Band 62, Heft 9

Monitoring systems for machines, plants, materials and equipment are increasingly used in production processes. These online condition monitoring systems can detect damage or excessive loads at an early stage and can drastically reduce or prevent long downtimes of plants and machines as well as high repair and maintenance costs. This paper depicts a method for online crack detection with pattern recognition methods for specimens joined by self-pierce riveting under cyclic load in fatigue tests (laboratory application). A software specially conceived for this application was developed. This software, AnrissMF, uses active acoustic testing with a structure-borne sensor to detect cracks in the joints at a very early stage. It is shown in this paper that this software can detect cracks much earlier than classical failure criteria for joints (i. e. before any drop in stiffness or frequency is observed). Furthermore, the successful application of software AnrissMF for online crack detection during the fatigue strength test is presented.

Hastelloy C-276 is a corrosion resistant nickel based solid solution hardened industrial alloy which has superior mechanical and corrosion properties. In this study, Hastelloy C-276 alloy was welded via the GTAW (Gas tungsten arc welding) method using ERNiCrMo-4 filler metal. Tensile, bending and notch impact tests were performed to determine the mechanical properties. The microstructure of the weld metal was investigated by light microscopy (LM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The impact toughness values of the heat affected zone (HAZ) showed a better performance than those of the weld metal. In addition, the corrosion properties of the weld metal and the base material were investigated by potentiodynamic polarization tests. Electrochemical potentiodynamic polarization parameters were determined according to corrosion behavior and microstructure properties. In particular, the corrosion rate of the weld metal increased because of the Mo-rich phases deposited in the weld metal microstructure.

In this study, resistance spot welding (RSW) process has been used on DP600 automotive steel at pressures of 2, 3, 4, 5 and 6 bar at a welding current of 7 kA. Microstructure, hardness and residual stress were examined to evaluate the effect of the welding pressure. An experiment at 5 kA and 6 bar was also conducted to investigate the effect of welding current on this behavior of steel sheet. The results show that microhardness values were higher in the welding region than in any other parts. There was no considerable change in the hardness behavior versus the welding pressure applied except for the sample welded at 7 kA and 4 bar. When the welding current changed from 7 to 5 kA, hardness improved. Residual stress evaluation reveals that compression stress was found in the specimens, and the specimen welded at 5 kA and 6 bar exhibited the highest residual stress.

The use of low transformation temperature (LTT) filler materials represents a smart approach for increasing the fatigue strength of welded high strength steel structures apart from the usual procedures of post weld treatment. The main mechanism is based on the effect of the low start temperature of martensite formation on the stress already present during welding. Thus, compressive residual stress formed due to constrained volume expansion in connection with phase transformation become highly effective. Furthermore, the weld metal has a high hardness that can delay the formation of fatigue cracks but also leads to low toughness. Fundamental investigations on the weldability of an LTT filler material are presented in this work, including the characterization of the weld microstructure, its hardness, phase transformation temperature and mechanical properties. Special attention was applied to avoid imperfections in order to ensure a high weld quality for subsequent fatigue testing. Fatigue tests were conducted on the welded joints of the base materials S355J2 and S960QL using conventional filler materials as a comparison to the LTT filler. Butt joints were used with a variation in the weld type (DY-weld and V-weld). In addition, a component-like specimen (longitudinal stiffener) was investigated where the LTT filler material was applied as an additional layer. The joints were characterized with respect to residual stress, its stability during cyclic loading and microstructure. The results show that the application of LTT consumables leads to a significant increase in fatigue strength when basic design guidelines are followed. This enables a benefit from the lightweight design potential of high-strength steel grades.

In this work, a hydroforming analysis of an exhaust pipe clamp adaptor made of galvalume steel was performed. Deformation and failure of galvalume exhaust pipe connectors and its coatings via the hydroforming process were studied. An explicit type (LsDyna) numerical analysis approach was adapted for the development of the hydroforming process. Pressure rate, unconstraint length, residual stress, strain hardening, and spring-back effects were considered in the analysis. The failure of tubular parts was evaluated via a forming limit diagram (FLD) according to Hill- Swift criteria. The hydroforming process was developed according to three models, which are found in the safety forming region of FLD studies. Minimum thickness values obtained from the experiments were compared with finite element analysis (FEM). The maximum percentage thinning ratio in the tubular section was more than 18 - % at 65 MPa internal pressure. The relationship between thickness reduction at plastically work hardening regions and residual stress was discussed. The deformation characteristics of the produced connector headers were also analyzed by scanning electron microscopy (SEM) and micro Vickers hardening measurements. The FEM analysis and FLD examination indicated that the failure did not occur in hydroformed specimens, whereas damage was observed at coated’ plastic work regions through microscopical investigation.

In this study, an alloyed steel surface was coated by thermal diffusion with titanium carbide powder at different temperatures (900, 1000 and 1100 °C) and times (1, 2 and 3 h) to determine their effect on the coating microstructure. The thermal coating was carried out by tube cementation designed and manufactured by us for the purpose of thermal coating. Coating thickness, element distribution and phase composition were analyzed by optical microscope, scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction (XRD) methods, respectively. Microhardness tests were performed to determine mechanical properties. The influence of different temperatures and production times on the coating thickness and microstructure were determined. The results illustrated that titanium carbide particles adhere almost completely and are diffused to the substrate during production. Thus, coating thickness increased through increasing time and temperatures. Moreover, microhardness increased with increasing temperature more than time did. The highest microhardness value achieved was 1301 HV for 3 hours. Temperatures of 1000 °C were reached due to the diffusion of TiC to the surface of the substrate, thus forming an extremely hard and protective layer.

The present study deals with the improvement of the mechanical and dielectric properties of low-density polyethylene (LDPE) by incorporating BiSr 2 CaCu 2O6.5 (coded as Bi-1212) ceramic powder. In this context, LDPE/Bi-1212 films with different ceramic weight contents were prepared by hot press technique. The ceramic reinforcing material was produced by a solid-state reaction technique. While the surface morphologies of the films were determined using a scanning electron microscope (SEM), the chemical structure analyses of the samples were performed by Fourier transform infrared (FTIR) spectroscopy. The mechanical performances of the specimens were evaluated by determining the tensile strength, Young’s modulus, percentage strain at break, and energy at the break values obtained by stress-strain curves. The real and imaginary components of the complex permittivity, as well as the alternating current (ac) conductivities of the samples, were measured within in the frequency interval of 1 Hz-1 MHz between 293 K and 353 K. As a result of tensile properties analyses as well as the investigations of ac electrical parameters, it was revealed that the LDPE/1.0 wt.-% Bi-1212 composite has a promising potential in electronic applications due to its improved mechanical durability, charge storage ability and conductivity.

The investigated alloy is modified by casting with Sr in order to finish the eutectic silicon from microstructure and furthermore subjected to solution treatment followed by natural or artificial aging to improve the usage properties. Metallographic investigations and X-ray diffraction analyses showed that the heat treated microstructure consists of α- solid solution crystals with aluminum base, α + Si eutectic and intermetallic Al-Mn-Si phases. Mechanical tests and ultrasonic cavitation measurements showed that the highest mechanical characteristics and cavitation erosion resistance properties are obtained by applying the solution treatment followed by artificial aging. In contrast, electrochemical tests carried out in a saline concentration of 3.5 % NaCl in order to determine the corrosion rate, indicated that although there are no significant differences between the three structural states, a slight improvement was found in the corrosion behavior after applying the solution treatment followed by both natural and artificial aging. The phenomenon was demonstrated by shifting the values of corrosion currents from 2.66 μm/cm2 for the as-cast state, to 1.81 μm/cm2 and 1.52 μm/cm2, respectively, for the aged states. Finally, analysis of the cavitation eroded surface highlights the presence in the areas with α-solid solution structure of some flat-bottomed striped pinches, characteristic of fatigue fracture and of microcraters in the micro-zones where the fragile intermetallic phases were dislocated.

The quality of grouting is important for the mechanical performance of prefabricated buildings. The impact echo method is used to study the internal condition of grouting sleeves. Mode analysis of the sleeves is carried out by finite element methods. Impact loads with durations of 12 μs, 20 μs and 40 μs were applied to the model for impact echo studies, and the echo signals were compared by FFT transformation and wavelet transform. In impact echo experiments, steel balls with different diameters were used to impact the grouting sleeve. It was found that the characteristic frequency polarization of the echo signal with a diameter of 5 mm was the most pronounced. The research shows that for smallsized inspected objects such as grouting sleeves, the object’s own vibration frequency has a major influence on the detection result.

The main objective of this experimental work was to evaluate the mechanical (tensile, flexural, impact, and microhardness) and water absorption behaviors of borassus/epoxy composites. The composites were fabricated by varying the weight percentages of borassus fruit fiber from 5 to 20 wt.-%. The surface morphology of the failed test specimens were also analyzed using FESEM. The composites reinforced with 15 wt.-% borassus fruit fiber were found to have superior mechanical properties when compared with other developed composites. The water absorption was higher for the composites having fiber weight percentage of 20.

This study addresses the effect of loading rates on cohesive parameters and microstructural composition of the Araldite 2015 adhesive. Double Cantilever Beam (DCB) samples were tested under the loading rates of 1, 5, 10, 20, 100 and 200 mm × min-1. The Park-Paulinho-Roeser model (PPR model) was used to get cohesive parameters. In modelling of the softening behavior, inverse analyses were performed using the date obtained from the PPR softening curves. It was seen that the fracture energy and cohesive parameters are decreasing with increasing the loading rate. However, there seems to be a transition region where the fracture energy nearly remains constant. Microstructural analyses were implemented in order to study the effects of the loading rates on the characteristics of the fracture surfaces. It was concluded that the loading rates greatly influence the distribution of micro-voids in the epoxy matrix. Moreover, the presence of voids in epoxy matrix improved the plastic deformation around the crack tip and increased the fracture toughness.

Failure modes of two kinds of unbonded prestressed reactive power concrete (UPRPC) beams were studied experimentally under static and fatigue loads, and compared. In the tests, deflections, crack widths, rebar strains, and beam strains were measured and analyzed. The fatigue test results show that the fatigue life of non-creeped beams is more than 2 million loading cycles and the creeped beam is damaged after 1.75 million loading cycles. The compression edge strain, mid-span deflection, and rebar strain are -814 μξ, 7.6 mm, and 484 μξ, respectively, for noncreeped UPRPC beams, and -1110 μξ, 11.2 mm, and 1169 μξ , respectively, for creeped UPRPC beams (μξ represents 1 × 10 -6 ). These values are much smaller than the static load failure values. Considering the fatigue residual strain, fatigue elastic modulus degradation, and fatigue strength degradation, a simplified analysis method for a full fatigue process is proposed, which predicts values in good agreement with the measured results.

This paper presents the effect of cutting tool, cutting speed and feed rate on the flank wear and surface roughness of austenitic stainless steel (AISI 304) during wet turning. Turning tests were designed based on the Taguchi method (L18). An orthogonal array, the signal-to-noise ratio (S/N) and the ANOVA were used to investigate the machinability of AISI 304 stainless steel with PVD and CVD coated tungsten carbide inserts. As a result of ANOVA, it was found that the feed rate was the most effective parameter on both flank wear and surface roughness.

High strength aluminum alloys have a face-centered cubic structure and it is not easy to form hydrides due to the extremely low solubility of hydrogen. Therefore, it is very difficult to test and characterize hydrogen in the high-strength aluminum alloys. In this work, the penetration of hydrogen and the distribution of hydrogen near the crack tip in the highstrength aluminum alloy were analyzed by time of flight secondary ion mass spectrometry (TOF-SIMS). Meanwhile, the test method of trace H in highstrength aluminum alloy was investigated by using TOF-SIMS technology.