Zengshan Li

Microscopic Progressive Damage Simulation and Scale-Span Analysis of Cross-Ply Laminate Based on the Elastic–Plastic Theory

Computational mechanics has been carried out to study the microscopic failure mechanisms of cross-ply laminate. A microscopic model of fiber regular distribution near the [90/0]8S laminate interlaminar zone is established, with two dominant damage mechanisms-matrix plastic deformation and interfacial debonding included in the simulation by the extended Drucker-Prager model and cohesive zone model respectively. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the damage of the [90/0]8S RVE under tension initiates in 90° ply, and then intralaminar damage cracks spread to interlaminar cohesive region, which causes delamination between adjacent plies. Meanwhile in 0° ply, matrix plastic deformation and interface debonding occurs near the zone of interlaminar delamination expansion. While the damage of the [90/0]8S RVE under compression initiates in 0° ply with fiber microbuckling and interfacial debonding, then the intralaminar degradations in 0° ply expand to interlaminar cohesive region, which produces a wide range of interlaminar delamination.

Multi-Scale Modeling and Damage Analysis of Composite with Thermal Residual Stress

In order to analysis thermal residual stress and its influence on the strength of composite, the hierarchical multi scale simulation method is applied. A microscopic computational model of single fiber composite with thermal residual stress is built to research the stress distribution. Then the damage initiation discipline details of unidirectional composite are researched, and the effects of different fiber arrangements on thermal residual stress distribution, damage initiation and the different final failure behaviors of fiber regular distribution and random distribution under tension and compression are researched in details. It shows that in fiber regular arrangement, damage initiation in interface appears evenly and in matrix it appears at somewhere randomly. But in fiber random arrangement, initial damage focuses at the resin pockets between closely packed fibers with both interface and matrix damage. The maximal thermal residual stress in fiber random arrangement model is larger than that in fiber regular arrangement model. And it reaches the normal strength of the interface and thus causing the initiation of interface damage. Also the failure modes of composites under transverse tension and compression with and without residual stress are quite different from each other. The strength and failure path of different RVE and loading are showing respectively in this paper.

Pull-Through Mechanical Behavior of Composite Fastener Threads

A method was proposed to test the pull-through mechanical behavior of fastener threads, which were fabricated from weave carbon/carbon (C/C) composites. The damage morphologies of the C/C fastener threads were observed through an optical microscope and high-resolution micro-CT systems. The acoustic emission (AE) technique was utilized to track the damage progression of threads during loading up to fracture in terms of AE event rate which has an exponential type profile. Finally, A 3D finite element damage evolution model of composite threads was established based on continuum damage mechanics to calculate the thread load distribution and damage progression. The relations between the pitch and the load distribution, as well as between different fabrication directions and ultimate loads, were investigated by using this model. The stress in the first thread was analyzed based on the tapered cantilever assumption. The results show that, the first thread is brittle fracture at the root where is the higher stress level region of the threads and it is the initial damage. The load distribution in C/C threads is not uniform and not improved as the value of pitch decreases. Load capacity of C/C threads is different result from the fabrication direction. Numerical results agree well with experimental results.

A Progressive Damage Model for Predicting Permanent Indentation and Impact Damage in Composite Laminates

In this paper, a progressive damage model was established on the basis of ABAQUS software for predicting permanent indentation and impact damage in composite laminates. Intralaminar and interlaminar damage was modelled based on the continuum damage mechanics (CDM) in the finite element model. For the verification of the model, low-velocity impact tests of quasi-isotropic laminates with material system of T300/5228A were conducted. Permanent indentation and impact damage of the laminates were simulated and the numerical results agree well with the experiments. It can be concluded that an obvious knee point can be identified on the curve of the indentation depth versus impact energy. Matrix cracking and delamination develops rapidly with the increasing impact energy, while considerable amount of fiber breakage only occurs when the impact energy exceeds the energy corresponding to the knee point. Predicted indentation depth after the knee point is very sensitive to the parameter μ which is proposed in this paper, and the acceptable value of this parameter is in range from 0.9 to 1.0.

The Experiment and Numerical Simulation of Composite Countersunk-head Fasteners Pull-through Mechanical Behavior

Fasteners made of the anisotropic carbon/carbon (C/C) composite material have been developed for joining C/C composite material components in the high-temperature environment. The fastener specimens are fabricated from the C/C composites which are made from laminated carbon cloths with Z-direction carbon fibers being punctured as perform. Densification process cycles such as the thermal gradient chemical vapor infiltration (CVI) technology were repeated to obtain high density C/C composites fastener. The fasteners were machined parallel to the carbon cloths (X-Y direction). A method was proposed to test pull-through mechanical behavior of the countersunk-head C/C composite material fasteners. The damage morphologies of the fasteners were observed through the charge coupled device (CCD) and the scanning electron microscope (SEM). The internal micro-structure were observed through the high-resolution Mirco-CT systems. Finally, an excellent simulation of the C/C composite countersunk-head fasteners were performed with the finite element method (FEM), in which the damage evolution model of the fastener was established based on continuum damage mechanics. The simulation is correspond well with the test result . The damage evolution process and the relation between the countersunk depth and the ultimate load was investigated.

Analytical model of adhesively bonded composite double-lap joints containing a fully or non-fully debond flaw

Adhesive bonding technology is widely applied in the assembly and repair processes of composite laminates. However, the debond flaws existing in the adhesive layer weakens the load-carrying capacity of adhesively bonded structures. This article presents an analytical model to investigate the adhesive shear behaviour of double-lap joints containing a debond flaw. Differential element method is employed to simulate the stress and strain distribution of adhesive layer, which considers anisotropy of each ply in the composite laminates and elastic-perfectly plastic behaviour of the adhesive. This model can compute the stress and stiffness of adhesive double-lap joints with different debond flaw conditions, covering fully debond ones as well as non-fully ones. Finite element model of double-lap joints is built in this paper to validate the accuracy of proposed analytical model. We also compared the debond flaw joints and non-flaw ones in terms of their adhesive strain and stress distribution.

Buckling of honeycomb structures under out-of-plane loads

The governing equations for the buckling of honeycomb cores with various cell geometries under combined compression and shear are established and three types of core including rectangular, hexagonal and triangular cores are under consideration. After invoking the Bloch wave representation form, the equations are simplified by the periodicity and the hypothesis that the out-of-plane displacement remains zero at the intersections. Different cell geometries and load cases are taken into account and numerical results offer validation for the analytical solutions. Moreover, the results of Finite Element (FE) models show that the fine results can only be acquired by models with appropriate cell numbers. Experimental study is conducted on the regular hexagonal honeycomb structures. Both the results of the numerical benchmarks and the experiments prove the effectiveness of the proposed analytical method and the hypothesis for predicting the buckling load of honeycomb structures.

Effects of chamfering, cold expansion, bolt clamping, and their combinations on fatigue life of aluminum–lithium alloy single plate:

The objective of this study was to establish the effects of cold expansion, chamfering, bolt clamping, and their combinations on the fatigue life of an aluminum–lithium alloy single plate. Fatigue tests were conducted to quantify the anti-fatigue effects of the different techniques. A scanning electron microscope was used to perform fracture analyses of the used specimens, and the residual stresses were measured using an X-ray diffraction device. In addition, three-dimensional finite element models of the specimens were established and used to characterize their stress states, and the Smith–Watson–Topper method was used to predict the fatigue lives of the specimens. The fatigue test results showed that all the considered processes improved the fatigue life of the pristine specimen. The most effective was a combination of 3.2% cold expansion, 1-mm chamfering, and bolt clamping using a 6.4-N m torque, which improved the fatigue life of the pristine specimen by a factor of 15.5. The finite element method results also revealed that this combination decreased the maximum stress and confirmed its superiority in relation to the other fatigue-life enhancement techniques in terms of the anti-fatigue effect. The Smith–Watson–Topper method underestimated the specimen fatigue life, but the accuracy satisfied engineering requirements.

An numerical investigation on the effect of the combination of cold expansion and interference fitting on fatigue life improvement of a 7075-T6 aluminum alloy single plate

Cold expansion and interference fitting both can improve the fatigue life of 7075-T6 aluminum alloy single plates. To study the combination effect of the two techniques on fatigue life improvement, a numerical analysis method was employed to simulate the specimens with different cold expansion and interference fit degrees. Finite element methods were conducted to capture the residual stress, and the Smith-Watson-Topper (SWT) method was used to predict the fatigue life. The simulation results revealed that, compared with single technique enhanced specimens, the combined enhanced specimens of 4% cold expansion and 2% interference fit increase the fatigue life by 12–38%.

Edgewise compression behavior of honeycomb sandwich structures

An analytical solution and numerical models are proposed for the buckling and post-buckling behavior of honeycomb sandwich structures under edgewise compression. General buckling and core crushing due to transverse shear are observed in the experiment. Based on these phenomena, the von Kármán plate theory are employed for the analysis for the buckling behavior and the analytical solution for the core under transverse shear are obtained for predicting the failure of the sandwich structures. Comparison between these solutions and the experiments shows that the imperfection of pre-bending deformation significantly affects the ultimate load for the sandwich structures, resulting the failure before the compression reaches the critical load for general buckling. Moreover, parameters such as core geometries, asymmetry face sheets are under discussions for the load carrying capacity of the specimens.