Statistical safety evaluation method for composite high-pressure hydrogen vessels considering stacking sequence effect and fiber strength development rate

In the service of composite materials, scenarios involving repeated impacts are frequently encountered. In our study, the time‐frequency characteristics of acoustic emission signals during repeated impact processes were investigated using wavelet packet transform. Principal Component Analysis was utilized to identify the frequency bands containing the most information. Shannon entropy was employed to select the optimal wavelet basis function. The terms DDD3, ADD3, and DAA3 correspond to matrix cracking, delamination, and fiber failure, respectively. The initial impact, due to the presence of numerous internal voids in the specimen, often leads to stress concentration, thus resulting in a prolonged duration of matrix cracking during the initial impact. Delamination becomes more active when matrix cracking initially occurs. Fiber failure typically accompanies unloading when it becomes active. Matrix cracking dissipates the most energy, followed by delamination and then fiber failure. When the final impact penetration occurs, the characteristic frequency band proportions for the quasi‐isotropic matrix cracking, delamination, and fiber failure are 48.99%, 22.39%, and 7.16%, respectively. For the cross‐ply laminate, the corresponding proportions of the characteristic frequency bands are 56.62%, 14.46%, and 6.98%, respectively. Highlights Multiple impact tests were conducted, and acoustic emission signals were recorded. Principal component analysis was performed to select the characteristic frequency bands. The time‐frequency characteristics of the two types of laminated composites during the impact process were analyzed. The trends of the characteristic frequency band proportions under multiple impacts on the composites were investigated.

Vegetal fibre composites are in significant demand by the industry due to their renewability and biodegradability. An important characteristic of fibre reinforced composites is that various mechanical responses can be obtained by altering the constitutive stacking sequence of laminates. The bio-inspiration of the composite designs represents an increasing trend due to their capacity to improve the overall strength of the structure. In this study, samples with bioinspired and non-inspired, quasi-isotropic, stacking configurations were manufactured. Hand layup of flax fibre epoxy pre-preg, consolidation using vacuum bagging and polymerisation in autoclave were used as the manufacturing method. The test samples have been tested under a constant quasi static regime of 2 mm/min with three mechanical response regions being identified. Two samples from each stacking configuration have been tested and the mean mechanical response has been considered in the investigation. The contact force-displacement data, energy absorbed, maximum contact force and related displacement are reported. The bio-inspired stacking sequence [±45/40/80] s laminates showed with 62% higher peak load than the conventional stacking laminate. The main energy dissipating mechanisms, observed by looking at the front and rear of the samples, were matrix cracking, delamination and fibre failure. It can be detected, in terms that the quasi-isotropic stacking sequence exhibited a limited damage area compared to the bio-inspired samples.

Pultruded glass fibre reinforced polymer (pGFRP) composites provide outstanding properties for composite polymer cross arms in power transmission line applications. This study has investigated the effects of various stacking sequences of fibres directions of pGFRP on flexural strength and creep behaviour. The use of static four-point bending flexural tests revealed that Stacking Sequence 2 (±45/0/90/0/90/0) had a significant flexural strength of 399.9 MPa while Stacking Sequence 1 (±45/90/0/±45) had a flexural strength of 242.5 MPa. Furthermore, the four-point bending creep experiments were performed at three distinct stress levels, notably 12%, 24%, and 37% of the ultimate flexural strength, to characterise the creep behaviour of distinct stacking sequences. Moreover, Findley’s power law equation for bending creep behaviour has revealed that the time-dependent reduction factor of Stacking Sequence 1 and Stacking Sequence 2 estimates a drop in flexural modulus of 23% and 10% respectively.

7 - Time-dependent nonlinear damage behavior of ceramic–matrix composites

In this work, the effect of water absorption and stacking sequences on the mechanical properties (i.e. impact, tensile and flexural) of hybrid sisal/glass polyester composites has been studied. Each composite laminate is prepared by the hand lay-up technique followed by static compression using about 20 wt% of fibre content. Different stacking sequences are obtained by changing the position and number of glass layers keeping a total of eight plies in each composite. Water absorption behaviour is investigated by soaking the composite specimens in the distilled water until saturation and its characteristics such as sorption, diffusion and permeability coefficient are measured. The obtained results suggested that the mechanical and water resistance properties of sisal composites are considerably enhanced by the incorporation of glass fibre. There is 23%, 29% and 46% improvement in tensile strength, flexural strength and impact strength respectively in hybrid composites than that of sisal composite. However, the effect of water absorption showed a significant reduction in the mechanical performance of all the composites. Morphological analysis by scanning electron microscope has been also performed to support the statement as weakening of fibre–matrix interfacial bonding by water absorption.

Effects of stacking sequence and rotation angle of patch on low velocity impact performance of scarf repaired laminates

Low-velocity impact predictions of composite laminates using a continuum shell based modeling approach Part b: BVID impact and compression after impact

Matrix damage of fibrous composites: Effects of thermal residual stresses and layer sequences

This study presents an experimental and analytical investigation into the impact behavior of electrospun polyacrylonitrile (PAN) nanofiber interleaved carbon fiber-reinforced polymer (CFRP) composites. Experimental testing was conducted using a single-stage gas gun, with photogrammetry employed to capture the high-speed impact dynamics. An energy-based analytical model was developed to identify dominant energy absorption mechanisms during impact. Results indicate that the nano-interleaved CFRP (Nano) exhibited superior ballistic performance compared to the baseline composite (Control). The ballistic limit increased from 70 m/s for Control to 80 m/s for Nano, a 14% improvement along with a 24% increase in energy absorption per unit areal density. Damage analysis revealed distinct failure patterns between the two. The analytical model accurately predicted energy absorption contributions: tensile failure of primary fibers (37%), secondary fiber deformation (33%), delamination (14%), and matrix cracking (15%) in Control; versus secondary fiber deformation (33%), delamination (26%), matrix cracking (25%), and minimal tensile failure (15%) in Nano. The strong correlation between experimental and analytical results confirms that PAN nanofiber interleaving significantly enhances the ballistic properties of CFRP, offering promising potential for future advancements in impact-resistant composite materials.

The goal of this study is to initiate a “test-calculation dialogue” on low velocity/low energy impact tests in laminated composites. The different types of impact damage developing during an impact test, i.e. matrix cracking, fiber failure, interface delamination and permanent indentation, are simulated. The bibliography shows a general lack of detailed validation of impact modeling and the originality of this work is to use refined and complementary experimental data to build and validate a numerical model. The good correlation between the model and this refined experimental database gave us relative confidence in the model, despite a few non-standard material parameters.

An approach was proposed to design high-pressure composite hydrogen vessels for fuel cell vehicles with damage tolerance. In addition, a stacking-sequence optimization method was developed, which was built upon the damage tolerance design. A no-growth approach, damage categorization, and residual strength requirements were proposed for the damage-tolerance design method for high-pressure hydrogen vessels, which were implemented in the damage-tolerance design method for composite aircrafts. The initial burst pressure was set at 180% of normal working pressure (NWP) assuming that the structural health monitoring (SHM) system with acoustic emission (AE) testing would immediately detect the occurrence of fiber failure. Stacking-sequence optimization was then conducted to minimize vessel thickness and meet the residual strength requirements. As a result of the optimization, the calculations confirmed that the vessel thickness could be reduced by approximately 52% of the existing vessel stacking sequence. The results show that the thickness of the vessel can be reduced by introducing a damage-tolerant design. Moreover, the proposed method for an optimal design based on the damage-tolerance design method is effective.

Interpreting acoustic energy emission in SiC/SiC minicomposites through modeling of fracture surface areas

The effect of progressive damages in composite materials is incorporated into a geometrically nonlinear solid shell element formulation to investigate the dynamic response of composite structures under the blast wave pressure loading. The effect of material damages such as fiber failures, matrix cracking, and fiber-matrix shearing failures on the behavior of composite structures is addressed via reducing their stiffness corresponding to detected failures. Two separate failure criteria are used for unidirectional and woven composites. In addition, the strain-rate effect on the material strength is taken into account. For the dynamic formulation, the trapezoidal rule is chosen for the time integration. Various numerical analyses of composite panels are conducted under the static and dynamic loading conditions to validate the effectiveness of the present formulation. The analysis results clearly exhibit the significant effect of material damages on the structural behavior of the composite panels. Also, the analysis results are in good agreement with the available experimental data.

The aim of this study is to improve the flexural performance and damage mechanism of nano-filled three-dimensional orthogonal woven E-glass/epoxy composites (3DOWC). The inherent brittleness of epoxy-based 3DOWC leads to the early onset of damage mechanisms such as matrix cracking, fibre-matrix debonding, and fibre failure. To overcome these limitations, epoxy resin has been modified with nano-fillers such as graphene nanoplatelets (GNP) and the novel nanostrength® (NS). The epoxy resin was infused in 3DOWC using VARI with different weight percentages of GNP (0.5, 1.0, and 1.5 wt.%) and NS (2.5, 5.0, and 7.5 wt.%). Three samples in each warp and weft direction of 3DOWC were tested in a three-point bend test. The results showed an increase of 48.4%, 56.2%, and 27.4% in flexural strength, final failure, and energy absorption under warp-loading with 0.5 wt.% GNP, respectively, whereas weft-loaded samples with 1.5 wt.% GNP exhibited 12.0% and 10.5% increases in flexural strength and final failure, respectively. However, 7.5 wt.% NS showed an increase in flexural strength in the warp and weft directions by 36.9% and 39.3%, respectively, and an increase in final failure and energy absorption in the warp direction by 39.4% and 12.3%, respectively. For weft-loaded samples, the final failure increased by 39.3% for the same weight percentage, but there was no significant increase in energy absorption in this direction. Scanning electron microscopy (SEM) of damaged samples revealed that crack reconnection by GNP and fibrils formation and plasticization by NS particles improved the overall flexural performance of 3DOWC.

Numerical/experimental impact events on filament wound composite pressure vessel