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• A criterion for simulation of delamination is proposed. • An imperfection induced pre-buckling must be incorporated into failure analysis of open-hole laminate under in-plane compression. • Rules for selection of a scale factor for the pre-buckling analysis are set forth. • Various failures of notched laminate under compression are predicted with no parameter adjustment and no iteration. Failure prediction of open-hole laminates under compression still remains a great challenge. A number of unique mechanics theories for composites developed by the author are successfully applied to analyze in plane compression induced various failures of the notched laminates in this paper. A buckling mode must be incorporated into the analysis of the compression induced delamination, and the rules for selecting a scale factor for the buckling analysis through ABAQUS are established. To simulate delamination, the interlaminar matrix stress modification method is applied. A more pertinent criterion for delamination is proposed. When and where is delamination initiated, how can the initiated delamination be propagated and how much is the delaminated area to be attained can be easily reproduced. Intralaminar failures are estimated based on Bridging Model and the matrix true stress theory. The ultimate strength of a notched laminate is assumed when any primary layer element outside neighborhoods of the stress singularity and weak singularity points firstly attains an ultimate failure. A limited, if not the minimum, number of inputs are required all measurable independently and following existing standards with no data calibration. Except for the pre-buckling analysis, no iteration is needed for prediction of all the other failures. The predicted failure modes and ultimate compressive loads of several laminates with single or double holes agree well with our measured counterparts. [ABSTRACT FROM AUTHOR]

We analyze progressive damage and failure of composite laminates by using a micromechanical bridging model and compare numerical and experimental results for three fracture problems, namely, the four-point bending, the simple tension, and the simple tension of a laminate with an open hole at its centroid. These problems involve fiber–matrix interface debonding, constituents' damage, interlayer delamination, and localized damage due to stress concentration. Macroscopic constitutive equations of unidirectional lamina, derived from those of the fiber and the matrix by using the bridging model with the fiber material assumed to be linearly elastic and the matrix to be elasto-plastic obeying the Drucker–Prager yield criterion, are employed. Strains in each constituent of the composite are assumed to be infinitesimal for the additive decomposition of strains into elastic and plastic parts to be valid, and the incremental plasticity theory is used. Stresses in the two constituents are found from their values in the homogenized material by using a dehomogenization technique. The intra-layer damage is assumed to initiate at a material point when the failure criterion for either the fiber or the matrix is satisfied. Young's modulus of a constituent is degraded by following a Weibull distribution. A finite element is deleted when an energy-based failure criterion is satisfied in it, and the analysis is continued till the structure fails. The delamination between adjacent plies is simulated by including a thin resin layer at the interface and studying failure initiation and propagation in it. The computed reaction force versus the displacement curves and the failure patterns in the three problems are found to agree with the corresponding experimental data. [ABSTRACT FROM AUTHOR]

A series of full-scale loading tests are performed for a prospective subway tunnel with a rectangular shape including two reliability tests: one stagger-jointed three-ring reliability test, and one ultimate failure test on a single ring. Comprehensive measuring programs are designed to record the deformation of both lining structure and joints and the stresses of concrete, bolts and reinforcements. Experimental results show that in both the single-ring and three-ring loading cases, the long sides of tunnel cross section bend inwards while the short sides of tunnel cross section bend outwards. The inner part of joints opens while the outer part of joints closes at places experiencing positive moment and vice versa. Joint’s rotational stiffness varies at different locations. Concrete cracking and crushing are the chief damage modes, and they are closely related to the distribution of bending moment. Stagger-jointed fabrication significantly increases the overall rigidity of lining system, which thereby greatly reduces the deformation of both concrete lining and joints in comparison with the single-ring case. It is shown that the routinely-used uniform rigidity model is conservative and the preliminary design can be optimized by applying an effective rigidity ratio (ERR) of 0.5. [ABSTRACT FROM AUTHOR]

Waterfront structures such as seawalls, dikes, and levees are frequently subjected to scouring at the toe of the slope, leading to deteriorated performance and increased failure potential. To this end, some model reinforced steep-faced slopes consisting of a two-dimensional backfill were brought to failure to explore the failure mechanisms of some geosynthetic-reinforced slopes subjected to simulated toe scouring. Results of model tests indicate that in the case of shallow scouring, a reinforcement length (L) increase from 0.4 to 1.0Ht(Ht, total height of reinforced walls) significantly increases the tolerance against toe scouring-induced failures. In this case, a local bearing capacity failure of facing is the dominant failure mode. In the case of deep scouring, an increase inLbeyond 0.7Htprovides no additional tolerance against toe scouring because the ultimate state is always associated with a global circular sliding in the unreinforced zone. Experimental values of the lateral pressure coefficient (Kt) converted from the measured reinforcement forces indicate that reinforcement forces consistently increase in response to toe scouring up to the final collapsing state and that the reinforcement forces forL = 1.0Htmobilize more effectively than those forL = 0.7Ht. [ABSTRACT FROM PUBLISHER]

• New method for determining interlaminar matrix stress modification coefficient before delamination initiation is proposed. • Delaminated area in any laminated structure is obtained and displayed with no iteration. • Ultimate strength of a laminated structure is assumed when any primary element outside neighborhood of singularity or weak singularity attains an ultimate failure. • Neighborhood sizes of singularity and weak singularity are determined. To simulate delamination in a laminate, a pure matrix secondary layer is inserted between its primary lamina layers. The stresses in the weakest secondary layer element obtained from a finite element (FE) approach must be modified since singularity or weak singularity exists at delamination point. The failed secondary layer element is deleted and delamination initiation or propagation is attained. We find in this paper that the modification coefficients before the delamination initiation can be determined by experimental data of a Short Beam Shear (SBS), whereas those after the initiation are obtained based on parameters of Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests on the unidirectional laminate. The ultimate strength of a notched laminate is assumed when any primary layer element outside neighborhoods of singularity and weak singularity attains an ultimate failure. It is shown that the neighborhood range for the weak singularity is 4 times of the primary layer thickness, while that for the singularity is 6 times of the former. No iteration is needed and all input data can be measured independently and following existing standards. Tensile strengths of four notched laminates with different lamination angles and hole diameters are studied. The simulation results agree well with the measured counterparts. [ABSTRACT FROM AUTHOR]

A novel design of out-of-plane composite π joints is proposed to elevate the load carrying capability. This design adopts an in-plane π-shaped section scheme along the loading direction to eliminate the delamination damage, which commonly occurs in traditional composite π joints, and uses bolts to connect the joint and other structural components. Static experiments of the novel bolted composite π joint and an aluminum counterpart were conducted. The bolted composite π joint exhibited a larger load carrying capability with a massive weight reduction. Compared with existing static experimental results from a traditional design, the novel bolted composite π joint also showed a greater load carrying capability. A progressive damage analysis of the bolted composite π joint was further performed and validated by experimental data. Based on numerical simulation, the progressive damage process and failure mechanism of the bolted composite π joint were discussed. It follows that the novel design not only eliminated potential weaknesses in deltoid fillers but also brought positive factors of composite into play and substantially strengthened the composite π joint. [ABSTRACT FROM AUTHOR]

We study how the competition of the disordered local strength and of the evolving inhomogeneous stress field affects the evolution of the series of breaking avalanches accompanying the fracture of heterogeneous materials. To generate fracture processes, we use a fiber bundle model of localized load sharing where the degree of strength disorder is controlled by varying two parameters of the distribution of the breaking threshold of fibers. Analyzing the record statistics of avalanches of breaking fibers, we demonstrate that both for low and high disorders the series of crackling events remains stationary until global failure making the collapse of the system unpredictable. Based on computer simulations, we determine a region of the parameter plane of strength disorder where global failure is preceded by an accelerating breaking activity. We show that the record avalanche with the longest lifetime can be used to identify the onset of acceleration of the fracture process towards the catastrophic failure. Comparison of the results to their equal load sharing counterparts reveals that the accelerating regime is shorter than in case of a homogeneous stress field due to the higher degree of brittleness of the system caused by stress localization. • The evolution of the sequence of breaking avalanches accompanying fracture is studied. • The statistics of record size avalanches is analyzed. • A range of disorder is identified where global failure is preceded by an acceleration. • The record with the longest lifetime can be used for forecasting global failure. [ABSTRACT FROM AUTHOR]

This paper deals with the studies on the state of displacement of symmetric and anti-symmetric angle-ply and cross-ply laminated composite plates during its ultimate failure, subjected to transverse static load. First-order shear deformation theory (FSDT) is employed in conjunction with the finite element approach using eight-noded quadratic isoparametric element. The free vibration analyses of isotropic and laminated composite plates are carried out to ensure the overall validity of the present finite element formulation. The mid surface of the laminate is considered as the reference plane. The principal material directions in different laminae are oriented to produce a laminated structural element capable of resisting loads in several directions. The stiffness of a composite laminate is obtained from the properties of the constituent laminae. The affected stiffness of the failed lamina is discarded completely after the failure of weakest ply. The rigidity matrix of the laminate with remaining laminae is re-established. The re-evaluation process continues until the laminate fails completely. To investigate the displacement behaviour of laminates during the ultimate failure, parametric studies are carried out for different cases by varying the stacking sequences, fiber orientations, layer thicknesses, aspect ratios and the number of layers in the laminate. The comparison of results in terms of non-dimensional natural frequencies and ply-by-ply failure analyses obtained from the present investigation are made with those available in the reported literature. [ABSTRACT FROM AUTHOR]

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Abstract: The behaviour of thin-walled compression members is known to be significantly influenced by the effects of local buckling. Thin-walled sections lose axial compressive stiffness and the compressive carrying capability of the members can be considerably reduced as a result of the effects of local buckling. Finite element simulation is employed in this paper to examine the post-buckled response of thin-walled I-section and box-section struts, giving due consideration to the influence of geometric imperfections and to elasto-plastic material behaviour. A detailed account of the growth and redistribution of stresses after local buckling as well as the initiation of yielding and yield propagation throughout loading is given in the paper. The influence of different in-plane displacement boundary conditions imposed in the simulations at the section plate junctions is also detailed in the paper as well as the effect of yielding on the post-buckled stiffness and failure of the sections. It is shown that the effects of geometrical imperfections are most prominent for strut designs with near simultaneous buckling and yielding and that the ultimate failure and unloading of strut designs, in general, is synonymous with the development of yielding at the section junctions along the full length of the struts and through to the middle surface of the section walls. [Copyright &y& Elsevier]

A new technique based on tube shrinkage is proposed for the fabrication of composite archwires. Compared with a traditional pultrusion method, this new technique can avoid any fiber damage during the fabrication and can provide the archwire with a required curvature in its final clinical usage. The present paper focuses on the technique development and mechanical design and validation in terms of constituent materials by using a micromechanics bridging model. Prototype archwire has been fabricated using fiberglass and an epoxy matrix, with a wire diameter of 0.5 mm and a 45% fiber volume fraction. Tensile and three-point bending tests have shown that the mechanical performance of the prototype composite archwire is comparable to that of a clinical Ni–Ti archwire. Another purpose of the present paper is to provide an efficient procedure for a critical design of composite archwires. For this to be possible, the ultimate load especially flexural load carrying ability of the composite archwire must be assessed from the knowledge of its constituent properties. However, difficulty exists in doing this, which comes from the fact that the failure of the utmost filament of the composite archwire subjected to initially the maximum bending stress does not imply its ultimate failure. Additional higher loads can still be applied and a progressive failure process is generated. In this paper, the circular archwire was discretized into a number of parallel laminae along its axis direction, and the bridging micromechanics model combined with the classical lamination theory has been applied to understand the progressive failure process with reasonable accuracy. Only the constituent fiber and matrix properties are required for this understanding. Nevertheless, the ultimate bending strength cannot be obtained only based on a stress failure criterion. This is because neither the first-ply nor the last-ply failure corresponds to the ultimate failure. An additional critical deflection (curvature) condition must be employed also. By using both the stress failure and the critical deflection conditions, the predicted load-deflection up to the ultimate failure agrees well with the measured data. Thereafter, different mechanical performances of composite archwires can be tailored before fabrication by choosing suitable constituent materials, their contents, and the archwire diameters. Several design examples have been shown in the paper. [Copyright &y& Elsevier]