Simulating low-velocity impact induced delamination in composites by a quasi-static load model with surface-based cohesive contact

https://research-information.bris.ac.uk/files/34722333/Yasaee_CST_2014_Single_z_pin_experimental_pre_print.pdf

Experimental characterisation of mixed mode traction–displacement relationships for a single carbon composite Z-pin

Finite element modelling of z-pinned composite T-joints

Z-pinned laminates and sandwich materials are an important class of composites with current applications in aircraft and emerging uses in other light-weight engineered products. This paper reviews research advances in the manufacture, microstructural chacterisation, mechanical properties, environmental durability and other functional properties (e.g. electrical, thermal, damage sensing) of z-pinned composite materials and structures. The review reveals that z-pins can be used in create novel multifunctional laminates that uniquely combine a multitude of important properties including high interlaminar fracture and fatigue resistance, superior impact damage tolerance, increased thermal and electrical conductivities, damage sensing, and high-performance joining. Similarly, z-pins can impart sandwich composites with a novel combination of multifunctional properties that include high structural, damage tolerance, electrical and damage sensing properties. Research gaps in our understanding of z-pinned laminates and sandwich composites are identified. Future research opportunities to create next-generation z-pinned materials with even more versatile multifunctional properties are described.

Review of z-pinned laminates and sandwich composites

This paper presents an experimental and analytical study into the importance of the skin–flange thickness on the strengthening mechanics and fracture modes of z-pinned composite T-joints The structural properties of unpinned and z-pinned carbon fibre–epoxy T-joints that had skin–flange thickness values between 2 mm (thin) and 8 mm (thick) were determined under tension (stiffener pull-off) loading Experimental testing revealed that the capacity of z-pins to improve the structural properties was strongly dependent on the T-joint thickness The joint properties increased at a quasi-linear rate with the skin–flange thickness, and z-pin pull-out tests showed that this was due to the increased crack bridging traction load and traction energy The increase to the structural properties of the z-pinned T-joints with increasing thickness is explained using the bridging traction laws for z-pinned laminates

Strengthening mechanics of thin and thick composite T-joints reinforced with z-pins

A cohesive zone model for predicting delamination suppression in z-pinned laminates

Predicting mode-II delamination suppression in z-pinned laminates

A numerical model is developed for predicting low-velocity impact damage in laminated composites. Stacked shell elements are employed to model laminate plies with discrete interface elements in pre-determined zones to model the onset and propagation of matrix cracks and delamination. These interface elements are governed by a bi-linear cohesive failure law. Cohesive element zone size is determined by a separate finite element analysis using solid elements to identify the stress concentration sites. In order to save the computational effort, low-velocity impact load is modelled by quasi-static loading. Influence of contact force induced friction on shear driven mode II delamination is modelled by a friction model. For a clustered cross-ply laminate, calculated impact force and damage area are in good agreement with the test results. It is shown that matrix cracks should be included in the model in order to simulate delamination in adjacent interface. The practical outcome of this research is a validated modelling approach that can be further improved for predicting low-velocity impact damage in other stacking sequences.

Predicting low-velocity impact damage in composites by a quasi-static load model with cohesive interface elements

This paper presents an FE model for predicting the performance and failure behaviour of a novel hybrid metal-composite joint with interlocking pins to increase resistance to debonding failure. A unit-strip model with cohesive interface elements at bond line was constructed to simulate the onset and propagation of debonding cracks. Two separate traction-separation laws for the cohesive elements are employed; one represents unreinforced adhesive properties and the other is used for the pin enhanced toughness. This approach can account for the so-called large scale bridging effect and also avoids using concentrated pin forces in the numerical models, thus removing mesh-size dependency and permitting more accurate and reliable computational solution. Predicted load-displacement relations are in good agreement with the test measurement especially the trend in the debonding damage and failure phases.

Francesco Bianchi

Papers

Finite element modelling of z-pinned composite T-joints

A cohesive zone model for predicting delamination suppression in z-pinned laminates

Predicting mode-II delamination suppression in z-pinned laminates

Predicting low-velocity impact damage in composites by a quasi-static load model with cohesive interface elements

A numerical model for hybrid metal-composite joints with through-thickness reinforcement