Low velocity impact behavior of composite sandwich panels

This paper is submitted in honor of Professor Jack R. Vinson of the University of Delaware. This is a tribute recognizing Prof. Vinson's outstanding and enduring contributions to the field of composite materials in general and more specifically to structural applications of composites and composite sandwich structures. In addition, one of the authors (Daniel) feels privileged and thankful to Professor Vinson for his personal friendship.
https://doi.org/10.1016/j.compositesa.2004.11.014Get rights and content

Abstract

Composite sandwich structures are susceptible to low velocity impact damage and thorough characterization of the loading and damage process during impact is important. The objective of this work is to study experimentally the low velocity impact behavior of sandwich panels consisting of woven carbon/epoxy facesheets and a PVC foam core. Instrumented panels were impacted with a drop mass setup and the load, strain, and deflection histories were recorded. Damage was characterized and quantified after the test. Results were compared with those of an equivalent static loading and showed that low velocity impact was generally quasi-static in nature except for localized damage. A straightforward peak impact load estimation method gave good agreement with experimental results. A contact force–indentation relationship was also investigated for the static loading case. Experimental results were compared with analytical and finite element model analysis to determine their effectiveness in predicting the indentation behavior of the sandwich panel.

Introduction

The use of composite sandwich structures is expanding in the aerospace and marine industries, as well as in other areas where a lightweight material with high in-plane and flexural stiffness is needed [1]. The concept behind these structures is the separation of relatively stiff, strong and thin facesheets by a lightweight and thicker flexible core. The proper design and application of sandwich construction depends on a thorough characterization and understanding of not only the sandwich constituent materials (facesheets, core, and adhesive), but also of the structure as a whole under quasi-static and dynamic loadings. Sandwich structures are known to be susceptible to impact damage by foreign objects [2], [3]. This type of damage and more specifically, the response of composite sandwich panels under low velocity impact, is the focus of this study. Although the induced damage may not be readily apparent, its effects on the strength and reliability of the structure can be quite detrimental. Several common failure modes have been identified, including core indentation/cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core [4], [5], [6].

Because of its complex nature, the investigation of low velocity impact on composite sandwich structures remains an active research topic and has received much attention. Composite sandwich beams have been studied to characterize failure processes and damage [7], [8], [9], [10], [11]. This two-dimensional approach somewhat simplifies analysis and gives a cross-sectional view of the damage. However, a sandwich panel can provide a more complete deformation and damage response. Most of the previous impact research conducted on panels has focused on the analysis of impact dynamics, the characterization of impact damage, and the determination of the post-impact mechanical properties of the composite structure [2]. A wide range of materials, geometries, and facesheet layups have been used [12], [13], [14], [15], [16], [17].

In this study, simply supported sandwich panels consisting of woven-carbon/epoxy facesheets and PVC foam core were loaded under central point impact in a drop weight apparatus. Identical sandwich panels were also tested under central point quasi-static loading for comparison. The low velocity impact response of the current sandwich panel was determined by means of a detailed load–strain analysis and damage characterization. Because of the relatively dense core and woven carbon facesheets, the contact forces, along with the impact energy required to produce damage, are quite high compared to the findings in the referenced works above. Although much research already exists on the study of low velocity impact of composite structures, new configurations are continually being developed and need to be characterized. The contact force–indentation relationship for sandwich panels is an area that requires additional investigation as well. Analytical models and a finite element analysis were compared with experimental results on indentation for the static loading case.

Section snippets

Constituent materials and fabrication

The facesheets of the sandwich panel were woven-carbon fabric/epoxy laminates (AGP370-5H/3501-6S). This AS4-based carbon fabric was a five-harness satin weave with the same tow count in the warp and fill directions. The matrix is an amine-cured epoxy resin. The facesheets were laminates made of four plies of prepreg, resulting in a cured plate with a thickness of 1.37 mm and a fiber volume ratio of 0.62. Table 1 lists measured mechanical properties of this woven carbon composite.

The core used

Quasi-static loading

Load vs. displacement curves for the static loading test are shown in Fig. 3. The displacements of both the top and bottom facesheets at the panel center are given. The difference between the top and bottom facesheet deflections is due to indentation of the facesheet with core crushing. The top deflection increases at a nearly linear rate and a portion is recovered upon unloading. The portion that is not recovered is due to permanent indentation. Facesheet damage was initiated at a load of 17.3 

Conclusions

Besides the localized effects caused by load contact characteristics, the quasi-static and low velocity impact behavior of composite sandwich panels composed of woven carbon fabric/epoxy facesheets and a PVC foam core investigated in the current study are quite similar. In this respect, the low velocity impact response of plates can be characterized as quasi-static in nature. This conclusion is based on the comparison of a quasi-static test and multiple impact tests on sandwich panels and an

Acknowledgements

This research was sponsored by the Office of Naval Research (ONR). We are grateful to Dr Y.D.S. Rajapakse of ONR for his encouragement and cooperation. We also acknowledge the efforts of Dr J.W. Yoo for developing and implementing the ABAQUS finite element model for this paper.

References (25)

  • J.A. Nemes et al.

    Low-velocity impact response of foam-core sandwich composites

    J Compos Mater

    (1992)
  • F. Edgren et al.

    Compressive failure of impacted NCF composite sandwich panels—characterisation of the failure process

    J Compos Mater

    (2004)
  • Cited by (0)

    View full text