Quasi-isotropic Panels Research Articles

Characterization and comparison of thin ply IM7/8552 composites processed by automated tape placement and hand layup

Manufacturing of large composite structures is labor-intensive and challenging. Traditional processing methods can be replaced by automated approaches such as Automated Tape Placement (ATP). The focal point of this effort is on thin-ply thermoset prepreg materials which have shown better resistance to damage tolerance under applied stress when compared to standard ply-thickness materials in recent times. However, composites processed by thin-ply materials require more placement iterations to achieve the same net volume of coverage as the standard ply-thickness materials. This can increase the probability of placement defects such as gaps and overlaps, which may degrade the mechanical performance of the composite. Modifications to the existing ATP system are made to address the issues of placement defects, which resulted in improved accuracy for thin ply placement. Unidirectional and quasi-isotropic panels are fabricated from 12’’ wide unidirectional prepreg sheet with the conventional hand layup (CHL) method for the baseline property comparison with panels fabricated by the ATP process using a ¼’’ wide slit tape. This paper investigates the effects of processing defects and material variability on the mechanical properties such as tensile, Open hole tension and Bearing tests of thin ply composites. In addition, the microstructure of the fabricated thin-ply composite panels is analysed using ultrasonic C-scans and confocal microscopy to examine the processing and material quality.

A numerical study on stress wave propagation in quasi-isotropic stacks of Dyneema® compliant composite laminates

This study aims to identify the ballistic mechanisms of quasi-isotropic (QI) panels made from compliant composite laminates, focusing on the propagation of various stress waves. Three hypotheses are proposed that in terms of the propagation of stress waves, incorporating ① longitudinal, ② transverse, and ③ bending waves, QI panels would perform better than the aligned (AL) counterparts, based on simplified theoretical analyses to provide a theoretical basis for the study. 2-, 3-, and 4-ply QI panels are found to absorb approximately 23 % more energy than the aligned (AL) counterparts in the ballistic tests. The finite element (FE) models are developed using hierarchical and global-local approaches, and the results are well agreed with the experimental ones. The results elucidate the influence of QI stacks on the propagation of stress waves. The ballistic mechanisms are ultimately explored and elucidated.

A numerical study on the influence of quasi-isotropic structures on the ballistic performance of para-aramid woven composites

Aimed at improving the ballistic performance of woven fabric reinforced composites, quasi-isotropic structures were developed. This paper reports on the ballistic impact performance and mechanisms of multi-ply para-aramid/epoxy composites with orthotropic and quasi-isotropic structures based on numerical exploration. A validated yarn level model was established using Abaqus® for the investigation of the ballistic performance in terms of the total energy absorption and the ballistic mechanisms regarding energy absorbed in different forms, stress dissipation, damage occurrence time and deformations. The results revealed that para-aramid/epoxy woven composites reinforced by quasi-isotropic structures exhibit improved ballistic performance with new ballistic mechanisms as the reinforcing structures changing from the conventional orthotropic to quasi-isotropic. In this research, up to an 8.9% increase in energy absorption is achieved by the quasi-isotropic composites comparing with the orthotropic composites under the same areal density, caused by enlarged stress dissipation and deformation areas as the yarn directions increase. Limited by the stress propagation in the thickness direction, there exists a maximum number of yarn directions in forming the most energy absorbent quasi-isotropic panel. This research fills the gap between the current study on quasi-isotropic woven composites and their applications for ballistic protections.

A numerical study on the mechanisms of Dyneema® quasi-isotropic woven panels under ballistic impact

Quasi-isotropic (QI) fabric panels were recently found to have a promising performance against ballistic impact, compared with the aligned panels, since more areas in the followed plies are transversely deformed in the QI panels forced by the front plies. However, the ballistic mechanisms of QI panels have not been completely elucidated. This study aims to identify the ballistic mechanisms of QI woven fabric panels using finite element method (FEM), focusing on the interference from the back plies towards the front. A yarn-level Dyneema® woven fabric model was created, and then validated by the ballistic test results. 2-ply, 3-ply and 4-ply aligned and QI panels were investigated in terms of energy absorption, the initial time of yarn failure, penetration time of panels, and stress distribution within different plies. ImageJ® software was used to quantify the areas under stress in different scale ranges in different plies. It was found that QI panels absorbed 6%–8% more energy than aligned panels, and were less vulnerable to be perforated. Moreover, compared with the aligned panels, QI panels overall had approximately 10% larger areas stressed in the front plies. These results were attributed to the fact that QI panels have higher efficiency in stress distribution along the primary yarns in the front and middle plies. The higher efficiency afterwards was ascribed to the higher interference between plies in the QI panels. This study demonstrated the importance of interference in QI panels on stress distribution, and the findings are of great significance for guiding further designs of fabric and composite panels for the ballistic protection.

Effect of core and facesheet thickness on mechanical property of composite sandwich structures subjected to thermal fatigue

Sandwich panels made of polymeric composite materials used in hostile thermal fatigue environments are prone to microcracking due to internal stresses. The impact of micro-cracking as a result of thermal fatigue is more severe in sandwich structures as opposed to solid laminates. Four sandwich panel configurations are studied. They are quasi-isotropic panels made of polymeric carbon fiber reinforced skin bonded by adhesive to honeycomb Kevlar cores. Different facesheet and core thicknesses are investigated. Samples are subjected to thermal cycles from −195 °C to 150 °C. Microscopic inspection is performed at the sample cross-section for a number of cycles to observe the location and density of cracks. It is observed that cracks are formed mainly at the adhesive/composite interface. Also, microcracks are formed more in the core ribbon direction compared to the core transverse direction. For all samples, after 40 thermal cycles, the total crack length becomes saturated and remains almost constant and no more damage happens. To study the effect of microcracks on mechanical property, flatwise tensile test was performed at room temperature. By increasing the number of cycles, the crack area increases and the flatwise strength decreases. Experimental data indicates that the samples with higher core to facesheet thickness ratio has higher microcrack lengths and lower flatwise strength. Therefore, sandwich panels with thinner facesheet and thicker core are more susceptible to the damage if subjected to the thermal fatigue.

Effect of fibre orientation on the low velocity impact response of thin Dyneema® composite laminates

Ultra-high molecular weight polyethylene (UHMWPE) fibre reinforced composite materials are widely used in ballistic impact and collision scenarios due to their extremely high specific strength and stiffness. Exceptional levels of protection are provided by controlling the damage and deformation mechanisms over several length scales. In this study, the role of UHMWPE fibre architecture (cross-ply, quasi-isotropic and rotational “helicoidal” layups) is considered on the damage and deformation mechanisms arising from low velocity impacts with 150J impact energy and clamped boundary conditions. Dyneema® panels approximately 2.2mm thick were impacted with a fully instrumented hemi-spherical impactor at velocities of 3.38m/s. Full field deformation of the panels was captured through digital image correlation (DIC). The results indicate that the cross-ply laminate [0°/90°] had the largest back face deflection, whilst quasi-isotropic architectures restricted and reduced the central deflection by an average of 43%. In the case of the [0°/90°] panel, the deformation mechanisms were dominated by large amounts of in-plane shear with limited load transfer from primary fibres. Conversely, the failure of the quasi-isotropic panels were dominated by large amounts of panel buckling over various length scales. The observed mechanisms of deformation with increasing length scale were; through thickness fibre compression, fibre micro-buckling, fibre re-orientation with large matrix deformation, lamina kink band formation, and laminate buckling. The helicoidal panels showed that bend-twist and extension-twist coupling were important factors in controlling clamped boundary conditions and the laminate buckling/wrinkling shape. Further examination of the impact zone indicated that the damage mechanisms appear to be fibre orientation dependent, with quasi-isotropic laminates having up to 37.5% smaller impact damage zones compared with [0°/90°]. The experimental observations highlight the importance of fibre orientation in controlling the deformation mechanisms under dynamic impact, in particular limiting the shear deformation of Dyneema® panels.

Influence of attenuation on acoustic emission signals in carbon fiber reinforced polymer panels

Influence of attenuation on acoustic emission (AE) signals in Carbon Fiber Reinforced Polymer (CFRP) crossply and quasi-isotropic panels is examined in this paper. Attenuation coefficients of the fundamental antisymmetric (A0) and symmetric (S0) wave modes were determined experimentally along different directions for the two types of CFRP panels. In the frequency range from 100kHz to 500kHz, the A0 mode undergoes significantly greater changes due to material related attenuation compared to the S0 mode. Moderate to strong changes in the attenuation levels were noted with propagation directions. Such mode and frequency dependent attenuation introduces major changes in the characteristics of AE signals depending on the position of the AE sensor relative to the source. Results from finite element simulations of a microscopic damage event in the composite laminates are used to illustrate attenuation related changes in modal and frequency components of AE signals.

Effect of lay-up orientation on ballistic impact behaviors of GLARE 5 FML beams

This paper examines experimental and numerical investigations on ballistic impact behaviors of GLARE 5 (3/2) fiber-metal laminated (FML) beams of various stacking sequences. The ballistic impact tests were conducted using a high-speed gas gun equipped with a high-speed camera. Optical imaging and mechanical sectioning techniques as well as high-speed-camera footage were used to characterize the impact damage in the specimens. The incident projectile impact velocity versus the residual velocity (VI ∼ VR) was plotted and numerically fitted according to the classical Lambert–Jonas equation for the determination of ballistic limit velocity, V50. The results showed that the unidirectional [90°4] specimen offered the least resistance to the projectile perforation. In addition, except for the unidirectional [90°4] specimen, the ballistic limit was almost insensitive with respect to change in stacking sequence. The interfacial delamination/debonding as well as bending and stretching in aluminum layers played significant roles in dissipating the impact energy in the GLARE 5 (3/2) FML beams. The ballistic impact tests were modeled using a 3D finite element (FE) code, LS-DYNA. The FE prediction was validated using the experimentally obtained VI ∼ VR trends, damage patterns and bullet residual length. Good agreement between experimental and numerical results was achieved. Moreover, the FE predicted V50 as a function of specimen stacking sequence was in good agreement with its experimental counterpart. The validated FE model was then utilized to obtain some insights to the ballistic impacts of which experimental results were not available. It was found that the cross-ply specimen offered the highest contact resistance force while the unidirectional [90°4] lay-up orientation offered the lowest. The maximum contact force for the unidirectional [0°4] and quasi-isotropic panels were almost the same. The results showed that when subjected to ballistic impact, the specimen with [0°/90°]s lay-up orientation dissipated more energy compared to the other lay-up orientations.

Prediction and verification strength in APC-2 composite laminates at elevated temperature

► We determined the strengths of APC-2 composite laminates with central holes. ► We combined the modified PSC model with the equation of thermal degradation. ► We predicted the tensile strength of notched APC-2 composite laminates. ► The predictions were in good agreement. ► The characteristic length that varied with the environmental conditions. The residual strength of notched AS-4/Polyetheretherketone (PEEK) composite (APC-2) laminates with a central hole at elevated temperature was systematically investigated by both analytical and empirical methods. First, [0/±45/90] 2s quasi-isotropic panels were fabricated and cut into samples. A circular hole with one of five diameters was drilled in the center of each sample. The samples were subjected to quasi-static tensile tests at elevated temperatures to measure their mechanical properties – tensile strength and longitudinal stiffness. Stress concentration and high temperature were found to remarkably affect the residual strength of composite. Point stress criterion (PSC) above was found to be unsatisfactory in predicting the residual strength. A modified point stress criterion combined with the equation of thermal degradation was able to predict the characteristic length at elevated temperature. The residual strengths predicted by the extended modified PSC model were in very close agreement with the empirical data.

Thermomechanical Design Optimization of Variable Stiffness Composite Panels for Buckling

Both numerical and experimental research have shown that variable stiffness laminates allow for significant design improvements. Variable stiffness laminates can be manufactured by exploiting the built-in steering capabilities of modern fiber placement machines (FPM). Such variable stiffness panels optimized for buckling were built and tested previously. Test results yielded higher buckling loads than those predicted numerically, which was later found to be related to the residual stresses present after curing. Hence, to benefit fully from the design freedom offered by variable stiffness panels built by FPMs it is essential to include the influence of thermal stresses in the optimization formulation. In this paper, an optimization framework, developed by the authors, is extended to include the influence of thermal loads. Numerical results confirm the importance of including thermal effects in the optimization process. Designs including thermal stresses are found to have improved buckling performance over a larger range of operating temperatures. Proper tailoring of both stiffness and thermal properties allows for ultimate buckling load improvements in the order of four to six times that of the corresponding quasi-isotropic panel.

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.

Mechanical Property Characterization and Impact Resistance of Selected Graphite/PEEK Composite Materials

To use graphite polyetheretherketone (PEEK) material on highly curved surfaces requires that the material be drapable and easily conformable to the surface. This paper presents the mechanical property characterization and impact resistance results for laminates made from two types of graphite/PEEK materials that will conform to a curved surface. These laminates were made from two different material forms. These forms are: (1) a fabric where each yarn is a co-mingled Celion G30-500 3K graphite fiber and PEEK thermoplastic fiber; and (2) an interleaved material of Celion G30-500 3K graphite fabric interleaved with PEEK thermoplastic film. The experimental results from the fabric laminates are compared with results for laminates made from AS4/PEEK unidirectional tape. The results indicate that the tension and compression moduli for quasi-isotropic and orthotropic laminates made from fabric materials are at least 79 percent of the modulus of equivalent laminates made from tape material. The strength of fabric material laminates is at least 80 percent of laminates made from tape material. The evaluation of fabric material for shear stiffness indicates that a tape material laminate could be replaced by a fabric material laminate and still maintain 89 percent of the shear stiffness of the tape material laminate. The notched quasi-isotropic compression panel failure strength is 42 to 46 percent of the unnotched quasi-isotropic laminate strength. Damage area after impact with 20 ft-lbs of impact energy is larger for the co-mingled panels than for the interleaved panels. The inerleaved panels have less damage than panels made from tape material. Residual compression strength of quasi-isotropic panels after impact of 20 ft-lbs of energy varies between 33 percent of the undamaged quasi-isotropic material strength for the tape material and 38 percent of the undamaged quasi-isotropic material strength for the co-mingled fabric material.