Synthesis and characterization of methacrylic acid‐based molecular imprinted polymers by positron lifetime spectroscopy

Three molecular imprinted polymers (MIPs) were prepared with different amounts of crosslinker (ethylene glycol dimethacrylate) in methacrylic acid functional monomer. Positron lifetime spectroscopy has been used to study the variation in free volume cavities (nanosize pores) in these MIPs of different monomer‐crosslinker ratio. The changes in free volume cavities after the template molecule 4‐chlorophenol extracted from the polymer matrix were measured. The results indicate that the optimum monomer‐crosslinker ratio for this MIP is 1:5. The free volume distribution derived from CONTIN program further reveals the presence and absence of template molecules in the system. The Fourier transform infrared spectra results have been used to compliment this through the absence of stretching frequencies at 648 cm^?1 and 825 cm^?1 of C? Cl suggesting the successful extraction of the template from the MIPs. Scanning electron microscope (SEM) scans indicate the grains are of heterogeneous shape and size in agreement with earlier studies. POLYM. ENG. SCI., 54:667–674, 2014. © 2013 Society of Plastics Engineers     

High speed insertion of bone fracture fixation pins: a finite element penetration model with experimental comparisons

A new method of bone fracture fixation is considered in which small pins/darts are dynamically inserted into bone to prevent translation and rotation at the fracture site. An ABAQUS model was developed to analyze dart penetration in cortical and cancellous bone for varying dart diameter, material, and velocity, and cortical thickness. The method is advocated for bioresorbable darts, so polylactide (PLA) and magnesium are the materials examined in this study. Numerical results showed that magnesium darts can achieve full penetration in bone while suffering little damage. The PLA darts penetrated thin bone well, but substantial deformation was seen as the cortical thickness increased, especially for small diameter darts. As partial validation, prototype PLA fixation darts were fired into cadaveric bone with a custom nailer. As in the model, the PLA darts could penetrate thin cortices but saw gross deformation when impacted against thicker bone.     

Failures of welded titanium aircraft ducts

An in-service failure of a thin-walled titanium bleed-air duct from a wide-bodied commercial aircraft has been investigated. Cracking had occurred in the heat-affected zone (HAZ) adjacent to a circumferential weld joining two sections of the duct which was manufactured from commercially pure (grade 3) titanium sheet. Specimens were cut from the duct to include an intact weld and tested under known conditions (overload, fatigue, sustained loading) for comparison with the failed duct. Metallographic observations showed that cracking occurred through an acicular HAZ, and fractographic observations revealed brittle, cleavage-like cracking with occasional areas of fatigue striations for both the failed duct and fatigued specimens. These observations, and the absence of sustained-load cracking in test specimens, suggested that the in-service failure had occurred primarily by fatigue. Observations also indicated that small, cleavage-like cracks had been present in the ducts prior to service, although whether these cracks were caused by overload tearing, stress-corrosion cracking, hot-salt cracking or fatigue was not clear. Other possible causes of cleavage-like cracking, e.g. the presence of hydrides as proposed for previous failures, contamination of HAZs by oxygen/nitrogen, are discussed. Possible ways of preventing further failures are then outlined.     

Localized tensile strain distribution and metallurgy of electron beam welded Ti–5Al–5V–5Mo–3Cr titanium alloys

The tensile properties, strain distribution, fracture mechanisms, and microstructure of electron beam welded Ti–5Al–5V–5Mo–3Cr (Ti–5553) in the as-welded condition were investigated in order to evaluate the weldability of the titanium alloy. Rolled sheets of Ti–5553 were electron beam welded perpendicular to the rolling direction and sub-size tensile specimens were machined from the sheets. Tensile tests were conducted in conjunction with the use of a non-contact 3D image correlation photogrammetry system, ARAMIS, to determine the as-welded tensile properties as well as localized strain in the vicinity of the weld zone. Microstructural examination showed the fusion zone and heat affected zones consisted of large grains of retained β phase whereas the base metals consisted of α surrounding small β grains containing dispersed α precipitates. Tensile tests revealed that elongation at fracture in the as-welded condition was comparable to un-welded Ti–5553 while also revealing that the tensile strength was lower. Hardness profiles across the welds showed a decrease in average hardness from the base metals to the fusion zone. The decrease in hardness can be attributed to the large grain size, retained β phase, and loss of Al during welding. Fracture occurred in the weld zone and was primarily due to microvoid coalescence. These results indicate that Ti–5553 is readily weldable and displays reasonable properties in the as-welded condition.