Papers by Keyword: Fracture

Abstract: Plastic bags are one of the most widely used packaging materials in industries. Lower production cost, lightweight and high strength envisage its use in food packaging, carrier, and transportation industries. However, these plastic bags are non-biodegradable and cause serious water and soil pollution. It is one of the most thrust areas of research to overcome this problem in the current world. In the present investigation, fabrication, and characterization of an environmentally friendly and biodegradable "jute fibre reinforced paper composite" and "reinforcement free paper laminate" subjected to drop test were done. The standard procedure of the "MIL-STD-810G 516.6" drop test was followed. The adhesion between the matrix (paper) and reinforcement (jute fibre) was produced by applying a chemical-free adhesive made of flour and water. There were four different types of samples: single paper (SP), reinforcement-free paper laminate (RFPL), single-layer jute fibre reinforced paper composite (SLJPC), and double-layer jute fibre reinforcement paper composite (DLJPC). The microstructural characterization of the fractured surface after the drop was done using a scanning electron microscope operated at a voltage of 20 kV. The DLJPC samples had the maximum density and envisage drop-strength of 7 times as compared to the SP samples having the lowest density. The drop-strength exhibited linear regression with density for all the samples. The microstructure of the as-received matrix showed a non-homogeneous distribution of fibres along with micro-voids which were susceptible sites for the fracture. Unlike as-received matrix fibres distribution, the reinforcement fibres were aligned in two mutually perpendicular directions which leads to its strengthening. Hence, it can be said that the non-uniform structural properties envisaged by the as-received matrix can be compensated by uniformly distributed structural properties of the as-received reinforcement when both come together as a composite. The primary fracture mechanism of SP samples exhibited fibre breaking along with a few fibres' delamination. However, in the case of the RFPL sample, the nature of adhesion applied was capable to hold both the interfaces and the primary fracture mechanism was fibre breaking. It is suggested that the adhesive applied transmitted load through the interfaces. Unlike SP sample, the RFPL sample exhibited some adhesive pull-off. The fractured surface of the SLJPC sample showed that the matrix did not subject to fracture, however, reinforcement did fracture hence the load was transferred from matrix to reinforcement followed by a rupture of the reinforcement. Therefore, the primary fracture mechanism for the SLJPC sample was reinforcement rupture. The DLJPC sample showed a delamination of matrix and reinforcement.

Abstract: The article is aimed at studying the effect of austempering temperature below and above Ms temperature on the phase-structural state and mechanical properties of 0.2 wt.% C TRIP-assisted steel micro-added with Nb, V, Mo, Cr. The samples were austenitized at a temperature close to the Ac₃ point (900 °C) and held at 300 °C (below M_s), 350 °C (close to M_s) and 400 °C (above M_s) for 5-20 min. The work was performed using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction, and tensile/impact testing. It was found that austempering at the aforementioned modes ensures the multiphase structure consisting of carbide-free bainite, tempered martensite, ferrite and retained austenite (in different combinations). The optimal was austempering at a temperature close to Ms which provided an advanced complex of tensile properties (PSE of 23.9 GPa×%) and V-notched impact toughness (95 J/cm²). TRIP-effect contributed to these properties while the strain hardening process tended to be prolonged with increasing the austempering temperature.

Abstract: Flanges are commonly used in aircrafts to provide stiffness and support for the assembly Incremental Sheet Forming (ISF) processes have been approached to produce both stretch and shrink flanges as a low-cost alternative in the fabrication of a small number of parts and prototypes. This work analyzes stretch and shrink flanges of AA2024-T3 sheet with different geometries manufactured by Single Point Incremental Forming (SPIF). The numerical simulation using Finite Elements of the flanges allows evaluating the stress in successful and failed flanges. On the one hand, the formability of stretch flanges is usually evaluated in terms of principal strains within the Forming Limit Diagram (FLD). However, this approach does not seem to capture all the physics to explain the enhancement in formability observed in SPIF over the conventional forming. A formability analysis is performed in the field of stress triaxiality versus equivalent plastic strain, discussing the differences between successful and fractured specimens. On the other hand, for shrink flanging, the appearance of wrinkles is analyzed in terms of the compressive stresses along the flange during the incremental forming. This allows to determine a critical limit stress of winkling to predict the failure in practice for a given geometry and forming condition.

Abstract: Fracture resistance curves (R-curves) have served as a robust tool in characterizing the entire fracture process of engineering materials. However, obtaining such curves for asphalt concrete (AC) mixtures is cumbersome due to the non-linear inelastic behavior of the mixtures. In this research, a single-specimen technique is developed based on the unloading compliance method which is used for metals. AC mixtures with limestone aggregate and PG58-22 binder were prepared. Beam specimens were fabricated and single-edge notched beam (SE(B)) fracture testing was conducted at low temperatures. A loading-partial unloading regime was used in the experiments and crack growth increments were captured by digital images throughout the tests. Using a multi-variable regression analysis, modified compliance equations were obtained for AC and R-curves of the mixtures could be constructed. It was revealed that the R-curve developed by ASTM E1820 compliance method could potentially overestimate the resistance of the mixtures against low-temperature fracture. The constructed R-curve exhibits a lower semi-vertical region addressing lower resistance of the mixture in the crack blunting phase. Also, the post-peak phase of the fracture shows a significantly lower slope in the constructed R-curve which denotes lower resistance of the mixture against unstable crack propagation.

Abstract: This paper considers the softening and flattening treatment of Schizostachyum Lumampao Bamboo as part of a process in its preparation for utilization. Bamboo half-culm samples with thicknesses of either 5.00 mm or 6.00 mm was soaked in palm oil at either of two temperatures (100°C or 160°C) with an application of 50 N load to simulate the flattening process for a period of either 2700 sec. (45 mins.) or 3600 sec. (60 mins.). By measuring the height of the bamboo half-culm before and after the thermal treatment, a relationship was derived between the total heat transferred into the bamboo culms and the flattening behavior. Furthermore, after the flattening, the bending strength of the culms exhibited an exponentially decaying trend and the fracture strength a bi-modal behavior which is confirmed by other studies. This study reports a derived parameter designated as the thermodynamic bending stiffness of about 4 MPa for the flattening process.

Abstract: The article is focused on the analysis of fracture mechanisms of specimens made of austenitic steel, which have been subjected to dynamic tests. Austenitic stainless steels are characterized as high corrosion resistant materials with high bio-tolerance and relatively high strength. They are made by cold working, where plastic deformation occurs and they are deformed especially by slipping and twinning. Deformed regions are characterized by deformation twins and slip deformation. Specimens were used in two states, in the initial state and after chemical-thermal treatment. Dynamic tests to which specimens were subjected were the impact test and the three-point bending test. Fracture areas were evaluated by scanning electron microscope.

Abstract: The effect of hydraulic fracturing on the permeability and fluid flow of various Iraqi oil storage cores has been studied. Rumaila, Majnoon, and Zubair fields with different permeability were included in this study. Experiments were performed to determine the breaking pressure of the permeable core. Therefore, numerical and experimental results were compared to test the constant pressure on the impermeable core. Finite Volume Method (EbFVM) corresponding to the Dual Porosity Dual Permeability Model (DPDP) was used. A fractured reservoir in the Iraqi fields was studied for its single-phase fluid flow behavior. To represent the pressure distributions, ANSYS-CFX program was used. The comparisons between the fractured and non-fractured cores has been presented. In this paper, the pressure for permeable cores of Rumaila field has been determined and verified during the experimental and numerical simulation due to its lower fracture pressure. Hence, the required pressure has been determined for impermeable cores of Majnoon and Zubair fields due to the difficulties to measure such high pressure. The results show that the expected pressure to make a flow for the core belongs to the Majnoon field reaches to 1.724×107 Pa (2500 psi), while the expected pressure for the fluid to flow into the core belongs to the Zubair field reaches 1.379×108 Pa (20000 psi).

Abstract: Self-reinforced polymeric composites have numerous advantages over traditional polymers, including increased mechanical strength, fracture toughness, and other mechanical properties. A special type of such composites can be obtained using non-conventional molding techniques. The investigated self-reinforced composites have layered structure, where layers are created by the same material in numerous molding cycles in the manufacturing process, and have gradient mechanical properties. According to this, it is of high interest to investigate structural behavior of these materials under fatigue loading, including temperature effects and fracture mechanisms occurring during fatigue. The results of the performed studies proved complex fracture mechanisms of self-reinforced polymeric composites manufactured using the applied unconventional injection molding technique due to variability of mechanical properties over the thickness.

Abstract: In this work, a new type of δ-TRIP steel was designed. The fatigue limit and S-N curve were measured by the fatigue testing machine. The fatigue fracture was observed by the scanning electron microscope (SEM), and influencing factors of fatigue crack initiation were analyzed. The results show that the fatigue limit of δ-TRIP steel is 361MPa, and fatigue cracks are initiated on the surface of the sample. In the initial stage of fatigue crack initiation, the crack growth rate of the specimen is relatively slow under the action of stress. As the crack continues to expand, the magnitude and direction of the stress on the crack tip change and the stress situation becomes more complicated. The crack begins to branch and grows in different directions The metastable retained austenite in δ-TRIP steel undergoes phase transformation under the action of external force, which will absorb a lot of energy. At the same time, there are many large angle grain boundaries in the steel hindering the crack propagation. These two characteristics make the δ-TRIP steel has excellent fatigue performance.

Abstract: This paper presents the peridynamic (PD) numerical model for simulating a tensile test until total fracture for a brittle polymeric material namely polymethyl methacrylate (PMMA). U-notched and V-notched specimens were used to investigate the effect of the notches on the elongation and fracture of PMMA. The tensile elongation of PMMA exhibits nonlinearity with respect to the applied load, while the fracture occurs when the material stress has reached the ultimate tensile stress of the material. Similar elongation and fracture properties were applied on PD simulations. Two types of elongation equation are used namely brittle and ductile equations to form PD-brittle and PD-ductile models. The published experimental data of tensile fracture test on notched PMMA specimens are used as reference to validate the simulations of the PD models. The PD numerical force-extension curves have good quantitative similarity for V-notched specimen but adequate quantitative similarity for U-notched specimen. As for the quality of the fractured specimen shape, the PD simulations have good similarity for the V-notched specimen but adequate similarity for the U-notched specimen. The plot of the internal force distribution from the simulations of PD shows good qualitative similarity to the plot of the stress distribution from the published data of FEM in terms of stress concentration. From the PD results, it is observed that the PD-ductile model has better capability in producing accurate simulation of the notched specimens than the PD-brittle model.