To improve the strength of the adhesive-bonded carbon fiber reinforced polymer (CFRP) joints, atmospheric pressure plasma treatment (APPT) was used to treat a CFRP substrate surface. This study investigated the effects of nozzle distance (i.e., the distance between plasma nozzle and CFRP substrate) and nozzle speed (i.e., the moving speed of plasma nozzle relative to CFRP substrate) of APPT on the lap-shear strength of adhesive-bonded CFRP joints. Results show that the lap-shear strength of plasma-treated CFRP joints increased to a peak value and then decreased as the nozzle distance increased, and the nozzle distance associated with the peaked joint strength depends on the applied nozzle speed. The lap-shear strength of plasma-treated adhesive-bonded CFRP joints reaches up to 31.6 MPa, compared to 8.6 MPa of the as-received adhesive-bonded CFRP joints. The surface morphology of plasma-treated CFRP substrates was investigated by scanning electron microscope observation, and the mechanism associated with the improved joint strength after applying APPT was revealed through surface chemistry analysis. It is found that APPT not only effectively removed the content of Si element and –CH₃ (i.e., the main compositions of release agent) from the as-received CFRP substrate surface, but also generated many polar groups (i.e., –NH₂, –OH, –COOH, etc.), which has a positive effect on increasing the wettability and interfacial bonding strength of CFRP substrates and consequently results in a significant improvement of lap-shear strength of plasma-treated CFRP joints. In addition, the result of differential scanning calorimetry (DSC) test shows that the surface temperature of CFRP substrate should not exceed 175.3 °C during APPT. In this study, an empirical model governing temperature, nozzle distance and nozzle speed was established to guide the selection of atmospheric pressure plasma treatment process parameters in industrial manufacture.

This work describes a new procedure to remove the differences in measured forming limits obtained from Marciniak and Nakazima tests, which are the two most frequently used testing methods to obtain necking limits for forming limit diagrams (FLD) used in formability analysis of sheet metal stamping processes. The procedure compensates for the combined effects of curvature and nonlinear strain path that occur during these tests, using measurements recorded by digital image correlation (DIC) throughout the deformation history of the point on the test specimen that eventually necks. The severity of forming is then determined by presenting the critical forming conditions in a stress diagram in order to account for the effects of through-thickness pressure that influences the onset of localized necking in the Nakazima test. These stress-based forming limits are then transformed back to the familiar strain limits (or FLD), but now representing the limits under the restriction of in-plane perfectly linear stretching and plane-stress conditions. Accounting for the effects of nonlinear strain path is particularly sensitive to the detection of the actual onset of localized necking, so this work also recommends the use of realistic methods to detect the actual onset of localized necking. The method adopted in this work is based on a new method described by Min et al. [13,14], in which a change in the surface curvature is used to detect a geometric effect associated with the onset of localized necking. In addition to demonstrating that the standard Marciniak and Nakazima tests (punch diameter of 101.6mm) produce essentially identical limit curves for a 980MPa grade multi-phase high strength steel after correction for curvature, nonlinear strain path, and pressure, the method is also applied to the analysis of data from a non-standard Nakazima test with a smaller punch diameter of 50.8mm, where the severity of these processing conditions are significantly enhanced. This additional test is further proof of the validity and comprehensive coverage of the corrections for the different processing conditions involved in measurement of forming limit curves

Lightweight sheet metals are attractive for aerospace and automotive applications due to their exceptional properties, such as low density and high strength. Sheet metal forming (SMF) is a key technology to manufacturing lightweight thin-walled complex-shaped components. With the development of SMF, numerical simulation and theoretical modelling are promoted to enhance the performance of new SMF technologies. Thus, it is extraordinarily valuable to present a comprehensive review of historical development in SMF followed by state-of-the-art advanced characterization and modelling approaches for lightweight metallic materials. First, the importance of lightweight materials and their relationship with SMF followed by the historical development of SMF are reviewed. Then, the progress of advanced finite element technologies for simulating metal forming with lightweight alloys is covered. The constitutive modelling of lightweight alloys with an explanation of state-of-the-art advanced characterization to identify the constitutive parameters are presented. Then, the formability of sheet metals with major influencing factors, the techniques for measuring surface strains in SMF and the experimental and modelling approaches for determining the formability limits are clarified. Finally, the review is concluded by affording discussion of the present and future trends which may be used in SMF for lightweight metallic materials.