Loaded carbon composite scarf joints subject to impact

Bonded composite scarf repairs are often used when a flush surface is required for aerodynamic or stealth reasons. Such repairs on the external surface of an aircraft are subject to the same impact risk as that of the parent structure. Consequently, it is essential to assess their durability in the case of impact. A previous preliminary experimental study found an instance of catastrophic failure of a composite scarf joint subject to impact whilst prestrained to 3000 μ. It was postulated that this phenomenon is a result of failure in the joint due to the combination of the prestrain and global structural oscillations resulting from the impact event. In this investigation, a previously applied finite element model is extended to more accurately replicate such catastrophic failure. The effect of lay-up sequence on adhesive failure is studied

Design of composite adhesive joints

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Printing and characterisation of Kagome lattice structures by fused deposition modelling

Sandwich structures with lattice cores exhibit high specific bending strength and stiffness when compared to monolithic structures. Additive manufacturing is able to further expand the available design space to fabricate novel core structures with complex features. In this paper, the compressive performance of the Kagome truss unit cell of acrylonitrile butadiene styrene (ABS) ABSplus™ fabricated by fused deposition modelling is investigated. The influences of part build orientation, truss radius and surface roughness on strength and stiffness are critically explored. The change in build orientation improved the average peak strength and effective stiffness by 23% and 19%, respectively. 90% (v/v) acetone was used to polish the printed surfaces and 5 min chemical treatment was optimal based on the measured surface roughness, strength and stiffness values. These single cell studies will help to understand the macroscopic behaviour of the beams and plates with Kagome cores under quasi-static bending and impact loading scenarios.MOE (Min. of Education, S’pore

Failure Mode Analysis Of Kagome Lattice Structures

Ultralight weight structures are today’s essential need in aerospace, marine and automotive industries. Strength and stiffness optimization of load bearing structures is made possible with the evolution of additive manufacturing technologies through shape or topology optimization. Further composite materials and sandwich constructions reduce the design weight. To fully realize the lightweight structure, core designs with low density and high strength are necessary for sandwich panel design. In this study, the performance of the 3D Kagome truss core structure in compression loading is experimentally investigated. These bio-inspired core structures are fabricated by Fused Deposition Modelling (FDM) with Acrylonitrile butadiene styrene (ABS) ABSplus® material for experimental validation purposes. The geometrical parameters of the Kagome structure in terms of its slenderness ratio are varied to study the switch of failure mechanism from yielding dominant behavior to buckling. The effective stiffness of the truss found from finite element modeling and based on experimental results are compared, and the reasons for their discrepancy are explored. The modulus of the Kagome unit-cell is found to be linearly related to its relative density. The result show that with the increase in the slenderness ratio (l/r), the strength of the Kagome structure decreases.MOE (Min. of Education, S’pore)Published versio

Prediction of moisture diffusion and failure in glass/steel adhesive joints

Glass/steel adhesive joints are being used increasingly in the construction industry as they offer significant structural advantages over conventional mechanical fastener approaches. However, adhesive joints are also known to be sensitive to moisture diffusion into the bondline, which reduces the interfacial bonding strength for hybrid glass/steel substrates. The effect of moisture on the performance degradation of glass/steel adhesive joints has been successfully predicted assuming adhesive property degradation but requires experimental determination of the affected moisture ingress zone. This study utilizes a multi-physics numerical approach implemented via the commercial finite element code Abaqus 2020, which firstly simulates moisture ingress into the adhesive/glass interface and subsequently couples the diffusion effects with a cohesive zone modelling approach for damage initiation and propagation. The numerical predictions are calibrated against experimental data on glass/steel Double Cantilever Beam (DCB) specimens, which are bonded with a ductile methacrylate adhesive (Araldite 2047–1). The modelling approach is then validated against the experimental response of large double lap shear joints of a significantly different bondline geometry. It is demonstrated that the numerical model successfully predicts the critical exposure time for partial to complete joint degradation enabling the development of engineering guidelines for life-time prediction of various joint geometries.</p

Abstract of: Strength evaluation and failure prediction of bolted and adhesive glass/steel joints

This paper investigates the use of bolted and brittle/ductile adhesive connections in glass structures. Two benchmark designs of shear connections are introduced and tested experimentally in quasi-static tensile tests. The designs consist oftempered glass and aluminium substrates while steel splices are used for the load application. In addition, material characterisation testing for the glass and the adhesive is performed and the outputs are used for the numerical simulation of the same joints. Pressure-sensitive, plasticity and failure models are introduced and calibrated to accurately capture the behaviour of the adhesives. Good agreement between the experimental observations and numerical predictions is achieved. The results show that both types of adhesive joints outperform bolted joints while counter-intuitively the lower strength ductile adhesive achieves consistently higher joint strength compared to the brittle adhesive. The numerical analyses highlight that while brittle adhesive joints fail once the fracture strain of the adhesive has been reached, while for ductile adhesives an extensive plastic zone develops near the areas of stress concentrations thereby delaying the damage initiation

Effects of Heat-treatments on the Mechanical Strength of Coated YSZ: An Experimental Assessment

The mechanical strength of thin, symmetric sandwich specimens consisting of a dense yttria-stabilized zirconia (YSZ) substrate coated with a porous NiO-YSZ layer at both major faces was investigated. Specimens were loaded in uniaxial tension to failure following heat treatments at various temperatures. In comparison with the YSZ material, the failure strength of coated specimens was found to increase for heat treatments at 1100°C, but decreased again with further increased heat-treatment temperatures