Darko Damjanović

Residual stress state in pipe cut ring specimens for fracture toughness testing

Abstract Thin-walled pipes are not suitable for measuring fracture toughness parameters of vital importance because longitudinal crack failure is the most common failure mode in pipes. This is due to the impossibility to manufacture standard specimens for measuring fracture toughness, such as SENB or CT specimens, from the thin wall of the pipe. Previous works noticed this problem, but until now, a good and convenient solution has not been found or developed. To overcome this problem, very good alternative solution was proposed, the so-called pipe ring notched bend specimen (PRNB) [1–5]. Until now, only the idealized geometry PRNB specimen is analyzed, i. e., a specimen which is not cut out from an actual pipe but produced from steel plate. Based on that, residual stresses are neglected along with the imperfections in geometry (elliptical and eccentricity). The aim of this research is to estimate the residual stress state(s) in real pipes used in the boiler industry produced by hot rolling technique. These types of pipes are delivered only in normalized condition, but not stress relieved. Therefore, there are residual stresses present due to the manufacturing technique, but also due to uneven cooling after the production process. Within this paper, residual stresses are estimated by three methods: the incremental hole drilling method (IHMD), X-ray diffraction (XRD) and the splitting method (SM). Knowing the residual stress state in the ring specimen, it is possible to assess their impact on fracture toughness measured on the corresponding PRNB specimen(s).

Analysis of wind influence to static stability of the eave framework

Eave framework is analyzed in this paper from the standpoint of statics. Loads taken into account are self-weight, snow weight and wind influence. 3D model of eave framework is generated in the software PartSolution (Cadenas) with certain simplifications in geometry. Simplifications are taken into account in order to reduce costs of the numerical calculation, especially the FSI calculation. The resulting pressure distribution solution obtained from CFD calculation is set as the boundary conditions on the eave framework when performing structural analysis. Solutions for equivalent stress as well as deformation are presented and analyzed. Also, solutions for airflow prediction around the eave framework are presented in order to analyze if it is possible to perform some changes in the construction in terms of reducing the air resistance and thus reducing the wind load on the construction.

Fatigue Damage Assessment of Bolted Joint Under Different Preload Forces and Variable Amplitude Eccentric Forces for High Reliability

Fatigue damage assessments of M10 bolted joint, made of 42CrMo4 heat treatable steel and strength class 10.9, were carried out for different preload forces and variable amplitude eccentric forces for high reliability. Assessments were done with preload forces of 50, 70 and 90 % of force at bolt yield point and without preload force. The nominal approaches from Eurocode standard and VDI 2230 guidelines are mostly used for fatigue assessment. These nominal approaches cannot consider and describe in detail the local stress state at the thread root, which on bolt M10 have a radius of 217 μm. Threaded joints have several peculiarities that complicate the fatigue damage assessments. Range of dispersion was used to describe material cyclic scatter band with Gaussian normal distribution in logarithmic scales. Range of dispersion for notched structure of 42CrMo4 steel was taken from measurements. In order to take multiaxial stress field in thread root with high notch effect, multiaxial fatigue stress criterion based on a critical plane theory was applied for fatigue damage assessment. Critical plane approach is used for estimation of the fatigue damage and fatigue fracture plane position. Decrease of fatigue strength beyond the S-N curve knee point at 2 × 106 cycles was considered for the damage calculation. The main difficulties encountered in threaded joint fatigue damage assessment are due to the uncertainties and therefore, statistics and probability were applied. Assessments were carried out for 50, 97.5, 99, 99.9, 99.99, 99.999, 99.9999, 99.99999 and 99.999999 % survival probability.