Abstract
The functional gradient is a concept often occurring in nature. This concept can be implemented in the design and fabrication of advanced materials with specific functionalities and properties. Functionally graded materials (FGMs) can effectively eliminate the interface problems in extremely hard–soft connections, and, thus, have numerous and diverse applications in high-tech industries, such as those in biomedical and aerospace fields. Here, using voxel-based multi-material additive manufacturing (AM, = 3D printing) techniques, which works on the basis of material jetting, we studied the fracture behavior of functionally graded soft–hard composites with a pre-existing crack colinear with the gradient direction. We designed, additively manufactured, and mechanically tested the two main types of functionally graded composites, namely, composites with step-wise and continuous gradients. In addition, we changed the length of the transition zone between the hard and soft materials such that it covered 5%, 25%, 50%, or 100% of the width (W) of the specimens. The results showed that except for the fracture strain, the fracture properties of the graded specimens decreased as the length of the transition zone increased. Additionally, it was found that specimens with abrupt hard–soft transitions have significantly better fracture properties than those with continuous gradients. Among the composites with gradients, those with step-wise gradients showed a slightly better fracture resistance compared to those with continuous gradients. In contrast, FGMs with continuous gradients showed higher values of elastic stiffness and fracture energy, which makes each gradient function suitable for different loading scenarios. Moreover, regardless of the gradient function used in the design of the specimens, decreasing the length of the transition zone from 100%W to 5%W increased the fracture resistance of FGMs. We discuss the important underlying fracture mechanisms using data collected from digital image correlation (DIC), digital image microscopy, and scanning electron microscopy (SEM), which were used to analyze the fracture surface.
Highlights
IntroductionGraded materials (FGMs) are (multi-phase) composites with spatially varying material properties designed to satisfy specific requirements (e.g., mechanical, thermal, or electrical [1,2,3]).These inhomogeneous materials have several applications in various disciplines, such as aerospaceMaterials 2019, 12, 2735; doi:10.3390/ma12172735 www.mdpi.com/journal/materialsMaterials 2019, 12, 2735 and biomedical engineering [4]
Graded materials (FGMs) are composites with spatially varying material properties designed to satisfy specific requirements.These inhomogeneous materials have several applications in various disciplines, such as aerospaceMaterials 2019, 12, 2735; doi:10.3390/ma12172735 www.mdpi.com/journal/materialsMaterials 2019, 12, 2735 and biomedical engineering [4]
In the second stage, the level of force remained almost constant until the end of the test, where the specimen broke apart (Figure S3b of the supplementary document)
Summary
Graded materials (FGMs) are (multi-phase) composites with spatially varying material properties designed to satisfy specific requirements (e.g., mechanical, thermal, or electrical [1,2,3]).These inhomogeneous materials have several applications in various disciplines, such as aerospaceMaterials 2019, 12, 2735; doi:10.3390/ma12172735 www.mdpi.com/journal/materialsMaterials 2019, 12, 2735 and biomedical engineering [4]. Examples are thermal barrier coatings [5], piezoelectric [6] and thermoelectric devices [7], non-uniform pressurized cylinders [8], (dental) implants [9], biocompatible graded hydrogels [10], soft robotics [11], and tissue engineering scaffolds [12] In addition to these engineering applications, functional gradients frequently occur in nature, including hard tissues such as bone and tooth [13]. Such functional gradients in the chemical composition of natural materials mainly appear through the selective occurrence of two elementary constituents (i.e., hard (inorganic) and soft (organic) building blocks [14,15]). The concept of functional gradients is, one of the design motifs that has its roots in nature [19] and can be implemented in the design of advanced functional materials with properties not achievable using homogenous materials
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