Thin-walled Cylindrical Specimens Research Articles

Buckling behavior of double-layered composite cylindrical shells

Composite shells of revolution are widely used as pressure vessels, reservoirs and tanks, rocket, spacecraft parts and in different elements of the building. The aim of this study is to investigate the buckling and post-buckling behavior of the cylindrical shell made of mild steel filled with melted waste polymer, polypropylene, filled with the silicon and the steel adhesive. Furthermore, seven thin-walled cylindrical shell specimens with different composite cover subject to hydrostatic pressure were tested in the present work. The results of testing under different theories and codes are compared. Also, one specimen was without composite cover (perfect model), which used to check other specimens. This study shows that the initial buckling value of all other specimen tests was higher than the perfect specimen and double layer CFRP coiling have a significant effect on the initial buckling value. However, using material between two cylinders instead of CFRP wrapping increased initial buckling value compared to the perfect specimen. The use of melted PP was more effective than other materials (silicone, steel adhesive glue and waste polymer).

Modeling of elastoplastic deformation of steel 45 along Archimedes spiral type trajectories

This paper addresses the mathematical modeling of complex elastoplastic deformation of steel 45 along the plane trajectory in the Ilyushin’s deviatoric space, which consists of sections of both constant and variable curvature (Archimedes spiral). The constitutive equations of the proposed mathematical model are based on the Ilyushin’s vector representation of strain and stress. An approximate model of the theory of elastoplastic processes is used in mathematical modeling for plane trajectories with approximations of process functionals, which depend on the initial value of the curvature, rather than on the current curvature of the deformation trajectory. The constitutive equations of the mathematical model are reduced to the Cauchy problem, a numerical solution to which is obtained using the fourth order Runge-Kutta method. The validity of the mathematical model for this class of curvilinear strain trajectories was verified by comparing the calculation results with the experimental data obtained on the automated test machine SN-EVM in the mechanical testing laboratory of the Tver State Technical University. The experiment was carried out on thin-walled cylindrical specimens of steel 45 under complex loading (combined tension-compression and torsion). The calculation results and experimental data characterizing the scalar and vector properties of the material are presented graphically. It has been established that the proposed approximate mathematical model is able to capture (both qualitatively and quantitatively) the main effects of complex plastic deformation for the considered class of strain trajectories in the areas of small and medium curvature. More accurate calculation results in the approximations of the plasticity functionals can be obtained by taking into account all complex loading parameters, including the current curvature of the strain trajectory, especially for strain trajectories with large curvature.

CFRP Effect on the Buckling Behavior of Dented Cylindrical Shells

Considering that the use of thin-walled shells is expanding every day, it is important to examine the problem of instability in this form of structure. Many steel structures such as high-water tanks, water and oil reservoirs, marine structures, and pressure vessels, including shell elements, are under stress tension. In addition, shell elements are subject to instability owing to the loads applied. Ten thin-walled cylindrical shell specimens in two groups with different dent depths of tc and 2tc, and the different dent number subject to uniform external pressure were tested in the present research (tc is the thickness of cylindrical shell). The samples were modified to include either one or two dent line with amplitudes of h/3 in height (h the height of cylinder shell). Moreover, CFRP Strips on the dent depth was used in one of the groups. The results of testing under different theories and codes are compared.

Experimental analysis of the effect of dent variation on the buckling capacity of thin-walled cylindrical shells

Tanks, silos, and most large steel-shell structures consist of smaller pieces connected together during the manufacturing process. This causes several types of malformations on the shell walls. Furthermore, thin-walled members can be easily deformed in wall surfaces owing to the thickness of the structure. Fourteen thin-walled cylindrical shell specimens in two groups with different dent depths and various longitudinal dent numbers subject to hydrostatic pressure were tested in the present work. The models were designed to demonstrate how repairing dents by using carbon-fibre-reinforced polymer can recover lost capacity. The results of testing under different theories and codes were compared. This study shows the decreasing effects of the longitudinal dent number on the buckling capacity of the shells. Using carbon-fiber-reinforced polymer strips resulted in softening or stiffening behaviour in the models. Furthermore, to obtain the initial and overall buckling according to theoretical formulas, coefficients were predicted to obtain the initial, overall, and collapse buckling without an experiment for the models that were beyond the scope of the theories.

Investigation of the Portevin-Le Chatelier effect in metals under additional vibration impact by using the DIC-technique and the IR-analysis

In the present work, the deformation processes and local heating were analyzed by the digital image correlation technique and the infrared thermography accordingly to characterize the spatial-time inhomogeneity associated with the Chernov-Lüders behavior and the Portevin-Le Chatelier (PLC) effect in metals. To estimate the influence of the additional vibration impact on the serrated flow, the set of flat dog-bone specimens and thin-walled cylindrical specimens were deformed in tension with a biaxial servo-hydraulic testing system Instron 8850 and with additional vibration impact provided with a tensile load and a frequency of 4 Hz with a sinusoidal waveform. The results show the influence of additional cyclic loading on the spatial-time inhomogeneity of the plastic flow in metals, it is reduced the quasi-periodic character of the plastic flow.

A Temporary Local Rigid Clamping Structure to Improve Antibuckling Ability of the Thin-Walled Cylinder Under External Pressure

Although the inner container of the cryogenic liquid semitrailer works under inner pressure, it needs to be vacuumed during the helium leak detection. Furthermore, the inner container usually cannot meet the stability requirements during the evacuation, though equipped with stiffening structures such as supporting rings for baffles inside the container. Therefore, a kind of temporary local rigid clamping structure was proposed to improve the antibuckling ability of the inner container during the helium leak detection. “Lulu” can was taken as the thin-walled cylindrical shell specimen under external pressure and was clamped with the temporary local rigid ring on the outside surface. The critical pressures were experimentally and numerically studied for the specimen with local clamping rings of different sizes, in which eigenvalue buckling analysis and nonlinear analysis were employed with the aid of ANSYS. It indicates that the critical pressure of the specimen with the local clamping ring is higher than that without the clamping ring. Finally, the optimal clamping scheme including size and location of clamping rings for the inner container of DC18 type cryogenic liquid semitrailer was studied with the finite element method, which aimed to improve the antibuckling capacity of the inner container during the helium leak detection.

SUS304鋼の高温多軸疲労き裂進展と疲労寿命に及ぼす応力成分効果

Under multiaxial loading conditions, high temperature fatigue crack growth tests for SUS 304 stainless steel was conducted. In this study, effects of applied stress level on multiaxial fatigue crack growth were examined experimentally. DC electrical potential drop method was applied to thin-walled cylindrical specimen subjected cyclic tension and torsion. Effects of stress components Δσ and Δτ , and applied equivalent stress range, Δσeq on multiaxial fatigue crack growth characteristics, fatigue life, Nf , and crack propagation life, Np are examined. Crack propagation life, Np can be correlated with maximum principal stress range,Δσ1, whereas relationship between fatigue crack growth rate, db/dN and stress intensity range, ΔKIb, which is expressed as a function of Δσ1 , was affected by ratio of combined stresses, Δτ/Δσ.

Augmentation of classical failure criteria for comprehensive applicability to interfacially deficient woven-roving composites

The dominant mode of failure in the fiber-reinforced composite materials of poor fiber-matrix adhesion is the interfacial shear failure. The main objective of the current study is to modify the classical failure criteria to comprise this mode of failure as they ignore the actual failure mechanisms, which are emerging microscopic deficiencies. An interfacial shear correction factor is proposed to augment the local shear component ( σ6) in the investigated failure criteria. Thus, the shear stress concentration at fiber-matrix interface in the boundary of the deficient zone is considered. To conduct the experimental part, thin-walled cylindrical specimens are fabricated from two layers of woven-roving E-glass fabrics of [±45°] and [0°, 90°] fiber directions. The fabrics are reinforced with polyester. For each fiber orientation, the specimens are divided into four groups: the first group includes the pristine specimens and the other three groups having circular inclusions of 5, 7, or 9 mm radii, respectively. Each specimen is subjected to fully reversed bending loading using a strain-controlled testing machine. The S-N curves and the relative damage vs. number of cycles to failure (N) curves are plotted for each group. The relative damage values are used to examine the theories applicability to the studied material and to calculate the values of the proposed interfacial shear correction factor.

J0310102 SUS304鋼の高温多軸疲労き裂進展に対する負荷位相差,及びスリット形状の影響(第2報)

Effects of phase difference of loading waveform and slit geometry of specimen on high temperature fatigue crack growth of SUS304 stainless steel were examined under multiaxial loading conditions. Fatigue crack growth tests were conducted by using thin-walled cylindrical specimen and DC electrical potential drop method. It was found that crack growth rate under the out-of-phase loading is slower than that under in-phase loading. Slit geometry influences degree of the effects of phase difference on crack initiation, as well as fatigue crack growth. These tendencies result in the characteristics of fatigue life which increases under the out-of phase loading. Concerning to the effects of slit geometry, in case of the slit of low stress concentration, crack initiation life increases and fatigue crack growth rate is decelerated.

Identification of the elastic modulus of polymeric materials by using thin-walled cylindrical specimens

A method for determining the elastic modulus of polymeric materials from deformation diagrams of thin-walled circular cylindrical shells in compression and tension in the region of geometrical nonlinearity has been elaborated. A numerical solution is found by the finite-element method (ANSYS.) The existence of a unified deformation diagram in generalized coordinates is established, from which the elastic modulus is determined. To validate the method, the eigenfrequencies of cylindrical specimens were found experimentally. The results obtained are compared with FEM calculations.

The Effects of Cooling Holes on the Creep Life in a Modeling Specimen of Single Crystal Air-Cooled Turbine Blade

This paper presents the study of the influences of cooling holes on the creep life behavior in the modeling specimen of single crystal cooling turbine blade at high temperature. Thin-walled cylindrical specimens with holes are tested to model the air-cooled turbine blade. Specimens without holes are also studied to make comparisons. Experimental results show that at 900°C, the creep lives of specimens with holes are longer than those of specimens without holes. Scanning Electron Microscopy (SEM) analyses reveal that creep deformations occur firstly around the cooling holes and finally rupture at the region with low stress and strain. Finite element analyses are used to study the creep damage development by a K-R damage model which has been implemented into the Abaqus user subroutine (UMAT). Simulation results show that stress concentration and redistribution occur around the cooling holes during the creep development. It is also shown that the maximum strain and stress are around the cooling holes which are the initial rupture region in the experiments.

Flexural modulus identification of thin polymer sheets

The method of determination of the flexural Young's modulus is based on a solution to the problem of compression of a thin-walled cylindrical specimen by two parallel planes (TWCS method). This method was employed to calculate the flexural modulus for PET polymer compositions. The flexural modules received by TWCS method were verified by comparing the experimentally measured eigenfrequencies by Polytec vibrometer with numerical results from ANSYS program.

OS0739 SUS304鋼の高温多軸疲労き裂進展の定量的評価(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

Fatigue crack growth tests under multiaxial loading condition were conducted at elevated temperature. Cyclic tension and torsion was applied to the thin-walled cylindrical specimens of SUS 304 stainless steeL By using DC electrical potential drop (PD) method, fatigue crack growth under multiaxial loading condition was evaluated. Under multiaxial loading condition, two types of fatigue crack growth were observed; "tensile type" crack, which propagates along the direction perpendicular to the maximum principal stress, σ1 and "shear type" crack, which propagates along the diiection of maximum shear stress, τ_<max> The type of the crack observed for a test depends on the value of △τ/△σ and shape of the initial notch Broadly speaking, the tensile type crack growth was observed under the conditions of △τ/△σ≦0.22 and △τ/△σ≧2.l6, whereas the shear type crack growth or mixture of the both growth type were observed under the intermediate condition of △τ/△σr. Using the crack length projected to the direction of τ_<max>, shear type fatigue crack growth was quantified. From these results, the characteristics of shear type fatigue crack growth were discussed.

Creep behavior of single crystal superalloy specimen under temperature gradient condition

In this paper, a thin-walled cylindrical single crystal superalloy specimen was used to investigate the creep behavior under high-temperature conditions. The outer specimen was heated, and the inner specimen was cooled in order to generate a temperature gradient in between. Temperature distributions of the specimen were obtained from coupled analysis of flow field and solid. The temperature was transferred to a finite-element analysis model by an interpolation method. Then the creep behavior under temperature gradient was simulated with special attention given to the distribution of creep strain and damage under temperature gradient. The result shows that there is an obvious strain gradient and a damage gradient as the result of temperature gradient. The creep strain in the higher-temperature zone is larger than that in the lower-temperature zone, while the opposite is true for the creep damage. Thus, the specimen fractures from the zone where temperature is lower. Furthermore, the stress and strain development at the outer and the inner wall was investigated. The stress at the outer wall is increasing with creep time, while the stress at the inner wall is decreasing with creep time. Furthermore, the outer creep strain increases faster than the inner.

Tensile and fatigue behavior of thin-walled cylindrical specimens under temperature gradient condition

In the present study, a temperature gradient system was designed with the aim of carrying out the tensile and fatigue test of thin-walled cylindrical alloyed steel (30CrMnSi). The tensile test under different temperatures was first carried out to obtain the static mechanical parameters. And then a three-dimensional (3D) finite element model was constructed to further study the deformation behavior under temperature gradient by the finite element analysis (FEA). The FE result was in good agreement with that of the experiment. Following this, the tensile fatigue test was performed under cooled air and no-cooled air to investigate the influence of the temperature gradient on fatigue life-time, respectively. The influence of cooled air under the lower nominal stress on fatigue life was not obvious than that of higher nominal stress. Finally, the scanning electron microscopy (SEM) was employed to investigate the fracture mechanism. The microstructure revealed that the fracture first occurred at the zone where there was a lower temperature.

Plastic Anisotropy of A2024BE-T6 and O Aluminum Alloys

Multiaxial stress tests were carried out on thin-walled cylindrical specimens of the A2024BE-T6 alloy aged at 120°C for 1 hr and the A2024BE-O alloy fully annealed at 495°C for 1hr. The shear strength of these alloys is much lower than the tensile one, and the equi-strain surfaces in the tension-torsion stress field lie within Tresca's criterion. Planar anisotropy was determined from two types of multiaxial stress tests by means of the combined loading of axial load, internal pressure and torsion, referred to as “off-axis tension test” and “off-axis torsion test”. The degree of anisotropy is expressed by using two different measures : one is the proof stress and the other is the angle deviation between the directions of the principal stress (or principal shear stress) and principal strain increment (or principal shear strain increment). The A2024BE-T6 alloy exhibits more severe anisotropy than the A2024BE-O alloy and both alloys reveal more severe anisotropy in off-axis tension test than in off-axis torsion test. The plastic deformation behavior can be analyzed precisely by the constitutive equation derived from the anisotropic yield function, hardening law and associated flow rule.

Multiaxial Plastic Behavior of A2024BE-T6 Aluminum Alloy Subjected to Torsional Prestrain

It has been found that the shear strength of the A2024BE-T6 aluminum alloy is much lower than the tensile strength and that this alloy exhibits severe initial anisotropy. The purpose of this paper is to clarify the relation between initial and subsequent anisotropies by comparing with the results of the previous paper. Off-axis torsion tests by means of the combined loading of compression, internal pressure and torsion are carried out on thin-walled cylindrical specimens of the A2024BE-T6 alloy subjected to a torsional prestrain. In this test method, it is possible to detect both Bauschinger effect and planar anisotropy by changing the principal shear stress direction from 0 deg to 90 deg. Furthermore, the anisotropic hardening behavior is analyzed precisely by applying the subsequent yield function to the experimental results, and the degree of anisotropy is expressed as a function of offset strain. It is proved that the maximum anisotropy based on the flow stress corresponds to the Bauschinger effect when small offset strains are used in specifying the equi-strain locus, while it is related to the residual initial anisotropy when large offset strains are used.

Multiaxial stress effects on fatigue behavior of filled natural rubber

This work explores multiaxial stress effects on fatigue crack nucleation and growth in filled natural rubber based on experiments using short thin-walled cylindrical specimens subjected to axial and twist displacements. Cyclic stress–strain response exhibits significant initial softening relative to the monotonic response, followed by a more gradual additional softening. Irreversible breakage of various types of bonds is believed to cause the initial softening, while the presence of fillers and their influence on network chain breakage is believed to cause the additional softening. Crack nucleation planes and crack growth paths were monitored and cracks were observed to form on specific plane(s). Correlations of crack nucleation lives and growth rates with respect to the peak maximum principal strain were obtained and are discussed.

Experiments conducted in the context of the strain-space formulation of plasticity

The experiments reported here were conducted using the framework of the strain-space formulation of plasticity. A servohydraulic multiaxial testing machine was used to perform combined axial-torsional experiments on a thin-walled cylindrical aluminum specimen. The specimen was machined from extruded aluminum 1100 tubing, and annealed at 650 °F for 1 h. Conical epoxy sections were molded on the ends of the specimen for gripping. The specimen was loaded at strain rates under 20×10 −6 per minute. An offset strain of 5×10 −6 was used to define yield. A novel technique for identifying yield surfaces is introduced, which makes use of an experimentally measured focal point through which all yield probes are made to pass. Yield surfaces were obtained both in stress space and in strain space at various stages of plastic deformation. Although all strains were under 1%, the inelastic states included situations in which the origin in stress space lies outside the elastic region. The yield surfaces in both spaces were found to change shape considerably with deformation, usually developing a region of high curvature near the preload point and a region of low curvature on the opposite side of the yield surface. Experiments were also designed and conducted to test the validity of a prescription for identifying plastic strain in the context of the strain-space formulation. Changes in plastic strain were induced under incremental loading conditions and measured using the prescription. To within experimental error, the measured plastic strain increments satisfy the normality condition. The plastic strain increments are consistent with a flow rule of the type postulated in the strain-space formulation of the theory. The experimental results lend support to the constitutive theory and to the prescription for identifying plastic strain.

New Concept on Fatigue Crack Growth Characteristics under Combined Stress Conditions

Using thin walled cylindrical specimens subjected cyclic tension and torsion, fatigue crack growth tests under combined stress conditions were carried out. The characteristics of the fatigue crack growth rate (FCGR) and morphology of the fatigue crack were identified.