Fatigue degradation modelling of plain woven glass/epoxy composites

Due to their anisotropic and inhomogeneous nature, the fatigue behaviour of fibre-reinforced composite materials is complicated and since many years a large research effort is being spent on this problem. Despite these efforts, fatigue design of fibre-reinforced composites still has to rely mostly on expensive time-consuming fatigue experiments and large safety factors have to be adopted. In this paper, a combined experimental/numerical investigation of the fatigue behaviour of plain woven glass/epoxy composites is presented. Bending fatigue tests were used to yield the experimental data. With the aid of an advanced phase-shift shadow Moiré technique, an out-of-plane displacement profile during fatigue life of the composite specimens was recorded at a number of intervals, as well as the bending force history. A residual stiffness model, which describes the fatigue damage behaviour of the composite material, was adopted. Next, a new finite element approach was developed to implement the fatigue damage model in a commercial finite element code that proves to be capable of simulating the observed experimental results.     

Fatigue Life Estimation of Structural Components Using Macrofibre Composite Sensors

Abstract: Recently, a number of different structural health monitoring (SHM) techniques have been developed for the online inspection of air, land and sea engineering structures. Various smart materials are employed for detecting eminent damage in situ. Fatigue cracks in structural components are the most common cause of structural failure when exposed to fatigue loading. Fatigue design of structural components is typically accomplished either using a set of stress cycle (S‐N) data obtained from prior fatigue tests or using the fracture mechanics approach. The fracture mechanics approach considers the fatigue life of structures as a summation of crack initiation life and crack propagation life. The stress intensity factor (SIF) is required for the estimation of fatigue crack propagation life from the linear elastic fracture mechanics (LEFM) perspective. However, the accurate prediction of the SIF is difficult especially when the geometry or the boundary conditions of a structure becomes complex. In this study, a SHM application of macrofibre composite (MFC) sensors is presented. A set of MFC sensors is used for the real‐time measurement of the SIF. The measured values of the SIF are later used for the prediction of the crack propagation life. The impedance‐based SHM technique using the same set of MFC sensors is employed for the detection of crack initiation life.     

Effect of initial damage level and patch configuration on the fatigue behaviour of reinforced steel plates

The application of carbon fibre reinforced polymer composites externally bonded on cracked steel plates is an effective system in extending the fatigue life of these structural elements. In particular, composite patches bonded on the crack tip region reduce the stress concentration and the crack opening displacement, leading to an extension of the fatigue life. In order to additionally show the effectiveness of this kind of reinforcing technique, experimental tests were performed at the laboratories of the Politecnico di Milano. Fatigue tests were executed on single edge notched tension specimens reinforced by pultruded strips bonded to a single side (non‐symmetric reinforcement). Different patch configurations (reinforcement stiffness and patch location) and initial damage levels were considered as parameters influencing the repair effectiveness in extending the fatigue life. The results showed that the use of carbon fibre reinforced polymer materials bonded around the tip region allows extending the fatigue life for different amount of initial damage level. Finally, this work provides some useful information for the more efficient repair configuration.     

Mechanical fatigue of an electronic component under random vibration

Electronic equipment, which is widely used in military applications, must be able to survive harsh environments. The endurance of such equipment is defined by the durability of their internal sensitive components. In this study, vibration induced fatigue life analysis of an axial leaded aluminium capacitor is performed. Three point bending tests are performed for the composite FR‐4 printed circuit boards (PCBs) material in order to determine bending modulus. Experimental modal analysis is used to validate a simulation model of the PCB. Step stress tests (SSTs) of reinforced and unreinforced capacitors which are mounted on the test PCBs are performed. It is found that the failure locations on the test PCBs are compatible among themselves and all the failures are due to flexure stress developed at the lead wires and solder joints. Numerical fatigue analyses are performed to define failure in terms of damage index. In addition, the Weibull model is used to define mean time to failure (MTTF) values. The comparison between MTTF values shows that the fatigue lives are strongly increased by the eccobond reinforcement. The last stage in this work is to focus on the influence of some design parameters on the fatigue life. An exponential equation is proposed to find the relation between lead‐wire diameter and the fatigue damage. It is shown that fatigue damage becomes a maximum for a square shaped PCB and it appears that component body diameter is more effective than the body length in increasing fatigue life.     

Assessment of the mechanical behaviour of glass fibre composites with a tough polydicyclopentadiene (PDCPD) matrix

The use of a tough thermoset polydicyclopentadiene (PDCPD) as a matrix material for composites was explored. A PDCPD–glass fibre composite and an equivalent epoxy composite were compared. Fibre–matrix adhesion quality was assessed by transverse bending tests. The materials were subjected to compression tests, impact tests, static tensile tests and tensile fatigue tests. The results indicate that the tough behaviour of the PDCPD matrix markedly influences the composite damage resistance. The size of the impact damage in the PDCPD composite was half of that in the epoxy composite. The tensile tests indicated no significant difference in tensile strength, but the damage before failure was found to be much more severe in the epoxy samples. The fatigue results showed a much lower variation in fatigue life for the PDCPD material than for the epoxy material, as well as clear differences in damage development for the two materials.     

The damage tolerance of carbon fiber reinforced composites—A workshop summary

The principal objective of this workshop was to identify the critical problem areas associated with the damage tolerance of carbon-fiber reinforced composite materials. The discussion was divided into six areas: (1) damage tolerant materials; (2) testing methods; (3) structural life prediction; (4) damage tolerant design concepts; (5) repair methods; and (6) nondestructive testing. Approximately 1 h was devoted to the discussion of each of these topics. Discussion was stimulated by having one, two, or three introductory presentations by the discussion leaders of each topic followed by open discussion. In this report, my impressions of the discussion on each topic are combined with those of the discussion leaders and presented in the following format: Summary of Presentations; Summary of Discussion; and Critical Issues. It is possible to obtain the final conclusions of the workshop by examining the critical issues listed at the end of each section.

The discussion was lively, controversial, and open. The main conclusions to be drawn from the workshop are: (1) damage from impact is the worst type of damage for these materials—significant reductions in the compressive strength will occur following impact; (2) fatigue damage at the present time does not limit the use of these materials but as new materials are developed fatigue failure may become an issue; (3) very little is understood about the micromechanics of damage, hence it is impossible to predict the effect of changing the properties of the individual components (fibers, matrix, and fiber-matrix interface) on the bulk material properties; (4) better analytical models to describe the formation and growth of impact-damage are badly needed; and (5) rapid NDT methods to inspect large components are required.     

Fail-safe design of integral metallic aircraft structures reinforced by bonded crack retarders

This paper presents an investigation on the effectiveness of crack growth retarders bonded to integral metallic structures. The study was performed by both numerical modelling and experimental tests. It focuses on aluminium alloy panels reinforced by bonded straps made of carbon-epoxy, glass-epoxy composite materials or a titanium alloy. The goal was to develop a fail-safe design for integrally stiffened skin-stringer panels applicable to aircraft wing structures. The modelling strategy and finite element models are presented and discussed. The requirements that the models should meet are also discussed. The study has focused on establishing the extent of crack retarder benefits, in terms of fatigue crack growth life improvement, by numerical simulation and experimental tests of various crack retarders. The results of predicted fatigue crack growth retardation have been validated by tests of laboratory samples. This study concludes that by bonding discrete straps to an integral structure, the fatigue crack growth life can be significantly improved.     

In situ monitoring in real time of fatigue-induced damage growth in composite materials by acoustography

There are growing concerns about the effects of accidental impact damage on the structural integrity of aerospace composites and about the possible growth of the damage due to in-service fatigue. There has been some success in the use of established methods (ultrasonic C-scan, thermography, X-rays) to monitor damage development during fatigue experiments by interrupting a test and removing the specimen for damage inspection but this stop-and-restart test procedure is far from satisfactory. Real-time damage monitoring in composite materials during fatigue has now become possible by the emergence of a new ultrasonic imaging technology, acoustography. The successful integration of acoustography and a servo-hydraulic fatigue test machine has resulted in a new measurement system which can be used for the in situ monitoring in real time of damage growth in composite specimens during long-term fatigue tests. Results are presented which show damage-area growth during fatigue cycling under high compressive loads. After an initial small enlargement (stage 1), damage grows at a constant rate (stage 2) until the third stage is reached when there is further growth at an increasing rate to final failure. However, a ‘fatigue limit’ has also been observed. At stresses below this fatigue limit, a zero damage-growth régime has been found in studies of >10⁶ fatigue cycles. The results obtained have important implications for the understanding of the effects of damage on fatigue life and for the design of ‘safe’ damage-tolerant structures.     

Describing the Flexural Behaviour of Cross-ply Laminates Under Cyclic Fatigue

The objective of this work is to derive modelling of the fatigue behaviour of cross-ply laminates from the experimental results obtained in the case of three-point bending tests. Modelling the fatigue behaviour is based on the stiffness reduction of test specimens. Firstly, experimental results are described using interpolation functions. Then, the characteristic coefficients of these functions are studied as function of the laminate properties and loading conditions. This approach allows to predict the fatigue life of composite laminates while avoiding a large number of fatigue tests. Wöhler curves are used to compare the experimental and analytical results, and a good agreement is found between the results. Next, a simple approach is considered to define a damage parameter. It is based on the analogy between the mechanical behaviour and the fatigue damage evolution of composite laminates during fatigue tests. The developed models are applied to analyse the influence of constituents on the fatigue behaviour and damage development of composite materials under fatigue loading.     

Fatigue analysis of railway coupling joint

A new type of railway coupling joint, developed by a leading international railway manufacturer, has shown limited fatigue lives in service, in a preliminary service assessment of fatigue life. To get more data about this problem a detailed study was carried out of the fatigue design of the component including stress analysis and fatigue life evaluation.The main objective of this paper consisted in getting accurate information concerning the level of operational reliability of the fatigue behaviour of this new type of coupling used in carriages for passenger transportation in suburban lines.The strains in service were obtained with strain gauge rosettes in a typical line. Values of the cumulative fatigue damage were obtained in this service conditions. Linear damage accumulation laws were used.A numerical stress analysis using FE methods were performed. Detailed stress distributions were obtained in the component. Hot spot areas of stress were obtained and also the values of critical fatigue damage in these critical points of stress distributions. The extrapolation method to the hot spot points, was used to obtain the critical values of fatigue damage.The results of fatigue damage gave an infinite value of fatigue life for the coupling.     

Characterization of fatigue damage in long fiber epoxy composite laminates

A study of damage characterization of a GFRC laminate is presented here. Forty fatigue tests were executed and S–N curves traced. Two parameters were chosen to monitor damage evolution during each test: stiffness and dissipated energy per cycle. Moreover, the presence of three zones in graphs of processed data can be observed and it is evident that the most important structural transformations take place only in the very final part of life. Adopting a continuum mechanics approach, the degradation through the whole life in composite is evaluated and it is shown that the two parameters are strictly related to damage state of composite material. A method for predicting the remaining life in a GFRC is here proposed.     

Crack growth induced delamination on steel members reinforced by prestressed composite patch

ABSTRACT Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce a crack closure effect. Moreover, it modifies the crack geometry by bridging the crack faces and so reduces the stress intensity range at the crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A crack growth induced debonded region in the adhesive‐plate interface was observed using an optical technique. Moreover, the size of the debonded region significantly influences the efficiency of the crack repair. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique (MVCCT). A parametric analysis is performed to investigate the influence of some design parameters such as the composite patch Young's modulus, the adhesive thickness and the pretension level on the adhesive‐plate interface debond.     

Damage mechanisms and life prediction in high temperature fatigue of a unidirectional SiC–Ti composite

Isothermal fatigue tests are performed in the longitudinal direction at 450°C on a unidirectional SiC/Ti composite. Three major damage mechanisms are identified: the matrix cyclic softening which overloads fibers leading to their progressive rupture; the interfacial degradation of these broken fibers and their oxidation by the environment. Damage kinetics are estimated using microscopic observations and acoustic emission. Finally, a micromechanical model is used in order to understand the respective influence of these damage mechanisms. It is shown that, at a sufficient load level, fatigue fracture of the composite strongly depends on the interfacial degradation kinetics and that fiber oxidation by the environment generates more progressive damage and considerably reduces the composite fatigue life compared to loading under vacuum.     

复合材料疲劳寿命预测

在疲劳载荷作用下,复合材料的弹性模量会随着载荷循环数的增加而不断下降,而材料中的内部损伤则不断增大。为此,本文提出复合材料的疲劳模量和累积应变的概念,并由此定义出三种预测复合材料疲劳寿命的疲劳损伤模型。文中应用这三种模型对单应力水平和多应力水平下的玻璃纤维增强环氧树脂复合材料的疲劳寿命进行了估算,并同实验结果进行了比较。     

Buffeting-induced fatigue damage assessment of a long suspension bridge

As modern suspension bridges become longer and longer, buffeting-induced fatigue damage problem for the bridges located in strong wind regions may have to be taken into consideration. Furthermore, there is a trend to install wind and structural health monitoring systems (WASHMS) to long suspension bridges for performance assessment. A systematic framework for assessing long-term buffeting-induced fatigue damage to a long suspension bridge is thus presented in this paper by integrating a few important wind/structural components with continuum damage mechanics (CDM)-based fatigue damage assessment method. By taking the Tsing Ma Bridge in Hong Kong as an example, a joint probability density function of wind speed and direction is first established based on wind data recorded by the WASHMS installed in the bridge. A structural health monitoring-oriented finite element model of the bridge and a numerical procedure for buffeting-induced stress analysis of the bridge are then used to identify stress characteristics at hot spots of critical steel members under different wind speeds and directions. The accumulative fatigue damage to the critical steel members at hot spots during the bridge design life is finally evaluated using a CDM-based fatigue damage evolution model. The proposed framework is found to be feasible and practical.     

Fatigue life simulation of a rear tow hook assembly of a passenger car

A comparison between laboratory test data on fatigue crack nucleation in a rear tow hook pin assembly of passenger vehicle and a computational methodology using commercial package software is presented. Fatigue damage is determined using local material response, measured during experimental tests. Experiments were performed simulating the actual conditions in the customer environment. Stress and strain were experimentally measured by using strain gages, bonded on the hook assembly. These experimental lives are compared with those obtained through numerical analysis using a commercial fatigue software. Fatigue analysis methods (S–N curves, rainflow counting and Miner rule) were used to determine the fatigue damage imposed on the component. Interpretation and evaluation of the measured strain and stresses, simulation tests and fatigue life assessments, on the basis of S–N curve, are described in this paper.     

Fatigue life prediction of woven composite laminates with initial delamination

An engineering approach for fatigue life prediction of fibre‐reinforced polymer composite materials is highly desirable for industries due to the complexity in damage mechanisms and their interactions. This paper presents a fatigue‐driven residual strength model considering the effect of initial delamination size and stress ratio. Static and constant amplitude fatigue tests of woven composite specimens with delamination diameters of 0, 4 and 6 mm were carried out to determine the model parameters. Good agreement with experimental results has been achieved when the modified residual strength model has been applied for fatigue life prediction of the woven composite laminate with an initial delamination diameter of 8 mm under constant amplitude load and block fatigue load. It has been demonstrated that the residual strength degradation‐based model can effectively reflect the load sequence effect on fatigue damage and hence provide more accurate fatigue life prediction than the traditional linear damage accumulation models.     

Continuous fatigue damage prediction of a rubber fibre composite structure using multiaxial energy‐based approach

This paper details an advanced method of continuous fatigue damage prediction of rubber fibre composite structures. A novel multiaxial energy‐based approach incorporating a mean stress correction is presented and also used to predict the fatigue life of a commercial vehicle air spring. The variations of elastic strain and complementary energies are joined to form the energy damage parameter. Material parameter α is introduced to adapt for any observed mean stress effect as well as being able to reproduce the well‐known Smith‐Watson‐Topper criterion. Since integration to calculate the energies is simplified, the approach can be employed regardless of the complexity of the thermo‐mechanical load history. Several numerical simulations and experimental tests were performed in order to obtain the required stress‐strain tensors and the corresponding fatigue lives, respectively. In simulations, the rubber material of the air spring was simulated as nonlinear elastic. The mean stress parameter α , which controls the influence of the mean stress on fatigue life, was adjusted with respect to those energy life curves obtained experimentally. The predicted fatigue life and the location of failure are in very good agreement with experimental observations.     

A numerical methodology for optimizing the geometry of composite structural parts with regard to strength

The increased use of composite materials in lightweight structures has generated the need for optimizing the geometry of composite structural parts with regard to strength, weight and cost. Most existing optimization methodologies focus on weight and cost mainly due to the difficulties in predicting strength of composite materials. In this paper, a numerical methodology for optimizing the geometry of composite structural parts with regard to strength by maintaining the initial weight is proposed. The methodology is a combination of the optimization module of the ANSYS FE code and a progressive damage modeling module. Both modules and the interface between them were programmed using the ANSYS programming language, thus enabling the implementation of the methodology in a single step. The parametric design language involves two verifications tests: one of the progressive damage model against experiments and one of the global optimization methodology performed by comparing the strength of the initial and the optimum geometry. There were made two applications of the numerical optimization methodology, both on H-shaped adhesively bonded joints subjected to quasi-static load. In the first application, the H-shaped joining profile was made from non-crimp fabric composite material while in the second from a novel fully interlaced 3D woven composite material. In the optimization of the joint’s geometry, failure in the composite material as well as debonding between the assembled parts was considered. For both cases, the optimization led to a considerable increase in joint’s strength.