Debond Growth Rates Research Articles

Prediction of crack growth at trailing edge bondlines of a wind turbine rotor blade for the assessment of remaining service life

Debonding along trailing-edge joint of a wind turbine rotor blade is one of the often reported failure modes during the rotor blade maintenance inspections. This study focuses on buckling-induced debonding along the trailing edge under in-plane compressive loads due to the gravity forces on the rotor blade. With a priori known debond crack length, an analytical model is used for predicting debond growth rate and assessing the time that takes the crack to reach a critical length. For the growth rate prediction, required input parameters are size of the initial debonding, laminate and adhesive layer thicknesses, elastic and interfacial fracture mechanics properties, gravitational load distribution and position of the initial debonding. The damage model is implemented in excel spreadsheet tool. The predictions of the analytical model were compared with the results from 3D finite element simulations and the difference between the results (energy release rate) was 8%. This study demonstrates how damage specific tools can be developed for predicting remaining useful life of rotor blades.

Low temperature mixed-mode debond fracture and fatigue characterisation of foam core sandwich

This paper experimentally investigates the effects of low temperature on fracture toughness and fatigue debond growth rate in foam core sandwich composites. Mixed-mode bending specimens were statically and cyclically tested inside a climatic chamber at a low temperature (−20°C) and at room temperature (23°C) as a reference. Testing was conducted in mode I (opening) and mixed-mode I/II (opening-sliding) mode mixities. The fatigue tests results are presented according to the modified Paris–Erdogan relation. Results showed substantial fracture toughness reduction due to low temperature. Low temperature furthermore elevated the cyclic crack growth rate.

Delamination of carbon-fiber strengthening layer from concrete beam during deformation (infrared thermography)

Technology of strengthening reinforced concrete structures with composite materials has found wide application. The effectiveness of strengthening of concrete structures with externally bonded reinforcement is supported by a great deal of experimental evidence. However, the problem of serviceability of such structures has not been adequately explored. The present work describes the results of experimental studies on the loadcarrying capacity of concrete beams strengthened with carbon fiber reinforced plastic (CFRP). Special emphasis is placed on studying the debonding of the strengthening layer from the concrete surface and analyzing its influence on the load-carrying capacity of beams. Infrared thermography is used to detect the first signs of debonding and to assess the debond growth rate.

Adhesion and debonding kinetics of photovoltaic encapsulation in moist environments

Debonding of photovoltaic (PV) encapsulation in moist environments is frequently reported but presently not well understood or quantified. Temperature cycling, moisture, and mechanical loads often cause loss of encapsulation adhesion and interfacial debonding, initially facilitating back-reflectance and reduced electrical current, but ultimately leading to internal corrosion and loss of module functionality. To investigate the effects of temperature (T) and relative humidity (RH) on the kinetics of encapsulation debonding, we developed a mechanics-based technique to measure encapsulation debond energy and debond growth rates in a chamber of controlled environment. The debond energy decreased from 2.15 to 1.75 kJ m−2 in poly(ethylene-co-vinyl acetate) (EVA) and from 0.67 to 0.52 kJ m−2 in polyvinyl butyral when T increased from 25 to 50°C and 20 to 40°C, respectively. The debond growth rates of EVA increased up to 1000-fold with small increases of T (10°C) and RH (15%). To elucidate the mechanisms of environmental debonding, we developed a fracture-kinetics model, where the viscoelastic relaxation processes at the debonding-tip are used to predict debond growth. The model and techniques constitute the fundamental basis for developing accelerated aging tests and long-term reliability predictions for PV encapsulation. Copyright © 2015 John Wiley & Sons, Ltd.

Environmental mechanisms of debonding in photovoltaic backsheets

The backsheets used in photovoltaic modules are exposed to aggressive field environments that may include combined temperature cycles, moisture, and mechanical loads. The effects of the field environment on backsheet debonding, which can lead to module degradation (corrosion) and loss of function, are still not well understood or quantified. Employing a newly developed quantitative mechanics technique, we report the effect of aging on backsheet debond energy, including the separate effect of temperature, mechanical stress and relative humidity on debond growth rate. The debond energy of the backsheet decreased dramatically from 1000 to 27J/m2 within the first 750h of exposure to hot (85°C) and humid (85% RH) aging treatments. The debond growth rate increased up to 500-fold with small changes of temperature (10°C) and relative humidity (20%). To elucidate the mechanisms of environmental debonding, we developed a fracture-kinetics model, where the molecular relaxation processes at the debond front are used to predict debond growth. The model and techniques form the fundamental basis to develop accelerated aging tests and long-term reliability predictions for photovoltaic backsheets.

FEM modeling of fiber/matrix debond growth in tension-tension cyclic loading of unidirectional composites

The fiber/matrix interface crack (debond) growth from fiber break in unidirectional composite subjected to high stress tension–tension cyclic loading is analyzed. The debond growth is simulated calculating the strain energy release rate GII by FEM in three-dimensional formulation and using power law with respect to the GII change to describe the debond growth rate. Two models were applied. In Model 1 the partially debonded fiber/matrix cylindrical unit with a fiber break is surrounded from all sides by an effective composite phase. In Model 2 the effective composite phase was used around the fiber/matrix unit except the region between the unit and the specimen surface, which contains neat matrix only. Calculations show that the average GII is slightly larger, when the analyzed fiber is closer to the specimen surface. The debond growth was simulated using interface fatigue parameters obtained from single fiber composite specimen tests in the literature. Simulation results show that debonds from fiber breaks close to the specimen surface grow faster than from fiber breaks inside the composite specimen.

Interface debond crack growth in tension–tension cyclic loading of single fiber polymer composites

Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension–tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.

Fatigue de-bond growth in adhesively bonded single lap joints

The fatigue de-bond growth studies have been conducted on adhesively bonded lap joint specimens between aluminium and aluminium with Redux-319A adhesive with a pre-defined crack of 3 mm at the bond end. The correlations between fracture parameters and the de-bond growth data are established using both numerical and experimental techniques. In the numerical method, geometrically non-linear finite element analyses were carried out on adhesively bonded joint specimen for various de-bond lengths measured from the lap end along the mid-bond line of the adhesive. The finite element results were post processed to estimate the SERR components G I and G II using the Modified Virtual Crack Closure Integral (MVCCI) procedure. In experimental work, specimens were fabricated and fatigue de-bond growth tests were conducted at a stress ratio R = − 1. The results obtained from both numerical analyses and testing have been used to generate de-bond growth curve and establish de-bond growth law in the Paris regime for such joints. The de-bond growth rate is primarily function of mode-I SERR component G I since the rate of growth in shear mode is relatively small. The value of Paris exponent m is found to be 6.55. The high value of de-bond growth exponent in Paris regime is expected, since the adhesive is less ductile than conventional metallic materials. This study is important for estimating the life of adhesively bonded joints under both constant and variable amplitude fatigue loads.

Fracture mechanics analysis of debond growth in a single-fiber composite under cyclic loading

A model is developed to analyze the growth of a fiber/matrix debond along a broken fiber interface in a single-fiber composite subjected to tension-tension fatigue. The Paris law expressed in terms of debond growth and strain energy release rates is used. An analytical solution for the Mode II energy release rate G II is obtained for long debonds, where the interface crack growth is self-similar. For short debonds, the interface crack interacts with the fiber break, and therefore a FEM modeling in combination with the virtual crack closure technique was performed to calculate the increase in G II . Finally, the calculated G II dependences are summarized in simple expressions that are used to simulate the debond growth in fatigue. A parametric study of the effect of Paris law parameters on the debond growth is performed.

Theoretical Solution for Fatigue Debonding Growth and Fatigue Life Prediction of FRP-Concrete Interfaces

This paper presents an analytical solution for the evolution and distribution of shear stresses along the entire bond length of FRP-concrete interfaces due to mode-II fatigue loading. The creep-fatigue interaction and fatigue crack growth after debonding initiation are incorporated into a nonlinear interfacial constitutive law. While the creep-fatigue interaction is represented by the degradation of the interfacial stiffness, the debond growth is governed by a form of the Paris equation and the fracture energy ratio, Gmax/Gc. Furthermore, a new form of energy ratio is adopted to be debond-dependent. Through a series of experimental double-lap shear specimens, the results showed that the debond growth rate ( da/dN) along the FRP-concrete interfaces diminishes with fatigue cycles and that 30% of the static bond capacity of the FRP-concrete interface can be considered as the endurance limit of fatigue loading for FRP-strengthened beams. The agreement between the theoretical predictions and experimental results is valid, with a good degree of accuracy.

Anomalous debonding behavior of a polymer/inorganic interface

Adhesion and subcritical debonding at the interface between a thin diglycidyl ether of bisphenol F polymer layer and either SiN x or SiO 2 passivated silicon substrates are described. The interface is characterized by weak hydrogen bonding. Prolonged exposure to a moist environment resulted in a time-dependent decrease in adhesion. Subcritical debond-growth rates as a function of applied loads were sensitive to temperature and relative humidity, and greatly accelerated in the presence of cyclic loading. Of particular interest was the occurrence of an anomalous region of persistent debonding that developed below ∼10 −8 m s −1 under both monotonic and cyclic loading. In this region, debond-growth rates were characterized by a weak dependence on the applied loads, a strong dependence on moisture activity, and the absence of a measurable threshold below which debonding could not be measured. We propose a new stress-dependent transport model that describes the moisture diffusion mechanism responsible for this anomalous behavior.

Novel capacitance sensor design for measuring coating debonding: the effects of thermal-hygroscopic cycling

The effects of combined thermal and hygroscopic cycling on the adhesion performance of an epoxy coating were measured using a novel electrode sensor. The sensor is uniquely designed, consisting of a series of independent interdigitated electrode traces which are arranged parallel to the sensor edges. Coupled with single-frequency capacitance measurements, the sensor detects changes in capacitance in the adhered coating–sensor interfacial region as a function of the distance from the edge of the sensor, x. Recently, this sensor was utilized by O'Brien and co-workers to measure interfacial diffusion and the concentration profile of fluid in an adhesive joint (Int. J. Adhesion Adhesives 23, 335–338 (2003)). In the present work, large capacitance changes due to debonding and displacement of the coating by fluids at the sensor surface were used to monitor coating delamination. The apparent debond growth rate and number of cycles until failure were determined as a function of coating thickness, fluid environment and sensor surface chemistry. The results show that the coating becomes more durable as the thickness is reduced; and also that thermal and hygroscopic cycling of coatings produces different results than conventional continuous adhesion tests. This study suggests that this novel sensor or a similar design is applicable for the study of adhesion loss and interfacial diffusion processes, and could be extended to other coatings or adhesives in a variety of environments. General trends about coating durability are also discussed.

Environmental Effects on Cracking and Delamination of Dielectric Films

The effects of temperature and moisture on the adhesive and cohesive strength of dielectric materials and the interfaces that are composed of those materials commonly found in thin-film interconnect structures are reviewed. Debond growth rate versus debond driving energy curves (V-G curves) were collected over a range of environmental conditions for both dielectrics and dielectric/metal interfaces. Both are found to exhibit characteristics consistent with stress corrosion cracking mechanisms found in the bulk glass literature. The mechanisms identified in both systems are explained in terms of the salient chemical reactions occurring at the debond tip.

Subcritical debonding of polymer/silica interfaces under monotonic and cyclic loading

The effects of interface chemistry and alternating mechanical loads on subcritical debonding of thin polymer layers from inorganic dielectrics are of significant interest for the integrity of devices used in a range of modern technologies. However, the mechanisms of subcritical debond growth at polymer interfaces, particularly under cyclic loads, are not well understood. Such subcritical debonding of a representative polymer dielectric (benzocyclobutene) to a silica-passivated silicon substrate was studied under cyclic and monotonic loading in moist environments. The effect of interface chemistry on subcritical debonding was evaluated using selected amino- and vinyl-functional silane adhesion promoters. Debond growth rates over the range of 10−3–10−9 m/s were characterized under Mode I and mixed Mode I/Mode II loading. AFM and XPS results suggest that the failure mode of the BCB was cohesive in a region very close to the interface. Fatigue loading was found to significantly accelerate subcritical debond growth rates, producing fatigue striations with a striation height of ~1–2 nm and a spacing that was correlated with the debond growth rate. A model is presented for the mechanism of striation formation.

Moisture-assisted subcritical debonding of a polymer/metal interface

Subcritical debonding of polymer/metal interfaces has important implications for the reliability of a range of modern device technologies containing thin layered structures, although the physics of the underlying crack-growth mechanisms are not well understood. This study investigates the effect of moisture on subcritical debonding at the interface between a silica-filled epoxy resin and a copper substrate electroplated with a nickel layer. Subcritical debond-growth rates in the range of 10−5 to 10−11 m/s were characterized as a function of the applied strain energy release rate, GA. Particular attention was given to the relationship between the debond tip strain energy release rate, Gtip, and the measured debond growth rates. The kinetics of interfacial debond growth were rationalized in terms of a stress-dependent chemical reaction and mass transport of the environmental species to the debond tip. The order of the chemical reaction with respect to water was determined and found to be surprisingly high (n∼5). In the transport-controlled region, the rate of debond growth was found to increase linearly with the partial pressure of water vapor. The effects of salient interface parameters including the peak stress in a cohesive zone used to model the fracture mechanism were explored.

Adhesion of benzocyclobutene-passivated silicon in epoxy layered structures

Adhesion and subcritical debonding at interfaces between a silica-filled epoxy underfill and a silicon die passivated by silicon nitride and benzocyclobutene (BCB) layers were investigated. Adhesion was measured in terms of a critical value of the applied strain energy release rate, G (J/m2). Subcritical debond-growth rates in the range of 10−9to 10−3m/s were characterized as a function of applied G. Adhesion and subcritical debonding were affected by changes in interfacial chemistry and environment. The surprisingly large effect of adjacent layer elastic properties on interfacial adhesion was demonstrated with simulations of interfacial fracture using a mechanics of materials approach. Interfacial chemistry was modified by using different adhesion promoters, by varying the BCB cure state, and by using different epoxy underfill resins. The effects of environmental variables were studied with temperature- and humidity-controlled environments in order to determine the separate roles of moisture activity and temperature.

The Effect of Fatigue on the Adhesion and Subcritical Debonding of Benzocyclobutene/Silicon Dioxide Interfaces

AbstractThe effect of fatigue loading on microelectronic thin film interfaces has until now been difficult to quantify. Most industrial fatigue testing uses HAST (Highly Accelerated Stress Testing) protocols, which inherently convolutes the effects of mechanical fatigue and the test environment. Our work focuses on isolating the deleterious effects of mechanical fatigue on interfaces, which we have found to be substantial. In this study, the integrity of a low-k polymer interface involving benzocyclobutene (BCB) and silica was examined under a variety of loading conditions. Critical (fast fracture) adhesion values were measured using standard interface fracture-mechanics geometries. Experiments were then conducted to measure the debond growth rate as a function of the applied strain energy release rate under both static and cyclic loading conditions. Our results show that even under room temperature conditions, debond growth rates measured under cyclic fatigue are considerably faster than those observed under static loading. Results are presented detailing the effects of interface chemistry (adhesion promoters), environmental moisture, and test temperature on the resistance of the interfaces to subcritical debonding. Strategies for increasing resistance of dielectric interfaces to fatigue debonding are outlined.

Subcritical Debonding of Multilayer Interconnect Structures: Temperature and Humidity Effects

AbstractThin film structures may fail by progressive or time-dependent debonding at stresses far below those required for catastrophic failure. Previous work has shown that progressive debonding in a typical interconnect structure occurs either along the TiN/SiO2 interface or parallel to this interface in the SiO2 Such subcritical debonding was found to span several orders of magnitude of debond growth rates and occur at significantly reduced driving forces. The presence of SiO2 at the failure location indicates that the mechanisms which give rise to stress corrosion cracking in bulk glasses may also play a role in the subcritical debonding behavior of multilayer interconnect structures. Accordingly, this work focuses on the effects of temperature and humidity on subcritical debonding and rationalizes them in terms of the relevant chemical reactions taking place at the debond tip.

The Effects of Environment and Fatigue on the Adhesion and Subcritical Debonding of Dielectric Polymers

AbstractThe adhesion of thin film polymers will be critical in the integration of low-κ materials into microelectronic processing. This study describes the adhesion of two promising low-κ polymers (polyimide and benzocyclobutene) to a silicon dioxide surface. Critical adhesion values were measured using interface fracture mechanics samples in a double cantilever beam geometry. The effect of subcritical (time-dependent) delamination was also evaluated for these systems. Subcritical debonding data are important in understanding the effect of environment and temperature on interface reliability. To that end, experiments were conducted over a range of humidities to elucidate the effect of moisture on interface delamination. The important effect of the acceleration of debond growth rates due to cyclic loading is also described. In addition, XPS studies are presented to characterize the debond path in these layered systems.

Adhesion and Reliability of Polymer/Inorganic Interfaces

The reliability of microelectronic components is profoundly influenced by the interfacial fracture resistance (adhesion) and associated progressive debonding behavior. In this study we examine the interfacial fracture properties of representative polymer interfaces commonly found in microelectronic applications. Specifically, interface fracture mechanics techniques are described to characterize adhesion and progressive bebonding behavior of a polymer/metal interface under monotonic and cyclic fatigue loading conditions. Cyclic fatigue debond-growth rates were measured from ~10−11 to 10−6 m/cycle and found to display a power–law dependence on the applied strain energy release rate range, ΔG. Fracture toughness test results show that the interfaces typically exhibit resistance-curve behavior, with a plateau interface fracture resistance, Gss, strongly dependent on the interface morphology and the thickness of the polymer layer. The effect of a chemical adhesion promoter on the fracture energy of a polymer/silicon interface was also characterized. Micromechanisms controlling interfacial adhesion and progressive debonding are discussed in terms of the prevailing deformation mechanisms and related to interface structure and morphology.