Experimental investigation of NCF-on-tool contact

The true fibre contact length for NCF-on-tool contact was measured for an area of 8.29 mm × 11.54 mm over a range of normal pressures using microscopy. Contact length was approximately 67% lower than for unidirectional tow-on-tool contact at the same nominal pressure (240 kPa). This is because NCF stitching and gaps between tows significantly reduce fibre contact length compared to tow-on-tool fibre contact measurements. A Hertzian approach was used to estimate contact area from contact length

Biomimetic-inspired CFRP to perforated steel joints

In many high-performance applications there is a need to join steel to CFRP parts. However the stiffness mismatch between these materials leads to high stress concentrations in such joints. This paper uses the biomimetics approach to help develop solutions to this problem. Nature has found many ingenious ways of joining dissimilar materials, with a transitional zone of stiffness at the insertion site commonly used. In engineering joints, one way to reduce the material stiffness mismatch is to gradually decrease the effective stiffness of the steel part of the joint by perforating it with holes. This paper investigates joining of flat perforated steel plates to a CFRP part by a co-infusion resin transfer moulding process. The possible effect of mechanical interlocking as resin fills the perforations is assessed by filling the holes with PTFE prior to moulding to prevent such resin ingress. The joints are tested under static tensile loading. The perforated steel joints show a 175% increase of joint strength comparing to non-perforated joints. Finite element analyses are used to interpret the experimental results. It has been found that the model is able to reproduce with accuracy the experimental load–displacement test curves and show the failure mechanisms of the joint.The authors acknowledge the financial support provided by the Engineering and Physical Sciences Research Council (EPSRC) and Dowty Propellers (part of GE Aviation).This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.compstruct.2016.06.01

Biomimetic-inspired CFRP to perforated steel joints

In many high-performance applications there is a need to join steel to CFRP parts. However the stiffness mismatch between these materials leads to high stress concentrations in such joints. This paper uses the biomimetics approach to help develop solutions to this problem. Nature has found many ingenious ways of joining dissimilar materials, with a transitional zone of stiffness at the insertion site commonly used. In engineering joints, one way to reduce the material stiffness mismatch is to gradually decrease the effective stiffness of the steel part of the joint by perforating it with holes. This paper investigates joining of flat perforated steel plates to a CFRP part by a co-infusion resin transfer moulding process. The possible effect of mechanical interlocking as resin fills the perforations is assessed by filling the holes with PTFE prior to moulding to prevent such resin ingress. The joints are tested under static tensile loading. The perforated steel joints show a 175% increase of joint strength comparing to non-perforated joints. Finite element analyses are used to interpret the experimental results. It has been found that the model is able to reproduce with accuracy the experimental load–displacement test curves and show the failure mechanisms of the joint.The authors acknowledge the financial support provided by the Engineering and Physical Sciences Research Council (EPSRC) and Dowty Propellers (part of GE Aviation).This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.compstruct.2016.06.01

Adhesive-based tendon-to-bone repair: failure modelling and materials selection.

Surgical reattachment of tendon to bone is a procedure marked by high failure rates. For example, nearly all rotator cuff repairs performed on elderly patients with massive tears ultimately result in recurrence of tearing. These high failure rates have been attributed to stress concentrations that arise due to the mechanical mismatch between tendon and bone. Although recent studies have identified potential adhesives with mechanical properties tuned to alleviate these stress concentrations, and thereby delay the onset of failure, resistance to the progression of failure has not been studied. Here, we refined the space of adhesive material properties that can improve surgical attachment by considering the fracture process. Using cohesive zone modelling and physiologically relevant values of mode I and mode II adhesive fracture toughnesses, we predicted the maximum displacement and strength at failure of idealized, adhesively bonded tendon-to-bone repairs. Repair failure occurred due to excessive relative displacement of the tendon and bone tissues for strong and compliant adhesives. The failure mechanism shifted to rupture of the entire repair for stiffer adhesives below a critical shear strength. Results identified a narrow range of materials on an Ashby chart that are suitable for adhesive repair of tendon to bone, including a range of elastomers and porous solids.EPSR

Frictional behaviour of non-crimp fabrics (NCFs) in contact with a forming tool

Microscopic observation and analysis are used to examine the role that contact conditions play in determining the frictional behaviour of non-crimp fabrics (NCFs). The true fibre contact length is measured over a range of normal pressures. For the NCF considered, the contact length is 67% lower than for a corresponding unidirectional tow-on-tool contact at a pressure of 240 kPa. The difference in contact behaviour is associated with the fabric architecture, specifically stitching and gaps between tows. These microscopic observations are used to predict friction using a constant interface shear strength model. These predictions are found to compare well with macroscopic friction measurements taken using a sliding sled arrangement, once the roughness of the sled tool is taken into account

A review of natural joint systems and numerical investigation of bio-inspired GFRP-to-steel joints

There are a great variety of joint types used in nature which can inspire engineering joints. In order to design such biomimetic joints, it is at first important to understand how biological joints work. A comprehensive literature review, considering natural joints from a mechanical point of view, was undertaken. This was used to develop a taxonomy based on the different methods/functions that nature successfully uses to attach dissimilar tissues. One of the key methods that nature uses to join dissimilar materials is a transitional zone of stiffness at the insertion site. This method was used to propose bio-inspired solutions with a transitional zone of stiffness at the joint site for several glass fibre reinforced plastic (GFRP) to steel adhesively bonded joint configurations. The transition zone was used to reduce the material stiffness mismatch of the joint parts. A numerical finite element model was used to identify the optimum variation in material stiffness that minimises potential failure of the joint. The best bio-inspired joints showed a 118% increase of joint strength compared to the standard joints

Biomimetic-inspired joining of composite with metal structures: A survey of natural joints and application to single lap joints

Joining composites with metal parts leads, inevitably, to high stress concentrations because of the material property mismatch. Since joining composite to metal is required in many high performance structures, there is a need to develop a new multifunctional approach to meet this challenge. This paper uses the biomimetics approach to help develop solutions to this problem. Nature has found many ingenious ways of joining dissimilar materials and making robust attachments, alleviating potential stress concentrations. A literature survey of natural joint systems has been carried out, identifying and analysing different natural joint methods from a mechanical perspective. A taxonomy table was developed based on the different methods/functions that nature successfully uses to attach dissimilar tissues (materials). This table is used to understand common themes or approaches used in nature for different joint configurations and functionalities. One of the key characteristics that nature uses to joint dissimilar materials is a transitional zone of stiffness in the insertion site. Several biomimetic-inspired metal-to-composite (steel-to-CFRP), adhesively bonded, Single Lap Joints (SLJs) were numerically investigated using a finite element analysis. The proposed solutions offer a transitional zone of stiffness of one joint part to reduce the material stiffness mismatch at the joint. An optimisation procedure was used to identify the variation in material stiffness which minimises potential failure of the joint. It was found that the proposed biomimetic SLJs reduce the asymmetry of the stress distribution along the adhesive area. © 2014 SPIE

Biomimetic-inspired composite to metal single lap joints

Joining composites with metal parts leads, inevitably, to high stress concentrations because of the material property mismatch. Since joining composite to metal is required in many high performance structures, there is a need to develop a new multifunctional approach to meet this challenge. This paper uses the biomimetics approach to help develop solutions to this problem. Nature has found many ingenious ways of joining dissimilar materials and making robust attachments, alleviating potential stress concentrations. One of the key characteristics that nature uses to joint dissimilar materials is a transitional zone of stiffness in the insertion site. Several biomimetic-inspired metal-to-composite (steel-to-CFRP), adhesively bonded, Single Lap Joints (SLJs) were numerically investigated using a finite element analysis. The proposed solutions offer a transitional zone of stiffness of one joint part to reduce the material stiffness mismatch at the joint. An optimisation procedure was used to identify the variation in material stiffness which minimises potential failure of the joint. It was found that the proposed biomimetic SLJs reduce the asymmetry of the stress distribution along the adhesive area, giving a 59% shear stress reduction in some configurations

Numerical investigation of bio-inspired tubular composite to steel joints

In many high-performance applications there is a need to join tubular steel and composite parts. Bonded joints between dissimilar tubular members under axial tension develop relatively high stresses with steep gradients localised at the joint ends. This is due to the large stiffness mismatch between the materials of the adherends. This paper uses the biomimetics approach to help develop solutions to this problem. One of the key methods that nature uses to join dissimilar materials is a transitional zone of stiffness at the insertion site. This method was used to propose bio-inspired solutions with a transitional zone of stiffness at the joint site for tubular CFRP-to-steel and GFRP-to-steel adhesively bonded joint configurations. The transition zone was used to reduce the material stiffness mismatch of the joint parts. Two-dimensional axisymmetric finite-element models of tubular CFRP-to-steel and GFRP-to-steel joints were developed. A cohesive zone degradation formulation was chosen to calculate accurately the load carrying capacity of the adhesive joints. The model was used to identify the optimum variation in material stiffness which minimises potential failure of the joint. The best bio-inspired CFRP-to-steel and GFRP-to-steel joints showed a 10% and 30% increase of joint strength comparing to the non-bioinspired ones

Review of natural joints and bio- inspired CFRP to steel joints

There is a great variety of joint types used in nature, from jaws, bones and tendons to root systems and tree branches. To understand how to optimise biomimetic-inspired engineering joints, fi rst it is important to understand how biological joints work. In this paper, a review based on the functions of natural joint systems is presented. Emphasis was given to understanding natural joints from a mechanical point of view, so as to inspire engineers to fi nd innovative methods of joining man-made structures. Nature has found many ingenious ways of joining dissimilar materials, with a transitional zone of stiffness at the insertion site commonly used. In engineering joints, one way to reduce the material stiffness mismatch is to gradually decrease the effective stiffness of the steel part of the joint by perforating it with holes. This paper investigates joining of fl at perforated steel plates to a CFRP part by a co-infusion resin transfer moulding process. The joints are tested under static tensile loading. The perforated steel joints show a 175 % increase of joint strength comparing to non- perforated ones. Finite element analyses are used to interpret the experimental results