Damage Modes Research Articles

Experimental investigation of uncertain factors on the mechanical properties of half-grouted sleeve connected with large-diameter high-strength reinforcement

The half-grouted sleeve connection (HGSC) is widely used to connect reinforcement in precast concrete (PC) structures. While some studies have examined the mechanical behavior of the HGSC, there is still a lack of research on uncertain factors with large-diameter high-strength reinforcement. To fill this gap, an extensive experimental study was conducted to investigate the mechanical properties of the HGSC under monotonic tensile loads. The impact of various uncertain factors, including the embedded length, water-binder ratio (w/b) of the grouting material, eccentricity of rebar, thread diameter, and thread length, was investigated. It was found that the HGS connected with large-diameter high-strength reinforcement has four damage modes, i.e., thread fracture, thread pullout, reinforcement fracture, and bond-slip failure. Notably, to obtain the target of equivalent reinforcement, the dispersion ratio of the w/b should be controlled within 0.3, the embedded length should be greater than 6d, and the thread diameter and the thread length should not be less than 1d. Based on the HGSC investigated in this paper, a model for predicting the axial force and circumferential confining strain was presented, and a method was provided to calculate the elastic and plastic lengths for the embedded reinforcement. This study provides valuable guidance for the application of large-diameter high-strength rebar in PC structures.

Tensile fatigue behavior and damage evolution of z-pinned carbon/phenolic woven composite using in-situ monitoring technology

Carbon/phenolic composite, as structural materials for aerospace, undergoes static and dynamic loading which can affect the structural integrity of components. This paper aims to comprehensively investigate the mechanical properties and damage evolution mechanisms of z-pinned carbon/phenolic woven composite subjected to static tensile and different levels of tension-tension fatigue loading. The essential correlation between damage modes and acoustic emission (AE) signals of carbon/phenolic composite was first established. The AE signals were processed using the Hilbert-Huang transform method to obtain the percentage of damage modes at various load levels. The results showed that the peak frequency ranges of the three damage modes, matrix cracking, debonding, and fiber breakage, were 100–200 kHz, 200–300 kHz, and over 300 kHz, respectively. Moreover, the damage evolution mechanisms can be revealed using the OM-AE monitoring method. It was found that the dominant damage modes were all matrix cracking. The fatigue damage occurs at the edge of the specimen, and the three damage modes interact with each other on a microscopic scale. Cracks expand from the edge to the center at low stress. The developed OM-AE online damage monitoring system allows real-time damage monitoring and accurate recognition of damage modes.

Failure analysis of gear using continuous wavelet transform applied in the context of wind turbines

One of the main reasons for failure in the wind turbine is the wear between the gear teeth during the power conversion and changes in the rotation speed, which is also generally associated with changes in the lubrication regimes. In this sense, vibration and signal analysis are frequently used in predictive maintenance as they usually permit the identification of deviations in the proper functioning of the equipment. Thus, this work aims to apply the continuous wavelet transform (CWT) to correlate gear wear and vibration signals, using visual and straightforward analysis. An experimental setup of a gear system was used to analyze vibration signals from different tooth gear damages. Gears with different levels and modes of damage were used in order to evaluate the sensitivity of vibration signals to them. The features from vibration signals were extracted by Morlet wavelet analysis. Results demonstrate that the proposed method accurately detected the early failure by visualization in frequency–time maps.

A fragile points method with a numerical-flux-based interface debonding model to simulate the delamination migration in composite laminates

Predicting damage in composite laminates is essential for a better utilization of the material, but it is challenging especially when the inter- and intra-ply damages are coupled. In this paper, a Fragile Points Method (FPM) with a numerical-flux-based interface debonding model is proposed to simulate the laminates’ damage. The FPM is a novel meshless method using the discontinuous Galerkin weak formulations and the point-based discontinuous polynomial trial functions. In this work, an orthotropic interface debonding model has been developed based on the numerical fluxes to simulate the mixed-mode inter- and intra-ply damage. For any interior interface in an FPM model, the interface damage is triggered when the damage criterion is satisfied and the interface is separated when the fracture energy is reached. Due to the discontinuous characteristics of the FPM, the inter- and intra-ply damage modes are introduced explicitly, and thus the coupling between them can be naturally captured. In this paper, the proposed FPM is verified by four benchmark examples for Mode I, Mode II and Mixed Mode delamination. Then the proposed method is applied to simulate the delamination migration, used as an example for the inter- and intra-ply damage. The FPM results show that this method is able to provide reliable prediction on the load–displacement curves as well as the damage and crack patterns, and the mechanism governing the delamination migration is also discussed based on the FPM results.

On the Micro-Stress Fields in Fibre-Hybrid Polymer Composite Laminae with Periodic Microstructure Under Transverse Tension and Transverse Shear Loading

AbstractThis paper investigates the effect of intra-laminar fibre hybridisation, i.e., primary and secondary fibres within a matrix, on the homogenised properties and micro-stress fields in uni-directional polymer composite laminae. The study is focused on S-glass/epoxy laminae which are hybridised with secondary fibres (e.g., polypropylene). Two-dimensional repeating unit cells (2D RUCs) with periodic microstructures are developed to conduct the micro-mechanical analyses under transverse tensile and transverse shear loading conditions. Uni-directional fibre-hybrid S-glass/epoxy laminae with different secondary fibres are studied by varying (a) the periodic microstructure and (b) the material properties of the constituent fibres to assess the effect of such geometric and material variations on the homogenised elastic lamina properties and intra-lamina micro-stress fields. The results show that intra-laminar fibre hybridisation significantly affects the elastic lamina properties and micro-stress fields. Notably, the presence of the secondary fibres significantly increases or reduces the stress fields in the matrix and at the fibre-matrix interfaces (i.e. normal and shears stress components)–depending on the microstructure and the stiffness of the secondary fibres–which could be explored to manipulate the damage modes and thus energy dissipation mechanisms.

Experimental study on resin pin-reinforced marine sandwich composites under quasi-static indentation load

This study aims to experimentally examine the indentation behaviour of marine sandwich composites with pin-reinforced polyvinyl chloride foam core and E-glass face sheets. The effects of indenter diameter, resin pin diameter, and arrangement on force-displacement, maximum contact force, and absorbed energy values of sandwich panels were evaluated by the indentation tests. Throughout the experiments, the contact forces increased as the pin diameter increased. The contact forces reduced as the distance between pin diameters increased. Additionally, the values of absorbed energy increased as the diameters of the resin pin and indenter increased. Placing the pins more closely together additionally allows the material to absorb more energy in the event of indentation. A visual inspection took place on the post-indentation cross-sections of the sandwich specimens to detect any damage modes. Damage modes varied depending on the size of the indenter and the hole pattern in the foam core.

Experimental study on the effect of water-induced deterioration on shear properties of bolted rock joints

The investigation into the impact of dry and wet cycles as well as prolonged immersion on the shear properties of bolted rock joints can facilitate comprehension of the mechanism behind shear damage in anchorage rock slopes located within dam sites, which is crucial for conducting long-term stability analyses of such slopes and ensuring safe operation of hydropower plants. In this study, the sandstone from the Three Gorges reservoir area was collected and utilized to made into bolted rock joint specimens, which underwent 12 cycles of wetting and drying as well as 180 days of immersion before undergoing direct shear tests to investigate the degradation of their mechanical properties. The test results indicate that both treatments for water deterioration can significantly reduce the peak shear strength of bolted sandstone joints. The shear deformation characteristics of bolts also vary with the decrease in rock mass strength. As the number of wet and dry cycles, as well as immersion time, increases, the plastic hinge length of the bolt gradually extends. The surface roughness of the joint has an impact on the fracture mode of the bolt. In a flat joint, the fracture section of the bolt exhibits a certain inclination angle, indicating a damage mode where positive and shear stresses act in concert; whereas in a rough joint, the fracture section of the anchor rod shows almost zero inclination angle and is damaged under positive stress. Finally, an attempt was made to apply the conclusions of this paper to engineering by conducting a time-varying reliability analysis of the generalized anchored slope model. The results obtained have significant guidance for evaluating the long-term stability of anchored slopes in hydropower projects.

Dynamic responses of steady-state transition structures under impact loading

Advancements in materials science and mechanical system have spurred widespread investigation into non-traditional structures. In particular, engineers have shown interest in steady-state transition structures with phase transition properties. In this paper, a three-dimensional steady-state transition structure was established using the genetic algorithm and three-dimensional expansion, on which the low-velocity impact tests were carried out. Force response and energy absorption of the steady-state transition structures under impact were studied. From the force response and energy absorption curves, it can be seen that the steady-state transition structures absorb and store portion of impact energy temporarily, gradually releasing it after impact peak. This unique energy behavior results in a steady state transition structure with a long impact response time and a low peak impact force under impact. In addition, investigation into energy transfer within these structures reveals that energy oscillations occur among multilayered bistable surfaces during steady-state transitions, further improving the energy dissipation. Further, the dynamic response and damage modes of the honeycomb structure and the steady state transition structure were compared, showing that the damage mode of the steady state transition structure under the impact is an overall damage, in contrast to the penetration damage of the honeycomb structure. Notably, the steady-state transition structures showcase a higher proportion of elastic strain energy during impacts, effectively delaying structural damage onset, and dispersing impact energy, thus enhancing impact resistance. In light of these findings, the demonstrated behavior of steady-state transition structures in absorbing and dissipating impact energy showcases their potential as promising avenues for the development of more effective impact protection strategies.

Seismic performance loss evaluation of reinforced concrete frame structure based on updatable damage model

AbstractAs structural seismic damage develops, the internal force transfer and damage weight relationship of component are constantly changing, and the fixed weighted coefficient is adopted by the traditional weighted method, which cannot reflect the changes in the structural damage state. The structure is a network formed by connecting a large number of components, and the network state undergoes complex changes with the development of component damage. In this paper, a novel method for component damage transfer to structural seismic performance loss is proposed using network shortest paths. First, a directed weighted complex network based on force transfer direction and component stiffness is constructed for reinforced concrete (RC) frame structures. Then, the component damage is calculated by the degradation of its stiffness and is used to update the weight distribution of the damage network. Finally, the structural performance loss is evaluated by the degradation of damage network efficiency. Results show that the proposed method with an updated component weight relationship is able to reflect the true loss of structural performance according to the finite element dynamic elastic‐plastic analysis of RC frame structures. For the structural performance loss degree (none, slight, moderate, severe, complete), the proposed method and the weighted method both exhibit high accuracy (number correctly classified/total number of samples) when structural performance loss is none, severe, or complete loss. The accuracy of the proposed method is significantly higher than that of the weighted method when structural performance loss is slight and moderate loss, with 58.1% and 84.2% improvement, respectively. For structural typical damage modes (global, local, and weak layer damage mode), the proposed method and the weighted method both exhibit high accuracy when structures exhibit global damage mode. The accuracy of the proposed method is significantly higher than that of the weighted method when structures exhibit local and weak layer damage mode. The accuracy of the proposed method is stable at 95.2%, while the accuracy of the weighted method is 76.2% and 70.2%, respectively. Overall, the weighted method exhibits high accuracy when structural performance loss is less than slight loss or greater than moderate loss, while the proposed method exhibits high accuracy throughout the entire process of structural performance loss, especially in moderate loss.

The relationship between repeated impacts and CAI strength of sandwich structure considering different impactor shape in individual impact

Sandwich structures are susceptible to repeated impact loads, and each individual impact is not nearly identical to the previous one, especially for repeated impacts under different impactor shapes such as hail impact. Therefore, the repeated impact behavior using different impactor shape in individual impact of polymethacrylimide (PMI) foam sandwich structure is studied and discussed through experiments and finite element (FE) simulations in this paper. The representative mechanical characteristics such as impact responses, damage modes, and energy absorption are investigated until penetration occurs, and the mechanism of impact damage accumulation between impact events with different impactor shapes is revealed. Besides, the compression after impact (CAI) strength is utilized to assess the residual strength of sandwich structure under repeated impacts. It indicates when the contact area between the specimen and impactor is larger, it can result in a higher impact force, a shorter contact duration, and potentially reduce the extent of damage. The matrix damage is the main damage mode, and the proportions of matrix tension and compression are nearly constant, with a ratio of approximately 2:1. In addition, the proportion of the impact energy absorbed by foam core increases as the number of impacts increases. When the impact energy is low, the CAI strength is sensitive to the number of impacts, the shape of impactor, and the sequence of impactors, but the similar phenomenon does not occur at higher impact energy. The present study provided a series of experimental and numerical results, which could be utilized as design reference guidance in the designing of composite sandwich structures.

Experimental Investigation of Damage and Failure Modes in Stirrupless Reinforced Concrete Beams under Varied Thermal-Mechanical Loadings

Experimental Investigation of Damage and Failure Modes in Stirrupless Reinforced Concrete Beams under Varied Thermal-Mechanical Loadings

Multiscale coupling analysis of energy absorption in 3D printed continuous fiber reinforced thermoplastic orthogonal fabric composites

Generally, serious delamination and defect propagation are the primary damage modes observed in 3D printed continuous fiber reinforced thermoplastic composites (3D printed CFTP composites) under low-velocity impact. The study of fabric structure design and its impact resistance is crucial for the practical application of 3D printed composites, yet the existing research in this area is limited. In this paper, the preparation scheme of 3D printed continuous fiber reinforced thermoplastic orthogonal fabric composites (3D printed CFTPOF composites) with the improved impact resistance is proposed initially. Then, the analysis models for the molding process and mechanical properties of the 3D printed CFTPOF composites are established. To address the limitations of current analysis methods for 3D printed composites, a coupled multiscale method (3DP-MS method), is proposed for predicting mechanical properties. The transfer of mechanical parameters between multiscale models is also established and validated through experiments. Finally, the impact performance and the energy absorption of 3D printed CFTPOF composites under different fabric paths and yarn distributions are analyzed and discussed. This paper provides theoretical basis and simulation method for the structural design of impact-resistant 3D printed CFTP composites.

Damage evolution and destabilization precursors in granite under splitting load at different temperatures

Brazilian splitting acoustic emission (AE) tests were conducted on the granite specimens that were heated at the temperatures from 50 to 600 °C to investigate the mechanical and damage characteristics of granite under high temperatures and splitting load in deep drilling. The results indicate that temperature variations significantly alter the mechanical characteristics and AE properties of granite, thereby influencing the evolution of fractures. At the temperature above 500 °C, the tensile strength of granite decreases significantly and the proportion of shear-induced damage modes increases. Furthermore, the rate of fracturing also exhibits a noticeable increase. Damage parameter mutation points on the damage parameter evolution curves of hit counts and energy counts were defined to determine the rock damage precursor. Mutation points based on hit counts arrive earlier than rock instability signals. The greater the damage is, the earlier the mutation points arrive. This time difference can be used to identify rock destabilization signals.

Experimental and Numerical Investigation of Energy Absorption Characteristics of a E-Glass/Epoxy Crash Box

Crash boxes are energy absorbing components generally placed at the front end of cars to reduce the amount of damage at especially low impact velocities. The number of electric vehicles has been increasing recently, so weight reduction studies are. For this reason, lighter glass or carbon fiber reinforced composite crash boxes are preferred instead of steel ones. In the current study, the dynamic compression behavior of a rectangular cross-section thin-walled composite crash box was investigated both experimentally and numerically. The main aim of the study was to understand the effective damage modes and monitor the deformation sequence experimentally and numerically. Once the numerical model is verified then it can be further used to reveal the behavior at different impact velocities and geometries. The methodology followed in the study first started with the static mechanical characterization of the composite material. Within the scope of this study, 2×2 twill-woven glass fiber/epoxy crash boxes were produced using the vacuum bagging method. Quasi-static compression and tension tests were carried out in accordance with ASTM D3039 and ASTM D6641 standards. In the numerical part, Radioss finite element package was used with the material model of MAT 25 along with the failure option of Tsai-Wu. Experimental dynamic crushing tests of the crash box was carried out using a custom made drop-weight tester at impact velocity of 4.4 m/s and dropping mass with 450 kg. The material model constants were obtained once the coupon based static and dynamic tests were completed. From the dynamic crushing tests, maximum and mean force values of 225 and 65.0 kN were noted, respectively. There is close agreement between the experimental and numerical results both in terms of force and displacement values. This verified numerical model can further be used to investigate the crushing characteristics at different impact conditions.

Dynamic Response Characteristics of Composite Concrete Structures Subjected to Reactive Jet Impact

Composite concrete structures, commonly found in urban infrastructures, such as highways and runways, are pivotal research object in the protection field. To study the dynamic response of composite concrete structures subjected to reactive jet penetration coupled with an explosive effect, a full-scale damage experiment of composite structures under the action of 150 mm caliber shaped charges was performed, to derive the dynamic damage modes of different concrete thicknesses under the combined kinetic and chemical energy damage effects. The results indicated that under aluminum jet penetration, concrete layers exhibited minor funnel craters and penetration holes. However, concrete layers displayed a variety of damage modes, including central penetration holes, funnel craters, bulges, and radial/circumferential cracks when subjected to the PTFE/Al jet. The area of the funnel crater expanded as the thickness of the concrete increased, while the height of the bulge and the number of radial cracks decreased. The diameter of penetration holes increased by 76.9% and the area of funnel crater increased by 578% in comparison to Al jet penetration damage. A modified-RHT concrete model that reflected concrete tensile failure was established, utilizing AUTODYN. Segmented numerical simulations of damage behavior were performed using the FEM-SPH algorithm and a restart approach combined with reactive jet characteristics. The spatial distribution characteristic of the reactive jet and the relationship between kinetic penetration and explosion-enhanced damage were obtained by the simulation, which showed good concordance with the experimental results. This study provides important reference data and a theoretical basis for the design of composite concrete structures to resist penetration and explosion.

Experimental study of fracture behaviors of granite at ultra-low temperatures

A good understanding of fracture characteristics of rocks at ultra-low temperatures is essential in selecting and designing cavernous liquefied natural gas (LNG) storage facilities. In this study, three-point bending tests on Notched Semi-Circular Bend (NSCB) granite specimens with various grain sizes and saturation levels were conducted at temperatures of 25°C, −90°C, −120°C, and −165°C, respectively. The test results show that the fracture characteristics of granite are significantly weakened at −165°C under saturated conditions. As the temperature goes low, granite’s peak strength, fracture toughness, and fracture energy increase under dry conditions. A robust linear relationship between the mode-I fracture toughness (KIC) and the fracture energy of granite at ultra-low temperatures is established in this study. This relationship demonstrates that specimens with higher fracture toughness require more energy to reach their fractures. In addition, the relationship, along with its test constraints, forms a theoretical basis that can be used to explain the fracture damage modes and the characteristics of granite at different temperatures. Furthermore, granite at ultra-low temperatures was also investigated from the view of pores with frozen water and the change of particle sizes. The data obtained from our experiments can serve as valuable parameters for simulating underground LNG storage. The research findings from this study provide a valuable foundation that can be used for designing and installing cavernous LNG storage tank facilities.

Scratch performance of natural rubber and natural rubber composites reinforced with nylon, kevlar, and carbon fabrics

The scratch performance of natural rubber (NR) and fibre-reinforced NR was investigated using a scratch test with acoustic emission (AE). Both maximum penetration depth and maximum tangential force were characterised by two procedures. Procedure 1 applied the corner of a steel cube at face leading orientation. Procedure 2 applied a steel pyramid indenter with a spherical tip at the edge leading orientation. AE was adopted in Procedure 2 for evaluation of cutting damage mode that varied from ploughing/tearing the matrix to cutting both the matrix and fabric. The scratched regions of all specimens were observed using an optical microscope to estimate the damage level and examine damage mechanisms. The results show that scratch resistance improved as fibre content and fibre stiffness increased. The maximum tangential force depended on the damage modes. Two or more fabric layers could further increase penetration resistance, but the damage level is more serious in most cases.

Effect of void defects on mechanical behavior and failure features of C/C honeycomb structure

The combination of honeycomb structure and carbon/carbon (C/C) composites offers unique high load-bearing and light-weight advantages in satellite-bearing platforms. However, the defects generated by the imperfect preparation process are inevitable. This paper employs the chemical vapor infiltration (CVI) technique to fabricate novel C/C honeycomb structures and considers the presence of internal void defects. Based on micro-computerized tomography (μ-CT) images, the void distribution and porosity as well as the micro-structure characteristics are acquired. Meso-scale representative volume element (RVE) models is established to predict the performance of honeycomb wall material. A secondary development of ABAQUS is performed to establish a model of honeycomb structure containing void defects. Multi-scale models are developed for investigating the mechanical performance of the C/C honeycomb structure. Experimental methods and finite element simulations are utilized to examine the effects of porosity and different weave structures on the compression and shear properties, as well as damage modes of C/C honeycomb structures in detail. The results demonstrate that the presence of low porosity ρ = 1 % severely diminishes the load-bearing capacity of the honeycomb and affects its damage pattern. Twill-2 weave C/C honeycomb inherits the advanced performance of the composites and exhibits superior comprehensive properties compared to plain weave and twill-1 weave honeycomb structures.

Tensile strength analysis of sandwich composite-to-metal joints based on experimental investigation and progressive failure method

In this study, the tensile strength characteristics of the hybrid sandwich composite-to-metal specimen consisting of two steel I-beams and a T-shaped sandwich composite structure were studied. The tensile strength of the sandwich hybrid joints under design load and ultimate load has been tested. The progressive failure model is established according to the modified Shokrieh-Hashin criterion. The interface cohesive zone model is applied to reveal the damage development of the bonding layer. The stresses, strains, ultimate strength, and main damage modes of the hybrid joint under design load and ultimate load conditions are simulated and the predicted results are compared with those from tests. The experimental and numerical results show that the strain, the trend of load-displacement curves and the damage modes are in good agreement. The strength of matrix is the main factor limiting ultimate strength. Therefore, the influences of the overlapping width and thickness on ultimate bearing capacity were analyzed to obtain the optimized structural strength of the sandwich composite-to-metal bolted joints.

Fracture characteristics of cement-stabilized macadam using three-point bending test

Semi-rigid base is the main component of highway pavements in China and is prone to cracking damage under vehicle loading and environmental effects. In this paper, three-point bending (TPB) tests on prefabricated cracked specimens of CSM beams were conducted to study the damage evolution and fracture characteristics of cement-stabilized macadam (CSM) under different loading rates. The acoustic emission (AE) technique and scanning electron microscope (SEM) analysis were utilized. The results indicate that the fracture energy and fracture toughness of CSM were enhanced at increasing loading rates, and the amplitude distribution of AE events became wider and more energetic. The damage modes of CSM can be classified into interface transition zone (ITZ) damage, cement mortar damage, and aggregate damage. The fracture surfaces were mainly ITZ damage. The brittle structure of CSM was caused by incomplete hydration of cement particles in ITZ. This study can provide a better understanding of the fracture mechanism of CSM at different loading rates. The findings can also guide the design, construction, operation and maintenance of semi-rigid roadbed pavement structures.