A study on the ductile fracture of Al-alloys 7075 and 2017

The mechanism of the ductile fracture is studied experimentally for the Al-alloys 7075-T6 and 2017-T4 specimen.The experiments are conducted for the 1CT specimen and they are analyzed by using the acoustic emission and FRASTA (Fracture Surface Topographic Analysis) technique and the process of the fracture is investigated in real time.By the frequency analysis there are 3 kinds of AE patterns of the AE occurrence.By using the FRASTA technique, the details of voids nucleation and growth and the coalescence of voids and crack and the crack propagation process is clarified visually.AE with small amplitude corresponds to the cracking and decohesion of the inclusions and the AE of the large energy with eminent low cycle components corresponds to the high speed coalescence of cracks and voids nearby and this energy corresponds to the propagation area of the crack.

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Résumé On étudie expérimentalement le mécanisme de la rupture ductile sur des échantillons d'alliages d'aluminium 7075-T6 et 2017-T4. Les essais sont effectués sur des éprouvettes ICT, analysées par émission acoustique et par examen topographique de la surface de rupture, le processus de la rupture étant étudié en temps réel.Trois types d'émissions acoustiques ressortent d'une analyse de fréquences. Le détail de la nucléation de cavités, puis de la croissance et de la coalescence des cavités en une fissure, puis du processus de la propagation de la fissure apparaissent clairement grâce à l'examen topographique.Une émission acoustique à forte énergie avec des composantes à basse fréquence correspond à une coalescence à grande vitesse de fissures et de cavités dans le voisinage, et cette énergie est en relation avec la surface de propagation de la fissure.

     

Direct observation of tensile and fatigue cracks

Crack growth in aluminum foil specimens 1 mil thick has been studied under monotonie tension and low cycle fatigue. Crack tip regions were observed during loading by optical microscopy. Following final fracture, the crack path and fracture surface were examined by scanning electron microscopy.

Both tensile and fatigue cracks are typically preceded by a narrow necked region. Voids develop in the neck, crack growth proceeding by their growth and coalescence. As the voids form and grow, the foil necks down to zero thickness along the path of the crack.

Crack growth in both tension and fatigue results from concentrated plastic strain. The plastic flow is heterogeneous to an extent that voids precede the crack. In monotonic tension new voids are continually nucleated in the region of high strain ahead of the growing crack, while in fatigue, the voids result from plastic instabilities under cyclic loading.     

Relationships between crack opening displacement,alloy variables,and crack detection sensitivity

Alloy grain size is known to influence both the propagation path and the closure stress of surface fatigue cracks in many alloys. The general trend is for the path to be more tortuous and the closure stress to be larger, the larger the grain size. By use of Ti Al-4V and Al 7075-T6, the effects of grain size on the nondestructive detection of surface cracks which might arise from closure stress and path irregularity variables were evaluated. Titanium specimens were inspected using an acoustic harmonic generation technique, and it was discovered that the major source of harmonic signals was grain sized crystallographic cracks. Harmonic signals were larger during fatigue in an 8-µm compared to a 4-µm grain sized alloy, as there were more grain sized cracks in the large grain material. Crack closure was found to be extremely important in determining the reflected acoustic amplitude obtained in inspecting small (100–1000 µm) cracks in Al 7075-T6 using a critical angle technique. Average received amplitudes were an order of magnitude smaller for cracks at zero load than for those opened by a tensile stress. The scatter in the reflected amplitude was also large, apparently as the result of variations in the degree of the closure from crack to crack. For the 7075 material, the important effect of larger grain size was to increase the irregularity of the crack path, making the small cracks more visible acoustically at azimuthal angles not normal to the crack plane.     

Fatigue voids and their significance

ABSTRACT Fractures from tests on 2014‐T6511 and 2024‐T3 test coupons under specially designed programmed loading reveal voids with distinct fatigue markings. These ‘fatigue voids’ appear to form as a consequence of the separation of noncoherent secondary particulates from the matrix in early fatigue. The process of their formation is through the initiation, growth and coalescence of multiple interfacial cracks around the particulate. Such voids become visible on the fatigue fracture surface if and when the crack front advances through them. In vacuum, each fatigue void is the potential initiator of an embedded penny‐shaped crack. The one closest to the specimen surface is likely to become the dominant crack, indicating that fatigue voids appear to be the likely origins of the dominant crack in vacuum. In air, the dominant crack forms at the notch surface and grows much faster, giving less opportunity for multiple internal cracks to spawn off from the innumerable internal fatigue‐voids. Thus in air, fatigue voids do not appear to affect the fatigue process at low and intermediate growth rates. At high crack growth rates involving considerable crack tip shear, slip planes with particulate concentration offer the path of least resistance. This explains the increasing density of fatigue voids with growth rate. Very high growth rates signal the onset of a quasi‐static crack growth component that manifests itself through growing clusters of microvoid coalescence associated with static fracture. Fatigue voids are likely to form in other Al‐alloys with secondary noncoherent particulates. They have nothing in common with microvoids associated with ductile fracture.     

Modeling of coalescence of dispersed surface cracks. Part 2. Simulation model for multiple fracture

Abstarct A simulation model for multiple fracture has been developed that reproduces random processes of initiation, growth, and coalescence of dispersed surface cracks. The model is based on the method of statistical simulation (Monte Carlo method) and on the fracture regularities determined experimentally. The main factor responsible for fracture is found to be the coalescence of dispersed cracks, especially at the final stage, which accounts for about 30% of the total life. The ultimate state of a structure is defined by the condition according to which the length of the largest of the available damages is bigger than the calculated value of the maximum crack length.Translated from Problemy Prochnosti, No. 1, pp. 108–117, January–February, 2005.     

Tensile deformation and fracture behaviors of high purity polycrystalline zinc

The damage and fracture behaviors of high purity polycrystalline zinc with two grain sizes during tension were investigated experimentally at different strain rates. It was found that specimens with coarse grains (1 mm) showed serrated flow behavior and failed in intermittent brittle cleavage fracture, while specimens with fine grains (70 μm) showed no cleavage crack initiation before necking even at high strain rate. It was observed that the fracture process of the fine-grained specimen was highly related to strain rates. With the strain rate increasing, the damage mechanism transformed from formation of tearing cracks along interfaces (including grain boundaries, twin boundaries and kink band boundaries) and cavity coalescence into abrupt quasi-cleavage fracture. Based on the observation, the inter-crystalline fracture of zinc was investigated, and the damage and fracture behaviors of polycrystalline zinc during tension at room temperature were discussed in general.     

Analysis of Acoustic Emissions from a Steel Bridge Hanger

A steel bridge hanger with three fatigue cracks was monitored for acoustic emission (AE) using combined source location, strain gauge monitoring, and waveform analysis. AE activities from all three cracks were clearly identified and classified as crack growth or noise signals using location, strain magnitude, position on strain cycle, and uniqueness of waveforms as the primary criteria. A vast majority of AE from the cracks was found to be due to crack face rubbing and the crushing of corrosion products between the crack faces while limited crack growth emissions were detected. Results from laboratory tests on A588 compact tension specimens under variable-amplitude tension-tension fatigue loading were used to aid in interpreting AE data from the hanger. Crack growth AEs from these tests were detected only on overload cycles mostly above 92% of the maximum load while AE due to crack face rubbing occurred throughout the load cycle.     

Acoustic emission monitoring of a fatigue crack in 50D steel in a sea-water environment

Four specimens of 4360-50D steel plates were fatigued under a range of simulated marine environments and loads. AE events were detected using four broadband point-contact transducers and located in 3D (< 1 mm accuracy) relative to the crack front. This information was used together with the load at which each emission occurred to distinguish between ‘primary’ and ‘secondary’ source events.Little or no emission was observed from a dry fatigue crack, indicating that ductile crack growth by mode I loading in 50D steel plate is relatively quiet. In the presence of sea water, however, secondary emissions were observed at discrete points behind the crack tip where subsequent chemical analysis indicated a high concentration of calcium carbonate deposit. A third, cathodically protected, specimen gave much more AE, consistent with an increase in calcium carbonate between the crack faces. The emissions occurred at loads between zero and maximum tension. The characteristics of the AE signals did not change on reducing the maximum load, confirming that crack advance was not responsible. Crack face debonding, arising from fracture of calcareous deposits present, is believed to be the source of AE. A fourth specimen, also cathodically protected, was fatigued entirely in tension and gave less AE, consistent with much weaker crack face bonding.Measurements of the compressional wave arrival strengths at each probe were compared with calculated radiation patterns for different types of AE source. The experimental radiation patterns were always consistent with a double monopole combined with a microcrack source in varying proportions.Under these testing conditions, secondary events associated with corrosion product fracture dominated the emission. Whilst little or no primary emission was detected during fatigue crack growth in parent plate, this may not be the case in welds, regions of low toughness or where the stress fields are more complex.     

Evaluation of the tendency toward cold-crack formation during the welding of steels

Conclusions Using the AE method, the moment of nucleation of cold cracks in the studied material has been determined.A theoretical model has been derived, explaining the correlation between the AE activity and the mechanisms of nucleation, coalescence, and propagation of cold cracks caused by own stresses.The theoretical model has been proved experimentally.Published in Fiziko-Khimicheskaya Mekhanika Materialov, No. 2, pp. 44–50, March–April, 1992.     

Application of X-ray refraction topography to fibre reinforced plastics

The effect of X-ray refraction employs an unconventional small angle X-ray scattering (SAXS) technique which has been developed and applied to meet actual problems for improved non-destructive characterisation of advanced materials. The X-ray refraction technique makes use of X-ray optical effects at micro interfaces of composite materials. This method reveals the inner surface and interface concentrations in nanometer dimensions due to the short X-ray wavelength near 10⁻⁴ μm. Sub-micron crack and pore sizes are easily determined by “X-ray refractometry” without destroying the structure by cutting or polishing for microscopic techniques. The non-destructive characterisation of microfailure e.g. voids, fibre debonding, fibre cracks and microcracks of a short glass fibre reinforced polyoximethylene (POM-GF) after mechanical loading and accelerated ageing is investigated. X-ray refraction topographs are illustrated, showing the damage accumulation of POM-GF specimens after the fatigue test.     

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

Effect of test method and crack size on the fracture toughness of a chain-silicate glass-ceramic

The fracture toughness of a canasite glass-ceramic with a highly acicular, interlocked grain structure was measured by a number of different methods. The values at room temperature obtained by the chevron-notch, short-bar and notched-beam methods ranged from 4 to 5 M Pa m^–1/2, well above literature values for other glass-ceramics. Similar values of toughness were obtained by the fracture of bars with indentation cracks introduced with loads ranging from 1.96 to 400 N, but only for crack sizes >200 m, with lower values for cracks of smaller size. The toughness values obtained by the direct measurement of the size of the indentation cracks were appreciably lower than the values obtained by all other methods over the total range of indentation loads and corresponding crack size. SEM fractography showed that the surface within the indentation cracks was appreciably smoother than the surrounding fracture surface. The high values of fracture toughness were attributed to the combined mechanisms of crack-deflection and microcrack-toughening due to the stress-enhanced creation of microcracks caused by the residual stresses which arise from the thermal expansion anisotropy of the canasite monoclonic crystal structure. The strong negative temperature dependence of the fracture toughness suggests that at room temperature microcrack toughening represents the primary contributing mechanism to the fracture toughness. The combined effects of crack-deflection and microcrack-toughening can lead to the development of glass-ceramics with greatly improved resistance to crack propagation.     

Crack velocity dependence of longitudinal fracture in bone

An evaluation of the fracture characteristics of bovine tibia compact tension specimens associated with controlled crack propagation in the longitudinal direction has been made. The fracture mechanics parameters of critical strain energy release rate (G _c) and critical stress intensity factor (K _c) were determined for a range of crack velocities. A comparative fracture energy (W) was also evaluated from the area under the load-deflection curve. It was found that an increase in the average crack velocity from 1.75 to 23.6×10^–5 m sec^–1 produced increases in G _c (from 1736 to 2796 J m^–2), K _c (from 4.46 to 5.38 MN m^–3/2) and W. At crack velocities >23.6×10^–5 m sec^–1, W decreased appreciably. Microstructural observations indicated that, for crack velocities <23.6 m sec^–1, relatively rough fracture surfaces were produced by the passage of the crack around intersecting osteons (or lamellae), together with some osteon pull-out. In contrast, at a higher crack velocity, fracture was characterized by relatively smooth surfaces, as the crack moved indiscriminately through the microstructural constituents.     

AE source location and orientation determination of tensile cracks from surface observation

A technique to determine the location and orientation of an acoustic emission (AE) source due to a tensile crack was studied. Theoretical AE waveforms from tensile cracks were synthesized on the basis of the generalized theory of AE. Numerical experiments were conducted using the waveforms. We considered a source location technique using only surface observations and took into account the effects of velocity anisotropy. The technique is based on the radiation pattern of P-waves and incorporates a procedure to determine the orientation of the AE source. The results accurately reproduced the location and orientation of simulated tensile cracks as well as the values of their moment tensor representation.     

Recrystallization-etch approach to study the plastic energy absorbtion at crack initiation and extension of pressure-vessel steel A533B-1

To evaluate the elastic-plastic fracture toughness parameter of nuclear pressure-vessel steel A533B-1, a newly developed technique (the recrystallization-etch technique) for plastic strain measurement was applied to different sizes of compact tension specimens with a crack length/specimen width of 0.6–0.5 that were tested to generate resistance curves for stable crack extensions. By means of the recrystallization-etch technique, the plastic energy dissipation or work done within an intense strain region at the crack tip during crack initiation and extension was measured experimentally. Furthermore, the thickness effects on this crack tip energy dissipation rate were examined in comparison with other fracture-parameter J integrals. Thickness effects on critical energy dissipation and energy dissipation rate during crack extension were obtained and the energy dissipation rate dW _p/da in the mid-section shows a constant value irrespective of specimen geometry and size, which can be used as a fracture parameter or crack resistance property.     

Fractal Nature of the Brittle Fracture Surfaces of Metal

We have studied the fracture surfaces of low-alloy low-perlite steel after impact bending tests and of aluminum wire after fatigue failure at different temperatures. We have established that the fracture surfaces after brittle destruction are fractal surfaces. On the basis of the fractal model of a crack and the determined fractal dimensionalities of the boundaries of fracture surfaces, we have evaluated the critical sizes of brittle cracks.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 1, pp. 58 – 62, January – February, 2005.     

Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC_p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC_p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC_p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC_p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC_p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.     

Acoustic emission monitoring of fatigue in 7010 aluminium alloys

Abstract

Acoustic emission (AE) has been monitored in test pieces of 7010 aluminium alloy containing growing fatigue cracks. Both high-purity and commercial casts were studied in underaged, peak-caged, and overaged conditions, using tension–tension fatigue with R = 0·1 and 0·5, at crack growth rates in the range 0·1–3 μm/cycle. The reduction of grip noise and discrimination of AE events with sources distant from the plane of the crack were achieved by careful design of test piece geometry and clamping, and by the use of two broadband point transducers with calibrated response. Analysis of the load distribution at which AE events occurred at the crack plane showed that most events occurred near peak loading. However, the frequency of detected AE events was sometimes greatly reduced by adding oil to the crack, implying that crack-face rubbing was one source of emission. This was confirmed by fractographic examination and suggests that the practice used by some workers of gating–out all but those signals generated close to peak load in order to eliminate rubbing signals is not necessarily effective. The frequency of events remaining after the removal of rubbing signals was shown to be consistent with the size of the plastic zone at the crack tip, the rate of crack growth, and the inclusion size/density distribution. It is estimated that the fracture of inclusions of area greater than ~40 μm² could be detected by the testing system. The commercial cast contained more inclusions of this size than the pure cast, in proportion to the increased AE event frequency.

MST/184     

Phases analysis and impact of phases on fracture mechanism of AZ61‐SiC composite

Phase composition of AZ61‐SiC composite with 5 wt.% of nanosized silicon carbide reinforcement was analysed and failure mechanism by in situ tensile test in scanning electron microscope was observed. Microstructure of the experimental materials was heterogeneous with grain size of 15 μm. Based on the quantitative analysis of composite, besides, silicon carbide strengthened particles added externally into the matrix magnesium silicide, magnesium oxide, and aluminium/manganese particles formed in situ were found in the matrix. In situ tensile test in scanning electron microscope has shown that reinforcing particles substantially influenced failure mechanism. Large, brittle magnesium silicide particles (size of 40 μm–50 μm) cracked during tensile deformation and at the same time, as a result of different physical properties, decohesion of the matrix and smaller aluminium/manganese, silicon carbide and magnesium oxide particles (size of 5 μm–10 μm, 10 μm and 50 nm respectively) occurred. Reinforcing particles and brittle secondary phases driven micro voids and their coalescence was found as a major cause of large cracks formation. Subsequently the increase of stress caused the cracks propagation by the coalescence of fractured particles and decohesively release smaller dispersed particles. The fracture propagated at approximately 90° angle to the direction of the tensile load direction. Fracture surface had feature of transcrystalline and intercrystalline failure.     

Nondestructive Techniques for Studying Fracture Processes in Concrete

Some laboratory nondestructive evaluation techniques have been invaluable in studying fracture processes in concrete. Several nondestructive evaluation methods including acoustic emission (AE), computer vision, digital speckle pattern interferometry (DSPI), and X-ray microtomography (XMT) were used to examine the fracture behavior of concrete in tension and compression. Acoustic emission testing was used in an attempt to characterize the fracture properties of individual microcracks. As the specimens were loaded, AE waveform data was recorded, and analyzed for source location and source characterization. While DSPI analysis is limited to the specimen surface, the resolution is detailed such that microcracks on the order of 0.25 μm can be detected. Computer vision is a very useful method to measure crack openings for multiple crack development. It also can be used in conjunction with a hydraulic testing machine, which often generates vibration problem for some sensitive techniques. Crack patterns in cement-based materials under various material compositions and testing conditions are examined. This revised version was published online in July 2006 with corrections to the Cover Date.