Damage tolerance of impacted curved panels
Introduction
Composite structures can be accidentally damaged by dropping tools, or in collisions with foreign objects in manufacturing, storage or operation. Although it is easy to detect this kind of damage in metallic structures, it is not in composite structures, particularly when the reinforcement used is carbon fibre. Therefore, since it is difficult to avoid accidents, it is necessary to evaluate the effects of damage. There are a number of damage tolerance studies for this kind of material [4], [9], [15], [16], [2], [14]. The procedure used consists of three steps: (i) damage initiation, either using real conditions or reproducing them as closely as possible, (ii) damage inspection, evaluating its nature and topology, and (iii) the quantification of the residual strength, when the structure is stressed under simulated real conditions. Having determined the maximum structural damage that can be tolerated for a given residual strength, there is often a need for structural re-design.
The aim of our study is to evaluate the influence of damage on the residual strength of carbon/epoxy vessels. This generally requires a great number of test results. Study of the real structures is impossible because of the excessive cost involved. Hence specimens are extracted from these structures. In this case, curved panel specimens are extracted from tubes.
Although the literature contains papers concerning the residual burst strength of impacted composite tubes [6], [10], few studies have focused on damage tolerance of composite curved panels, especially when loaded in tension along the longitudinal axis. Our study is a first step to understanding the physical phenomena which take place when shear or normal stresses (tensile or compressive) are induced at the mesoscale of the multilayered composite. The main damage mechanisms observed, in impacted vessels filled with propellant, are fibre failure and localized delamination. Since fibre failure is the most critical damage mechanism for the tension strength of composite specimens, defect initiation tests were chosen to generate this damage mode and thus cause a significant loss of strength during quasi-static tension tests. Delamination is deliberately limited here to isolate the influence of fibre failure on the residual strength.
Two different methods can be used to predict structural damage due to impact. The first is semi-analytical modelling, in which the interactions between damage mechanisms (fibre failure, delamination …) and the coupling between local damage and the global response of the structure, are considered. However, the boundary conditions and the contact conditions in impact tests are difficult to evaluate. Since the phenomena occur on a millisecond timescale, it is only possible to identify them experimentally by doing interrupted tests. This strategy was not used because of the small number of samples available.
The second modelling method is empirical using an experimental design [11]. This “black-box” model links quantities, representative of damage, to impact parameters. Impact kinetic energy was not chosen as an input, since very different modes of failure can correspond to the same kinetic energy, according to the mass–velocity couple [11]. Therefore, we chose to consider the mass and velocity as independent parameters. It is possible to explore a parameter domain, consistent with the real conditions, while minimizing the number of tests required.
Section snippets
Material and specimens
The material investigated was made of T800HB carbon fibre and epoxy resin (class 120 °C). Known as CS603 W, its mechanical properties are listed in Table 1. The fibre volume fraction is 60%. The specimens were cut from tubes manufactured by filament winding.
The tubes were manufactured by winding circumferential layers and longitudinal layers on to a mandrel. With the mandrel rotation axis referred to as the 0° axis, the circumferential layers have 90° orientation and the longitudinal layers have
Damage initiation
The first step in a damage tolerance study is damage initiation. The experimental device used to simulate potential accidents is as follows.
Damage assessment
The damage assessment was the second step of the study. The suspected damage mechanisms were delamination and fibre failure. To detect these, ultrasonic non-destructive inspection was used for delamination and optical microscopy examination for fibre failure.
Residual tensile strength
The determination of the residual tensile strength of impacted specimens is the last phase of the study.
Conclusions
For this study, a specific procedure has been set up to quantify the residual tensile strength of impacted composite vessels filled with propellant. In order to reduce the cost of the experimental tests, the specimens used were curved plates extracted from tubes. An experimental design was used to link impact parameters to the residual tensile strength of specimens. This methodology reduces the number of specimens required.
Impact tests reproduced specific types of accidental damage such as from
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