Damage Initiation Point Research Articles

Multi-scale fatigue damage analysis in filament-wound carbon fiber reinforced epoxy composites for hydrogen storage tanks

This article presents the findings of a multi-scale experimental study on carbon fiber-reinforced epoxy composites (CFRP) used in lightweight hydrogen storage pressure vessels produced via filament winding. The research employs a combination of tension-tension load-controlled fatigue tests and high-resolution physical-chemical characterization and porosity quantification to assess the impact of porosity on mechanical performance. The findings demonstrate that porosity has a detrimental impact on mechanical properties, acting as nucleation sites for damage mechanisms such as crack initiation, fiber-matrix separation and fiber breakage. At the mesoscopic level, microdefects coalesce into transverse cracks and delamination, resulting in complex failure modes under cyclic loading. The results of the tensile tests demonstrated that the orientation of the fibers has a significant impact on the mechanical behavior of the material. The ±15° configuration demonstrated superior tensile strength and modulus, while the ±30° and multilayer configurations exhibited higher ductility. The results of the fatigue testing confirmed that fiber orientation has a significant impact on fatigue life, with the ±15° configuration proving to be the most resistant. Microscopic analysis indicated that pores act as damage initiation points, accelerating failure through matrix cracking, fiber-matrix debonding, and delamination. This study highlights the need for improved porosity control during manufacturing to enhance the durability of hydrogen storage systems. Additionally, it provides valuable insights for optimizing fiber orientation to improve fatigue performance in practical applications.

Effects of damage initiation points of depth-damage function on flood risk assessment

The flood depth in a structure is a key factor in flood loss models, influencing the estimation of building and contents losses, as well as overall flood risk. Recent studies have emphasized the importance of determining the damage initiation point (DIP) of depth-damage functions, where the flood damage is assumed to initiate with respect to the first-floor height of the building. Here we investigate the effects of DIP selection on the flood risk assessment of buildings located in Special Flood Hazard Areas. We characterize flood using the Gumbel extreme value distribution’s location (μ) and scale (α) parameters. Results reveal that average annual flood loss (AAL) values do not depend on μ, but instead follow an exponential decay pattern with α when damage initiates below the first-floor height of a building (i.e., negative DIP). A linear increasing pattern of the AAL with α is achieved by changing the DIP to the first-floor height (i.e., DIP = 0). The study also demonstrates that negative DIPs have larger associated AAL, thus contributing substantially to the overall AAL, compared to positive DIPs. The study underscores the significance of proper DIP selection in flood risk assessment.

Experimental study and numerical simulation of transverse tensile behavior for thermoplastic glass fiber reinforced plastics

In order to predict the mechanical behavior of thermoplastic composites, the phases of glass fiber reinforced plastics (GFRP) were discretized within the computational micromechanics framework. This approach incorporated the random distribution of fibers, nonlinear matrix behavior, and interface damage behavior in a synergistic manner. Subsequently, a three-dimensional representative volume element (RVE) model was established. To verify the accuracy of the numerical model, macroscopic transverse tensile experiments on GFRP were designed. Building upon this foundation, a detailed study on the damage failure of GFRP under transverse tensile loads was conducted to reveal the damage evolution mechanisms. Furthermore, the influence of the random distribution of fibers, nonlinear characteristics of the matrix, and interfacial parameters on the transverse damage mechanical behavior of GFRP was elucidated. The results demonstrate that the random distribution of fibers significantly affects the damage initiation point and crack evolution path. Moreover, GFRP with higher interfacial performance exhibits substantially improved transverse tensile strength compared to those with lower interfacial performance.

Strength characteristics and damage constitutive model of sandstone under hydro-mechanical coupling

Abstract To study the mechanical properties of saturated sandstone, experiments were conducted under hydro-mechanical coupling on saturated sandstone. A damage constitutive model was established to describe the response of saturated sandstone under pore pressure, and its validity was verified using the results of the triaxial tests. The results indicate that the peak strength (σ p), effective peak strength (σ p′), residual strength (σ r), effective normal stress (σ n′), effective shear strength (τ n′), elasticity modulus (E), and rupture angle (θ) of sandstone are positively correlated with the confining pressure (σ 3) and negatively correlated with the pore pressure (P). Conversely, Poisson’s ratio (μ) exhibits an opposite relationship. The model parameters exhibit non-linear relationships with the confining pressure (σ 3), with the parameter m decreasing gradually as the confining pressure increases, and the parameter F 0 increasing with higher confining pressure (σ 3). Moreover, the pore pressure (P) and the confining pressure (σ 3) significantly affect the damage variables (D), with the stress value at the damage initiation point increasing with increasing confining pressure (σ 3), while the strain value at the damage initiation point decreasing with increasing pore pressure (P), indicating that pore pressure induces damage development in rocks.

Influence of Curing Agents Molecular Structures on Interfacial Characteristics of Graphene/Epoxy Nanocomposites: A Molecular Dynamics Framework

AbstractThis paper presents a comprehensive molecular dynamics study on the effects of the stoichiometric ratio of epoxy:hardener, hardener's linear and cyclic structure, and number of aromatic rings on the interfacial characteristics of graphene/epoxy nanocomposite. The van der Waals gap and polymer peak density as a function of the type of the hardener is calculated by analyzing the local mass density profile. Additionally, steered molecular dynamics are used to conduct normal pull‐out of graphene to study the effect of the mentioned features of hardeners on the interfacial mechanical properties of nanocomposites, including traction force, separation distance, and distribution quality of reacted epoxide rings in the epoxy. Influence of the hardeners on the damage mechanism and its initiation point are also studied by analyzing the evolution of local mass density profile during the normal pull‐out simulation. It is seen that stoichiometric ratio and geometrical structure of the hardeners affect the interfacial strength. It is also revealed that the hardener type can change the epoxy damage initiation point. The damage occurs in the interphase region for a higher stoichiometric ratio or cyclic structure of hardener. In comparison, for hardener's lower stoichiometric ratio and non‐cyclic structure, failure begins in the epoxy near graphene layers.

Failure analysis and multi-objective optimization of crashworthiness of variable thickness Al-CFRP hybrid tubes under multiple loading conditions

Different structural parameters and interlaminar damage will have significant effects on the crushing performance of metal-composite hybrid structures. A hybrid gradient structure is proposed in this paper. By varying the external CFRP thickness of the hybrid tube, realize the gradient of the crushing performance of AI-CFRP hybrid tube. Considering the above effects, under the different loading angles, it has realized that the optimization design of comprehensive crushing performance of Al-CFRP hybrid gradient structure. The results indicated that compared with pure Al tube and hybrid structure, hybrid gradient structure has obvious performance advantages; during the compression process, the damage initiation point is at the tip of the crushing end of the tube. The matrix damage evolution shows strong directionality, and the matrix tensile stress of CFRP is the main cause of interlaminar damage. Compared with the specific energy absorption, the variation pattern of the delamination rate of the hybrid gradient tube shows similar regulation. Compared with the gradient hybrid tube under the structural parameters of the specimen, the peak crushing force under the optimal structural parameters (in Case 1) is reduced by 36.07%, and the specific energy absorption (in Case 3) is increased by 23.24%.

Composite Material Failure Model Updating Approach Leveraging Nondestructive Evaluation Data

Abstract A novel failure model updating methodology is presented in this paper for composite materials. The innovation in the approach presented is found in both the experimental and computational methods used. Specifically, a dominant bottleneck in data-driven failure model development relates to the types of data inputs that could be used for model calibration or updating. To address this issue, nondestructive evaluation data obtained while performing mechanical testing at the laboratory scale are used in this paper to form a damage metric based on a series of processing steps that leverage raw sensing inputs and provide progressive failure curves that are then used to calibrate the damage initiation point computed by full-field three-dimensional finite element simulations of fiber-reinforced composite material that take into account both intra- and interlayer damage. Such curves defined based on nondestructive evaluation data are found to effectively monitor the progressive failure process, and therefore, they could be used as a way to form modeling inputs at different length scales.

A parametric analysis of damage evolution for pull-out of a rigid fiber from an elastomer matrix

The structures composed by metal fiber and auxiliary elastomer are stretchable and have potential application under larger strain. Nowadays, interfacial failure is still the key problem affecting practicability of the structures. The result of typical pull-out test indicates that three stages exist in this pull-out process: elastic deformation, interfacial debonding and interfacial friction. For the sake of determination of interfacial parameters, a numerical method has been proposed to characterize the damage initiation point by using the slope of force-displacement curve of pull-out experiment. Furthermore, based on the basic material and interfacial parameters, extensive simulations have been conducted to study parametric effect on the general mechanical behavior of structures, including interfacial elastic modulus, failure strength and damage evolution law and friction. The results indicate both interfacial cohesive and friction contribute to the force increasing after damage initiation points.

Discussion on Determination Method of Long-Term Strength of Rock Salt

Due to the extremely low permeability and the excellent creep behavior, rock salt is the optimal surrounding rock of underground energy storage. The long-term safe operation of the rock salt energy storage is closely related to the creep behavior and long-term strength of rock salt, but few researches focus on the long-term strength of rock salt. In order to more accurately predict the long-term strength of rock salt, the isochronous stress–strain curve method and the volume expansion method for determining the long-term strength were analyzed and discussed based on axial compression tests and axial creep tests. The results show that the isochronous stress–strain curve method is intuitive but will greatly increase the test cost and test time to obtain a satisfactory result. The volume expansion method is simple, but the long-term strength obtained according to the inflection point of volumetric strain is much greater than the actual long-term strength of rock salt. Therefore, a new method applicable to rock salt was proposed based on the evolution of damage in rock salt in this paper, which takes the corresponding stress value at the damage initiation point as the long-term strength. The long-term strength determined by this method is consistent with that by the isochronous stress–strain curve method. The method is more economical and convenient and aims to provide a reference for the long-term stability study of underground salt caverns.

Experimental and numerical investigation of the impact response of elastomer layered fiber metal laminates (EFMLs)

The aim of the present study is to investigate the effect of introducing an elastomer layer into conventional fiber metal laminates on their perforation resistance. Natural compounded rubber as elastomeric media and glass/epoxy composite were sandwiched in between two layers of aluminum 6061-T6 and then the resulted structure was perforated by a 10 mm diameter hemispherical projectile at different impact velocities. Residual velocities were recorded by a high speed camera via a shadowing technique. Results showed that an elastomer layer located nearer to frontal face had a better energy absorbing performance due to load spreading; besides, by increasing the impact velocity the elastomer performs more efficiently because of the elastomer damage initiation point movement toward the periphery of the stretched area. Numerical simulation of penetration process was accomplished using the advanced finite element code of LS-DYNA. Finally, a numerical parametric study was performed to assess the effect of elastomer thickness on the energy absorption efficiency (EAE) of the whole structure. Based on the obtained results, adding an elastomeric layer into the structure is more beneficial than composite thickening at the same thickness in terms of improving EAE and reducing areal density.

Effect of interface properties on micromechanical damage behavior of fiber reinforced composites

This paper investigates the micromechanical damage behavior of carbon-epoxy composite on the transverse tensile of representative volume elements (RVEs) with various fiber volume fractions. A new algorithm is developed to generate the random distribution of fibers in the RVE, and it is possible to create fiber distributions with high fiber volume fractions. The fibers and matrix are considered linear elastic and elastoplastic with progressive damage criterion, respectively. Fiber-matrix debonding and matrix crack are considered as the dominant damage modes. The normal cohesive properties distribution is applied for interfaces between fibers and the matrix in order to achieve a more realistic microstructure. To investigate the effect of the position of fibers with the weakest cohesive strengths, sensitivity analyses concerning the different arrangements of specific normal cohesive properties on the RVE’s strength are performed. Moreover, the effects of different damage parameters, such as random fiber distributions and various cohesive parameters on the overall damage behavior of the RVE, are described in detail. It is revealed that the application of a normal cohesive distribution sharply reduces the maximum strength of the RVE and shifts the strain of damage initiation point and crack propagation path.

Experimental and numerical study of mode-I and mixed-mode fracture of ductile U-notched functionally graded materials

In the present study, ductile fracture of the U-notched samples made from bainitic Functionally Graded Steel (FGS) has been studied experimentally and numerically under the mode-I and mixed-mode conditions. The desired FGS plates were fabricated using the Electro Slag Remelting (ESR) technique. Then, the notched specimens were tested under the three-point bending condition. Effects of both the notch geometry and loading point position have also been investigated. A numerical scheme based on kinematic hardening constitutive model as well as Hollomon's equation has been utilized to define the material response including damage evolution. This numerical scheme is also able to predict the damage initiation point based on a method that uses both the stress triaxiality and equivalent stress values. Moreover, material properties of each point within the graded regions have been specified using the strain gradient plasticity theory, instability point concept and area under stress–strain curve. Comparing the experimental results with the numerical ones shows the capability of the numerical scheme. Also, the critical fracture loads of the notched samples made from FGS show the superiority of the present graded material over the homogeneous one.

Life Prediction of Rocket Combustion-Chamber-Type Thermomechanical Fatigue Panels

Thermomechanical fatigue panels represent small actively cooled sections of the hot-gas wall of a regeneratively cooled liquid rocket engine. In combination with cyclic laser heating, the panels are used to study both the fatigue life and thinning and bulging phenomena (the so-called doghouse effect) without the need for testing a full-scale engine. To improve the prediction of the thermomechanical fatigue panel’s fatigue life, a viscoplastic model coupled with isotropic damage and thermal aging was implemented. The temperature-dependent material parameters are determined using experimental data to take into account softening effects due to material degradation. The number of cycles to failure is computed numerically and compared to the results of a thermomechanical fatigue panel test. The damage parameter-based finite element analysis successfully predicts the damage initiation point in the middle cooling channel of the thermomechanical fatigue panel. Critical damage occurs at the 150th cycle, whereas the failure is experimentally observed at the 174th cycle. The effect of the increase of the wall thickness in the fin areas is also obtained by this numerical analysis. This study shows that cost-efficient thermomechanical fatigue panel tests have the potential to validate numerical fatigue life predictions for novel rocket combustion-chamber wall materials.

A tensorial based progressive damage model for fiber reinforced polymers

Progressive damage simulation is of major importance for predicting the damage behavior of composite laminates. This work faces the challenge to apply a progressive damage law for oblique fracture planes. Based on Puck’s or Camanho’s failure condition the damage initiation point and the fracture plane are determined. The presented damage evolution techniques for degradation of the elasticity tensor and their deficiencies are introduced. A new degradation approach based on an eighth order damage tensor is derived in this work. For efficient application in a finite element simulation a mapping of the fourth order material tensor to the fracture plane coordinates is conducted. The degradation is applied in the fracture plane system. Improved numerical stability and physically plausible behavior can be achieved by this approach. Both is demonstrated by simulation of a composite cube under out-of-plane compression load and a low velocity impact simulation. For evaluation and validation, the achieved results are compared with experimental data.

Strain-based plastic instability acceptance criteria for ferritic steel safety class 1 nuclear components under level D service loads

This paper proposes strain-based acceptance criteria for assessing plastic instability of the safety class 1 nuclear components made of ferritic steel during level D service loads. The strain-based criteria were proposed with two approaches: (1) a section average approach and (2) a critical location approach. Both approaches were based on the damage initiation point corresponding to the maximum load-carrying capability point instead of the fracture point via tensile tests and finite element analysis (FEA) for the notched specimen under uni-axial tensile loading. The two proposed criteria were reviewed from the viewpoint of design practice and philosophy to select a more appropriate criterion. As a result of the review, it was found that the section average approach is more appropriate than the critical location approach from the viewpoint of design practice and philosophy. Finally, the criterion based on the section average approach was applied to a simplified reactor pressure vessel (RPV) outlet nozzle subject to SSE loads. The application shows that the strain-based acceptance criteria can consider cumulative damages caused by the sequential loads unlike the stress-based acceptance criteria and can reduce the overconservatism of the stress-based acceptance criteria, which often occurs for level D service loads.

Optical Fiber Distributed Sensing - Physical Principles and Applications

Obtaining the strain data all along the optical fiber, with adequate spatial resolution and strain accuracy, opens new possibilities for structural tests and for structural health monitoring. Formerly, only point sensors, as strain gages or fiber Bragg grating, were available, and information about the response to loads was restricted only to those points on which the sensors were bonded. Unless a sensor was located near the damage initiation point, details about the failure initiation and growth were lost. With a distributed system, the information is given as an array of data with the position in the optical fiber and the strain or temperature data at this point. In this article, the physical principles underlying the different techniques for distributed sensing are discussed, a classification is done based on the backscattered wavelength; this is important to understand its possibilities and performances. The definition of performance for distributed sensors is more difficult than for traditional point sensors because the performance depends on a combination of related measurement parameters. For example, accuracy depends on the spatial resolution, acquisition time, distance range, or cumulated loss prior to measurement location. The field of applications of this new technology is very wide; results of the structural tests of a 40 m long wind turbine blade, detecting the location and load of onset of buckling, and the results of the delamination detection in a composite plate, are presented as examples.

Incremental Damage Development in a 3D Woven Carbon Fibre Composite

Interest in 3D woven carbon fibre composites has increased within industries such as aerospace, automotive and marine, due to their high strength to weight ratio, their increased tailorability and their capacity to be manufactured into near net shape preforms, thereby reducing parts count, assembly time, labour intensity and costs. It is vital that critical areas of concern such as damage (and in particular damage initiation and development) are studied and understood, thereby reducing the limiting factors to their acceptance. The damage initiation and subsequent intervals of development for ILSS (Interlaminar Shear Strength) were determined experimentally. Particular focus is paid to the significance of binder edge and binder middle testing and the influence of through-the-thickness (T-T-T) reinforcement in relation to damage types and development. Control samples for binder edge and binder middle loading locations were tested to failure as a means of determining an average point of failure, allowing the generation of testing intervals. The performance and architecture of samples from each incremental interval were characterised using a combination of graphical analysis and optical microscopy with the aid of dye-penetrant to highlight fibre damage and matrix cracking. Samples displayed specific damage initiation points, thus allowing the generation of a damage guide relating to applied force. In addition, the results imply that a relationship exists between the location of applied load and subsequent damage, thus showing the significant influence played by the T-T-T binder loading location on damage development within 3D woven carbon fibre composites. Some of the preliminary data shown in this paper was presented at IMC23 at the University of Ulster, UK in August 2006 and at Texcomp 8 in Nottingham, UK October 2006.

Fatigue Damage Progression of Woven Fabric Composites under Cyclic Loading (Observation of Initial Damage Process by Thermoelastic Image Technique)

Initial fatigue damage process of plain woven CF/epoxy composites was studied with thermoelastic image technique. Thermoelastic damage analysis was conducted to evaluate the fatigue damage in the specimen. The experimental results showed that the damage initiation points were widely scattered in the location of the material. The each area, where the damage initiated, had different damage level at any fatigue cycles. The thermoelastic damage response can be an indicator for the state of fatigue damage of the composites. The thermoelastic damage analysis also predicted that the initial fatigue damage process of plain woven fabric composites changed when the local damage area was distributed in lateral direction of the specimen.

Interlaminar Stresses in Composite Notched and Unnotched Laminates

The purpose of this paper is to determine the interlaminar stress distribution at curved and straight free edges for quasi-isotropic and zero-dominated laminates. The interlaminar stresses are calculated using a three-dimensional finite-element code with error control. For straight free edges the results are compared to existing semi-analytical methods. Another objective with this investigation is to evaluate a quadratic failure criterion for prediction of initial load for delamination and the most likely position for the delamination. In this paper it is found, for quasi-isotropic stacking sequences, that it is not possible to tailor a stacking sequence so that o, becomes compressive near hole boundaries or at straight free edges. On the contrary both tensile and compressive stresses appear in a periodic manner through the thickness. It can also be concluded that interlaminar stresses exhibit a thickness effect for repeating bundles. The singularity of the interlaminar components is concentrated to a narrow region, approximately 1-2 fibre diameters in thickness and radial direction. Since the interlaminar stresses are so localised, a continuum approach will not provide useful solutions in this area. Consequently the material cannot be treated as homogeneous in this region, instead the heterogeneous nature within each ply should be modelled. The smallest element size in the present analysis for the refined mesh is less than 1 pum, i.e., far less than a fibre diameter. Utilising a quadratic failure criterion, for the studied quasi-isotropic laminates, in conjunction with obtained FE-results, it is shown that global strain to initiate delamination does not depend on the stacking sequence, but so does the number of possible initiation points. For quasi-isotropic layups with a circular cut-out, initiation of delaminations occurs at approximately the same global strain, 0.25%. For unnotched zero-dominated layups initiation and fracture strain are almost the same, i.e., -1.2%. The investigation also shows that the number of ±450/00 or ±45°/900 alterations should be kept at a minimum to minimise the interlaminar stresses and possible damage initiation points. It is likely that the primary cause for delamination initiation for notched plates are the high interlaminar shear stresses and not the normal stresses. Finally it is demonstrated that available semi-analytical methods do not correctly describe the interlaminar stress distribution.