2.2. Rotating Fatigue Tests

The specimens for the rotating bending fatigue test were prepared in the same manner as for the tensile test, using the same printing equipment and setup. Since there is no specific standard, by authors’ knowledge, for laminated polymer and plastic fatigue test, the test specimens were prepared as depicted in Figure 2., i.e., as defined by the testing equipment manufacturer. The surface roughness was also an important parameter in fatigue life analysis. Scratched surfaces cause stress concertation, thus smooth surfaces provide a better stress distribution and reduce fatigue fracture. The surface roughness of FFF polymer specimens depends on two parameters, namely the layer height and nozzle diameter, and it can be determined either experimentally or calculated using the proposed term in [35] R_a = 112.5 × t, where R_a is the surface roughness in µm and t the layer height in mm. With the nozzle diameter of the 3D printer used for preparing samples equal to 0.2 mm with a layer height of 0.2 mm, the surface roughness of the used specimens was R_a = 22.5 µm, which is in accordance with the surface roughness that can be found in the literature for similar FFF parameters and polymers [36].

Rotating bending fatigue specimen, with the dimensions (mm).

The rotating bending fatigue machine clamps the specimen as a cantilever, as shown in Figure 3. The test machine is TQ SM1090 (TecQuipment Ltd., Nottingham, UK) with versatile data acquisition system, which uses an adjustable dead weight for applying vertical, downward load on the specimen, using a self-aligning bearing inside a gimbal. The dead weight can be adjusted on the toothed leverage, which makes sure that the tests, i.e., the load on the specimen, are repeatable. The load on its free end creates tension on the upper half of the specimen, and likewise compression on the lower half. The test machine repeatedly stresses a test specimen for a known number of cycles, with alternate compressive and tensile stress on any given part along the unsupported length of the test specimen. The point inside the cross section of the specimen, except the point on the neutral axis, moves from zero stress to the maximum tensile stress and back through the zero stress and maximum compressive stress. This span represents one cycle with two reversals, where the reversal represents the path from the maximal positive to the maximal negative stress and vice versa. Thus, the applied stress on specimen vs. cycle is described with sinusoidal function.

Rotating bending fatigue machine setup.

The stress at the neck of the specimen can easily be calculated using the standard bending Equation (1), where F is the load on the free end of the specimen, and l is the distance from the neck to the load and d is the diameter at the specimen neck: