A CAI study on transition zones of conventional and Double-Double laminates

Laminate-thickness tapering opportunities of Double-Double (DD) laminates are unique, compared to conventional laminates (denoted as Quad) in aerospace, which are typically composed of 0°, 45°, −45°, 90°plies. The more aggressive tapering concept of DD, with drop-offs located on laminate’s outer surfaces, promises simplification in terms of manufacturing. However, the DD concept bears the risk to impede crack propagation after impacts negatively, as no full plies cover building-block run outs.

The present article utilizes conventional CAI (AITM-1-0010) infrastructure to examine how the characteristics of DD and Quad laminates deviate, when laminate transition zones experience impact loads.

Sample dimensions and the overall testing procedure was executed as close as possible to the AITM norm, which is usually intended for testing quasi-isotropic, 4 mm thick laminates. The study focuses on M21E/IMA UD carbon-fiber epoxy prepreg.

A tapered sample represents the key object of the present experimental study. It features a laminate transition from 16- to a 32-ply region, with a 1:10 ramp. Both regions are quasi-isotropic. The individual ply run outs are distributed along the transition zone (staggering), as it is done in industry. The examined DD laminate represents a structural equivalent of the Quad laminate (identical $\left[A\right]$ matrix). The transition zone shows 4-building-block run outs.

The tapered samples are impacted from both sides, to assess the effects of the differences in laminate architecture. Constant-thickness, 16-ply and 32-ply, samples complement the tests of the tapered samples. The study features a delamination area assessment, based on ultra-sonic scans, as well as the analysis of CAI tests.