Ply Thickness Research Articles

Optimizing performance of hybrid fiber reinforced overwrapped pressure vessel

Composite overwrapped pressure vessels (COPVs) offer significant weight advantages over traditional all-metal vessels particularly in industries like aerospace, automotive and pharmaceutical, but pose unique challenges in mechanical design, fabrication, and testing. Despite their benefits, COPVs are susceptible to stress rupture failure, which can lead to catastrophic consequences during operation. This failure mode is not fully understood, with burst pressure-induced extreme stress concentrations and dynamic loading primarily contributing to rapid deformation and strain. To address these concerns, a comprehensive investigation was conducted, focusing on optimizing and examining the effects of optimum winding angle and lay-up pattern configuration on burst pressure in vessels under internal pressure. Finite element modeling analyzed the burst pressure behavior of COPVs with a 4 mm thick aluminum core cylinder and varying layers of carbon/fiber epoxy, each with a constant thickness. ABAQUS composite modeler generated 18 COPV models with carbon fiber/epoxy plies, ensuring uniform thicknesses while exploring different fiber orientations. The effects of ply stacking sequence were analyzed through finite element analysis for all models, comprising varying layers with uniform thicknesses. Attention was paid to failure criteria, methodically evaluating the burst strength of the Al/CFRPC COPVs This exhaustive analysis yielded an optimal COPV design profile characterized by a precisely calibrated ply stacking sequence of [24.50, −24.50] PP winding pattern and six (6) arranged layers. The stress-strain distribution analysis of the Composite Overwrapped Pressure Vessel (COPV) revealed both a uniform distribution of stress across its surface and extreme stress concentration, particularly peaking towards the polar boss region. This concentration is discernible due to variations in ply thickness induced by the overwrapped fiber orientation. This investigation provides valuable insights for optimizing COPV design and enhancing its performance and reliability in various applications.

X-ray micro-computed tomography for mechanical behaviour analysis of Automated Fiber Placement (AFP) laminates with integrated gaps and overlaps

Automated Fiber Placement (AFP) is a manufacturing technique widely used for the serial production of aerospace parts. A deep understanding of the effect of lay-up defects is crucial for part and lay-up design. Currently, numerical models for structural simulation lack a precise representation of the internal structure of AFP laminates, which is crucial for understanding the impact of defects on mechanical properties. This paper presents a novel approach based on high-resolution micro-computed tomography (micro-CT) scans from specimens manufactured with AFP, which automatically creates a mesoscale numerical model incorporating as-fabricated defect morphologies. The hexahedral mesh, generated from the segmented plies of the micro-CT volume, accounts for ply thickness and out-of-plane fiber orientation. This approach is verified with mechanical testing and digital image correlation (DIC) under tensile loading. The simulation results align closely with experimental testing and accurately illustrate the influence of fiber waviness in various defect configurations, such as gaps and overlaps. The study shows that lay-up defects can lead to knockdown factors of up to 12% in tensile properties, with each defect creating a distinct pattern in the local strain. This model can serve as a benchmark for further numerical simulations and surrogate models of defect configurations under varying loading conditions.

Assessment of ply thickness and aluminum foils interleaving on the impact response of CFRP composites designed for cryogenic pressure vessels

Carbon fiber reinforced polymer (CFRP) proved to be the best choice to produce cryogenic pressure vessels coupling a reduced weight with superior mechanical performance. Despite this, CFRPs are prone to fuel leakage due to interconnected matrix cracks resulting from mechanical and thermal stresses. Two strategies were combined in this work to reduce fuel permeability, i.e., increase of plies number and metallic film interleaving, evaluating the cryogenic impact response of these new configurations. At first, the only ply thickness effect was assessed and the hybrid sandwich-like (SL) configuration with traditional CFRP (RC) skins and a thin-ply (TP) core displayed impact and residual flexural properties close to RC ones. Then, the effect of aluminum foils interleaving was investigated, and the interleaved SL configuration displayed higher residual flexural properties than interleaved RC for all temperatures, e.g., the residual flexural strength was 30.3 % higher at room temperature and 14.6 % higher at −70 °C, respectively.

Local reinforcement of adhesively bonded square shape aluminum energy absorber with CFRP under quasi-static axial loading: An experimental and numerical study

This study presents experimental and numerical investigations on the effects of composite ply angle, thickness and number of layers on energy absorption behavior of adhesively bonded carbon fiber-reinforced plastic/aluminum hybrid structure under quasi-static axial loading. This paper aims to use the advantages of adhesive bonding for the local strengthening of a square shaped aluminum energy absorber by composite and as a result achieving desirable energy absorption behavior prior to failure in the connection and without heterogeneous deformation of the structure while reducing the weight of the structure and usage of material and at the same time obtaining favorable results compared to the traditional ways like reinforcing the aluminum structure with composite globally. For this purpose, two models were made and analyzed; one reinforced with four L-shaped composites adhesively bonded using Araldite 2015 to four outside corners of the aluminum square and the other reinforced locally to four corners from within the square. Finite element model was developed for analysis of these hybrid structures. Five different ply angles for composite and multiple number of composite layers varied from 2 to 8 layers under the same and different thicknesses were investigated. Moreover an alpha factor has been determined to measure the importance of local reinforement of the square shape aluminum energy absorber by L-shaped composites. The results showed that compared to aluminum and aluminum with global composite reinforcement energy absorbers, due to the adhesive connection between aluminum and composite in the locally reinforced CFRP/aluminum specimen, the composite had better energy absorption by following the collapse pattern of aluminum and creating a continuous collapse and failure modes. The results indicated good coordination and agreement between the simulated models and the experimental tests. The findings from this experiment demonstrates significant potential for local reinforcement of structures employing adhesives. Utilizing such connections and the ability to strengthen specific areas based on arbitrary geometry and strengthening location could offer substantial opportunities in constructing lightweight structures with high energy absorption across diverse applications.

Improving tensile properties of glass fiber-reinforced epoxy resin composites based on enhanced multiphase structure: The modification of resin systems and glass fibers

Glass fiber-reinforced polymer (GFRP) composites have been widely used as reinforced materials in marine engineering due to their good corrosion resistance and economic benefits. However, the mechanical properties of GFRP are lower than that of composites reinforced by carbon and aramid fibers etc. Methods to improve the mechanical properties of GFRP received ongoing attention. GFRP have various mechanical properties due to the characteristics of its multiple phases. This study developed an optimized processing method based on multiphase structures for GFRP composites, significantly enhancing the tensile properties of GFRP laminates. The modified GFRP can be produced efficiently by this method to meet more utilization situations in marine engineering. The influence of curing agents, silane coupling agents (SCA)-treated glass fibers (GFs), and nanomaterials on GFRP's tensile properties was investigated. The findings reveal a 10.64 % increase in the characteristic load of GF bundles due to the improved bonding between fibers by SCA. Besides, the contact angle between SCA-treated GFs and epoxy resin shows a significant reduction (26.69 %). Variations in ply thickness and resin system distinctly influence the tensile properties and failure modes of GFRP laminates. Laminates with 4-ply GF fabrics facilitate more effective load transfer across the matrix-fiber boundary, yielding optimal tensile properties for GFRP. Epoxy/phenolic amine resin system with more reactive functional groups (-OH) enhances the fiber-resin bonding, further improving the tensile properties of GFRP. The enhancement mechanism of the multiphase structure in GFRP was elucidated by microscale characterization, indicating that the multiphase structures in GFRP, enhanced by SCA, nanomaterials, and functional curing agents, significantly improve its tensile properties.

Simulation for Composite Cylindrical Pressure Hull using Progressive Response Surface Method

Response surface method (RSM) as a fitting method had been applied to composite cylindrical buckling problem. There are little researches focusing on the accuracy of the RSM. For most cylinder buckling problems, only angles of the fiber orientation had been considered as the important variables. This paper proposes a progressive response surface method (RSM) to approximate the buckling critical pressure for composite cylindrical pressure hull taking ply angles and ply thicknesses into account. This method is adopted to replace the finite element method (FEM) to rise the calculation efficiency, and the fitting model result in this method can be proved to be effective and accurate compared with 6 different RSM models. The results can demonstrate its accuracy and feasibility under ply angles and thicknesses for composite cylindrical buckling problem.

An integrated approach to analyzing matrix-crack-induced stiffness degradation and medium leakage in linerless composite vessels

This paper proposes an integrated analysis method for linerless composite vessels considering matrix-crack-induced stiffness degradation and permeation. Stiffness degradation models are obtained using machine learning regression, the data sets of which consist of finite element analysis (FEA) results and virtual samples. Then, the effective leakage conductance is obtained by using the linear elastic fracture mechanics and the micro-cracking leakage mechanism. The proposed integrated method is applied to analyze vessel structures designed by the grid theory, which has variable ply thicknesses and layup angles. Simulation results show that whether the structure at cryogenic temperature cracks and leaks more easily depends on its dimensions and stacking sequences.

A Forward Sensitivity Analysis and Design of U-Shaped Bellows Considering the Radial Compensation Capability Under Internal Pressure

Abstract Bellows serve as integral component within a piping system, which withstand radial displacement loads and internal pressure loads. This study investigates the sensitivity analysis of structure design parameters, such as convolution height, convolution pitch, ply thickness, and convolution number, on radial compensation capability of U-shaped bellows under internal pressure. The influences of the geometric parameters on the strength performance of U-shaped bellows under internal pressure and radial displacement loads are also investigated. A forward design process considering the sensitivity of the structural parameters of the bellows to the bending stiffness and strength is provided. In order to improve the radial displacement compensation ability and to obtain better pressure resistance performance, a mathematical model is established to describe the performance of bellows, and multi-object optimization of bellows is conducted. The results show that the ply thickness has the greatest impact on bending stiffness and strength, and the optimized model has lower stiffness and higher strength compared to the initial model.

Investigating factors influencing interlaminar stresses and energy release rates of semi-elliptical cracks at free edges

Traditional approaches that rely on stress or energy-based criteria to predict delamination require the determination of stresses before crack onset or energy release rates after crack initiation, respectively. While a relatively new coupled criterion, Finite Fracture Mechanics, necessitates the computation of both stresses and energy release rates. This study discusses the numerical determination of the interlaminar stresses and energy release rates of interlaminar semi-elliptical cracks emanating at the free edge in carbon fibre-reinforced polymer materials. The method establishes a new semi-analytical framework, based on dimensional analysis, which permits obtaining a functional expression for the stresses and the energy release rates in function of material and geometrical parameters. This framework eliminates the need to re-solve the underlying boundary value problem for a different material, geometry, and load. The non-dimensional functions are obtained by performing careful interpolation from a set of finite element solutions, ensuring predictions are accurate and within acceptable numerical limits. The influence of material contrast and relative ply thickness on interlaminar stresses and energy release rates are investigated using the semi-analytical framework. Additionally, the fracture mechanics analysis is also conducted to investigate the influence of semi-axes of semi-elliptical crack on energy release rate distributions. Understanding the interlaminar stresses and energy release rates is essential for applying either stress, energy, or coupled criterion to predict the structural strength of a layered body.

The stacking sequence optimisation of a filament wound composite bicycle frame using the data-driven evolutionary algorithm EvoDN2

ABSTRACT This work focusses on identifying the optimal stacking sequence for composite tubes in mountain bike frames using a data-driven model combined with evolutionary algorithms. The objective is to find a frame that is sufficiently stiff while meeting the requirements of weight, strength, and minimum tube wall thickness. The decision variables are the ply winding angles and the ply thicknesses of each tube. The study performs designs for two load cases – Starting and Uphill – and explores two types of winding: the gradual winding of individual layers (1ply) and the winding of layers between predefined inner and outer layers with variable thicknesses (TW). Additionally, the design process is applied to frames made of isotropic materials, such as steel, aluminium, and titanium, using the same methodology to allow for comparison of results. The article demonstrates the successful application of this methodology to common sports equipment, suggesting its potential for beneficial use in other common composite frame structures.

Eliminating cracks in Ti–47Al–2Cr–2Nb/Ti–6Al–4V micro-laminated composites fabricated by dual-material laser powder bed fusion

The low ductility of titanium aluminide at room temperature has been one of the factors that severely limit its applications. In this study, Ti–6Al–4V (Ti64) alloys were combined with Ti–47Al–2Cr–2Nb (TiAl) alloys to make micro-laminated composites with optimal ply configurations to eliminate crack formation in TiAl and improve the ductility. A dual-material laser powder bed fusion (LPBF) system was used to fabricate the TiAl/Ti64 micro-laminate samples. The presence of cracks, elemental composition and mechanical properties were experimentally characterised and analysed to understand the effect of ply configuration and the mechanism of eliminating cracks. It was found that smaller TiAl ply thickness and lower TiAl volume fraction result in fewer cracks formed in the micro-laminates. Elemental composition analysis reveals that the Al content in the first two powder layers (i.e., 200 μm thick in total) of a TiAl ply is reduced to 10%–35% due to the partial mixture of the TiAl ply and the proceeding Ti64 ply in the LPBF. The reduced Al content leads to combined α-Ti, β-Ti and α2-Ti3Al phases instead of γ-TiAl phases, thus eliminating the formation of cracks within the first 200 μm thick region of the TiAl ply. This study demonstrates that crack-free and dense TiAl/Ti6 micro-laminated composites can be produced with the TiAl ply thickness reduced to 200 μm and the TiAl volume fraction reduced to 25% (Ti64 ply thickness 400 μm). Ductility of the crack-free micro-laminates at room temperature is comparable to or even exceeds the properties of TiAl/Ti64-type laminates made by other manufacturing processes, which may imply good machinability. Note that further research is required to investigate the properties and applications of the micro-laminates at elevated temperatures.

Tensile behavior of unidirectional thick-Ply all-carbon hybrid laminates: a systematic experimental and numerical study

Unidirectional long fiber reinforced polymers generally exhibit unfavorable abrupt and brittle failure under mechanical stresses without pre-warning which currently limits their use in safety critical applications. The lack of ductility of such composites can be overcome by interlayer hybridization where Low Strain (LS) material is sandwiched between High Strain (HS) material. This results in complex failure mechanisms, including multiple interacting damage modes, such as ply fragmentation and delamination. All-carbon unidirectional hybrid laminates with different layup sequences were designed and manufactured to study the pseudo-ductile behavior. An available analytical model was exploited to predict the damage scenarios of the laminates, both with stress-strain diagrams and damage mode maps. Tensile tests were carried out using different measurement and observation techniques including digital image correlation (DIC), embedded distributed fiber optic sensors (dFOS) and helicoidal X-ray computed tomography (CT). A finite element model was also developed to predict the damage mechanisms. Validated by experimental results, the numerical model was found to accurately predict the tensile damage modes and their evolution in the considered unidirectional thick ply all-carbon hybrid laminates.

Study on tensile properties of ultra‐thin‐ply carbon fiber‐reinforced composite laminates under static load

AbstractThe thinning of carbon fiber‐reinforced composites has been shown to enhance their strength and damage resistance. In this study, cross‐ply and quasi‐isotropic laminate tensile experiments are conducted using composites with ply thicknesses of 24, 55, 75, and 100 μm to investigate the relationship between the strength of laminates and ply thickness. By analyzing the acoustic emission amplitude and energy signals during the loading process of the specimens, the damage evolution of each ply thickness composite under two different stacking methods is studied, revealing the failure mechanisms of the materials. The results show that under both cross‐ply and quasi‐isotropic stacking methods, the ultra‐thin‐ply composites exhibited higher strength. The thickness of ply has a significant influence on the fracture mode of the quasi‐isotropic laminates and the mechanical properties of quasi‐isotropic laminates are more affected by ply thickness than those of cross‐ply laminates.Highlights Quasi‐isotropic laminate strength is affected more by ply thickness than cross‐ply. Reduced ply thickness changes fracture mode in quasi‐isotropic laminates. Ultra‐thin‐ply composites exhibit flat fractures resulting in higher strength.

In-situ automated fiber placement gap defects filled by fused granular fabrication

This paper presents an investigation into the effect of Fused Granular Fabrication gap filling on the composition and mechanical performance of in-situ consolidation Automated Fiber Placement-manufactured laminates. Two laminates with staggered triangular gap defects were manufactured using in-situ consolidation Automated Fiber Placement. In the first laminate, the defects remained unmodified whereas in the second laminate Fused Granular Fabrication was used to fill the gap defects to mitigate out-of-plane waviness in subsequent plies and large pores which can initiate cracks. The process was successfully demonstrated. Fused Granular Fabrication layer thickness was however hard to adjust to match consolidated ply thickness of the AFP process due to the slightly uneven surface of the AFP substrate resulting in out-of-plane undulations. Compressive strength remained at the same level for both, reference and gap specimens. Tensile strength decreased 11 % and 23 % for gap defects and filled gap defects, respectively. Further work is required to utilize the full potential of the process combination.

Study on biaxial tensile failure behavior of ultra-thin-ply carbon fiber reinforced composites

The superior mechanical properties of ultra-thin-ply carbon fiber reinforced composites under uniaxial stress have been widely recognized. However, composite structures are often subjected to multiaxial loading in practical applications. Therefore, this study focuses on investigating the mechanical properties of ultra-thin-ply carbon fiber reinforced composites under biaxial stress conditions. Firstly, the geometric structure of a cruciform biaxial test specimen for composites is designed, and its rationality is demonstrated. Subsequently, cruciform biaxial test specimens with ply thicknesses of 24 μm and 100 μm are prepared for quasi-isotropic laminated plates, and biaxial tensile experiments with stress ratios of 0:1, 1:1, and 2:1 are conducted. This study establishes the failure envelope of ultra-thin-ply composites under biaxial tensile conditions and compare it with the experimental results of standard-ply composites. Finally, the fracture surfaces of the specimens and acoustic emission signals are analyzed to reveal the failure mechanisms of the composites under biaxial stress conditions. The results indicate that ultra-thin-ply carbon fiber reinforced composites exhibit higher interaction degree under biaxial stress, and their ability to suppress delamination is stronger.

Ultrasonic metrics for large-area rapid wrinkle detection and classification in composites

Due to their high strength-to-weight ratio, composite materials are now in use in many high-stress applications, particularly where light weight is also a requirement. In these situations, the detrimental knock-down in mechanical strength due to an out-of-plane wrinkle defect can have serious consequences and is the reason for a requirement to rapidly detect any such wrinkles at manufacture. Unfortunately, current ultrasonic inspection techniques used for quality control at manufacture are not sensitive enough to detect these wrinkles above coherent structural noise variations. This paper exploits the ply resonance that is a characteristic of multi-layer structures to generate two new metrics for both detection and classification of out-of-plane wrinkles, due to their perturbations of the ply spacing. These can be measured at every location on a structure using the instantaneous frequency, which is the rate of change of phase in the pulse-echo ultrasonic response. The proposed two new metrics for detection and classification of wrinkles are mean spacing and spacing difference and they can be applied to each waveform in real time, as it is acquired. Use of an analytical model to predict the ultrasonic response of the structure has allowed an understanding of how these metrics will be affected by various wrinkle types and how they can not only detect wrinkles but also classify the type of wrinkle and provide an approximate indication of severity. Three main types of wrinkle are considered: classic wrinkles near the mid-plane of a structure, back-surface wrinkles formed from a resin bulge near the back of a structure and folded wrinkles where several plies can be folded over completely in the bulk of the structure. Both simulations and experimental results demonstrate the effectiveness of these metrics on various types of structure, including carbon-fibre and hybrid carbon/glass-fibre composites with a range of ply thicknesses and wrinkle types.

Fracture toughness and fiber bridging mechanism for mode-I interlaminar failure of spread-tow woven composites

Given the current lack of research data on the delamination behavior of spread-tow woven composites, this study aims to systematically characterize the mode-I interlaminar fracture behavior of plain weave spread-tow fabrics (STFs) composites. DCB tests were conducted to investigate the influence of different ply thicknesses, unit cell sizes, and interface angles on delamination behavior and fiber bridging. The experimental results revealed a stick–slip crack propagation behavior in mode-I delamination of studied material, and a detailed analysis was performed on the influence mechanism of unit cell size on this behavior. Fiber bridging was found to be the primary mechanism responsible for the increased interlaminar fracture toughness in the plain weave STFs composites. Fractography analysis indicated that regions rich in fibers and resin in thicker-ply laminates promote fiber bridging, while an interface angle inconsistent with the direction of crack propagation inhibits fiber bridging. The concept of average steady-state fracture toughness (average Gs) was introduced as a means to assess propagation fracture toughness and two methods were proposed to obtain bridging tractions for characterizing the fiber bridging phenomenon. The bridging tractions and average Gs were used as key parameters to establish a trilinear cohesive zone model (CZM) for simulating mode-I delamination. The numerical results aligned well with the experimental results, demonstrating the applicability of the parameter determination strategy based on the average Gs and bridging tractions for the selection of parameters to use in the CZM.

A review on gas permeability of polymer matrix composites for cryogenic applications

Fiber-reinforced polymer matrix composites are increasingly considered for lightweight cryogenic pressure vessels due to their excellent combination of tailorability, specific mechanical properties, and relatively low coefficients of thermal expansion. However, significant challenges must be overcome to fully utilize PMCs for cryogenic fuel tanks in terms of transverse microcracking and subsequent permeation of cryogenic fuel. Gas permeation and microcrack densities of cryogenically cycled composites are highly influenced by their layup, ply thickness, load case, and manufacturing defects like voids and resin rich zones. There has been a significant amount of research on measuring gas permeation of composites fatigued under pure thermal or uniaxial thermo-mechanical stresses. However, results demonstrate that the gas permeability should be measured under biaxial thermo-mechanical stresses to properly gauge the leakage characteristics of damaged composites. This paper summarizes the results from over a hundred papers on the key parameters that influence the gas permeability of composites, appropriate testing methods to cycle composites for permeability measurement, methods to limit the evolution of transverse microcracks, and materials traditionally used for the fabrication of all-composite cryogenic fuel tanks. Thin plies and nanofiller-toughening of the matrix have been shown to provide significant improvements in transverse microcrack suppression within cryogenically cycled composites.

Mechanical property distribution of composites considering cured ply thickness variability

Variability is an essential feature of composites and directly affects the application of composite structures. Only a few studies have been conducted on the mechanical properties of composites considering cured ply thickness variability. This paper analyses the phenomena of cured ply thickness and the mechanical properties of materials obeying normal distribution after normalization. The distribution function and probability density function of the mechanical properties of composites considering the variability of cured ply thickness are derived. The correctness of the derived distributions is verified by Monte Carlo simulation, which compares the distribution of mechanical properties of composites after normalization with that considering the variability of cured ply thickness. The results showed that after considering the cured ply thickness variability, the mechanical properties showed a skewed distribution. At the lower percentile of the distribution, the mechanical properties are low compared to the normalized values, which reduces the permissible values of the composites.

Hybrid and gradient design of ultra-thin-ply composite laminates for synergistic suppression of delamination and fiber fracture damage modes

Ply thickness hybridization is a common strategy in the layup structure of composite laminates. The rational allocation of thin-ply and thick-ply to effectively improve the performance of composite materials is a shared goal. Compared to standard laminates, ultra-thin-ply laminates have many advantages, including design flexibility, better damage tolerance, and enhanced in-situ strength. However, the risk of fiber fracture triggered by ultra-thin-ply requires cautious application. Effectively reducing the risk of fiber fracture caused by ultra-thin-ply laminates has become a challenge in the field of composite materials. Therefore, this paper proposes a thick/thin/thick gradient structure to balance delamination and fiber fracture. Short-beam bending test results demonstrate that this novel gradient structure exhibits good bending load-bearing performance and damage resistance, achieving the coexistence of multiple damage couplings and effectively enhancing damage tolerance.