Fatigue Cracks In Aluminum Alloy Research Articles

Acoustic Emission Characteristics Determination for Online Structural Monitoring of Fatigue Crack Growth inAluminium Alloy 2025

In the use of metals, due to industrial advances and the application of more dynamic loads, it is necessary to pay more attention to the fatigue issue. Structural health monitoring (SHM) system is a method of evaluating and monitoring structural health. It has been widely applied in various engineering sectors due to its ability to respond to adverse structural changes, improving structural reliability and life cycle management. Non-destructive inspection methods are used to condition and health monitoring structures at the time of production and even during the service life of parts. Among non-destructive methods, the acoustic emission method has become a standard and reliable method in recent years. The stimulated internal energy of the structure is received in this acoustic emission technique as health monitoring features. A dominating attribute of the acoustic emission technique is its application ability in its loading condition. Therefore, it provides instant damage information within a short period of time. Thus, acoustic emission monitoring tests are often performed in the operating conditions of the structure. Acoustic emissions can inform us of the changes that occur before the final failure and prevent much financial and human damage. In this project, the characteristics of acoustic emission in the fatigue crack growth of aluminum alloy 2025 for online structural monitoring have been investigated and determined. Acoustic emission tests have been performed in two parts: bending fatigue test with the aim of initiation of fatigue cracks in aluminum alloy 2025 specimens and following tensile tests with the aim of growth of fatigue cracks. The acoustic emission signals and parameters sent by the acoustic emission sensor during both tests were received and recorded by the acoustic emission software. According to the received acoustic emission information, various diagrams are plotted. Analyzing the results from online acoustic emission monitoring showed the acoustic emission method can be considered a suitable and reliable technique for detecting crack initiation and crack growth in aluminum alloy 2025.

Characterization of internal fatigue cracks in aluminum alloys by simulation of phase contrast tomography

Synchrotron Radiation Computed Tomography (SRCT) allows a better detection of fatigue cracks in metals than laboratory CT due to the existence of phase contrast. However the presence in reconstructed images of fringes at the edges of objects generated by Fresnel diffraction makes it difficult to identify and analyze the cracks quantitatively. Simulations of phase contrast synchrotron tomography images containing cracks with different sizes and shapes are obtained by using GATE software. Analyzing the simulation results, firstly, we confirmed that the bright parts with strong contrast in SRCT image are streak artifacts; secondly, we found that the gray scale values within the cracks in SRCT images are related to the crack size; these simulation results are used to analyse SRCT images of internal fatigue cracks in a cast Al alloy, providing a clearer visualisation of damage.

Propagation of corrosion induced fatigue crack in aluminum alloy

<abstract> <p>Aluminium is considered a green metal due to its environmental responsive characteristics. The 7475-T7351 aluminum alloy is extensively used in automotive and aerospace applications due to its light weight and high strength. In the present work, the effects of the corrosive environment on the high cycle fatigue (HCF) behaviors of the 7475-T7351 aluminum alloy was investigated. The aqueous solution of sodium chloride was used for solution treatment. The HCF test was performed on pre-cracked specimens using a servo-hydraulic universal testing machine, Instron 8800. The fractured specimens were characterized using a scanning electron microscope. It was observed that the crack propagation occurred through anodic dissolution at high stress and a significant crack tip blunting and crack extension occurred. However, no appreciable change in crack growth was noticed over the lower frequency range of 0.1 to 0.9 Hz. The slower growth rate envisages oxide debris formation between the cracked faces. When the alloy was treated under corrosive environments, the HCF tests depicted that the fatigue life reduces up to two orders of magnitude. The corrosion pits induced the crack initiation in stage-I at lower alternating stress; however, the fatigue crack growth rate (FCGR) was increased in the corrosive environment. The transition from stage-I to stage-II occurred at a lower stress intensity range (∆<italic>K</italic>) level; it was due to the combined effects of corrosion, hydrogen embrittlement, active path dissolution, and stress concentration. The corrosion fatigue test at low frequency also depicted a slower FCGR as compared to its moderate frequency counterpart and showed an irregular crack growth behavior.</p> </abstract>

Effect of Laser Peening on the Mechanical Properties of Aluminum Alloys Probed by Synchrotron Radiation and X-Ray Free Electron Laser

Synchrotron radiation (SR) and X-ray free electron laser (XFEL) are indispensable tools not only for the exploration of science but also for the evolution of industry. We used SR and XFEL to elucidate the mechanism and the effects of laser peening without coating (LPwC) which enhances the durability of metallic materials. X-ray diffraction (XRD) employing SR revealed that the residual stress (RS) in the top surface became compressive as the laser pulse irradiation density increased with appropriate overlapping of adjacent laser pulses. SR-based computed tomography (CT) was used to nondestructively reconstruct three-dimensional (3D) images of fatigue cracks in aluminum alloy, revealing that LPwC retarded crack propagation on the surface and inside of the sample. SR-based computed laminography (CL) was applied to friction stir welded (FSWed) aluminum alloy plates to visualize fatigue cracks propagating along the welds. The fatigue crack had complicated shape; however, it became a semi-ellipsoid once projected onto a plane perpendicular to the fatigue loading direction. Ultra-fast XRD using an XFEL was conducted to investigate the dynamic response of aluminum alloy to an impulsive pressure wave simulating the LPwC condition. The diffraction pattern changed from spotty to smooth, implying grain refinement or subgrain formation. Shifts in diffraction angles were also observed, coinciding with the pressure history of laser irradiation. The durations of the dynamic phenomena were less than 1 µs; it may be possible to use high-repetition lasers at frequencies greater than kHz to reduce LPwC processing times.

Effect of pulsed electric current on the growth behavior of fatigue crack in Al alloy

The growth of fatigue crack in aluminum alloy, 6061-T6, under the application of pulsed electric current was studied. To examine the effect of pulsed electric current, different electric current densities were used at different fatigue cycles, where the electric current densities were subjected as 90 A/mm2 and 150 A/mm2. With the electric current density of 90 A/mm2, the fatigue life was increased. However, when the current density levels became higher, that is, 150 A/mm2, the fatigue life was decreased. From the fracture surface observations, the local melting was found on crack surface. It is concluded that the increase of fatigue life is attributed to the crack shielding effect resulted from the local melting induced by the pulsed electric current. In addition, the effect of pulsed electric current was more remarkable in small crack region.

A Simple Algorithm to Eliminate Ambiguities in EBSD Orientation Map Visualization and Analyses: Application to Fatigue Cracks in Aluminum Alloys

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Imaging of Fatigue Cracks in Aluminum Alloy by Micro Computed Tomography with Synchrotron Radiation

The authors have applied micro computed tomography (μCT) with synchrotron radiation of SPring-8 to imaging of fatigue cracks in cast aluminum alloy AC4CH. Theoretical evaluation showed that the edge of a sample apart from an X-ray detector was enhanced in the radiographic image due to the refraction of X-rays. The edge enhancement was confirmed with 28keV monochromatic X-ray under the arrangement where the detector was set behind the sample by 800mm. The enhancement also appeared on the tomographic image reconstructed by FBP (filtered back projection) algorithm. The fatigue cracks were clearly observed in both reconstructed images of laser-peened and unpeened samples. The crack image of the unpeened sample was evidently larger than that of the laser-peened sample, which implies that laser peening retarded the fatigue crack propagation.

Investigation of fatigue cracks in aluminium alloys 2024 and 6013 in laboratory air and corrosive environment

Fatigue cracks have been generated in the commercial aluminium alloys AA2024 and AA6013 and analysed with a positron microprobe. This instrument provides laterally resolved positron annihilation measurements, which are sensitive to lattice defects like vacancies and dislocations. These commercial alloys have undergone a solution heat treatment and quenching prior to fatigue testing. Subsequently, they have been aged at room temperature and 190°C for AA2024 and AA6013, respectively. We performed the fatigue crack generation both in air and under the influence of a chemically aggressive environment (artificial seawater). Due to the corrosive environment hydrogen is probably produced at the fresh fractured surface in the vicinity of the crack tip. We discuss the possible implications of in-diffused hydrogen on the produced lattice defects, especially when there is a delayed migration of vacancies in the lattice, due to a reduced mobility.

Evaluation of Closed Cracks by Model Analysis of Subharmonic Ultrasound

Cracks in solids can be detected by ultrasound if they are open. However, their detection is not easy when they are closed with a closure stress, and thus it is a fundamental problem in ultrasonic testing. Subharmonics with half the input frequency is potentially useful in the detection and evaluation of such cracks, although quantitative analysis has not been established. In this work, we develop analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, for the first time. We proved their validity by comparison with experiments on a well-defined fatigue crack in aluminum alloy, finding reasonable agreements. Based on these theories, it will be possible to estimate important parameters of partially closed cracks by fitting measured waveforms to theoretical predictions, which solves the fundamental problem in ultrasonic testing of cracks.

Direct Observations of Cracks and Voids in Structural Materials by X-ray Imaging Using Ultra-bright Synchrotron Radiation

Refraction contrast X-ray imaging experiments were conducted on acrylic resin with an artificial cylindrical hole, A7075 aluminum alloy, A6063 aluminum castings, mild steel with cracks or voids, and low alloy steel with inclusions, using a ultra-bright synchrotron radiation X-ray beam in BL24XU hutch C of SPring-8. Conventional absorption contrast X-ray imaging experiments were also done for the comparison. The X-ray beam was controlled to be monochromatic by Si double-crystals and collimated by a slit. The distance between the sample and the detector was changed from 0 to 3 m, and the X-ray energy was 15 to 25 keV. Photographs were taken by X-ray film and/or X-ray CCD camera. As a result, the refraction imaging method gave a much more distinct image of the artificial cylindrical hole in acrylic resin as compared with the absorption method. The fatigue cracks in aluminum alloy and mild steel were also distinctly observed. The X-ray imaging revealed the presence of MnS nonmetallic inclusions in low alloy steel. Void defects in aluminum castings were clearly detected by the imaging. In addition, in-situ observation of tensile fracture of aluminum alloys using a high resolution X-ray CCD camera system was successfully conducted. The observations by use of asymmetric reflection technique for X-ray imaging experiment were also well performed. From above, the X-ray imaging method using ultra-bright synchrotron radiation is concluded to be very useful for fracture research of materials.

An optical-fibre fatigue crack-detection and monitoring system

An assessment has been made of the reliability of an optical-fibre crack-monitoring system for detecting the initiation and growth of fatigue cracks in aluminium alloys. The sensors are capable of detecting crack-width increases in the range 5-30 mu m in aluminium alloys subjected to cyclic loading with a high degree of confidence. The performance of the sensors is unchanged by large cyclic loading cycles up to about 100 microstrain, over the temperature range -196-+120 degrees C, up to 100% relative humidity conditions or immersed in normal or sea water. Parallel work showed that the same sensors can be used for detecting cracks in steel, concrete and composites. In the case of composites and also concrete, the sensors can be embedded within the structures during manufacture or construction.

Small and large fatigue cracks in aluminum alloys

Crack opening loads and the displacements near the crack tip have been measured by stereoimaging for large (through) cracks and small (surface) cracks. These results have been published. The present paper combines these observations and considers them in a new way, showing the relationship between small and large cracks, and provides a method for computing an effective driving force (ΔKeff) for small and large fatigue cracks.

Optimization of the Gel Electrode for Repeatable Imaging of Fatigue Cracks in Aluminum Alloys

Abstract The gel electrode method of detecting and imaging very small fatigue cracks in aluminum alloys provides the basis for a more rapid evaluation of fatigue performance and offers the convenience of periodic inspections during a test to monitor the initiation and growth of cracks. However, repetitive imaging of fatigue cracks can yield images with successively weaker contrast. This problem can be avoided by the correct selection of the operating parameters. This report describes the influence of various parameters on the spatial resolution and repeatability of the image. The spatial resolution is increased by increasing the starch concentration and reducing the charge flow. To obtain repeatable images the pulse duration (and charge) must be adjusted to balance the effects of the processes of passivation and corrosion. Sodium borate, which was previously included as an electrochemical buffer, should be omitted because it promotes too much passivation. Increasing the concentration of potassium iodide deters passivation so that images can be obtained repeatably with a shorter voltage pulse, less charge, and better spatial resolution. A compromise must be made between the spatial resolution and the visibility of the image. For routine inspection of components a highly visible image can be obtained with a gel electrode containing 0.06 M potassium iodide (KI) and 0.2 M starch upon application of a 10 V, 50 ms pulse. This combination yields repeatable images of cracks in aluminum alloys, with a spatial resolution ≈ 6 × 10−2 mm.

Gel electrode imaging of fatigue cracks in aluminium alloys

Previous papers have described a gel electrode technique recently devised for detecting and imaging fatigue cracks in aluminium tested in simple bending. In this study, the technique is shown to be applicable to testing in both bending and torsion and to high strength aluminium alloys 7075-T6, 2024-T3 and 2024-T4. Fatigue cracks as short as 10 μm in length are consistently detected and located. The flow of charge during image formation under standard conditions provides a quantitative measure of crack length, which is independent of alloy composition. A crack 100 μm long can be reliably detected by charge flow measurement; thus, this approach is not as sensitive as the information contained in the actual images.

A unified description of micro and macroscopic fatigue crack behaviour

In this paper, the importance of crack front length as a factor controlling growth rate is emphasized. It is shown that fatigue cracks in aluminium alloys do not advance in a coherent manner, but the front is divided into sectors, each of which relates to an individual cracking element. These elements act with some degree of mechanical isolation from their neighbours, and such an arrangement leads to crack front fragmentation and to an increase in the real crack front length. Even on this microscopic scale the crack segments extend as though in a continuum, and, certainly at low stress intensity factors, crack paths are dictated by the local stress direction.

Gel electrode imaging of metal fatigue: Part i. cracks in 6061-T6 aluminum

A simple electrochemical technique is described which produces high-resolution, visible images of fatigue cracks in aluminum alloys, without conventional metallographic surface preparation. The only preparation required is the formation of a thin (14 nm) anodic oxide film on the surface. After fatigue cycling, a semisolid electrolyte is placed in contact with the specimen and a voltage applied. As the current flows to the fatigue cracks a clearly defined image is formed. The capabilities of the technique are demonstrated by measurements on6061-T6 aluminum, and the images are correlated with scanning electron micrographs of the specimens. The images are reproducible and record features of the cracks which are barely discernible with a scanning electron microscope. Fatigue cracks only ∼30 µm long have been detected. Measurement of the charge flow during imaging is a quantitative measure of the crack length. These measurements indicate that considerably smaller cracks should also be detectable.