Course Description: Fundamentals of fracture mechanics and failure analysis, including Griffith theory, establishment of stress field at crack tip, stress intensity factor, fracture toughness, J-integral, R-curve and their measurements, fatigue and fatigue crack propagation, da/dn vs ΔK relationship, micro-mechanisms of fracture process and failure analysis as well as case studies; computational modeling of fracture.
Course Description: In this course, graduate students are taught the theory and application of computer modeling of materials at the atomic scale. Specific topics include: classical and modern first principles atomistic modeling approaches, statistical mechanics, molecular statics and dynamics, density functional theory and kinetic Monte Carlo sampling. The approximations, advantages and limitations involved with each approach are highlighted. A significant focus of the course is to provide a hands-on training in these computational techniques. To achieve this, a number of practical case studies from advanced materials and nanotechnology are presented in detail. The course includes an individual or group project. Another important focus is the computer modeling of material failure at the atomic scale. Towards the later portion of the course, some advanced topics, such as accelerated molecular dynamics, multiscale modeling, and coarse-graining approaches, are covered.
The average tensile strength, modulus and elongation of chemically retted single bamboo fibers were obtained as 1.77 GPa, 23.56 GPa and 2.89 %, respectively. In the comparison of bamboo fiber bundle and bamboo single fiber, the tensile strength was decreased by 65.5 %, the modulus by 12.3 %, and the elongation by 9.7 %. For the mechanically retted single bamboo fibers, an averaged tensile strength of 0.93 GPa, modulus of 34.6 GPa and elongation of 4.3 % were obtained as shown in Fig. 11. For both mechanically retted bamboo fiber bundles and single bamboo fibers, the tensile strength, modulus and elongation of bamboo fiber bundles were obtained as 68.8, 52.3 and 60.9 %, respectively, which were lower than that of single bamboo fibers. Most failures for the tested specimens happened in the parenchyma between the single fibers. Figure 12 shows the failure modes of the specimens for the bamboo fiber bundles. Most failures were that fibers being pulled out. Shao et al. studied the behaviors of Mode I (crack opening mode) interlaminar fracture parallel to grain of moso bamboo, and observed that the crack propagation developed along the longitudinal interface between the fibers and ground tissue, indicating that the longitudinal interfacial strength was weak among bamboo cells .
As shown in Fig. 13, some bamboo fiber bundles contain parenchyma with lower tensile strength in comparison with the single bamboo fibers . As the existence of weak interface among the parenchyma, the crack propagation easily happened resulting in an ultimate failure . The isolation method also affected the property reduction rate from single fibers to fiber bundles. The chemical treatment had less property reduction compared to the mechanical one.
As shown in Fig. 11, the tensile strength of bamboo strips was 67.7 % of that of the single fibers when retted mechanically, whereas the modulus was 42.7 %. Bamboo is made up of vascular bundles with parenchyma in between, where the interface between vascular bundles and parenchyma is weak . During the tensile testing, the crack propagation in the specimen first happened in the parenchyma area. As the stress continues to increase, slips happened at the interface between bamboo fiber bundles and parenchyma, then cracks appeared at the other weak interface, such as interface between bamboo fibers, interface between parenchymas, and in the end, the bamboo fiber bundles were pulled out slowly from parenchyma . Compared with single bamboo fibers, weak interface area presented in bamboo and bamboo fiber bundles that lead to a huge reduction in the mechanical properties.
Concept of elastic stress intensity factor, Griffith energy balance, determination of the elastic field at a sharp crack tip via eigenfunction expansion methods, J integrals analysis, experimental determination of fracture toughness, fatigue crack growth, elastic-plastic crack tip fields. Emphasis on modern numerical methods for determination of stress intensity factors, critical crack sizes and fatigue crack propagation rate predictions.
In order to determine the influence of ductility on the fatigue crack growth rate of aluminum alloys, fatigue tests were carried out on central notched specimens of 2024-T3 and 2024-T8 sheet material. The 2024-T8 material was obtained by an additional heat treatment applied on 2024-T3 (18 hours at 192 C), which increased the static yield strength from 43.6 to 48.9 kgf/sq mm. A change in the ultimate strength was not observed. Fatigue tests were carried out on both materials in humid air and in high vacuum. According to a new crack propagation model, crack extension is supported to be caused by a slip-related process and debonding triggered by the environment. This model predicts an effect of the ductility on the crack growth rate which should be smaller in vacuum than in humid air; however, this was not confirmed. In humid air the crack-growth rate in 2024-T8 was about 2 times faster than in 2024-T3, while in vacuum the ratio was about 2.5. Crack closure measurements gave no indications that crack closure played a significant role in both materials. Some speculative explanations are briefly discussed.
The dynamic behavior of three commonly used airplane fuselage materials is investigated, namely of Al2024-T3, Glare-3 and CFRP. Dynamic tensile tests using a servo-hydraulic and a light weight shock testing machine (LSM) have been performed. The results showed no strain rate effect on Al2024-T3 and an increase in the failure strain and failure strength of Glare-3, but no stiffening. The LSM results on CFRP were inconclusive. Two types of fracture tests were carried out to determine the dynamic crack propagation behavior of these materials, using prestressed plates and pressurized barrels, both with the help of explosives. The prestressed plates proved to be not suitable, whereas the barrel tests were quite reliable, allowing to measure the crack speeds. The tougher, more ductile materials, Al2024-T3 and Glare-3, showed lower crack speeds than CFRP, which failed in a brittle manner.
Comlekci, Tugrul and Marin Perez, Jonatan and Milne, Lewis and Gorash, Yevgen and MacKenzie, Donald (2023)Structural steel crack propagation experimental and numerical analysis. Procedia Structural Integrity, 42 (2022). pp. 694-701. ISSN 2452-3216 2b1af7f3a8