Fracture and fatigue of inhomogeneous materials
Research output: Thesis › Doctoral Thesis
Many technical materials have an inhomogeneous build-up, e.g. welded or glued joints, or all sorts of composites. A deeper knowledge of the fracture and fatigue behavior is necessary for a proper design of such structures. Knowledge about the influence a material inhomogeneity has on the fracture and fatigue behavior can also contribute to the development of new high-strength and simultaneously damage resistant composites. Deep-sea glass-sponges serve as model materials as they consist mainly of brittle silica but have a high fracture resistance. This behavior results from the microstructure of the sponge, consisting of glass layers connected by thin protein layers. Analytical studies showed that the change in the mechanical properties between the high-strength glass and the soft protein has a strong influence on the crack driving force. The change in the material properties causes a strong reduction of the driving force acting on the tip of a crack which has grown into a soft interlayer. This “material inhomogeneity effect” leads to crack arrest and an increase in fracture toughness. This effect occurs also if no delamination occurs at the interfaces between the layers and is, therefore, different from the classical toughening mechanisms of composites. The current experimental work is devoted to the investigation of the material inhomogeneity effect on the fracture and fatigue behavior of multilayers. Multilayers are produced that either have a difference in Young’s modulus or yield strength of the constituents. Three multilayered composites are investigated: First, a model system with a Young’s modulus inhomogeneity based on paper, where air serves as interlayer material. The other two systems are based on sheets of a high-strength aluminum alloy (Al7075-T6), which are either bonded with thin adhesive layers or thin layers of pure aluminum (Al1050). The Al7075/Adhesive composite is an example for a technically interesting composite with Young’s modulus plus yield strength inhomogeneity; the Al7075/Al1050 composite represents a system with equal Young’s modulus but a high variation in yield strength. Fracture mechanics tests are carried out to determine crack growth resistance curves in terms of the J-integral for all three systems. To determine the fracture toughness of paper or other short fiber reinforced composites, two new methods have been developed to determine physically meaningful crack growth resistance curves. All three multilayers show a strong increase in fracture resistance after crack arrest in the soft interlayers. Equations have been derived that allow an estimate of the maximum fracture toughness from the specimen geometry and the mechanical properties for the composites. Fatigue tests are carried out on Al7075/Adhesive and Al7075/Al1050 multilayers to study the material inhomogeneity effect in cyclically loaded components. For both multilayers an increase in the fatigue life is measured compared to homogeneous Al7075. The increase in fatigue life is much stronger for the multilayer with the Young’s modulus inhomogeneity than for the one with the yield strength inhomogeneity. This difference has been confirmed by numerical modeling of the fatigue tests, which shows a stronger decrease in the crack driving force for the Al7075/Adhesive- than for the Al7075/Al1050 composite. In summary it can be concluded that the introduction of thin compliant or soft interlayers may improve strongly the fracture and fatigue behavior of materials.