A new project management system dynamics model and simulator

Thesis: S.M. in Engineering and Management, Massachusetts Institute of Technology, System Design and Management Program, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 56-57).Simulators generated from project management system dynamics models are exercised for training the future project managers. In today's' high dynamic, vibrant and complex markets, the models should incorporate more business dynamics and also provide more tools to the players who can flexibly steer in the project games. Along with that objective, this study brings new dynamics and modeling approach to the original multi-phase project system dynamics model of Ford and Sterman, 1998. The new dynamics include the development of new features in the market growing the customer expectation, reflection of customer expectation to project economics, supersede of project concurrencies by rushing the tasks, allowing the defects delivered to customers to be compensated by lifetime repair cost and free positioning of the phase schedules while maintaining concurrencies. A new formulation for completion putthrough, option to include final downstream defect correction and elaborate project econometrics are also included. The model is built in modules that can be utilized to increase the number of phases and/or explain the model to the trainees more easily. The project model employs two options; a zero-defect policy and allowed defect policy where the latter is newly introduced by the repair cost. The system dynamics model is tested by proposed extreme project manager traits which are implemented as table function to use one or more modules to pursue an ultimate objective within a certain logic. A construction project principally mimicking the cases provided by Parvan et al. 2015 is simulated with the manager traits. The results initiate interesting tradeoffs such as the influence of project delivery time versus repair cost, accepting new tasks versus creating more defects or rescheduling the project or positioning the workforce before the ramping up of testing and defective task correction activities. The model necessitates a deeper understanding and analyses of long-term phenomenon such as the lifetime repair cost, the financial consequences of defects and lifetime earnings of products as well as the continuous feature development in the market and its economic value. It is found that the current model proposes an enhanced tool for the training of future project managers. Keywords: System dynamics, project management, simulation, defect policy, numerical modeling.by Burak Gozluklu.S.M. in Engineering and Managemen

MODELING OF DYNAMIC DELAMINATION IN L-SHAPED COMPOSITE BRACKETS

One of the widely used geometrically complex parts in recent civil passenger aircrafts is the L-shaped composite brackets connecting ribs to skins. Due to the sharp curved geometry, interlaminar opening stresses are induced and delamination occurs under considerable mode-mixities at the corner. Dynamic phenomena during delamination initiation and propagation of L-shaped beams are investigated using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods (CZM). In ABAQUS a sequential explicit analysis followed by static (implicit) solution is used where the solution duration is considerably reduced. The thickness of the specimens is varied from 1.0 mm to 4.0 mm while the inner radius is kept same. Loading is applied parallel to one of the arms quasi-statically. Even though the crack is at the very middle of the specimen, this specific loading type yields variable traction fields and mode-mixities in the two sides of the crack in which delamination occurs under shear stress dominated loading on one crack tip and opening stress dominated loading on the other. It is observed that the delamination propagation is highly dynamic even though the loading is quasi-static. The speed of the delamination under shear dominated loading at one side can reach 800 m/s and under normal stress dominated loading is 50 m/s in dedicated thickness levels. In addition, moving elasto-dynamic radial compressive waves along the interface are observed. An important observation for design applications, a typical solution of adding more plies to the laminate might yield failure transition to a secondary crack nucleating at the arm and propagating towards the center crack

Modeling of the dynamic delamination of L-shaped unidirectional laminated composites

One of the widely used geometrically complex parts in advanced commercial aircraft is the L-shaped composite. Due to the sharp curved geometry, interlaminar opening stresses are induced and delamination occurs under considerable mode-mixities in L-shaped beams. Dynamic phenomena during delamination initiation and propagation of L-shaped beams are investigated using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods. The 2-D model consists of 24 plies of unidirectional CFRP laminate with an initial 1 mm crack at the center of the laminate at the bend. Loading is applied parallel to one of the arms quasi-statically. The loading type yields different traction fields and mode-mixities in the two sides of the crack in which delamination occurs under shear stress dominated loading on one crack tip and opening stress dominated loading on the other. The speed of the delamination under shear dominated loading at one side is 800 m/s and under normal stress dominated loading is 50 m/s. In addition radial compressive waves at the interface are observed. Finally, as the thickness is changed, a different failure mode is observed in which a secondary crack nucleates at the arm and propagates towards the center crack

Modeling of dynamic crack propagation using rate dependent interface model

Influence of crack tip separation rate-dependent cohesive zone modeling at mesoscale and macroscale behaviors are investigated for sub-Rayleigh, intersonic and sub-Rayleigh to intersonic dynamic delamination growth speeds. A simple rate-dependency factor, k, is proposed for bilinear cohesive zone model (CZM) using the interface model of Corigliano et al. (2006). The factor is defined by the ratio of dynamic to static fracture toughness which is found to be related to the square of the ratio of pure-mode interfacial strengths for the selected bilinear CZM. Experimental cases from the literature with different loading rates are parametrically studied by varying the rate-dependency factor from unity, which is rate independent, to infinity, a non-physical but theoretical condition. The first case is the three-point impact bending test that exemplifies the low-speed mode-I dynamic fracture. Next, asymmetric impact loading of polymer-composite experiment providing a mode-II dominated high speed intersonic propagation is simulated with various k. After comparing with mode-I low speed and mode-II dominated high-speed dynamic fracture experiments, an parametric analysis is carried out for a mixed-mode sub-Rayleigh to intersonic dynamic delamination in composite L-beams and compared with the experiments from Gozluklu et al. (2015). For this case, it is observed that the macroscopic aspects of fracture do not change with the rate-dependency factor. The effect of rate-dependency on crack growth kinetics is also found to be negligible for crack initiation and early stages of propagation, although the energy release rate increases. For one of the crack tips growing at intersonic speeds, the crack tip slows to sub-Rayleigh as the rate-dependency factor goes to infinity. The crack tip speeds fork > 1 provides slightly better results compared with the experiments. In conclusion, rate-dependency is observed to be necessary in accurately modeling low speed, and low-to-high speed crack propagation cases but not in high-speed intersonic crack propagation

FAILURE MODE TRANSITION DURING DELAMINATION OF THICK UNIDIRECTIONAL L-SHAPED COMPOSITE LAMINATES

Curved composite laminates such as L-beams are frequently used in wind turbine blade structures such as spars and ribs. It is widely assumed that delamination initiates at the curved region of the L-shaped laminate leading to loss of loading carrying capacity. However, as shown in this paper, under certain conditions a second failure mode in thick L-shaped laminates is observed in which a secondary crack initiates at the arm region. Delamination in L-shaped laminates is modeled using a sequential analysis with implicit analysis followed by explicit dynamic (explicit) finite element analysis in conjunction with cohesive zone methods. The 2-D model consists of 24 plies of unidirectional CFRP laminate with an initial crack at the center of curved region. Loading is applied parallel to one arm quasi-statically and the observed delamination occurs dynamically. For thin laminates and larger precracks, delamination starts from the initial crack and propagates towards the arms. For thicker L-shaped laminates and smaller precracks at the center of curved region, formation of secondary crack in the arm region is observed. Therefore the size of the initial crack as a function of thickness at the center of the curved region mainly decides the failure mode

Intersonic delamination in curved thick composite laminates under quasi-static loading

Dynamic delamination in curved composite laminates is investigated experimentally and numerically. The laminate is 12-ply graphite/epoxy woven fabric L-shaped laminate subject to quasi-static loading perpendicular to one arm. Delamination initiation and propagation are observed using high speed camera and load displacement data is recorded. The quasistatic shear loading initiates delamination at the curved region which propagates faster than the shear wave speed of the material, leading to intersonic delamination in the arms. In the numerical part, the experiments are simulated with finite element analysis and a bilinear cohesive zone model. Cohesive interface elements are used between all plies with the interface properties obtained from tests. The simulations predict a single delamination initiating at the corner under pure mode-I stress field propagating to the arms under pure mode-II stress field. The crack tip speeds transition from sub-Rayleigh to intersonic in conjunction with mode change. In addition to intersonic mode-II delamination, shear Mach waves emanating from the crack tips in the arms are observed. The simulations and experiments are found to be in good agreement at the macro-scale, in terms of load-displacement behavior and failure load, and at the meso-scale, in terms of delamination initiation location and crack propagation speeds. Finally, a mode dependent crack tip definition is proposed and observation of vibrations during delamination is presented. This paper presents the first conclusive evidence of intersonic delamination in composite laminates triggered under quasi-static loading

Effect of thickness and pre-crack length on delamination of unidirectional l-shaped laminated composites

It is widely assumed that delamination initiates at the curved region of the Lshaped laminate leading to loss of loading carrying capacity. However, as shown in this paper, under certain conditions a second failure mode in thick L-shaped laminates is observed in which a secondary crack initiates at the arm region. Delamination in L-shaped laminates is modeled using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods. The 2-D model consists of 24 plies of unidirectional CFRP laminate with an initial crack at the center of curved region. Loading is applied parallel to one arm quasi-statically and the observed delamination occurs dynamically. Moreover a sequential analysis with implicit method followed by explicit is used for the sake of efficient computational analysis. For thicker L-shaped laminates with an initial crack at the center of curved region, formation of secondary crack in the arm region is observed. After the length of the initial crack reaches a critical value, initial crack at the curved region starts propagating. Therefore the size of the initial crack at the center of the curved region mainly decides the failure mode. The failure at the initial crack is under opening stresses whereas the failure due to secondary crack is under shear stresses. In addition, the change of the failure mode from the failure at the initial crack to failure due to secondary crack result in a decrease of the maximum load that the L-shaped laminate can resist. Thus the assumption of failure at the initial crack leads to the underestimation of failure load in the load displacement curve