Impact Fatigue Loading Research Articles

Mechanism of concrete damage under the coupled action of freeze-thaw cycle and low-stress impact fatigue load：From pore structure to energy dissipation

The interaction mechanism behind concrete damage induced by the combined effects of freeze-thaw cycles and fatigue load (FTF) remains insufficiently understood. This study aims to shed light on this mechanism by employing three sets of specimens, each subjected to different conditions: freeze-thaw cycles alone, low-stress impact fatigue (LIF) load alone, and FTF coupled action. Macro performance and microstructural changes of these specimens were measured after each testing round to analyze the evolution of concrete damage. Additionally, the influence of load duration on the damage under the coupled action was also explored. Results indicated that under FTF action, when LIF loading is applied for a short duration, freeze-thaw cycles play a dominant role in concrete damage, while the influence of LIF loading is minimal. However, when LIF loading is sustained for an extended period coupled with freeze-thaw cycles, it can accelerate concrete degradation. This can be attributed to the accumulation of micro-cracks during longer loading duration, eventually manifesting as visible cracks. These cracks link adjacent pores and increase the number of interconnected pores. This, coupled with freeze-thaw damage increasing the probability of crack forming, results in accelerated deterioration of concrete. The mechanism underlying this concrete deterioration was also analyzed from an energy dissipation prospect.

Impact Fatigue Life of Adhesively Bonded Composite-Steel Joints Enhanced with the Bi-Adhesive Technique

One of the most common loading conditions that bonded joints experience in service is repeated impact. Despite the destructive effects of impact fatigue, the behavior of metal-composite bonded joints subjected to repeated impact loads has rarely been studied in the literature. Therefore, it is of utmost importance to pay attention to this phenomenon on the one hand and to find solutions to improve the impact fatigue life of bonded composite metal components on the other hand. Accordingly, in this study, the use of the bi-adhesive technique is proposed to improve the durability of composite-metal single-lap joints (SLJs) under impact fatigue loading conditions. J-N (energy-life) method is also used to analyze the experimental data obtained. Accordingly, in the present study, the impact fatigue behavior of single adhesive metal to composite joints was analyzed experimentally based on the J-N method and also numerically using the finite element method (FEM). By using two adhesives along a single overlap, the impact fatigue life of joints between dissimilar composite and metal joints was also analyzed experimentally. The results show that the double adhesives technique can significantly improve the impact fatigue life of the tested joints. It was also found that the optimum length ratio of the adhesives (the length covered by the ductile adhesive relative to the total overlap size) is a function of the stiffness of the joint and is more pronounced for less stiff bonded joints. A linear elastic numerical analysis was also conducted to evaluate the stress state along the bloodline of the bonded joints. Results show that the compressive peel stress made at the boundary of the two adhesives can be a possible reason behind the different results observed.

Study on impact fatigue test and life prediction method of TC18 titanium alloy

To study the impact fatigue properties of TC18 titanium alloy and propose a new damage model for the impact fatigue life prediction of material, the impact fatigue test method and the impact response measurement scheme are designed. The impact fatigue tests are then implemented on a drop hammer impact test machine to obtain the fatigue lives and impact responses of the tested specimens under different impact conditions. A new impact damage model is proposed based on the theory of continuum damage mechanics considering the effect of strain rate. The calibration method of material parameters is presented, and the parameters of the impact damage model are obtained according to the test data of TC18. Afterward, the numerical implementation of impact damage calculation is established, and an ABAQUS user-defined subroutine is used to realize the finite element numerical simulation of impact fatigue damage accumulation. Finally, the damage evolution of TC18 under tensile impact fatigue loading is simulated and the corresponding fatigue life is predicted. The errors between the calculated impact fatigue lives and the experimental lives are less than 25%, which demonstrates the good validity and applicability of the proposed damage model and the numerical method.

Life Prediction Method of Dissimilar Lightweight Materials Welded Joints with Precrack under Coupled Impact-Fatigue Loading.

This paper aims to explore the fatigue life estimation approach of welded joints with precrack under coupled impact and fatigue loading, and the base metal is dissimilar 5083H111 and 5754 aluminum alloy. Impact tests are first carried out on the dissimilar lightweight materials welded joint with precrack located in the middle of the specimen, and a stress and strain field is obtained to determine the fatigue damage model parameters by using finite element dynamic analysis to simulate the impact process. Based, on the S-N curve of welded joints, the predicted life expectancy is found to be inconsistent with the experimental results. According to the continuum damage mechanics, the lifetime assessment model is presented to calculate both impact and fatigue damage. The estimated results agree well with the experimental ones.

Damage investigation of cross-ply flax/epoxy composite under impact fatigue

This paper aims at studying the damage evolution of a [0/90]3S cross-ply flax/epoxy laminate repeatedly impacted at three energy levels of 5, 6 and 7 J. Several specimen plates were prepared by the vacuum infusion technique and then subjected to repeated low-velocity impacts. To assess external damage mechanisms in the impacted plates, an inspection of the back and front faces damage was conducted by using a high-resolution digital reflex camera. Moreover, different techniques were used to evaluate the internal damage such as intensive light exposure, infrared thermal imaging, and computed X-ray tomography. The obtained results show that damage is visible even at low energy level and starts to appear from the first impacts, especially in the back face of the flax/epoxy samples. In addition, after front and back faces cracks appear, delamination is the most predominant damage mechanism that occurs during the different phases of impact fatigue loading. Its size is found to extend with the energy level and the multiplication of impacts.

МОДЕЛИРОВАНИЕ УСКОРЕННЫХ ИСПЫТАНИЙ НОЖЕЙ РЕЖУЩЕ-ИЗМЕЛЬЧАЮЩИХ АППАРАТОВ КОРМОУБОРОЧНЫХ КОМБАЙНОВ

A method is proposed and a technique is developed for accelerated bench tests in laboratory conditions of knives of cutting-reducing devices of forage harvesters. One of the main purposes of the developed method is an experimental comparative assessment of the long-term performance (according to the wear resistance criterion) of various knife options (according to the materials for their manufacture and structural and geometric parameters). The proposed test method solves the problem of assessing the wear resistance of agricultural harvester knives by two parameters: physical depreciation and dulling of their cutting edge. A flexible belt with an abrasive fixed on one of its surfaces is used as counter-specimens during testing, which allows for wear with sliding impact and thereby significantly accelerates the process of knives damage. The technique makes it possible to test simultaneously two knives made using different technologies, under comparable conditions. In addition, to speed up testing, not one, but several counter-specimens are mounted on the test equipment. As a result of such tests, the process of impact-fatigue loading with slipping is simulated and the damage to the knife is reproduced, which is close to the operational one: wear and an increase in the cutting edge radius. Using the developed technique, comparative accelerated tests of imported steel and domestic cast iron (produced by OJSC “Gomselmash”) knives made using various technologies were carried out. The tests conducted made it possible to quantify the difference in the service characteristics of the cutting drum knives made using various technologies, which confirms its usefulness and efficiency. The developed technique was approved in accordance with the established procedure and is used for bench testing of knives at OJSC “Gomselmash”.

Evaluation of damage within sandwich honeycomb panels subjected to impact fatigue loading

This work was aimed at investigating damage evolution within sandwich panels consisting of aluminum skins and Nomex™ honeycomb core, with three different values of the core densities, subjected to multiple impacts. Repeated impacts at low energy were conducted using an impact fatigue machine. Bending tests were conducted to determine the residual stiffness after impacts in order to analyze the evolution of a damage parameter D. A model was therefore proposed for describing the changes in this parameter as a function of impact cycles N. After repeated impacts, the D(N) curves are characterized by an S-shaped curve. A good agreement is observed between model and experimental results, the maximum standard deviation being less than 7% for different densities. Microscopic observations of the impacted specimens were conducted in order to evaluate the crater growth (associated with permanent indentation). The influence of the number of impacts on the dimensions of the impact zone has also been evaluated. For all the core densities, the permanent indentation gradually increases as a function of impact cycles.

Study on low velocity cyclic impact wear of amorphous carbon films with different mechanical properties

The reproducible wear characterization methods were used to evaluate and analyze the cyclic dynamic loads damage and sliding wear process of the amorphous carbon films. The films with different properties (a-C, a-C:H and a-C: H/WC) on copper substrate (H62) were tested. The wear rates of the three different films have the same order of magnitude under sliding friction test. However, the films failure degrees were quite different in low velocity impact tests based on energy control. It is necessary to select the appropriate film for a given tribological application. The test results show: the a-C: H film has better impact wear resistance, while the damage of a-C:H/WC film is the worst due to the existence of WC interlayer. In the impact test, it was found that the fracture toughness of the a-C:H/WC film was inversely correlation to the H3/E2 and H/E ratio. Test results indicated that the film failure behavior includes fatigue cracking, chipping, and spalling. In addition, soft film is more durable than hard film under impact fatigue load.

Post- Impact Fatigue Damage Analysis of Quasi Isotropic CFRP Laminates through Infrared Thermography

Carbon fiber reinforced plastic (CFRP) materials used for lightweight aircraft structures are susceptible to low velocity accidental impacts during manufacture and while in service. In this study the CFRP laminates subjected to low velocity impact conditions of three different energy levels were tested under various fatigue loading conditions till it reached the failure state. The infrared thermography NDT technique was used for the damage analysis of these post-impact fatigue loaded CFRP specimens to understand the damage distribution across the specimen. The cooling response curves obtained from the active thermography of the post impact fatigue specimens under four different combinations (transmission or reflection mode with impacted or un-impacted surface facing camera) for each of the specimens were analyzed. The rate of cooling in the temperature response curves is different for each specimens subjected with various post impact-fatigue loading conditions and it seems to be correlated with the extent of post-impact fatigue damage present in each specimen quantified through X-ray CT image processing. The temperature rise observed in the passive thermography at the fiber/ply breakage during static strength tests were proportional to corresponding load drops and the non-homogeneous spread of damage at the onset of tensile failure was seen from the thermographs.

Experimental Characterization of Impact Fatigue Damage in an Adhesive Bonding Using the Second Harmonics

An adhesive bonding is widely used in various industrial applications. In this paper, an application of non-destructive evaluation (NDE) technique based on the second harmonics for experimental characterization of impact fatigue damage in an adhesive bonding is presented. Adhesively bonded specimens made from AZ31 magnesium–aluminum alloy were firstly subjected to impact fatigue loading; the ultrasonic harmonics generated due to impact fatigue damage within the adhesive layer were thereafter measured. The acoustic nonlinearity parameter (ANP) based on the fundamental and second harmonics was thus obtained. The experimental results show that the normalized ANP, which is an indicator of material properties, increases with the impact fatigue life. Further discussion based on a theoretical model with different interfacial compression and tension stiffness was also conducted. The experimental results consist well with the theoretical ones. The results in this paper demonstrate that the nonlinear ultrasonic method based on the second harmonic generation technique can be used to characterize the impact fatigue damage in an adhesive bonding.

Experimental Research on CFRP Laminates with Different Impact Energy Levels

The effects of thickness and impact energy on the impact damage of CFRP laminates were studied in this paper. Impact tests for the CFRP laminates with the size of 600 mm×700 mm with five different thicknesses were subjected to impact fatigue loading at different energy levels from 5 J to 65 J. The crater depth and matrix length were investigated according to different energy levels and different thicknesses. The impact damage was evaluated by visual inspection, three-dimensional microscope. The experimental results reveal that the crater depth and the crack length increase with the increasing impact energy. For the same impact energy, the crater depth and the crack length decreased with the increasing thickness of specimens.

Study on Damage Modes of a Sandwich Panel Impacted Repeatedly

This paper describes an experimental investigation for determining the damage modes under low energy impact-fatigue of sandwich panels consisting of aluminum skins supported by honeycomb core made of aluminum. Square samples of 125mm by 125mm sides and 10mm thickness (skin of 0.6mm and 8.8mm of core) were subjected to impact fatigue loading using a testing machine at four different energy levels (2J, 3J, 5J and 7J). The square plates are clamped in a fixture system over a 100mm diameter hole. Three different diameters of impactor head (15mm, 25mm and 35mm) are used to study their influence on life duration of the sandwich plates. Results showed that damage area at impacted face and propagation of multi-cracks at rear face are greatly affected by energy level and impactor diameter.

Tensile and Impact Behavior and Fracture Characteristics of FGH96 Powder Metallurgy Superalloy

Powder metallurgy (P/M) superalloy has not only been the primary material of turbine disk but also the main material of packing disk, packing ring and baffle plate of the aero-engine. Among these components, some parts are very thin, for example, the thinnest of baffle plate is only 2mm. The thin plate component was subjected to complex load, and the failure mode was synthetical. Some parts of component are subjected to impact load or impact-fatigue load although the nominal load of this component is fatigue load, more than one baffle plates have cracked through fast expansion and burst multiple debris because of the different local load type for the same component. It is very useful to investigate the effect on tensile and impact properties by specimen shape and shot peening. In this article, the different tests were carried out, including different specimen shape, temperature and surfaces. The results indicated that fracture strength σb and elongation δ5 were not affected by shape of specimen of P/M superalloy. Rod or plate specimens may be used to characterize the static properties of material. Shot peening would decrease the elongation,δ5 (at room temperature and 650°C) and impact work, but it did not reduce the fracture strength σb. In order to utilize adequately the component subjected to complicated loads, the surface state of the component may be treated distinctively according to the specific local load.

Special Features of Strain and Fracture in Steels under Impact Fatigue Loading

Processes of carbon steels fracture under their repetitive dynamic loading according to the compression scheme have been studied. It has been ascertained that the main cause of fracture of cyclically compressed specimens is crack initiation and propagation on exposure to tensile internal stresses forming at the crack tip during the external load removal stages. The crack growth rate is maximal at the initial stage of its advancement near the notch. When the crack propagates depthward the specimen, its rate decelerates and weakly depends on the current specimen cross section. A model of the steels fatigue fracture process under repetitive dynamic loading has been proposed. It has been revealed that medium-and high-carbon steels quenched and tempered at 300 °С have long life duration.

Delamination-Crater Interaction in Damage of Glass/Epoxy Composite Plates Subjected to Impact Fatigue

The use of composite materials is increasing although their behavior under impact fatigue loading remains rather unknown. This study is to assess the evolution of damage, especially delamination and crater, in a composite Glass/Epoxy woven fabric, using repeated impact tests at low energy levels (<10J). Both types of damage that arise and grow within the material cannot be independent from each other. Our objective in this work is to establish the interaction between two damages (delamination and crater) on laminate damage, and understand the contribution of each of them in the different phases through which passes the composite before perforation. To do this, impact fatigue tests are carried out on composite plates and measures of the crater size (diameter and depth) and the size of the delaminated area (diagonals from a diamond shape) are collected for different numbers of impacts and impact energies. A question worth asking; can we foster one of these damages over the other? especially when we are interesting to the “structure applications”, where one "prefers" perforation to delamination (while completing correctly the function's intended to the structure), or “shielding applications”, where one "prefers" the delamination to perforation. Although the range of impact velocities is not the same, it is still interesting to consider the synergy between these two damages at low impact velocities, always in the case of “structure applications” and “shielding applications”.

Damage assessment in CFRP laminates exposed to impact fatigue loading

Demand for advanced engineering composites in the aerospace industry is increasing continuously. Lately, carbon fibre reinforced polymers (CFRPs) became one of the most important structural materials in the industry due to a combination of characteristics such as: excellent stiffness, high strength-to-weight ratio, and ease of manufacture according to application. In service, aerospace composite components and structures are exposed to various transient loads, some of which can propagate in them as cyclic impacts. A typical example is an effect of the wind gusts during flight.This type of loading is known as impact fatigue (IF); it is a repetition of low-energy impacts. Such loads can cause various types of damage in composites: fibre breaking, transverse matrix cracking, de-bonding between fibres and matrix and delamination resulting in reduction of residual stiffness and loss of functionality. Furthermore, this damage is often sub-surface, which reinforces the need for more regular inspection.The effects of IF are of major importance due its detrimental effect on the structural integrity of components that can be generated after relatively few impacts at low force levels compared to those in a standard fatigue regime. This study utilises an innovative testing system with the capability of subjecting specimens to a series of repetitive impacts. The primary subject of this paper is to assess the damaging effect of IF on the behaviour of drilled CFRP specimens, exposed to such loading. A detailed damage analysis is implemented utilising an X-ray micro computed tomography system. The main findings suggested that at early stages of life damage is governed by o degree splits along the length of the specimens resulting in a 20% reduction of stiffness. The final failure damage scenario indicated that transverse crasks in the 90 degree plies are the main reason for complete delamination which can be translated to a 50% stiffness reduction.

Evaluation of impact fatigue damage in glass/epoxy composite laminate

In the current study, experimental investigations are carried out to determine the response of glass/epoxy cross ply laminates to repeated low-velocity impact. The material response is evaluated in terms of damage progression with the number of impacts. Damage is evaluated by visual and SEM inspection. Damage parameter D is based on the evaluation of the loss of material bending stiffness and evolution of damage parameter is given versus number of impacts. Rectangular composite laminates of size 280 mm by 180 mm and nominal thickness of 5.4 mm are subjected to low-velocity impact fatigue loading at two energy levels (4 and 5 J). Delamination has been found as the main factor of increasing of damage parameter. Impact crater participate less to this increase at the beginning of live duration but has greater influence on D parameter evolution at the end. Experimental results indicate that the damage evolution versus number of impacts can be represented by “S” shape curve and thus indicates the importance of the first impacts.

Mixed-mode crack growth in bonded composite joints under standard and impact-fatigue loading

Carbon fibre reinforced polymers (CFRPs) are now well established in many high-performance applications and look set to see increased usage in the future, especially if lower cost manufacturing and solutions to certain technical issues, such as poor out-of-plane strength, can be achieved. A significant question when manufacturing with CFRP is the best joining technique to use, with adhesive bonding and mechanical fastening currently the two most popular methods. It is a common view that mechanical fastening is preferred for thicker sections and adhesive bonding for thinner ones; however, advances in the technology and better understanding of ways to design joints have lead to increasing consideration of adhesive bonding for traditionally mechanically fastened joints. In high-performance applications fatigue loading is likely and in some cases repetitive low-energy impacts, or impact fatigue, can appear in the load spectrum. This article looks at mixed-mode crack growth in epoxy bonded CFRP joints in standard and impact fatigue. It is shown that the back-face strain technique can be used to monitor cracking in lap-strap joints (LSJs) and piezo strain gauges can be used to measure the strain response of impacted samples. It is seen that there is significant variation in the failure modes seen in the samples and that the crack propagation rate is highly dependent on the fracture mode. Furthermore, it is found that the crack propagation rate is higher in impact fatigue than in standard fatigue even when the maximum load is significantly lower.

Fracture morphology and deformation characteristics of repeatedly impacted thermoplastic matrix composites

Deformations of thermoplastic matrix composites during impact fatigue loading were investigated by instrumented impacts tester and scanning electron and optic microscopy (SEM). Curves of damage evolution against the number of repeated impacts to fracture the composites revealed three distinct zones: fibre micro buckling and shear fracture of fibres (1st region), initiation and propagation of delaminations and matrix deformations (2nd region), propagation of delaminations and fibre cracking and pull out especially in tensile area (3rd region). Intensive deformations observed primarily in compression region during impact-fatigue loading can be explained by lower compressive strength of composites compared to their tensile strength.

The Effects of Thermal Cycles on the Impact Fatigue Properties of Thermoplastic Matrix Composites

The effects of thermal cycles on the impact fatigue properties of unidirectional carbon fibre reinforced polyetherimide (PEI) matrix composites were investigated. During the thermal cycles, samples were immersed into boiling water (100 °C) and subsequently to ice water (0 °C), 50, 200 and 500 times. The changes in viscoelastic properties of the composites were investigated by means of dynamic mechanical thermal analyzer (DMTA). At the second step, thermal cycled composites were subjected to repeated impact loadings, with different impact energies. Instrumented impact test results were presented as a function of force, energy, deformation during the experiments. The scanning electron microscope (SEM) studies were done in order to understand the morphology of fractured samples after impact fatigue loading. The number of thermal cycles and applied impact energy of the hammer are found to have a great importance on the fracture morphology of repeatedly impacted material, as expected.