Theoretical and numerical investigation of the mode I delamination of composite laminate with uneven thickness

In this paper, a theoretical analysis model and two simulation methods are applied to characterize the quasi-static and fatigue delamination of composite laminate with uneven thicknesses. The test data of partially reinforced double-cantilever beam (DCB) were used as benchmark to verify the analysis model and simulation, and cohesive zone models (CZMs) and virtual crack closure technique (VCCT) are used in simulation. It’s shown that the partially reinforced DCB has a unique double-peak load-displacement relationship, and produces instability development during the delamination. By comparing the results of simulation and experiment, it is found that the simulation based on the exponential CZM can simulate the delamination process of partially reinforced DCB under both quasi-static and fatigue loading; while VCCT method will generate a straight delamination front edge in the area of reinforcement, and lost the micro-damage of the previous loading step between load steps, and result in an incorrect delamination behavior.

Experimental Study of Mullins Effect in Natural Rubber for Different Stretch Conditions

We subjected rubber coupons to cyclical uniaxial tension to investigate the softening effect, where the primary loading at its initial position was followed by additional unloading and reloading. Less stress was required upon reloading than that required in the previous loading for the same degree of stretch, reached on the first loading. This stress softening is significant when reloading follows virgin loading. The magnitude of stress softening is related to the maximum stretch elastomers can achieve in each cycle. To investigate this phenomenon, rubber coupons were subjected to four cycles of simple tension until the desired stretch was reached. We expected that several tests under the same conditions would provide almost identical results. However, we observed different stress requirements for different degrees of stretch when multiple cycles of the same stretch were performed. For three different experimental tests of the same amount of stretch, we saw huge differences in each cycle of loading-relaxation-reloading, a phenomenon that was more obvious during stress relaxation.

Buckling Analyses of Spherical Shells by the Finite Element Method Based on the Willis-Form Equations

Buckling analysis of spherical shells under external pressure is a crucial problem in mechanical and aerospace engineering. It is widely known that the buckling loads obtained by classical methods are much higher than experimental results. The main reason for this large discrepancy is customarily attributed to initial geometrical imperfections, and the impact of inhomogeneously distributed stresses during loading process is usually ignored. In order to investigate the effect of this ignored factor, the buckling loads of several spherical shells are analyzed by the geometrically nonlinear finite element method (FEM) based on the Willis-form equations, which explicitly contain the stress gradients at previous loading step. It can be shown that the buckling loads from the Willis-form FEM are about 10% lower than the values from classical FEM. This finding may give better understandings to the differences between theoretical and experimental results for nearly perfect spherical shells and may be helpful to obtain more accurate buckling loads for shells with initial geometrical imperfections.

THE ELASTOPLASTIC CALCULATION OF FRAMES USING THE DISPLACEMENT METHOD

We proposed a method for calculating statical indeterminacy frames taking into account plastic defor­mations, which is based on the use of a schematized diagram of material with hardening. Two types of standard beams with supports are used during the implementation of the displacement method (DM) and the elastic solu­tion of the problem: “fixed” - “pinned” and “fixed” - “fixed”, but unlike the elastic solution, standard beams con­tain plastic zones (PZs). So as the stresses in these zones did not exceed the limit of yielding in the nonlinear frame calculation, we took measures to transform the PZs into equal strength plastic zones (ESPZ). The calcula­tions were made for both types of beams for all single and load impacts. The frame calculation consists of two stages (elastic and plastic). At the elastic stage, we determine an elastic moment diagram and the corresponding load. For a practical use of the DM in a nonlinear frame calculation, we introduced a simplifying prerequisite sup­plementing the well-known hypotheses of the classical version of the method, and formulated a Statement of the limiting load. According to the Statement, each length of the PZ can correspond to the lower boundary of the lim­iting load. The plastic stage of the calculation is performed at a given length of the PZ using the method of se­quential loadings. At each loading stage, incremental equations are written using the DM equations, which estab­lish relations between incremental moments and the incremental load, that allows you to get the resulting moment diagram. This diagram represents a sum of the elastic diagram and the diagrams of incremental moments at all previous loading stages. According to the resulting diagram, we calculate the length of the PZ, together with the limiting load. The calculation is considered complete if the length of the PZ does not exceed the specified value within the margin of error.

Corrosion of RC Beams During Time

This article presented results of impact corrosion on reinforce concrete beams during time with previous loading. Before testing the RC beams were loaded by 37% and 75% from bearing-capacity load of undamaged corrosion beams. There were tested two types of beams. First type beams were tested under force loading action and second type beams were tested under force loading and corrosion action. As an aggressive environment were used the 10% solution of sulfuric acid H2SO4. Such environment takes place in separate chemical manufactures, galvanic workshops, flue pipes of thermal power plants. The results of experimental studies have established that the effect of the aggressive environment with simultaneous loading significantly impairs the stress-strain state of reinforced concrete beams. Bearing capacity due to the yielding of reinforcement bars for specimens with the simultaneous action of aggressive environment and previous loading of 37% and 75% takes place on 51…53 and 58…60 days. The destruction of these specimens, by crushing compressive concrete, at previous loading 37% occurred after 75…79 and at previous loading 75% - after 79…88 days. The history loading does not impact significantly on beams failure, on finite deflections and timing exhaustion bearing capacity since the start of the simultaneous action of aggressive environment and loading.

Effects of Detwinning on the Inelasticity of AZ31B Sheets During Cyclic Loading and Unloading

Magnesium alloys exhibit significant inelastic behavior during unloading, especially when twinning and detwinning are involved. It is commonly accepted that noteworthy inelastic behavior will be observed during unloading if twinning occurs during previous loading. However, this phenomenon is not always observed for Mg sheets with strong rolled texture. Therefore, the inelasticity of AZ31B rolled sheets with different rolled textures during cyclic loading-unloading are investigated by elastic viscoplastic self-consistent polycrystal plasticity model. The incorporation of the twinning and detwinning model enables the treatment of detwinning, which plays an important role for inelastic behavior during unloading. The effects of texture, deformation history, and especially twinning and detwinning on the inelastic behaviors are carefully investigated and found to be remarkable. The simulated results are in agreement with the available experimental observations, which reveals that the inelastic behavior for strongly rolled sheets is very different than the extruded bars.

A Study of Creep Behavior of TSV-Cu Based on Nanoindentaion Creep Test

Through-Silicon-Via (TSV) is considered to be the most potential solution for 3D electronic packaging, and the mechanical properties of TSV-Cu are critical for TSV reliability improving. In this paper, to make deeply understand the creep behavior of TSV-Cu, nanoindentation creep tests were conducted to obtain its creep parameters. At first, the TSV specimens were fabricated by means of a typical TSV manufacturing process. Then a combination programmable procedure of the constant indentation strain rate method and the constant load method was employed to study the creep behavior of TSV-Cu. To understand the influence of the previous loading schemes, including the different values of the indentation strain and the maximum depths, the nanoindentation creep tests under different loading conditions were conducted. The values of creep strain rate sensitivity m were derived from the corresponding displacement-holding time curves, and the mean value of m finally determined was 0.0149. The value of m is considered no obvious correlation with the different indentation strain rates and the maximum depths by this method. Furthermore, the mechanism for the room temperature creep was also discussed, and the grain boundaries might play an significant role in this creep behavior.

Experimental Study on the Secondary Consolidation Properties of Marine Soft

Through the one dimensional consolidation of creep test, the paper presents and analyzes the deformation law of reloading under different stress level, the variation of the coefficient of consolidation and the relation between the secondary consolidation deformation and loading history. These test results show that the secondary compression during previous loading has great influence on the subsequent deformation of soil.

Soil Phosphorus Storage Capacity for Environmental Risk Assessment

Reliable techniques must be developed to predict phosphorus (P) storage and release from soils of uplands, ditches, streams, and wetlands in order to better understand the natural, anthropogenic, and legacy sources of P and their impact on water quality at a field/plot as well as larger scales. A concept called the “safe” soil phosphorus storage capacity (SPSC) that is based on a threshold phosphorus saturation ratio (PSR) has been developed; the PSR is the molar ratio of P to Fe and Al, and SPSC is a PSR-based calculation of the remaining soil P storage capacity that captures risks arising from previous loading as well as inherently low P sorption capacity of a soil. Zero SPSC amounts to a threshold value below which P runoff or leaching risk increases precipitously. In addition to the use of the PSR/SPSC concept for P risk assessment and management, and its ability to predict isotherm parameters such as the Langmuir strength of bonding,KL, and the equilibrium P concentration, EPC0, this simple, cost-effective, and quantitative approach has the potential to be used as an agronomic tool for more precise application of P for plant uptake.