## 4 Results and Discussion
Figure 6: (**a**) Testing set-up of the tensile tests with the clamped multi-material test specimen made of PLA (white) and ABS (blue) and (**b**) corresponding force-displacement curve recorded by Instron. However, each specimen shows a minor defect at the starting and ending point of the outer shell in the fracture pattern. That can cause a general reduction of the maximum strength because they are located at the stress peak of the interface (see Figure 7). As specimen 5 shows no abnormalities in fracture pattern, the reduction of tensile strength could be the result of an error in clamping, for instance, because of an additional bending moment caused by an inclination of the specimen. The lap-shear specimens are clamped with a clamping length of 50 mm. The thickness of specimens is selected in such a way that the bending moment is minimized. However, in the experimental set-up of the lap-shear tests (see Figure 8a), a deformation of the PLA part can be seen as a result of the shrinkage of ABS caused by cooling after the printing process. Essentially, the highest deformation is located on the edges, even in the material transition. As a result of the thermal shrinkage, residual stresses are introduced. However, these stresses are probably reduced by the build table temperature that warms up the PLA part of the specimen to close too the glass transition temperature. The results of the lap-shear tests are shown in Figure 8. These experimental results show a low deviation of maximum force, except for specimen 4. The values for specimen 1, 2, 3, and 5 range between 1.03 kN and 1.07 kN. Compared to the tensile tests, the displacement is about three times higher. In addition, the increase of the force is not regular, but jagged with few decreases of the shear force, which indicates a partial separation of the two materials. Figure 8: (**a**) Testing set-up of the lap-shear tests with the clamped multi-material test specimen made of PLA (white) and ABS (blue), and (**b**) corresponding force-displacement curve recorded by Instron. Figure 7: Failure pattern of multi-material tensile specimen 4 made of PLA (white) and ABS (blue) with illustration of the imprinted infill pattern of the PLA part on the ABS surface and the defects on starting and ending points. Figure 9 illustrates the failure pattern of lap-shear specimen 3 (a) and specimen 4 (b). In contrast to the tensile tests, material residues of PLA are visible on the ABS surface. This can result from both a higher temperature in the material transition due to the heated build platform, and a force transmission parallel to the layer. In the case of shear force, the surface roughness supports the mechanical coupling and improves the maximum lap-shear strength. The decrease of force of specimen 3 is most likely initiated by a defect in the material transition due to small unwanted strands of plastic called stringing (see Figure 9b). An underextrusion, on the other hand, in the PLA part, can be the reason for premature failure of specimen 4 (Figure 9b). Besides, a decrease of force at 0.3 mm displacement is noticeable for all specimens, which is comparable to the point of failure of the tensile specimens. This decrease probably results from the front surface's bonding of the PLA part to the ABS interface because material residues of PLA are visible in the material transition of the ABS part (see Figure 9a,b). The load-displacement curve of the compression-shear tests in Figure 10 shows that the load remains at zero until the specimens reach an extension of about 0.3 mm, because they are not clamped in the clamping jaws at the beginning of testing. For specimen 1, 2, 4, and 5, the values of maximum force range from 2.34 kN to 2.75 kN, at which the displacement lies in a range of about 0.8 mm to 0.9 mm, which corresponds to the displacement of the lap-shear tests. However, specimen 3 already fails at 2.03 kN and a displacement of about 0.7 mm. The standard deviation of maximum transmittable force is about 10.2%, and is consequently higher than in the tensile and lap-shear tests.