K13
Crack Growth

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15:30
conference time (CEST, Berlin)
Fatigue Crack Growth of a Bolt Loaded in Tension using The Boundary Element Method
27/10/2021 15:30 conference time (CEST, Berlin)
Room: K
S. Haugland, R. Grant, K.E. Frøysa (Western Norway University of Applied Sciences, NOR); R. Nordrik (Bergen Engines, NOR)
S. Haugland, R. Grant, K.E. Frøysa (Western Norway University of Applied Sciences, NOR); R. Nordrik (Bergen Engines, NOR)
Threaded fasteners are some of the most important machine elements. They often take on critical tasks where they are subjected to cyclical loading and are thus susceptible to fatigue damage. Typically fatigue cracks initiate in the thread root on the first loaded thread and grows in a thumbnail shape until fatigue fracture occurs. Both the Wöhlerian approach and the Fracture Mechanics approach may be used to study this phenomenon. Using the Fracture Mechanics approach requires Stress Intensity Factor (SIF) solutions, and these have been available in the literature for a long time. These solutions can be found from theoretical, experimental, and numerical methods. However, much of it is limited to opening mode (mode I) cracking only. There are several studies using numerical methods to obtain SIF for a fixed crack shape and size, some even including mode II and III; but numerical studies featuring growing cracks in threaded fasteners are sparse. Using numerical tools to model fatigue crack growth offers a highly useful capability, as experimental procedures can be very time consuming, allowing testing and design processes to be greatly accelerated with the possibility of crack growth modelling added to the toolbox. This work investigates crack growth in a threaded fastener using numerical methods. The software used is BEASY, which utilises the Boundary Element Method (BEM) and provide analysis tools which specialise in fatigue analysis. In the analyses the BEM models using surface mesh rather than 3D elements to model the continuum, which greatly reduces the number of elements needed for the model and lends itself well to coping with geometrical irregularities which may be present in a fatigue crack. The bolt is loaded in axial tension. Cracks are inserted into the root of the first loaded thread and cyclic loading is applied to induce growth. The growth is dominated by the opening mode, but mode II and mode III are also present. It is of interest to study how the crack develops; in particular its shape, critical size, and the influence of mode II and III over the course of the crack growth. The results are validated using data from literature.
Threaded fastener, BEM, SIF, Fatigue, Crack growth
15:50
conference time (CEST, Berlin)
Computational Study of Temperature-dependent Fiber/Matrix Interfacial Debonding in CFRPs
27/10/2021 15:50 conference time (CEST, Berlin)
Room: K
G. Zaverdinos, D. Dragatogiannis, C. Charitidis, NTUA, Greece (Athens National Technical University, GRC)
G. Zaverdinos, D. Dragatogiannis, C. Charitidis, NTUA, Greece (Athens National Technical University, GRC)
The parameters concerning the interface debonding between fiber and matrix have been of great scientific interest in the field of composite materials. The study of this phenomenon has applications on both the determination of the failure conditions of a composite in microscale, as well as offering the possibility of exploring the reusability of fiber reinforced polymers through the reclamation of the individual materials that consist the composite. The focus of the present study is the effect of combined local heating and axial, pull-out load application, on the fiber/matrix debonding. As a result of these conditions, the adhesion of the fiber onto the thermoplastic matrix is degraded, resulting in controllable fiber/matrix separation which facilitates the above mentioned recycling purposes. The separation mechanism is studied numerically at different temperatures, utilizing the Virtual Crack Closure Technique (VCCT) to derive the energy release rates (ERR) that characterize the crack propagation, as well as the Benzeggagh – Kennane fracture criterion to define the critical ERR values depending on the thermomechanical properties of the materials involved. Those properties’ values are exploited to discuss optimal material design in an effort to enhance interface debonding performance. The numerical model is solved with finite element method in a three-dimensional geometry concept, as a means to investigate the effect of the presence of neighboring fibers, in the separation mechanism. Crack initiation is approached with a pre-meshed crack, in accordance with a suggested failure criterion, after relevant failure criteria evaluation for this specific application. Crack propagation is studied with the aid of interface cohesive elements, whose behavior ultimately determines the debonding. The simulation results contribute in the evaluation of physical parameters like initial crack length, strain rate, loading conditions as well as model parameters like the order of finite elements used and the appropriateness of the debonding theory applied, for the determination of useful post-processed quantities.
FEM, FEA, interface, CFRP, thermoplastic, carbon fiber, VCCT, Benzeggagh Kennane, ERR, CZM
16:10
conference time (CEST, Berlin)
Implementation and Validation of a Finite Element Method to Model Interlaminar Fatigue Damage for Continuous Composite Material
27/10/2021 16:10 conference time (CEST, Berlin)
Room: K
C. Lequesne (Siemens Digital Industries Software, BEL); H. Xiong (Samtech SA, a Siemens Company, BEL); L. Carreras, E. Lindgaard, B. L. V. Bak (2. The CraCS Research group, Aalborg University, DNK)
C. Lequesne (Siemens Digital Industries Software, BEL); H. Xiong (Samtech SA, a Siemens Company, BEL); L. Carreras, E. Lindgaard, B. L. V. Bak (2. The CraCS Research group, Aalborg University, DNK)
Composite materials have been used for many years in industries such as the wind power, automotive and aeronautics to design lightweight structures with high mechanical performances, i.e. stiffness and strength. However, the microstructural complexity of the composite materials challenges the estimation of the load carrying capabilities of composite structures. Failure in laminated composites is usually caused by inter-laminar fractures, such as delamination or adhesive joint debonding, promoted by or coexisting with intra-laminar damage mechanisms, like matrix cracking and fibre failure. This paper presents a method for modelling fatigue-driven delamination using finite elements and a cohesive zone model approach. The fatigue damage rate is linked to a Paris’ law-like expression for the crack growth rate evaluated using the energy release rate and constant load ratio. A J-integral formulation that defines an integration path across the length of the cohesive zone allows accurate computation of the energy release rate. Integration paths are tracked following the growth driving direction at each integration point, which is computed as the gradient of the ratio of the total performed work to the interfacial fracture toughness. Only the maximum loads of the fatigue cycles are simulated and a cycle jump algorithm is applied to avoid prohibitive CPU time together the adaptive cycle integration algorithm in order to account for static damage during the cycle jump. The jump criterion is based on a target crack growth increment chosen by the user. This method was implemented in the solver Simcenter Samcef. To validate the implementation, a batch of two double cantilever beam (DCB) specimens made of a non-crimp fabric laminate were tested applying a constant mode I bending moment on each arm. The specimens included a partial reinforcement at the mid-width to promote a curved delamination front during propagation. The experimental crack front shape evolution was well reproduced by the simulation method. The crack front propagation was slightly underestimated compared to the experimental results. However, the difference between experimental and simulation results at the end of the analysis is comparable to the variation in crack propagation among the different demonstrator specimens experimentally tested.
Composite, fatigue, interlaminar, FEM, Cohesive zone model
16:30
conference time (CEST, Berlin)
Fracture Mechanics Analysis and Fitness-for-Service of Cracking in the High Pressure Stage 1 Blade T-Hooks of a Steam Turbine Rotor
27/10/2021 16:30 conference time (CEST, Berlin)
Room: K
E. Jensen, G. Thorwald (Quest Integrity, LLC, USA)
E. Jensen, G. Thorwald (Quest Integrity, LLC, USA)
Following a bearing failure in a steam turbine, internal inspection of the unit post-failure additionally uncovered cracking in nearly all of the 76 High Pressure (HP) Stage 1 blade T-hooks (at the base of each blade) used to retain the blades in the rotor. Considering the similar design, manufacture, installation, and operation of a sister turbine, it was assumed that similar cracking was likely present in that unit’s blades. A fracture mechanics assessment using procedures from the fitness-for-service standard API 579-1/ASME FFS-1 was performed to analyze the stability of the assumed cracking and the susceptibility to fatigue crack growth caused by continued operation. The second unit’s rotor was to be replaced in 30 months during which the blades would also be replaced. Continued operation of the second turbine using the existing blades until the new rotor was ready represented a significant financial gain by reducing the unit’s outage time minimizing the loss of power generation as well as saving on the cost of replacement blades. A cyclic symmetric finite element model of the full rotor and HP1 blades was used to model the two primary load cycles of interest: cold start through shutdown and a daily load swing. Mechanical and thermal loads provided by stream temperature and pressure data and rotational forces due to the spinning rotor were applied to coupled heat transfer and static stress analyses. Although fully transient processes, static analyses at specific, relevant “snapshots” over the transients were performed to analyze the turbine’s behavior over the two load cycles. Using custom 3-D crack meshes, a 9 mm deep crack was inserted into the trailing edge of the HP1 Blade’s T-hook. From the finite element analysis results, stress intensity values along the crack front were compared against material fatigue data to determine if the particular load cycle was susceptible to fatigue crack growth. It was determined that the daily load swing was unlikely to cause crack growth as the change in stress intensity over the cycle was less than the fatigue crack growth threshold, but a small amount of growth was predicted to occur during each cold start/shutdown cycle. However, due to the relative infrequency of the cold start/shutdown load cycle (typical operation saw the unit shutdown on average three times per year) and size of the predicted growth, it was determined that continued operation blades was acceptable for at least 30 months assuming a maximum of 10 cold start/shutdown cycles per year. The analysis results supported continued operation of the turbine until the replacement rotor was available and helped provide operating guidelines to minimize potential crack growth in the turbine blades.
fracture mechanics, fitness-for-service, steam turbine, fatigue crack growth, heat transfer, stress analysis, finite element analysis
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