Initial Failure Mechanism Research Articles

High‐speed observation of dynamic delamination failure in tapered composite laminates under tensile loading

AbstractThis article presents high‐fidelity observations of the time evolution of unstable delamination growth in tapered composite beams under tensile loading via ultra‐high‐speed camera in conjunction with digital image correlation. Asymmetrically tapered GFRP laminates, with a thickness drop from 18 plies to 12 plies, are manufactured in three different layup configurations with grouped drop‐offs: [06/(02)3/06], [06/06/06], and [06/(±45)3/06]. Beam specimens are subjected to quasi‐static tensile loading in a servo‐hydraulic axial testing machine, and the damage evolution on one edge of the tapered beam is monitored in situ at 124,000 fps with a high‐speed camera (HSC). In selected tests, digital image correlation (DIC) analyses are conducted either on the edge or on the tapered surface to obtain the full‐field strain and elucidate failure mechanisms. Real‐time HSC observations of the damage sequence consistently affirm that the initial failure mechanism is a transverse crack at the nearest drop‐off to the thin section, which is followed by delaminations at interfaces of sublaminates. Unstable delamination growth is characterized by measurements of lower bounds of crack tip speeds. The unique crack speed data provide evidence for the existing effect of through‐the‐thickness stresses on delamination growth. The detailed sequence and patterns of dynamic failure presented in this paper can serve as a benchmark to validate damage models used in simulations of tapered laminates.Highlights The sequence of unstable delamination growth in tapered GFRP beams is investigated experimentally. Characterized unstable delamination growth based on crack tip speed measurements. Strain fields are evaluated along beam edges using DIC in real‐time. In‐situ observations of dynamic failure can serve as a benchmark to validate damage models.

Initial stiffness, ultimate capacity and failure mechanisms of tubular X-joints with external ring or plates at fire conditions

ABSTRACT This research work evaluates the effect of the ring, the plate and the joint geometry on the initial stiffness, ultimate strength and failure mechanisms of tubular X-joints subjected to axially compressive load at different fire conditions. At the first step, a finite element model was generated and verified by 19 experimental tests and 8 numerical models carried out by the present first author and other researchers. At the next step, 1170 numerical models were conducted. Using the generated FE models, the structural behaviour under different fire conditions (200°C, 400°C, 600°C and 800°C) was evaluated and compared to the corresponding behaviour at normal temperature (20°C). Results showed that both of the external ring and plate can notably increase the initial stiffness and ultimate capacity at all different elevated temperatures. Also, the use of these methods significantly improves the failure mechanisms at fire conditions. Two theoretical design formulas with high accuracy were proposed, based on the yield volume model, to determine the ultimate capacity in ring reinforced X-joints and plate reinforced X-joints at elevated temperatures.

Static Strength of Tubular K-Joints Reinforced with Outer Plates under Axial Loads at Ambient and Fire Conditions

In this paper, the effect of the outer reinforcing plate on the initial stiffness, ultimate strength, and failure mechanisms of tubular K-joints under axial load at ambient and different fire conditions is evaluated. In the first phase, a finite element (FE) model was generated and verified by 13 experimental tests. In the next step, 1057 numerical models were generated. In these models, the welds joining the chord and braces were modeled. Using the produced FE models, the structural behavior under ambient and different elevated temperatures (20, 150, 300, 450, 600, 750, and 900 °C) was evaluated. The results showed that the outer plate can enhance the ultimate strength by up to 319% under fire conditions. Despite the considerable effect of the outer plate on the stiffness, ultimate strength, failure modes, and the frequent usage of the K-joints in tubular structures, the static response of the reinforced K-joints at ambient and elevated temperatures has not been studied. Hence, according to the extensive parametric studies, a highly precise practical design equation has been proposed based on the yield volume model for determining the ultimate strength.

A failure mechanism of IGBT module in MMC and improvement

In modular multi-level converter valves, as the blocked mode happens, the failure in IGBT modules often causes a chain explosion reaction, which masks the initial failure mechanism. The root cause of the failure was that the voltage, consisting of the diode forward voltage and the induced voltage on the parasitic inductance, exceeded the reverse blocking capability of the anti-parallel IGBT. The methods against the failure were studied from three aspects: decreasing the diode’s peak forward voltage, increasing the reverse blocking capability of IGBT, and minimizing the parasitic inductance of the package. It was found that the doping concentration’s effect on the diode’s peak forward voltage is not monotonic. It can be explained from the carrier density distribution during forward recovery transient that lower doping concentration reduces the carrier injection, and higher doping concentration reduces the excess carrier concentration in the doping region. Then, the influence of two buffer designs on the reverse blocking capability of IGBT was compared. It was found that a higher reverse blocking voltage would be achieved when a multi-proton implantation buffer replaced the diffused buffer. And the new design has almost no degradation in forward blocking capability.

Failure prediction of syntactic foam core L-shaped sandwich structures

L-shaped laminates with a syntactic foam core were manufactured to investigate the bending behavior of the sandwich structures including sharp corners. The bending behavior, initial, and progressive failure modes of the sandwich structure were analyzed by four-point bending tests and the finite element method. The cohesive zone model and Hashin’s failure criteria were used for inter-laminar and intra-laminar failure of the laminates, respectively. The concrete damage plasticity model was proposed to model the foam material with considering different material properties in tension and compression. Mode-I and Mode-II fracture toughness of the foam was obtained to characterize the foam/laminate interface. Load-displacement curves predicted under bending simulation were quite well compliant with experimental results. The difference between numerical and experimental failure loads was 10.6 %, 1.2 %, and 1.9 % for the foam with facesheet stacking sequence of [0/90]2s, [0/90]3s, and [0/45/-45/90]2s, respectively. Predicted radial and hoop direction strain fields in the foam section were also compared to the measured one using a digital image correlation technique. Strain fields and failed regions and their mechanisms were captured well using the model. The initial failure mechanism was delamination at the interface between the foam and facesheet followed by foam tensile fracture.

Centrifuge modeling of the run-out behavior of fly ash deposits subjected to a loss of confinement

Numerous industries are concerned by the humanitarian, environmental, and economic consequences of debris flows, landslides, and material run-outs. However, the run-out behavior following a loss of confinement, e.g., dam failure, is not well understood. The material’s complex constitutive behavior means multiple mechanisms contribute to the deformation, e.g., slope instability, seepage forces, erosion, or static liquefaction. In this study, centrifuge models of fly ash deposits, with varying initial deposit density and water table height, were subjected to a rapid loss of lateral confinement. Deformation, pore pressure, and water content measurements were used to investigate and separate the mechanisms governing the run-out. Static liquefaction was observed in deposits initially looser than the critical state and led to a rapid material outflow. Slope instability was the initial failure mechanism for denser deposits or those with reduced water tables, with transient stability due to dilation-induced negative excess pore pressures followed by progressive failures caused by seepage pressures from drainage and pore pressure dissipation. Cone penetration tests, performed before the loss of confinement at different penetration rates, were used to characterize the material run-out and volume change tendencies and demonstrate a practical tool for assessing the run-out risk of deposits in the field.

Mechanisms for the Formation of an Exceptionally Gently Inclined Basal Shear Zone of a Landslide in Glacial Sediments—The Ludoialm Case Study

The Ludoialm landslide, which is located in the municipality of Münster in Tyrol, Austria, represents a large-scale translational landslide in glacial soil sediments characterised by an exceptionally low inclined basal shear zone of only 12°. Although a temporal coincidence between meteorological events and slope displacement is obvious, the hydromechanical coupled processes responsible for the initial landslide formation and the ongoing movement characteristics have not yet been identified. This article provides a comprehensive analysis of the predisposition factors and the initial failure mechanism of this landslide from geological and geotechnical perspectives. We use a prefailure geometry of the cross section to simulate the initial slope failure process by a limit equilibrium analysis (LEA), a strength-reduction finite element method (SRFEM), and a finite element limit analysis (FELA). The shape and location of the computationally obtained basal sliding zone compare well with the geologically assumed one. Based on the computational study, it turns out that a high groundwater table probably caused by snow melting in combination with different permeabilities for the different layers is needed for the formation of the exceptionally low inclined basal shear zone. This paper presents the failure mechanism of the Ludoialm landslide and discusses the role of the shear band propagation in the process of slope destabilization.

Remote sensing-based deformation monitoring and geological characterisation of an active deep-seated rock slide (Tellakopf/Cima di Tella, South Tyrol, Italy)

This study presents a 5-year long field campaign combined with 75 years of remote sensing data on the active, deep-seated Tellakopf rock slide in South Tyrol, Italy. Detailed field data and remote sensing techniques from airborne and terrestrial laser scanning (ALS, TLS) and orthoimages from federal flight campaigns and a helicopter-based monitoring campaign, allowed the development of a geometrical-kinematical rock slide model to investigate the initial failure and temporal-variable deformation mechanisms. The rock compound slide formed in foliated, highly fractured metamorphic rock masses at the SE-facing Tellakopf slope, directly above the commuter-rich national road SS 41. A total volume of approximately 3–4 Mm3 was estimated based on geographic information system (GIS) analysis by comparing the pre-failure surface topography and the reconstructed basal shear zone. Multi-temporal deformation analyses based on orthoimages, ALS and TLS data show alarmingly high velocities of at least 9 m/month during the initial formation phase in 2014, followed by a continuous velocity reduction to the current mean annual values of 1–2.5 m/year. Results indicate internal slab formation along discrete shear zones displayed on surface as main and minor scarps. These slabs show a translational movement behaviour along a fully persistent, slightly curvilinear basal shear zone. Rock mass yielding and fracturing conditions extensive mass loss manifested by high rock fall activity and large rock scree deposits at the rock slide toe. The cause for rock slide formation is attributed to retrogressive processes caused by long-term stress release due to topographical and hydrogeological changes by adjacent rock slides.

Assessment of ductile dike behavior as a novel flood risk reduction measure.

Dikes are an effective flood risk reduction measure in deltaic areas. Present risk analyses consist often of decoupled calculations of probabilities of dike failure and calculation of consequences of flooding given dike failure. However, the flood defense design determines not only the probability of failure, but influences the consequences of flooding as well. Especially when the dike has a ductile failure and breach growth behavior, due to a structural robust design, the consequences of flooding reduce. In this article, we present a novel assessment of risks and investments, valuing structural robustness of a construction type, represented by its ductile behavior during high loads. Therefore, the consecutive occurrence of initial dike failure mechanisms, failure path development, breach growth, and consequences is modeled integral and time-dependent. The investments consist of the costs to reinforce or reconstruct the flood defense to behave relatively ductile. This integral assessment enables to compare flood impacts of different construction types and different dimensions of designs. We applied it on a case in a riverine area in the Netherlands. The results show that the total societal costs and the individual risks on victims are very dependent on the construction type. The risk profile of a polder protected by a brittle or a ductile dike differs significantly. The brittle sand dike in the case requires larger dimensions than the more ductile dike with a claycore.

Failure analysis of brazed sandwich structures with square honeycomb-corrugation hybrid cores under three-point bending

In this work, the bending stiffness, initial failure load, initial failure modes and minimum mass design of square honeycomb-corrugation hybrid core sandwich structures (SHCH) subjected to bending were explored by a combined theoretical analysis, experimental testing, and numerical prediction method. Samples were fabricated by brazing process and measured under three-point bending. To theoretically obtain the initial failure mechanism map, six different initial failure modes were taken into account, which agree well with associated experimental data and numerical results. It is found that honeycomb filler not only changed the failure mode of corrugated core sandwiches but also enhanced dramatically its bending resistance. Then the minimum weight design is achieved as a function of load index, and the effect of key factors, including honeycomb relative density, loading platen width, material parameters, and inclination angle are quantified. Furthermore, the mechanical performances of SHCHs are compared with the competing structures.

Geographic-information-system-based topographic reconstruction and geomechanical modelling of the Köfels rockslide

Abstract. The Köfels rockslide in the Ötztal Valley (Tyrol, Austria) represents the largest known extremely rapid landslide in metamorphic rock masses in the Alps. Although many hypotheses for the trigger were discussed in the past, until now no scientifically proven trigger factor has been identified. This study provides new data about the (i) pre-failure and failure topography, (ii) failure volume and porosity of the sliding mass, and (iii) numerical models on initial deformation and failure mechanism, as well as shear strength properties of the basal shear zone obtained by back-calculations. Geographic information system (GIS) methods were used to reconstruct the slope topographies before, during and after the event. Comparing the resulting digital terrain models leads to volume estimates of the failure and deposition masses of 3100 and 4000 million m3, respectively, and a sliding mass porosity of 26 %. For the 2D numerical investigation the distinct element method was applied to study the geomechanical characteristics of the initial failure process (i.e. model runs without a basal shear zone) and to determine the shear strength properties of the reconstructed basal shear zone. Based on numerous model runs by varying the block and joint input parameters, the failure process of the rock slope could be plausibly reconstructed; however, the exact geometry of the rockslide, especially in view of thickness, could not be fully reproduced. Our results suggest that both failure of rock blocks and shearing along dipping joints moderately to the east were responsible for the formation or the rockslide. The progressive failure process may have taken place by fracturing and loosening of the rock mass, advancing from shallow to deep-seated zones, especially by the development of internal shear zones, as well as localized domains of increased block failure. The simulations further highlighted the importance of considering the dominant structural features of the rock mass. Considering back-calculations of the strength properties, i.e. the friction angle of the basal shear zone, the results indicated that under no groundwater flow conditions, an exceptionally low friction angle of 21 to 24∘ or below is required to promote failure, depending on how much internal shearing of the sliding mass is allowed. Model runs considering groundwater flow resulted in approximately 6∘ higher back-calculated critical friction angles ranging from 27 to 30∘. Such low friction angles of the basal failure zone are unexpected from a rock mechanical perspective for this strong rock, and groundwater flow, even if high water pressures are assumed, may not be able to trigger this rockslide. In addition, the rock mass properties needed to induce failure in the model runs if no basal shear zone was implemented are significantly lower than those which would be obtained by classical rock mechanical considerations. Additional conditioning and triggering factors such as the impact of earthquakes acting as precursors for progressive rock mass weakening may have been involved in causing this gigantic rockslide.

Sturzprozesse ausgehend von Felsböschungen: Abschätzung der Reichweite mithilfe numerischer Modelle

AbstractRunout processes originating from rock slopes can be classified according to their process behaviour into rockfall, rock mass fall and rock avalanche. Their process behaviour is primarily determined by the volume of the detached rock mass, the degree of fragmentation of the moving mass and the morphology and nature of the runout path. The initial conditions of these processes depend on the initial failure mechanism. It is difficult to predict the runout and magnitudes of these types of rapid landslides since the complexity of the processes requires major simplifications in the calculation approaches. A good understanding of the process and clear mechanical model conceptions are therefore crucial in the development and choice of prediction models. The paper presents a classification of the runout processes originating from rock slopes on the basis of clearly defined mechanical models and provides an overview of the currently available models for prediction.

Towards an International Levee Performance Database (ILPD) and Its Use for Macro-Scale Analysis of Levee Breaches and Failures

Understanding levee failures can be significantly improved by analysing historical failures, experiments and performance observations. Individual efforts have been undertaken to document flood defence failures but no systematically gathered large scale, open access dataset is currently available for thorough scientific research. Here, we introduce an efficiently structured, global database, called International Levee Performance Database (ILPD), which aims to become a valuable knowledge platform in the field of levee safety. It comprises information on levee characteristics, failure mechanisms, geotechnical investigations and breach processes for more than 1500 cases (October 2019). We provide a macro-scale analysis of the available data, aiming to provide insights on levee behaviour based on historical records. We outline common failure mechanisms of which external erosion is identified as the most frequent for levees. As an example, we investigate flood events occurred in Germany (2002, 2013) and examine breach characteristics of hundreds of failures. It is found that initial failure mechanisms have an influence on breach characteristics and that failures due to instability and internal erosion are less frequent but lead to larger breaches. Moreover, a relation between the return period and the expected breach density during a flood event is identified. These insights could complement flood risk assessments.

Static strength of tubular T/Y-joints reinforced with collar plates at fire induced elevated temperature

This paper aims to investigate the structural behavior of T/Y-joints reinforced with collar plates subjected to compressive loading at fire induced elevated temperatures. At the first step, a finite element (FE) model was developed and verified by results of six experimental tests carried out in the present author's previous study. Also, the FE model was validated using experimental data reported in available research works. At the next step, a total number of 360 FE analyses were performed to investigate the effect of elevated temperatures, dimensionless geometrical parameters (η, τc, β, γ), and brace inclination angle (θ) on the initial stiffness, ultimate strength, and failure mechanisms. The joints were analyzed under axially compressive load and five different temperatures (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C). Results showed that at elevated temperatures, the increase of the collar plate length and thickness significantly results in the increase of the initial stiffness. Also, at elevated temperatures, the ultimate strength of T/Y-joints reinforced with collar plate can be up to 254% of the ultimate strength of the corresponding unreinforced joint. Despite this significant difference between the static strength of unreinforced and collar plate reinforced T/Y-joints at elevated temperatures, there has not been any study on the initial stiffness and ultimate capacity of the T/Y-joints reinforced with collar plate at elevated temperatures. Also, there is no design equation for determining the ultimate strength of the collar plate reinforced T/Y-joints at elevated temperatures. Therefore, the results of the parametric study were used to develop a design formula, through nonlinear regression analyses, to calculate the ultimate strength of tubular T/Y-joints reinforced with collar plates subjected to axially compressive loading at elevated temperatures.

Low-velocity impact response of sandwich beams with a metal foam core: Experimental and theoretical investigations

Dynamic response of fully clamped sandwich beams with a metal foam core under low-velocity impact is investigated experimentally and theoretically. Failure modes of metal sandwich beams are identified and competing initial failure mechanism map depending on the geometries and material properties is constructed to characterize the initial failure mechanism. A fracture criterion of the maximum allowed deflection is proposed based on the maximum tensile strain of face sheets to evaluate the fracture-resistance of sandwich beams. Effects of loading location, geometries, and material properties are also considered for both initial failure and final fracture. Good agreement is achieved between the experimental results and theoretical predictions. It is shown that the loading location, geometries, and material properties have significant effects on low-velocity impact response of sandwich beams.

Simulation of Drop-Weight Impact Test on Composite Laminates using Finite Element Method

This study presents the simulation of standard drop-weight impact test on a [0/90/0]s composite laminate. For this purpose, a three-dimensional virtual test setup is developed in ABAQUS/Explicit finite element tool. Hemispherical impactor and specimen fixture are modeled as rigid bodies. Composite plate is modeled as a 3-D deformable solid and discretized using a biased mesh for computational efficiency. For simulation of ply damage in the composite laminate, a continuum damage mechanics based damage model is developed and implemented into the analysis via a user-written subroutine VUMAT. Delamination damage is simulated by inserting cohesive elements at the interfaces of plies having different orientations. Results show that the initial failure mechanism in the 3-D low-velocity impact event is the matrix cracking in the lowermost plies independent from the stacking sequence of the laminate. Furthermore, the simulation accurately predict that delaminated regions expand mainly in the same direction as of the fibers of the lower adjacent layer in accordance with the bending stiffness mismatching concept.

A numerical assessment of the progressive collapse resistance of RC frames with respect to the number of stories

Abstract This paper presents the results of a numerical investigation into the influence of the number of stories on the progressive collapse resistance of reinforced concrete planar frames. Starting from the specifications provided by Yi et al. (2008) in their experimental program, an initial numerical model was developed in Midas FEA software package. To assure a high results accuracy for the current and the following investigations based on this set-up, this initial numerical model was successfully validated against the data’s revealed by the experimental test. To achieve the proposed objective, five numerical models are developed starting from the initial one by increasing/decreasing the number of structure’s stories. To simulate the gradual failure of the first story column caused by the abnormal loadings such as explosions or impact, a target displacement of 50mm is imposed and a nonlinear static “push-down” analysis is conducted in each case. As a result, the ultimate load carrying capacity to progressive collapse for each numerical model is determined and the additional capacity of the RC frames with respect to the number of stories is assessed. The activation of the supplementary resisting mechanisms (Compressive Arch Action – CAA and Catenary Action – CA) of the planar frames to better resist progressive collapse is also discussed. The obtained results indicate that, as expected, the structure’s load carrying capacity increases with the number of stories but, when expressed in terms of percentage with respect to the activation of the initial failure mechanism (three-hinge mechanism) the value decreases. Also, based on the results obtained herein, a simplified approach is proposed to estimate the peak load that can be sustained by the planar RC frames without collapse.

Structural behavior of tubular X-joints strengthened with collar plate under axially compressive load at elevated temperatures

This paper investigates elevated temperature resistance of circular hollow section X-joints reinforced with collar plates subjected to brace compression. A finite element (FE) model was developed and the results were validated against the data available from seven experimental tests. Also, the results of the present FE model were compared with the numerical results of other researchers. Afterwards, several 3-D FE-models of the collar plate reinforced X-joints were generated to investigate the effect of joint geometry and collar plate size on the initial stiffness, ultimate strength, and failure mechanisms at different elevated temperatures. Results showed that the use of the collar plates in X-joints at elevated temperatures (200 °C, 400 °C, and 600 °C, and 800 °C) leads to the significant increase of the initial stiffness and ultimate strength. Also, the increase of the plate length results in the less reduction in ultimate strength, at all elevated temperatures. But, the effect of the plate thickness on the reduction factor can be ignored. Moreover, the decrease of the β in the plate reinforced X-joints at elevated temperature can lead to the shifting of the maximum plastic strain from joint intersection to the toe of the weld joining the collar plate to chord member. The parametric investigation was followed by the non-linear regression analysis to propose a parametric equation for determining the ultimate strength of the collar plate reinforced X-joints under brace compression at elevated temperatures.

The effect of protrusion density on composite-metal joints with surfi-sculpt reinforcement

The effect of protrusion density on the static mechanical properties of composite-metal joints strengthened by surfi-sculpt protrusions has been experimentally studied with single lap joints. The CFRP composite adherends were constant thickness with a quasi-isotropic layup. The metallic adherends were Ti-6Al-4V alloy with a variable number of protrusions per unit area, manufactured by electron beam surfi-sculpt. Digital image correlation was used to measure the debonding on the overlap during the tests. Although the surfi-sculpt protrusions did not significantly affect the onset of debonding, they did resist the initial unstable failure mechanism and converted it into stable growth. The analysis indicated that the efficiency of the surface protrusions was different at the metal and composite ends of the overlap. This finding opens the possibility to vary the protrusion density across the overlap to meet specific damage tolerance criteria and optimise joint efficiency. Increasing the protrusion density significantly increased the ultimate failure load, joint extension and hence absorbed energy.

Fixation strength of a polyetheretherketone femoral component in total knee arthroplasty

IntroductionIntroducing polyetheretherketone (PEEK) polymer as a material for femoral components in total knee arthroplasty (TKA) could potentially lead to a reduction of the cemented fixation strength. A PEEK implant is more likely to deform under high loads, rendering geometrical locking features less effective. Fixation strength may be enhanced by adding more undercuts or specific surface treatments. The aim of this study is to measure the initial fixation strength and investigate the associated failure patterns of three different iterations of PEEK-OPTIMA® implants compared with a Cobalt–Chromium (CoCr) component. MethodsFemoral components were cemented onto trabecular bone analogue foam blocks and preconditioned with 86,400 cycles of compressive loading (2600 N–260 N at 1 Hz). They were then extracted while the force was measured and the initial failure mechanism was recorded. Four groups were compared: CoCr, regular PEEK, PEEK with an enhanced cement-bonding surface and the latter with additional surface primer. ResultsThe mean pull-off forces for the four groups were 3814 N, 688 N, 2525 N and 2552 N, respectively. The initial failure patterns for groups 1, 3 and 4 were the same; posterior condylar foam fracture and cement–bone debonding. Implants from group 2 failed at the cement–implant interface. ConclusionsThis study has shown that a PEEK-OPTIMA® femoral TKA component with enhanced macro- and microtexture is able to replicate the main failure mechanism of a conventional CoCr femoral implant. The fixation strength is lower than for a CoCr implant, but substantially higher than loads occurring under in-vivo conditions.