67 7.5 Microscopy of Welds To optically assess the fractures in the lap shear samples, scanning electron microscopy of the fracture was conducted with a Zeiss Supra 40 scanning electron microscope. This was done to compare with the results of Suzuki et al. [24], who investigated the fracture surface of a composite material subjected to bending stresses. A stitched image of the fracture surface of sample LS-1 at ×60 magnification can be seen in Fig. 7.10. The highlighted areas indicated in Fig. 7.10 show areas that have been studied more closely. The top area is close to where the highest tensile stresses were due to the bending and stress concentration from the results of the FEA. The lower area is near an area with the lowest stresses due to the bending stress distribution. In Fig. 7.10 (top), the dark spots (circled) are voids left after fiber pullout, which is expected in high tensile stress regions [25]. In Fig. 7.10 (bottom), a region near lower stresses is shown, where fewer fiber pullout areas are present, suggesting that this region failed due to crack propagation through this region of the composite [25]. The scanning electron microscopy of the fracture surface supports the results from the FEA, showing a region of high tensile stresses close to the weld due to the bending stresses and stress concentration. These stresses were the driving force of the failure of the lap joints [18]. 7.6 Conclusions The results of this work have successfully shown the use of ultrasonic spot welding to join chopped fiber thermoplastic composite materials. The results from the experiments showed that welding composite materials are similar to welding the thermoplastic material of the composite matrix, and welding parameters used to join thermoplastics may be used to join composites with the same thermoplastic matrix. Lap joints were manufactured by ultrasonically spot welding 6.35 mm thick Tecanat GF20 and characterized by lap shear and impact testing to determine their mechanical strength. A high repeatability suggests that the ultrasonic spot welding process may be well suited for high volume production. The lap shear samples all failed in a consistent manner, due to bending stress and a stress concentration at the interface between the weld and the base material. This theory was further confirmed by conducting FEA and scanning electron microscopy of the fracture surfaces. The stresses in the experimentally tested lap joints were calculated, and agreed with the results from the finite element model. Fig. 7.8 Isometric view of lap joint (left), isometric view of half of the joint (center), and side view (right) of the joint showing plot of normal stress in the z direction. Dark colors indicate compressive stress, light colors indicate tensile stress, red colored areas are subjected to the highest tensile stress, and the thickness of material is 6.35 mm Fig. 7.9 Stresses in z direction plotted on the y-z cross section. Arrows in the negative z-direction are tensile stresses, arrows in the positive z-direction are compressive stresses, and the thickness of material is 6.35 mm 7 Impact and Lap Shear Properties of Ultrasonically Spot Welded Composite Lap Joints
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