No abstract

Using the asymmetric semi-circular bend specimen (ASCB) with vertical crack, a series of mixed mode I/II fracture tests were performed on two different rigid polyurethanes materials. The experimental results obtained for brittle fracture are compared with the theoretical predictions based on MTS and SED criteria. The results demonstrate that the SED criterion gives better results for mode II loading conditions, while for mode I and predominantly mode I both criteria are accurate.Keywords: ASCB specimen, mixed mode I/II, brittle fracture. Introduction.Brittle mixed mode fracture is a frequently mode of failure in engineering materials and components containing cracks and defects. Many different fracture criteria were proposed for mixed mode I/II fracture investigation, in order to determine the critical conditions at which a crack will initiate. Erdogan and Sih (1963) have formulated the maximum tangential stress (MTS) criterion, which was generalized by Smith et al. (2001) by taking into account the influence of the non-singular T-stress. The minimum strain energy density (SED) criterion introduced by Sih (1974) is another criterion, which has received researchers attention. Several different types of specimens were used in experiments. Thus, Mahajan and Ravi-Chandar (1989) have conducted their study on PMMA and Homalite-100 employing a center-cracked panel specimen. Aliha and Ayatollahi (2012) have analyzed the fracture initiation angle in some ceramics using the cracked Brazilian disc specimen. Introduced by Chong and Kuruppu (1984) for testing mode I fracture toughness, the semi-circular bend specimen (SCB) with angled-crack was also employed by Ayatollahi and Aliha (2006) to the fracture behaviour investigation of rock materials. Recently, Ayatollahi et al. (2011) have proposed the ASCB specimen with vertical crack for study of mixed mode brittle fracture on PMMA. Taking into account the advantages of the ASCB specimen (Ayatollahi et al., 2011) the experimental tests reported in this paper employ this type of specimen.Mixed mode fracture behavior for two types of rigid polyurethanes was investigated based on MTS and SED criteria. The elastic modulus E and the

The paper is focused on failure assessment of the shaft of a pumped storage unit. The shaft failure occurred during normal operation of the hydropower unit after 35 years of service. Three different methods are applied in order to assess its lifetime. Firstly, a finite element analysis was performed in order to obtain the static stress distribution and to apply a strain‐based crack initiation for the shaft under torsion load. The number of cycles for crack propagation was obtained using the Paris law for mode III. More precisely, the lifetime assessment of the shaft is obtained based on a low‐cycle fatigue analysis for estimating the number of cycles for crack initiation and then followed by that for fatigue crack propagation. Secondly, an analytical estimation based on failure assessment diagram is carried out for circumferential crack in order to determine the safe/unsafe region, where the shaft can operate with a crack. The failure assessment diagram for mode III loading was plotted in order to obtain the critical circumferential crack using the stress intensity factor solution and three analytical solutions for plastic zone. Thirdly, the theory of critical distance is employed for estimating the shaft life under torsional loading. A good agreement is obtained between the estimated results and experimental data for the shaft life and the crack length.

No abstract

Abstract. The key ingredient for any turbine runners design procedure and failure analysis is the material and its mechanical and fatigue properties. Starting from this observation, a series of static and dynamic tests were performed on the T10CuNiCr180 stainless steel after more than three decades of service. The fatigue tests were carried out under constant amplitude at a stress ratio of 0.1 and a frequency of 20 Hz. In the high cycle fatigue range, the S-N median curve was plotted and the fatigue limit for 3⋅10 6 cycles to failure was determined at 93.35 [MPa] expressed in stress amplitude. The casting and welding defects that were observed on fracture surfaces explained this lower fatigue limit. The fatigue crack growth tests were performed on compact tension specimens and a three-parameter model was used to plot the crack length versus number of cycles curves. In addition, the material constants from Paris' law were determined. Finally, the experimental results are compared against published data for another stainless steel.