Dahsin Liu

Impact-Induced Delamination—A View of Bending Stiffness Mismatching

Fiber-reinforced composite materials are very susceptible to impact loading. The damage characteristics of a composite material subjected to subperforation impact can be classified as indentation, fiber breakage, matrix cracking, fiber-matrix debonding and delamination. Among these damage modes, delamination has been found to be important for all cases of loading. In this study, the damage characteristics of many composite plates made of glass/epoxy, Kevlar/epoxy, and graphite/epoxy were investigated. Delaminations in the impacted composite plates were inspected by high-intensity light, X-ray radiog raphy, ultrasonic imaging system and edge replication. It was found that delamination in every interface of impacted composite plates had a peanut-like shape. Since membrane ef fect could be ignored in this study, stresses caused by bending were the major stresses which should be responsible for the delamination. Although the interlaminar stresses had been analyzed by many investigators for the determination of delamination, in this study, however, the mismatching of bending stiffness between two adjacent laminae was pro posed as an indicator of delamination in the composite laminates subjected to subperfora tion impact. The effects of material properties, stacking sequence and thickness on the delamination were discussed. The justification of the hypothesis of bending stiffness mis matching was also presented.

AN OVERALL VIEW OF LAMINATE THEORIES BASED ON DISPLACEMENT HYPOTHESIS

With the use of a recursive technique, this paper presents an overall comparison of laminate theories based on displacement hypothesis. A generalized polynominal form is used to unify the displacement hypothesis. Both theories available in the literature and inferable from the existing theories are addressed. Firstly, Shear Deformation Theories are recognized to give good results for in-plane stresses but poor results for interlaminar stresses. However, Layerwise Theories give excellent results for both global and local distributions of displacement and stress (both in-plane and out-of-plane). A compromising theory, the Generalized Zigzag Theory, is presented. Due to its success in laminate analysis, a series of Quasi-layerwise Theories are presented. Unfortunately, a physical impossibility-coordinate dependency-takes place. It then requires a Global-Local Superposition Technique to formulate the laminate theories. By examining the results of Superposition Theories, it is concluded that the completeness of the terms is very important. Based on a Hypothesis for Double Superposition, this study presents three Double-Superposition Theories. They are verified to give excellent numerical accuracy, along with computational efficiency, when compared with elasticity solutions.

Matrix Cracking in Impacted Glass/Epoxy Plates

In addition to delamination, which has been found to be the major failure mode in a fiber-reinforced laminated composite plate subjected to subperforation impact, matrix cracking and fiber breakage have also been observed. Each failure mode, however, does not generate independently. On the contrary, the different modes interact with one another. Therefore, in order to have an overview of the failure mechanism in an impacted com posite plate, it is necessary to find out the mechanism of every failure mode and the in teractions among them. Microscopic observations of the cross sections of impacted com posite materials have revealed the interaction between matrix cracking and delamination. In this study, however, several techniques have been used to present the matrix cracking patterns in impacted glass/epoxy plates with different thickness, geometries, boundary conditions, stacking sequences, and types of loading. The matrix cracking patterns on the surfaces of impacted plates have been enhanced by dye penetrant while those of the in ternal laminae have been investigated by edge replication. Results have shown that the delamination area on the second interface of a three-lamina glass/epoxy plate coincides with both the central matrix cracking zone of the impacted lamina and the middle crack- free zone of the non-impacted lamina. The matrix cracks in the internal lamina are also restricted by the projected delamination area. The association between delamination and matrix cracking has been discussed and also the effect on matrix cracking of the tensile stresses introduced by the flexural wave.

Delamination Resistance in Stitched and Unstitched Composite Plates Subjected to Impact Loading

The delamination resistance of a composite plate subjected to low-velocity impact is dependent on the interlaminar strengths and the level of interlaminar stress. The former is strongly influenced by the material properties such as matrix toughness and fiber-matrix bonding strength while the latter can be affected by the specimen geometry and loading parameters. In this study, some thin composite plates were fabricated and sub jected to low-velocity impact. The delamination in every specimen was examined. Experi mental results revealed that the delamination area was proportional to the lamina thickness and the difference of fiber angle between adjacent laminae. A Mismatch Theory based on the difference of bending stiffness between adjacent laminae was used to interpret the cause of high interlaminar stress level. The effect of interlaminar strengths on the delami nation resistance was also discussed. It was found that through-the-thickness stitching could reduce the delamination area in a composite plate subjected to low-velocity impact as high as 40%.

On the relationship between impact energy and delamination area

Thin glass/epoxy plates fabricated from 3M prepreg tape were subjected to low-velocity impact. Delamination in the impacted composite plates was measured by edge replication and was identified as the major damage mode. The effects of fiber orientation, thickness, and lamination on the delamination resistance were investigated. Experimental results verified three previous findings: (1) a linear relationship holds between delamination area and impact energy, (2) the mismatch of bending stiffness between adjacent laminae can be correlated with the delamination area on the interface, and (3) the behavior of a thin composite plate under low-velocity impact is very similar to that caused by global bending. In addition, based on the calculation of impact energy per unit delamination area, the dynamic fracture energy and the energy of dissipation in the thin glass/epoxy plates can be examined.

Size effects on impact response of composite laminates

Delamination was known to be one of the most important damage modes in composite laminates subjected to impact loading. In an effort to further understand the impact response of composite laminates, various degrees of impact ranging from subperforation to perforation were introduced to glass/epoxy laminates through an instrumented drop-weight impactor. In addition, composite laminates of various in-plane dimensions and thicknesses were examined for in-plane dimensional and thickness effects, respectively. Experimental results showed that in-plane dimensional effect was not as significant as thickness effect. The impacted composite laminates were then subjected to compression after impact (CAI) tests for characterizations of residual mechanical properties. Experimental results showed that perforation was the most important damage stage in composite laminates subjected to impact loading since impact characteristics (peak force, contact duration and absorbed energy) and mechanical properties degradation (residual compressive maximum force and residual compressive absorbed energy) of composite laminates became stable once perforation took place. However, it was also found that delamination played a very important role in the characterizations of mechanical properties degradation. Since the impact response of composite laminates is due to plate bending to some extent, bending analysis was used to explain the greater influence of thickness effect to in-plane dimensional effect. It was also found that bending analysis was feasible for interpretation of delamination in mechanical properties degradation.

Characterization of Impact Properties and Damage Process of Glass/Epoxy Composite Laminates:

This paper investigates the responses of glass/epoxy composite laminates subjected to impact loading. It presents a new technique to characterize the impact properties and to correlate them with the damage process of the composite laminates. The technique, called the energy profiling technique, is based on instrumented impact tests and the least-squares method. The technique gives specific definitions of the penetration and perforation thresholds of the composite laminates. It also defines the range of the penetration process and quantifies the energy absorption efficiency of the composite laminates. The primary damage modes in the damage process of the composite laminates can also be correlated with the impact properties by using the energy profiling technique. The advantages of using this technique to analyze the experimental data obtained from impact tests are demonstrated by investigating the glass/epoxy composite laminates with various impactor sizes, laminate thicknesses, fiber orientations, and a sandwich construction. Results from the energy profiling technique show that the penetration and perforation thresholds increase nearly linearly with the size of the impactor while they increase nonlinearly with the thickness. A sandwich composite made of two glass/epoxy laminates and a foam material is found to have higher energy absorption efficiency than a bonded laminate. Among the five [05/θ 5/05] composite laminates investigated, where θ = 0, 15, 30, 45, and 90, [05/155/05] has the highest resistance to delamination and has the highest penetration and perforation thresholds. All these results clearly indicate the advantages of using the energy profiling technique for the analysis of composite laminates subjected to impact loading.

Impact perforation resistance of laminated and assembled composite plates

A previous study on impact response of composite laminates concluded that impact perforation was the most important damage stage in composite laminates subjected to impact loading since impact characteristics and degradation of mechanical properties of composite laminates reached critical levels once perforation took place. It was also found that thickness had a greater influence on impact perforation resistance than in-plane dimensions. However, as the composite laminates became very thick, the manufacturing cost for obtaining high-quality composite laminates could become unaffordable. In an effort to meet design requirements and to reduce manufacturing costs, assembled composite plates, which were organized by assembling multiple thin composite laminates together, were considered as alternatives to thick laminated composite plates. Various joining techniques including mechanical riveting, adhesive bonding, stitching and their combinations were used in assembling two- and four-laminate plates. Experimental results revealed that epoxy bonding outperformed other joining techniques. Although good bonding resulted in a higher impact bending stiffness and subsequently a higher perforation threshold, increasing the laminate thickness, or the number of laminates, was found to be more efficient in raising perforation threshold than improving the joining stiffness. As a major finding of the study, the assembled composite plates were found to have perforation thresholds similar to the laminated counterpart. Hence, the former could be used to replace the latter, at least, as far as perforation threshold was concerned.

Interlayer shear slip theory for cross-ply laminates with nonrigid interfaces

In the conventional analysis for laminated composite materials, the composite interface is always assumed to be rigidly bonded. However, due to the low shear modulus and poor bonding, the composite interface can be nonrigid. Based on the previously developed inter laminar shear stress continuity theory and a linear shear slip law, this study presents a so-called interlayer shear slip theory to investigate the effect of the interfacial bonding on the behavior of cross-ply laminates. Closed-form solutions for the cases of the cylindrical bending of long composite strips with [0/0] and [0/90] sequences are obtained. Numerical results for the laminates with different length-to-thickness ratios are presented. They reveal that the interlayer shear slip theory is valid for cross-ply laminates with rigid and nonrigid interfaces. It is also concluded from this study that at some special locations, namely, singular points, the transverse shear stress or in-plane normal stress remains insensitive to the condition of interfacial bonding.

Tensile Strength of Stitching Joint in Woven Glass Fabrics

Stitching has been found to be able to improve the interlaminar strength of composite laminates. Its application as composite joining has been also explored. This study examined the tensile strength of some stitched composite beams made of vomen glass fabric and epoxy matrix. The effects of stitching parameters on joining strength were evaluated by both experimental technique and finite element method