicate that the particle gradation tended to be reasonable gradually. Besides, for a given stress, the particle breakage rate presented a slow increasing trend initially while accelerated later under different temperatures; whereas, for the given temperature, it exhibited an opposite trend vary with the increasing stress load. For the given stress, the changing amplitude for the mass of the raw coal sample with various sizes were respectively 4.64%, 4.94%, and 7.41% under the temperature intervals of 30–60 °C, 60–90 °C, and 90–120 °C; meanwhile, for the given temperature, the changing amplitude for the mass fraction of raw coal particles were respectively 14.61%, 8.67%, and 5.9% under the stress intervals of 3–6 MPa, 6–9 MPa, and 9–12 MPa. In addition, stress played a dominant part in the breakage for coal particles, and the increase in coal temperature strengthened the stress-dominated breakage to a certain extent. Moreover, the breakage degree of coal particles under the stress-thermal synergy conditions was larger than that under pure stress conditions, and the Weibull distribution was appropriate for describing the breakage features of crushed particles in the initial stage while fractal distribution was applicable for that in the later stage. With the assistance of the scanning electron microscope (SEM), the mechanism of coal particle breakage induced by thermal stress was theoretically analyzed. Results demonstrated that with the increasing temperature, the size of cracks on coal particle surface increased while the number of cracks decreased instead. Also, the superimposition effect of the confinement stress and thermal stress primarily contributed to the stress distribution of the crushed coal particles.