Threshold Strain Energy Release Rate Research Articles

Effects of hygrothermal aging on the fatigue behavior of two toughened epoxy adhesives

Aged open-faced asymmetric double cantilever beam (ADCB) specimens made with two different rubber-toughened epoxy adhesives were subject to cyclic loading under mixed-mode conditions. The contrasting results illustrated the effects of environmental degradation on the matrix and toughener. The fatigue threshold strain energy release rate, G th , and the crack growth rates of adhesive 1 degraded in two stages: G th initially decreased with aging time until it reached a constant minimum value at long aging times. Similarly, fatigue crack growth rates initially increased with aging time until reaching a limiting upper value. However, G th reached the minimum value sooner than did the crack growth rate. In contrast, the G th of adhesive 2 decreased significantly with aging while the crack growth rates remained unchanged even after prolonged aging. These differences in fatigue threshold and crack growth rate behavior were attributed to changes in the size of the plastic zone at the crack tip as the applied loads changed. The differences in the degradation behavior of both adhesives were explained using gravimetric and dynamic mechanical thermal analysis (DMTA). The degradation of the toughening mechanism of adhesive 1 was related to retained, bound water disrupting the chemical bonds at the rubber/matrix interface. Having no retained water, adhesive 2 was unaffected by this degradation.

The effect of carbon nanotubes on the fracture toughness and fatigue performance of a thermosetting epoxy polymer

Multi-walled carbon nanotubes, with a typical length of 140 μm and a diameter of 120 nm, have been used to modify an anhydride-cured epoxy polymer. The modulus, fracture energy and the fatigue performance of the modified polymers have been investigated. Microscopy showed that these long nanotubes were agglomerated, and that increasing the nanotube content increased the severity of the agglomeration. The addition of nanotubes increased the modulus of the epoxy, but the glass transition temperature was unaffected. The measured fracture energy was also increased, from 133 to 223 J/m2 with the addition of 0.5 wt% of nanotubes. The addition of the carbon nanotubes also resulted in an increase in the fatigue performance. The threshold strain-energy release-rate, G th, increased from 24 J/m2 for the unmodified material to 73 J/m2 for the epoxy with 0.5 wt% of nanotubes. Electron microscopy of the fracture surfaces showed clear evidence of nanotube debonding and pull-out, plus void growth around the nanotubes, in both the fracture and fatigue tests. The modelling study showed that the modified Halpin–Tsai equation can fit very well with the measured values of the Young’s modulus, when the orientation and agglomeration of the nanotubes are considered. The fracture energy of the nanotube-modified epoxies was predicted, by considering the contributions of the toughening mechanisms of nanotube debonding, nanotube pull-out and plastic void growth of the epoxy. This indicated that debonding and pull-out contribute to the toughening effect, but the contribution of void growth is not significant. There was excellent agreement between the predictions and the experimental results.

Effect of Silane Adhesion Promoters on Subcritical Debonding of Epoxy/Glass Interfaces After Hygrothermal Aging

Subcritical debonding is of particular concern for microelectronic packaging, coating, and adhesive applications. Time-dependent subcritical debonding at polymer/substrate interfaces occurs at lower mechanical loads and strain energy release rates compared with those required for catastrophic interface fracture. In this work, the role of organosilane adhesion promoters, 3-aminopropyltriethoxysilane and glycidoxypropyltrimethoxysilane, in subcritical debonding of epoxy/glass interfaces under hygrothermal condition is investigated. The epoxy systems studied included a thermally cured bisphenol F-based epoxy resin and a bisphenol F-based epoxy resin cured with a UV active curing agent. Subcritical debonding results revealed that there are two regions, the threshold strain energy release rate (G TH) and the power law region, observed in subcritical debonding curves. Hygrothermal aging not only lowers the critical debonding driving energy required for debond extension (GC) but also lowers G TH, below which interfacial crack growth does not occur. Interestingly, applying silane adhesion promoters on glass surfaces increased G TH values and decreased debonding growth rate. Therefore, the subcritical debonding growth rate mechanism was found to be sensitive to interface chemistry. An attempt to correlate the results of critical and subcritical debonding was undertaken. It was found that as aging time increased, the role of subcritical debonding became less important compared with the critical debonding component. However, the presence of subcritical debonding at applied driving energies significantly below G C has important implications for the long-term reliability of interfaces.

Mixed-mode fatigue behavior of degraded toughened epoxy adhesive joints

Open-faced asymmetric double cantilever beam (ADCB) specimens of toughened epoxy-aluminum adhesive joints were aged either in a constant humidity environment or a cyclically changing environment to study the mixed-mode fatigue behavior. Under constant humidity environments, the fatigue threshold strain energy release rate initially decreased with aging time until it reached a constant minimum value for long times. In contrast, the crack growth rates continued to increase with aging time. It is hypothesized that at crack growth rates close to threshold the fatigue behavior is governed by the epoxy matrix, whereas at relatively high crack growth rates the fatigue behavior is governed by the loss of the rubber toughening mechanism. Increasing the aging temperature accelerated the degradation of the joints leading to a reduction in the time to reach the constant minimum value and increased the crack growth rates. Under a cyclic aging environment with intermittent salt spray, neither the threshold strain energy release rate nor the crack growth rates degraded until four weeks of aging. The superior fatigue performance of these joints compared to joints aged in constant humidity environments was due to the lower water concentrations in the adhesive while aging. This conclusion was supported by moisture uptake measurements of the adhesive in deionised and salt water environments that showed simple Fickian behavior at room temperature and dual-Fickian behavior at higher temperature. The salt water environment produced osmotic pressure that decreased the moisture concentration in the second stage of diffusion.

Effect of Surface Roughness on the Performance of Adhesive Joints Under Static and Cyclic Loading

The role of surface roughness on the fatigue and fracture behavior of a toughened epoxy adhesive system was investigated experimentally. Fatigue studies covered both the fatigue threshold strain energy release rate, G th , and fatigue crack growth rates, while the strain energy release rate for crack initiation, , and the steady-state value, , were measured under quasi-static loading. Mixed-mode fatigue results showed a significant dependency on surface roughness. G th increased with roughness, reached a plateau, and then decreased for very rough surfaces. This increase in G th was explained in terms of the increase in bonding and fracture surface area, crack growth retardation due to the microtopography of the substrate, and crack path deviation from the interface. The decrease in G th for very rough substrates was attributed to void formation and stress concentration at the tip of asperities. The effect of roughness on fatigue diminished as the applied strain energy release rate increased. This was a result of the crack path becoming more cohesive, moving away from the interface. Similarly, no effect of surface roughness was observed in the mode-I fatigue results and the mixed-mode fracture results, since the crack path in these cases was far enough from the interface.

Modified DCB and CLS specimens for mixed-mode fatigue testing of adhesively bonded thin sheets

The fatigue behavior of adhesive joints made with commercially coil-coated thin aluminum sheets has been measured using modified double cantilever beam (DCB) and cracked lap shear (CLS) specimens that avoided yielding. The thin sheets were bonded to a stiffener using a reinforcing adhesive layer. Experimental and finite element analyses demonstrated that the reinforcing adhesive layer had an insignificant effect on the stress state at the crack tip and the resulting fatigue behavior. The modified DCB test geometry was then used to measure the effect of the loading phase angle and test environment on the fatigue performance of the pretreated aluminum sheet. It was observed that the phase angle affected the threshold strain energy release rate, G th , and the crack growth rates. The presence of moisture in the test environment greatly reduced the fatigue threshold. It was also observed that the G th was sensitive to the rolling line orientation on the surface of the sheet.

Fracture and fatigue study of unidirectional glass/epoxy laminate under different mode of loading

ABSTRACTInterlaminar fracture is the dominant failure mechanism in most advanced composite materials. The delaminating behaviour of materials is quantified in terms of the strain energy release rate G. In this paper, the experimental measurements of the fatigue delaminating growth for some combinations of energy release rate mode ratio have been carried out on unidirectional glass/epoxy laminates. On this base the constants in the Paris equation have been determined for each GII/GT considered modal ratio. The fatigue threshold strain energy release rate Δ GTth, below which delaminating doesn't occur, were measured. Three type specimens were tested, namely: double cantilever beam (DCB), end‐loaded split (ELS) and mixed‐mode bending (MMB) under mode I, mode II and mixed‐mode (I + II) loading, respectively. Scanning electron microscopy techniques were used to identify the fatigue delamination growth mechanisms and to define the differences between the various modes of fracture.

The effect of mode ratio and bond interface on the fatigue behavior of a highly-toughened epoxy

The fatigue threshold and the cyclic crack growth of a highly-toughened epoxy adhesive were studied under mode I and several mixed-mode loading cases and compared with the quasi-static critical fracture energies. Four different adhesive systems were examined using steel and aluminum substrates having different surface roughness, and surface treatment. The effect of increasing the amount of mode II (increasing the phase angle) on the fatigue threshold strain energy release rate and the cyclic crack growth rate was found to be insignificant at low phase angles. However, a significant increase in the fatigue threshold and decrease in the cyclic crack growth rate was observed at higher phase angles. These trends were similar to that seen in adhesive joint fracture. Adherend surface roughness and surface preparation affected the fatigue behavior significantly, particularly at low crack speeds and high phase angles. The fatigue properties were essentially the same for both steel and aluminum adherends provided that the crack paths were cohesive. A general observation was that the fatigue crack path moved progressively closer to the more highly strained adherend under mixed-mode loading as the applied strain energy release rate and hence the crack speed, decreased. This caused mixed-mode cracks to be nearly interfacial in the threshold region.

Loading variable effects on mode—I fatigue of wood–FRP composite bonded interface

In this paper, the effect of fatigue loading variables for waveform, load ratio and frequency is evaluated for red maple wood–phenolic fiber reinforced polymer (FRP) composite bonded interface. Experiments are performed with a contoured double cantilever beam (CDCB) specimen under load control, and the crack propagation rate is obtained by the compliance method. Three groups of tests are performed to study the effects of waveform, load ratio and frequency on crack propagation rate. In addition, the modified load shedding technique is used to explore the fatigue threshold and the load ratio effect on fatigue threshold. The tests are carried out under constant amplitude cyclic loading at room temperature and laboratory conditions. The results indicate that while waveform has negligible effect, both load ratio and frequency have significant effects on crack propagation rate of the bonded interfaces. A modified Paris Law model is proposed to include both load ratio and frequency effects. The dependence of the threshold strain energy release rate range, Δ G th, on load ratio is explained effectively by the crack closure concept.

The effect of moisture on the fracture energy of TiN/SiO 2 interfaces in multi-layer thin films

The fracture energy of TiN/SiO 2 interfaces, prepared as part of multi-layer structures by conventional thin-film device processing, has been measured using a residually stressed, micro-strip decohesion test. Both the decohesion energy and the decohesion velocity, at fixed strain energy release rate, are found to be affected by the presence of moisture and are functions of relative humidity. A threshold strain energy release rate for interface decohesion in the presence of moisture is determined. Also, the diffusivity of moisture along the TiN/SiO 2 interface, at room temperature, is determined to be (6±2)×10 −13 cm 2/s from sequential images of the 2D concentration profiles formed by dynamic secondary ion mass spectroscopy (SIMS). Evidence is presented indicating that the TiN/SiO 2 interface is weakened by in-diffusion of moisture ahead of the advancing decohesion front.

Mixed-Mode Delamination Behavior of Carbon/Epoxy Composites

The mixed-mode bending (MMB) test was employed in an investigation of delamination fracture behavior in static and fatigue loading for carbon/epoxy composites 1300/M10 and HTA/6376. As specimens were loaded at different ratios of alL for different mixed-mode ratios, both investigated composites exhibited brittle unstable or brittle stable behavior of crack growth. Mixed-mode delamination fracture toughness was determined at a IL = 0.5. Fracture surfaces of MMB specimens were examined using scanning electron microscopy to distinguish fracture features. Stable crack growth was observed over the entire range of crack lengths investigated in the cyclic tests. A consistent relation of crack growth per cycle to the cyclic strain energy release rate was obtained. The trend of reduction in crack growth rate data with diminution in cyclic strain energy release rate suggests the presence of a threshold strain energy release rate.

Moisture-assisted crack propagation at polymer/glass interfaces

Moisture-assisted crack growth at various polymer/glass interfaces was measured as a function of applied strain energy release rate and relative humidity using a four-point flexure apparatus coupled with an inverted microscope. The specimens consisted of two glass plates bonded together with a thin layer of commercially available epoxy-acrylate, urethane-acrylate or epoxy adhesive. The crack front at the epoxy-acrylate and epoxy interfaces was relatively smooth and, above a threshold strain energy release rate, the crack growth rate was dependent on the applied strain energy release rate via a power law relationship. Crack growth along the urethane-acrylate interface was characterized by the development of finger-like perturbations along the advancing crack front. These finger-like perturbations grew until they reached a steady-state length. Once the fingers reached steady-state, the crack growth rate of the overallcrack front was dependent on the applied strain energy release rate via a power law function. With all the polymer adhesives crack growth rates increased with higher relative humidities.

Subcritical crack growth along epoxy/glass interfaces

Subcritical crack growth behavior along polymer/glass interfaces was measured for various epoxy adhesives at different relative humidities. A four-point flexure apparatus coupled with an inverted microscope allowed for observation in situ of the subcritical crack growth at the polymer/glass interface. The specimens consisted of soda-lime glass plates bonded together with epoxy acrylate, epoxy (Devcon), or epoxy (Shell) adhesives. Above a threshold strain energy release rate, the subcritical crack velocity was dependent on the strain energy release rate via a power law relationship where the exponent was independent of the adhesive tested and the test humidity (n = 3). However, the multiplicative constant A in the power law relation varied by over three orders of magnitude between the various adhesives with epoxy (Shell) having the smallest value and the epoxy (Devcon) the greatest value; in addition, A was very sensitive to humidity, decreasing by over two orders of magnitude from 80% to 15% relative humidity. At high strain energy release rates, the subcritical crack velocity reached a plateau at approximately 10−6 m/s. The use of this subcritical crack velocity data in predicting thin film delamination is discussed.

Time dependent crack growth in soldered joint

Time dependent crack growth in a soldered joint, 60/40 Sn/Pb and unplated, silver plated and gold plated beryllium copper, is experimentally investigated. Double cantilever beam specimens selected for the test are machined by wire EDM with the center split deeply cut almost clear to one end. This center slit is then soldered by vapor phase reflow after depositing the solder paste. After creating an initial crack in the solder layer, a subcritical load in the opening mode is applied and the corresponding displacement at the loading point is held constant. Subsequent load decrease and increase in the crack length are recorded with time. Thus observed rate of crack growth per unit time is found to relate proportionally to the third power of the strain energy release rate. Furthermore, there seems to exist a threshold strain energy release rate of 200–300 J/m 2 under which no crack growth is observed.