T-joint Structure Research Articles

Comparative study on submillimeter flaws in stitched T-joint carbon fiber reinforced polymer by infrared thermography, microcomputed tomography, ultrasonic c-scan and microscopic inspection

Stitching is used to reduce dry-core (incomplete infusion of T-joint core) and reinforce T-joint structure. However, it may cause new types of flaws, especially submillimeter flaws. Microscopic inspection, ultrasonic c-scan, pulsed thermography, vibrothermography, and laser spot thermography are used to investigate the internal flaws in a stitched T-joint carbon fiber-reinforced polymer (CFRP) matrix composites. Then, a new microlaser line thermography is proposed. Microcomputed tomography (microCT) is used to validate the infrared results. A comparison between microlaser line thermography and microCT is performed. It was concluded that microlaser line thermography can detect the internal submillimeter defects. However, the depth and size of the defects can affect the detection results. The microporosities with a diameter of less than 54 μm are not detected in the microlaser line thermography results. Microlaser line thermography can detect the microporosity (a diameter of 0.162 mm) from a depth of 90 μm. However, it cannot detect the internal microporosity (a diameter of 0.216 mm) from a depth of 0.18 mm. The potential causes are given. Finally, a comparative study is conducted.

An experimental and finite element study of the transverse bending behaviour of CFRP composite T-joints in vehicle structures

The behaviour of a woven fabric carbon/epoxy composite T-joint (representing a simplified version the T-joint located at the connection between the B-pillar and the longitudinal rocker in a car body structure) is investigated using experimental and numerical methods. Details of the manufacturing process and experimental design factors are considered to understand their influence on the performance of the T-joint structure. The experimental results reveal the influence of manufacturing process and experimental setup on the load-carrying capacity and failure mode of the T-joint. Numerical simulation accurately predicts the stress distribution and load-carrying capacity of the T-joint obtained from experimental tests. The FEM model, which includes the adhesive interface layers at the edges, convincingly represents the experimentally found stiffness: the error is less than 3%. According to Hashin matrix tension criteria, the first ply failure occurs at 3.746 kN when the Hashin failure index (R) becomes equal to 1. Whereas, in the case of experimental tests, the first ply failure occurs around 3.4 kN, at which force the first load drop is observed.

Brittle crack propagation/arrest behavior in T-joint structure of heavy gauge steel plate

Brittle crack arrestability is extremely important in welded joints of heavy gauge steel plates used in large container ships. Recently, much attention has been focused on potential crack propagation along welds fabricated by large heat input welding. This paper examines the application of a T-joint to the strength deck structure of container ships to enhance crack arrestability. The unwelded face and the crack arrest toughness, K ca, for crack arrest were varied. In the case of a T-joint with an unwelded face, the ESSO test of the T-joint components showed that a brittle crack arrested at the T-joint embedding the unwelded face. In the case of a full penetration T-joint, the brittle crack was arrested if the steel plate used for the flange had a high K ca value in the range from 4900 to 7300 N/mm3/2. FEM analysis of the stress intensity factor K indicated that brittle crack propagation was arrested under the condition that the K value at the running crack tip was less than the K ca of the material. In the T-joint, it was noted that the K value around the deepest point of the crack decreased and was finally less than the K ca of the flange plate when the brittle crack penetrated into the flange plate. This phenomenon shows the advantage of using a T-joint for brittle crack arrest in the flange plates of strength deck structures. In order to verify the long brittle crack arrestability of the T-joint structure, ultra-large-scale tests were carried out under the recommended conditions obtained from a dynamic FEM analysis. The test results showed that long brittle cracks could arrest at the aforementioned T-joint structures.

Numerical and experimental investigation into failure of T700/bismaleimide composite T-joints under tensile loading

A finite element model for a composite T-joint structure under tensile loading has been established to simulate and analyze failure mechanisms, carrying capacities, in-plane and interlaminar damage behaviors based on the Tsai-Wu failure criterion and a cohesive zone model. Strains on the stress-concentration zone of the T-joint structure during loading were measured to validate the FEM model. The results showed that the simulated results had a good agreement with that of the measurements. The carrying capacity of the composite T-joint is mainly determined by failure modes, which can be changed by reducing fillet-filling ratio in the fillet of the T-joint and initiation and propagation paths of delamination cracks between the fillet and the L-ribs. The maximum tensile load of the T-joint calculated by FEM methods was 1854N, while the experimental value was 1862N.

Brittle Crack Propagation/Arrest Behavior in T-Joint Structure of Heavy Gauge Steel Plates

Brittle crack arrestability is extremely important in welded joints of heavy gauge steel plates used in large container ships. Recently, much attention has been focused on potential crack propagation along welds using large heat input. This paper examines the application of a T-joint to the strength deck structure of container ships to enhance crack arrestability. The crack arrest toughness, Kca, for crack arrest was varied. The ESSO test of T-joint components showed that brittle crack was arrested at the T-joint if the steel plate used for the flange had a high Kca value in the range from 4900 to 7300N/mm3/2. FE-analysis of the stress intensity factor K indicated that brittle crack propagation was arrested under the condition that the K-value at the running crack tip was less than the Kca of the material. In the T-joint, it was noted that the K-value around the area of the deepest point of the crack decreased and was finally less than the Kca of the flange plate when the brittle crack penetrated suddenly into the flange plate to a 10mm depth. This phenomenon shows the advantage of using a T-joint for brittle crack arrest in the flange plates of strength deck structures.

Effect of unwelded length on behaviour of brittle crack arrest in T-joint structure

In the large heat input weld joint of heavy gauge steel plate used for large container carriers, a brittle crack possibly propagates straight along a weld joint without diverging to the base metal. This phenomenon is discussed for its application to the large heat input weld of heavy gauge plate to ship structures. In this study, to arrest a brittle crack at the T-joint embedding unwelded face, the effect of the unwelded face on behaviour of brittle crack propagation/arrest in the T-joint structure was investigated and analysed. The ESSO test of the T-joint structure was carried out, supposing that a brittle crack which propagates along a weld joint of hatch coaming rush into T-joint of hatch coaming and strength deck. The test results showed that the brittle crack arrested at the T-joint embedding the unwelded face, and the brittle crack arrested easier, the longer the unwelded face. The results of static FEM analysis showed that the stress intensity factor of the brittle crack was increased by the unwelded face, until the brittle crack reached a flange. However, when the brittle crack propagated into the flange, the stress intensity factor of the brittle crack was decreased by the unwelded face. The crack-arrest effect of the unwelded face appears after the brittle crack propagates into the flange.

A Comparative Study of Statistical Outlier Analysis and ANN Methods for Damage Detection and Assessment in Composite Structures

This article introduces some of the experimental and analytical work in damage detection applied to polymeric composite T-joints used in maritime structures. Two methods of damage detection are discussed — A statistics-based outlier technique and one using artificial neural networks (ANNs). The SHM using ANNs system was found to be capable of not only detecting the presence of multiple delaminations in a composite structure, but also capable of determining the location and extent of all the delaminations present in the T-joint structure, regardless of the load (angle and magnitude) acting on the structure. The system developed, relies on the examination of the strain distribution of the structure under operational loading. Finally, on testing the SHM system developed with strain signatures of composite T-joint structures, subjected to variable loading, embedded with all possible damage configurations (including multiple damage scenarios), an overall damage (location and extent) prediction accuracy of 88.32% was achieved. These results are presented and discussed in detail in this article.

Strain-based Structural Health Monitoring of Complex Composite Structures

The use of composite structures in engineering applications has proliferated over the past few decades due to its distinct advantages, namely: high structural performance, corrosion resistance, and high strength/weight ratio. However, they also come with a set of disadvantages, i.e., they are prone to fiber breakage, matrix cracking, and delaminations. These types of damage are often invisible and if undetected, could lead to catastrophic failures of structures. Although there are systems to detect such damage, the criticality assessment and prognosis of the damage is often much more difficult to achieve. This article discusses the research study conducted, which resulted in the development of a Structural Health Monitoring (SHM) system for a 2D polymeric composite T-joint, used in maritime structures. The SHM system was found to be capable of not only detecting the presence of multiple delaminations in a composite structure, but also capable of determining the location and extent of all the delaminations present in the T-joint structure, regardless of the load (angle and magnitude) acting on the structure. The system developed relies on the examination of the strain distribution of the structure under operational loading.

Structural Health Monitoring of Composite Structures using Artificial Intelligence Protocols

This study discusses a structural health monitoring (SHM) system developed to detect the presence of delamination, and predict its location and size in a composite structure. Two structures are considered in this study: a composite beam and a T-joint structure used in ships. Finite element (FE) models of these structures are created, embedded with delaminations, and the strain distribution along the bond-line and surface of the structures is used as a damage characteristic, to get information about the structures' condition. Experimental tests are then conducted to verify the FE model, an excellent corroboration is achieved between the two. Artificial neural networks is then used in tandem with a pre-processing program developed, called the damage relativity assessment technique (DRAT), to determine the presence of the damage and then predict its size and location. This SHM system developed is completely independent of the structures' loading condition and it detected the presence, and predicted the size and location of delaminations with an acceptable level of accuracy.

Damage detection in T-joint composite structures

The use of composite structures in engineering applications has proliferated over the past few decades. This is mainly due to their distinct advantages of high structural performance, high corrosion resistance, and high strength/weight ratio. They are however prone to fibre breakage, matrix cracking and delaminations which are often invisible. Although there are systems to detect such damage, the characterisation of the damage is often much more difficult to achieve. A study is presented of the strain distribution of a GFRP T-joint structure under tensile pull-out loads and the determination of the presence and the extent of disbonds. Finite element analysis (FEA) has been conducted by placing delaminations of different sizes at various locations along the structure. The FEA results are also validated experimentally. The resulting strain distribution from the FEA is pre-processed by a method developed called the damage relativity assessment technique (DRAT). Artificial neural networks (ANNs) were used to determine the extent of damage. A real-time system has been developed which detects the presence, location and extent of damage from the longitudinal strains obtained from a set of sensors placed on the surface of the structure. The system developed is also independent of the magnitude of load acting on the structure.

Geometry and damage effects in a composite marine T-joint

The structural integrity and damage tolerance of typical composite T-joints found in ships constructed from glass fibre reinforced plastic was the subject of this investigation. The effect of the geometry of the T-joint on the strain distribution was investigated using finite element (FE) analysis. The results, reported in this paper, showed that the critical strains were significantly affected by the joint geometry. Results of the FE analysis conducted to investigate the effect of disbonds between the filler and overlaminate are also reported. This showed that particular defects led to large changes in the strains in the T-joint structure, which would encourage disbond progression. The FE model was validated by mechanical tests on a representative T-joint, instrumented with surface strain gauges and displacement transducers, into which a range of defects was progressively introduced.

Free Vibration Analysis of a T Joint Using Thin-Walled Beam and Shell Elements

This paper proposes an efficient beam-shell modeling technique for the free vibration analysis of a T-joint thin-walled beam structure. Except a small portion of a T-joint which is modeled by shell elements, the structure is modeled by thin-walled beam elements that can describe warping and distortion. In order to match the shell and thin-walled beam elements at the interface of the dissimilar elements, a technique based on a pseudo inverse matrix is formulated. This paper also examines the role of the thin-walled element taking into account the distortion and warping deformation degrees of freedom in predicting accurately the dynamic characteristics of a T-joint thin-walled structure.

Processing aid

Stitching is used to reduce incomplete infusion of T-joint core (dry-core) and reinforce T-joint structure. However, it might cause new types of flaws, especially micro-sized flaws. In this paper, a new micro-laser line thermography (micro-LLT) is presented. X-ray micro-computed tomography (micro-CT) was used to validate the infrared results. The micro-LLT and micro-CT inspection are compared. Then, a finite element analysis (FEA) is performed. The geometrical model needed for finite element discretization was developed from micro-CT measurements. The model is validated for the experimental results. Finally a comparison of the experiments and simulation is conducted. The infrared experimental phenomenon and results are explained based on the FEA results.