Experimental study of the effect of automated fiber placement induced defects on performance of composite laminates

Abstract

The Automated Fiber placement (AFP) process shows great potential for efficient manufacturing of large composite structures. However, uncertainties exist on mechanical performance of final product that are associated with the process induced defects. This experimental work investigates the effect of four principal defect types, namely gap, overlap, half gap/overlap and twisted tow on the ultimate strengths. Tests are executed at the lamina level (fiber tension, fiber compression and in-plane shear), as well as at the laminate level (open hole tension and open hole compression). Then each test is compared with a baseline configuration exempt from defects. Tests have revealed the minimal effects of a single and isolated defects on mechanical performance especially at the lamina level (around 5%) compared to the laminate level (up to 13%).

Introduction

Composite materials are now increasingly used as large and complex primary structures in the aeronautics industry, because of their advantages over traditional materials. However, their well known manufacturing weaknesses drive many companies to develop new methods to obtain better part quality. The recent introduction of airplanes with parts made by the Automated Fiber Placement (AFP) process has increased the demand for this method. An AFP machine consists of a computer controlled robotic arm with a placement head end effector that lays bands of prepreg strips onto a mould in order to construct the layup. The bands are made with 8–32 prepreg strips, called tows, which are aligned side-by-side by the placement head. For each ply, the machine accurately places the bands on the mould respecting the proper ply angles and the covering technique. The part is then placed in an autoclave to polymerize the resin material and consolidate the plies. Another advantage of this process is the possibility of making variable-stiffness laminates with curvilinear fiber path for the purpose of optimizing the composite structure. This technique has been proven to be effective to enhance the buckling load [1], [2], [3], reduce the effect of stress concentration [4], [5], [6], reduce the notch sensitivity [7], [8] and maximize the fundamental frequency [9]. However, in order to manufacture complex shapes or variable-stiffness parts, misalignments are induced on the band edges, which introduce gaps and/or overlaps. To overcome this situation, the software provided with the machine includes a cut-and-restart option that gives different approaches to limit these gaps and/or overlaps. Depending on the strategy chosen during the path generation [10], the machine can cut individual tows to create full gaps, full overlaps or a ratio of full gaps and full overlaps versus the initial defects. This option gives more control on the type of defect and on its size. In addition, the material and machine tolerances induce small random gaps and overlaps within the entire structure that cannot be removed, because they are an integral part of the processing. Furthermore, missing, twisted, or spliced tows are sometimes laid down during manufacturing, which create uncertainties and must be repaired during the process. Several researchers have established methods to reduce the effects of these defects on a composite structure. Some examples are: the staggering technique [6], [11], [12] and placement of ±45° layers on the top and bottom layers [13]. However, even with these techniques, the defects are still present and their influence on the performance of the structures is not well understood.

Over the years, research has demonstrated the feasibility of using the AFP process by testing panels and aerospace parts. From these studies it can be concluded that, overall, the parts perform similarly or better than those made by hand layup [7], [14], [15]. In a case study, Measom and Sewell [14] demonstrated the effect of the tow width (12.7 mm [0.5″], 19.1 mm [0.75″] and 25.4 mm [1″]) and the cut-and-restart option, both on the interlaminar shear performance. They revealed that the size of the bandwidth had minor effects on the interlaminar shear tests, which confirms the good consolidation obtained at the edges. Also, the gaps-fiber-placed configuration had equivalent strength to the hand layup tape, which draws attention to the small effect of the gap type of defect on the structural performance. These results confirmed the potential of this technology; however, more recent machines now use smaller tow width of 3.2 mm [0.125″] or 6.35 mm [0.25″].

Sawicki and Minguet [11] explored the effect of a gap width (0.76 mm [0.03″] and 2.5 mm [0.10″]) by means of the compression strength test (notched and unnotched specimens). It is concluded that any gap size induced a strength decrease. It is shown that the rate of this decrease is pronounced for gap sizes smaller than 0.76 mm [0.03″] and the strength reduction is relatively constant for larger gap sizes. They found that the compression strength reduction for unnotched and notched laminates was the same. The main failure mechanism for specimens in compression was caused by the out-of-plane waviness induced by the defects on the subsequent plies. Similarly, numerical analysis has demonstrated, on a variable-stiffness panel, that the stiffness is proportional to the amount of the tow-drop area and that cracks propagate more easily in a surface ply [12].

This can be explained by the strain perturbation of defects with the approximate ratio of 0.97 for the strain gap (resin rich area) and 0.89 for the strain overlap (stiff inclusion) region compared to the baseline average strain [5]. More recently, Turoski [16] conducted an experimental and numerical analysis to obtain the effect of the number of gaps for several mechanical properties, such as tension and compression for notched and unnotched quasi-isotropic laminates. It is concluded that in general, the unnotched specimens are more influenced by the defects than the notched specimens. This indicates that the stress concentration associated to the notch induces more strength reduction than the defects. Moreover, the amount of defects on the unnotched specimens is related to the strength reduction and confirms the performance reduction of gaps on structures. Overall, this recent research has given a small overview of the failure mechanisms induced by the gap type of defect; however, other types of defects have not been investigated experimentally and most of the results have been obtained by means of numerical analysis.

The lack of results from previous research creates uncertainties which justify the need to understand the real effects of the main defects. For this reason, the purpose of this investigation is to quantify the effect of the manufacturing induced defects by testing four principal types of defects, namely gap, overlap, half gap/overlap and twisted tow. To do so, all these defects have been tested at three lamina levels (fiber tension, fiber compression and in-plane shear) and two laminate levels (open hole tension and open hole compression). Experimental results have been compared with a baseline configuration without defects. To quantify the effect of defects on ultimate strength, a high toughness autoclave cure carbon–epoxy prepreg have been used.