Calvin Rans

Riveting Process Induced Residual Stresses Around Solid Rivets in Mechanical Joints

The interference fit provided by solid rivets introduces a residual stress field beneficial to the fatigue life of riveted joints. Evolution in riveting technology has led to force-controlled riveters which provide greater consistency over the rivet installation process and the resulting residual stress field. By reexamining the rivet installation process and its effects on the formation of residual stresses, the fatigue benefits of rivets could be further exploited. Using a 3-D finite element model, installation of universal and countersunk rivets in monolithic aluminum sheet has been studied. Aspects of accepted riveting practice, including the degree of rivet flushness and the rivet squeeze force were found to play significant roles in the formation of residual stresses. Residual stresses beneath the rivet head were also found to be influenced primarily by through-thickness compression of the joined sheets during riveting, challenging the traditional analogy of riveting to radial expansion processes.

Fatigue performance of additively manufactured meta-biomaterials: The effects of topology and material type

Additive manufacturing (AM) techniques enable fabrication of bone-mimicking meta-biomaterials with unprecedented combinations of topological, mechanical, and mass transport properties. The mechanical performance of AM meta-biomaterials is a direct function of their topological design. It is, however, not clear to what extent the material type is important in determining the fatigue behavior of such biomaterials. We therefore aimed to determine the isolated and modulated effects of topological design and material type on the fatigue response of metallic meta-biomaterials fabricated with selective laser melting. Towards that end, we designed and additively manufactured Co-Cr meta-biomaterials with three types of repeating unit cells and three to four porosities per type of repeating unit cell. The AM meta-biomaterials were then mechanically tested to obtain their normalized S-N curves. The obtained S-N curves of Co-Cr meta-biomaterials were compared to those of meta-biomaterials with same topological designs but made from other materials, i.e. Ti-6Al-4V, tantalum, and pure titanium, available from our previous studies. We found the material type to be far more important than the topological design in determining the normalized fatigue strength of our AM metallic meta-biomaterials. This is the opposite of what we have found for the quasi-static mechanical properties of the same meta-biomaterials. The effects of material type, manufacturing imperfections, and topological design were different in the high and low cycle fatigue regions. That is likely because the cyclic response of meta-biomaterials depends not only on the static and fatigue strengths of the bulk material but also on other factors that may include strut roughness, distribution of the micro-pores created inside the struts during the AM process, and plasticity. STATEMENT OF SIGNIFICANCE Meta-biomaterials are a special class of metamaterials with unusual or unprecedented combinations of mechanical, physical (e.g. mass transport), and biological properties. Topologically complex and additively manufactured meta-biomaterials have been shown to improve bone regeneration and osseointegration. The mechanical properties of such biomaterials are directly related to their topological design and material type. However, previous studies of such biomaterials have largely neglected the effects of material type, instead focusing on topological design. We show here that neglecting the effects of material type is unjustified. We studied the isolated and combined effects of topological design and material type on the normalized S-N curves of metallic bone-mimicking biomaterials and found them to be more strongly dependent on the material type than topological design.

Effects of applied stress ratio on the fatigue behavior of additively manufactured porous biomaterials under compressive loading

Additively manufactured (AM) porous metallic biomaterials are considered promising candidates for bone substitution. In particular, AM porous titanium can be designed to exhibit mechanical properties similar to bone. There is some experimental data available in the literature regarding the fatigue behavior of AM porous titanium, but the effect of stress ratio on the fatigue behavior of those materials has not been studied before. In this paper, we study the effect of applied stress ratio on the compression-compression fatigue behavior of selective laser melted porous titanium (Ti-6Al-4V) based on the diamond unit cell. The porous titanium biomaterial is treated as a meta-material in the context of this work, meaning that R-ratios are calculated based on the applied stresses acting on a homogenized volume. After morphological characterization using micro computed tomography and quasi-static mechanical testing, the porous structures were tested under cyclic loading using five different stress ratios, i.e. R = 0.1, 0.3, 0.5, 0.7 and 0.8, to determine their S-N curves. Feature tracking algorithms were used for full-field deformation measurements during the fatigue tests. It was observed that the S-N curves of the porous structures shift upwards as the stress ratio increases. The stress amplitude was the most important factor determining the fatigue life. Constant fatigue life diagrams were constructed and compared with similar diagrams for bulk Ti-6Al-4V. Contrary to the bulk material, there was limited dependency of the constant life diagrams to mean stress. The notches present in the AM biomaterials were the sites of crack initiation. This observation and other evidence suggest that the notches created by the AM process cause the insensitivity of the fatigue life diagrams to mean stress. Feature tracking algorithms visualized the deformation during fatigue tests and demonstrated the root cause of inclined (45°) planes of specimen failure. In conclusion, the R-ratio behavior of AM porous biomaterials is both quantitatively and qualitatively different from that of bulk materials.

Isolated and modulated effects of topology and material type on the mechanical properties of additively manufactured porous biomaterials

In this study, we tried to quantify the isolated and modulated effects of topological design and material type on the mechanical properties of AM porous biomaterials. Towards this aim, we assembled a large dataset comprising the mechanical properties of AM porous biomaterials with different topological designs (i.e. different unit cell types and relative densities) and material types. Porous structures were additively manufactured from Co-Cr using a selective laser melting (SLM) machine and tested under quasi-static compression. The normalized mechanical properties obtained from those structures were compared with mechanical properties available from our previous studies for porous structures made from Ti-6Al-4V and pure titanium as well as with analytical solutions. The normalized values of elastic modulus and yield stress were found to be relatively close to each other as well as in agreement with analytical solutions regardless of material type. However, the material type was found to systematically affect the mechanical properties of AM porous biomaterials in general and the post-elastic/post-yield range (plateau stress and energy absorption capacity) in particular. To put this in perspective, topological design could cause up to 10-fold difference in the mechanical properties of AM porous biomaterials while up to 2-fold difference was observed as a consequence of changing the material type.

Fatigue Behavior of Fiber/Metal Laminate Panels Containing Internal Carbon Tear Straps

This paper presents a novel hybrid structural concept for improving the damage tolerance of a thin panels subjected to fatigue, such as the skin of an aircraft fuselage. The concept is based on the hybrid fiber/metal laminate technology used as a fuselage skin material in the Airbus A380. By tailoring the stiffness of an fiber/metal laminate locally by variation of the reinforcing fiber, a damage-tolerant feature analogous to the bonded titanium tear strap in monolithic metallic fuselage skins can be introduced internally within the laminate, without variations in laminate thickness. The overallmerits of this concept are discussed, including a preliminary experimental investigation into its improvements to damage tolerance of standard fiber/metal laminates. Overall, the concept was shown to result in nearly a twofold reduction in crack growth and up to a 25% increase in residual strength for fatigue cracks located near the damage-tolerant feature.

Influence of Fabric Carrier on the Fatigue Disbond Behavior of Metal-to-Metal Bonded Interfaces

The effect of an adhesive support on the Mixed-Mode fatigue properties of a metal-to-metal bonded interface was evaluated. Pure Mode I, pure Mode II, and Mixed-Mode fatigue disbond tests were conducted on supported and unsupported bonded samples. The fracture surfaces were observed and related to the fatigue disbond growth rate. The supporting system analyzed was found not to affect the fracture surface under Mode I loading, resulting in similar disbond growth rates for both supported and unsupported bonds. However, under Mixed-Mode and Mode II loading the supporting system improved the fatigue resistance. The improvement was related to different failure mechanisms caused by the supporting system such as Mode II load bearing and a change in the disbond path.

Finite Element Modeling of Fatigue in Fiber–Metal Laminates

Innovative hybrid materials developed at Delft University of Technology (e.g., ARALL and GLARE) dramatically reduce life-cycle costs and offer a great opportunity for service life extension of legacy aircraft. Replacement or repair of damaged aircraft components requires high-strength composite materials with high tailorability, fatigue, and impact-damage resistance, all of which are offered by the advanced hybrid materials. In addition, a reliable fatigue-life evaluation methodology for hybrid structures of arbitrary layup, configuration, constituent materials, and geometry is necessary. An efficient computational framework is presented for simulation of fatigue fracture in fiber–metal laminates based on the homogenized laminate modeled with large shell elements and cohesive zone used to simulate crack propagation. The cohesive traction–separation relationship is calibrated against the analytical solution for the strain-energy release rate, which explicitly accounts for the effect of fiber bridging. Appr...

Evaluation of mode II fatigue disbonding using Central Cut Plies specimen and distributed strain sensing technology

ABSTRACT The lack of a widely-accepted test standard for characterizing the mode II fatigue disbond growth behavior of adhesively bonded interfaces is a challenge to the research community in terms of producing consistent and repeatable results. Typically, researchers apply the End Notch Flexure specimen, which is already used for static delamination studies. However, the needs for static and fatigue disbond growth characterization are not the same, resulting in some undesirable effects in such specimen. This study looks at a particular mode II test configuration known as the Central Cut Plies (CCP) specimen. A critical evaluation of the suitability of this specimen, including the influence of geometry, disbond measurement approaches and the stability of the disbond growth is carried out through a combination of numerical and experimental investigations. A distributed strain sensing system based on Rayleigh Backscattering provided a surface strain profile from which disbond growth rate data was obtained. A finite element model was used to verify the experimental results and determine the disbond length from the strain profiles. Results of this evaluation have shown that the CCP specimen is a promising specimen configuration for characterizing fatigue disbond growth; however, it also presents several challenges that require consideration in its application.

A Paradigm Shift in Teaching Aerospace Engineering: From Campus Learners to Professional Learners – a Case Study on Online Courses in Smart Structures and Air Safety Investigation

In this paper, the transition from teaching on-campus to an online audience consisting of working professionals in an Aerospace Engineering context is described. The differences in the learner’s needs and the transition in teaching methods and style that is required from teaching staff is discussed. This is illustrated by two case studies: for Smart Structures and for Air Safety Investigation. Recommendations on how universities can contribute to Life Long Learning are given.

Forensic engineering: Learning by accident. Teaching investigation skills to graduate students using real-life accident simulations

This paper relates the experiences of lecturers at Delft University of Technology in the designing and running of a Master course in Forensic Engineering. Rather than traditional face-to-face lectures, use of real-life evidence-based learning was made in the form of training for and execution of a mock aircraft accident investigation. The culmination of this learning experience for the students was the group exam, which took the form of the examination of a recreated accident scene. Students were required to organize their investigation groups, document the scene, and collect evidence while on scene. Subsequently, they were given time to analyze the results and prepare a standard accident investigation report. Evaluation results show satisfied students and good learning outcomes, making this a course worth repeating.