Transverse Cyclic Loading Research Articles

Effects of the presence of compression in transverse cyclic loading on fibre–matrix debonding in unidirectional composite plies

Fatigue of composite materials is of great concern in load-carrying structures. The first type of damage to appear is generally transverse cracks in off-axis plies. These cracks form when fibre-matrix debonds coalesce. The underlying mechanism is hence fatigue growth of debonds at the fibre-matrix interfaces. In the present study, debond growth has been characterized under tensile and compressive cyclic loading of single glass fibres embedded in polymer matrix. The debond length was deter-mined by in situ microscopy with transmitted polarized light showing the more damaging effect of tension-compression cyclic loading than tension-tension cyclic loading. A boundary element model has been developed and interfacial fracture mechanics concepts applied over the numerical results aiming to give an explanation of this experimental fact. These results may be used to formulate a fatigue growth law at a local microscopic level, at a stage prior to the formation of any visible damage, i.e. transverse cracks. Ideas of how to develop this methodology further are also discussed.

Study of the Effect of Hole Clearance and Thread Fit on the Self-Loosening of Threaded Fasteners

This study provides an experimental and theoretical investigation of the effect of hole clearance and thread fit on the self-loosening of tightened threaded fasteners that are subjected to a cyclic transverse service load. An experimental procedure and test setup are developed in order to collect real-time data on the rate of clamp load loss per cycle as well as the loosening rotation of the bolt head. Three levels of hole clearance are investigated; namely, 3%, 6%, and 10% of the bolt nominal diameter. For the commonly used 2A thread fit for a selected bolt size, three classes of the nut thread fit are considered; namely, 1B, 2B, and 3B. A simplified mathematical model is used for the analytical investigation of the effect of the hole clearance and thread fit on threaded fasteners self-loosening. The experimental and theoretical results are presented and discussed.

Testing and life prediction for composite rotor hub flexbeams

A summary of several studies of delamination in tapered composite laminates with internal ply-drops is presented. Initial studies used 2D FE models to calculate interlaminar stresses at the ply-ending locations in linear tapered laminates under tension loading. Strain energy release rates for delamination in these laminates indicated that delamination would likely start at the juncture of the tapered and thin regions and grow unstably in both directions. Tests of glass/epoxy and graphite/epoxy linear tapered laminates under axial tension delaminated as predicted. Nonlinear tapered specimens were cut from a full-size helicopter rotor hub and were tested under combined constant axial tension and cyclic transverse bending loading to simulate the loading experienced by a rotor hub flexbeam in flight. For all the tested specimens, delamination began at the tip of the outermost dropped-ply group and grew first toward the tapered region. A 2D FE model was created that duplicated the test flexbeam layup, geometry, and loading. Surface strains calculated by the model agreed very closely with the measured surface strains in the specimens. The delamination patterns observed in the tests were simulated in the model by releasing pairs of multi-point constraints (MPCs) along those interfaces. Strain energy release rates associated with the delamination growth were calculated for several configurations and using two different FE analysis codes. Calculations from the codes agreed very closely. The strain energy release rate results were used with material characterization data to predict fatigue delamination onset lives for nonlinear tapered flexbeams with two different ply-dropping schemes. The predicted curves agreed well with the test data for each case studied.

Parametric study of threaded fastener loosening due to cyclic transverse loads

This paper presents results from an experimental investigation of mechanical loosening in bolted joints due to cyclic transverse loads. The influence on the resistance to loosening of basic parameters such as preload, fastener material elastic modulus, nominal diameter, thread pitch, hole fit and lubrication is quantified. Sixty-four tests have been performed as part of a nested-factorial design in which the nominal diameter is the nesting factor of preload, thread pitch and hole fit. A statistical analysis identifies the factors and interactions that significantly affect the resistance to loosening and it is found that the preload and the fastener elasticity are the most influencing parameters. A statistical model is developed that predicts the level of loosening reached by a threaded fastener under defined conditions. The analysis shows that optimum conditions to avoid fastener loosening are high preload, low modulus of elasticity, large diameter, lubrication, tight fit and fine threads.

Finite Element Modeling of Self-Loosening of Bolted Joints

A three-dimensional finite element (FE) model with the consideration of the helix angle of the threads was developed to simulate the second stage self-loosening of a bolted joint. The second stage self-loosening refers to the gradual reduction in clamping force due to the back-off of the nut. The simulations were conducted for two plates jointed by a bolt and a nut and the joint was subjected to transverse or shear loading. An M12×1.75 bolt was used. The application of the preload was simulated by using an orthogonal temperature expansion method. FE simulations were conducted for several loading conditions with different preloads and relative displacements between the two clamped plates. It was found that due to the application of the cyclic transverse load, microslip occurred between the contacting surfaces of the engaged threads of the bolt and the nut. In addition, a cyclic bending moment was introduced on the bolted joint. The cyclic bending moment resulted in an oscillation of the contact pressure on the contacting surfaces of the engaged threads. The microslip between the engaged threads and the variation of the contact pressure were identified to be the major mechanisms responsible for the self-loosening of a bolted joint. Simplified finite element models were developed that confirmed the mechanisms discovered. The major self-loosening behavior of a bolted joint can be properly reproduced with the FE model developed. The results obtained agree quantitatively with the experimental observations.

1208 周期的加熱と周期的横荷重を受ける不均質長方形板の熱誘起振動問題(GS-3,4 熱弾性)

1208 周期的加熱と周期的横荷重を受ける不均質長方形板の熱誘起振動問題(GS-3,4 熱弾性)

Effect of Thread Pitch and Initial Tension on the Self-Loosening of Threaded Fasteners

A mathematical model and an experimental procedure are presented to study the self-loosening phenomenon of threaded fasteners that are subjected to cyclic transverse loads. The study investigates the effect of thread pitch, initial bolt tension, and the amplitude of the external excitation on the loosening of a single-bolt joint. The rate of drop in the joint clamp load (fastener tension) per cycle, as well as the total number of cycles that would cause the complete loss of clamp load, are monitored. In the mathematical model, the differential equations of linear and angular motion of the bolt are formulated in terms of the system properties and the external cyclic transverse excitation. Numerical integration of the equation of angular motion provides the bolt rotation in the loosening direction, which causes the partial or full loss of the clamp load. An iterative MATLAB code is developed and used for the calculation of tension loss in the fastener tension due to the self-loosening. Analytical and experimental results are discussed.

901 周期的加熱と周期的横荷重を受ける不均質長方形板の熱誘起振動問題(OS-5 周期的熱応力変動,研究発表講演)

901 周期的加熱と周期的横荷重を受ける不均質長方形板の熱誘起振動問題(OS-5 周期的熱応力変動,研究発表講演)

2221 軸直角方向負荷下でのねじ締結部の回転ゆるみ挙動評価(J10-4 機械的締結体信頼性,J10 締結・接合部の力学とプロセス)

2221 軸直角方向負荷下でのねじ締結部の回転ゆるみ挙動評価(J10-4 機械的締結体信頼性,J10 締結・接合部の力学とプロセス)

235 軸直角方向負荷下におけるねじ締結部ゆるみ挙動の実験解析(OS4-5 応力・ひずみ測定,OS4 実験力学手法の新展開3)

235 軸直角方向負荷下におけるねじ締結部ゆるみ挙動の実験解析(OS4-5 応力・ひずみ測定,OS4 実験力学手法の新展開3)

An Experimental Study of Self-Loosening of Bolted Joints

The self-loosening process of a bolted joint consists of two distinct stages. The early stage of self-loosening is due to the cyclic plastic deformation of the materials. The second stage of self-loosening is characterized by the backing off of the nut. The current work is concentrated on an experimental investigation of the second stage self-loosening. Over one hundred bolted joints with M12×1.75 bolts and nuts were experimentally tested using a specially designed testing apparatus. The experiments mimicked two plates jointed by a bolt and a nut and were subjected to cyclic transverse shear loading. During an experiment, the relative displacement between the two clamped plates, denoted by δ, was a controlling parameter. For a given preload, the relationship between, Δδ/2, the amplitude of the relative displacement between the two clamped plates, and, NL, the number of loading cycles to loosening followed a pattern similar to a fatigue curve. There existed an endurance limit below which self-loosening would not persist. A larger preload resulted in a larger endurance limit. However, a large preload increased the possibility for the bolt to fail in fatigue. The results suggest that the use of a regular nut is superior to the use of a flange nut in terms of self-loosening resistance.

Electromagnetic Performance of Sub-Size NbTi CICC's Subjected to Transverse Cyclic Loading

It was demonstrated previously that transverse cable loading due to electromagnetic forces in coils has a strong impact on the inter-strand contact resistance R/sub c/ of poloidal field (PF) cable-in-conduit conductors (CICC) for the International Thermonuclear Experimental Reactor (ITER). Continuing the study, two NbTi medium-size CICC's were subjected to cyclic loading up to 40,000 cycles in the Twente Cryogenic Press in order to simulate transversal forces on the strands and to verify their influence on the conductors' contact resistances R/sub c/ and AC loss behavior. The results are presented and compared with the data obtained on the other section of the same conductor lengths in the SULTAN test facility and on full size ITER cables tested in the cryogenic press.

Change of interstrand contact resistance and coupling loss in various prototype ITER NbTi conductors with transverse loading in the Twente Cryogenic Cable Press up to 40,000 cycles

The multistrand NbTi conductors for the Poloidal Field (PF) Coils of the International Thermonuclear Experimental Reactor (ITER) are subjected to heavy transverse loading due to the Lorentz forces in the coils. The current in the multistage Cable-In-Conduit Conductors (CICC) exceeds 50 kA and the magnetic field reaches up to more than 6 T for a few tens of thousands of pulses. The large transverse forces, accumulating from strand to strand over the cable cross-section, cause a severe deformation of the cable bundle inside the conduit and this goes along with electromagnetic, mechanical, and thermohydraulic effects. In order to study the electromagnetic and mechanical behaviour in more detail, a Cryogenic Cable Press is build to simulate the effect of the Lorentz forces on a conductor comparable to the present design for ITER in magnet operating conditions. The magnetisation of the conductor (and from this the coupling loss expressed in nτ) and the interstrand resistance ( R c) between various strands and strand bundles inside the cable can be measured along the loading history, starting at virgin condition and accordingly subjected to various loads. The results, all obtained on eight full-size ITER type NbTi conductor samples with a variety of cable strand layout and coatings, are reported here. A consistent correlation is found between the experimental AC loss and interstrand contact resistance ( R c) results. It is also observed that there is a strong impact of cyclic loading on the AC loss and R c which may change up to orders of magnitude. The variation of the AC loss due to transverse cyclic loading of CICC conductors in ITER coils can be accomplished by reducing the void fraction. The results point out that cyclic loading with a significant number of cycles, sufficient to reach a saturation after having passed the peak transverse resistance, should be included in next tests on large NbTi CICC's and PF Model Coils as the AC loss and ability of current sharing among strands will vary along the loading history.

Shakedown, self-stresses, and unilateral contact in a dental implant problem

Some issues related to the low-cycle fatigue failure of dental implants, subjected to cyclic axial and transverse loads, are studied by means of Finite Element shakedown analyses. The unilateral contact conditions existing at the interfaces between the various parts of the implant, which depend on the degree of tightening of the internal screw connecting the abutment to the fixture of the implant, make it impossible to apply in a straightforward way the classical shakedown theorems. We illustrate a way of reasoning which allows the combination of shakedown analyses results with considerations about the contact conditions. Shakedown envelopes associated to several values of the internal screw tightening can be constructed; these suggest that a common implant design is not fully safe against cyclic loading conditions.

A Study of Early Stage Self-Loosening of Bolted Joints

Both experimental investigation and finite element analysis were conducted to explore the mechanisms for the early stage self-loosening of bolted joints under transverse cyclic loading. The nuts were glued to the bolts using a strong thread locker in the self-loosening experiments to ensure that no backing-off of the nut occurred. Depending on the loading magnitude, the clamping force reduction ranged from 10% to more than 40% of the initial preload after 200 loading cycles. Three-dimensional elastic-plastic finite element analysis was conducted with the implementation of an advanced cyclic plasticity model. The finite element results revealed that the local cyclic plasticity occurring near the roots of the engaged threads resulted in cyclic strain ratcheting. The localized cyclic plastic deformation caused the stresses to redistribute in the bolt, and the result was the gradual loss of clamping force with loading cycles. The finite element results agreed with the experimental observations quantitatively. When the two clamped plates started to slip and the slip displacement was controlled, both experiments and finite element simulations suggested that the friction between the clamped plates has an insignificant influence on the early stage self-loosening.

One-dimensional model of cable-in-conduit superconductors under cyclic loading using artificial neural networks

An artificial neural network with two hidden layers is trained to define a mechanical constitutive relation for superconducting cable under transverse cyclic loading. The training is performed using a set of experimental data. The behaviour of the cable is strongly non-linear. Irreversible phenomena result with complicated loops of hysteresis. The performance of the ANN, which is applied as a tool for storage, interpolation and interpretation of experimental data is investigated, both from numerical, as well as from physical viewpoints.

Failure Analysis of Reinforced Concrete Shell Structures using Layered Shell Element with Pressure Node

In this paper, a finite element analysis technique is presented for the path-dependent nonlinear failure analysis of reinforced concrete shell structures. A so-called pressure node is added into a layered shell element utilizing in-plane constitutive models of reinforced concrete and layered formulation in the failure analysis. By controlling the volume of the shell structures using the pressure node, postpeak softening behavior after the ultimate load of the shell structures is obtained. Since the constitutive models cover loading, unloading, and reloading paths, the element is capable of predicting the behaviors of reinforced concrete shells under cyclic loading. For verification of the techniques in this paper, failure analyses of reinforced concrete slabs subjected to in-plane and out-of-plane loads and cyclic transverse loads are performed and numerical results are compared with experimental data. In addition, reinforced concrete dome structures designed with different reinforcement ratios are also analyzed to check the applicability of the technique in this paper. Results show that the techniques can be applied effectively to the failure analysis of various types of reinforced concrete shell structures.

Estimation of Back-Off Loosening Limit for Bolted Joints under Transverse Cyclic Loading.

In many bolted joint structures the mismatch of the thermal expansion coefficients between each component cause transverse cyclic relative slippage and back-off loosening of bolts. In the previous papers we expressed that the loosening behavior of these bolted joints under transverse loading can be estimated taking into account the relative slippage on the contact interface between the bolt head and the jointed body. The amount of this relative slippage can be calculated using the mismatch of the thermal expansion between both jointed bodies and the bending elastic deformation of the bolts. In the calculations of the bending elastic deformations of the bolts the estimation of the bending compliance on the bolt head contact areas are indispensable. So, in this paper we calculate this bending compliance on the bolt head contact area using finite element method considering contact conditions. Using these results we can derive a simple estimation expressions for bending compliance on bolt head contact areas. By using this estimation method of bending compliance on bolt head contact areas we can estimate the critical relative slippage for any bolted joint structures.

Eccentrically Loaded Steel Columns under Cyclic In-Plane Loading

This paper is aimed at investigating the ultimate strength and ductility capacity correlation between centrally loaded and eccentrically loaded steel columns subjected to cyclic transverse (horizontal) load at their top. The columns considered are of the box and pipe sections. A finite-element formulation accounting for both geometrical and material nonlinearity is developed to obtain the cyclic hysteretic behavior of the columns. In the analysis, a modified two-surface plasticity model is employed to model material nonlinearity. Based on the numerical results, the transverse load versus transverse displacement relationship between centrally and eccentrically loaded columns has been proposed. According to the proposed equations, the ultimate strength and ductility capacity of the eccentrically loaded columns can be conveniently obtained from those of the centrally loaded columns.

Electromagnetic and mechanical AC loss of an ITER TF model coil conductor (DP4) under transverse cyclic loading

Energising a coil results in a transverse force on the strands pushing the cable towards one side of the jacket. This load causes a transverse compressive strain in strands and in particular in strand crossover points. Besides this, contact surfaces interfere by micro-sliding resulting in friction and anomalous contact resistance behaviour versus force. Two Central Solenoid Model Coil conductors have been tested previously in a cryogenic press and now the experimental results are presented for the Toroidal Field Model Coil (TFMC) conductor (DP4). The press can transmit a variable (cyclic) force of at least 650 kN/m directly to a cable section of 400 mm at 4.2 K. The magnetisation of the conductor and the interstrand resistance (R/sub c/) between various strands inside the cable can be measured by varying pressure. The force on the cable and the displacement are monitored simultaneously in order to determine the effective cable Young's modulus and the mechanical heat generation due to friction and deformation. The mechanical heat generation, the coupling loss time constant n/spl tau/ and the R/sub c/ of the full-size ITER TFMC conductor have been studied under load up to 40 full loading cycles. The evolution of R/sub c/ is comparable to the behaviour found for the CS Model Coil type of conductors. A significant decrease of the cable coupling current time constant, n/spl tau/ and mechanical heat generation after cyclic loading is found.