Yigeng Xu

A self-aligning four-point bend testing rig and sample geometry effect in four-point bend fatigue

Abstract A self-aligning four-point bend testing rig was designed and made which can minimise the possible misalignment associated with a four-point bend test and be used to study the fatigue of materials both at room and elevated temperatures. The stress distribution between the inner-rollers in a specimen under four-point bend, that is the nominal pure-bending section length, was analysed with respect to various load-span/specimen-thickness ratios ( t / h ) and support-span/load-span ratios ( L / t ) using a finite element method. It was found that the stress distribution could vary with both t / h and L / t . It was found that values of t / h and L / t between 1.2 and 1.5 and between 4 and 5, respectively, were the optimum testing geometry which led to a relatively uniform stress distribution consistent with the value calculated by beam theory. Fatigue tests ( R =0.1 and frequency=20 Hz) were carried out on samples with different thickness in a peak-aged 8090 Al–Li alloy using the rig. The results appear to support the finite element results. The S – N curve of the 8090 Al–Li alloy was measured using the optimum testing geometry in the four-point bend, and it was found to be consistent with that reported in the literature.

Systematic assessment and validation of compliance-based crack closure measurements in fatigue

Different methods of closure measurement and issues related to the identification of closure points are examined. Based on compliance curves obtained from crack mouth clip gauges and near-tip strain gauges, a systematic assessment of closure measurements has been made by a variety of non-subjective methods. Closure results obtained from compliance curves are also compared with direct crack tip observations. For results obtained on a commercial Al alloy, AA2024-T351 plate, it is shown that curve fitting methods based on a combination of linear and quadratic functions provide particularly sensitive and consistent closure measurements. The potential separation of plane stress and plane strain closure effects that arise in real, three-dimensional specimens is demonstrated via a combination of global compliance measurements and the use of side-grooved specimens.

Atmospheric Ice Loading and Its Impact on Natural Frequencies of Wind Turbines

Wind energy is a promising way in the middle of growing demand for clean energy in high north, where atmospheric icing is a hazard for safe operations of wind turbines. In this research work, the vibrational effects due to atmospheric ice accretion on a multi megawatt large wind turbine are numerically investigated using a finite element based approach. Three different icing scenarios were addressed and accreted on-blade ice mass was calculated analytically based upon ISO 95214 standards. Special attentions are given to the first natural frequency of the wind turbine blade and its interaction when the exciting frequency approaches the natural frequency of the wind turbine tower. Results show variations of natural frequencies due to different accreted icing loads scenarios, which may have different implications to the integrity of the structure.

An Analytical Model for the Identification of the Threshold of Stress Intensity Factor Range for Crack Growth

The value of the stress intensity factor (SIF) range threshold for fatigue crack growth (FCG) depends highly on its experimental identification. The identification and application of are not well established as its determination depends on various factors including experimental, numerical, or analytical techniques used. A new analytical model which can fit the raw FCG experimental data is proposed. The analytical model proposed is suitable to fit with high accuracy the experimental data and is capable of estimating the threshold SIF range. The comparison between the threshold SIF range identified with the model proposed and those found in the literature is also discussed. identified is found to be quite accurate and consistent when compared to the literature with a maximum deviation of 5.61%. The accuracy with which the analytical model is able to fit the raw data is also briefly discussed.

A Review of the Effects of Ice Accretion on the Structural Behavior of Wind Turbines

Icing of wind turbines happens occasionally at different latitudes and locations in the world and consequently affects the wind turbine fatigue loads. Large ice accretion may cause wind turbine vibration due to uneven ice shedding, which could lead to structural failures in addition to hazardous issues accompanied with ice being shed off wind turbine blades. In this paper, a review study of the effects of ice accretion on the structural behavior of the wind turbines is presented.

The use of pseudo-inverse methods in reconstructing loads on a missile structure

A missile during air carriage is subjected to high vibratory forces. These forces consequently impose high levels of stress on the attachment points to the aircraft. A repetitive application of stress causes fatigue. However, since the vibratory forces cannot be measured directly at the interface, an inverse method utilising missile accelerometer data has been used to determine the forces. This paper presents two frequency domain techniques of reconstructing loads. Both techniques utilise the Moore-Penrose pseudo-inverse method, although the second procedure incorporating a normal modes analysis is perhaps better suited for embedding within a health and usage monitoring system (HUMS). Both techniques enable discrete dynamic loads, which are applied to a finite element model in the time domain, to be successfully reconstructed. Consequently, by identifying forces on a missile structure successfully, the presented techniques enable a better structural integrity assessment to be undertaken.

Load sequence effect on fatigue damage

Reliable damage tolerant design of airframe structures relies on the accurate life prediction of fatigue cracks propagating from a detectable size to the critical size, which is challenging due to the complex load sequence effect under spectrum loading. This paper aims at gaining a further understanding of the complex influence of the loading history on fatigue damage through a detailed numerical simulation of the near-tip crack behaviour using the crack closure concept. The spectrum loading is broken down into a number of simple yet representative loading scenarios with overload/underload superimposed onto the baseline constant amplitude fatigue loading. Detailed finite element (FE) simulation of the plasticity-induced crack closure (PICC) has been carried out to catch the transient behaviour of PICC and link it to the fatigue damage. The load interaction effect has been analysed with the aim to identify the possible dominant loading cycle which could simplify the fatigue life prediction process in the industry. It is concluded that more reliable damage tolerant design can be achieved if the load sequence effect on fatigue damage can be taken into account more accurately for a structure under spectrum loadings.

Simulation of near-tip crack behaviour and its correlation to fatigue crack growth with a modified strip-yield model

A modified strip-yield model has been developed to simulate the plasticity-induced crack closure under the constant amplitude (CA) and a single overload loading conditions. The paper focuses on the simulation of the near tip crack profiles and stress distributions during the fatigue process. Detailed information on near-tip stress and displacement fields at the maximum load (Pmax), the minimum load (Pmin), and the crack opening load (Pop) of a fatigue load cycle have been presented. The correlation of the crack closure to the near-tip material fatigue damage has been investigated and used to rationalise the crack growth behaviour under the CA and a single overload loading conditions.

Effect of Plastic Deformation on Compliance Curve Based Crack Closure Measurement

Fatigue crack growth depends heavily on near tip stress-strain behavior controlled by many micromechanical and microstructural factors. Crack closure is widely used to rationalize crack growth behaviour under complex loading conditions. Reliable crack closure measurement is essential for enhanced damage tolerance design and remains a challenge to the industry. This paper focuses on the effect of plastic deformation ahead of a notch/crack on the non-linearity of compliance curves of 6082-T651 aluminium alloy specimens to highlight a potential issue in the conventional compliance curve based crack closure measurement technique. Experimental and numerical simulation results demonstrate that plastic deformation ahead of the notch will introduce non-linear stress-strain behavior in the absence of crack closure. It is proposed that the effect of crack tip plasticity on the non-linearity of the compliance curve be separated to obtain reliable crack closure measurement.

Effect of Electromagnetic Treatment on Fatigue Resistance of 2011 Aluminum Alloy

Beneficial effects of the electromagnetic treatment on fatigue resistance were reported on several engineering alloys. These could be linked to the dislocation activity and the rearrangement of the crystal structure of the material under the electromagnetic field (EMF), resulting in delayed crack initiation. This paper presents an experimental study on the effect of pulsed electromagnetic treatment on the fatigue resistance of 2011 aluminum alloy. Circular cantilever specimens with loads at their ends were tested on rotating fatigue machine SM1090. Fatigue lives of treated and untreated specimens were analyzed and compared systematically. It has been found that the effect of the pulsed electromagnetic treatment on the fatigue resistance is dependent on the intensity of the pulsed EMF and the number of the treatment applied. Clear beneficial effect of the pulsed electromagnetic treatment on the fatigue resistance of the aluminum alloys has been observed, demonstrating a potential new technique to industries for fatigue life extension.