*3.2. Flexural Response*

Out-of-plane loading is one of the main forces that acts on the structural components in outdoor environments. This requires a thorough investigation of the flexural response of 3D printed polymer structures under the bending load. Figure 6a shows that the flexural modulus of ABS specimens with or without ASA coating was relatively unchanged during the environmental exposure. This can be attributed to the fact that modulus is a bulk material property, while the environmental effect is primarily a surface phenomenon. The embrittlement of polymers on the surface area due to UV radiation can cause a slight increase in the modulus of specimens, as illustrated in Figure 6a.

**Figure 6.** Flexural properties of uncoated and ASA-coated 3D printed ABS samples during exposure to UV radiation and moisture. (**a**) Flexural modulus; (**b**) flexural strength; (**c**) failure strain; (**d**) flexural toughness.

Figure 6b demonstrates that uncoated ABS specimens lost 5.5% of their flexural strength throughout the exposure, while no noticeable changes in the flexural strength of ASA-coated ABS specimens were observed. The failure strain of specimens over the exposure time is given in Figure 6c. The failure strain of uncoated specimens decreased significantly during exposure to UV radiation and moisture with a maximum reduction of 32.1% after 1200 h of environmental aging. However, the ductility of ASA-coated specimens showed a slight reduction throughout the exposure, with a maximum decrease of 5.1%. This can be attributed to the extensive propagation of microcracks on the surface areas of uncoated ABS specimens that can significantly impair the elongation in the outer layers which are subjected to maximum tensile stresses.

Flexural toughness was measured as the integral of the area under the stress–strain curves of samples during the three-point bending test. Figure 6d shows that the flexural toughness is the most susceptible mechanical properties of uncoated 3D printed ABS specimens to the environmental aging with a maximum reduction of 43.8% after 1200 h of exposure to UV radiation and moisture. The ASA-coated ABS specimens could significantly preserve their flexural toughness after 1200 h of environmental exposure with a retention of 95.9% of the initial flexural toughness. Figure 7 presents the average stress–strain curves of uncoated and ASA-coated ABS specimens after 0, 300, 600, and 1200 h of environmental exposure that are obtained from the flexural test. It can be observed that both uncoated and ASA-coated ABS specimens show ductile failure before and after environmental aging. The significant reduction in the flexural toughness of uncoated specimens was a result of a decrease in both flexural strength and ductility of specimens that largely reduced the area under the stress–strain curves, as observed in Figure 7.

**Figure 7.** The average stress-strain curves of uncoated and ASA-coated 3D printed ABS specimens after 0, 300, 600, and 1200 h of environmental exposure to UV radiation and moisture.
