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The crack growth behavior of the aluminum alloy 6061 T651 was investigated using cruciform specimens with a measurement area of 120 × 120 × 2 mm3 with two center crack configurations of the starting notch parallel to one of the loading axes and under an angle of 45°, respectively. For the case with crack direction in one of the loading axes the load ratio R = σmin/σmax as well as the force parallel to the crack direction (resulting in different T-stresses) were changed. Crack growth rate was studied under varying T-stress. Also the retardation after single overloads was determined for R = 0.1, R = 0.5 and R = 0.8. As a result a change in T-stress does not significantly affect crack growth rate on high R ratios (R = 0.5) for constant DF loading. In case of lower R-ratios (R = 0.1) crack growth retardation was observed at presence of a static tensile load parallel to the crack growth direction due to higher influence of crack closure. Furthermore, such tensile load results in longer retardation periods after applying an overload at R = 0.1. Less pronounced overload retardation can be assumed with tensile force FX for R = 0.8 and 1.3 times overloads. Non proportional loading with a phase shift in time between the two axes of 45° and 90° results in a mixed mode situation (mode I / mode II) at the crack tip of a crack which is orientated under 45° to the loading axes. Mode I and mode II fractions change during every cycle. A phase change of 45° did not change crack growth significantly compared with proportional load. Crack branching occurred when changing from proportional loading to non-proportional 90° phase shifted loading. The two crack tips of the center crack under 45° divided in 4 crack tips under approximately 90° to the loading axes which were simultaneous propagating for more than 10 mm. Finally, two crack tips propagated faster than the remaining two. The stress intensity factors KI and KII as well as the T-stress where calculated by FEA (ABAQUS). For the 45° crack orientation and the non-proportional load case with 90° phase shift linear elastic FEA calculations show that there are time dependent rotating principal stress axes on the crack tip during one cycle. In the unnotched (uncracked) specimen there are fixed principal stress axes also in the phase shifted loading case. The configuration with 4 cracks has a significant higher ΔKI than the configuration with two crack tips while ΔKII is significantly lower. In addition uniaxial crack growth measurements were performed on SENB specimen in the size of 10 × 20 × 100 mm3 covering the threshold and Paris-region for loading ratios R = 0.1, 0.3, 0.5, 0.8. [ABSTRACT FROM AUTHOR]

The fracture mechanical properties of coarse-grained MgO–C were investigated in four-point bending tests at single-edge V-notched beams (SEVNB) at room and high temperature. For measurement of the crack mouth opening displacement (CMOD) an optical system was used. Room temperature tests were accompanied with microstructural observations of the crack path with the use of digital image correlation to investigate the mechanisms of crack propagation and to determine the crack length. MgO–C showed stable crack propagation at room temperature as well as up to 1500 °C. Increased fracture toughness K Ic and work of fracture were observed in the temperature range from 700 °C to 1200 °C. This behavior was correlated with crack closure, quasi-plastic deformation as well as the activation of visco-plastic creep mechanisms above 1200 °C. Additionally, the results were discussed in terms of Hasselman's thermal shock parameters. [ABSTRACT FROM AUTHOR]

The fracture mechanical properties of fine-grained carbon-bonded alumina (Al 2 O 3 –C) were investigated in four-point bending tests at single edged V-notched beams (SEVNB) at room and high temperature. Graphite served as a reference material. For measurement of the crack mouth opening displacement (CMOD) an optical system was used. The fracture behavior of Al 2 O 3 –C changed from brittle at room temperature to a crack resistance behavior at 1400 °C due to visco-plastic phenomena. The fracture toughness K Ic was determined as 0.69 ± 0.07 MPa m1/2 at room temperature and 0.55 ± 0.21 MPa m1/2 at 1400 °C. At room temperature, the work of fracture γ wof was determined as 26.3 ± 6.3 J/m2, and Hasselman's thermal shock parameters R st and R '''' were 8.6 K m1/2 and 0.9 mm, respectively. Although a quantitative analysis of the work of fracture at 1400 °C was hindered by heat shimmer within the chamber, the results indicated significant increases of this parameter as well as Hasselman's thermal shock parameters up to 1400 °C. [ABSTRACT FROM AUTHOR]

The fatigue life of welded joint was calculated based on numerical integration of simple Paris' law and a reliable solution of the stress intensity factor (SIF). The initial crack length ( a) was assumed to be equal to 0.1 mm in case of weld toe. This length was satisfactory for different butt joints geometries. The comparisons with the available data from standards and literature were demonstrated. It was shown numerically that the machining of weld reinforcements will increase the fatigue life. The increase of plate thickness decreases the fatigue strength (FAT) and the number of cycles to failure when using the proportional scaling of crack length. The validation processes of the current calculations have been shown. Therefore, it can be concluded that it will prevent the unnecessary waste of time consumed to carry out the experiments. [ABSTRACT FROM AUTHOR]

With welded joints, stress concentrations occur at the weld toe and at the weld root, which make these regions the points from which fatigue cracks may initiate. To calculate the fatigue life of welded structures and to analyze the progress of these cracks using fracture mechanics technique requires an accurate calculation of the stress intensity factor SIF. The existing SIFs were usually derived for one particular geometry and type of loading. In this study, the finite element method (FEM) was used to calculate the SIF. The stress intensity factors during the crack propagation phase were calculated by using the software FRANC2D, which is shown to be highly accurate, with the direction of crack propagation being predicted by using the maximum normal stress criterion. In the current work, a new analytical approach for the weld toe crack in cruciform welded joints has been used. The SIF results from FRANC2D were compared with those from the International Institute of Welding-IIW, and literature. A good correlation was obtained and the work results have bench marked which made it possible to use FRANC2D to simulate different weld geometries. The results of these comparisons are shown and the agreement is clearly well. [ABSTRACT FROM AUTHOR]

Abstract: The observation of cracks with curved crack path has to be done optically. It is shown that a modified commercial flat bed scanner and a combination of a high-resolution scanner camera system with 10,000×10,000 pixels with a telecentric lens are appropriate for high-resolution (up to 8μm/pixel) optical recording of multiple crack ends on sample areas up to 210mm×290mm. The high-resolution photographs are suitable for determination of crack lengths. It is also possible to observe crack paths or geometrical crack tip parameters and strain fields by image correlation. The method is used to determine static crack resistance and cyclic crack growth curves on center crack tension and biaxial cruciform samples. Furthermore, the paper presents an improved finite element technique for the simulation of curved fatigue crack growth in a multiple arbitrarily pre-cracked isotropic sheet under biaxial plane stress loading applying a predictor–corrector procedure in combination with the modified virtual crack closure integral (MVCCI) method including the consideration of the plastic limit loads. For this, the program PCCS-2D was extended to analyse the crack growth and the plastic limit load for each crack propagation step in a fully automatic simulation. The proposed solution algorithm provides a powerful tool for flaw assessment with the failure assessment diagram procedure in combination with a numerical crack path simulation. Finally, the simulation is verified experimentally. [Copyright &y& Elsevier]

When determining static fracture mechanics parameters, specimens are typically designed to maximize crack-tip constraint and consequently induce high stress triaxiality at the crack front. This is often achieved using deeply cracked bending specimens with side grooves. The crack resistance curves evaluated from such specimens represent a lower bound, enabling a conservative and reliable assessment of cracks in components. Depending on the geometry of the component and the crack configuration being evaluated, these bounds can be, however, overly conservative. Numerous studies over the past 30 years have investigated the influence of crack-tip constraint on the crack resistance behavior of steels, aiming to incorporate these effects into the evaluation of components. In contrast, there is a significant lack of corresponding data for nodular cast iron, a material widely used in mechanical engineering. This work aims to address this gap by presenting an experimental program complemented by numerical simulations. Single-edge bending SE(B) specimens with varying crack depths and side-groove configurations, as well as middle-cracked tension M(T) specimens are tested. Additionally, some specimens are tested in-situ using synchrotron-radiation computed laminography (SRCL) to gain insights into the processes occurring in the fracture process zone (FPZ). This work also evaluates the applicability of a non-local version of the Gurson–Tvergaard–Needleman (GTN) ductile damage model for predicting the effects of constraint. Correlations between fracture values and constraint-parameters, such as the T -stress, Q -parameter and stress triaxiality h are derived. Finally, an empirical relation is proposed to account for the loss of crack-tip constraint in the engineering assessment of cracks in components made of ductile cast iron. • Crack-tip constraint effects on the crack resistance behavior of nodular cast iron. • Experimental program complemented by numerical simulations. • In-situ analysis using synchrotron radiation computed laminography. • Correlation between fracture toughness and constraint parameters (T-stress, Q, h). [ABSTRACT FROM AUTHOR]

The thermo-mechanical uniaxial and biaxial-planar fatigue behavior of the forged nickel base superalloy Waspaloy™ was investigated for in-phase and out-of-phase loading between 673 K and 923 K. At these temperatures also the uniaxial and biaxial-planar isothermal material behavior was studied. In order to determine the influence of the multiaxial stress state on the fatigue life biaxial-planar isothermal tests were carried out at three different strain ratios. The uniaxial isothermal lifetimes coincide very well with literature data. A conservative lifetime description for thermo-mechanical in-phase and out-of-phase fatigue test was set based on the isothermal tests at the upper temperature. The comparison of lifetimes from uniaxial and biaxial-planar tests shows that the equivalent strain hypothesis according to von Mises correlates the fatigue lives of the different stress states within a scatter band of two. The failure mechanism including crack initiation, crack growth and the macroscopic crack path were studied by scanning electron microscopy. A change from a mainly transgranular crack growth at 673 K to a mostly intergranular fracture at 923 K as well as under thermo-mechanical fatigue loading was found. The fatigue lives of the isothermal and thermo-mechanical tests under both uniaxial and biaxial-planar loading were correlated by a new lifetime model which is based on a stress–strain approach. [ABSTRACT FROM AUTHOR]

Highlights: [•] We applied a 3D imaging technique in SEM for analyzing of fracture surfaces. [•] We determined both the stretch zone height and the stretch zone width. [•] We obtained good agreement of J i value determined by J iBl method and the 3D measurements. [ABSTRACT FROM AUTHOR]