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In this study, we analyzed the effect of a single overload fatigue cycle on the efficiency of bonded composite patch repair on a cracked Al 2024 T3 aluminum plate. Experimental and numerical analyses were performed to achieve the objective of the study. In the experimental part of the study, fatigue tests were conducted on a V-notched aluminum plate subjected to a constant amplitude fatigue loading with an overload applied at a relative crack length of 0.06 (a/W = 0.06). The plate was repaired with a carbon/epoxy composite patch. A fatigue life curve (a = f(N)) was plotted for the patched and unpatched specimens using the results of the experimental tests, and scanning electron microscopy images of the fracture surfaces of the failed specimens were observed. In the numerical part, the elastic–plastic finite element method was used to evaluate the plasticity induced by the overload on the patched plate and to analyze the adhesive damage after this overload was applied. The obtained results demonstrate that the patch deposition of the overload can lead to an extended fatigue life, and the adhesive damage after the overload is quite significant, but the risk of total failure of the adhesive layer is not high.

Friction stir welding is recent yet spectacular process, which assumes accrescent expanse to evolve as a multi-purpose process. Its potential is greatly being tapped through large-scale experimental and computer simulation-based investigations. Several simulation and empirical models have been proposed but exact fundamental analyses on forces, material flow and strain are still absent. Complexities associated with the process are perhaps the main reason that a fundamental analysis is difficult. A comprehensive analysis of this kind is critical for understanding the evolution of microstructure, mechanical properties of joint and defect formation. This study presents an analysis of material flow, process forces and strains using first principle approach. Results have been presented as exact mathematical expressions in terms of material properties and process parameters. It was demonstrated that the material during stirring experiences direct and shear strains both when it moves from advancing side to retreating side in front of the tool and after rotation deposits behind the tool. It was also demonstrated that the strain significantly reduced from advancing to retreating side; for a typical case the shear strain greater than 10,000% prevails in advancing side and the maximum shear strain on retreating side is of the order of 6000%.

The effect of single overload on the fatigue crack growth in 2024-T3 and 7075-T6 Al alloys was analyzed. Fatigue tests under constant-amplitude loading with overload peak were carried out on V-notched specimens. Fractographic analysis was used as a principal approach to explain the crack growth retardation due to the overload. Scanning electron microscopy (SEM) analyses were conducted on the fractured surface of failed specimens to study the retardation effect. The obtained results show that the overload application generates a plastic zone in both aluminum alloys. The generated plastic zone is three times larger in the case of 2024-T3 compared to 7075-T6, and thus, a significant crack retardation was induced for 2024-T3. The retardation effect due to the overload for 2024-T3 and 7075-T6 lasted for about 10 mm and 1 mm, respectively, from the point of overload application.

The objective of the present study was to evaluate strain-controlled low cycle fatigue behavior of a high zinc-containing Mg-10Zn-5Al (wt.%) cast magnesium alloy in relation to the microstructure characterized via EBSD and XRD. The addition of Zn with a Zn/Al ratio of ∼2 suppressed the presence of β-Mg17Al12 phase, and the as-cast alloy consisted mainly of primary α-Mg and eutectic-like τ-Mg32(Al,Zn)49 phase in a characteristic network form. The alloy exhibited superior tensile properties compared with several commercial cast alloys. Unlike extruded magnesium alloys, the alloy exhibited almost symmetrical stress-strain hysteresis loops due to the presence of τ-Mg32(Al,Zn)49 phase and nearly random textures. Although slight cyclic softening occurred at higher strain amplitudes, cyclic stabilization basically remained. This was also reflected by the nearly overlapped cyclic and monotonic stress-strain curves. The fatigue life predicted via strain energy based approach was in good agreement with the experimental value. Fatigue crack initiated from the near-surface imperfections, and crack propagation was characterized by fatigue striation-like features along with tear ridges.

Boron carbon oxynitride (BCNO) and boron carbon nitrogen (BCN) nanocompounds are successfully synthesized by a solid state sintering method using boric acid, urea and dextrose as the sources of B, N and C respectively under inert atmosphere. X-ray diffraction (XRD) and infrared spectroscopy (FTIR) analysis show that nanomaterials are synthesized at the high temperature range of 1350 °C. Sufficient amount of oxygen impurities are present in the sample, which is manifested in their photoluminescence spectra. Increase in temperature to 1350 °C under N2 atmosphere causes elimination of significant amount of O-species from the system. The presence of ample amount of carbon in the system results in the formation of high amount of polycrystalline h-BCN with minor amount of h-BN. However, some residual oxygen in the sample, which is detected in the FTIR study, leads to the formation of BCNO compound. These semiconductor BCNO compounds act as the potential luminescence centers for the blue-green emissions of the particles. Thus, these materials may find potential application as a phosphor material in various opto-electronic devices.

The technology of bonded composite repair has displayed its effectiveness in increasing the residual fatigue life of cracked aeronautical structures. However, two major phenomena reduce the performance of this technique; the adhesive disbond by the fatigue load and the thermal residual stresses due to the adhesive curing. In this study, the effects of the adhesive disbond and the thermal residual stresses on the fatigue life of repaired cracked aluminum plate were analyzed experimentally and numerically. Fatigue tests were performed on cracked 2024-T3 aluminum alloy specimens repaired using carbon/epoxy patches that have artificial disbonds, to highlight the adhesive disbond effect. Fatigue tests were performed on repaired specimen bonded with elevated temperature cure and ambient temperature cure adhesives to evaluate the effect of the thermal residual stresses. To analyze the effects of the two phenomena, the finite element method was used to compute the stress intensity factor at the front of repaired cracks. The experimental results indicate that fatigue life is significantly reduced by the presence of an initial adhesive disbond and thermal residual stresses. The numerical results are consistent with the experimental observations, and reveal that the stress intensity factor increases with an increase in the disbond width. Moreover, it also increases in the presence of thermal residual stresses.

The plasticity in the crack region is a great influence on the fatigue life of aircraft structures, and the adhesive damage has negative effects on the durability of bonded joints. In this paper, the interaction effect between the crack tip’s plasticity and adhesive damage in a bonded composite repair of cracked metallic panels was analyzed using the three-dimensional finite element (FE) method. A damage criterion was implemented in the FE model to evaluate the damaged zone in the adhesive layer, and the plastic strain around the crack was extracted from the FE calculations. In this research, we have demonstrated that the applied load has very significant effects both on crack plasticity and adhesive damage, but the ductility of the repaired material has no significant effect on adhesive failure. Fatigue tests and SEM observations were performed to validate the numerical results.

In this study, the effect of a single overload on the fatigue life of cracked aluminum 2024-T3 alloy repaired with a bonded composite patch were studied. Special consideration was given to the load history, sequence of the bonded patch repair and the overload application (repair-overload and overload-repair). The two cases were compared to a reference case involving a single overload on unrepaired specimen. Fatigue tests were performed on V-notched repaired and unrepaired specimens naturally pre-cracked under constant amplitude loading. Fractographic analysis on the failed specimens was carried out to analyze the obtained fatigue results. It was found that when the crack is repaired before the overload application, the retardation effect due to the overload is attenuated by the patch. Nonetheless, when the patch precedes the overload, very high fatigue life extension of about 38.9 times is observed due to a cumulative retardation effect of the two events (patching and overload).

Under real working conditions, structures are subjected to variable and peak loads. It is well known that cracks growth rate is load history dependent. Overloads generate crack growth retardation while underloads produce acceleration in crack growth. The effect of load history on patch repair is not covered so far. In this work, we investigated experimentally, how a single overload peak affects the performance of a bonded composite patch repair in cracked Al 2024-T3 and 7075-T6 samples fatigued under a cyclic loading. We studied the effect of three overload peaks; 9, 12 and 14 kN, applied just before and soon after the bonded composite repair. The fatigue lives and retardation cycle numbers exhibited that subjecting the test sample to overload before the repair will cause more retardation of the crack growth, while the reverse will result in less retardation, yet still better than that subjected to an overload alone. We carried out SEM observations to analyse the fractographs of failed specimens subjected to overloads and observed mixed mode fracture type.

This study analyzes the effect of the patch length the repair performance of bonded composite repair. Experimental and numerical approaches are used. In the experimental part of the study, fatigue tests with a constant amplitude were performed on notched specimens of 2024 T3 and 7075 T6 aeronautical aluminum alloys. Two stress ratios of R = 0 and R=0.1 were applied. The value of the patch length was varied to highlight its effect on the fatigue life. In the numerical part, the stress intensity factor and adhesive stresses were computed for different sizes of the patch length. The experimental results show that the fatigue life of repaired plates is significantly reduced with increases in patch length irrespective of the type of aluminum alloy and the stress ratio. The numerical results show that the stress intensity factor increases with increases in the patch length, and this is in agreement with the experimental results. This effect is explained by the presence of a secondary bending moment generated by the application of a single sided composite patch.

The mechanical response of a servo-hydraulic testing system is affected by the stiffness of test specimens. An adaptive controller helps in auto-tuning the system by setting the optimal proportional-integral-derivative values for the subsequent test as the stiffness changes. This paper presents the effect of auto-tuning of various channels on the flow response of several commercial Al and Mg alloys and a mild steel. Strain-controlled monotonic tensile tests were performed at a given strain rate of 1 &times, 10&minus, 4 s&minus, 1 after auto-tuning of position, load, and strain channels in different combinations. Serrated flow or Portevin&ndash, Le Chatelier effect was observed in the Al alloys after auto-tuning of either position channel only or position and load channels. However, the serrations of Al alloys were shielded after further auto-tuning of strain channel. The stress-strain curves of Mg alloys and mild steel were observed to be basically free of serrations under any combinations of auto-tuning, which confirms that the serrated flow is a property of specific materials rather than a machine system noise.

In this study, the effects of patch shape on the performance of bonded composite repairs on aircraft structures are investigated using experimental and numerical approaches. Fatigue tests under constant amplitude loading, with a positive stress ratio, are performed on V-notch cracked specimens using two aluminum alloys, 2024-T3 and 7075-T6 that are repaired with bonded carbon/epoxy patches. Three composite patch shapes are used: rectangular, trapezoidal, and triangular. Left- and right-oriented triangular patch shapes are used to analyze the effects of the patch orientation on the repair efficiency. Numerical models are built with these patch shapes and orientations. The stress intensity factors and adhesive stresses are computed to numerically evaluate the repair performances obtained with the different patch shapes and orientations. The experimental results demonstrate that rectangular patches provide the most efficient repair, whereas the triangular shape with a left-orientation may lead to catastrophic results, particularly with the 2024-T3 aluminum alloy. The numerical results concur with the experimental observations.

The purpose of this study was to examine weldability of an AA7072-cladded high-strength AA7075-T6 via ultrasonic spot welding, focusing on the influence of welding energy on mechanical properties and failure mechanisms. Defect-free sound joints were achieved with robust bonding and grain refinement via dynamic recrystallization at the weld interface, leading to superior static and fatigue properties. The nominal tensile lap shear strength increased with increasing welding energy, and exceeded AWS D17.2 standards at higher energy levels. Interfacial failure at higher cyclic loads with distinctive elongated shear dimples and fatigue striations and transverse through-thickness failure at lower cyclic loads were observed.

The purpose of this investigation was to study the low-cycle fatigue (LCF) behavior of a newly developed high-pressure die-cast (HPDC) Al-5.5Mg-2.5Si-0.6Mn-0.2Fe (AlMgSiMnFe) alloy. The effect of heat-treatment in comparison with its as-cast counterpart was also identified. The layered (&alpha, Al + Mg2Si) eutectic structure plus a small amount of Al8(Fe,Mn)2Si phase in the as-cast condition became an in-situ Mg2Si particulate-reinforced aluminum composite with spherical Mg2Si particles uniformly distributed in the &alpha, Al matrix after heat treatment. Due to the spheroidization of intermetallic phases including both Mg2Si and Al8(Fe,Mn)2Si, the ductility and hardening capacity increased while the yield stress (YS) and ultimate tensile strength (UTS) decreased. Portevin&ndash, Le Chatelier effect (or serrated flow) was observed in both tensile stress&ndash, strain curves and initial hysteresis loops during cyclic deformation because of dynamic strain aging caused by strong dislocation&ndash, precipitate interactions. The alloy exhibited cyclic hardening in both as-cast and heat-treated conditions when the applied total strain amplitude was above 0.4%, below which cyclic stabilization was sustained. The heat-treated alloy displayed a larger plastic strain amplitude and a lower stress amplitude at a given total strain amplitude, demonstrating a superior fatigue resistance in the LCF regime. A simple equation based on the stress amplitude of the first and mid-life cycles ((&Delta, &sigma, /2)first, (&Delta, /2)mid) was proposed to characterize the degree of cyclic hardening/softening (D): D=&plusmn, (&Delta, /2)mid&nbsp, &minus, &nbsp, /2)first(&Delta, /2)first, where the positive sign &ldquo, +&rdquo, represents cyclic hardening and the negative sign &ldquo, &ldquo, reflects cyclic softening.

The aim of this investigation was to assess the corrosion behavior of gray cast iron (GCI) alloyed with copper. Alloyed GCI specimens were austempered isothermally at varying temperatures. After austenitizing at 927 &deg, C, the samples were austempered at different temperatures ranging from 260 to 385 &deg, C with an interval of 25 &deg, C for 60 minutes. As a result, these samples developed an ausferrite matrix with different percentages of austenite. The resulting microstructures were evaluated and characterized by optical microscope (OM), scanning electron microscope (SEM), and X-ray diffraction (XRD). The corrosion characteristics were determined using potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) of these samples. These tests were carried out in a medium of 0.5 M H2SO4 and 3.5% NaCl solution. It was observed from the potentiodynamic polarization results that with increasing austempering temperature, the corrosion rate decreased. All results of the EIS were in accordance with a constant phase element (CPE) model. It was found that with an increase in austempering temperature, the polarization resistance (Rp) increased. The austenite content was also found to influence the corrosion behavior of the austempered gray cast iron (AGCI).

In the recent years, metal foams have become promising candidate materials in the engineering sector owing to their light weight and excellent energy absorption properties. Friction stir processing (FSP) has emerged as a cost-effective route to fabricate metal foam precursors from bulk substrates. Although the short processing time in FSP is able to provide high productivity, the cost of the foaming agent, TiH2 in the case of aluminum foams is still high. This paper introduces flame heating to achieve localized foaming of aluminum alloy AA5754 to explore the possibility of using magnesium carbonate as a foaming agent stirred using multi-pass FSP. A specially designed slot based strategy using two plates arranged in lap configuration is devised to stir the foaming agent and understand the material movement after each subsequent pass. Microscopy techniques were carried out to evaluate the distribution of the foaming agent after each pass and the resulting microstructure of the processed plates as well as the morphology of the foamed sample. EDX results showed higher Mg and O content around the pore walls.

The aim of this study is to find out the microstructure, hardness, and corrosion resistance of Pb-5%Sb spine alloy. The alloy has been produced by high pressure die casting (HPDC), medium pressure die casting (AS) and low pressure die casting (GS) methods, respectively. The microstructure was characterized by using optical microscopy and scanning electron microscopy (SEM). The hardness was also reported. The corrosion resistance of the spines in 0.5M H2SO4 solution has been analyzed by measuring the weight loss, impedance spectroscopy and the potentiodynamic polarization techniques. It has been found that the spine produced by HPDC has defect-free fine grain structure resulting improvement in hardness and excellent corrosion resistance.