Engineering analysis and design software
Composites analysis

Case Study

Estimating Impact Damage in Honeycomb Sandwich Panels

Composite sandwich construction is used increasingly for aerospace applications in components such as the trailing-edge wedges of aircraft flaps. However, sandwich structures are generally poor at resisting impact damage. The Centre for Polymer and Composites Research at the University of Auckland used LUSAS Composite to develop a damage material model that will simulate core failure in sandwich panels under a range of impact loading.

On aircraft flaps, low speed impacts can occur from tool drops, hail, or debris thrown up from runways. High speed impacts can also occur with low mass bodies such as birds. Two different numerical simulations using the developed damage material model have been performed to give a comparative assessment of both types of impact with experimental testing.

The damage material model is based upon a combination of elastic and inelastic strain accumulation together with degradation of the modulus matrix. It requires an initial damage threshold, the gradient of the damage function, and the maximum permissible value of damage to be defined. From experimental data, the reduction in stiffness was found to be 60% for typical Nomex honeycomb, giving a modulus ratio of 40%. The damage material model was embedded within an axisymmetric, explicit dynamic analysis, additionally taking account of finite strain and large deformation behaviour.

In the soft body experiment, a rubber membrane filled with water was used as the impact projectile. To model this, an isotropic, elastic material with a low Young's modulus and a Poisson's ratio close to 0.5 was defined to allow constant volume deformation. Clamps on the panel boundary were represented by appropriate supports in the LUSAS model. Slidelines automatically handled the contact between the impacting body and the sandwich, and also tied the components of the sandwich model together. In all, 35,706 time increments over a total elapsed time of 6.44ms modelled the impact event.

During a soft body impact the projectile deforms over the surface of the sandwich causing a shallow, large plan area defect to develop and remain in the sandwich. The diameter and depth of the internal core damage predicted by LUSAS from this event compared very well with the experimental results. By plotting the contours of damage using LUSAS the extent of honeycomb crushing in the core of the sandwich could be easily seen.

Download an AVI file of the soft body impact (1.5Mb)

Animation of a Soft Body Impact

Overall, the analysis of soft impacts using the damage material model showed good correlation with experimental results in terms of the damage diameter and core crushing depth. The permanent deformation calculated using the damage model was found to be about twice that of the experimental panels. A combination of time dependant recovery and the sectioning of the panels to make measurements may account for this. The LUSAS model does, however, provide a value for the maximum deformation in the panel at peak impact.

In a hard body impact study using the same damage material model, a projectile impacting with a velocity of 15.3ms-1 normal to the sandwich surface was analysed using 28,000 time increments, over a total elapsed time of 2.4ms. The extent of the core crushing is shown by the arrows on the accomanying diagram and the left-hand boundary of core damage is shown by the circled area.

Results from this LUSAS analysis appear to underestimate the diameter of internal core damage. This underestimate occurs because the model, currently, does not sufficiently calculate low level core damage in a thin layer at the defect boundary that occurs in the experimental case. A component of the lower numerical estimate may also be attributed to differences in modelling the damage formation in a continuum as opposed to a cellular structure that exhibits discrete 3mm jumps in the size of damage. In this instance the damage is underestimated by approximately three cell diameters.

More detailed information on this case study

Hard impact damage


 

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