Investigation on the Deformation and Failure Characteristics of Concrete in Dynamic Splitting Tests

The dynamic response behavior of concrete is constantly concerned because of seismic, impact and explosion events in the service of constructions. As a classic device for testing the dynamic mechanical properties of materials, the splitting Hopkinson pressure bar was used to carry out dynamic splitting tests on concrete in this paper. The variation of the dynamic tensile strength against the stress rate was fitted by the incubation time criterion. The full-field strain distribution on the observed surface of the specimen at the crack initiation stage was obtained by the digital image correlation (DIC) method. Morphological characteristics of the fragmentized process of concrete specimens in splitting processes were obtained by combining the image processing techniques and the FracPaQ. The size distribution of fragments of concrete specimens was obtained by sieving. The results show that the strain concentration zone and crack initiation appear along the loading direction through the center of the specimen. The secondary cracks initiated occurred at the contact end of the specimen, which expanded along the strain concentration zone and then interacted with the main crack. At the early stage of crack extension, the main crack dominates the normalized length of fracture traces in the horizontal direction. The normalized length of the vertical fracture trace increases with the main cracks opening width and the expansion of the secondary crack. The relationship between the length and angle of fracture traces in the dynamic splitting process of concrete conforms to the Gaussian function. Finally, the fragment sizes decrease with the stress rates of impact loads.     

Experimental Study on Mechanical Properties and Failure Laws of Granite with Artificial Flaws under Coupled Static and Dynamic Loads

Rock is the main construction material of rock engineering, such as the engineering of mines and tunnels; in addition, its mechanical properties and failure laws are of great significance to the stability evaluation of rock engineering, especially under the conditions of coupled static–static stresses. In this study, granite specimens were manufactured with artificial flaws. Coupled static and dynamic loads tests were carried out with a modified split Hopkinson pressure bar (SHPB) apparatus; and six typical levels of axial pre-stresses and three crack inclination angles were designed. Three-dimensional digital image correlation (3D-DIC) was also applied to record and analyze the fracturing process and damage evolution of the specimens. The test results show that there was no compaction stage in the stress–strain curve under combined dynamic and static loading. The dynamic strength of the specimens increased first and then decreased with the increase in the static pressure; moreover, the specimens reached the maximum dynamic strength when the static pressure was 10% UCS. The dynamic strength decreased first and then increased with the increase in the crack inclination angle; and the lowest strength appeared when the inclination angle was 45°. The change in axial compression had a significant influence on the failure mode, and the failure mode gradually transformed from shear–tensile failure to shear failure with the increase in the pre-stress. The tensile strain was usually generated at the end of the fractures or near the rock bridge. When the axial pressure was small, the tensile strain zone parallel to the loading direction was easily generated; and when the axial pressure was large, a shear strain zone developed, extending along the diagonal direction. The research results can provide a theoretical reference for the correct understanding of the failure mechanisms of granite and its engineering stability under actual conditions.     

Discrete Element Modelling of Cold Crushing Tests Considering Various Interface Property Distributions in Ordinary Refractory Ceramics

The microstructures and local properties of ordinary refractory ceramic materials are heterogeneous and play a role in the fracture behavior of ordinary refractory ceramic materials. It is important to consider them in numerical modeling. Herein, the discrete element (DE) method was applied to determine the influences of heterogeneity of ordinary refractory ceramic materials by applying statistically distributed interface properties (uniform, Weibull), as opposed to constant interface properties, among the elements. Uniaxial cold crushing tests were performed as a case study. A reasonable loading strain rate for receiving quasi-static loading conditions and computation efficiency was evaluated. The loading wall displacement was recorded to present the stress–strain curves of cold crushing tests. Furthermore, the effects of the interface property distributions on the load/displacement curve, fracture energy, cold crushing strength, and fracture events were investigated. The results reveal that the DE method is a promising method for visualizing and quantifying the post–peak fracture process and crack events in ordinary refractory ceramics. Different interface property distributions contribute to significant variances in the load/displacement curve shape and fracture pattern. The heterogeneity of ordinary refractory ceramics can be further determined by comparing the experimental curves and fracture propagation along with an inverse identification approach.     

Multiscale Pore Structure Characteristics and Crack Propagation Behavior of Coal Samples from High Gas Seam

Investigation on the pore-fracture features and crack propagation behavior of coal is necessary to prevent coal mine disasters. The pore structure features of coal samples taken from high gas seam were obtained by mercury injection porosimetry (MIP) and gas adsorption methods. The process of deformation and failure for coal samples under three-point bending conditions were obtained. The results demonstrate that the adsorption pores with diameter less than 100 nm are the most developed and their surfaces are the roughest (the average surface fractal dimension D_s is 2.933). The surface of micro-cracks is smoother (D_s is 2.481), which is conducive to gas seepage. It may be the explanation for that 14-3# coal seam is a high gas seam, while there was almost no gas outburst accident so far. At the initial stage of crack propagation, the main crack on the coal sample expanded along the direction of the natural cracks. In the process of crack propagation, the surface fractal dimension of the main crack increased, suggesting that the bending degree of the main crack enhanced. The brittle characteristics of coal samples can be reflected by the ratio of the dissipated energy to the accumulated energy.     

Effect of Particle Size Distributions and Shapes on the Failure Behavior of Dry Coke Aggregates

Carbon anodes participate in chemical reactions to reduce alumina in the Hall–Héroult process, of which coke aggregates make up a major part. The failure analysis of coke aggregates not only leads to a better understanding of the deformation mechanisms of anode paste under compressive loading but also can identify potential causes of structural defects in carbon anodes, such as horizontal cracks. The coke aggregates are composed of particles with different size distributions and shapes, which may strongly affect the failure behavior of the anode during compaction. In this paper, the effects of particle size distributions and shapes on the mechanical behavior and the failure of coke aggregates are investigated using the discrete element method modeling technique. The numerical results reveal that, although the mechanical behavior of coke mixtures is generally dependent on larger particles, the presence of fine particles in the coke aggregates reduces fluctuations in the stress–strain diagram. In addition, the rolling resistance model is employed as a parameter representing the effect of particle shape. It is shown that the rolling resistance model can be an alternative to the overlapped spheres model, which has a higher computational cost than the rolling resistance model. The second-order work criterion is used to evaluate the stability of the coke aggregates, the results of which indicate that the addition of fine particles as well as increasing the rolling resistance between the particles increases the stability range of the coke aggregates. Moreover, by using the analysis of micro-strain contour evaluations during the compaction process, it is shown that, both by adding fine particles to the coke mixture and by increasing the rolling resistance between the particles, the possibility of creating a compression band in the coke aggregates is reduced. Since the presence of the compaction bands in the anode paste creates an area prone to horizontal crack generations, the results of this study could lead to the production of carbon anodes with fewer structural defects.     

Damage Evolution Constitutive Behavior of Rock in Thermo-Mechanical Coupling Processes

For thermal and loaded rock in engineering structures for some projects, triple-shear Drucker–Prager yield criteria, compaction coefficient K, damage variable correction factor δ, and thermal damage variable D_T are introduced in a new thermomechanical (TM) constitutive model for the entire process. The compaction stage of rock in uniaxial compression test and the strain softening of rock caused by thermal attack are considered in this article. The damage evolution of rocks is described by a damage variable and a constitutive equation, which are in agreement with the actual thermal experimental breakage. The uniaxial compressive strength of granite subjected to a TM coupling effect can be predicted properly by this new unified constitutive model. The new TM unified constitutive model considering the compaction stage and post-failure stage is in good agreement with the test curves throughout the entire process. The coupling effect of heat and load in the total damage of rock has obvious nonlinear properties, but the coupling effect significantly weakens the specimens. By using the new TM unified constitutive model, the whole process of changes in rock damage with strain after high temperature can be calculated. Meanwhile, the model well represents the stress–strain curve at the post-failure stage. It is expected that this model can provide references for studying the mechanical response of the rock damage propagation characteristics in the future.     

Reducing Friction in Orthodontic Brackets: A Matter of Material or Type of Ligation Selection? In-Vitro Comparative Study

(1) Background: Orthodontic appliances have changed and improved with the increasing demand for orthodontic treatment of the general population. Patients desire for shorter orthodontic treatments and for the wearing of more aesthetic devices has led to the technological development of orthodontic brackets; these were manufactured from aesthetic materials (ceramics, composite polymers) and presented different designs regarding the way archwires are ligated to the bracket. The aim of this study was to determine whether there were any differences between the static frictional forces generated by stainless steel (metallic) and polycrystalline alumina (ceramics) conventional and self-ligating brackets. (2) Methods: Static friction assessment was carried out in vitro with a universal testing machine, HV-500N-S (Schmidt Control Instruments, Hans Schmidt & Co. GmbH), intended for measuring compression and traction forces. (3) Results: The study revealed significant differences in static frictional forces at the bracket-archwire interface between the tested brackets. Stainless steel brackets produced lower static friction forces than polycrystalline alumina and self-ligating brackets generally produced lower static frictional forces than conventional brackets. The reduction of frictional forces was noticeable in the first stages of treatment, when thin, flexible orthodontic archwires (0.016” NiTi) are used. Engaged with large rectangular stainless steel archwires, (0.019 × 0.025” SS), the frictional forces produced by conventional and self-ligating metal brackets were similar, no significant differences being observed between the two types of metallic design. However, in the case of tested ceramic brackets, the results showed that the self-ligating type allows a reduction in frictional forces even in advanced stages of treatment compared to conventionally ligation. (4) Conclusions: From the perspective of an orthodontic system with low frictional forces, metal brackets are preferable to aesthetic ones, and self-ligating ceramic brackets are preferable to conventional ceramic brackets.     

Experimental Study on Basalt Fiber Crack Resistance of Asphalt Concrete Based on Acoustic Emission

In this study, the semicircle three-point bending tests of ordinary asphalt concrete and basalt fiber asphalt concrete were carried out and acoustic emission parameters were collected during the test. The differences of the characteristics of acoustic emission parameters between basalt fiber asphalt concrete and ordinary asphalt concrete were analyzed, and the damage stages were divided based on the variation of acoustic emission parameters; Rise Angle and Average Frequency were introduced to study the cracking mode and crack resistance mechanism of asphalt concrete with basalt fiber. The results show that the acoustic emission parameters can well represent the toughening and crack resistance effect of basalt fiber in asphalt concrete, and the damage stages can be divided into three stages: microcrack initiation stage, fracture stage, and residual stage. The duration of the fracture stage and the load resistance time of the specimen were greatly prolonged. The proportion of shear events in the whole failure process increased greatly after the basalt fibers were added, especially in the fracture stage, which reduced the tensile failure tendency of the specimens, and thus improved the bending and tensile performance of the specimens and played a toughening and crack resistance role in the fracture stage.     

The Influence of Microstructure on the Flexural Properties of 3D Printed Zirconia Part via Digital Light Processing Technology

In recent years, additive manufacturing of ceramics is becoming of increasing interest due to the possibility of the fabrication of complex shaped parts. However, the fabrication of a fully dense bulk ceramic part without cracks and defects is still challenging. In the presented work, the digital light processing method was introduced for fabricating zirconia parts. The flexural properties of the printed zirconia were systematically investigated via a three-point bending test with the digital image correlation method, scanning electron microscopy observation and fractography analysis. Due to the anisotropy of the sample, the bending deformation behaviors of the zirconia samples in the parallel and vertical printing directions were significantly different. The flexural strength and the related elastic modulus of the samples under vertical loading were higher than that of the parallel loading, as the in-plane strength is higher than that of the interlayer strength. The maximum horizontal strain always appeared at the bottom center before the failure for the parallel loading case; while the maximum horizontal strain for the vertical loading moved upward from the bottom center to the top center. There was a clear dividing line between the minimum perpendicular strain and the maximum perpendicular strain of the samples under parallel loading; however, under vertical loading, the perpendicular strain declined from the bottom to the top along the crack path. The surrounding dense part of the sintered sample (a few hundred microns) was mainly composed of large and straight cracks between printing layers, whereas the interior contained numerous small winding cracks. The intense cracks inside the sample led to a low flexural property compared to other well-prepared zirconia samples, which the inadequate additive formulations would be the main reason for the generation of cracks. A better understanding of the additive formulation (particularly the dispersant) and the debinding-sintering process are necessary for future improvement.     

Calibration of Drucker–Prager Cap Constitutive Model for Ceramic Powder Compaction through Inverse Analysis

Phenomenological plasticity models that relate relative density to plastic strain are frequently used to simulate ceramic powder compaction. With respect to the form implemented in finite element codes, they need to be modified in order to define governing parameters as functions of relative densities. Such a modification increases the number of constitutive parameters and makes their calibration a demanding task that involves a large number of experiments. The novel calibration procedure investigated in this paper is based on inverse analysis methodology, centered on the minimization of a discrepancy function that quantifies the difference between experimentally measured and numerically computed quantities. In order to capture the influence of sought parameters on measured quantities, three different geometries of die and punches are proposed, resulting from a sensitivity analysis performed using numerical simulations of the test. The formulated calibration protocol requires only data that can be collected during the compaction test and, thus, involves a relatively smaller number of experiments. The developed procedure is tested on an alumina powder mixture, used for refractory products, by making a reference to the modified Drucker–Prager Cap model. The assessed parameters are compared to reference values, obtained through more laborious destructive tests performed on green bodies, and are further used to simulate the compaction test with arbitrary geometries. Both comparisons evidenced excellent agreement.     

Influence of Sample Wetting Method on ESC-Behavior of PMMA under Dynamic Fatigue Crack Propagation

Environmental stress cracking (ESC) is one of the most prominent failure mechanisms for polymer components. The high sensitivity of plastics in regard to environmental influences has always meant that plastics as materials have been viewed very critically in outdoor applications. Recently, the massive occurrence of microplastics in the environment means that questions about the long-term stability of plastic parts and the studies of plastic fragmentation are of great scientific interest. ESC behavior also plays an important role in connection with the formation of microplastics. In this work, the influence of two different sample wetting methods on ESC behavior was investigated. In case A, the sample was in situ wetted with the medium during the measurement by using a sponge. In case B, the sample was wetted by storage in the medium prior to measurement. Different stress cracking agents (SCA) were examined for polymethylmethacrylate (PMMA). Fracture-mechanical fatigue crack propagation (FCP) tests were carried out to quantitatively determine the sensitivity to ESC. Correlations between the absorption behavior and the ESC behavior of the SCA and the resulting morphological phenomena were established. Depending on the wetting method, significant differences in FCP were observed. The in situ wetting of the samples (case A) during the FCP measurement with ethylene glycol (EG) and with deionized water (DI) led to a significant shift in the crack propagation curves to higher ∆K—compared to the PMMA reference. In the case of n-heptane (NH), a more brittle crack propagation behavior was observed due to the chemical interaction with PMMA. The previously immersed samples (case B) give different results. Storage in NH and EG showed no influence on the crack propagation behavior. Samples immersed in DI showed a completely different course of crack growth. At a certain load, a sudden deceleration of the crack propagation and thus a horizontal curve could be seen. Above a certain ∆K value, crack growth began again. Depending on the immersion time (14, 30, or 60 days), this so-called stepped behavior shifted to lower da/dN values.     

A Flow Stress Model of the AA3104-H19 Alloy for the FEM Simulation of the Beverage Can Manufacturing Process under Large Plastic Deformations

This paper discusses the development of a flow stress model to simulate the AA3104-H19 alloy under the conditions of large plastic deformations characteristic of the beverage can manufacturing process. This study focuses on the first five steps of this process: cupping, redrawing, ironing #1, ironing #2, ironing #3. These are the stages that reduce the thickness of the base material to the maximum, resulting in an effective strain of more than 2.0, unattainable in conventional plastometric tests. To solve this problem, the specific calculation-experimental method for the development of the flow stress model was proposed. Based on the FEM modeling of the technological process, data on the history of deformation and the trajectory of movement of the selected points of the material at all stages of the production were obtained. Microspecimens for the tensile tests were taken from the points of the beverage can wall that were determined in this way. The initial strain of each sample was taken from the FEM simulation. In this way, the tensile curves were obtained for the material points at different stages of the production. The processing of these curves allowed the creation of a flow stress model for large strains, corresponding to production conditions. The tensile tests were performed on a Zwick Z250 machine at room temperature and strain rate of 0.005 s⁻¹. The FEM-based algorithm for the determination of empirical coefficients of the analytical flow stress model is presented. The final flow stress model covers the range of effective strain from 0–2. Validation of the developed model based on the measured beverage can thicknesses showed that a flow stress model was developed that correctly and accurately describes the forming process.     

嗜人按蚊不同发育阶段溴氰菊酯敏感性分析

目的 研究嗜人按蚊不同发育期溴氰菊酯敏感性动态变化。方法选用室内选育的嗜人按蚊溴氰菊酯敏感品系和抗性品系，以浸渍法测试幼虫溴氰菊酯敏感性，以WHO推荐的接触筒强迫接触法测试成蚊溴氰菊酯敏感性。结果幼虫期，溴氰菊酯敏感品系和抗性品系对杀虫剂的敏感性由低至高依次均为Ⅳ、Ⅲ、Ⅱ龄幼虫；成蚊期，两种品系的敏感性均以早期成蚊最低，中期成蚊次之，晚期成蚊最高。结论嗜人按蚊两品系不同生长期溴氰菊酯敏感性消长相似，敏感性水平随幼虫的发育而上升，随成蚊蚊龄增长而下降。     

嗜人按蚊不同发育阶段溴氰菊酯敏感性分析

目的研究嗜人按蚊不同发育期溴氰菊酯敏感性动态变化。方法选用室内选育的嗜人按蚊溴氰菊酯敏感品系和抗性品系,以浸渍法测试幼虫溴氰菊酯敏感性,以WHO推荐的接触筒强迫接触法测试成蚊溴氰菊酯敏感性。结果幼虫期,溴氰菊酯敏感品系和抗性品系对杀虫剂的敏感性由低至高依次均为Ⅳ、Ⅲ、Ⅱ龄幼虫;成蚊期,两种品系的敏感性均以早期成蚊最低,中期成蚊次之,晚期成蚊最高。结论嗜人按蚊两品系不同生长期溴氰菊酯敏感性消长相似,敏感性水平随幼虫的发育而上升,随成蚊蚊龄增长而下降。     

Research on the Evolution of Shield Segment Cracks Based on Acoustic Emission and CMOD

In order to explore the cracking law and failure characteristics of segments, a model test of shield segment cracking was conducted. The microscopic and macroscopic crack evolution process of the segment is studied by using acoustic emission detection technology and crack opening displacement (CMOD). According to the acoustic emission signal and CMOD, characteristics generated in the process of segment cracking, in the form of numerical value, the evolution characteristics of each stage of segment cracking are directly reflected. Based on acoustic emission energy and CMOD, the segment cracking damage model was established to determine the segment fracture damage degree. The result shows that segment cracking can be divided into three stages, and the acoustic emission detection results and CMOD have different degrees of change in each cracking stage. This proves that both the acoustic emission acquisition results and CMOD can be used as evaluation indicators of damage degree. Acoustic emission can accurately identify the crack evolution process, and the yield strengthening is an important stage of crack damage evolution. The damage data points in this stage account for 76.83% of all the damage data points, the occurrence rate of damage data points is 0.225 s, and the density of data points in the damaged area is 3.219 × 10⁻⁴ mm³, which is larger than the other two stages. The segment cracking damage model can effectively reflect the segment cracking degree and provide a reference for the actual segment cracking assessment.     

Large Deformation and Energy Absorption Behaviour of Perforated Hollow Sphere Structures under Quasi-Static Compression

Hollow sphere structures with perforations (PHSSs) in three different arrangements (simple cubic (SC), body-centred cubic (BCC), and face-centred cubic (FCC)) were fabricated through three-dimensional (3D) printing, and the mechanical behaviours of these PHSSs under quasi-static compression were investigated experimentally and numerically. The results indicated that under uniaxial compression, the PHSSs mainly undergo three stages, i.e., a linear elastic stage, a large deformation or plateau stage, and a densification stage. During the stage of large deformation, the SC and BCC PHSSs experience a preliminary compaction sub-stage after layer-by-layer buckling, while for the FCC PHSS, layer-by-layer collapse and compaction are the dominant deformation behaviours. A numerical simulation was employed to study the mechanical properties of PHSSs with different geometric parameters under quasi-static compression and to explore the effect of the wall thickness, hole diameter, and sphere arrangement on the first peak stress, plateau stress, and specific energy absorption (SEA) of the PHSSs. The results reveal that the geometric parameters have a significant impact on the large deformation behaviour and energy absorption capacity of PHSSs. The presented PHSS is also proven to be much lighter than traditional metallic hollow sphere structure (MHSS) and has higher specific strength and SEA.     

Prevalence of Co-morbidities and Severity of COPD

We aimed at exploring whether the prevalence of co-morbidities of chronic obstructive pulmonary disease (COPD) increases with COPD severity. Analysis of medical records of outpatients with established diagnosis of COPD was retrospectively performed. The lower limit of normality (LLN) for FEV1/FVC was applied to establish the occurrence of airway obstruction in the elderly population. The prevalence of co-morbidities was calculated, and the proportion of patients with each co-morbidity along with GOLD stages was analysed by chi-square for trend. A total of 326 (M/F: 256/70) consecutive outpatients with COPD (stage GOLD I to IV), aging 71.8 ± 9.2 years, were included in the analysis. The most frequent co-morbidities in the entire sample were systemic hypertension (64.7%), diabetes (28.5%), coronary artery disease (19.9%), arrhythmias (16.6%) and congestive heart failure (13.8%). Underweight patients were 8.0% of the sample while obese patients were 22.4%. None of the analyzed co-morbidities showed a trend towards increasing prevalence with COPD severity, except for nutritional problems. The current findings suggest that the occurrence and prevalence of co-morbidities is independent from the COPD severity, and encourage to assess co-morbidities even in the early stages of the COPD.     

The Room Temperature Fracture Behaviors of GNPs/TA15 Composites by Pre-Sintering and Hot Extrusion

Graphene nanoplates (GNPs)/TA15 composites were fabricated by pre-sintering and hot extrusion. During a room temperature tensile test, the dislocation was generated in grains. With increasing strain, the dislocation piled up along the interface between GNPs and Ti matrix, leading to stress concentration and microcracks. Then, the microcracks extended to GNPs or along the interface. The GNPs cracked under the shear force and the GNPs pulled out along with the crack propagation along the interface. This work provides a new sight in the room temperature tensile fracture behaviors.     

Linear Elastic Fracture Mechanics Assessment of a Gas Turbine Vane

This work assesses the crack propagation at the most critical point of a second stage of a gas turbine blade by means of linear elastic fracture mechanics (LEFM). The most critical zone where the crack may nucleate, due to a combination of thermo-mechanical loads, is detected with an uncracked finite element (FE) model pre-analysis. Then the sub-modelling technique is used to obtain more precise results in terms of stresses within the area of interest. Simulations of the state of stress at the crack apex are performed through an FE model, using the Fracture Tool within ANSYS Workbench, and the stress intensity factors (SIFs) are determined accordingly. The Fracture Tool was previously verified on a simple model, and the results were compared with its analytical solution. Finally, the evaluation of the crack growth due to fatigue stress, creep, and oxidation is performed through in-house software called Propagangui. The crack behavior is estimated along with the component life. Results show an unexpected decrease in KI with increasing crack length and slowing of the crack growth rate with crack propagation. A detailed analysis of this behavior emphasizes that the redistribution of the stresses at the crack apex means that unstable propagation is not expected.     

Strain Rate and Stress Amplitude Effects on the Mechanical Behavior of Carbon Paste Used in the Hall–Héroult Process and Subjected to Cyclic Loadings

Carbon products such as anodes and ramming paste must have well-defined physical, mechanical, chemical, and electrical properties to perform their functions effectively in the aluminum electrolysis cell. The physical and mechanical properties of these products are assigned during the shaping procedure in which compaction stresses are applied to the green carbon paste. The optimization of the shaping process is crucial to improving the properties of the carbon products and consequently to increasing the energy efficiency and decreasing the greenhouse gas emissions of the Hall–Héroult process. The objective of this study is to experimentally investigate the effect(s) of the strain rate, of the stress maximum amplitude, and of the unloading level on the behavior of a green carbon paste subjected to cyclic loading. To this end, experiments consisting of (1) cyclic compaction tests at different maximum stress amplitudes and strain rates, and (2) cyclic compaction tests with different unloading levels were carried out. The study obtained the following findings about the behavior of carbon paste subjected to cyclic loads. The strain rate in the studied range had no effect either on the evolution of the permanent strain as a function of the cycle number, nor on the shape of the stress–strain hysteresis during the cyclic loading. Moreover, samples of the same density that had been subjected to different maximum stress amplitudes in their loading history did not have the same shape of the stress–strain curve. On the other hand, despite having different densities, samples subjected to the same number of cycles produce the same stress–strain curve during loading even though they were subjected to different maximum stress amplitudes in their loading histories. Finally, the level of unloading during each cycle of a cyclic test proved significant; when the sample was unloaded to a lower level of stress during each cycle, the permanent strain as a function of the cycle number was higher.