Fatigue performance of DIARC® plasma coated bonded metal specimens

The fatigue performance of DIARC^® vacuum plasma surface treatment for titanium, stainless-steel and aluminium structural bonding was tested with double lap shear test specimens. In the metal surface treatment, a nanoscale DIARC Bindo coating is deposited on the substrates in a vacuum chamber. The DIARC-treated surface is ready for bonding and does not require any additional treatments, chemicals or primers containing hazardous CrVI chromium. Finite element method was used in analysing the test specimens. The tests were performed at room temperature dry and at room temperature after hot and wet exposure. The specimens were cycled with constant amplitude loading until failure or until 10 million cycles. The fatigue performance of the DIARC coating was found acceptable. There were no interfacial failures between the DIARC coating and metal or between the DIARC coating and adhesive. The residual strengths of all specimens after 10 million cycles were comparable to the static strength.     

Interfacial shear strength studies of nicalon fibers in epoxy matrix using single fiber composite test

Interfacial properties of Nicalon (SiC) fiber in epoxy matrices of varying stiffnesses were studied using the single fiber composite test, in conjunction with stress birefringence patterns. Extensive debonding was observed with hard epoxies, but transverse matrix cracks were found in the more flexible epoxies, with the interface remaining intact. Micromechanical modeling and Monte Carlo simulation of the single fiber composite fragmentation process provided a basis to compute the interfacial shear stress from the final fragmentation length distribution. The interfacial shear stress appeared to decrease moderately with increasing matrix ductility. The large diameter Nicalon fibers create transverse cracks in the single fiber composite specimens made with flexible epoxies. Consequently, there is a high possibility of premature failure of the specimen before fiber break saturation is reached. This poses some difficulty in interpreting the results for flexible epoxies. It was also found that the interfacial shear stress values from the single fiber composite tests were always considerably higher than the ultimate shear stress values obtained from bulk epoxy (without fiber) tension tests. This effect is similar to what was seen earlier for single fiber composite tests based on graphite fibers and similar epoxy blends, though the difference between the two values was not as great.     

Fracture and fatigue behavior of scrim cloth adhesively bonded joints with and without rivet holes

_—Tensile and fatigue disbond propagation studies on scrim cloth structural adhesive lap joints without and with rivet holes were performed. The geometry of the rivet holes is similar to that in a fuselage part of an aircraft. The joints were cycled in tension-tension fatigue at a frequency of 3 Hz and a maximum load, below the linear limit of the joint, which was obtained from the tensile tests of similar joints. The disbond length at each corner of the joint was viewed using a travelling optical microscope attached to a video camera. It was found that the static-tensile behavior of both types of joints (without and with rivet holes) consists of three stages: a linear stage followed by a region of increased non-linearity and then a 'yield' region. It is within this yield region that the rivet holes affect the strength of the joint. Stress analysis of the disbond problem under static loading revealed a strong mixed mode between the opening and shear mode stress intensity factors for both types of joints. The fatigue disbond kinetics of adhesively bonded joints without and with rivet holes were found to display an S-shaped curve with three stages of the disbond propagation rate. Failure analysis of the fatigue failed joints (without and with rivet holes) revealed three distinct regions on each half of the failed joint: an interfacial region with bare metal, a cohesive region, and an interfacial region with the adhesive adhered to the substrate. Scanning electron microscopic analysis of the disbond surface showed that the cohesive region of the fatigue fractured joints is more tortuous compared with the statically failed joints.     

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP-blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF-PP with different amounts of maleic anhydride grafted PP (PP-g-MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF-blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF-PPs with varying PP-g-MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.     

Effects of Microwave Processing on Fiber-matrix Adhesion in Composites

Experiments have been done using a single mode (TE₁₁₁, 2.45 GHz) cylindrial microwave cavity with single fiber composite specimens. After obtaining a cure cycle with microwaves to match that achieved with a conventional thermal cure cycle as measured by tensile tests, dynamic mechanical analysis and differential scanning calorimetry, quantitative measurements of interfacial shear strength and physical properties have been carried out and compared with the results from conventional thermally-cured systems. Under the conditions studied for single fiber specimens, the fiber-matrix interfacial shear strength decreases slightly in both glass-epoxy and aramid-epoxy cases as comapared with thermally-cured specimens. Graphite fiber-epoxy adhesion, on the other hand, increases significantly in these single fiber studies in microwave processed specimens as indicated by an increase in the interfacial shear strength. The failure mode changes from interfacial (thermal curing) to matrix failure.     

Damage assessment of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue at ambient and elevated temperatures

The damage evaluation behaviour of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue was investigated by means of acoustic emission (AE) monitoring and forced resonance techniques. AE technique provided sufficient information about the damage initiation and progression in real time whilst the forced resonance (FR) technique allowed the detection of changes in elastic modulus (E) and internal friction (Q⁻¹) that occurred with increasing number of cyclic fatigue at room temperature. From the two non-destructive detection techniques results combined with microstructural observations, it is concluded that the composite cyclic fatigue damage evolution begins with multiple crack formation within the matrix and is followed by delamination (interfacial failure). Final failure of the composite is caused by fibre fracture and extensive cyclic sliding along the fibre/matrix interface. The strong bonding between mullite matrix and alumina fibre caused by the glassy phase within the mullite matrix determined the fatigue performance of the composite at 1350°C. Regions with glassy phase failed catastrophically as a result of early fibre fracture.     

Effects of Microwave Processing on Fiber-matrix Adhesion in Composites

Experiments have been done using a single mode (TE₁₁₁, 2.45 GHz) cylindrial microwave cavity with single fiber composite specimens. After obtaining a cure cycle with microwaves to match that achieved with a conventional thermal cure cycle as measured by tensile tests, dynamic mechanical analysis and differential scanning calorimetry, quantitative measurements of interfacial shear strength and physical properties have been carried out and compared with the results from conventional thermally-cured systems. Under the conditions studied for single fiber specimens, the fiber-matrix interfacial shear strength decreases slightly in both glass-epoxy and aramid-epoxy cases as comapared with thermally-cured specimens. Graphite fiber-epoxy adhesion, on the other hand, increases significantly in these single fiber studies in microwave processed specimens as indicated by an increase in the interfacial shear strength. The failure mode changes from interfacial (thermal curing) to matrix failure.     

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.     

Fatigue strength of vibration‐welded unreinforced nylon butt joints

The fatigue properties of vibration‐welded butt joints in two thermoplastics, nylon 6 and nylon 6,6, are examined in this work. Injection molded plaques were welded under high and low pressure conditions at 212 Hz and at an amplitude of 1.8 mm to a weld penetration of 1.5 mm. Dog‐bone coupons were machined from welded and unwelded plaques and then fatigue cycled in load control at a stress ratio of R = 0.1. The test frequency ranged from 1 to 10 Hz to avoid hysteretic heating. When the temperature rise in the weld region during testing was insignificant, no physical thermal damage was observed. It was found that the nonwelded specimens have longer fatigue lives than the welded ones, while the welded specimens appear to have similar fatigue behavior, except for nylon 6 welded in high pressure, which was slightly inferior. Vibration welding of these materials appears to be viable for structural applications requiring fatigue resistance. POLYM. ENG. SCI. 45:935–944, 2005. © 2005 Society of Plastics Engineers     

Fatigue behavior of medium-density polyethylene pipes for gas distribution

This paper illustrates the factors that control brittle failure under fatigue loading for test specimens cut from medium-density polyethylene pipes for gas distribution. A square bar specimen cut from a pipe with a notch was made and a fatigue test was conducted to cause a brittle failure. To obtain the correlation among stress range, frequency, temperature, and cycles to failure in this fatigue test, Coffin-Manson's frequency-modified fatigue life equation was adopted and the material constants were determined. By gradually lowering the frequency, the resistance to creep can be estimated because cycles to failure—indicating the fatigue damage—decreased, and the actual loading time—indicating the creep damage—increased.     

Monotonic and fatigue performance in bending of fiber-reinforced engineered cementitious composite in overlay system

This paper presents an experimental study and theoretical analyses on the monotonic and fatigue performance in bending of a polyvinyl alcohol (PVA) fiber-reinforced engineered cementitious composite (ECC) overlay system. The influence of the interfacial characteristics between overlay and old concrete substrate on the overall bending performance is investigated. The experimental results show that when ECC is used as overlay material, both load carrying capacity and deformability represented by deflection at peak load of the overlaid beams in flexure are significantly increased compared to those of plain concrete (PC) overlaid beams. The fatigue life of ECC overlaid beams in flexure is not influenced by the layer/base interfacial characteristics, such as smooth cutting surface or sand-blasted rough surface. However, the deformation ability of the overlaid beams, such as deflection at midpoint of beam, in both static and cyclic loading cases are influenced by the interfacial property. The smooth casting surface leads to larger deformation at peak load under monotonic loading and at failure under fatigue loading than the corresponding values for beams with a rough casting surface. The present study demonstrates that reflective cracking failure in pavement overlays can be eliminated by the use of a ductile material such as ECC.     

Micromechanical analysis of the kink‐band performance at the interface of a thermoplastic composite under tensile deformation

The manufacture of thermoplastic composites normally involves compression molding that generates fiber dislocations known as kink‐bands, which create stress concentrations able to cause the premature compression failure of a composite; nevertheless, the kink‐band influence over the interfacial performance or failure of a composite tested under tension is not fully understood. This work uses Raman spectroscopy as a tool to map the axial stress distribution around a kink‐band in an aramid/low density polyethylene single fiber composite. The stress distribution along the fiber was fitted to a generalized shear‐lag model to calculate the interfacial shear stress; its maximum was found around the kink‐band where the fiber interface was still bonded to the matrix, defining a localized stress concentration. POLYM. COMPOS., 31:1817–1817, 2010. © 2010 Society of Plastics Engineers.     

Thermomechanical Fatigue Behavior of a Silicon Carbide Fiber-Reinforced Calcium Aluminosilicate Composite

Isothermal fatigue and in-phase thermomechanical fatigue (TMF) tests were performed on a unidirectional, continuous-fiber, Nicalon®-reinforced calcium aluminosilicate glass-ceramic composite ([O]_16, SiC/CAS-II). Monotonic tensile tests were performed at 1100°C (2012°F) and 100 MPa/s (14.5 ksi/s) to determine the material's ultimate strength (σ_ult) and proportional limit (σ_pl). Isothermal fatigue tests at 1100°C employed two loading profiles, a triangular waveform with ramp times of 60 s and a similar profile with a superimposed 60-s hold time at σ_max. All fatigue tests used a σ_max of 100 MPa (40% of σ_pl), R = 0.1. TMF loading profiles were identical to the isothermal loading profiles, but the temperature was cycled between 500° and 1100°C (932° and 2012°F). All fatigued specimens reached run-out (1000 cycles) and were tested in tension at 1100°C immediately following the fatigue tests. Residual modulus, residual strength, cyclic stress-strain modulus, and strain accumulation were all examined as possible damage indicators. Strain accumulation allowed for the greatest distinction to be made among the types of tests performed. Fiber and matrix stress analyses and creep data for this material suggest that matrix creep is the primary source of damage for the fatigue loading histories investigated.     

The fracture properties of a smart fiber metal laminate

This paper investigates the interfacial, tensile, and fatigue properties of a novel smart fiber‐metal laminate (FML) based on a nickel‐titanium (Ni‐Ti) shape memory alloy and a woven glass fiber reinforced epoxy. Initial tests, using the single cantilever beam (SCB) geometry, have shown that this unique system offers high values of metal‐composite interfacial fracture toughness. Tensile tests have shown that the mechanical properties of these FMLs lie between those offered by its constituent materials and that their tensile modulus and strength can be easily predicted using a rule of mixtures approach. Tension‐tension fatigue tests have shown that the fatigue performance of notched smart FMLs is superior to that offered by the plain Ni‐Ti alloy. A subsequent optical examination of unnotched laminates tested to failure under tension‐tension fatigue loading has shown that the fracture mechanisms occurring within the Ni‐Ti FMLs are strongly dependent on the applied cyclic stress. POLYM. COMPOS., 28:534–544, 2007. © 2007 Society of Plastics Engineers     

Use of acoustic emission to characterize corrosion fatigue damage accumulation in glass fiber reinforced polyester laminates

The acoustic emission technique has been used to characterize fatigue damage accumulation in glass fiber woven roving (0/90°) polyester laminates after prolonged exposure in sea water. Comparisons were made with fatigue tests of “as-received” laminate under similar loading conditions. Pre-exposure has been found to substantially reduce the fatigue strength of the composite. Acoustic emission monitoring during fatigue testing has shown that the amplitude distribution of the acoustic events shifts from predominantly low amplitude (40–55 dB), associated with matrix cracking, in as-received specimens, to intermediate amplitude (55–75 dB) associated with delamination and debonding after pre-exposure. Optical microscopy of fatigued samples has verified these failure mode changes. The number of recorded high amplitude events (≥ 80 dB) associated with fiber fracture is the same in both cases, which indicates that the glass reinforcement is unaffected by pre-exposure.     

钛合金‑芳纶纤维复合材料单搭接胶接接头渐进破坏分析

采用芳纶纤维复合材料与钛合金制备单搭接胶接连接实验件。利用万能实验机、DIC、应变采集系统等手段,对胶接接头的极限载荷、应变场、应变分布和破坏模式进行表征,分析了拉伸载荷下胶接接头的应变分布规律和复合材料层合板刚度折减规律,探究了异质材料单搭接胶接接头的破坏过程。结果表明,胶接接头破坏模式为搭接接头两端胶层界面破坏,中间部位复合材料层间破坏。接头破坏过程为渐进破坏,受载时复合材料端头产生较大的剪切应变,裂纹在此处萌生,并不断向钛合金端头扩展,扩展部位复合材料层合板刚度不断折减,直到搭接面积过小胶层突然发生界面破坏。     

Fatigue characteristics of a co-cured single lap joint subjected to cyclic tensile loads

In this paper, the effect of bond parameters on the fatigue characteristics of a steel-composite co-cured single lap joint under cyclic tensile loads was experimentally investigated. We considered the surface roughness of the steel adherend and the stacking sequence of the composite adherend as bond parameters. A fatigue failure mechanism of the co-cured single lap joint was explained systematically by investigating the surfaces of failed specimens.     

Effects of cross‐section design and loading direction on the creep and fatigue properties of wood/PVC composite beams

The creep and fatigue properties of two wood/poly(vinyl chloride) (WPVC) composite beams were studied under flexural and cyclic deformations. The effects of cross‐section design and load direction were the main interests. The weight ratio of the wood and PVC compound used was 1:1, and the composites were produced by using an industrial‐scale twin‐screw extruder. In creep testing, the changes in WPVC beam displacement for the edgewise and flatwise directions increased with time. The WPVC composite with a greater size (thickness) and number of cores had the higher creep resistance. Testing a WPVC composite in the flatwise direction gave less time‐dependence than testing in the edgewise direction. The recommended applied loads for optimum creep resistance of the WPVC specimens were found to be 20 and 30% of the ultimate load to failure, depending on the size and number of cores for the cross‐section used. In fatigue testing, the number of cycles to failure for both WPVC composite specimens tested in the flatwise direction was greater than that for testing in the edgewise direction. J. VINYL ADDIT. TECHNOL., 2010. © 2010 Society of Plastics Engineers     

基于混凝土基体和界面过渡区性质的疲劳方程

根据混凝土材料在静态荷载与疲劳荷载作用下破坏的相似性,结合其在静态荷载作用下的破坏分析了疲劳破坏过程,通过引入基体和界面过渡区对疲劳性能的影响因子f1,f2,定义基体性质特征参数I和界面过渡区性质特征参数M,应用数学模型描述f1,f2随疲劳寿命对数值lgN的变化趋势,建立混凝土材料基体、界面过渡区性质与疲劳性能之间的定量关系,并得到基于基体与界面过渡区性质的疲劳方程.测试了水胶比为0.35,不同矿物掺合料掺量混凝土在不同应力水平下的疲劳寿命.应用所建立的疲劳方程能较好地拟合S-N关系,尤其是大矿物掺合料掺量的情况下,反映了低周疲劳向高周疲劳过渡的非线性变化.     

Interfacial shear strength studies using the single-filament-composite test. I: Experiments on graphite fibers in epoxy

The failure of the interface in a carbon fiber-epoxy system was studied for six different epoxy blends using the single-filament-composite technique. The blends were formulated to yield a wide range of stiffnesses, and their effect on interfacial failure was examined. Specimens were made from Hercules IM6-G carbon fiber and the different blends of epoxy, and then strained to obtain a distribution of fiber fragment lengths. Birefringence patterns near the fiber breaks were observed and recorded. Some of the specimens were strained until they failed and the resulting fracture surfaces were observed under a scanning electron microscope to determine fracture patterns and the existence of debonding. The fragment length distributions were interpreted using a Monte-Carlo simulation of a Poisson/Weibull model for fiber strength and flaw occurrence. The results were used to calculate an effective interfacial shear strength. From this analysis we conclude that one cannot accurately predict the interfacial properties of a composite based solely upon conventional single fiber and bulk matrix properties. Local matrix properties and fiber/matrix interactions, on a microscale, play a key role in composite strength.