Pejo Konjatić

This paper examines the influence of cooling time in the temperature interval from 800°C to 500°C (Δ t _8/5 ) on the Paris law parameters C and m for HSLA steel. Fatigue crack growth tests were performed on specimens machined from S960QL steel plates subjected to different cooling times. The test specimens were prepared in accordance with ISO 148‐1:2009, with a 2 mm deep V‐notch introduced into each specimen. Experimental data obtained using a RUMUL Cracktronic 160‐Nm resonant testing machine were used to determine crack growth rate diagrams as a function of the stress intensity factor range. Based on these diagrams, the Paris law parameters were calculated for different Δ t _8/5 values. The results show that variations in cooling time significantly influence the parameters C and m and the fatigue crack growth behavior. In addition, a numerical analysis using the extended finite element method (XFEM) was carried out to verify fatigue life simulation models by implementing experimentally determined parameters for each cooling condition.

The influence of infill density, number of layers, and fibre type on the tensile mechanical properties of composite parts produced by FDM 3D printing with continuous fibre reinforcement (CFR) was investigated. Specimens made of ONYX composite material reinforced with carbon, glass, and aramid fibres were tested using a static tensile test according to ISO 527. A linear regression model was developed to correlate mechanical properties with 3D printing parameters. However, the influence of infill density could not be reliably determined due to the automatic generation of solid infill around fibre layers and at boundary layers in the utilized 3D printing software. These reinforcements provided varying degrees of enhancement, with carbon and glass fibres showing the highest increase in strength, glass fibres offering the best enhancement in fracture strain, and carbon fibres in stiffness. The obtained models for tensile strength, strain at maximum stress, and modulus of elasticity can be useful in the design of 3D printed parts, offering a simple solution for the prediction of mechanical properties.

Dimensional accuracy is a critical challenge across many additive manufacturing (AM) technologies, directly impacting the assembly of 3D-printed parts. This study investigates these deviations for stereolithography (SLA) on a desktop printer. To anal...

Additive manufacturing via LCD vat photopolymerisation enables direct bonding of photopolymer to textile substrates, but optimal processing parameters remain unclear. A 3 × 3 factorial design investigated the effects of layer thickness (0.01, 0.025, 0.05 mm) and UV exposure time (40, 80, 120 s) on the single-lap shear strength of woven fabric-photopolymer joints (65% polyester/35% cotton) using a novel pause-and-bond methodology, following the EN ISO 4587:2003 standard. Five replicate specimens per condition yielded 45 samples for mechanical testing. All specimens (45/45) exhibited adherend-limited failure within the textile substrate rather than at the polymer-textile interface, yielding consistent shear strengths of 1.38 ± 0.04 MPa (range: 1.30–1.45 MPa). Two-way ANOVA revealed no significant parametric effects (p > 0.05), indicating that interfacial bond strength consistently exceeded textile cohesive strength across all parameter combinations. The minimum resource-efficient condition (0.01 mm/40 s) achieves equivalent performance to higher-parameter combinations, enabling substantial process optimisation for textile-integrated photopolymer sandwich structures while reducing material and processing time requirements.

Manufacturing flaw in pressure vessels and pipelines, especially in welded joints, poses a major challenge to structural integrity and operational safety. This paper examines the effects of such flaws, focussing on a specific case study of inclusions...

The research presented in this paper is based on the need for personalized medical implants, whose serial production is impossible, so the need for production process adjustments is inevitable. Conventional production technologies usually set geometrical limitations and generate a lot of waste material, which leads to great expenses, especially when the material used for production is an expensive Ti alloy. Additive technologies offer the possibility to produce a product almost without waste material and geometrical limitations. Nevertheless, the methods developed for additive production using metal powder are not significantly used in biomedicine because there is insufficient data published regarding the properties of additively produced parts, especially from the fatigue and fracture standpoint. The aim of this research is the experimental determination of fracture mechanics properties of additively produced parts and their comparison with the properties of parts produced by conventional technologies. Drawing is the first production process in the comparison, and the second one is selective laser melting (SLM). The Ti-alloy Ti-6Al-7Nb, used for medical implants, was selected for this research. Experimental testing was performed in order to determine ΔK_th fracture mechanics parameters and resistance curves according to ASTM E1820. Test specimen dimensioning and the experiments were carried out according to the respective standards. For the drawn test specimen, the value obtained was ΔK_th = 3.84 MPam^0.5, and the fracture toughness was K_c = 84 MPam^0.5, while for SLM produced test specimens the values were ΔK_th = 4.53 MPam^0.5, and K_c = 21.9 MPam^0.5.

The research presented in this paper is based on the need for personalized medical implants, whose serial production is impossible, so the need for production process adjustments is inevitable. Conventional production technologies usually set geometrical limitations and generate a lot of waste material, which leads to large expenses, especially when the material used for production is expensive Ti-alloy. Additive technologies offer a possibility to produce a product almost without waste material and geometrical limitations. Nevertheless, the methods developed for additive production using metal powder are not significantly used in biomedicine because there is insufficient data published regarding the properties of additively produced parts, especially from the fatigue and fracture standpoint. The aim of this research is experimental determination of fracture mechanics properties of additively produced parts and their comparison with the properties of parts produced by conventional technologies. Drawing is the first production process in comparison, and the second one is selective laser melting (SLM). Ti-alloy Ti-6Al-7Nb used for medical implants is selected for this research. Experimental testing has been performed in order to determine ΔKth, fracture mechanics parameters and resistance curves according to ASTM E1820. Test specimens dimensioning and the experiments are carried out according to the respective standards.

Ensuring the structural integrity of piping systems is crucial in industrial operations to prevent catastrophic failures and minimize shutdown time. This study investigates a transportation-damaged pipe exposed to high-temperature conditions and cyclic loading, representing a realistic challenge in plant operation. The objective was to evaluate the service life and integrity assessment parameters of the damaged pipe, subjected to 22,000 operational cycles under two daily charge and discharge conditions. The flaw size in the damaged pipe was determined based on a failure assessment procedure, ensuring a conservative and reliable input. The damage was characterized as a long axial surface crack with a depth of a = 2 mm and half-length c = 50 mm (c/a = 25), a geometry not well covered by existing Stress Intensity Factor solutions. To address this limitation, a modified magnification factor (M*) was introduced and tested for the present damage case (c/a = 25) and for additional crack geometries (c/a = 28–70), which showed improved agreement with Finite Element Analysis (FEA) than Newman’s original formulation. Stress Intensity Factor and Plastic Limit Pressure, essential parameters for structural integrity assessment, were computed numerically using FEA and validated against analytical predictions. Fatigue crack growth was evaluated using the Paris law with crack propagation simulated numerically by Ansys’s S.M.A.R.T. The Failure Assessment Diagram (FAD) was used to assess service life, incorporating constant working pressure and fracture toughness while considering evolving crack size during propagation. Results showed that analytical predictions with the modified magnification factor matched FEA within 5%, while the original Newman formulation overestimated results. The analytical service life solution predicted approximately 8500 fewer cycles than the numerical, remaining conservative but efficient. These findings are based on the present case of a long axial surface crack with high aspect ratios (c/a = 25–70, depending on crack depth), and while the modified magnification factor may also improve predictions for other geometries, this requires structured validation in future studies.

The aim of this study is to provide a solution for the plastic limit pressure and stress intensity factor of the elbows containing a part-through axial semi-elliptical crack by considering various crack sizes. The supporting system and loading conditions of the pipeline are described. The critical part of the observed pipeline was isolated for analysis and subjected to various sizes of semi-elliptical cracks. By performing numerical analysis, results were obtained for crack dimension ratios of c/a, and depth/thickness ratios of a/t. The obtained results include plastic limit pressure and stress intensity factor. The results were analyzed with a symbolic regression algorithm, and closed-form solutions for the limit pressure and stress intensity factor were proposed. To validate pipeline integrity, the Structural Integrity Assessment Procedure (SINTAP) was applied, and the FAD (Failure Assessment Diagram) was generated for cracks below the FAD function. The failure pressure was calculated by determining the points where the loading paths intersect the FAD function.