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Postprint
This is the accepted version of a paper presented at CFRAC, 14 – 16 June 2017, Nantes.
Citation for the original published paper:
Carlsson, J., Isaksson, P. (2017)
High-velocity crack speed in wood fibre composites: an experimental and numerical study
In: CFRAC 2017, International Conference on Computational Fracture and Failure of Materials and Structures, Book of abstracts
N.B. When citing this work, cite the original published paper.
Permanent link to this version:
http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-367100
CFRAC 2017 - the Fifth International Conference on Computational Modeling of Fracture and Failure of Materials and Structures
J. Carlsson
1, P. Isaksson
1 Solid Mechanics, The Ångström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden – jenny.carlsson@angstrom.uu.se
How fast does a mode I crack propagate? Here, the question is addressed by a series of experiments, using a high-speed camera to track the crack propagation. A strip specimen geometry was chosen for the experiments, since it provides fairly easy interpretation of the results [1]. Two different materials were studied: one wood fibre composite, consisting of a PLA matrix and natural wood fibres, and one homogenous plastic, consisting of the PLA matrix material without fibres. Experiments were performed under different humidity conditions, and for different initial crack lengths.
The experimental results are compared to numerical results. In the numerical analysis, a dynamic phase field model is used to simulate crack propagation. In a phase field model, a crack is represented by a phase field 𝑑, which is defined over the entire body Ω, and has values 1 in the proximity of the crack and 0 elsewhere. The method is based on the principle of energy minimisation, and the governing equations are obtained from variation of the Lagrangian,
𝐿 = 𝜓& 𝑑𝛺 −
*
[ 1 − 𝑑 - 𝜓./+ 𝜓.1} 𝑑𝛺 −
*
𝐺4 𝑑- 2𝑙 + 𝑙
2𝛻𝑑 ∙ 𝛻𝑑 𝑑𝛺
*
.
Here 𝜓
&is the kinetic energy density, 𝐺
4the fracture toughness and 𝜓
.the elastic energy.
The elastic energy is split into 𝜓
./and 𝜓
.1, and only 𝜓
./is degraded by the crack phase field 𝑑. The use of a phase field extends the crack evolution to all possible crack states in the body, and thus the model is capable of capturing e.g. branching phenomena without additional criteria.
It is observed that crack branching is more common in the PLA specimens than in the fibre composites, for both wet and dry specimens. This implies that there are other energy consuming mechanisms present in the composites, such as fibre fracture and fibre pull- out. The experiments also support previous results, in that the crack propagation velocity increases for shorter initial crack lengths and that the energy required to cause fracture increases as the initial crack length decreases [1]. In sum, the observations indicate that while the composites qualitatively behave according to prevailing theories, for complex and fibrous materials such as wood and fibre composites, microstructural effects need to be taken into account.
References
[1] F. Nilsson, Crack propagation experiments on strip specimens, Eng. Fract. Mech. 6 (1974) 397–403.
