Paul A. Hooper

Modelling the behaviour of composite sandwich structures when subject to air blast loading

Large-scale glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) sandwich structures (1.6 m x 1.3 m) were subject to explosive air blast (100 kg TNT equivalent) at stand-off distances of 14 m. Digital image correlation (DIC) was used to obtain full-field data for the rear-face of each deforming target. A steel plate of comparable mass per unit area was also subjected to the same blast conditions for comparison. The experimental data was then verified with finite element models generated in Abaqus/Explicit. Close agreement was obtained between the numerical and experimental results, confirming that the CFRP panels had a superior blast performance to the GFRP panels. Moreover all composite targets sustained localised failures (that were more severe in the GFRP targets) but retained their original shape post blast. The rear-skins remained intact for each composite target with core shear failure present.

3D printing with flying robots

Extensive work has been devoted recently to the development of 3D printing or additive layer manufacturing technologies, as well as to the field of flying robots. However, to the best of the authors7 knowledge, no robotic prototype has been presented so far that combines additive layer manufacturing techniques with aerial robotics. In this paper, we examine the feasibility of such a hybrid approach and present the design and characterisation of an aerial 3D printer; a flying robot capable of depositing polyurethane expanding foam in mid-flight. We evaluate various printing materials and describe the design and integration of a lightweight printing module onto a quadcopter, as well as discuss the limitations and opportunities for aerial construction with flying robots using the developed technologies. Potential applications include ad-hoc construction of first response structures in search and rescue scenarios, printing structures to bridge gaps in discontinuous terrain, and repairing damaged surfaces in areas that are inaccessible by ground-based robots.

Compressive strength after blast of sandwich composite materials

Composite sandwich materials have yet to be widely adopted in the construction of naval vessels despite their excellent strength-to-weight ratio and low radar return. One barrier to their wider use is our limited understanding of their performance when subjected to air blast. This paper focuses on this problem and specifically the strength remaining after damage caused during an explosion. Carbon-fibre-reinforced polymer (CFRP) composite skins on a styrene–acrylonitrile (SAN) polymer closed-cell foam core are the primary composite system evaluated. Glass-fibre-reinforced polymer (GFRP) composite skins were also included for comparison in a comparable sandwich configuration. Full-scale blast experiments were conducted, where 1.6×1.3 m sized panels were subjected to blast of a Hopkinson–Cranz scaled distance of 3.02 m kg−1/3, 100 kg TNT equivalent at a stand-off distance of 14 m. This explosive blast represents a surface blast threat, where the shockwave propagates in air towards the naval vessel. Hopkinson was the first to investigate the characteristics of this explosive air-blast pulse (Hopkinson 1948 Proc. R. Soc. Lond. A 89, 411–413 (doi:10.1098/rspa.1914.0008)). Further analysis is provided on the performance of the CFRP sandwich panel relative to the GFRP sandwich panel when subjected to blast loading through use of high-speed speckle strain mapping. After the blast events, the residual compressive load-bearing capacity is investigated experimentally, using appropriate loading conditions that an in-service vessel may have to sustain. Residual strength testing is well established for post-impact ballistic assessment, but there has been less research performed on the residual strength of sandwich composites after blast.

The effect of implant position on bone strain following lateral unicompartmental knee arthroplasty: A Biomechanical Model Using Digital Image Correlation

Objectives Unicompartmental knee arthroplasty (UKA) is a demanding procedure, with tibial component subsidence or pain from high tibial strain being potential causes of revision. The optimal position in terms of load transfer has not been documented for lateral UKA. Our aim was to determine the effect of tibial component position on proximal tibial strain. Methods A total of 16 composite tibias were implanted with an Oxford Domed Lateral Partial Knee implant using cutting guides to define tibial slope and resection depth. Four implant positions were assessed: standard (5° posterior slope); 10° posterior slope; 5° reverse tibial slope; and 4 mm increased tibial resection. Using an electrodynamic axial-torsional materials testing machine (Instron 5565), a compressive load of 1.5 kN was applied at 60 N/s on a meniscal bearing via a matching femoral component. Tibial strain beneath the implant was measured using a calibrated Digital Image Correlation system. Results A 5° increase in tibial component posterior slope resulted in a 53% increase in mean major principal strain in the posterior tibial zone adjacent to the implant (p = 0.003). The highest strains for all implant positions were recorded in the anterior cortex 2 cm to 3 cm distal to the implant. Posteriorly, strain tended to decrease with increasing distance from the implant. Lateral cortical strain showed no significant relationship with implant position. Conclusion Relatively small changes in implant position and orientation may significantly affect tibial cortical strain. Avoidance of excessive posterior tibial slope may be advisable during lateral UKA. Cite this article: A. M. Ali, S. D. S. Newman, P. A. Hooper, C. M. Davies, J. P. Cobb. The effect of implant position on bone strain following lateral unicompartmental knee arthroplasty: A Biomechanical Model Using Digital Image Correlation. Bone Joint Res 2017;6:522–529. DOI: 10.1302/2046-3758.68.BJR-2017-0067.R1.

Blast Loading of Sandwich Structures and Composite Tubes

This chapter reviews blast impact experimentation on glass fibre reinforced polymer (GFRP) and carbon-fibre reinforced polymer (CFRP) sandwich composite materials and laminate composite tubes. Explosive charges of 0.64–100 kg TNT equivalent were used during these air- and underwater-blast tests. The difference in response and damage inflicted from underwater- and air-blast loading was assessed from strain-field measurements and post-blast specimen analysis. Procedures for monitoring the structural response of such materials during blast events have been devised. High-speed photography was employed during the air-blast loading of GFRP and CFRP sandwich panels, in conjunction with digital image correlation (DIC), to monitor the deformation of these structures under shock loading. Failure mechanisms have been revealed using DIC and confirmed in post-test sectioning. The improved performance of composite sandwich structures with CFRP skins compared to GFRP equivalent constructions is demonstrated for air-blast experiments. Strain gauges were used to monitor the structural response of similar sandwich materials and GFRP tubular laminates during underwater shocks. The effect of the supporting/backing medium (air or water) of the target facing the shock has been identified during these studies. Mechanisms of failure have been established such as core crushing, skin/core cracking, delamination and fibre breakage. Strain gauge data supported the mechanisms for such damage. A transition in behaviour was observed in the sandwich panels when subject to an underwater blast as opposed to an air-blast load. Damage mechanisms notably shifted from distributed core shear failure originating from regions of high shear in air blast to global core crushing in underwater blast. The full-scale experimental results presented here will assist in the development of analytical and computational models. Furthermore, the research highlights the importance of boundary conditions with regards to blast resistant design.

Blast Performance and Damage Assessment of Composite Sandwich Structures

This chapter reviews blast experiments that have been carried out on composite sandwich panels with glass-fiber reinforced polymer (GFRP) face-sheets, carbon-fiber reinforced polymer (CFRP) face-sheets and face-sheets containing a mixture of fiber fabrics. The panels were subjected to explosive charges ranging from 1.28 to 100 kg TNT equivalent during the air and underwater blast tests. The difference in panel response was recorded using high-speed photography and digital image correlation (DIC) during the air blast tests. More conventional instrumentation using strain gauges was required for the underwater blast tests. Following each experiment, the panel damage was analyzed and compared either visually or using X-ray computed tomography (CT) scanning. The addition of polypropylene (PP) Innegra interlayers into a GFRP front face-sheet has been shown to be advantageous during air blast loading. Due to the increased thickness of the front face-sheet, the panel deflects less and experiences less front face-sheet damage. The replacement of GFRP plies with aramid plies during underwater blast loading was shown to be detrimental to the panel performance. The panel suffered from more severe damage compared to a fully GFRP panel. The comparison of CFRP against GFRP panels during underwater blast revealed that the CFRP has a greater stiffness but this result in greater face-sheet debonding due to its lower strain to failure. These experiments have shown that an optimal blast resilience response could be achieved through the combination of different fiber fabrics.

Roles of microstructures on deformation response of 316 stainless steel made by 3D printing

One of the main challenges in additive manufacturing (AM) of metals is to manufacture high quality materials and ensure the performance of AM materials in service duties. This challenge can only be solved when the relationships between build process parameters, microstructure and deformation behaviour are understood. This present study is part of holistic efforts at Imperial College to reveal such relationships. In this study, we present our study of porosity condition, grain morphology, texture and metastable phases in AM stainless steel 316. To provide samples for mechanical and microstructural study, cylindrical samples of stainless steel 316 were printed by powder-bed laser melting with a bi-directional hatch pattern. Scanning electron microscopy and electron backscattered diffraction were used to investigate fine microstructures (such as grain morphology, texture and crystal phases) after 3D printing and deformation. Subsequently, a detailed 3D structure of columnar grains in as-printed 316 steel is ...

A Novel Image Processing Method for ARCMAC Point to Point Optical Strain Measurement

The remaining life of high pressure steam pipes in power stations is heavily dependent upon material creep rates. Monitoring strain in these pipes is difficult due to demanding operational conditions and has resulted in the development of a rugged optical strain gauge system by E.ON UK. The E.ON UK auto-reference creep management and control (ARCMAC) gauge is a point-to-point biaxial creep strain measurement technique. Room temperature validation of the gauge was done using a UK National Physical Laboratory (NPL) extensometer calibration rig (ECR). These gauges have been successfully installed and measurements have been acquired on high temperature (∼600 °C) low-alloy ferritic steel power plant piping.In conjunction with the ARCMAC gauges, research has been ongoing to seek effective methods to process the corresponding ARCMAC images. This paper describes a novel image processing method for the analysis of ARCMAC images. This method processes ARCMAC images with high efficiency and accuracy even if images were taken in demanding environmental conditions. This novel image processing method has been developed into a standalone software program named ARCMAC Assistant to assist both experiment and field work. Experiments involving calibration test work and in-situ creep tests have been used to validate its accuracy.Copyright