Explicit Finite Element Simulation of Impact Damage in Composite Laminates
Fiber-reinforced composite laminates undergo internal damage under low velocity impact. Expeiments show that when impact energy exceeds a threshold, damage occurs in the forms of matrix cracking and inter-ply delamination. Interlaminar bonding, inter-ply layup and loading modes are found to have significant influences on failure behavior and energy dissipation associated with impact damage.
A cohesive finite element method (CFEM) is used to provide explicit modeling of the damage process. This model combines a cohesive crack surface relation and orthotropic material constitutive characterization. The effects of interlaminar bonding strength, laminate layup, loading mode and loading rate on the damage initiation and evolution under low-velocity impact are analyzed. Explicit CFEM simulations track the time-resolved history of damage initiation and growth. Finite deformation kinetics and a full dynamic framework of analysis are used. Details of the formulation can be in papers on the Publications list.
The following movie illustrates the process of the dynamic failure development in a composite beam under three-point bend loading due to low-velocity impact. The laminate consists of three plies. The top and bottom layers are 0-degree plies and the middle layer is a 90 degree ply. Damage initiates in the 90-degree ply and subsequently propagates and causes interlaminar delamination.
Damage evolution: matrix cracking followed by interlaminar delamination in Graphite/Epoxy composite beam with a 0/90/0 layup.
Time-resolved Impact Response and damage of Fiber-reinforced composite laminates
Fiber-reinforced polymer matrix composites are used in aerospace vehicles, marine vessels, automobiles and a variety of other structures. With their high specific strengths and elastic moduli, flexibility in design, and continually advancing manufacturing techniques, these materials offer definite advantages over other materials. However, composite laminates also have inherent weaknesses, susceptibility to impact damage being one example. Since many structures using laminated composites are likely to encounter impact by foreign objects in service, characterizing the impact response and quantifying the induced damage in these materials are important issues which have attracted significant attention of the researchers.
The analysis of impact response and damage entails the determination of the histories of relevant mechanical quantities such as contact force and displacement. Most impact experiments reported have been conducted using a drop-weight impactor or a propelled projectile system. Force profiles are usually measured using load transducers or inferred indirectly from readings from accelerometers or strain gages. Response models based on certain assumptions are often needed to determine or refine force or deformation histories. Inaccuracies often arise in such operations due to the indirect nature of the measurements and the need to use approximate models.
In the current investigation, a split Hopkinson pressure bar (SHPB) apparatus is used to characterize the impact response and damage of composite laminates. This technique provides direct measurement or inference of contact forces, contact-point velocities, displacement, mechanical work and energy dissipation within a fully dynamic framework. The mechanical quantities are measured in real-time with submicrosecond resolutions. Since the analysis is based on the theory of one-dimensional wave propagation associated with the elastic bars of the SHPB, no complicated data processing is necessary. The approach yields more accurate force and displacement measurements with better time resolutions than experiments using load cells or accelerometers. In addition, no assumption concerning the behavior of specimen material is needed. In the current analysis, while the full histories of contact force, contact-point velocity and displacement are determined, the focus is on the histories of work transfers between the impactor and specimen and energy absorbed in the specimen during impact.
The configuration uses a three-point bend fixture in a SHPB apparatus for controlled loading and real-time diagnosis [Fig.1]. The materials analyzed are IM7/K3B, a graphite-fiber polyimide-matrix composite, and S2 glass/5250-4, a glass-fiber bismaleimide-matrix composite. Experiments conducted characterize the responses of the materials over a range of impact velocity or impact energy. The post-impact tensile experiments indicate that, for given impact, the glass fiber composite retains a higher percentage of its tensile strength and stiffness than the graphite-fiber composite [Fig.2]. A monotonic relation is observed between the amount of energy absorbed by the specimen and the post-impact tensile strength of the laminate [Fig.3]. Postmortem ultrasonic vibration tests [Fig.4] are used to assess the extent of damage and its effect on post-impact strength. The mean square amplitude of the transverse vibratory response of impacted specimens is found to increase monotonically with increasing post-impact strength. A similar correlation is also observed between the post-impact strength and through-transmission wave speed in impacted specimens.