Experimental Study on Special Testing Strength Curve for Compressive Strength Evaluation by Rebound Method

This article aims to improve the accuracy and predictability of rebound method in compressive strength evaluation of concrete members of the Yanchong Expressway, which is an important traffic guarantee project for the 2022 Winter Olympics Games in Zhangjiakou city, Hebei Province, China. For this purpose, concrete specimens were prepared by the mixing ratios of C35, C40, and C50. First of all, the paper investigated the effects of curing age, types, and dosage of mineral admixtures (fly ash and slag) on concrete rebound number and compressive strength. Then, the special testing strength curves of 7 d and 90 d curing age were proposed. The results showed that the average relative errors of the special testing strength curves were reduced by 35.99% and 20.26% respectively, compared with the national testing strength curve. Fly ash and slag were beneficial to the growth of concrete rebound numbers but cannot contribute to the growth of concrete compressive strength (except for fly ash single-mixed with 15% dosage). The present results can provide a significant reference for the compressive strength evaluation and later maintenance of concrete members of the Yanchong Expressway or other projects with tight construction period in this area.

Environment-assisted fatigue of steel bridges: A conceptual framework for life assessment

This paper presents a framework based on a recently proposed fatigue strength curve of corroded steel to assess the life of an existing steel bridge exposed to environment-assisted fatigue. Environment-assisted cracking (EAC) and how it affects the structural integrity of steel bridges are introduced by the framework. Determination of both corroded and uncorroded details in a corrosive environment are also included in this framework. To conform the applicability and significance, a fatigue life of a railway bridge was assessed by methods given in the framework. The obtained fatigue lives were compared. The difference of the estimated fatigue lives emphasizes the importance of having this framework to consider the interaction of corrosion and fatigue mechanisms.

Development of the Sequential Voids Creation Approach in Axisymmetric Forming Dies

The study deals with the problem of topological optimization of forming dies with a limitation on fatigue strength. As a model of a stamp, a typical geometric configuration of stamps for the manufacture of parts of the " cup " type is considered. The algorithm for finding the optimal topology is proposed to be built separately in the internal areas under the stamp flanges and under the bottom of the "cup" of forming. Mathematical regularities are presented, according to which elements that fall into the area of predicted removal have a very small elastic modulus, which is widely used in topological optimization methods, then the stress state level is analyzed according to the fatigue strength curve. In the area of the flanges, there is a "build-up" of the mass according to the quadratic law with a variation in the depth of removal of the material. In the area of the bottom of the " cup " of forming, a step-by-step addition of rod elements is proposed until the level of the stress state meets the specified restrictions. Thus, this study is a modification of the topological optimization method. The novelty of the research lies in the construction of a new geometric scheme for determining the area of stored and deleted elements. The results of the study can be further developed in the development of methods for effective redistribution of material, as well as significantly reduce the material costs for the production of metal forming dies.

Fatigue Strength Curve for Tubular Joints of Offshore Structures under Dynamic Loading

Offshore structures are subjected to dynamic environmental loads (wave and wind loads). A stress-life fatigue strength curve is proposed for tubular joints which are in the splash zone area of offshore jacket structures. The Det Norske Veritas (DNV) offshore structures standards given design T-curve in the air is modified with the environment-dependent parameters to obtain this fatigue strength curve. Validity of the curve is done by comparing fatigue lives given by the proposed curve with experimental fatigue lives of tubular joints tested in seawater under different loading conditions. The fatigue assessment of a case study tubular joint is performed using the proposed curve. Nominal stress ranges of the members, which are connected to the joint, are obtained by dynamic analysis of the jacket structure. Stress concentration factors are utilized with the nominal stresses to obtain the hot spot stress ranges. Fatigue lives are calculated and compared with the conventional approach. Hence the applicability and significance of the proposed fatigue strength curve are discussed.

Bearing Strength and Failure Mechanisms of Riveted Woven Carbon Composite Joints

This research aimed to determine riveted carbon/epoxy composites’ mechanical performance when fabricated by resin transfer molding (RTM). As this manufacturing process is gaining importance in the aeronautics and automotive industries, assembly methods and their reliability must be studied in terms of their airworthiness and transportation implementation. The study case resumes the determination of the bearing strength of RTM-woven carbon composites for different rivet joint diameters (1/8, 5/32 and 3/16 in). The joint shear strength was obtained following the ASTM D5961 instructions, and post-failure analysis was carried out by a computerized tomography scan. A residual strength curve is provided with the results to infer the bearing strength for the riveted composites as a function of the rivet width-to-diameter ratio. A discussion of the fracture mechanism and tensile strength is carried out to assess the understanding of the riveted woven composites.

Experimental Study on Rebound Curve of High-Strength Concrete

As the application of high-strength concrete in civil engineering becomes more and more extensive in our country, it needs to cooperate with the popularization and application of new high-strength concrete technology so as to solve the practical problems of high-strength concrete in the engineering application of the strength detection and master the national strength measurement curve of high-strength concrete. A representative commercial concrete manufacturer in Guangzhou was selected for the detection accuracy of this area. And the general raw materials in Guangzhou were used to make concrete specimens. The 4.5J high-strength rebound tester and 5.5J high-strength rebound tester were used for testing the rebound value of the specimen and the compressive strength of the specimen at different ages. The basic research results can be used as an important basis for establishing the strength curve of high-strength concrete areas.

Analysis of free vibrations of sandwich panel with electrorheological layer based on two models of laminated shells

Based on two models of laminated shells, free low frequency vibrations of a three-layered cylindrical panel with the internal layer fabricated of an electrorheological composite are studied. Both models lead to the same governing equations accounting for shears in layers, but differ in equations for calculating the reduced shear parameter which depends on the electric field strength and the temperature of a composite. In the case of a simple support of all edges, the formula for the complex natural frequency is obtained explicitly. The influence of the electric field strength and the temperature of the electrorheological composite on the lowest natural frequencies and associated vibration decrements is investigated. It was detected that both models give very close results for the heated composite at an electric field strength of more than 1.5 kV/mm. It is also shown that the frequency of natural vibrations of the electrorheological panel is a monotonically increasing function of the electric field strength, while the decrement – strength curve shows the presence of a local maximum corresponding to the best damping of viscoelastic vibrations.

Weibull S-N Fatigue Strength Curve Analysis for A572 Gr. 50 Steel, Based on the True Stress—True Strain Approach

In this paper a Weibull methodology to determine the probabilistic percentiles for the S-N curve of the A572 Gr. 50 steel is formulated. The given Weibull/S-N formulation is based on the true stress and true strain values, which are both determined from the stress-strain analysis. For the analysis, the Weibull β and η parameters are both determined directly from the maximum and minimum addressed stresses values. The S-N curve parameters are determined for 103 and 106 cycles. In the application, published experimental data for the CSA G40.21 Gr. 350W steel is used to derive the true stress and true strain parameters of the A572 Gr. 50 steel. Additionally, the application of the S-N curve, its probabilistic percentiles and the Weibull parameters that represent these percentiles are all determined step by step. Since the proposed method is flexible, then it can be applied to determine the probabilistic percentiles of any other material.