Vinay K. Goyal

Intralaminar and interlaminar progressive failure analyses of composite panels with circular cutouts

A progressive failure methodology is developed and demonstrated to simulate the initiation and material degradation of a laminated panel due to intralaminar and interlaminar failures. Initiation of intralaminar failure can be by a matrix-cracking mode, a fiber-matrix shear mode, and a fiber failure mode. Subsequent material degradation is modeled using damage parameters for each mode to selectively reduce lamina material properties. The interlaminar failure mechanism such as delamination is simulated by positioning interface elements between adjacent sublaminates. A nonlinear constitutive law is postulated for the interface element that accounts for a multi-axial stress criteria to detect the initiation of delamination, a mixed-mode fracture criteria for delamination progression, and a damage parameter to prevent restoration of a previous cohesive state. The methodology is validated using experimental data available in the literature on the response and failure of quasi-isotropic panels with centrally located circular cutouts loaded into the postbuckling regime. Very good agreement between the progressive failure analyses and the experimental results is achieved if the failure analyses includes the interaction of intralaminar and interlaminar failures.

An Irreversible Constitutive Law for Modeling the Delamination Process using Interface Elements

An irreversible constitutive law is postulated for the formulation of interface elements to predict initiation and progression of delamination in composite structures. An exponential function is used for the constitutive law such that it satisfies a multi-axial stress criterion for the onset of delamination, and satisfies a mixed mode fracture criterion for the progression of delamination. A damage parameter is included to prevent the restoration of the previous cohesive state between the interfacial surfaces. To demonstrate the irreversibility capability of the constitutive law, steady-state crack growth is simulated for quasi-static loading-unloading cycle of various fracture test specimens.

COHESIVE-DECOHESIVE INTERFACIAL CONSTITUTIVE LAW FOR THE ANALYSES OF FATIGUE CRACK INITIATION AND GROWTH

A mechanistic analytical model for crack initiation and growth due to either monotonic loading or fatigue loading is presented. A cohesive-decohesive constitutive law is postulated for the mechanical behavior of interfacial surfaces. The interfacial surfaces enable for the possible formation of discrete new crack surfaces. The softening law satisfies a multi-axial stress criterion for crack initiation and a mixed mode fracture criterion for crack growth under monotonic interfacial loading. A damage evolution law is postulated to prevent the restoration of the previous cohesive state between the interfacial surfaces, and to account for the energy dissipated associated with a possible interfacial unloadingreloading cycle. The evolution law for damage accumulation is a function of the maximum and minimum effective interfacial displacement jumps and it is a function of crack growth rate material characterization properties. Damage accumulates not only along the softening path of the constitutive law but also along an unloading path, enabling the simulation of subcritical crack growth with energy dissipation levels less than the material fracture toughness. The cyclic crack growth rate versus applied stress intensity factor range curve emerges naturally including the threshold and Paris law regimes. The ability of the method to reproduce qualitatively the experimentally observed fatigue life curves of the double cantilever beam and the center crack tension are particularly noteworthy.

Predicting Failure of Damaged Composite Sandwich Structures Using Compression-After-Impact Strength Data

*The damage tolerance properties of a composite sandwich structure are investigated with a Progressive Failure Methodology (PFM) to predict the compression-after-impact (CAI) strength of a composite sandwich panel. The PFM, previously correlated to coupon-level CAI strength data, is incorporated within a finite element model to establish the existing level of damage and predict the initiation and propagation of damage. The modeling techniques have been shown to match the CAI response of a coupon-level sandwich and to quantify important failure parameters. This paper extends the analysis tool to the response of a larger structure. The relative size of the damage to the overall structure was studied with a panel-level model. The model was used to predict the failure of an impacted panel, showing local instability at the delamination location followed by a rapid propagation of fiber failure as compression continued. The analysis predicted a significant decrease in strength of the impacted panel, although not as severe as predicted by a coupon-level model.

Modeling Failure of 3D Fiber Reinforced Foam Core Sandwich Structures with Defects

Foam core sandwich composites are widely used in primary structural components of launch vehicles and spacecraft. These structures exhibit complex failure modes that are sensitive to butt-joints, core mismatches, impact damage, voids and facesheet delaminations. 3D Fiber Reinforced Foam Core (3DFRFC) represents a new class of core material designed to replace standard foam core in future aerospace structures. An analysis, test, and nondestructive evaluation program was developed to estimate the strength reduction for debonds between the facesheet and the 3DFRFC. A compression test method was selected to evaluate fully bonded samples and samples with debonds of varying diameters: 0.5 inch, 1.0 inch, 1.5 inch, and 2.0 inch. The manufactured debonds were verified using throughtransmission ultrasonic inspection. Nonlinear finite element analysis with a progressive failure methodology was used to understand the failure processes and predict strength reduction. The typical failure mode for 0.5 inch, 1.0 inch, and 1.5 inch debonds was facesheet compression failure, while analysis showed that for larger debonds failure consisted of a rapid sequence of buckling, delamination, and fiber failure. Testing and analysis demonstrated significant strength reductions for the 2.0 inch debond.

Predicting and Measuring the Strength Reduction of Sandwich Structures with Spliced Foam Cores

An analytical and test program was designed to gain understanding of the failure mechanisms and strength reductions that occur in sandwich structures containing single core splices (butt joints) or intersecting core splices (T-joints). A three point bend test configuration was used to induce a state of nearly uniform shear stress and to evaluate the effect of butt joints. Finite element modeling with a damage zone criterion was used to predict the strength reduction due to these features. The predicted strength reduction was in excellent agreement with test data. The analysis also provided insight into several unexpected phenomena observed during testing. Furthermore, the location of failure initiation was identified through analysis and confirmed with high speed photography. This investigation demonstrated that core splices with certain adhesive materials may only result in slight strength reductions.

Techniques for Finite Element Analysis of Clamp Band Systems

This paper presents two computational techniques for determining the structural capability of clamp band systems, which are typically used in launch vehicle applications . Both techniques utilize three -dimensional finite element models (3D FEM) and consider contact between all the components of the clamp band system. One technique models the fu ll circumference of the clamp band structure , while the other technique models a circumferential slice of the clamp band system and uses cyclic symmetry. The analytical predictions for the full 3D model show reasonable agreement with test data. The cycli c symmetry model underestimates the capability of the hardware to withstand applied loads . Numerical s tudies were also performed to investigate how physical parameters , such as friction and band pre -tension, affect the structural capability of the clamp ba nd system .

Identification of Failure Mechanisms in Sandwich Structures with Foam Core Thickness Mismatches

An analytical and test program was designed to gain understanding of the failure mechanisms and strength reductions that occur in sandwich structures containing two sections of foam having different thicknesses, typically joined by a butt joint. A three point bend test configuration was used to induce a state of nearly uniform shear stress and to evaluate the effect of butt joints. Three distinct failure modes were identified using finite element analysis and these modes were confirmed with high speed photography of three point bend test specimens. The investigation demonstrated that the failure mode changed as the mismatch height increased. A distinct failure criterion was proposed for each failure mode and the resulting strength predictions were in reasonable agreement to test data. This study found that sufficiently small thickness mismatches do not cause any strength reduction, while a sufficiently large thickness mismatch can cause significant strength reduction.

Predicting Compression-After-Impact Strength of Composite Sandwich Structures

In order to gain a better understanding of the damage tolerance properties of a composite sandwich structure, a computational Progressive Failure Methodology (PFM) was incorporated into a finite element model representing a Compression-After-Impact (CAI) test. The damage due to a low-velocity impact was simulated to serve as the initial condition for the CAI investigation. The results from the present study indicate that delamination size and location play a crucial role in the CAI strength of a composite sandwich panel. Other parameters such as matrix damage, coupon geometry and residual stresses are shown to influence the response of the CAI test. The model predicts a local instability due to delaminated plies leading to a premature failure of the damaged facesheet. The analysis results compares favorably with experiments. I. Introduction As composite sandwich structures are increasingly used in structural applications within the aerospace, transportation and marine industries, understanding their response after sustaining damage becomes more and more critical. However, quantifying the effects of damage on their residual mechanical properties remains a difficult task. The benefits in structural performance and efficiency gained through the use of composite sandwich construction can be quickly lost if the structure suffers damage to its constituent materials. This damage can be difficult to detect, adding to the complexity of damage tolerance characterization. Particularly detrimental to the residual properties of composite materials is impact damage. Specifically, low velocity impacts on sandwich structures can induce fiber breakage and matrix cracking in the facesheets, delamination between facesheet plies, debonding between the facesheets and core, and core crushing. Understanding these damage processes and how they act to reduce the loadcarrying capability of the structure is of utmost importance in structural assessments. The residual compressive strength of a composite sandwich has been shown to be the property most affected by low velocity impact damage 1 .The most common and effective method of measuring a composite structure’s damage tolerance is through a Compression-After-Impact (CAI) test. CAI testing is focused on characterizing the effect of impact damage by determining the reduction of compressive strength due to foreign object impacts on a composite structure. Various methods are available to characterize the CAI strength of a structure but in every case, a postimpact strength is compared to the compressive strength of an undamaged specimen. CAI testing can be sensitive to the test method used and, in this respect, results are somewhat qualitative when comparing different studies and material systems. Nevertheless, CAI properties are an essential part of determining how a structure and its constituent materials will retain strength after an impact event. In this study, a computational method that accounts for damage and its progression in a composite material will be employed to model the damage and compressive failure of a composite sandwich structure previously subjected to a low velocity impact event. This computational Progressive Failure Methodology (PFM) can predict not only the initiation of damage but also its progression as the damage process evolves. A finite element model (FEM) was constructed and integrated with the PFM to model the configuration of the CAI test. In a previous study, the PFM was incorporated into a model of a composite sandwich panel subjected to a low velocity impact event that revealed important physics of the impact damage process 2 . The current study represents an expansion of that low velocity impact characterization by predicting the residual strength of the impacted sample. The damage state of the composite sandwich structure following the impact was preserved and a portion of the model was then loaded in compression to replicate the CAI test process.

Enhancement to the Interfacial Element Formulation for the Prediction of Delamination

The effect of through-the-thickness compressive/tensile loads on the strength and response of textile composite blocks loaded in an off-axis configuration with respect to the stack direction is investigated using interface elements. Published data suggests that the composite blocks fail primarily by delamination. Thus, to predict the strength of the blocks, interface elements are placed between adjacent lamina. Recently postulated interfacial constitutive laws utilize a simple quadratic strength-based failure criterion that is shown to significantly underpredict the compressive strength of a composite block loaded 25-degrees off-axis with respect to the stack direction. The generally-used failure criterion is inadequate because it does not consider the apparent increase in shear strength when a composite is subjected to through-the-thickness compressive stress. An empirical failure criterion that considers the increase in shear strength due to the compressive stress and produces excellent correlation with experimental data was adopted and incorporated into the interfacial constitutive law. Strength predictions obtained from numerical simulations using the enhanced constitutive law were in reasonable agreement with published test data for rectangular specimens loaded in tension and compression. The numerical simulations captured the rapid delamination growth and catastrophic failure of the test specimen configurations. The predicted structural response is in good agreement with the measured response in compression when the finite element models consider the transverse nonlinear shear constitutive behavior.