Fazil O. Sonmez

Modeling of Heat Transfer and Crystallization in Thermoplastic Composite Tape Placement Process

The crystallization behavior of thermoplastic composites during the tape placement process has been analyzed. A heat transfer analysis has been carried out using a finite element method. The heat transfer analysis is coupled with a crystallization kinetics model to predict the resulting morphology. In order to determine the heat intensity that could be applied to the laminate, a degradation kinetics model is added. The degradation kinetics model thus provides an upper bound for the heat input. The models relate the process parameters (e.g., roller velocity, roller pressure, heat input) to temperature and crystallinity distributions and degradation weight loss. These models are incorporated into a computer code to generate numerical results. By using this code, process conditions which are favorable to consolidation and also likely to produce optimum crystallinity levels are identified.

Analysis of the On-Line Consolidation Process in Thermoplastic Composite Tape Placement:

The main objective of the present study is to establish the relationships between the process variables and the quality of thermoplastic composite laminates fabricated by tape placement. The quality parameters considered in the process modeling are interlaminar bond strength, weight loss through thermal degradation, and crystallinity. Stress, heat transfer, crystallization, degradation, and bonding models are developed. These models relate the process parameters (e.g., roller velocity, roller pressure, heat input) to temperature, stress, and crystallinity distributions, degradation weight loss, and degree of bonding within the composite. These relationships are used to develop a process window to ensure product quality. The process parameters are then optimized to reduce the lay-up time.

Analysis of Process-Induced Residual Stresses in Tape Placement

The tape placement for thermoplastic composites involves heating, melting, and cooling steps just as do the other manufacturing processes. Consequently, development of residual stresses is unavoidable due to disparate thermal characteristics of matrix and fiber materials and also due to nonuniform cooling rates. From the product quality standpoint, such as interlaminar strength, dimensional accuracy etc., these stresses should be kept within allowable limits. In this study, a thermoviscoelastic finite element model was developed to predict residual stresses induced during the placement of thermoplastic composite tapes. The process, being continuous, was considered to be under a quasi steady state where process conditions do not change with respect to the moving roller. Relaxation of the residual stresses in previously laid layers was also allowed for. Results were obtained for both unidirectional and cross-ply laminates. They show the residual stress distributions through the thickness for a number of chosen sets of process parameters (e.g., roller velocity and heat input). Therefore, residual stresses in a laminate can be controlled by modifying these process parameters.

Thermomechanical analysis of the laminated object manufacturing (LOM) process

LOM is one of the techniques for rapid prototyping, which is used to build three‐dimensional solid objects. In this paper, a numerical analysis of the thermomechanical behavior of a laminate during the LOM process is presented. Models were developed to describe heat transfer and deformation phenomena. These models are incorporated into computer codes to generate numerical results. The numerical results show how the process parameters (e.g. roller temperature, velocity, indentation) are related to temperature and stress distributions within the laminate. By using this code, process conditions which are favorable to bonding are identified.

Process Modeling and Optimization of Resistance Welding for Thermoplastic Composites

Resistance welding is a suitable technique for joining thermoplastic composites. Like other fusion bonding processes, it involves heating, melting and cooling steps. Productivity depends on the time that passes during these steps. This is the first study that tries to increase the productivity of the process in a systematic way. The objective of the present study is to determine the optimum set of process parameters to minimize the processing time. In order to ensure that the resulting joint satisfied the requirements of quality, the relationship between process variables and quality of the welded joint was established through process modeling. First, a one-dimensional transient heat transfer analysis was carried out using a finite difference method to find the temperature profiles across the thickness of the welding stack. Then, the heat transfer analysis was coupled with a degradation kinetics model in order to determine whether the resulting part has undergone excessive thermal degradation, or not. Finally, a bonding model adapted to the resistance welding process was used to determine the degree of bonding between the laminates. The process model was eventually combined with an optimization algorithm to minimize the processing time without violating the quality requirements. The algorithm was based on a search method called Nelder–Mead. Finally, optimum process parameters were obtained for different thicknesses of APC-2 laminates.

Measurement and Assessment of Fatigue Life of Spot-Weld Joints

Spot-weld joints are commonly used to fasten together metal sheets. Because fatigue fracture is the most critical failure mode for these joints under fluctuating loads, understanding their fatigue failure behavior and assessment of their fatigue lives are crucial from the viewpoint of failure prevention in design. In this study, a series of experiments was conducted to study the fatigue failure of spot-welded modified tensile-shear specimens made of a low carbon steel. Two different types of resistance spot welding were investigated (manual and automated). Tests were repeated under different load ranges, and the corresponding fatigue lives were determined. The specimens were also examined under an optical microscope. In the numerical part of this study, a finite element analysis was carried out using commercial software, ANSYS , to determine the stress and strain states within the specimens. The material nonlinearity, local plastic deformations around the welds during loading, and the residual stresses and strains developed after unloading as a result of plastic deformations were taken into account. Based on the predicted stress and strain states, fatigue analyses were performed using several models for life assessment. Then, the measured and predicted fatigue lives were compared, and the suitability of the models was discussed. Among the strain-based models, Coffin–Manson and Morrow’s means stress models yielded the best predictions.

Thermoviscoelastic Analysis of the Thermoplastic Composite Tape Placement Process

The thermomechanical behavior of a thermoplastic composite laminate during the tape placement process has been studied. Stresses induced by the compaction roller are predicted using a finite element method. These stresses control the compaction and bonding behavior of the laminate. In the analysis, the tape placement process is considered as a quasi-steady state rolling contact problem. The effect of temperature on creep compliance is included by treating the composite as a thermorheologicaliy simple material. Thermal expansion coefficients are allowed to be temperature dependent. The effect of friction at the contact surface is also taken into account. The numerical results show how the process parameters (e.g., roller velocity, roller pressure, heat input) are related to temperature and stress distributions within the composite. These relationships can be used to predict the bonding behavior of the laminate.

Optimal post-manufacturing cooling paths for thermoplastic composites

The objective of this study is to determine the optimal cooling scheme to minimise the processing time during the cooling stage in press moulding. The optimisation is subject to the quality constraints concerning maximum allowable residual stress and recommended levels of crystallinity. Since they depend on temperature state, a one-dimensional transient heat transfer analysis was carried out using a finite difference method so as to obtain the transient temperature profile through the thickness of the laminate. The heat transfer analysis was then coupled with a crystallisation-kinetics model. Using a plane-strain viscoelastic model, residual stress distribution was obtained corresponding to a given cooling scheme. These models were then integrated into an optimisation algorithm called sequential simplex, to minimise the cooling time without compromising quality requirements. Significant reduction in processing time was obtained for APC-2 laminates.

Optimal shape design of shoulder fillets for flat and round bars under various loadings

Abstract Fillets are usually the most critical regions in mechanical parts especially under fatigue loading, considering that an increase in the maximum stress level considerably shortens the fatigue life of a part. The aim of this study is to find the best shape for a fillet in a shouldered shaft or plate so that the maximum equivalent stress has the lowest possible value. Optimization is achieved using a stochastic global search algorithm called the direct search simulated annealing. The boundary is defined using spline curves passing through a number of key points. The method is also applicable to shape optimization problems in which geometric constraints are imposed and, for this reason, tangential stress is not uniform along the optimal fillet boundary. Optimal shapes are obtained for flat and round bars subject to axial, bending, torsional, or combined loads. The results show that stress concentration factors close to one can be achieved even for bars with significant variations in cross-section. Besides, the region occupied by the optimum fillets is much smaller in comparison to circular or elliptical ones.

Detailed and simplified models of bolted joints under impact loading

Mechanical components are commonly fastened together using bolts. In many applications, they are subjected to impact loads during their service life. Their response and failure behaviour under these conditions needs to be known for their safe use. The objective of this study was to develop computationally efficient and accurate finite element models for bolted joints under impact loading. First, a three-dimensional detailed finite element model for a bolted joint was developed using solid elements. With this full modelling, the aim was to simulate the physics of the impact event as accurately as possible without any concern about computational cost. In the design of mechanical structures containing numerous fastening elements, use of detailed models is not practicable, because the computational cost of the analysis dramatically increases with the increased number of complex interacting parts. Instead, simplified models that only account for dominating effects should be utilized so that the analysis time can be significantly reduced without compromising the level of accuracy. Accordingly, a number of simplified finite element bolt models were developed and then compared with the full model with regard to the solution accuracy and computational cost to select the most representative and cost-effective simplified model.