Ply Thickness Research Articles

Modelling the damage evolution in unidirectional all-carbon hybrid laminates

Hybrid reinforcements for composites have been extensively studied and adopted to overcome the lack of ductility via pseudo-ductility. Thin-ply all-carbon interlayer hybrid laminates have attracted attention for their peculiar pseudo-ductile tensile response. At the same time, conventional thick plies have been barely considered. This work developed a finite element model to simulate the complex tensile damage scenario of unidirectional thin- and thick-ply all-carbon interlayer hybrid laminates. The damage modes intended in the numerical model were fragmentation in the low-elongation (LE) plies, and delamination of LE and high-elongation (HE) ply interfaces. Thin- and thick-ply hybrid laminates were modelled and compared to available experiments. The numerical model was also adopted to simulate different layups to predict the effect of LE thickness fraction on the pseudo-ductile tensile behaviour and the evolution of damage modes. As suggested in the literature, the results allowed us to depict the damage mode map of the considered hybrid laminates. The map distinguishes the all-carbon hybrid laminate configurations with pseudo-ductile and brittle tensile responses.

Performance of Carbon Fiber-Reinforced Polymer Plates Against Impact Loading

The carbon-fiber laminated composites are widely used in various industrial sectors due to their magnificent mechanical and engineering properties. While designing the structures of strategic importance, the impact perforation study and prediction of impact performance of such composites are of utmost importance. This work basically concentrates on two major areas of impact perforation study, determining impact perforation characteristics and understanding the perforation process. The objective has been achieved through experimental as well as numerical investigations. Microstructural analysis have also been carried out to understand different energy-absorbing mechanisms. The pneumatic gun setup is utilized to fire the projectile. In addition, square-shaped carbon fiber laminates and projectiles are modeled in ABAQUS/Explicit. The carbon fiber laminates of different thicknesses and layup sequences are impacted with three different types of projectiles. The cross ply, angle ply and quasi-isotropic layup sequences are considered in this study. The conical, blunt and ogive nose with caliber radius head (CRH) 1.5 projectiles are modeled as analytical rigid. Hashin criteria are adopted to replicate the behavior of carbon fiber laminates. The experimental and numerical calculations are compared and are found to be in good agreement. The different perforation features are calculated and the influence of ply orientations, thickness, projectile shape and impact velocities on the performance of targets against impact are discussed. An efficient and better laminate configuration under various ballistic conditions has been sought.

Ballistic performance of spaced multi-ply soft fabrics: Experimental and numerical investigation

It has been reported that the ply gap influences the ballistic resistance of spaced multi-ply fabric systems, but its working mechanism was not well-understood. This paper reports the experimental and numerical approaches and results of an investigation on the mechanisms that enable the improved ballistic performance of spaced multi-ply systems. Penetration tests were performed over a range of impact velocities ranging from 200 m/s to 400 m/s. The results confirmed that the ply gap is beneficial to the energy absorption capability of the systems. This is because the front plies tend to absorb more energy when they are not immediately constrained by the rear plies. During a ballistic event, the gap relieves the reflection of the compressive pulse, prolonging the projectile engagement time with the front plies; on the other hand, the rear plies become increasingly less active in dissipating energy as the gap increases. When the gap is sufficiently widened to avoid any interference between the plies before the failure of the front ply, the responses of the whole system no longer vary. It was also found that the ballistic performance of the spaced systems is influenced by ply thickness, impact velocity, and the stacking order of the ply gap.

Bioinspired hybrid helical structure in lobster Homarus Americanus: Enhancing penetration resistance and protective performance

The high impact and penetration resistance of the cuticle in the lobster Homarus americanus can be attributed to its unique twisted plywood structure. Hybridization of two helical structures was observed in the exoskeleton, with different pitch angles and ply thickness of the exocuticle and endocuticle within the exoskeleton. Given the survival challenges posed by the natural environment, it is worthwhile to study why it evolved into a hybrid helical rather than a uniform helical structure. In light of this, we have proposed an innovative bionic hybrid helical structure design strategy that goes beyond simply reducing the pitch angle. In this paper, we replicated complex biological microstructures by introducing two types of pitch angles and ply thicknesses for regional configurations. Quasi Static Indentation (QSI) full-penetration experiments were conducted. Experimental results revealed that the exocuticle with a large pitch angle and small ply thickness possesses a higher knee load under the penetration test, which provides a higher initial damage threshold. On the other hand, the endocuticle with a small pitch angle and large ply thickness induces higher damage dissipation, which provides superior protective performance. These research findings demonstrate that ply hybridization can be a promising method for microstructure design to improve the penetration resistance of thin laminates and serve as a reference for developing more refined gradient helical structure designs.

Underwater Implosion and Energy Mitigation of Hybrid Glass-Carbon Composite Shells

Experiments were conducted to investigate the dynamic buckling behavior of underwater hybrid composite tubes. The study focused on roll-wrapped hybrid layered glass-carbon fiber epoxy composite shells with a six-layer quasi-isotropic layup configuration. In addition to control specimens consisting of fully glass fiber-reinforced polymer and carbon fiber-reinforced polymer, four different hybrid layup patterns were examined. These specimens fitted with custom endcaps were placed inside a 7-kiloliter pressure vessel and subjected to increasing hydrostatic pressure until dynamic implosion occurred. High-speed cameras captured the failure event, and the resulting images were analyzed using Digital Image Correlation (DIC) techniques to obtain full-field displacement data. Additionally, tourmaline pressure transducers positioned around the specimens recorded local dynamic pressure histories. The results revealed that the contribution of each ply location varied in the overall failure behavior of the structures. The thickness of the internal plies played a dominant role in enhancing the structural performance, while the stiffness of the outer plies greatly influenced the bending stiffness. The energy released during the collapse was highly dependent on the failure mechanism of the internal plies. Specifically, for the considered geometries, tubes with glass fiber internal plies exhibited significantly lower energy emissions compared to carbon fiber inner plies.

The Role of Thin Plies in Traditional Composite Structures Under Different Loading Conditions

In the current paper, composites with both thin and thick plies are designed to meet specific criteria and tested under both in plane and out of plan critical loading conditions. The tests adopted are bolted joint, drop-weight impact, compression after impact, Charpy impact, bending, and bending after thermal aging. The results of the proposed design are compared with that of the traditional composites and show higher improvement in most of the load cases by using thin plies inside the lamination process with the traditional ones. The results showed that the two proposed alternatives with thin plies are of higher advantage for the bolted joints problem. On the other hand, the alternative with thin plies distributes to be surrounding each traditional ply is of high advantages for both the impact and the bending problems. The alternative with a core of thin plies at the middle of the laminate is of high advantage for the compression after impact.

Numerical and experimental analysis of ultra-thin plies in carbon/epoxy cross-ply laminates subjected to bending after tensile cyclic loading

As a result of a fatigue testing campaign in cross-ply laminates with different thicknesses in the 90° ply block (conventional and ultra-thin), different cracking patterns were observed. While laminates with conventional 90° ply thickness show a damage pattern with transverse cracks in the 90° ply block and delaminations along the 0°/90° interface, laminates with ultra-thin plies show paths of debonds inside the ultra-thin 90° ply block both transverse and longitudinal to the loading direction. In order to see the influence that these different fatigue damage patterns could have on the mechanical behaviour of the laminates, static bending tests have been carried out after cyclic loading, and apparent flexural stiffness loss (concerning the pristine state) was measured. Numerical simulations of the bending test, including the cyclic damage pattern (and detailed non-linear frictionless contact conditions) were also carried out for a deeper understanding of the experimental evidence. Although the morphology of the fatigue damages encountered is different for the two studied 90° ply block thicknesses, their flexural behaviours are quite similar, in terms of apparent flexural stiffness loss, for both cross-ply laminates, independently of its 90° ply block thickness.

Optimization of Hybrid Composite Laminates with Distinct Ply Thicknesses Using Integer Programming

Optimization of constant-stiffness composite laminates with discrete values of fiber orientations and ply thicknesses is a nonconvex combinatorial problem. Recent studies show that this problem can be efficiently solved using the mixed integer program (MIP) by reformulating lamination parameters as functions of decision variables that represent fiber orientation and thickness choices. These formulations are limited to analyzing laminates having plies of the same thickness and material. This paper extends the previously employed lamination parameter reformulation to enable laminate designs with plies of different thicknesses and materials, thereby leading to MIP with cubic constraints. The linearization techniques are employed to convert cubic constraints of buckling load factors in terms of binary variables into sets of linear constraints that are efficiently solved with optimization algorithms for MIPs. It is shown that the use of distinct ply thicknesses in the optimization further reduces the mass of single-material laminates made with carbon-fiber-reinforced polymer when compared with previous studies using the same material. Additionally, the combination of plies made of glass-fiber-reinforced polymer and carbon-fiber-reinforced polymer with distinct thicknesses is shown to reduce the weight penalty when optimized for cost minimization in comparison to plies with uniform thickness.

Out-of-autoclave molding of carbon fiber laminates by consolidation with shape memory polymer foams

Carbon fiber reinforced (CFR) laminates have been manufactured by an innovative out-of-autoclave (OOA) process thanks to the use of smart materials. In this study, for the first time, a shape memory polymer (SMP) foam has been used into a pressure dome to apply the consolidation pressure to the laminate. Heat for resin curing was supplied by a thermal blanket between the laminate and the SMP foam. In order to test the OOA molding process, 24-ply CFR laminates were manufactured and tested. For comparison and reference, CFR laminates were also molded by traditional vacuum bagging and autoclave curing. Samples for bending tests were extracted from OOA and autoclave molded plates. Results show that the SMP-OOA process is able to provide optimal laminate consolidation, despite of the low applied pressure (about 1 bar). A comparable density and ply thickness was measured on samples by the innovative and the conventional molding process, whereas a higher bending strength (+5%) was found on OOA samples. A higher dispersion was also observed for the properties of SMP-OOA molded laminates in comparison with autoclave, probably due to the prototyping nature of the molding technique in comparison with the well-known autoclave practice. Nevertheless, the final quality of the molded laminates is very high, and the SMP-OOA process is a very promising candidate as a technology to repair CFR structures.

Deep reinforcement learning for stacking sequence optimization of composite laminates

Fiber reinforced polymer (FRP) composite laminates are increasingly used in a wide range of safety–critical products due to their excellent material properties. The stacking sequence of FRP composite laminates plays a critical role in the resulting part’s mechanical properties. Despite this, composite engineers still commonly fabricate parts with “classic” ply layups (e.g. four ply laminate with fiber orientation angles: [-45/0/45/90°]), which may have sub-optimal mechanical performance in the expected loading/use cases. Finding the composite stacking sequence that achieves the best material and mechanical properties possible is a challenging optimization problem characterized by the large domain space involved in solving an inverse design problem. This paper introduces a novel approach to optimizing stacking sequence for composite plate stiffness by applying off-policy deep reinforcement learning (DRL). We formulate the problem as a sequential decision making process. The state of the system is based on the stiffness of the composite for the currently selected stacking sequence and the action is to select a new stacking sequence using our reward function, formulated as the offset, normalized stiffness modulus. We compare our DRL model to two classical stacking sequences, a randomized baseline model, and a competitive genetic algorithm (NSGA-II). For maximizing longitudinal composite plate stiffness, the DRL model finds the optimum solution for all ply thicknesses and the genetic algorithm comes within 0.5% of the optimum. The DRL model determines the optimum stacking sequence significantly faster and is less sensitive to parameter tuning. The DRL model and the genetic algorithm both outperform the random baseline algorithm by over 5.7 standard deviations in the most conservative case. This research demonstrates the ability of DRL to effectively and efficiently optimize composite laminate stacking sequence.

Evolution of the damage onset and morphology in [0/90n/0] laminates when increasing the ply thickness

Evolution of the damage onset and morphology in [0/90n/0] laminates when increasing the ply thickness

Effects of ply thickness and interference-fit on the bearing strength of single-lap countersunk composite joints

The Countersunk composite joints are widely utilized in the load-bearing components of aerospace structures. However, hole-making causes local stress concentration, affecting joints’ bearing capacity. Therefore, a detailed experimental investigation was conducted on the strength under varying parameters including ply thickness, stacking sequence, and interference size in the single-lap countersunk composite joint. The installation force of the thin-ply joint was smaller than that of the thick-ply at the same interference. The initial damage of the thin-ply hole wall caused by the bolt-inserting process was less. The strengths of the two thick-ply joints improved by 10.12% and 9.09% (1.00% interference), respectively, compared with those of thick-ply neat-fit joints. The strengths of the two thin-ply joints increased by 18.18% and 21.78%, respectively, when the interference was increased to 1.44%. SEM imaging revealed that the damage extent of the thin-ply area is smaller with the same interference. Moreover, the fibre kinking angle of the 0° plies at the thick-ply section was more extensive, leading to fibre fracture. Using interference-fit technology to thin-ply laminates could improve the damage resistance of bolted joints and provide a novel idea for applying thin-ply laminates in thin-walled structural components.

Micro-mechanical analysis of the effect of ply thickness on curing micro-residual stresses in a carbon/epoxy composite laminate

During the manufacturing of thermoset-based carbon fiber reinforced polymers (CFRPs), micro-residual stresses are developed in the material, due a mismatch of the chemical-thermal-mechanical properties of the fibers and the polymer matrix. This ultimately leads to a reduction in the mechanical performance of the composite material. In this work, a representative volume element (RVE) of a composite sub-laminate with a 90° ply discretized at micro-scale level, stacked within homogenized plies, is proposed to study the micro-residual stresses and the ply mechanical response considering the in-situ effect caused by the constraining adjacent layers. A model framework is developed using finite elements in Abaqus®/Standard with user-subroutines. The retrieved micro-residual stresses in the 90° layer are generated by the dissimilar chemical shrinkage and thermal expansions, plus the additional residual meso-scale stresses caused by the constraining effect of adjacent plies. The residual stresses in the constrained condition can achieve almost 80 % of the ply failure load for the 0/90/0 sub-laminate, meaning lower sub-laminate strain to failure. Thick laminates retrieve more residual stresses in the transverse 90° layer and exhibit lower constrained strength and more unstable post-failure response, demonstrating that thin-ply laminates exhibit beneficial constraining effects during the curing process.

Quasi-brittle fracture performance of glass fiber composites based on boundary effect model

Light surface scratch around 50 μm or less in-depth was a formidable engineering challenge for the application of quasi-brittle materials. In the present study, a boundary effect model (BEM) was used to analyze the fracture performance of glass fiber reinforced polymer (GFRP) composites with shallow surface scratch. The ply thickness of single-layer prepreg was introduced as the controlling GFRP microstructure parameter or characteristic composite unit Cch of the model, and the parameter of ply thickness was set to 1.2 mm. Quasi-brittle transverse fracture parameters of the tensile strength ft and the fracture toughness KIC of GFRP were discussed. Finally, the average values of the fracture parameters uf and uk of the GFRP were obtained by normal distribution analysis. The means, and upper and lower bounds of the fracture parameters were within the range of 95% reliability covering almost all test discrete points. The laboratory samples of conventional sizes were used to predict the fracture parameters of GFRP. The result reflected that the error between the test results of all samples is only 5.7%. In addition, as the ratio α of the gap height of the GFRP sample increases, the fracture parameters showed a trend of increase firstly and then declining. The average fracture parameter estimated value has decreased by 14.5%, which was in good agreement with the test results.

Hierarchical mode I interlaminar toughening of unidirectional CFRP laminates by the synergistic effects of CNT powders and veils

While carbon fiber reinforced polymer (CFRP) composites are widely used in structural components because of their excellent mechanical properties, delamination between reinforcing plies remains a well-known Achilles heel, weakening the composite structure and limiting their more widespread applications. This work tried to improve the interlaminar properties of unidirectional CFRP laminates by the synergistic effects of CNT powders and veils. The short CNT powders (average length <1 μm) were adopted for toughening the matrix, which effectively avoids the “self-filtration effect” commonly found in the VARTM process and allows the CNTs to penetrate between the tiny gap of the carbon fibers and be uniformly dispersed in the composites. In addition, an ultrathin CNT fiber veil (∼100 nm in thickness) was used as the interleaf material, offering the opportunity to reinforce the interlaminar bonding with minimal weight penalty and effectively avoid reductions in in-plane properties. The results showed that 0.5 wt% is the optimum loading level of CNT powders for matrix toughening. The Mode I interlaminar fracture toughness (propagation value, GIC,prop) of the unidirectional CFRP composites increased as much as 32 %, 62 % and 123 % when integrating CNT veil, CNT powder and CNT veil + powder, respectively. The figure of merit for interlaminar reinforcement, consisting of the change in interlaminar properties normalized by interleaf thickness and ply thickness, comes out as high as 2460, which far outweighs the state of the art.

In search of the quasi-isotropic laminate with optimal delamination resistance under off-axis loads

Delamination is one of the most feared failure modes in laminated composites and yet there is a lack of well established procedures to find the ply configuration with the highest delamination resistance for a given geometry of the component and a set of elastic and strength constraints. This paper presents a methodology for determining the quasi-isotropic ply sequence more resistant to delamination under off-axis uniaxial tension. Delamination is considered to be triggered by interlaminar stresses at free-edges or by matrix cracks. Two different ply thicknesses (standard and thin) and two sets of ply orientations (multiples of π/4 or π/8) are considered. The results show that the use of thin plies enlarges the design domain by up to 70% and generates a practically isotropic safe space. The methodology presented is also suitable for optimizing the delamination resistance of other load cases with stress singularities.

Electrochemical performance of supercapacitor electrodes based on carbon aerogel-reinforced spread tow carbon fiber fabrics

Fabric-based supercapacitor electrodes were fabricated by embedding spread tow carbon fiber fabrics, in monolithic, bicontinuous carbon aerogels (CAG). The incorporation of CAG, at less than 30 wt%, increased the specific surface area of the CAG-CF fabric to above 230 m2 g−1 and the pore volume to about 0.35 cm3 g−1, orders of magnitude higher than that for the as-received carbon fibres. The presence of the CAG not only improves the electrochemical performance of the composite electrodes but may enhance the mechanical response due to the high stiffness of the aerogel structure. Cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance measurements were performed on symmetric supercapacitor cells consisting of two CAG-reinforced fabrics in an ionic liquid electrolyte. The specific capacitance of the symmetric supercapacitor was determined to be in the range 3–5 F g−1, considerably higher than that for the plain carbon fibers. Since optimum structural electrolytes are not yet available, this value was normalized to the total mass of both electrodes to place an upper bound on future structural supercapacitors using this spread tow CAG-CF system. The maximum specific energy and specific power, normalized to the total mass of the electrodes, were around 2.64 W h kg−1 and 0.44 kW kg−1, respectively. These performance metrics demonstrate that the thin CAG-modified spread tow fabrics are promising electrodes for future use in structural supercapacitors. In principle, in future devices, the reduced ply thickness offers both improved mechanical properties and shorter ion diffusion distance, as well as opportunities to fabricate higher voltage multicell assemblies within a given component geometry.

Micro-mechanical analysis of transverse tensile in-situ effect of thin-ply composites considering interlaminar resin region

The mechanical property of the thin-ply composite are improved, which has more uniform microstructure and fewer manufacturing defects compared with the standard ply composite. Based on the micromechanics theory, an RVE analysis method is proposed considering the interlaminar resin region in this paper. The influence of the interlaminar resin region on the RVE model of thin-ply composites is studied. The initiation and propagation of cracks are discussed in the thin-ply laminate under transverse tensile, and the in-situ transverse strength of 90° ply is predicted with different thicknesses. The results show that considering the interlaminar resin region has a great influence on the in-situ strength prediction results. Reducing the ply thickness can change the fracture mechanism and improve the transverse in-situ strength of the material.

Static and Fatigue Tensile Properties of Cross-Ply Carbon-Fiber-Reinforced Epoxy-Matrix-Composite Laminates with Thin Plies

Carbon-fiber-reinforced epoxy-matrix composite (CFRP) laminates with thin plies have strong damage-resistance properties compared with standard prepregs. The static and fatigue tensile fracture behavior of cross-ply CFRP laminates with thin plies should be further studied to establish the applicability of thin-ply prepregs for industrial structures. In this study, the static and fatigue tensile properties of cross-ply, high-strength polyacrylonitrile (PAN)-based carbon-fiber (T800SC)-reinforced epoxy-matrix composites with thin plies were investigated. The fiber orientations of the CFRP specimens were set to cross-ply with [0/90]10S (subscript S means symmetry), [(0)5/(90)5]2S, and [(0)10/(90)10]S. The static and fatigue tensile characteristics of the cross-ply CFRPs with thick plies with [0/90]2S and [(0)2/(90)2]S were also investigated for comparison. Under static loading, the tensile strength and failure strain of the thinnest 90°-ply-CFRP specimens were more than 5% higher than those of the other 90°-ply-thickness specimens. However, the tensile moduli and Poisson’s ratios were comparable between the cross-ply CFRPs with thin and thick plies. Under fatigue loading, the fatigue responses of the thinnest 90°-ply-CFRP specimens were 3% higher than those of the other 90°-ply-thickness specimens during lower-fatigue-cycle testing (&lt;105 cycles). However, during higher-fatigue-cycle testing (&gt;105 cycles), the fatigue responses decreased, with a decrease in the 90°-ply thickness, and the fatigue characteristics of the thinnest 90°-ply-CFRP specimen were 7% lower than those of the other cross-ply thin- and thick-ply-CFRP specimens.

Thickness effect on flexural strengths of laminar carbon fibre composites

The thickness dependence of flexural strengths of laminar carbon fibre reinforced polymer (CFRP) plates was investigated experimentally and analytically in this study. This thickness effect was linked to the ply thickness by a simple composite formula. Four different sample thicknesses of 1.48, 2.08, 2.48 and 2.96 mm were tested, and thickness dependent flexural strengths were evaluated by the Strength of Materials (SOM) formula and modelled by the simple composite formula containing the ply thickness. The ASTM standard D7264, recommending 4 mm thick CFRP samples, was justified analytically, i.e., the thickness effect is negligible if W≥ 4 mm for CFRP plates with typical ply thickness around 120–130μm. The intrinsic tensile strength, independent of the sample thickness, was deduced from all flexural tests by the composite formula, which compared well with the tensile strength from unidirectional tensile tests and the tensile strength estimated from notched flexural samples. The specimen and notch/crack size requirements in ASTM standards for the fracture toughness were also explained analytically.