In this paper, an experimental investigation of fatigue crack growth behavior and fatigue life of Coach Peel (CP) specimens is reported. In this investigation, the CP specimens with 1 and 1.5 mm thickness and from mild steel are used. Specimens are tested by a 25 kN capacity INSTRON 8802 servohydraulic fatigue test machine under constant amplitude but various level loadings. During loading, the crack length and cycle numbers are measured accurately by a 100X light microscope. Initiation and propagation of spot weld specimens are measured by drawing vertical displacement of the specimen's vs cycle's curves. Results show that initiation and propagation of cracks often occur in the plate with less thickness. Also in low level loadings, cracks propagate longtitudinaly, but in high level loadings, usually nugget pull out and joints, fail catastrophically. The crack lengths during propagation are nearly unique in both plates with the same thickness.

The axial force measurement plays an important role in tool designing and identification of its restrictions. Also, it is vital in design of machine mechanism and optimization of welding process parameters. In this study, a friction stir welded butt joint on AA7075-T651 aluminum alloy plate is investigated. With change of some parameters, factors including axial force, mechanical properties, microhardness and weld morphology, are studied. For measuring the axial force, load cell was utilized and a servohydraulic machine was applied to evaluate the mechanical properties. The results show that by increasing the welding speed and in the same rotational rate, the axial force and microhardness increase but the weld appearance quality decreases. However, by increasing the rotational rate in the same welding speed, there is not any certain relationship among the axial force, hardness and weld morphology. Also, by changing the tool tilt angle from zero to 2.5 degrees, the weld morphological feature is enhanced. The results illustrate that the proper change in welding speed and tool rotational rate have great effects on optimizing the welding friction stir process.

Nonlinear models of the flow control valves relate the control flows through the valve ports to the valve parameters. An integrated model covers thoroughly the critical-lapped, over-lapped and under-lapped valves. The non-equality of spool lands and incorrect null position of spools can also be simulated using this model. Moreover, the simplified model can be used base on some assumptions for ease of analysis. In this paper, an accuracy analysis is performed to compare a fully nonlinear valve model to the conventional simplified one. According to the second order linear model of a servohydraulic system, a non-dimensional analysis for the fully nonlinear model shows that the accuracy of the model is independent of the specific categories of its parameters. In fact, the model error for a certain amount of hydraulic damping coefficient does not depend on the model parameters, such that a combination of these parameters leads to a constant damping coefficient. Therefore, the accuracy of the model can already be determined using simulation. This point provides a high flexibility to the hydraulic control system designer to put up the possibility of using simplified models without any need to determine the model errors. Because the hydraulic systems are barely driven beyond the scope of the desired frequency, it can be concluded that for some applications, the simplified model of the valves can accurately express the valve dynamic.