Fatigue crack propagation behavior of RC beams strengthened with CFRP under cyclic bending loads

A fatigue crack propagation equation of reinforced concrete (RC) beams strengthened with a new type carbon fiber reinforced polymer was proposed in this paper on the basis of experimental and numerical methods. Fatigue crack propagation tests were performed to obtain the crack propagation rate of the strengthened RC beams. Digital image correlation method was used to capture the fatigue crack pattern. Finite element model of RC beam strengthened with carbon fiber reinforced polymer was established to determinate J‐integral of a main crack considering material nonlinearities and degradation of material properties under cyclic loading. Paris law with a parameter of J‐integral was developed on the basis of the fatigue tests and finite element analysis. This law was preliminarily verified, which can be applied for prediction of fatigue lives of the strengthened RC beams.     

An experimental investigation on the impact behavior of hybrid composite plates

In this experimental study, the impact behavior of hybrid composite plates has been investigated. The increasing impact energy was performed on two types of hybrid composite plates (glass–carbon/epoxy) until complete perforation of specimens. An energy profiling method, showing the relationship between impact energy and absorbed energy, was used together with load–deflection curves to determine the penetration and perforation thresholds of hybrid composites. The failure processes of damaged specimens for different impact energies were evaluated by comparing load–deflection curves and images of damaged samples taken from impacted sides and non-impacted sides. Cross-sections of damaged specimens were also inspected visually and discussed to assess the extent of damage, such as fiber fracture in layers, expansion of delaminations between adjacent layers. The perforation threshold of hybrid composite impacted from surface with carbon fibers was found approximately 30% higher than that of surface with glass fibers.     

Evaluation of fatigue life of aluminium alloy wheels under bending loads

This paper presents the details of S–N curve for aluminium alloy (Al) A356.2‐T6 and fatigue life of alloy wheels under bending load of cornering fatigue test (CFT). Development of S–N curve has been carried out by conducting rotary bending fatigue test at different stress levels as per Standards IS 5075. The rotary bending fatigue test has been performed under constant amplitude fatigue loading. The CFT of the wheel in normal driving mode has been carried out as per the procedure given in Japanese Industrial Standard Disc Wheels (JIS D_4103). It has been observed from the test that the cracks are initiated at the spoke and hub joining area closer to spanner hole on the front face of the wheel. Fatigue life of the alloy wheel has been predicted by finite element analysis (FEA), simulating the realistic test conditions. From finite element analysis, it has been observed that the maximum stress occurs at the mounting face of the wheel. Further, it has been observed that there is significant difference between the computed fatigue life and experimental value. Parametric study has been carried out for reliable fatigue life estimation and proposed an appropriate safety factor for fatigue life estimation under rotary bending test.     

Plastic loads of elbows with local wall thinning under in-plane bending

Based on systematic three-dimensional (3-D), large strain FE limit analyses using elastic- perfectly plastic materials, this paper quantifies the effect of local wall thinning on plastic behavior and TES (twice-elastic-slope) plastic loads for 90° elbows under in-plane bending. The thinning geometry is assumed to be rectangular rather than circular, but the nonlinear geometry effect is fully considered. Results from systematic analyses lead to simple approximations for TES plastic loads, covering a wide range of elbow and thinning geometries. Although the proposed approximations are developed for the case when wall thinning locates in the center of the elbow, it is also shown that they can be equally applied to the case when thinning exists anywhere within the elbow. Brief discussion is made on application of the proposed approximations to estimate maximum load-carrying capacities of elbows with local wall thinning.     

On the redesign of a shear pin under cyclic bending loads

An integrated motor and gearbox package drives a 3.9 kN flocculator train at a water filtration plant. The 4–20 rpm output shaft turns the flocculators at 1–5 rpm. Mounted on this shaft is a torque limiting coupling, utilizing a SAE 1040 steel shear pin. Analogous to a fuse in an electrical circuit, this shear pin fails if the flocculator paddles jam, thus protecting the equipment from overload. With the current design, a shear pin only last about one week. Analysis suggests that cyclic fatigue, caused by misalignment between driving and driven coupling elements, causes premature pin failure. A redesigned shear pin made from PH 13-8 Mo stainless steel, processed to improve fatigue resistance, is described.     

Delamination characterization of laminates under tension,bending and torsion loads

A two-dimensional finite-element model has been developed to calculate interlaminar stresses and strain energy release rates for the study of delamination in composite laminates subjected to tension, bending and torsion loads. This paper addresses the formulation, implementation, and verification of this element to investigate the variation of the interlaminar stresses and strain energy release rates for composite laminates. The study concentrates on establishing relationships of G_I, G_II, and G_III as functions of crack length and stacking sequence in laminates due to beding and torsion.     

Plastic loads of pipe bends under combined pressure and out-of-plane bending

This paper provides plastic limit and TES (twice-elastic-slope) plastic load solutions for 90° pipe bends under combined pressure and out-of-plane bending, via three-dimensional non-linear FE analyses using elastic-perfectly plastic materials. Without internal pressure, a closed-form approximation is given. For combined pressure and out-of-plane bending, tabulated data are given, from which TES plastic loads can be interpolated. It is found that TES plastic loads for pipe bends under out-of-plane bending are lower than those under in-plane opening bending, but are higher than those under in-plane closing bending. It suggests that the in-plane closing bending mode is the most critical loading mode for 90° pipe bends, which is fully consistent to existing findings.     

An efficient hybrid quadrilateral Kirchhoff plate bending element

This paper discusses the formulation of a hybrid stress quadrilateral Kirchhoff plate bending element based on an extended complementary energy functional first proposed by Tong. With the inclusion of a Lagrange multiplier in the functional to enforce a constraint on the assumed moment space, the construction of the C¹ deflection profile inside the element is no longer necessary. The continuity requirement on the deflection across the element interfaces is fulfilled by interpolating the generalized nodal displacements over the element boundary in the usual way. Special attention is paid to the selection of assumed moment space such that the element stability, convergency, invariance and nodal point numbering insensitivity are secured while the implementational cost of the element is kept low. Quadratic serendipity interpolation of the transverse deflection is adopted to discretize the distributed transverse loading. Numerical examples are presented and the accuracy achieved is found to be satisfactory.     

Fatigue analysis of unidirectional GFRP composites under combined bending and torsional loads

Fatigue behavior of unidirectional glass fiber reinforced polyester (GFRP) composites at room temperature under in-phase combined torsion/bending loading was investigated. All fatigue tests were carried out on constant-deflection fatigue machine with frequency of 25 Hz. A 30% reduction from the initial applied moments was taken as a failure criterion in the combined torsion/bending fatigue tests of the composite materials. A series of pure torsional fatigue tests were conducted to construct the failure contour of GFRP composites using different failure theories. The obtained S–N curves from combined torsion/bending tests were compared with both, pure torsion fatigue test results and published results of pure bending fatigue tests of GFRP rods. Pictures by scanning electron microscope were used to closely examine the failure mode of the tested specimens under combined torsion/bending loading.

The results showed that, the unidirectional glass fiber reinforced polyester composites have poor torsional fatigue strength compared with the published results of pure bending fatigue strength. Endurance limit value (calculated from S–N equation at N = 10⁷ cycles) of GFRP specimens tested under combined torsion/bending loading equals 8.5 times the endurance limit of pure torsion fatigue. On the other hand the endurance limit of combined torsion/bending fatigue strength approximately half the fatigue limit of pure bending fatigue strength. The predicted values of combined torsion/bending fatigue strength at different number of cycles, using the published failure theory are in good agreement with the experimental data. For the investigated range of fiber volume fractions (V_f) it was found that higher stress levels are needed to produce fatigue failure after the same number of cycles as V_f increases.     

An investigation on rotatory bending fretting fatigue damage of railway axles

Fretting damage failure analysis of a Chinese carbon railway axle RD₂ was carried out. The wheel hub was in situ cut to expose the damaged surface of the wheel seat to avoid additional damage. A small‐scale axle test rig was developed, and simulation tests were performed at different rotator speeds of 1800 and 2100 rpm. The wear mechanism of fretting damage areas was a combination of abrasive wear, oxidative wear and delamination. The fracture surfaces exhibited characterization of multisource and step‐profile. The fretting fatigue crack initiated at the subsurface and propagated along an inclined angle at the first stage. The fretting damage at the higher speed was more severe compared with the lower speed, which lead to a relatively shorter fatigue life. The damage morphologies of the axle in the simulation tests were in good agreement with that observed in the failure analysis on real axle.     

Finite element plastic loads for circumferential cracked pipe bends under in-plane bending

This paper proposes plastic loads (limit load and twice-elastic-slope (TES) plastic load) for pipe bends with circumferential through-wall and part-through surface cracks under in-plane bending, based on three-dimensional FE limit analyses. The material is assumed to be elastic-perfectly plastic, and both the geometrically linear (small strain) and nonlinear (large geometry change) effects are considered. Regarding a crack location, both extrados and intrados cracks are considered. Based on the FE results, closed-form approximations of limit and TES plastic loads are proposed for practical applications, and compared with corresponding solutions for straight pipes.     

弯剪复合载荷作用下复合材料层合板屈曲的强度校核方法

基于复合材料各向异性板的大挠度方程,采用Ritz法和最小势能原理推导出了正交各向异性复合材料层合板在面内弯曲载荷作用下临界屈曲弯矩的解析解;给出了面内弯剪复合载荷作用下复合材料层合板屈曲的强度校核方法,并将本文方法所得结果与用有限元法（FEM）所得结果进行了比较。结果表明,本文方法与有限元方法相吻合,并且形式简洁,便于工程应用。     

Dynamic constitutive behavior of Hastelloy X under thermo-mechanical loads

An experimental investigation has been conducted to study the dynamic constitutive behavior of Hastelloy X (AMS 5754) at room and elevated temperatures under varying rates of loading. A split Hopkinson pressure bar (SHPB) apparatus was used in conjunction with an induction coil heating system for applying dynamic loads at elevated temperatures. Experiments were carried out at different temperatures ranging from room temperature (25 °C) to 1,100 °C at an average strain rate of 5000/s. Room temperature experiments were carried out at varying strain rates from 1000 to 4000/s. The results show that as the strain rate increases from quasi-static to 4000/s, the yield strength increases by approximately 50%. Also, under dynamic loading, the yield stress decreases with temperature up to 700 °C, after which it shows a peak at 900 °C before beginning to decrease again as the temperature is further increased. The Johnson–Cook model was used to predict the dynamic plastic response under varying rates of loading and at different temperatures.     

Closed-form plastic collapse loads of pipe bends under combined pressure and in-plane bending

Based on three-dimensional (3-D) FE limit analyses, this paper provides plastic limit, collapse and instability load solutions for pipe bends under combined pressure and in-plane bending. The plastic limit loads are determined from FE limit analyses based on elastic-perfectly-plastic materials using the small geometry change option, and the FE limit analyses using the large geometry change option provide plastic collapse loads (using the twice-elastic-slope method) and instability loads. For the bending mode, both closing bending and opening bending are considered, and a wide range of parameters related to the bend geometry is considered. Based on the FE results, closed-form approximations of plastic limit and collapse load solutions for pipe bends under combined pressure and bending are proposed.     

Flexural fatigue behavior of synthesized graphene/carbon-nanofiber/epoxy hybrid nanocomposites

In the present research, effects of adding a combination of synthesized graphene nanosheets and carbon nanofibers (CNFs) on the flexural fatigue behavior of epoxy polymer have been investigated. Graphene nanosheets are synthesized based on a changing magnetic field. The flexural bending fatigue life of 0.5 wt.% of graphene/CNF/epoxy hybrid nanocomposites has been considered at room temperature. The samples were subjected to different displacement amplitudes fatigue loadings. Due to the addition of hybrid nanoparticles, a remarkable improvement in fatigue life of epoxy resin was observed in comparison with results obtained by adding 0.25 wt.% graphene or 0.25 wt.% CNF into the resin. Experimental observations show that at a strength ratio equal to 43% by using 0.5 wt.% of hybrid nanoparticles; 37.3-fold improvement in flexural bending fatigue life of the neat epoxy was observed. While, enhancement of adding only graphene or CNF was 27.4 and 24-fold, respectively.     

Developments of piezoelectric ultrasonic actuators operating under bending hybrid vibration modes

The exciting methods for the sandwich type piezoelectric ultrasonic actuators used bending hybrid modes are investigated. Five exciting methods are discussed and compared in detail, in which the polarizations and the partitions of the ceramic rings are different. Harmonic analyses are developed to obtain the effects of the exciting methods on the electromechanical coupling factors and the vibration amplitudes. At last, five prototypes are manufactured to verify their differences. Both the simulation and experiment results prove that the exciting method using ceramic rings with four separated partitions achieves great improvements on the efficiency, velocity and force, synchronously.     

Enhanced fracture assessment under biaxial external loads using small scale cruciform bending specimens

The present study is concerned with an enhanced fracture mechanics characterization of engineering materials using small scale cruciform bending specimens. Based on the regular SE(B) specimen geometry with a shallow crack, two additional loading legs allow the application of an additional stress component acting longitudinally to the crack front. Compared to standard specimen types, the biaxial loading conditions for the cruciform specimens are in general closer to the situation in pressurized vessels and pipes, especially under thermal shock loading conditions. In a combined experimental and numerical approach, detailed assessments of the local stress and strain fields in comparison to the crack front stress and strain states of standard specimens with deep and shallow cracks are provided. The cruciform bending specimen geometry is demonstrated to be suitable even in small scale dimensions. It permits the application of different combined external loading situations and thus a fracture assessment under conditions close to various situations in engineering application. Due to its small size, the specimen geometry can be employed even if only a limited amount of material is available.