Zhidong Guan

Damage tolerance analysis of adhesively bonded composite single lap joints containing a debond flaw

ABSTRACT Adhesive bonding technology is being increasingly used in the assembly and repair processes of composite structures. The existence of debond flaws weakens the performance of adhesively bonded structures. This article presents the results from an investigation into the effects of debond flaws on the mechanical performance of adhesively bonded single lap joints. The experimental results show that both the load-carrying capability and the failure mode of the single lap joints vary with the location of the debond flaws. Three-dimensional progressive damage finite-element models were developed in ABAQUS to simulate the tensile behaviour of single lap joints. The simulation results agree with the experimental data. The flaws located at 1/4 lap length result in a more pronounced reduction in the load-carrying capability than those located at the edge and the middle portion of the bond region. Compared with the other two locations, the residual strength of the single lap joint with a flaw at 1/2 lap length possesses a higher value. Moreover, the effects of flaws on strength reduction are more prominent for damage propagation than damage initiation.

Micro-Mechanical Analysis About Kink Band in Carbon Fiber/Epoxy Composites Under Longitudinal Compression

Kink band is a typical phenomenon for composites under longitudinal compression. In this paper, theoretical analysis and finite element simulation were conducted to analyze kink angle as well as compressive strength of composites. Kink angle was considered to be an important character throughout longitudinal compression process. Three factors including plastic matrix, initial fiber misalignment and rotation due to loading were considered for theoretical analysis. Besides, the relationship between kink angle and fiber volume fraction was improved and optimized by theoretical derivation. In addition, finite element models considering fiber stochastic strength and Drucker-Prager constitutive model for matrix were conducted in ABAQUS to analyze kink band formation process, which corresponded with the experimental results. Through simulation, the loading and failure procedure can be evidently divided into three stages: elastic stage, softening stage, and fiber break stage. It also shows that kink band is a result of fiber misalignment and plastic matrix. Different values of initial fiber misalignment angle, wavelength and fiber volume fraction were considered to explore the effects on compressive strength and kink angle. Results show that compressive strength increases with the decreasing of initial fiber misalignment angle, the decreasing of initial fiber misalignment wavelength and the increasing of fiber volume fraction, while kink angle decreases in these situations. Orthogonal array in statistics was also built to distinguish the effect degree of these factors. It indicates that initial fiber misalignment angle has the largest impact on compressive strength and kink angle.

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.

Study on ASTM Shear-loaded Adhesive Lap Joints

Finite element analyses and experiments are conducted to analyze the mechanical behavior of ASTM shear-loaded adhesive lap joints. Adhesive is characterized for the stress-strain relation by comparing the apparent shear-strain relations obtained from finite element analysis and experiments following ASTM D 5656 Standard. With the established stress-strain relation, two failure criteria using equivalent plastic strain and J-integral are adopted to predict the failure loads for joint specimens following ASTM D 5656 and ASTM D 3165 Standard, respectively. Good correlation is found between the finite element results and the experimental results. The strength of ASTM D 3165 specimens with debonding defects is also studied. Calculation results shows that experiment data following the standards provide only relative material constants, such as apparent shear modulus and strengths. Further investigation is required to find out the engineering properties needed for actual joint design. For the specimens with debonding defects, the locations of defects have great effects on their load bearing ability.

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.

Impact and compression after impact behavior of single-stiffener composite panels

The purpose of this paper was to investigate the damage tolerance of single-stiffener panels. The impact response and the influence of impact damage on compression behavior were studied. Barely visible impact damage (BVID) was introduced to the stiffener flange tip on the smooth side. By applications of visual inspections and C-scan, massive damage was detected. After the impact experiment, compression tests were carried on the intact specimens and impacted specimens. The experimental results reveal that impact damage has no significant effect on the buckling mode. but decreases the buckling load by about 10%. The results also show that impact damage has an important influence on the compression failure mechanism. Under the compressive loading, impact damage propagates inside the skin after the buckling, which is a trigger for final fracture of damaged stiffened panels. Consequently, the impact damage causes a dramatical decrease in the failure load by 34%, approximately.

Time-temperature dependent mechanical properties of cured epoxy resin and unidirectional CFRP

For composite structure design, determining the relationships between failure time and mechanical properties in the full range of working temperatures is essential. Dynamic mechanical analysis (DMA) and tensile tests were performed to get the dynamic and static mechanical properties of resin 5228A respectively at various temperatures. The master curves of storage modulus and Youngs modulus were constructed at selected reference temperature 25°C using closed form shifting (CFS) method based on the time-temperature superposition principle (TTSP). During the shifting procedure of storage modulus, corresponding time-temperature shift factors were obtained, which meet the Arrhenius equation very well. Finally, the master curves of transverse and longitudinal compressive strength were constructed at 25°C by shifting the compression tests data of unidirectional laminates CCF300/5228A horizontally with the time-temperature shift factors obtained previously. Results show that both dynamic and static mechanical properties of the resin 5228A are obviously time-temperature dependent, higher frequencies induce more elastic-like behavior, transverse and longitudinal compressive strength decreases significantly with increasing failure time.

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.