Material characterisation for crash modelling of composites
Doktorsavhandling, 2018

The transport industry must find solutions to reduce its impact on climate change. A promising way to reduce the weight of vehicles and therefore to reduce the CO2 emissions is to introduce components made of lightweight composite materials, in particular carbon fibre reinforced plastics (CFRPs). Aside from the new design possibilities for lighter vehicle structures, CFRPs can also potentially offer improvements in terms of crash performance in comparison to traditional metallic structures.

During crushing of composite structures, energy is absorbed through the stable progressive failure of the structure. The crushing process is a complex phenomenon involving the interaction of different competing failure mechanisms and frictional interactions taking place at different scales in the material. Today there is no reliable numerical tool to predict the behaviour of composite structures in crash scenarios, which is a hindrance to the introduction of composite materials in mass-produced vehicles. Joint research efforts from both numerical and experimental perspectives are needed to fill this gap.

In this doctoral thesis experiments are carried out to extract relevant material properties for crash modelling, and to assist in the development and the validation of numerical models as a first step of a building block approach with increasing structural complexity. The material selected for the study is a carbon fibre/epoxy uni-weave non-crimp fabric (NCF) composite. The first step in the material characterisation is to extract the different strengths and stiffnesses of the material, which requires dedicated tests because of the orthotropic nature of NCFs. Because several compressive failure mechanisms are driven by the shearing of the matrix polymer, a methodology is presented to extract the damage evolution laws from Iosipescu shear tests and indirect shear tests (uniaxial and biaxial compression tests). A quasi-static test method that uses crush coupons of simple geometry is proposed to measure the crush stress of composite plies for different fibre orientations and to characterise the associated crushing mechanisms. The experimental results of the crush coupons are then compared to blind predictions from finite element simulations to assess the predictive capabilities of a ply-based material model coupling damage and friction in a continuum damage approach. This material model is currently being developed in parallel to this thesis. Its aim is to pre-emptively simulate structural tests in order to optimise the design of crashworthy structures and to limit the number of physical tests.

mechanical testing

finite element analysis (FEA)

non-crimp fabric (NCF)

damage mechanics

carbon fibre composite

crushing

VDL, Tvärgata 4C, Chalmers
Opponent: Prof. Ivana Partridge, University of Bristol, UK

Författare

Thomas Bru

Chalmers, Industri- och materialvetenskap, Material- och beräkningsmekanik

In 2015, the transport sector contributed to nearly 30% of the total EU-28 greenhouse gas emissions. The figure decreases to 21% if international aviation and maritime emissions are excluded. The transport industry must therefore find solutions to reduce its impact on climate change.

A promising method to reduce the weight of vehicles and therefore to their CO2 emissions is to introduce components made of lightweight composite materials, in particular carbon fibre reinforced plastics. On medium size cars, weight savings as high as 35% can be achieved by replacing steel structures with structures made of composite materials, and so without any loss in mechanical performances (strength and stiffness). In addition, it has been shown that composites structures can potentially absorb more energy than metallic structures in crash situations. Higher energy absorption in crash yields higher safety of the occupants thanks to reduced deceleration loads.

Unfortunately, reliable simulation of the crash behaviour of composite structure has been identified as one the bottle necks for the introduction of composite materials in cars. With the aim of increasing the level of confidence in crash simulations, physical tests must be carried out in order to 1) extract relevant material properties to input to the simulation tools and to 2) validate the predictions of the numerical crash simulations.

In this work, a simple test method is developed to experimentally characterise the crushing behaviour of composites. The experimental results are compared the simulation results obtained from a project conducted in parallel to this thesis. The aim of the simulations is to pre-emptively predict the crushing behaviour of composite structures in order to optimise their design in terms of energy absorption and to reduce the number of physical tests which are associated with high costs. In addition, experimental methods are developed with the aim of extracting material parameters required as input to material models in simulation codes. It is important to carefully measure the mechanical response of composite materials under shear forces (shear forces are pairs of equal and opposing forces acting on opposite sides of an object, like the forces created when using a pair of scissors). Therefore, a methodology is proposed to characterise the shear response of composite materials and to calibrate crash models for composites from the measured shear response.

Drivkrafter

Hållbar utveckling

Ämneskategorier

Teknisk mekanik

Kompositmaterial och -teknik

Styrkeområden

Materialvetenskap

ISBN

978-91-7597-805-5

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4486

Utgivare

Chalmers tekniska högskola

VDL, Tvärgata 4C, Chalmers

Opponent: Prof. Ivana Partridge, University of Bristol, UK

Mer information

Senast uppdaterat

2018-11-20