Hierarchical tailoring of strut architecture to control permeability of additive manufactured titanium implants

Porous titanium implants are a common choice for bone augmentation. Implants for spinal fusion and repair of non-union fractures must encourage blood flow after implantation so that there is sufficient cell migration, nutrient and growth factor transport to stimulate bone ingrowth. Additive manufacturing techniques allow a large number of pore network designs. This study investigates how the design factors offered by selective laser melting technique can be used to alter the implant architecture on multiple length scales to control and even tailor the flow. Permeability is a convenient parameter that characterises flow, correlating to structure openness (interconnectivity and pore window size), tortuosity and hence flow shear rates. Using experimentally validated computational simulations, we demonstrate how additive manufacturing can be used to tailor implant properties by controlling surface roughness at a microstructual level (microns), and by altering the strut ordering and density at a mesoscopic level (millimetre).     

The three-prong method: a novel assessment of residual stress in laser powder bed fusion

ABSTRACT

Residual stress is a major problem for most metal-based laser powder bed fusion (L-PBF) components. Residual stress can be reduced by appropriate build planning and post-process heat treatments; however, it is not always avoidable and can lead to build failures due to distortion and cracking. Accurate measurement of residual stress levels can be difficult due to high equipment set-up costs and long processing times. This paper introduces a simple but novel method of measuring residual stresses via a three-pronged cantilever component, the three-prong method (TPM). The method allows for a quick and easy characterisation of residual stress for a wide range of machine parameters, build strategies and materials. Many different cantilever designs have been used to indicate residual stress within additive manufacturing techniques. All of which share the same shortcoming that they indicate stress in one direction. If the principal component of stress is not aligned with the beam geometry, it will underestimate peak stress values. A novel three-prong design is proposed which covers two dimensions by utilising three adjoined cantilever beams, a configuration which echoes that of hole-drilling where three measurements are used to calculate the stress field around a drilled hole. Each arm of the component resembles a curved bridge-like structure; one end of each bridge is cut away from the base plate leaving the centre intact. Deformation of the beams is then measured using a co-ordinate measurement machine. Stress profiles are then estimated using finite element analysis by meshing the deflected structure and forcing it back to its original shape. In this paper, the new TPM is used to compare the residual stress levels of components built in Ti–6Al–4V with different hatch patterns, powers and exposure times.     

The domain theory for requirements engineering

Retrieval, validation, and explanation tools are described for cooperative assistance during requirements engineering and are illustrated by a library system case study. Generic models of applications are reused as templates for modeling and critiquing requirements for new applications. The validation tools depend on a matching process which takes facts describing a new application and retrieves the appropriate generic model from the system library. The algorithms of the matcher, which implement a computational theory of analogical structure matching, are described. A theory of domain knowledge is proposed to define the semantics and composition of generic domain models in the context of requirements engineering. A modeling language and a library of models arranged in families of classes are described. The models represent the basic transaction processing or `use case' for a class of applications. Critical difference rules are given to distinguish between families and hierarchical levels. Related work and future directions of the domain theory are discussed     

Has the web transformed experience goods into search goods?

Klein (Journal of Business Research 41(3): 195–203, 1998) posited that the Web can transform experience goods into search goods (ES shifts). We examine her proposition in three ways. First, we critically assess the background of her proposition in light of the Web evolution in the past decade. Second, we conduct a comparison of past studies that measured the extent of search, experience, and credence (SEC) characteristics of goods. Third, we report the results of an exploratory survey on a set of commonly purchased products to benchmark possible ES shifts against the past studies. Their results indicate that SEC classification changes do not seem significant.     

Automated verification of refinement laws

Demonic refinement algebras are variants of Kleene algebras. Introduced by von Wright as a light-weight variant of the refinement calculus, their intended semantics are positively disjunctive predicate transformers, and their calculus is entirely within first-order equational logic. So, for the first time, off-the-shelf automated theorem proving (ATP) becomes available for refinement proofs. We used ATP to verify a toolkit of basic refinement laws. Based on this toolkit, we then verified two classical complex refinement laws for action systems by ATP: a data refinement law and Back’s atomicity refinement law. We also present a refinement law for infinite loops that has been discovered through automated analysis. Our proof experiments not only demonstrate that refinement can effectively be automated, they also compare eleven different ATP systems and suggest that program verification with variants of Kleene algebras yields interesting theorem proving benchmarks. Finally, we apply hypothesis learning techniques that seem indispensable for automating more complex proofs.     

Estimating farfield organic chemical exposures,intake rates and intake fractions to human age classes

This study describes the development and application of the Farfield Human Exposure (FHX) model. This screening level model brings together information on chemical partitioning, degradation, environmental fate and transport, and food web bioaccumulation for assessing human age class-specific exposures from chemicals released to the environment. The Level III model calculates steady-state concentrations in multimedia compartments such as air, water, and soil, and various food groups including fruits, vegetables, milk, dairy, fish, and meats based on a unit emission rate. Human multimedia intake and ingestion rates determine unit emission based chemical exposures, intake rates and intake fractions for various age classes representing the general population. The FHX model is generally applicable to regional areas and is designed to use readily available chemical input data for screening level and priority setting evaluations. In the present study the model is parameterized for an evaluative regional area using Health Canada exposure factors to determine multimedia intake rates for seven selected age classes. The model outputs, human chemical intake rates and human intake fractions are described for four case study chemicals. Biotransformation and biomagnification processes in food webs are highlighted as key factors influencing human exposure. Future research needs to improve the model are discussed.     

Investigating user experience in Second Life for collaborative learning

Two studies on collaborative learning in Second Life are reported. The first is an ecological study of Second Life used in an undergraduate class, by observation, interviews, and limit surveys. Use of Second Life motivated students with good user experience, although they viewed it as a games technology. Second Life was used to prepare virtual meetings and presentations but not for online discussion, with Blackboard and especially Facebook providing collaborative support. In the second experimental study, the effectiveness and user experience with Second Life and Blackboard were compared, including a face-to-face control condition. There were no performance differences overall, although face-to-face was quicker and was preferred by users, followed by Blackboard and Second Life. Blackboard was perceived to be more usable, whereas Second Life provided a better user experience. Worst performance was indicated by dislike of avatar interaction in Second Life, and poor user experience in Blackboard, whereas better performance was associated with engagement with avatars, and better usability in Blackboard. The results of both studies are reviewed using Salmon's model for online learning, suggesting that Second Life helped motivation and socialisation stages, although integration with other technologies is necessary for knowledge construction. Preliminary guidelines are proposed for configuration and management of Second Life in collaborative learning. The affordances for collaboration in virtual worlds are discussed, with reflections on user experience and functional support provided by Second Life, as an exemplar of a virtual world for collaborative learning support.     

The CADE-18 ATP System Competition

The CADE ATP System Competition (CASC) is an annual evaluation of fully automatic, first-order automated theorem-proving (ATP) systems. CASC-18 was the seventh competition in the CASC series. Twenty-four ATP system variants competed in the various competition and demonstration divisions. An outline of the design and a commentated summary of the results are presented. This revised version was published online in August 2006 with corrections to the Cover Date.