D. H. Kohn

Development and characterization of a biodegradable polyphosphate

A biodegradable polyphosphate polymer (Mn = 18,000, Mw/Mn = 3.2) matrix system was developed as a potential delivery vehicle for growth factors. As a model system, release of recombinant human osteogenic protein-1 (OP-1) from this polymer was evaluated. The polyphosphate was synthesized using a triethylamine catalyst in an argon environment, and characterized using elemental analysis, gel permeation chromatography (GPC), and Fourier transform infrared spectroscopy (FTIR). Degradation kinetics of the polyphosphate polymer in phosphate-buffered saline (PBS) were represented by a second-order polynomial while degradation in bovine serum was linear with time. The polymer degraded faster in PBS than in bovine serum. In vitro release of OP-1 was also faster in PBS than in serum. Release kinetics of OP-1 in PBS and serum were represented by second-order polynomials. The OP-1 release from this physically dispersed polymeric matrix may be described by several possible mechanisms: diffusion, bulk polymer degradation, ion complexation, and interactions among the protein (OP-1), polymer, proteins, and enzymes in the media. This polyphosphate may be an effective carrier for morphogens, growth factors, or other classes of bioactive molecules.

Tensile and fatigue strength of hydrogen-treated Ti-6Al-4V alloy

Tensile, fatigue and fractographic data on Ti-6Al-4V microstructures attained through a series of post-β-annealing treatments which used hydrogen as a temporary alloying element are presented. Hydrogen-alloying treatments break up the continuous grain boundary α and colony structure, and produce a homogeneous microstructure consisting of refined α-grains in a matrix of discontinuous β. These changes in microstructural morphology result in significant increases of the yield strength (974 to 1119 MPa), ultimate strength (1025 to 1152 MPa) and high cycle fatigue strength (643 to 669 MPa) compared to respective values for lamellar microstructures (902, 994, 497 MPa). The strengths are also significantly greater than the strengths of equiaxed microstructures (914, 1000, 590 MPa). The strengths of hydrogen-alloy treated samples are therefore superior to strengths attainable via other thermal cycling techniques.The fatigue fracture surfaces of the hydrogen-alloy treated samples were topographically similar to equiaxed samples. Fatigue crack initiation was characterized by faceted regions. As crack length and ΔK increased, the crack surface changed to a rounded, ductile topology, with microcracks and locally striated regions. Fracture primarily followed the α-β interfaces. This is rationalized by the fact that hydrogen-alloyed microstructures are very fine Widmanstatten microstructures having reduced aspect ratios, and these microstructures fail along α-β interfaces.

Acoustic emission during fatigue of Ti-6Al-4V : incipient fatigue crack detection limits and generalized data analysis methodology

The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6Al-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 10 μm. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.

Microstructural refinement of β-sintered and Ti-6Al-4V porous-coated by temporary alloying with hydrogen

A series of thermochemical treatments, in which hydrogen was used as a temporary alloying element to refine the lamellar microstructure of β-sintered and porous-coated Ti-6Al-4V was formulated. Each step of the treatment sequence (hydrogenation, eutectoid decomposition and dehydrogenation) was studied separately, on uncoated specimens and then on porous-coated specimens. The resultant microstructures can have α-grain sizes less than 1 μm, aspect ratios near unity and discontinuous grain boundary α (GBα), microstructural attributes which increase the fatigue strength. Microstructural refinement occurs because hydrogen-alloying reduces the (α+β)↔β transition temperature and enables a eutectoid decomposition reaction to occur. The optimal hydrogenation temperature is 850 °C, because hydrogen concentrations of 0.71 to 0.85 wt% are in-diffused and β-transformation is achieved. These weight percentages are in the optimal range for efficient eutectoid decomposition kinetics, β-transformation obviates the need for a separate β-transformation treatment step. A separate eutectoid decomposition treatment step may be used, or eutectoid decomposition may be combined with dehydrogenation. The finest eutectoid microstructures are obtained if hydrogen concentrations are in the range 0.5 to 0.8 wt%. The criteria for dehydrogenation are efficient removal of hydrogen, with minimal grain growth and absence of GBα. These criteria are best met by using dehydrogenation temperatures <700 °C. Altering the sintering temperature or adding a porous coating does not affect the parameters of the hydrogen-alloying treatment steps.

Sources of acoustic emission during fatigue of Ti-6Al-4V: effect of microstructure

The fundamentals of acoustic emission (AE) analysis of fatigue cracking were applied to Ti-6Al-4V. The effect of microstructure on the characteristics of the AE events generated and the failure mechanisms which produced AE in Ti-6Al-4V were established. Lamellar microstructures generated one to two orders of magnitude more emission than equiaxed microstructures. The combination of larger grain size, more continuous α/β interfaces, more tortuous crack-front geometry, cleavage and intergranular fracture in lamellar microstructures accounts for the greater amount of emission. For lamellar microstructures, most AE events were generated in the upper 20% of the stress range, whereas in equiaxed microstructures, most events were generated at lower stresses. Most AE events were generated during crack opening and also at low stresses. AE events having high level intensities were also generated at stresses other than the peak stress. This is because in titanium alloys, which have both high strength and toughness, AE events are generated from both plastic zone extension and crack extension.

Acoustic emission during fatigue of Ti-6Al-4V: incipient fatigue crack detection limits and generalized data analysis methodology

The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6AI-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 1 0 gm. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.