Papers by Keyword: Crack Closure

Abstract: An experimental campaign was developed to evaluate fatigue crack growth in Haynes 230. The effects of plasticity induced crack closure were investigated with Digital Image Correlation. In particular, crack opening levels were measured with the digital extensometer technique, which allowed the evaluation of crack flanks relative displacements. Experimental results were compared with a reference da/dn – ΔK_eff curve and with the data of a previous study, which analyzed single crystal propagation. It was found that the adoption of crack closure local measurements provided an accurate estimation of crack propagation driving forces, since all the experimental points from single crystals and polycrystals collapse onto the da/dn – ΔK_eff curve.

Abstract: Fatigue crack growth depends heavily on near tip stress-strain behavior controlled by many micromechanical and microstructural factors. Crack closure is widely used to rationalize crack growth behaviour under complex loading conditions. Reliable crack closure measurement is essential for enhanced damage tolerance design and remains a challenge to the industry. This paper focuses on the effect of plastic deformation ahead of a notch/crack on the non-linearity of compliance curves of 6082-T651 aluminium alloy specimens to highlight a potential issue in the conventional compliance curve based crack closure measurement technique. Experimental and numerical simulation results demonstrate that plastic deformation ahead of the notch will introduce non-linear stress-strain behavior in the absence of crack closure. It is proposed that the effect of crack tip plasticity on the non-linearity of the compliance curve be separated to obtain reliable crack closure measurement.

Abstract: Fatigue crack growth (FCG) behavior of three high manganese austenitic twin-induced plasticity (TWIP) steels with different stacking fault energy (SFE) was investigated, aiming at studying the correlation between the FCG resistance and the SFE of the steels. FCG tests were performed using three-point bending specimens at room temperature at stress ratio of 0.1 under the control of stress intensity factor range. Test results showed that the fatigue threshold values of these steels decrease with increasing the SFE. However, in the Paris regime, the crack growth rates of the steels do not appear to correlate directly with SFE. These results are discussed according to the degree of fatigue crack closure and the deformation mode of crack tip zone.

Abstract: The crack propagation behavior in a 2024 T351 Aluminum Alloy under constant amplitude loading has been studied. This study is analyzed in term of crack opening load measurements using a compliance technique. The results obtained under constant amplitude fatigue tests show that different crack propagation stages can be identified. Significant effects due to load ratio changes have been quantified.

Abstract: This paper deals with fatigue crack propagation under 4 bar hydrogen atmosphere in low alloyed steel (3.5Ni-1.5Cr-0.5Mo-V) used for turbine generator of nuclear plant. The tests are conducted in the same way in ambient air and high vacuum on CT specimens and the fatigue crack growth rate specially investigated in the near threshold range is plotted with respect to the applied stress intensity factor. It is shown that the propagation under hydrogen atmosphere is similar to that obtained in air up to a K_max value of 16,5 MPam^1/2 with increased growth rate compared to that in high vacuum leading to a threshold value lower that in vacuum, this effect being related to residual water vapor. For K_max higher than 16,5 MPam^1/2, much faster growth rates under hydrogen atmosphere becomes are associated to an intergranular propagation mechanism induced by an hydrogen effect. The results are discussed on the basis of available models.

Abstract: In this study, fatigue crack closure behavior was investigated in the aluminum alloy 6061-T6 and the carbon steel JIS. S25C. It was found that crack closure in the aluminum alloy 6061-T6 showed the characteristics of plasticity-induced fatigue crack closure (PIFCC), whereas the carbon steel JIS. S25C showed the characteristics of roughness-induced fatigue crack closure (RIFCC). The experiments included the determination of the crack-opening levels K_op as a function of stress intensity factor range ΔK and the effect of surface removal on the crack-opening level. In order to simulate the behaviors of the plasticity-and the roughness-induced fatigue crack closure, the finite element method was adopted. The results of FEM were in good agreement with the experimental results. It was cconcluded that at a given yield strength level , a low Youngs modulus and a low work-hardening coefficient will favor PIFCC, whereas a high Youngs modulus and a high work-hardening coefficient will favor RIFCC.

Abstract: The all-metal web-core sandwich structure consists of two face plates stiffened by one-directional system of web plates. These web core sandwich structures are used in many structural applications such as ship hulls, offshore platforms, bridge decks, and industrial platforms. However, the stress variation caused by the service loadings can be a determinant factor for crack initiation and growth until early failure of the entire structure. This paper presents an experimental study on fatigue crack growth rate in base material from a face plate after rolling and welding. The study is focused on the analysis of the stress ratio and crack closure effect on the fatigue crack growth rate in two directions. There is a significant stress ratio effect on fatigue crack growth rate, much more pronounced in the case of crack propagation in the longitudinal direction than in the transverse propagation. For all tests, the crack closure effect is more pronounced at low stress intensity factor range (in the threshold domain).

Abstract: Finite element simulation of stable fatigue crack growth using critical crack tip opening displacement (CTOD) was done. In the preliminary finite element simulation without crack growth, the critical CTOD was determined by monitoring the ratio between the displacement increments at the nodes above the crack tip and behind the crack tip in the neighborhood of the crack tip. The critical CTOD was determined as the vertical displacement at the node on the crack surface just behind the crack tip at the maximum ratio. In the main finite element simulation with crack growth, the crack growth rate with respect to the effective stress intensity factor range considering crack closure yielded more consistent result. The exponents m in the Paris law were determined.

Abstract: This paper discusses the physical meaning of crack opening corrections in fatigue crack growth analyses proposed in the literature. To provide an explanation concerning the physical relevance of these corrections, an energy approach is proposed to evaluate fatigue loading conditions. Considering cyclic loading in terms of cyclic energy rather than in stress cycle, explains the origin of the crack opening corrections.

Abstract: Aluminum alloys, widely used for constitutive parts of aircrafts are confronted to a wide range of temperature depending on altitude and climate, from room temperature on the ground down to some 223K at high altitude. The fatigue crack growth behavior of two new generation aluminum alloys, 2024A in T351 temper and 2022 in T351 and T851 tempers, have been investigated at both temperatures. It is shown that temperature and air humidity do not affect the crack growth resistance of the peak aged 2022 while these two parameters widely influence the crack growth in the under-aged alloys which exhibit in cold air a crystallographic retarded propagation similar to that in vacuum. The respective role of microstructure, temperature, atmosphere residual humidity and crack closure is discussed on the basis of a preexisting modeling framework for environmentally assisted fatigue.