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.

A B S T R A C T Retardation in fatigue crack growth rate following the application of single and periodic tensile overloads was studied for 2024-T3 and 7075-T6 aluminium alloys. Tests were performed at constant stress and at constant stress intensity factor ranges, at a load ratio of R = 0.1, at a baseline K in the 10-20 MPa √ m range which corresponds to the Paris regime. Overload ratios of 1.3-1.65 were studied with overload spacing, n, varying from 20 to 10 000 cycles. 2024-T3 displayed an order of magnitude higher retardation, N d , due to single tensile overloads compared to 7075-T6. Periodic overloads induced maximum retardation when n/N d ≈ 0.5 for both alloys, the magnitude being only 15% higher for 2024-T3. a = crack length CA = constant amplitude da/dN = crack growth rate K = stress intensity factor K min , K max = minimum and maximum stress intensity factor for base cycling K ol = stress intensity factor corresponding to overload peak M(T) = middle cracked tension specimen n = number of cycles between overloads N = number of cycles N d = number of retarded (delayed) cycles N CA = number of constant amplitude cycles for the crack to grow by an amount equal to the overload-induced plastic zone size OL = overload OLR = overload ratio (K ol /K max ) POL = periodic overload R = stress or load ratio (P min /P max ) S = nominal stress, load/cross-sectional area 2r ol y = plane strain plastic zone size associated with the overload S y = yield strength β = ratio of the constant amplitude crack growth rate to the rate during periodic overloads-retardation factor K = stress intensity factor range corresponding to base cycling, K max -K min K ol = K ol -K min N d = transient portion of the retarded growth ( Fig. 1) S = stress range (= load range/cross-sectional area)

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To predict crack opening stresses, contact elements are incorporated into the elastic-plastic finite element analysis of fatigue crack closure. A code using ANSYS Parametric Design Language (APDL) is developed. In spite of the fact that the use of contact elements in modelling crack surface contact and crack closure is inherently natural, efforts to incorporate them in the finite element analysis of crack closure are not widespread. The traditional method of modelling crack closure is based on placing truss elements at the crack surface nodes. In the present research, contact elements are used to model crack surface contact. While the load is applied incrementally, crack opening stress is determined by monitoring the state of the contact elements. The results of two-dimensional plane strain finite element analyses are in good agreement with previous work reported in the literature. Instead of finding crack opening stress at every load cycle, an algorithm to find crack opening stress at predetermined load cycle intervals is developed. With the developed algorithm it was possible to analyze crack closure behaviour during a larger number of load cycles with less execution time. The algorithm that is implemented is parametrically analysed. The effect of load increment is investigated. Since crack opening stress is not evaluated at every cycle, the effect of how often opening stresses are determined is another issue that is investigated. As a result of the simulation with a relatively high number of cycles, it was possible to observe the final stabilization in the crack opening stress values that follows a decay after the initial plateau.