New Training Tasks for Stepwise Loading in Isometric Bodyweight Squat with Active Posture Control

Isometric bodyweight squats are fundamental exercises in athletic training and rehabilitation. Previously, we measured muscle activity in a normal squat posture (NSP) and a squat posture with the center of foot pressure (COP) intentionally shifted forward as far as possible (forward-shifted posture: FSP), and the muscle activity patterns varied significantly according to the COP location. This study focused on stepwise loading as a training strategy. Ten healthy male participants performed isometric bodyweight squats in a previous study, adopting the NSP and FSP, with three knee flexion angles (30°, 60°, and 90°). The muscle activities of the vastus medialis (VM), semitendinosus, tibialis anterior (TA), and gastrocnemius muscle lateral head were measured using surface electromyogram. This study further explored the relationship between COP shifting and knee flexion angles on electromyogram changes using three-dimensional diagrams. In one-way repeated measures analysis of variance by ranks, knee flexion angles affected the muscle activities of the VM and TA in the NSP and muscle activities of the VM in the FSP. Combining these findings, stepwise loading tasks were created to train individual target muscles. The ten male participants examined all the tasks, and the feasibility was confirmed accordingly.

On the relationship between mechanical energy rate and heat dissipated rate during fatigue for a C45 steel depending on stress ratio

This work deals with the analysis in the frequency domain of the temperature signal and mechanical energy rate of C45 steel under two different fatigue stepwise loading series at stress ratios of 0.1 and -1. It was first investigated the energy distribution among the harmonic components of the signals to understand possible variations caused by a different stress ratio. In addition, the second amplitude harmonic (SAH) of heat dissipated and mechanical energy rates have been considered in the analysis and their relationship was investigated. It has been shown as it depends only on the material, hence it is valid whatever the kind of the test is without any assumption on the energy supplied to the material or material hysteresis loop stabilisation. The adopted approach allows the analysis of intrinsic dissipations by means of rapid, full-field and contactless techniques without any specific requirement on loading condition or temperature signal stabilisation.

MODELING OF THE COOL CREEPIN THE FRAMEWORK OF THE HARDENING THEORIES ON THE STEPWISE LOADING

In the framework of the hardening hypothesis, the creep behavior of a titanium alloy at room temperature is modeled at stepwise stress change both increasing and decreasing load. Methods for the identification of material constants and functions included in constitutive equations of the hardening theories are considered in detail. The prospects for the computational-experimental method proposed by Rybakina for determination of material constants in some versions of the hardening theory are noted. The results of the creep modeling of titanium alloy confirm the existing theoretical possibilities and limitations of the hardening hypothesis fully.

Compaction and Breakage Characteristics of Crushed Stone Used as the Backfill Material of Urban Pavement Subsidence

The subsidence of urban pavement is becoming frequent accidents, and backfill is the primary means of remedy. Crushed stone is a commonly used aggregate for backfill material in engineering, and its compaction behavior under load needs to be well understood. In this work, a series of compaction tests were carried out on the same batch of crushed stone samples with the same gradation. The content changes of particles with different sizes were analyzed, the particle breakage characteristics during the compaction process were discussed, and the difference in particle breakage caused by loading speed and loading mode was examined. It shows the following: (1) For all samples, the content of particles that were crushed during compaction was always less than 40%. The particles with the strongest breakage varied with sample gradation. (2) The particle breakage could be divided into four categories: complete fragmentation, complete rupture, local fragmentation, and surface grinding. They affected the particle size distribution after compaction to varying degrees. (3) The particle breakage could be expressed as a cubical parabola of loading speed, whose coefficients are related to the sample gradation. (4) Stepwise loading rendered stronger particle breakage than direct loading, and the increase of particle breakage due to loading mode was more evident for continuous grading samples than discontinuous grading samples. This study will provide an experimental basis and reference for the selection and use of backfill aggregate in urban subsidence areas.

Thermal Insulating and Constructive Foamed Concrete on a Composite Gypsum Binder

The article discusses the possibility of obtaining the effective thermal insulating and constructive foamed concrete on a composite gypsum binder. The composition was selected, the stepwise loading scheme of the foam concrete mixture components was proposed, the properties and microstructure of cellular concrete based on a composite gypsum binder with mineral addition of finely-dispersed concrete scrap were studied.

EFFECTIVE CELLULAR CONCRETE ON THE COMPOSITE GYPSUM BINDER

The dynamically developing construction of the Russian Federation makes it necessary to expand the range of alternative types of binders and materials based on them. Such binders include a composite gypsum binder used for the production of materials for various functional purposes. The manufacture and use of products based on composite gypsum binders is made possible by studying the Portland cement – gypsum – water system, the stability of which is ensured by introducing of an appropriate amount of active mineral additives. Such additives reduce the concentration of Са (ОН) 2 in the liquid phase of the hardening system and create the possibility of hardening under certain conditions without dangerous internal tensions. This article discusses the obtaining of effective cellular concrete on a composite gypsum binder. Cellular concrete surpasses some traditional materials in its structure, properties, methods of preparation, and they are universal in terms of operational properties. The possibility and expediency of using thin-ground concrete scrap as a mineral additive in the composition of composite gypsum binder for cellular concrete is established in the work. Thermal insulation and structural cellular concrete of D600 and D700 grades are obtained. It is revealed that the stepwise loading of the components of the concrete mixture with the initial introduction of a gypsum binder is rational.

Stepwise loading-unloading creep testing of fly ash concrete and its constitutive model

Stepwise loading-unloading creep testing of concrete with fly ash content of 35%, and 50% was conducted. The time course curve of stepwise creep in fly ash concrete was obtained. Analyses have revealed that it had decelerated creep, constant velocity, and accelerated creep properties. Based on rheological theory, a non-linear viscoelastic-plastic rheological model (MSSB-NVPB) was constructed, and its constitutive relations and creep equations were obtained. Combined with experimental data, the model parameters were determined. The results showed that this model can characterize the creep properties of the fly ash concrete fairly well.

The Effect of Confining Pressure and Water Content on Energy Evolution Characteristics of Sandstone under Stepwise Loading and Unloading

To investigate the mechanical properties and energy evolution characteristics of sandstone depending on the water contents and confining pressure, the uniaxial and triaxial tests were conducted. The test results show that the strain energy was stored in the sandstone samples at the prepeak stage, and that is suddenly released when the failure occurred, and energy dissipation is sharply increased at the postpeak stage. The damage and energy dissipation characteristics of the samples are observed clearly under the stepwise loading and unloading process. The critical strain energy and energy dissipation show a clear exponential relationship. The critical elastic energy decreases linearly as the water content increases. As the confining pressure increases, the critical elastic energy of the samples transforms from linear to exponential. The concept of energy enhancement factor is proposed to characterize the strengthening effect induced by the confining pressure on the energy storage capacity of the rock samples. The energy evolution of the sandstone samples is more sensitive to the confining pressure than that of the water content.