Abstract: In this paper, the scattering of P-waves by hidden cracks located near to a free surface is investigated. The Indirect Boundary Element Method was used in order to study the wave propagation phenomenon in a halfspace considering various crack models. A near free-surface crack generates scattered surface waves whose amplitude spectra show conspicuous resonance peaks. Such effect has been attributed to local resonances of a virtual layer formed by the free surface and the up crack´s face. For the case of two-near-free-surface cracks the amplitude spectra show additional peaks, which can be associated to the presence of the second crack. Given similar sizes between these two cracks, the characteristic resonance frequency measured at the free surface corresponds mainly to the equivalent layer formed by the shallowest crack and the free surface. However, when the deepest crack becomes sufficiently large with respect to the shallow crack, two characteristic resonance frequency peaks appear. To illustrate the response and multiple scattering effects due to the presence of a second crack, results in frequency domain are provided. For the case of a near free-surface crack, results obtained by means of the formulation shown here have been verified against those obtained by using an analytical solution and good agreement was obtained.
Abstract: The fatigue crack growth of longitudinal SAW welds of a API 5L X42 steel pipe was evaluated using curved three point bend test specimens, where the radius of curvature of the specimens was equal to the pipe radius, so that no extensive machining or flattening was required. The fatigue crack growth tests were done in a servohydraulic closed loop machine under load control, in air at room temperature. The da/dN vs ∆K plots and Paris’s equation constants were obtained for the base metal, deposited metal and heat affected zone in the CR orientation, which corresponds to the short transverse direction of the pipe. The greater resistance to fatigue crack growth was the base metal, whereas the deposited metal showed the least resistance to the fatigue crack propagation. The deposited metal and the heat affected zone behaved according to Paris’s law, unlike the base metal, that showed a high dispersion of the fatigue crack growth rates. This behavior was due to the propagation of the crack in the direction transverse to the banded microstructure. Furthermore, the differences in fatigue crack growth rates among each zone were also related to the roughness of the fracture surfaces and fracture toughness.
Abstract: Little work has been published concerning the transferability of Gurson’s ductile damage model parameters in specimens tested at different strain rates and in the rolling direction of a Grade A ship plate steel. In order to investigate the transferability of the damage model parameters of Gurson’s model, tensile specimens with different constraint level and impact Charpy specimens were simulated to investigate the effect of the strain rate on the damage model parameters of Gurson model. The simulations were performed with the finite element program ABAQUS Explicit . ABAQUS Explicit is ideally suited for the solution of complex nonlinear dynamic and quasi–static problems , especially those involving impact and other highly discontinuous events. ABAQUS Explicit supports not only stress–displacement analyses but also fully coupled transient dynamic temperature, displacement, acoustic and coupled acoustic–structural analyses. This makes the program very suitable for modelling fracture initiation and propagation. In ABAQUS Explicit, the element deletion technique is provided, so the damaged or dead elements are removed from the analysis once the failure criterion is locally reached. This simulates crack growth through the microstructure. It was found that the variation of the strain rate affects slightly the value of the damage model parameters of Gurson model.
Abstract: In this work, the mechanical properties of randomly folded thin sheets in the hydrostatic and non-hydrostatic stress states were studied. It is pointed out that under the hydrostatic compression the sheet, rigidity is governed by the volume dependence of its enthalpy, whereas in a non-hydrostatic stress state, the rigidity of folded sheets is controlled by the shape dependence of the Edwards entropy of the network of crumpling creases. Furthermore, the stress relaxation in folded sheets after uni-axial compression was studied. It was found that stress relaxation in folded elasto-plastic sheets differs from this in the folded predominantly plastic sheets and obeys an unusual relaxation law with the universal characteristic exponent.
Abstract: This work focuses on determining the influence of mixed stress conditions on the initiation of fatigue cracks from corrosion pits in a UNS S31603 stainless steel. Fatigue limit was established as a threshold stress criteria for fatigue crack initiation from semi-elliptic surface pits. The strain energy release rate range (G) around semi-elliptic surface pits was determined as a function of the pit aspect ratio (depth/major width axes: a/c), pit contour and the orientation of the nominal stress from a linear elastic fracture mechanics approach. Results indicated that stress conditions for crack initiation in mixed mode overcome the minimum stress level or single threshold stress established from the fatigue limit concept. Under mixed mode condition a crack will grow at the surface zone (φ = 0˚) when a/c ≥ 0.60 and for orientation angles from 5˚ to 75.
Abstract: The fracture toughness of AISI H13 borided steel and the strength adhesion of the coated system were estimated in the present work. The formation of the layers was carried out by the powder pack boriding process at 1273 K with 8 h of treatment. The fracture toughness (KC) of the layer is estimated at 25 and 45 m from the surface using four different Vickers indentation loads. The KC values were estimated by the extension of Palmqvist cracks parallel and perpendicular to the surface obtained at the indentation corners. The adherence of the layer/substrate was evaluated in qualitative form through the Rockwell-C indentation technique. The results obtained by both techniques, show, in first instance, that the fracture toughness of the boride layer can be expressed in the form (KC) (π/2) > (KC) > (KC) (0). Also, high delamination is obtained around the Rockwell-C indentation prints that denote poor adhesion in the coating-substrate interface.
Abstract: The Navarro-Rios micromechanical model was used to assess the bounds of two different damage zones: crack arrest region and crack propagation region of controlled shot peening (CSP) of high strength aluminium alloys. Performance of CSP in terms of fatigue resistance was investigated. This comparison indicated that CSP in terms of fatigue depends on the competition between its beneficial and detrimental products, i.e. surface roughness and compressive residual stresses respectively. The gathered information can be used for safe load determinations in design.
Abstract: Some mechanical properties of AISI 1045 borided steels were estimated in the present work. The boriding process was carried out by the powder pack method at 950°C with 8 h of treatment. The fatigue strength on borided notched specimens was evaluated with rotating bending tests (R=1) considering a stress concentration factor (Kt) of 2.53. Likewise, the presence of residual stresses in boride layers was established by the XRD technique. The Daimler-Benz Rockwell C test was used, also, to estimate the strength adhesion of the coated system. The results show a decrease in the fatigue strength of AISI borided steels due to the presence of high porosity in the layers. Finally, the Rockwell-C adhesion test showed no coating failure for the boride layer.