Interfacial Failure Criterion Research Articles

Effect of curing degree on forming of clinch-adhesive joint

ABSTRACT The clinch-adhesive joint has good mechanical properties and the curing degree of the adhesive during the clinching process shows significant impacts on the joint forming and its mechanical performance. During the clinching process, the interface of the cured adhesive layer restricts the flow of the metal thus shows a negative influence on the forming quality of the clinch-adhesive joint. This paper focuses on the interface layers change during the clinching process and the infulence of curing degree on the forming of clinch-adhesive joint by the finite element simulation model. Firstly, the constitutive model and interfacial failure criterion of Araldite 2015 epoxy adhesive considering curing degree were fitted through tensile tests for adhesive dumbbell specimens and bonding joints. Then, the forming process of the clinch-adhesive joints with fully cured adhesive were simulated. Finally, the effects of curing degree on the joint shape and interface failure were investigated. The results show that interface damage occurs at the location of the self-locking, and the curing degree mainly shows influences on the sizes of adhesive pocket and undercut, the damaged width of the interface layers, and the maximum damage of the interface layers.

An experimental and numerical study on the age depended bond-slip behavior between nano-silica modified carbon fibers and cementitious matrices

Electrophoretic deposition of nano-silica on the surfaces of carbon fibers (CF) is a novel approach for modifying the interfacial bond properties between the fibers and cementitious matrices. To understand its pullout behavior with increasing age, single fiber pullout tests and scanning electron microscopy were performed at various curing times for pristine and modified fibers. In this article, a new approach for calculating the debonding energy is introduced and two theoretical models – a stress-controlled and an energy-controlled – were employed to obtain the critical interface failure criterion. The results showed that interfacial adhesion at fiber-matrix interface was increasingly improved with age compared to the untreated fiber, due to the formation of C–S–H gel in the vicinity of the fiber surface. Meanwhile, the untreated fibers exhibited only a slightly increasing trend of interfacial bonding with longer curing time. A finite element model was applied to simulate the fiber pullout failure, which incorporated a discrete cohesive concept into the smeared phase-field method in a Representative Crack Element (RCE) framework. Using a phenomenologically defined cohesive traction-separation law, the numerical simulations successfully captured the characteristics of the experimental pullout curves. According to proper parametric calibrations, good agreement was obtained between the numerical results and the respective experimental evaluations.

Accurate 3D thermal stress analysis of thermal barrier coatings

An accurate and efficient calculation method for the thermoelastic field of thermal barrier coating structures has a great contribution to their accurate design and quality promotion. However, because of the thin coating thickness and large stress gradient in the coatings, it is too hard to guarantee the calculation accuracy especially on the interface by using the traditional commercial software. With this motivation, an accurate method for 3D thermoelastic field of thermal barrier coating structure under arbitrary heat loads is proposed in this paper. The numerical results reveal that the method has good convergence, high accuracy, and high stability. Based on this method, the effects of material parameters and coating thickness on the stress field are investigated to provide a refined guideline for the selection of coating materials. Furthermore, the optimal thickness of coating can be designed accurately by considering the interfacial failure criterion. Numerical results also reveal that the appropriate coating materials and coating thickness can largely reduce coating and interface stress. These studies indicate that the highly precise calculation method is useful for extremely fine structure design, parameter analysis and failure evaluation of the thermal barrier coating structures and enhancing their quality.

Determination of fracture toughness of polymer coating using micro-scale digital image correlation technique

This paper presents a comprehensive study on determining the interface adhesive strength of the polymer coating/substrate system to improve its durability. Interfacial stress intensity factors based on the crack tip opening displacement (CTOD) method were theoretically analysed and verified by the finite element simulation. Thereafter, an experimental setup was established with the micro-scale digital image correlation (DIC) method to measure the CTODs and the designed loading fixture for multi-angle loading. The apparent specimen rotation and out-of-plane displacement were significantly restrained in the test, and the measured displacement accuracy of the micro-scale DIC method was approximately 1/10 of the minimum CTOD value. Mixed-mode stress intensity factors and an interfacial failure criterion curve in terms of stress intensity factors were obtained. Furthermore, the test method was applied to evaluate the interfacial fracture toughness of nanoparticle-reinforced epoxy coatings. The experimental results demonstrate that determining the stress intensity factors on the interface of the polymer coating/substrate system based on the CTODs is an effective and reliable approach.

Mechanisms and control of edge interfacial delamination in a multilayer system containing a functionally graded interlayer

The edge interfacial delamination mechanisms for a system comprising of a metallic substrate (316 L stainless steel), stepwise functionally graded (FG) interlayer (316 L stainless steel and alumina) and top coat (silicon carbide (SiC)) were studied for the first time utilizing an axisymmetric cylindrical model and interfacial failure criteria. Finite element analysis was used for this investigation with the assumptions of ideal bonding between layers and an initial stress-free temperature from which the model was cooled down (as might be achieved from plasma spray coating). The influence of graded interlayer composition factor on delamination behaviour of the two system interfaces was investigated using four separate interfacial failure criteria. The SiC top coat thickness was then considered and studied as another critical parameter in the system. Finally, a novel delamination failure map was obtained for the studied system with a FG layer based on interfacial fracture toughness ratios as a function of SiC thickness and composition factor. It was shown using this map that delamination in a multilayer system comprising a FG interlayer and top coat can be controlled by modifying the FG interlayer composition factor and thickness of the top coat.

3D analytical solutions of piezoelectric thin film structure under charged cylinder flat head contact

In the system of the transversely isotropic piezoelectric thin film structure under charged cylinder flat head contact, the 3D analytical solutions for the stresses, displacements, electric displacements and electric potential are derived, which is a further development of the contact mechanics theory. We find that the variation tendency of the stresses and electrical components not always monotonically while thickening the film thickness. Even if the structure is under pressure (no tangential force loading on the contact area), the shear stress is singularity at the contact boundary. The optimal thickness can be determined with an optimization procedure. Interfacial delamination area can be predicted with interfacial failure criterion. The theoretical results are helpful for the prediction of some transient electrical effects, which can be combined with experiments to estimate some elastic constants, dielectric constants and piezoelectric constants in the indentation process.

Two- and Three-Dimensional Failure Criteria for Laminate Composites

Abstract Several two- and three-dimensional mixed-mode interface failure criteria are proposed for predicting delamination failure in multidirectional, laminate composites. The proposed criteria, based on the stress intensity factors K1, K2, and KIII, as well as the critical interface energy release rate Gic and phase angles ψ and ϕ, are examined using results obtained from Brazilian disk mixed-mode fracture toughness tests. Two material systems are considered. The first contains a delamination along an interface between a unidirectional fabric and a plain woven fabric. The second is composed of a plain woven fabric with fibers oriented in different directions in succeeding plies. The former was manufactured by means of a wet-layup and the latter is a prepreg. Finally, a statistical analysis is carried out to obtain a failure curve or surface with a 10% probability of unexpected failure and a 95% confidence. These curves or surfaces may be used to predict failure of structures containing these laminates and to assist in composite design.

3D Axisymmetric exact solutions of the piezo-coating sensors for coating/substrate system under charged conical contact

A complete set of frictionless conical contact (indentation) theory of the piezoelectric contact mechanics (PCM) for transversely isotropic piezoelectric elasticity coated system (TIPECS) under charged conical contact is established. 3D exact solutions for the stresses, displacements, electric displacements and electric potential are derived. The contact radius VS the total load relation is obtained, which is valuable to develop the technology of indentation. The relations of coating thickness VS voltage and coating thickness VS voltage are studied. The optimal thickness can be determined with an optimization procedure. Interfacial delamination area of the TIPECS can be predicted with interfacial failure criterion. The theoretical results are helpful for the prediction of some transient electrical effects, which can be combined with experiments to estimate some elastic constants, dielectric constants and piezoelectric constants in the indentation process.

The electro-mechanics of a coating/substrate system under charged spherical contact

A general theory of the piezo-electric contact mechanics for a transversely isotropic piezo-electric elasticity coated system (TIPECS) under charged spherical contact is presented. Three-dimensional exact solutions for the stresses, displacements, electric displacements and electric potential are derived. The contact radius versus the total load relation is obtained to guide the indentation experiments. The relations of coating thickness versus voltage and coating thickness versus voltage are studied. The optimal thickness can be determined with an optimization procedure. The interfacial delamination area of the TIPECS can be predicted with the interfacial failure criterion. The theoretical results are helpful for the prediction of some transient electrical effects, which can be combined with experiments to estimate some elastic constants, dielectric constants and piezo-electric constants in the indentation process.

Peridynamic simulation of two-dimensional axisymmetric pull-out tests

In this study, a systematic pull-out simulation scheme in the formwork of two dimensional (2D) axisymmetric domain using peridynamics (PD) is presented. An axisymmetric ordinary state-based peridynamic model is used to model the pull-out deformation just like the axisymmetric stress element in the finite element method (FEM). A failure criterion based on the bond energy density is adopted to predict the interfacial debonding, while a new PD contact model in the 2D axisymmetric domain is proposed to simulate the friction sliding during the pull-out process. The PD pull-out simulation scheme based on the respective 2D axisymmetric model, interfacial failure criterion and contact model is quantitatively validated in comparisons with either the FEM or analytical solutions. The whole process of the pull-out test is simulated using the proposed PD scheme, and the influences of both the interfacial fracture toughness and friction coefficient on the bond-slip behavior are investigated. The developed PD pull-out modeling scheme is capable of effectively simulating the pull-out tests and predicting the bond-slip behavior.

Laboratory investigation of interface shearing in chalk

Chalk, a soft fine-grained Cretaceous limestone, is encountered across northern Europe where recent offshore windfarm, oil, gas and onshore developments have called for better foundation design methods, particularly for driven piles whose shaft capacities are controlled by an effective stress Coulomb interface failure criterion. Interface type and roughness is known to affect both interface friction angles, δ′ and the magnitude of dilation required for shaft failure to develop. Site-specific interface ring-shear tests are recommended for offshore pile design in sands and clays to account for driven pile shaft materials, roughnesses and shear displacements. However, few such tests have been reported for chalks and it is also unclear whether δ′ angle changes contribute to the striking axial capacity increases, or set-up, noted over time with piles driven in chalk. This paper describes an interface shear study on low-to-medium density chalk from the St. Nicholas-at-Wade research test site in Kent, UK, where extensive field driven pile studies have been conducted [1, 2]. Direct shear and Bishop ring shear apparatus were employed to investigate the influences of interface material and surface roughness, as well as ageing under constant normal effective stresses (σn'). It is shown that the high relative roughness of the interface compared to the chalk grain size results in the ultimate interface shearing angles falling close to the chalk-chalk shearing resistance angles. The δ′ angles also increased by up to 5° over 38 days of ageing.

Fatigue performance of interface bonding between asphalt pavement layers using four-point shear test set-up

This paper aims to study the fatigue performance of interface bonding between asphalt pavement layers. To fulfill this objective, a four-point shear test (4PST) set-up was developed, and a series of fatigue tests were conducted on composite asphalt beam samples made up of stone mastic asphalt (SMA) and asphalt concrete (AC) mixtures. The combined effect of test temperature (3 levels), normal pressure (6 levels), loading frequency (5 levels), and shear stress (6 levels) with single tack coat type and application rate were considered for evaluation. Two interface failure criteria were successfully adopted in this study, and high correlation was relatively found between the fatigue test results and the corresponding fatigue life criteria. Findings of this study revealed that firstly, both the interface fatigue life and shear stiffness increased at all temperatures with increasing normal stress. Secondly, with increasing loading frequency, the interface fatigue life increased at each temperature. Thirdly, shear stiffness decreased with increasing shear stress for each temperature; the decrease was more pronounced at lower temperatures. Power laws were also proposed to characterize interface fatigue life at each temperature. Finally, the results of the analysis of variance (ANOVA) indicated that unlike the fatigue life, all factors had a significant effect on the shear stiffness. A multiple regression model was established successfully to predict the initial shear stiffness.

Experimental Study on the Shear Strength Between the Steel Bridge Deck and the Asphalt Overlay

Abstract A new steel–concrete interface shear (SCIS) test was used for measuring the interface shear strength between the steel bridge deck and the asphalt concrete overlay. This paper describes the test apparatus, specimen preparation, and testing procedure, and illustrates the interface shear strengths determined with the proposed test method. Three factors were investigated: interface type (a layer of epoxy resin tack coat with aggregate chips, a thin layer of epoxy asphalt tack coat, and a thick layer of epoxy asphalt tack coat with aggregate chips), temperature (25°C and 60°C), and normal stress level (0, 0.2, 0.4, and 0.7 MPa). Interface failure criteria and failure patterns are also reported and analyzed. The experimental results show that the SCIS test exhibits good repeatability. The steel–asphalt interface shows strain-softening behavior until it reaches the sliding state. The interface with epoxy resin tack coat provides the highest shear strengths compared to other types of interface. Substrate temperature has a dominant effect on interface shear strength. On average, shear strengths determined at 60°C are about 25 % to 33 % of those determined at 25°C for all interface types. Increased normal stress levels result in increased interface shear strengths. The normal stress effect is more evident at elevated temperature. Three types of interface show different failure patterns: cohesive failures of the epoxy zinc-rich primer, the adhesive failures at the primer–tack coat interface, and the cohesive failure of the tack coat.

A linear elastic-brittle interface model: application for the onset and propagation of a fibre-matrix interface crack under biaxial transverse loads

A new linear elastic and perfectly brittle interface model for mixed mode is presented and analysed. In this model, the interface is represented by a continuous distribution of springs which simulates the presence of a thin elastic layer. The constitutive law for the continuous distribution of normal and tangential initially-linear-elastic springs takes into account possible frictionless elastic contact between adherents once a portion of the interface is broken. A perfectly brittle failure criterion is employed for the springs, which enables the study of crack onset and propagation. This interface failure criterion takes into account the variation of the interface fracture toughness with the fracture mode mixity. A unified way to represent several phenomenological both energy and stress based failure criteria is introduced. A proof relating the energy release rate and tractions at an interface point (not necessarily a crack tip point) is introduced for this interface model by adapting Irwin’s crack closure technique for the first time. The main advantages of the present interface model are its simplicity, robustness and computational efficiency, even in the presence of snap-back and snap-through instabilities, when the so-called sequentially linear (elastic) analysis is applied. This model is applied here in order to study crack onset and propagation at the fibre-matrix interface in a composite under tensile/compressive remote biaxial transverse loads. Firstly, this model is used to obtain analytical predictions about interface crack onset, while investigating a single fibre embedded in a matrix which is subjected to uniform remote transverse loads. Then, numerical results provided by a 2D boundary element analysis show that a fibre-matrix interface failure is initiated by the onset of a finite debond in the neighbourhood of the interface point where the failure criterion is first reached (under increasing proportional load); this debond further propagates along the interface in mixed mode or even, in some configurations, with the crack tip under compression. The analytical predictions of the debond onset position and associated critical load are used for several parametric studies of the influence of load biaxiality, fracture-mode sensitivity and brittleness number, and for checking the computational procedure implemented.

A genetic algorithm based multi-objective shape optimization scheme for cementless femoral implant.

The shape and geometry of femoral implant influence implant-induced periprosthetic bone resorption and implant-bone interface stresses, which are potential causes of aseptic loosening in cementless total hip arthroplasty (THA). Development of a shape optimization scheme is necessary to achieve a trade-off between these two conflicting objectives. The objective of this study was to develop a novel multi-objective custom-based shape optimization scheme for cementless femoral implant by integrating finite element (FE) analysis and a multi-objective genetic algorithm (GA). The FE model of a proximal femur was based on a subject-specific CT-scan dataset. Eighteen parameters describing the nature of four key sections of the implant were identified as design variables. Two objective functions, one based on implant-bone interface failure criterion, and the other based on resorbed proximal bone mass fraction (BMF), were formulated. The results predicted by the two objective functions were found to be contradictory; a reduction in the proximal bone resorption was accompanied by a greater chance of interface failure. The resorbed proximal BMF was found to be between 23% and 27% for the trade-off geometries as compared to ∼39% for a generic implant. Moreover, the overall chances of interface failure have been minimized for the optimal designs, compared to the generic implant. The adaptive bone remodeling was also found to be minimal for the optimally designed implants and, further with remodeling, the chances of interface debonding increased only marginally.

Dynamic failure characterization of laser welds under combined loading conditions

This paper proposes a dynamic failure criterion for laser welds to describe the strain-rate dependent failure behavior of laser welds in the crash analysis of an auto-body. The failure criterion has been developed from experiments with nine different conditions of combined normal and shear loading including the normal loading and the pure shear loading. The experiments were carried out with a high speed material testing machine at intermediate strain rates ranging from quasi-static to 100 s−1. The dynamic failure tests were examined with four cases of the 25 mm stitch-type laser weld of CR340 1.2t, the 20 mm stitch-type laser weld of CR340 1.2t, the 25 mm stitch-type laser weld of SPCC 1.0t and the O-type laser weld with the 8 mm diameter of CR340 1.2t in order to evaluate the influence of the strain rate on the failure loads with respect to the base metal and the bead geometry. Based on the experimental data, a novel failure criterion was proposed for the description of the failure behavior of laser welds on the basis of the quasi-static failure criterion. The failure criteria are divided into two regions: the base metal failure criterion is expressed as a β-norm function; and the interfacial failure criterion is expressed as an exponential function. For the both failure criteria, the strain-rate sensitivity of the normal and the shear failure load are assigned as an exponential function of the logarithm of the strain rate. The change of the shear failure load with the increase of the strain rate is also considered to accurately construct the failure criteria. The failure mode and the onset of failure of the laser welds are determined when the maximum value reaches unity. The dynamic failure criterion proposed provides a fairly accurate description of the failure load obtained from the experiments under dynamic combined loading conditions.

Crack Onset and Propagation in Composite Materials Using Finite Fracture Mechanics on Elastic Interfaces

A method for the prediction of crack onset and propagation along an interface in a fibre-reinforced composite system is devel- oped. The method is based on the Linear Elastic-(Perfectly) Brittle Interface Model (LEBIM) and a coupled stress and energy criterion of the Finite Fracture Mechanics (FFM). Models similar to LEBIM, also called weak or imperfect interface models, are frequently applied to describe the behaviour of adhesively bonded joints. In the present paper the interface is modelled by a con- tinuous distribution of linear elastic-brittle springs where the normal and tangential stresses across the undamaged interface are, respectively, proportional to the relative normal and tangential displacements. The interface failure criterion applied takes into account the variation of the fracture toughness with the fracture mode mixity. In the present work a 2D collocational boundary element method (BEM) code is used to analyse debond onset and propagation at a fibre embedded in a matrix subjected to remote uniaxial transverse tension. One of the advantages of the method proposed here by applying the FFM criterion to the LEBIM is that we are able to make independent the two governing parameters of the model (critical stress and fracture toughness of the interface), which are related by an equation in the original LEBIM formulation. It seems that the predictions provided by FFM with LEBIM are quite similar to those obtained by the well-known Cohesive Crack Model (CCM). The advantage of the present method with respect to the CCM is evident, since a troublesome nonlinear analysis due to the softening can be avoided.

Study of Interfacial Stress Distribution in SiC Fiber Reinforced Titanium Matrix Composites on Transverse Tensile Tests

A combination of the transverse tensile test and the unilaterally coupled finite element method was used to evaluate the interfacial normal bond strength and stress distribution of titanium matrix composites (TMCs). In addition, in order to identify the interface shear failure mode of TMCs under transverse loading, both the push-out test and the finite element method have been developed to characterize the interfacial shear strength of TMCs, which is the interfacial shear failure criterion. This article studies the results of the experiments, which suggested that the interfacial normal bond and shear strength of SiCf/Ti-6Al-4V were 300 and 350 MPa, respectively, and the interface failure mode of TMCs under the transverse tensile test was radial failure rather than shear failure. Moreover, the effect of residual stress on the radial stress is also discussed in detail in this article.

Determination of mixed-mode interfacial fracture toughness for thermal barrier coatings

In this work, a test method was developed to determine the interfacial fracture toughness of the air plasma sprayed (APS) thermal barrier coatings (TBCs) over a wide range of mode mixities. For this mixed-mode test method, the analytical expressions for the energy release rate and stress intensity factors were derived based on the energy theory and the concept of “equivalence”. The fidelity of these expressions was affirmed by selected finite element analysis. The experimental results showed that the critical energy release rate increased with the increase of the positive mode mixity, which was mainly due to the increase in contact/friction effect and plastic work dissipation with increasing shear mode loading. Furthermore, an elliptical interfacial failure criterion in terms of the stress intensity factors was proposed. The agreement between the experimental results in the literature and those in our work indicated that our test method and the corresponding analytical solutions can well determine the interfacial fracture toughness of the TBCs over a wide range of mode mixities.

十字型試験片法によるガラス繊維／エポキシ界面のはく離発生位置を考慮した界面はく離クライテリオンの検討

This paper evaluates interfacial failure criterion under combined stress state and precise location along the fiber circumference of interfacial failure (debonding) initiation at glass fiber fiber/epoxy interface by the cruciform specimen method. The effects of both thermal residual stress occurred during specimen preparation and inelastic stress-strain constitutive relationship of matrix resin on the evaluation of interfacial failure criterion are considered. The interfacial stresses distribute along with fiber circumference. In this study, the location of interfacial failure initiation is specified and the interfacial failure criterion is evaluated. By changing the cruciform arm angle (the angle between the fiber and the loading directions), various combined stress states are obtained which make it possible to evaluate the interfacial failure criterion. Both the quadratic and parabolic criteria are assumed and discussed.