CAE-Companion-2018-2019

Engineering WISSEN CAE

Computational Approaches and Simulation of Progressive Damage in Composite Structural Components NEW

Simulation of Fatigue Damage in CompositeMaterials Composite materials impose new demands on testing, pro- duction and simulation. The combination of various materials, various fiber topologies and manufacturing methods results in numerous influences that simulations must be taken into account to manufacture components with optimum properties. Different damage mechanisms must be distinguished. On a material (micro-scale) level: the damage mechanism can be, for example, a matrix crack, fiber-matrix debonding, fiber breakage, or delamination, Figure 1

fact that material parameters are used at the layer level and thus need only be determined once. No further tests are necessary when the layup design is changed. Fatigue Behaviour in three stages Various damage mechanisms are at work in composite mate- rials. Early damage with substantial loss of stiffness is typically followed by an extended phase of stability prior to the failure phase, Figure 2. Comparative testing is more feasible for a stiffness fall-off curve graph than a SN-curve, which is usually used for fatigue, as the point of failure is scattering much more than the stiffness behavior of the test specimen.

Figure 2: Typical phases of stiffness fall-off (© Siemens PLM) A simplified (i.e. one-dimensional without interaction to other damage modes) model with fatigue damage behavior in three stages as derived from [2] can be written as = 1 (− 2 √ )+ 3 2 ( 5 〈 − 4 〉) Where dD/dN denotes the damage of one cycle (here in stresses σ a ) at the pre-damage D . These curves could also be replaced by virtual tests instead of using graphed stiffness reduction measurements. These would calculate the stiffness reduction curves based on pure material characteristics and a precise production simulation. Research projects investigating this issue are underway [4]. Hybrid approaches – measuring master data that are then adjusted based on local properties such as voids, moisture etc. – are also key options within this process. For short-fiber reinforced components, see [5] as an example.

Figure 1: Damage in the layer (© Siemens PLM) Different types of load, for example, static load, shock (impact) and fatigue loading lead to different damages, which may also interact. Although such damage may emerge early on in the useful life of the component, complete failure only happens much later. Nevertheless, changes in local stiffness and stress redistri- bution can develop, which, in turn, may affect the behavior during the lifetime. Accordingly, different approaches to simulation are required. Continuous DamageModel In the continuous damage model (CDM), damage influences are homogenized at the layer level in the directions dictated by fiber alignment, In other words damage is considered parallel and transverse to the fiber direction as well as shear damage. The advantage of these approaches is the ability to describe damage based on different load types (static, shock, fatigue) using the same or compatible models and therefore can be combined. This also means that many material parameters for the different types of load only need to be determined once. A further crucial advantage of this modeling approach is the

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