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.     

Effect of fatigue damage on static and dynamic tensile behaviour of electro‐discharge machined Ti‐6Al‐4V

The fatigue performance of electro‐discharge machined Ti‐6Al‐4V and, more specifically, the effect of cyclic damage on the static and dynamic tensile properties of the material have been investigated. In a first step, fatigue failure was studied. Afterwards, tensile tests were performed on specimens that had been previously subjected to cyclic loading during predefined fractions of the fatigue life. In addition to conventional experiments at quasi‐static strain rate, dynamic tests were performed using a split Hopkinson tensile bar setup. The edges of some of the specimens were removed after cyclic loading to discriminate between the effects of damage at the edges and in the bulk of the material. Results revealed that early fatigue failure is due to the growth of cracks on the machined edges of the specimens. Edge cracks can randomly reduce fracture strain and energy absorbing capacity. However, cyclic damage does not affect the tensile properties of the bulk material.     

Fatigue and post-fatigue behaviour of carbon/epoxy non-crimp fabric composites

This paper focuses on the static, fatigue and post-fatigue tensile properties of a biaxial carbon/epoxy non-crimp fabric composite. In a series of quasi-static tensile tests, the stress–strain level where damage initiates was determined. This stress level was then used as the maximum stress level in tensile–tensile fatigue tests in the fibre direction. It was found that in fibre direction, this load level can be considered safe for fatigue up to very high cycle numbers. The damage evolution during the tests was monitored at certain cycle times with X-ray radiography. The post-fatigue residual static tensile properties were determined after different numbers of cycles. A series of tensile–tensile fatigue tests at various higher stress levels allowed for the fatigue life curves to be constructed in each of the four testing directions. This revealed that the damage initiation load level is well below the practical fatigue limit of the material.     

Synergistic effect of cyclic mechanical loading and moisture absorption on the bending fatigue performance of carbon/epoxy composites

This study investigates the effects of hygrothermal condition on the static bending strength, the bending fatigue and the residual bending strength of carbon/epoxy composite laminates. Displacement-controlled three-point bending fatigue tests were conducted on carbon/epoxy composite laminates of immersion for 0, 7 and 14 days, respectively. After 40000 cycles the fatigue test was stopped and the residual properties were measured on the tested specimens. The effects of hygrothermal condition and fatigue on the micrographs of the specimens have been studied by the metallurgical microscope and scanning electron microscope. Experimental results reveal that moisture absorption can accelerate damage propagation of the composite; the accumulation of irreversible structural damage under the cyclic loading leads to a change in the macroscopic mechanical properties of the composites; the bending strength and the residual strength retention decreased with increased immersion time; hygrothermal aging lowered the threshold level for the onset of fatigue.     

The mechanical characteristics of smart composite structures with embedded optical fiber sensors

In this study, the mechanical characteristics of composite laminates with embedded optical fiber sensors were evaluated to investigate the effect of embedded optical fiber on the mechanical properties of composite laminates under the static tensile and the low cycle fatigue load. Testing specimens were fabricated with glass fiber/epoxy composites with embedded optical fiber sensors to observe initiation and growth of damage in the specimens and laser signal behavior transmitted through the optical fiber visually and directly. By using this transparency of glass fiber/epoxy composites, the damage of sensors and associated laser signal behavior was observed. Under the static load, the embedded optical fibers do not have significant effect on the stiffness and the strength, while the embedded optical fibers show significant effect on the fatigue life of composite specimens. Especially, the embedded optical fiber sensors show the very low resistance to the fatigue load.     

Tensile and Fatigue Properties of Free-Standing Cu Foils

Tensile and fatigue properties of free-standing as-rolled Cu foils were investigated by means of uniaxial tensile and dynamic bending tests. A special testing system was established to evaluate fatigue behavior of a mi-croscale material subjected to dynamic bending load. The experimental results show that the yield strength increases, but the fracture strain and fatigue resistance decrease with decreasing foil thickness. Deformation and fatigue damage behavior was characterized. The size effect on tensile a...     

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

Impact and post-impact properties of a carbon fibre non-crimp fabric and a twill weave composite

The impact and post-impact static and fatigue tensile properties of a carbon fibre/epoxy NCF composite were determined and compared to those of a carbon fibre/epoxy woven fabric composite, for two impact energies (3.5 and 7 J). The projected damage area after impact was larger for the NCF composite than that for the woven fabric composite for both impact energies. Impacted samples were subjected to static tensile tests and tensile–tensile fatigue tests. It was found that even a relatively low energy impact has already a significant negative influence on the residual properties in both static and fatigue tests, in the fibre direction as well as in the matrix dominated direction. In the matrix dominated directions the post-impact behaviour of the two materials is very similar. In the fibre direction, however, the properties of the non-crimp fabric composite are degraded more by an impact than those of the woven fabric composite.     

High-temperature properties of ferritic spheroidal graphite cast iron

The behaviour of fracture mode and intermediate temperature embrittlement of ferritic spheroidal graphite cast iron is influenced by many factors. From the experimental results, intermediate temperature embrittlement can be considered to be dominated by dynamic strain ageing and the triaxial stress field developed in the ferrite matrix amongst the graphite particles. In order to understand the effect of dynamic strain ageing on high-temperature properties, tensile properties, push-pull low-cycle fatigue properties, rotary bending fatigue properties and creep-rupture properties were investigated from room temperature to 500° C. It was found that all the properties investigated were influenced by dynamic strain ageing. The intermediate temperature embrittlement of ferritic spheroidal graphite cast iron found in different load conditions is reported.     

Response of notched carbon/PEEK and carbon/epoxy laminates subjected to tension fatigue loading

In this study a comparison is made between the tensile static and fatigue behaviours of quasi-isotropic carbon/PEEK and carbon/epoxy notched laminates, selected as separate representatives of both tough and brittle matrix composites. Damage progression was monitored by various non-destructive (ultrasonic scanning and x-radiography) and destructive (deply and microscopic examinations) techniques, and by continuously measuring the change in stiffness, in order to identify the effect of damage on mechanical properties.
The experimental observations indicated that fatigue damage in carbon/epoxy laminates consists of a combination of matrix cracks, longitudinal splitting and delaminations which attenuate the stress concentration and suppress fibre fracture at the notch; as a consequence, fatigue failure can be reached only after very high numbers of cycles while tensile residual strengths continuously increase over the range of lives investigated (10³–10⁶ cycles). Due to the superior matrix toughness and the high fibre-matrix adhesion, the nature of fatigue damage in carbon/PEEK laminates strongly depends on the stress level. At high stresses the absence of early splitting and delaminations promotes the propagation of fibre fracture therefore resulting in poor fatigue performances and significant strength reductions; while at low stress levels damage modes are matrix controlled and this again translates into very long fatigue lives. These results indicate a strong influence of the major damage mechanisms typical of the two material systems on the behaviour of the laminates, with the nature, more than the amount, of damage appearing as the controlling parameter of the material response up to failure.     

Investigation of chemical ageing and its effect on static and fatigue strength of continuous fibre reinforced plastics

The long term durability of continuous glass fibre reinforced thermosets with epoxy resin at high temperature and an oxygen-environment is investigated in detail. An extensive ageing-study is performed to detect the ageing effects on the residual strength in different laminate layups and to identify the corresponding damage mechanisms. Thermal degradation is found to be the dominant damage mechanism and the weight loss is found to be the most suitable measure of damage. A new phenomenological model that is based on the time-temperature-superposition principle and a master-relation between the residual strength and weight loss is presented to model the ageing effect on the residual strength. Fatigue experiments of pre-aged specimen are performed to verify the ageing effects on the fatigue strength. A simple methodology predicting the fatigue life of pre-aged specimen from the residual static strength is demonstrated and validated experimentally for several layups.     

Fabrication and characterization of carbon fiber reinforced clay/epoxy composite

In this study, dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and flexural tests were performed on unfilled, 1, 2, 3, and 4 wt% clay filled SC-15 epoxy to identify the effect of clay weight fraction on thermal and mechanical properties of the epoxy matrix. The flexural results indicate that 2.0 wt% clay filled epoxy showed the highest improvement in flexural strength. DMA studies also revealed that 2.0 wt% system exhibit the highest storage modulus and T _g as compared to neat and other weight fraction. However, TGA results show that thermal stability of composite is insensitive to the clay content. Based on these results, the nanophased epoxy with 2 wt% clay was then utilized in a vacuum assisted resin transfer molding set up with carbon fabric to fabricate laminated composites. The effectiveness of clay addition on thermal and mechanical properties of composites has been evaluated by TGA, DMA, tensile, flexural, and fatigue test. 5 °C increase in glass transition temperature was found in nanocomposite, and the tensile and flexural strengths improved by 5.7 and 13.5 %, respectively as compared to the neat composite. The fatigue strength was also improved significantly. Based on the experimental result, a linear damage model combined with the Weibull distribution function has been established to describe static failure processing of neat and nanophased carbon/epoxy. The simulated stress–strain curves from the model are in good agreement with the test data. Simulated results show that damage processing of neat and nanophased carbon/epoxy described by bimodal Weibull distribution function.     

Damage behaviour of laminated composites during fatigue loading

This study deals with the modelling of damage evolution in the carbon/epoxy laminated composites under static and fatigue loading. A cumulative damage model is developed on the basis of damage evolution due to static and fatigue during cyclic loading. A continuum damage mechanics (CDM)‐based damage model coupling with the micromechanics has been utilized to predict the fatigue behaviour of laminate composites. A multicriterion approach has been introduced to predict the damage behaviour in the longitudinal, transverse, and shear direction at the ply scale. Extensive experimental results on T300/EPL1012 carbon/epoxy laminates are prepared to characterize under static and fatigue loading and to evaluate the proposed model in different conditions. The obtained results show that at the beginning of the cyclic loading, the damage grows suddenly and increases until final failure, which justifies the proposed method is able to predict the evolution of the damage due to static and fatigue loading separately during cyclic loading. The obtained results show that considering damage due to static loading leads to more accurate results, particularly in low‐cycle fatigue.     

In situ analysis of delayed fibre failure within water-aged GFRP under static fatigue conditions

A stress corrosion model has been applied to the microscopic analysis of the delayed fibre failure processes occurring within a water-aged unidirectional glass/epoxy composite under static fatigue loading (i.e. relaxation). By means of in situ microscopic observations, the individual fibre failures within an elementary volume located on the tensile side of the flexural specimens have been quantified as a function of time under various applied strain levels. It was found that the time dependence of the in situ fibre failure processes obeyed a stress corrosion model. From the microscopic observations, it was possible to assess consistent values of the parameters characterising the in situ fibre strength distribution and the subcritical crack propagation law. A comparison with separate static fatigue experiments using unimpregnated fibre bundles demonstrated that the specific physico-chemical environment encountered by the glass fibres within the aged epoxy matrix can induce significant changes in the subcritical crack propagation rates, as compared to stress corrosion cracking data collected in humid air.     

Improvement of fatigue life by incorporation of nanoparticles in glass fibre reinforced epoxy

The fatigue properties of glass fibre reinforced epoxy laminates modified with small amounts (0.3 wt.%) of nanoparticles (fumed silica SiO₂ and multi-wall carbon nanotubes (MWCNT)) were evaluated by means of static (90°-tensile and stepped tensile) and dynamic fatigue tests. For the MWCNT-modified matrix, the electrical conductivity was measured in situ. The addition of nanoparticles lead to increases in inter fibre fracture strength of up to 16%. More significantly, the high cycle fatigue life is increased by several orders of magnitude in number of load cycles. The increased inter fibre fracture strength could be correlated to the improved fatigue behaviour, as final failure in high cycle fatigue is strongly correlated to matrix cracks. For the MWCNT-modified composites, the state of load and damage state was monitored by conductivity measurements. A correlation between the onset of matrix cracking and increase in electrical resistivity could be drawn enabling self sensing capabilities.     

Flexural properties of z-pinned laminates

This paper examines the effect of pinning on the flexural properties, fatigue life and failure mechanisms of carbon/epoxy laminates. Five-harness satin weave carbon/epoxy laminates were reinforced in the through-thickness direction with different volume fractions and sizes of fibrous composite pins. Microscopic examination of the laminates before flexural testing revealed that the pins caused considerable damage to the microstructure, including out-of-plane crimping, in-plane distortion and breakage of the fibres and the formation of resin-rich zones around each pin. The pins also caused swelling of the laminate that reduced the fibre volume content. Despite the damage, the pins did not affect the flexural modulus of the laminate. However, increasing the volume content or diameter of the pins caused a steady decline in the flexural strength and fatigue life, which appear to be governed by fiber rupture on the tensile side of the laminate. Property changes are discussed in terms of transitions in the dominant failure mechanisms due to the presence of pins.     

Experimental study on fatigue failure and damage of sandwich structure with PMI foam core

Sandwich structures consisting of aluminium skin sheets and polymethacrylimide foam core have been gradually used in the high‐speed trains. The static mechanical properties and fatigue damage of the sandwich structures with polymethacrylimide foam core were experimented in three‐point bending and were discussed. The failure mode is identified as local indentation. The static strength was obtained, and it showed good consistency with the forecasting formula. The fatigue property and damage evolution were also researched under cyclic loading. The fatigue life curve and the fitting formula were submitted. The fatigue damage evolution started from the skin sheet fracture and then the foam core indentation. The displacement at the midpoint as the damage parameter was discussed, and the evolution prediction formula was submitted, which showed great agreement with the experimental results.     

CALL混杂复合材料的弯曲试验研究

本文用高灵敏度云纹干涉法对CALL混杂复合材料在纤维方向和垂直于纤维方向的弯曲及破坏特性进行了实验研究,得到了弯曲试件横截面上的剪应变分布规律及破坏形式。实验结果表明:碳纤维/环氧树脂层的剪应变明显大于铝层的剪应变,但各自沿截面呈抛物线分布。纤维方向弯曲试件的破坏形式是分层或碳纤维/环氧树脂层剪切破坏;垂直于纤维方向弯曲试件的破坏由受拉面碳纤维/环氧树脂层的拉伸破坏所致。本文工作为进一步深入研究CALL材料的力学性能提供了重要的实验依据。     

A comparative study of the fatigue and post-fatigue behavior of carbon–glass/epoxy hybrid RTM and hand lay-up composites

The paper presents a study of the fatigue and post-fatigue behavior of a hybrid carbon–glass biaxial fabric reinforced epoxy composite manufactured by the resin transfer molding (RTM) and the hand lay-up (HL) processes, with the main objective of assessing whether a material characterization run at the prototype level of a handicraft technology could be significant for a mass production technology and whether a comparison on static properties (a viable task at an industrial level) could ensure the same level of agreement for the fatigue life and residual properties. Tensile and flexural static tests as well as displacement-controlled bending fatigue tests (R ratio of 0.10) were conducted on two sets of standard specimens, having fiber orientation parallel to the loading direction (on-axis specimens) and at 45° to the loading direction (off-axis specimens). Specimens were subjected to different fatigue loading, with the maximum load level up to 60% of the average ultimate flexural strength, and damage in the laminate was continuously monitored through the loss of bending moment during cycling. After 10⁶ cycles, the fatigue test was stopped and residual properties were measured. Micrographs of sample sections revealed some voidage for HL specimens while resin rich areas were observed for RTM specimens. Results of the static tensile and flexural tests pointed out lower mechanical properties for the RTM specimens when tested on-axis and slightly higher properties when tested off-axis. Regardless of specimen fiber orientation, the fatigue and post-fatigue performance of RTM samples was inferior to that of HL specimens with the gap increasing for increasing fatigue load levels. The result was ascribed to the presence in RTM samples of resin-rich areas, which are reported to have limited influence on the laminate static properties but which may act as initiation sites for fatigue cracks.