Constant Displacement Amplitude Research Articles

Characterisation of fatigue delamination growth in plain woven hybrid laminated composites subjected to Mode-I loading

Effect of similar and dissimilar crack plane interface configuration on fatigue delamination growth in plain woven hybrid composite laminates under Mode-I has been investigated. Constant displacement amplitude fatigue testing with displacement ratio of 0.1 was carried out on 3 configurations of plain woven glass/carbon epoxy composite laminates. A power law like fit between recorded delamination length and corresponding cycle was used to predict crack length for each of the cycle. Delamination growth rate,da/dN is computed by differentiating the expression of power-law like fit.The obtained crack growth rate for each of the specimens were plotted with respect to two normalised functions, G^Imax=GImax(a)/GIR(a) where GImax is maximum mode-I energy release rate and GIR(a) is the interlaminar fracture toughness resistance and ΔG^Ieff=G^Imax-G^Imin2 as crack driving parameters computed on the basis of Modified Beam Theory (MBT) and Valvo’s mode partition method (MPV) are used in Paris relation to quantify delamination propagation. It is observed that the exponent values predicted by MBT method for G^Imax is lower as compared to ΔG^Ieff. Whereas, exponent values predicted for G^Imax is higher as compared to ΔG^Ieff predicted by MPV method. The higher the exponent value, the higher is the sensitivity of the model leading to uncertainties in the crack growth prediction. Also, it is to be noted that cyclic loading effect is when both GImax and GImin is considered, the use of ΔG^Ieff as crack driving parameter to quantify delamination propagation is justified. Secondly, MBT method does not account for the mode-mixity arising due to hybrid material configuration as in the case of Local Symmetry Fatigue (LSF) and Asymmetry Fatigue (ASF) specimens. Hence, results in higher exponent as compared to MPV method. On the other side, the G^Imax and ΔG^Ieff computed on the basis of MPV method is the pure Mode-I component deduced from the total energy release rate of mode-mixity.The equations of curve fitting is very much the same for Simple Symmetry Fatigue (SSF) specimens indicating that MBT and MPV methods predict pure Mode-I behaviour for symmetric configuration for delamination growth under fatigue Mode-I loading. From the composite laminate configuration point of view, LSF specimens have higher exponents as compared to ASF and SSF specimens indicating, local symmetry configuration laminates are highly sensitive to the small uncertainties and results in unstable crack growth.Comparison of results of all hybrid composite laminates shows that the normalised functions of G^Imax and ΔG^Ieff as crack driving parameters computed on the basis of MPV method is able to capture the effect of interlayers and stacking effect on the delamination growth in hybrid plain woven composites in fatigue loading and MPV method is found to be not sensitive to G^Imax and ΔG^Ieff for displacement ratio of 0.1.

An experimental and comparative study of the self-loosening of bolted-joints under cyclic transverse loading

The capabilities of analytic models in predicting the experimental critical displacements of the self-loosening of bolted-joints were investigated experimentally and numerically. The experimental loosening rates were determined in a Junker test bench at a constant transverse displacement amplitude (0.45 mm) and under varying initial clamp force and clamp length and controlled bearing and thread friction coefficients. The analytic critical displacements were then calculated using experimental parameters. In addition, a three-dimensional accompanying finite element (FE) model was developed in order to calculate the ratio of spring constants engaging the thread to spring. The results showed relatively low capabilities of present analytic model in the prediction of the critical displacements of the self-loosening of bolted-joints. The efforts to modify the nut reaction moment and the inclination compliance of bolt head portion in the investigated equations however resulted moderate increase in the correct predictions. On the other side, the use of the reaction moment determined by FE model increased the correct prediction from 58.3 to 73.4%. The accuracy of the equations was further increased by the use of an appropriate kw value, but the increase in this case was only ~4%.

Friction Properties at the Contact Interfaces of Overhead Line Aluminium Conductors

Overhead conductors may undergo fretting fatigue failures at contact interfaces located at or near the suspension clamps. Some recent experimental studies were carried out on individual wires to understand the fretting fatigue phenomenon. However, the majority of the available results are for the wire-to-wire contact configuration, while little interest has been brought to the wire-to-clamp contact configuration, which is typically a critical interface for conductor fatigue. A new experimental test bench intended to perform fretting fatigue tests on individual strands of overhead conductors is used to study the wire-to-clamp and the wire-to-wire contact configurations. Variable displacement amplitude (VDA) tests under different normal contact forces were carried out on 1350-H19 aluminium wires for both contact types. The experimental results showed a slight difference between the studied contact configurations in terms of the friction coefficient values and the transition sliding amplitude. To complete this analysis, constant displacement amplitude (CDA) tests were carried out sweeping the displacement range and replacing the tested specimen for each test. The resulting contact marks are observed using an optical microscope and a good correlation is found between both types of tests in terms of friction coefficient and transition sliding amplitude. This study allowed characterizing the sliding conditions of wire-to-clamp contact in the context of fretting fatigue of aluminium conductors and is an important step in the development of a method to evaluate the fatigue life of conductors using fretting fatigue tests of individual wire-to-clamp contacts.

Finite element modeling of fretting wear in anisotropic composite coatings: Application to HVOF Cr3C2–NiCr coating

This paper presents a two-dimensional (2D) plane strain finite element model to simulate fretting wear in composite cermet coating. The coating considered in this investigation is High Velocity Oxy-Fuel (HVOF) sprayed Cr3C2–NiCr with 55% volume fraction of Cr3C2. The material microstructure is modelled using Voronoi tessellations with a log-normal variation of grain size. Moreover, the individual phases of the material in the coating were assigned randomly to resemble the microstructure from an actual SEM micrograph. The ceramic carbide phase is orthorhombic and the cubic matrix possesses a high anisotropy index. As a result, each grain was modelled with random orientation to account for material anisotropy. The RVE dimensions were chosen such that its elastic response represented the overall response of a poly-aggregate. In order to simulate debonding of the ceramic carbide phase from the matrix, cohesive elements were used at the grain boundaries. Damage mechanics was used to model degradation of cohesive elements resulting from repeated fretting cycles. A grain deletion algorithm was developed to simulate removal of material from fretting wear. The crack patterns predicted from the model match closely with the patterns observed in experimental studies on wear of HVOF Cr3C2–NiCr coating. The model also predicts carbide pullout, a major damage mechanism in HVOF Cr3C2–NiCr coating subjected to wear. Experiments were also conducted to evaluate and corroborate the wear rate of HVOF Cr3C2–NiCr coating. The wear rate from the model matches closely with experiments at a constant load and displacement amplitude. The results from the model were then extended to obtain a fretting wear map under a combination of various loads and displacement amplitudes.

Model Testing on the Effects of Section Geometry and Stiffness on the Cyclic Lateral Behavior of Piles in Loose Sand

In this study, two-way cyclic lateral loading tests with constant displacement amplitude are performed to investigate the effect of cross section geometry and modulus of elasticity on the behavior of model piles in sand. Various laboratory pile models were tested in a metal test tank equipped with various facilities including cyclic lateral loading system, devices to measure displacement and pressure along the pile, an inverter to adjust or change loading frequency, sand raining system, etc. Different cross section geometries, including square and circular shapes, were used for piles made of polyethylene and polyurethane. A loading frequency of 0.29 Hz and a total number of 145 loading cycles were used for all the tests. Results revealed that in constant displacement amplitude tests, both lateral load resistance of the pile head and stiffness of the pile–soil system increase with an increase in the number of loading cycles; however, the rate of increase gradually decreases. It is shown that these variations may be formulated using a logarithmic relationship, which includes a degradation parameter reflecting rate of these changes. For tests with different pile cross section shapes, dimensions, and moduli of elasticity, the degradation parameter varied from 0.08 to 0.18. The pile head load and stiffness of the pile–soil system at the end of the 145th cycle varied from 1.4 to 1.85 times those of the first cycle for different section shapes, dimensions, and moduli of elasticity.

Frictional behavior of sandstone fractures under forward-backward pre-peak cyclic loading

The objective of this study is to determine the effect of cyclic loading under pre-peak condition on shearing behavior of fractures in Phra Wihan sandstone, primarily to improve an understanding of the stability conditions of slope embankments along roadways and railways in the north of Thailand where seismic activities have often occurred. The fractures were artificially made in the laboratory by tension inducing method and cyclically sheared under forward to fully backward loading paths with constant normal stresses between 1 and 4 MPa. Two cyclic loading conditions were used on separate sets of specimens: constant stress amplitude and constant displacement amplitude. Fifty shear cycles were applied. The results indicated that the pre-peak cyclic loading under both loading conditions slightly reduce shear strengths and stiffness of the sandstone fractures due to the degradation of the second order fracture asperities. Fracture roughness notably reduced particularly under large shear stress and displacement amplitudes due to the deposition and packing of rock power. The subsequent monotonic loading performed after cyclic loading showed further reduction of fracture roughness due to the degradations of the first order asperities.

Accumulated Damage-based Experimental Study on Seismic Performance of HRBF500 Rebar Reinforced RC Bridge Columns

The evolution of the damage and seismic behaviour of the reinforcement concrete (RC) bridge columns reinforced with HRBF500 rebar was investigated in this paper. Nine bridge column specimens with various strengths of longitudinal reinforcement, reinforcement ratios, axial compression ratios and stirrup ratios were tested under horizontal cyclic loading. The influences of the designed parameters of the mechanical properties such as failure mode, loading-displacement curve, skeleton curve, strength and stiffness degradation and energy dissipation were analysed in the view of damage. There are four conclusions brought by the tested results. The damage process of specimens could be divided into turn-crack development stage, damage accumulation stage and failure stage. The damage will accumulate gradually and the strength and stiffness will also be degenerated, resulting in lower ductility and energy dissipation capacity of the specimens, as the displacement amplitude increases. The failure modes are similar, with plastic hinges occurrence varying from 30 to 260 mm above the concrete bases, accompanied by the steel buckling, fractured and the concrete local crushing. The good ductility and energy dissipation capacity of the HRBF500 reinforced concrete bridge columns are illustrated by the relatively plumper shuttle-shaped hysteresis curves obtained. The damage development was faster and energy was comparably larger of the specimens under the variation of displacement amplitude cycle loading, comparing with the constant displacement amplitude cycle loading. The bearing capacity of the specimens will be certain influenced by the increased reinforcement ratios and the larger energy dissipation capacity, with more longitudinal bars. The axial compression ratio also has an important influence on the damage development of the specimens. There are not considerable increments of bearing capacity, with the increment of reinforcement ratio, however, the ductility and energy dissipation ability will be improved obviously. The laws of damage development analysed by the experimental results were shown in this paper. The seismic damage assessment and seismic damage model of this kind of structural members were established by the experimental results above.

A novel test method based on small specimens for obtaining low-cycle-fatigue properties of materials

In this study, a novel method for low-cycle-fatigue (LCF) tests on small funnel and Ring specimens to obtain the Manson–Coffin law of materials is proposed based on the median energy equivalence principle. It includes a method for solving the cyclic stress-strain relationship of the material according to the cycling stability curve of load amplitude–displacement amplitude, and another method for acquiring the conversion relationship between the measuring displacement amplitudes and local strain amplitudes of the specimen. LCF experiments under constant displacement amplitude loading were performed on small funnel and Ring specimens, as well as standard straight-round bar specimens of the two materials, G115 and 16Mn. The results indicated that the uniaxial cyclic stress–strain relationship and Manson–Coffin law results of the G115 and 16Mn materials obtained by the new method were highly consistent with those obtained by the traditional standard method.

A novel magnetic resonance elastography transducer concept based on a rotational eccentric mass: preliminary experiences with the gravitational transducer

Background. Magnetic resonance elastography (MRE) is used to non-invasively estimate biomechanical tissue properties via the imaging of propagating mechanical shear waves. Several factors including mechanical transducer design, MRI sequence design and viscoelastic reconstruction influence data quality and hence the reliability of the derived biomechanical properties.Purpose. To design and characterize a novel mechanical MRE transducer concept based on a rotational eccentric mass, coined the gravitational transducer.Materials and methods. Table top measurements were performed using accelerometers to characterize the frequency response of the new transducer concept at different driving frequencies (f VIB) and different rotating masses. These were compared to a commercially available pneumatically driven MRE transducer. MR data were acquired on a 3T scanner using a fractionally encoded gradient echo MRE sequence in three healthy volunteers. Acceleration and displacement spectra were plotted in units of g and mm, respectively, and visually compared, emphasizing the ratio between the peaks at f VIB and its 2nd harmonic, a known cause of error in the reconstruction of biomechanical properties as is explored in more detail in numerical simulations here. No formal statistical testing was performed in this proof-of-principle paper.Results. The new transducer concept shows—as expected from theory—a quadratic or linear increase of acceleration amplitude with increase in f VIB or mass, respectively. Furthermore, different versions of the transducer show markedly lower 2nd harmonic-to-f VIB ratios compared to the commercially available pneumatically driven transducer. Displacement was constant over a range of f VIB, in accordance with theory. Phantom and in vivo data show low nonlinearity and excellent data quality.Conclusion. The table top measurements are in concordance with the theory behind a transducer based on a rotational eccentric mass. The resulting constant displacement amplitude irrespective of f VIB and low 2nd harmonic-to-f VIB ratio result in low nonlinearity and high data fidelity in both phantom and in vivo examples.

Fretting damage of Ni-rich ultrafine grained NiTi superelastic wires

The effects of fretting on Ni-rich ultra-fine grained NiTi superelastic wires have been characterized. Fretting tests have been performed using wire on wire in 90° cross-cylinder configuration until 105 cycles in air at 25 °C. Constant displacement amplitude of 50 µm and normal loads of 10, 20 and 50 N were considered. For a normal load of 10 N, the tribosystem performed in Gross Slip Regime and the predominance of wear damage was observed. Mixed Fretting Regime was instead observed for normal loads of 20 N and 50 N. In these cases, the predominant damage mechanism was crack formation with the cracks oriented normal to the displacement direction. Occurrence of martensitic transformation in the contact region was inferred from the particular shape of the fretting loops. Due to their possible impact on biocompatibility, the debris detached from the tribosystem during the different experiments were collected and characterized by TEM. They consisted in agglomerations of nano-crystalline TiO2 (rutile) and NiO oxide particles sized between 10 and 20 nm.

Toward broadband vibration energy harvesting via mechanical motion-rectification induced inertia nonlinearity

Broad frequency bandwidth is a desired feature for most energy harvesting systems. Rotational electromagnetic generators are widely used in energy harvesting systems and the generator rotor is considered as an inerter. While a lot of research striving for increasing frequency bandwidth, we found out that the inerter makes the bandwidth narrow. To solve this problem, this paper proposes using inertia nonlinearity which is realized by mechanical motion rectification (MMR). The influence of the MMR on energy harvesting performance in inerter-based systems was numerically and experimentally investigated with harmonic excitations of constant displacement amplitude. Simulation is done by transforming the mechanical system to an analogous electrical system. The simulation results show that the bandwidth of the MMR based system is broader than that of the counterpart without MMR. System parameter was identified by parameter fitting and experiment was conducted to verify the numerical simulation. Moreover, in the MMR based system, the force transmitted from the harvester to the base was decreased compared to the counterpart without MMR. For excitations with constant force amplitude, MMR based energy harvesting systems also have much broader frequency bandwidth compared to the counterpart without MMR.

In situ observation on formation process of nanoscale cracking during tension-compression fatigue of single crystal copper micron-scale specimen

This study aims to develop an in situ observation technique of micron-scale metal specimens under fully-reversed tension-compression cyclic loading and to investigate the unique cracking process in low-cycle fatigue. The fatigue experiment was conducted on a single crystal copper micron-scale specimen under a constant displacement amplitude. While crystallographic slip spread over the entire test section during the tensile first half-cycle, a locally concentrated slip band appeared during reverse loading (compression). In situ observations using scanning electron microscopy revealed that crystallographic slip occurred only near the localized band in consecutive cycles, and strain localization due to slip bands led to nanoscale extrusion/intrusion at the surface. This indicates that, unlike during the fatigue of a typical bulk pure metal, no characteristic dislocation substructure was formed during the extrusion/intrusion process. The extrusion/intrusion was the cause of failure of the micron-scale metal specimen in fatigue. Moreover, the process required a much higher stress amplitude than in the case of bulk copper specimen. Thus, the fatigue cracking process for copper micro-scale specimen differs significantly from that for bulk copper specimen.

Cyclic fatigue of silica refractories – effect of test method on failure process

Silica refractories serving in high temperature industrial installations fail due to thermo-mechanical cyclic loads. The failure process was investigated by performing cyclic fatigue tests of several methods. In uni-axial compression the samples were tested either with constant force or displacement amplitude or with fixed upper displacement limit. In bending constant displacement amplitude tests were done. The investigation was supported by monotonic loading tests and microstructural analysis. It was determined that the fatigue failure occurs due to the degradation of interlocking in the crack wake. Cracking of larger grains is important for the crack initiation. The test set-up and the loading procedures significantly influence the potential to resist the crack propagation. Cyclic loading produces less brittle failure than monotonic loading. The displacement controlled method allows more gradual, less brittle, failure than the force controlled method. The potential of the crack arrest is less developed in bending than in compression.

Cost-competitive hysteretic devices for seismic energy dissipation in steel bracings: experimental tests and low-cycle fatigue characterisation

This paper presents the results of experimental tests intended at investigating the behaviour of two cost-competitive solutions for realising dissipative devices to be adopted as link elements in Y-shaped eccentric braces (EB), widely employed both for seismic protection of newly designed structures and retrofitting of existing ones. These structural components, generally referred to as Steel Slit Shape (SSS) and Short Link (SL) devices, were realised through standard welding and cutting procedures generally employed in steel workshops. Experimental tests with both constant and variable displacement amplitude protocols were carried out for observing the mechanical performance for these devices under cyclic actions. Since a fast degradation in shear strength was generally observed for all the tested specimens, a low-cycle fatigue was identified: it resulted that the same curve can be employed describing the number of cycles leading to low-fatigue failure for a given amplitude of the imposed cyclic displacement history. This curve is an essential tool for describing the behaviour of this devices that, although characterised by poorer mechanical performance under cyclic actions, have the potential to be utilised as dissipative components, for instance in Y-shaped steel bracings adopted for retrofitting reinforced concrete structures in medium-to-low seismic intensity area.

일정 변위 진폭조건에서의 은도금한 커넥터의 미동마멸부식 거동

Fretting corrosion tests are conducted with a constant displacement amplitude using silver-plated brass coupons to investigate the effect of contact pressure on fretting corrosion. Three behaviors are identified based on the change in electric resistance and friction coefficient during the fretting test period, and the identified behaviors are dependent on the magnitude of the applied load. The failure cycle (Nf) with an electric resistance of 0.1 D cannot be achieved due to the adhesion behavior of the metal and metal contact under the higher applied load of 0.45 N. This suggests that an average contact pressure higher than 159 MPa for the silver-coated con- nector is desirable to gain an almost infinite lifetime. The relationship between the electric contact resistance (R) and the average contact pressure (p) can be written as p = 106.2 × Ω �1.5 . Keywordscontact pressure (접촉압력 ), fretting corrosion (미동마멸부식 ), friction coefficient (마찰계수 ), electric connector (전기 커넥터), electric resistance (전기저항)

Mixed-mode fatigue failure criteria for adhesively-bonded pultruded GFRP joints

The behavior of adhesively-bonded pultruded GFRP joints under cyclic loading was experimentally investigated. Three specimen types were examined: double cantilever beam, end load split, and mixed-mode bending specimens for Mode I, Mode II, and mixed-Mode I/II loading conditions respectively. The crack length was determined during the fatigue experiments by a dynamic compliance method for DCB specimens and a video extensometer for ELS and MMB specimens. Fatigue experiments were conducted at constant displacement amplitude. A significant difference was found between the quasi-static strain energy release rates and corresponding fatigue threshold values for all the examined cases. A total fatigue life model was used to simulate the fatigue crack growth curve under each mode-mixity. The experimental results and the analytical models were used to establish fatigue mixed-mode fracture failure criteria that can be used for the prediction of the fatigue behavior of other joint configurations comprising the same adhesive and adherends.

Quasistatic and dynamic mechanical properties of Al–Si–Cu structural films in uniaxial tension

In this article, the quasistatic and dynamic tensile properties of aluminum–silicon–copper (Al–Si–Cu) alloy films are described. The films were deposited by sputtering onto thermally oxidized Si wafers, and then half of the wafers were heat treated at 623 K in nitrogen gas for 1 h. Specially developed environment-controlled uniaxial tensile test equipment was used to carry out the quasistatic tensile test, stress relaxation test, and cyclic loading test at temperatures ranging from room temperature (RT) to 573 K in high vacuum, and the influence of annealing on the mechanical characteristics was investigated. The Young’s modulus did not show annealing dependency. The mean value was 65 GPa at RT, and gradually decreased with increasing test temperature. The yield stresses of nonannealed and annealed films were 168.5 and 129.6 MPa, respectively, which also decreased with temperature rise. In stress relaxation test results, creep exponents in respective films were obtained from curve fitting using the Norton law, which indicated that creep deformation was restricted by annealing. The cyclic loading test was performed under stress- and displacement-amplitude-constant modes. The stress-amplitude-constant mode test provided creep deformation acceleration to failure, whereas the displacement-amplitude-constant mode test showed a gradual drop of stress amplitude. The stress-amplitude change was compared with stress relaxation curves; consequently, the creep deformation was dominant to the degradation of Al–Si–Cu films subjected to cycling loading with constant displacement amplitude.

Bending Fatigue Behaviour of Laminated Sandwich Beams

In this study, the bending fatigue behaviour of laminated sandwich beams, which are made of carbon/epoxy face sheets and aramid honeycomb core, has been investigated experimentally. The wet hand lay-up technique is used and curing is processed on the heated vacuum table at an elevated temperature to manufacture the sandwich beams. The experimental set-up for bending fatigue test provides a cantilever in one end and a cyclic load at the free end with constant displacement amplitude at room temperature. The load applied to the beam is measured using a load cell during the bending test. Different displacement amplitudes are performed. Mechanical properties, bending stiffness and free vibration frequency of the sandwich beam are investigated. The bending test of the beams and vibration identification test using a vibration analyzer, a hammer and an accelerometer are performed to measure the bending stiffness and determination of free vibration frequency of the clamped sandwich beams before starting and after completion of the fatigue tests. The bending stiffness and free vibration frequencies before and after the fatigue tests are compared to understand the effect of repeated loadings on the mechanical performance of the laminated sandwich beams.

Effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposite

Humidity affects the mechanical properties of polymers and their composites. Understanding the influence of humidity on the strength, stiffness and fatigue characteristics will aid in better product design. The effect of relative humidity (RH) on indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposites is reported. Indentation hardness and indentation modulus of the material reduces up to ∼50% in the samples conditioned in water due to the plasticization and associated increased polymer chain mobility. Cantilever bending fatigue tests conducted at different relative humidity levels at constant displacement amplitude revealed increased fatigue life for polyamide 6 nanocomposites at high humidity. Hysteresis heating and molecular reorientation lowers the modulus during fatigue process and causes a reduction in the force amplitude at high humidity levels. The failure mechanisms at different humidity levels are discussed.

Evaluation of Fatigue Damage in Adhesive Bonding: Part 1: Bulk Adhesive

The measurement of fatigue damage in adhesive bonding has been investigated. Bulk adhesive was used in this study for two reasons: the stress distribution of adhesives in bulk is simpler to investigate than adhesives in joints; and the specimen dimensions met fatigue test standards. Bulk adhesive was made from a film type of epoxy resin. In general, the characteristics and the behaviour of bulk adhesive may differ from adhesive in joint because of the presence of voids and the constraints imposed by the substrates. Low cycle fatigue tests with a load amplitude ratio of 0.1 at a frequency of 5 Hz were performed to determine the damage as a function of the number of cycles. Damage curves, i.e., the evolution of the damage variable as a function of number of cycles, were derived and plotted using an isotropic damage equation. Damage was evaluated using the decrease of stress range during the lifecycles of a constant displacement amplitude test. It was found that the damage curves were well fitted by a low cycle, fatigue damage evolution law equation. This equation was derived from a dissipation potential function. Curve fitting was performed using the robust least square technique rather than ordinary linear least square technique because damage curves have extreme points (usually near the failure point). It was found that the fitting process would not converge for adhesive fractures at high cycle values (N f > 9000). Two damage constants A and β were found from the fitting process. Each fatigue set of data, at a certain level of von-Mises stress range for the undamaged state or at the stabilized hardened state, (Δσ*eq), had a different set of damage parameters A and β. Linear regression of these points was used to express A and β as a function of Δσ*eq. Using these expressions, damage curves for different levels of Δσ*eq could be predicted.