Browsing by Subject "crack initiation"

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Access status: Open Access ,
Observations on fatigue crack growth and fatigue life of riveted lap joints
(2012) Skorupa, Andrzej; Skorupa, Małgorzata; Machniewicz, Tomasz; Korbel, Adam
Effects of variables related to design and production of riveted lap joints representative of longitudinal sheet connections for a pressurized transport aircraft fuselage were experimentally investigated. The specimens from an aircraft Al alloy D16 Alclad sheets of three different thicknesses (1.9, 1.2 and 0.8 mm) were assembled under load control using round head rivets and rivets with the compensator from a P24 Al alloy. For the joints from 1.9 mm thick sheets fatigue tests indicated a dependency of the crack initiation site and crack path on the squeeze force level and on the rivet type. At the same time, increasing the squeeze force led to improved fatigue properties of the joints, specimens assembled using the rivets with the compensator showing fatigue lives consistently longer than joints with the round head rivets. All observed trends have been explained based on hole expansion and load transfer measurements. For thin sheets connected using the round head rivets, local deformations and indentations under the driven rivet head promoted crack initiation and failure in the adjacent sheet. Fatigue test results indicated that the detrimental effect of this type imperfections could outweigh the benefits associated with a decrease in secondary bending due to thinning the sheets. The rivets with the compensator were observed to cause significant local imperfections beneath the manufactured head, which adversely affected the joint fatigue performance.
Access status: Open Access ,
Thermomechanical modeling on the crack initiation of NiTi shape memory alloy
(Wydawnictwa AGH, 2022) Jiang, Dongjie; Xiao, Yao
The fracture of shape memory alloys is distinct from that of conventional metals, owing to the coexistence and interaction of multiple special features such as martensitic transformation, dislocation-induced plasticity, thermomechanical coupling and others. In this paper, the impact of thermomechanical behavior upon the crack initiation of a NiTi shape memory alloy under Mode I loading is investigated numerically and verified experimentally. A constitutive model incorporating phase transformation, plasticity and thermomechanical coupling is established. Via backward Euler integration and finite-element implementation, the longitudinal strain, martensite volume fraction and temperature field in the vicinity of the crack tip are furnished. The effects of grain size and loading rate on J-integral are revealed. The grain size dependence of crack initiation is non-monotonic. For the samples with grain sizes of 1500 nm, 18 nm and 10 nm, the shielding effect takes place in front of the crack. Additionally, the anti-shielding effect is detected for samples with grain sizes of 80 nm and 42 nm. The parametric study shows that loading rate imposes limited influence on J-integral, which is attributed to a small scale transformation. The decrement of yield stress and the increment of transformation hardening modulus can alleviate the anti-shielding effect and arouse the shielding effect upon crack initiation. The presented results shed light on the design and fabrication of high toughness phase transformable materials.