PSI - Issue 17

Feiyang He et al. / Procedia Structural Integrity 17 (2019) 72–79 Feiyang He/ Structural Integrity Procedia 00 (2019) 000 – 000

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under the practical complex load conditions. It involves the fracture mechanics: when crack propagation occurs and how to determine the crack growth rate in the structure. On the other hand, with the development of the manufacturing process, more and more new materials are used for 3D printing and additive manufacturing. With many advantages, the polymeric materials are widely used in manufacturing and industry. Therefore, it is also important to study the fatigue performance of them under different loads. This review paper presents the latest research focusing on the crack propagation of the structures under complex dynamic loads. The reviewed publications involve the variable methodologies to study the fracture mechanics of structures under the fatigue stress regime. Besides, the paper discusses the fatigue crack growth under different loads. The paper presents the research about the thermo-mechanical fatigue and fracture of the structure with metal, composite and polymer materials. In the end, the challenges and the current research gaps are summarised. Most of the structures used in engineering application are made of metallic materials and hence it is quite significant to study the effect of complex or stochastic loads on crack propagation in metals. This section focuses the research efforts on the metal’s structural integrity under the thermo-mechanical loads. Griffith (1920) proposed the most famous energy-based crack propagation criteria which are considered to be the birth of the field of fracture mechanics. The Griffith's failure criterion only considers the brittle structures with static mechanical loads. Irwin (1948) and Orowan (1948) extended the criterion to ductile materials by adding the energy due to plastic deformation at the crack tip to Griffith's initial development. In the past few decades, many researchers estimated the crack propagation through the criterion or its modifications. Most of the research studied thermo-mechanical fatigue (TMF) through the experimental method (Kersey et al., 2013). Haddar et al. (2005) tested the thermal fatigue of AISI 304L stainless steel applied in Pressurized Water Reactors (PWRs). Based on the experimental cracking network under different temperature change conditions, the authors developed suitable numerical models with modified stress intensity fact or and generalised Paris’ law to fit the test results from their experiments. Furthermore, Platts et al. (2017) developed the experiment with the same material under thermo-mechanical loads to measure the thermo-mechanical fatigue life and crack growth rate in PWRs. In their experimental approach, the in-phase (IP) and out-of-phase (OP) loads were considered apart from variable temperature and strain range. Apart from the structure in PWRs. More components in aerospace and automotive industries commonly work under extreme temperature and mechanical loads. With the consideration of different positive or negative load ratios, temperatures, frequencies and stress intensity factor values, Merhy et al. (2013) performed several long crack TMF tests on A356-T7 cast aluminium alloy, used in automotive cylinder heads. The paper presented the impact and contribution of each parameter to the crack propagation. Ewest et al. (2019) performed both isothermal and non-isothermal TMF tests for the Haynes 230 alloy material us ed in gas turbine combustor and described a numerical simulation to calculate the effective nonlinear fatigue crack growth parameter. On the other hand, Riva et al. (2018) produced TMF crack growth tests for a cast Ni-base superalloy gas turbine blades and vanes. The research developed the linear elastic fracture mechanics model based on isothermal experimental results to predict crack propagation. Similarly, Pretty et al. (2017) completed TMF crack growth tests for polycrystalline nickel-based superalloy RR1000 used in a gas turbine engine. The main results pointed out that IP loads promoted the crack growth rate compared with the OP loads condition. A significant number of researchers such as Foletti et al. (2015), Huter et al. (2016), Fedelich et al. (2017) and Ravi (2017) applied the high cycle fatigue (HCF) and low cycle fatigue (LCF) test to study the effect of temperature to metal material and structures. Schlesinger et al. (2017) presented the experiment to investigate the time and temperature dependent growth of fatigue cracks in nickel base alloy 718 with LCF test and TMF test respectively. With the developed crack growth and lifetime model, they validated the measured crack growth curves in an experiment in the temperature range from room temperature to 650 ℃ . Similarly, Wang et al. (2017) observed the impact of temperature on the microstructure evolution and proposed a model to investigate the fatigue life and LCF behaviours of Al-Si piston alloy with the experiment. 2. Fracture Mechanics under Dynamic Loads for Metal Material