Fatigue crack growth behavior of titanium foams for medical applications

Abstract

There is an urgent need to understand the failure behavior of titanium foams because of their promising application as load-bearing implant materials in biomedical applications. Following our recent study on fracture toughness of titanium foams [1], this paper investigates the mode I fatigue crack propagation in 60% porous open pore titanium foams both with and without solid coated surface. Fatigue crack propagation tests were performed on compact tension specimens at load ratios of R = 0.1 and R = 0.5 and the fracture surfaces were examined using scanning electron microscopy. The crack growth rate, da/dN, versus the stress intensity factor range, ΔK, curves were measured and compared using two different techniques; image processing and compliance methods. The crack extension rates were well described by ΔK, using the Paris-power law approach. Coated and non-coated titanium foams with 60% porosity had a significantly higher Paris exponent than solid titanium, which can be explained by crack closure and crack bridging. It was also shown that the fatigue crack grows along the centerline, following the weakest path throughout the foam. The results obtained from this work provide important information for evaluating the structural integrity of porous titanium components in the future biomedical applications.

Introduction

Intensive physical activity can lead to stress fracture or micro cracks in load-bearing bones and joints such as the human hip and knee [2]. These fractures are the result of continued repetitive or cyclic fatigue loading [3], [4], and damage leading to failure may develop over numerous years. As such, the cyclic fatigue loading properties of biomaterials used for bone replacement is an important consideration.

In recent years, new manufacturing processes and improvements in quality have made foam materials an interesting option for biomedical applications, particularly in load bearing functions such as hip and knee joint replacements. Foam materials offer significant advantages compared to solid materials as the porosity can be varied to match the strength and stiffness of the surrounding bone to minimize stress-shielding [5], as well as the porosity allowing ingrowth of tissue into the implant [5].

Numerous studies have been done on the mechanical properties of metal foams used in industries other than biomedical engineering [6]. For instance, Motz et al. [6], Zettl et al. [7], Olurin et al. [8], and McCullough et al. [9] have investigated fracture mechanics and fatigue properties of different aluminum foams. Alternatively, there are few studies [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] on the mechanical properties of titanium foams. For instance, Imwinkelried [17] carried out static compression, cyclic compression, bending, tension, and torsion tests on titanium foam, while Teoh et al. [18] have investigated the effect of pore sizes and cholesterol lipid solution on fracture toughness of pure titanium foam. Pore sizes in the pure titanium samples ranged from 25 μm to 103 μm with porosity of 8.5–35% and it was discovered that the fracture toughness of samples with smaller pore sizes was twice the fracture toughness values of samples with larger pore sizes [18].

While the high cycle fatigue properties of titanium foams have been studied (S–N approach) [17], to-date there are no studies on the fracture mechanics of highly porous titanium foams. As foam structures often have inherent flaws, there is usually no crack initiation stage. Thus, use of S–N approach, which typically incorporates initiation of a crack, is not practical. A realistic way to investigate foam materials is to examine the number of cycles needed to propagate these inherent flaws to failure. Fracture mechanics or damage-tolerant method is usually used for such predictions. For fatigue crack growth (FCG) analysis, the number of cycles needed for a crack to grow sub-critically to a critical size is calculated from information relating the crack velocity to the mechanical driving force, and the stress intensity factor, K [20]. Studying the crack growth of titanium foam will examine whether they are compatible for biomedical implantations.

In this study, the mechanical properties of pure titanium foams with 60% porosity (relative density of 0.40) have been examined by using mode I of crack growth testing, and the results are compared against a range of skeletal components. Foams were manufactured using the space-holder method [21] both with and without a solid coating. Typically, low porous metals have higher stiffness and strength than high porous metals; but do not have space for bone ingrowth [22]. Conversely, high porous metals have more space for bone regeneration with lower stiffness and strength. A good implant material should have both permeability and strength. For growth of bone tissue into the pores and connectivity of macropores, the material must have at least 55% porosity [15], [23]. Titanium foam with 60% porosity was chosen because this porosity not only is suitable for bone ingrowth but its strength and toughness is shown to be appropriate for biomedical applications [1], [17], [21]. The objective of this research is to understand the mechanisms of fatigue crack propagation in titanium foams and to compare these results with the fatigue data on the bone, dentine, and current implant materials from literature.