Damage Initiation Load Research Articles

A Novel Preparation Method of Composite Bolted T-Joint with High Bending Performance Based on the Prepreg-RTM Co-Curing Process.

A co-curing resin system consisting of 9368 epoxy resin for prepreg and 6808 epoxy resin for resin transfer molding (RTM) was developed. A corresponding preparation method for a novel polymer composite bolted T-joint with internal skeleton and external skin was proposed based on the prepreg-RTM co-curing process, and novel T-joints were fabricated. A series of conventional configuration T-joints based on the RTM process and T-joints made of 2A12 aluminum alloy were prepared simultaneously. Bending performances were studied on these T-joints experimentally. The results indicate that 9368 epoxy resin and 6808 epoxy resin exhibit good compatibility in rheological and thermophysical properties. The novel T-joints prepared with the prepreg-RTM co-curing process show no obvious fiber local winding or resin-rich regions inside, and the interface quality between the internal skeleton and the external skin is excellent. The main failure modes of the novel T-joint under bending load include the separation of the skin and skeleton and the fracture along the thickness on the base panel; the skeleton carries the main bending load, but there is still load transfer between external skin and internal skeleton through their interface. The internal damages of the novel T-joint are highly consistent with surface damages observed visually, facilitating the detection and timely discovery of damages. The initial stiffness, damage initiation load, and ultimate load of the novel T-joint are 1.65 times, 5.89 times, and 3.45 times that of the conventional T-joint, respectively. When considering the influence of the density, the relative initial stiffness and relative ultimate load of the novel T-joint are 1.44 times and 2.07 times that of the aluminum alloy T-joint, respectively.

Experimental tests and numerical simulation of delamination and fiber breakage in AP-PLY composite laminates

The behavior of delaminations and fiber breakage resulting from three-point bending test is a major concern in the study of composites. This research focused on analyzing fiber breakage and delaminations in advanced placed ply composite laminates through a series of tests. In order to compare simulation results with the experimental data, the cohesive zone model was implemented for analyzing the damage in the model. The results of the simulation were validated using experimental results. The predicted damage initiation load exhibited an approximate 20% deviation from the actual test loads and the discrepancy stems are analyzed.

Fracture toughness-based models for web-crippling of pultruded GFRP profiles

This paper presents a numerical study on the web-crippling failure of pultruded glass fibre reinforced polymer (GFRP) profiles, under external-two-flange (ETF) and internal-two-flange (ITF) configurations. The numerical study is validated through experimental tests recently conducted by the authors on four I-section profiles and one U-section profile, obtained from four different suppliers. These web-crippling tests were simulated through shell finite element (FE) models where the transverse compressive fracture toughness of each GFRP material was implemented as a damage evolution parameter. In order to quantify the influence of material damage and instability effects on the web-crippling failure, three different analyses were implemented, accounting for (i) material damage, (ii) instability, and (iii) both effects. The results obtained through the analysis accounting for both material damage and instability effects showed a good agreement with experimental results, in terms of stiffness, failure modes and ultimate loads; moreover, the transverse compressive strain distributions obtained from the numerical models also agreed well with experimental results. Finally, the transverse compressive stresses were found to influence web-crippling the most, regarding both damage initiation and ultimate load stages.

Can a 3D woven GFRP composite really provide better impact resistance compared to a 2D woven GFRP composite at all of the thicknesses?

The large majority of previous studies about comparison of impact response of 2D and 3D woven composites focused on thin laminates but not thicker ones. Thus, the aim of this study is to determine whether thicker 3D laminates exhibit better impact resistance compared to 2D ones or not. For this purpose, firstly several tensile tests were performed on the composite specimens in nine different thicknesses. In fact, the longitudinal tensile modulus extracted from tensile tests was used to determine delamination threshold load of them. Then, 2D and 3D laminates with an identical thickness were tested by drop weight tower under several energies to specify the perforation and penetration energy thresholds. Finally, 2D and 3D composites with nine different thicknesses were subjected to impact loading to investigate their impact behavior. The experimental results revealed that the relationship between thickness and delamination threshold load of 2D woven E-glass/unsaturated polyester composites can be fitted with a power regression function. The extracted empirical function was also validated with the analytical results obtained from a simple method based on mode II fracture toughness of laminates. Contrary to what is normally believed, the value of energy absorption in 2D specimens has no dependency on their thickness. In addition, analysis of results in terms of energy absorption, damage area and damage initiation load indicated that thicker 2D woven fabrics exhibit higher impact resistance compared to 3D ones unlike thinner laminates.

Timber beams with openings: Laboratory testing and nonlocal finite element modelling

Introducing openings in joists of flooring systems is usually necessary to pass through building services without increasing the floor-to-floor height. The openings reduce the stiffness and limits the ultimate load-carrying capacity of the timber beams by promoting tensile stresses perpendicular to the grain and longitudinal splitting shear stresses. Furthermore, quasi-brittle failure of the timber in shear and tension (promoted by openings) leads to localisation of strain, spurious numerical sensitivities and ultimately poor or erroneous damage initiation and failure load predicted by finite element (FE) models. In this paper, a nonlocal continuum damage model is proposed and implemented to alleviate the FE mesh dependency and strain localisation problems in failure analysis of the timber beams with openings. Benchmark experimental results obtained from bending test on Laminated Veneer Lumber (LVL) and Glued Laminated Timber (GLT or glulam) beams with openings are briefly presented and then accuracy of the implemented nonlocal FE model is validated against the benchmark test results and experimental data available in the literature. It is demonstrated that the proposed nonlocal model can effectively resolve the spurious mesh sensitivity and accurately capture the failure modes and peak load-carrying capacity of timber beams with openings.

Impact of Boundary Conditions on the Behavior of Thin-Walled Laminated Angle Column under Uniform Shortening.

Determining the appropriate boundary conditions of a structure is a very important aspect in the failure analysis. In experimental tests, the method of compressing composite samples significantly influences the obtained results. In numerical studies, there is a problem of correctly defining the boundary conditions applied in real object. Therefore, many numerical tests on samples should be undertaken to observe their behavior and to determine ultimate load. The present work includes study to determine the impact of boundary conditions on the thin-walled laminated angle column under compression. The phenomenon of buckling and the post-buckling bahavior of columns were investigated experimentally and numerically. First, the real simply supported angle columns subjected to uniform shortening are tested. Due to the stress concentration between the real sample and the grips, a flexible pads were used. Experimental tests are carried out on the universal testing machine. The deformations of columns were measured using the non-contact Aramis System. The composite material condition was monitored by acoustic emission using the Vallen Systeme with piezoelectric sensors. Next, the numerical calculations in Abaqus software based on the finite element method are performed to validate the empirical results. To determine the influence of the boundary conditions, two numerical models of the system with and without flexible pads are developed. To estimate damage initiation load in numerical models a different damage criteria ( Tsai-Hill, Tsai-Wu, Azzi-Tsai-Hill, Hashin) are used. Based on the results specified that the model with elastic pads more accurately reflects the actual behavior of the L-profile element under compression. It was supported, i.e., by good agreement of flanges deflection (the equilibrium paths) with experimental results. Furthermore, a qualitative and quantitative agreement of damage initiation load were obtained using Hashin criteria (error 4.61%).

Progressive damage analysis of adhesively bonded patch repaired carbon fibre–reinforced polymer specimen under compression involving cohesive zone model

In this paper, the in-plane compression behaviour of open-hole carbon fibre composite specimens adhesively bonded with the external carbon fibre composite patches on the single- and double side are studied. Uniaxial compression tests are conducted on MTS machine using ASTM anti-buckling fixture. A 3D progressive damage model is developed to predict the damage initiation and failure in both unrepaired open cutout and repaired carbon fibre composite specimens under compressive load. Stress-based 3D-Hashin's failure criteria are used for predicting the fibre and matrix damage in carbon fibre composite. The cohesive zone model element is used for modelling the interlaminar delamination in carbon fibre composite specimen and also the adhesive layer between patch and specimen. Initial stiffness, damage initiation load and ultimate load of the specimen are obtained using progressive damage model based on finite element analysis, and they are compared against the experimental values. The load–deflection curve and the damage progression obtained from finite element analysis using progressive damage model is found to be in good coherence with the experimental predictions. In case of patch bonded carbon fibre composite specimens, failure mechanism starts with partial patch debonding followed by complete specimen failure.

An analytical approach to predict the mechanical behavior of single-lap single-bolt composite joints reinforced with carbon nanofibers

In the present research, the stiffness and the damage initiation load of single-lap composite bolted joints (SLJs) reinforced with carbon nanofibers (CNFs) were studied. To this end, an analytical approach was developed and in the first step, the spring-mass model was modified. By having the elastic properties of the fiber, matrix and nano reinforcement, layup and geometrical parameters of the joint and using the modified spring-mass model, the stiffness of the joint was predicted. Then, by calculating the modulus and the stress concentration factor of a unidirectional ply with a pin-loaded hole and using the maximum stress failure criteria, the damage initiation load of the joint was predicted. Two different SLJs with [−45/0/+45/90]s and [90/−452/+45]s layups with and without nanoparticles were studied. The present analytical approach was evaluated by the finite element model as well as conducting an experimental program.

Comparative effect of implant-abutment connections, abutment angulations, and screw lengths on preloaded abutment screw using three-dimensional finite element analysis: An in vitro study.

Purpose:The purpose of this study is to compare the effect of implant-abutment connections, abutment angulations, and screw lengths on screw loosening (SL) of preloaded abutment using three dimensional (3D) finite element analysis.Materials and Methods:3D models of implants (conical connection with hex/trilobed connections), abutments (straight/angulated), abutment screws (short/long), and crown and bone were designed using software Parametric Technology Corporation Creo and assembled to form 8 simulations. After discretization, the contact stresses developed for 150 N vertical and 100 N oblique load applications were analyzed, using ABAQUS. By assessing damage initiation and shortest fatigue load on screw threads, the SL for 2.5, 5, and 10 lakh cyclic loads were estimated, using fe-safe program. The obtained values were compared for influence of connection design, abutment angulation, and screw length.Results:In straight abutment models, conical connection showed more damage (14.3%–72.3%) when compared to trilobe (10.1%–65.73%) at 2.5, 5, and 10 lakh cycles for both vertical and oblique loads, whereas in angulated abutments, trilobe (16.1%–76.9%) demonstrated more damage compared to conical (13.5%–70%). Irrespective of the connection type and abutment angulation, short screws showed more percentage of damage compared to long screws.Conclusions:The present study suggests selecting appropriate implant-abutment connection based on the abutment angulation, as well as preferring long screws with more number of threads for effective preload retention by the screws.

Vpliv parametrov varjenja in sredstva za spajanje na varjenje kompozitov

This paper presents an experimental investigation of the welding of a glass-fiber-reinforced PP composite. The goals of this paper are to investigate the issues of local changes of the welding strength that depend on the heating time and the coupling agent (MAPP). Composite samples were prepared by using an extruding (for mixing) process and a hot-press method. The PP matrix was reinforced by unidirectional short glass fibers. The welding process for the specimens was carried out using a non-contact heated tool butt welding process. Tensile and fatigue tests were conducted to investigate the effects of the heating time parameter and the coupling agent. The highest weld strength dependent on the heating time was achieved with 94 % relative to the base strength of the material. The fatigue behavior of short-fiber-reinforced thermoplastic composites (polypropylene/20 % of volume fractions of E-glass fiber) is presented in terms of stress versus the number of cycles to failure. The specimens were fatigue tested at various percentages of their static tensile strengths at a load ratio R = 0.1 and frequency f= 5 Hz. An indefinite fatigue life was obtained at 35 % of the static damage initiation load for glass-fiber-reinforced specimens. Then, these specimen’s maximum welding strengths and fatigue properties that were dependent on the heating time were compared.

Strengthening of composite T-joints using novel ply design approaches

This study assesses three different and novel design approaches to increase the failure stress and elastic damage limit of T-shaped structural joints made of carbon fibre-epoxy composite. The ply orientations to the laminate within the T-joint radius bend, which is highly susceptible to delamination failure, were designed using a numerical optimisation method and two design-of-experiment methods to increase the failure load. Finite element modelling and experimental testing proved that all three designs successfully tailored the ply orientations within the radius bend region to increase the delamination initiation load of the T-joint, while maintaining its original stiffness and weight. The new designs resulted in large improvements to the damage initiation load under both bending loading (up to ∼125% increase) and tensile loading (up to ∼85%) compared to a conventional composite T-joint with a quasi-isotropic ply design. The three new designs also increased the elastic damage limit and absorbed strain energy capacity of the T-joint. The improved designs lower and generate a more uniform interlaminar stress field within the radius bend region, and this increases the initiation load for delamination failure.

The effect of composite orientation on the mechanical properties of hybrid joints strengthened by surfi-sculpt

Advanced hybrid joints strengthened by surfi-sculpt (manufactured by electron beam surfi-sculpt (EBS)) have been developed to address the challenges in joining fibre reinforced polymer (FRP) composites to metals. In the present work, the effect of composite orientation on the mechanical properties of the joints is experimentally studied. In the study, the thickness of the composite adherend is kept constant and the volume content of ±45° ply is increased from 11.1% to 88.9%. Joints without surfi-sculpts are manufactured as reference joints. The result indicates that surfi-sculpt is able to delay the damage initiation and improve the joints ultimate failure load, failure strain, and absorbed energy. Composite orientation is able to vary joints mechanical properties significantly. With the volume content of ±45° ply increase, joints damage mechanisms change from bare composite matrix crush to the combination of surfi-sculpts bending, surfi-sculpts breakage and composite compression. With optimum composite orientation, the joints damage initiation load is increased by 24.84%, the joints ultimate failure load is increased by 134.5% and the joint energy absorption is increased by 257.39%. Scanning electron microscope (SEM) observation shows that surfi-sculpts experience three damage stages.

The influence of prosthesis stem design on femoral neck fracture by using finite element analysis

Objective To investigate the influence of prosthesis metaphyseal stem design on femoral neck fracture with a range of implant orientations. Methods A finite element model of the hip resurfacing arthroplasty was reconstructed on the basis of data of a normal femur(from a 63-year-old male)and a 50 mm birmingham hip resurfacing(BHR)prosthesis.And this modeling was used to indicate the effects of prosthesis metaphyseal stem design on femoral neck fracture with a given load of 0-6 k N, with a range of varus-valgus implant orientations. Results The analysis suggested that the intact femoral neck strength could be maintained across a wider range of varusvalgus orientations for short-stemmed and stemless prostheses.The model predicted that the damage initiation load would be decreased by approximately 20% with varus implant orientation and 10% in neutral orientation.Compared with the stemmed, traditional-design prosthesis, the short-stemmed and stemless designs had less weakening effect upon the femoral neck.For the stemless design in all orientations, the femoral neck strength was predicted to be within 5% of that for the intact bone, and the strength with the short-stemmed design was within 3% of the intact case. Conclusion The study suggests that a short-stemmed resurfacing head offers improved tolerance to misalignment over traditional designs.While femoral neck fractures are multifactorial, biomechanical factors are of clear importance to the clinical outcome, so this may reduce the risk for patients at the edge of the indications for hip resurfacing. Key words: Hip resurfacing; Neck fracture; Prosthesis

Experimental Analysis of Debonding of Skin/Stringer Interfaces under Cyclic Loading and Ageing

An experimental investigation aiming to characterize the fatigue failure mechanisms and effect of ageing of skin/stringer interfaces is presented. A simplified specimen known as a “stringer foot specimen” is used. The effects of local design, of the angle of plies located at the interface and of moisture ageing are studied. Among other results, it is shown that a quasi-infinite fatigue life can be obtained under 33% of the static damage initiation load for all designs. In the framework of multi-level analysis, this study is a preliminary investigation to study cyclic buckling of composite stiffened structures.

음향방출법을 응용한 복합재 날개 구조물의 정적구조 건전성 평가

복합재 날개 구조물에 대한 구조 건전성 및 손상을 평가하기 위하여 정적 구조강도시험에 음향방출(AE)법이 응용되었다. 시험중 스트레인과 변위측정기법을 통하여 정적구조강도를 확인하였고, 음향방출요소 분석과 위치표정기법을 통하여 구조물의 내부 손상을 평가하고 손상위치를 찾을 수 있었다. 시험은 설계제한하중시험, 2차에 걸친 설계극한 하중시험, 파단시험이 수행되었다. 주요한 AE신호는 front lug근처의 표면에 부착된 센서에 의하여 감지되었다. 특히 1차 설계극한하중시험에서 스트레인 및 변위결과는 내부 손상을 보이지 않았으나 AE신호는 내부 손상이 이미 형성된 현상을 나타내었다. 파단시험에서는 AE활성도가 매우 활발하였고, 스트레인 및 변위의 결과는 심한 손상에 의하여 하중경로가 바뀌는 경향을 나타내었다. 음향방출법을 적용하여 정적 구조시험이 진행되는 동안 내부손상이 발생되는 하중과 위치를 정확하게 평가할 수 있었다. 본 연구로부터 음향방출법은 정적 구조강도시험에 있어 내부 손상을 평가하는데 유용한 기법임이 확인되었다. AE technique was applied to the static structural test of the composite wing structure to evaluate the structural integrity and damage. During the test, strain and displacements measurement technique were used to figure out for static structural strength. AE parameter analysis and source location technique were used to evaluate the internal damage and find out damage source location. Design limit load test, the 1st and 2nd design ultimate load tests and fracture test were performed. Main AE source was detected by an sensor attached on skin near by front lug. Especially, at the 1st design ultimate test, strain and displacements results didn't show internal damage but AE signal presented a phenomenon that the internal damage was formed. At the fracture test, AE activities were very lively, and strain and displacements results showed a tendency that the load path was changed by severe damage. The internal damage initiation load and location were accurately evaluated during the static structural test using AE technique. It is certified from this paper that AE technique is useful technique for evaluation of internal damage at static structural strength test.

Fatigue and post-fatigue behaviour of carbon/epoxy non-crimp fabric composites

This paper focuses on the static, fatigue and post-fatigue tensile properties of a biaxial carbon/epoxy non-crimp fabric composite. In a series of quasi-static tensile tests, the stress–strain level where damage initiates was determined. This stress level was then used as the maximum stress level in tensile–tensile fatigue tests in the fibre direction. It was found that in fibre direction, this load level can be considered safe for fatigue up to very high cycle numbers. The damage evolution during the tests was monitored at certain cycle times with X-ray radiography. The post-fatigue residual static tensile properties were determined after different numbers of cycles. A series of tensile–tensile fatigue tests at various higher stress levels allowed for the fatigue life curves to be constructed in each of the four testing directions. This revealed that the damage initiation load level is well below the practical fatigue limit of the material.

Failure analysis in postbuckled composite T-sections

Blade-stiffened skin designs made of composite materials have the potential to produce highly efficient structures, when the large strength reserves in the postbuckling range are utilised. This paper investigates the failure under postbuckling deformations of T-section specimens cut from a blade-stiffened panel, by comparing experimental results to finite element models. In the experimental work, T-section specimens with a particular lay-up and geometry were tested to failure in antisymmetric and symmetric loading rigs. These loading rigs simulate deformations on skin-stiffener interfaces during panel postbuckling. For the numerical analysis, two-dimensional models of the interface cross-section were used with a strength-based criterion that monitored failure within each ply. The use of a zero-thickness layer of cohesive elements has also been investigated in order to simulate the delamination behaviour. The numerical predictions are compared to the experimental results in terms of the failure load, specimen stiffness and specimen behaviour. The analysis approach is shown to be capable of predicting the critical damage locations and initiation loads for both antisymmetric and symmetric loadings. The successful prediction of failure in skin-stiffener interfaces can be linked to a global-local approach for efficient analysis of large, fuselage-representative composite structures.

Fatigue failure characterisation of resistance-welded thermoplastic composites skin/stringer joints

An experimental investigation characterising the fatigue failure mechanisms of resistance-welded thermoplastic composites skin/stringer joints is presented. Unidirectional (UD) and quasi-isotropic adherends were welded using stainless steel meshes as heating elements. The specimen geometry consisted of a flange laminate, representing a stringer, welded onto a skin laminate. In order to avoid current leakage to the electrically conductive adherends, a ceramic-coated heating element (TiO 2 HE) was used for welding the UD specimens and some of the quasi-isotropic specimens. The fatigue performance of the welded joints was investigated under three-point bending. An indefinite fatigue life was obtained at 40% and 35% of the static damage initiation load for the UD and quasi-isotropic specimens, respectively. The failure mechanisms were documented based on observation of the fatigue cracks initiation and growth. UD specimens failed at the weld interface while quasi-isotropic specimens showed delaminations both in the flange or skin laminates and at the weld interface. The TiO 2 HE did not show any fatigue mechanical performance reduction. However, debonding at the weld interface was shown to occur between the metal mesh wires and the TiO 2 coating instead of between the laminates and the weld.

Progressive damage arrest mechanism in tailored laminated plates with a cutout

This study addresses the effectiveness of a simple stiffness tailoring concept to delay damage initiation, control damage progression, and improve residual strength in tensile-loaded composite plates with a central circular cutout. The tailoring concept is to simply reposit all axially oriented (0°) material into regions near the edge of the plate away from the cutout. This tailoring is done in a way so as not to affect the weight of the plate. This accomplishes several beneficial changes in the way that the plate resists loading with no increases in material cost or weight. Lowering the axial stiffness of the laminate surrounding the cutout lowers the stress concentration. Increasing the axial stiffness near edges of the plate attracts loading away from the vicinity of the cutout to further lower stresses in the critical cutout region. This study focuses on in-plane response including damage progression and residual strength as a function of the degree of tailoring and cutout size. Strength and stiffness properties typical of IM7/8551-7 preperg material were assumed and a modified version of the Hashin failure criteria was used to identify the local damage. Results show that tailoring can significantly increase the damage initiation load and the residual strength. In some cases, observed evidence shows that tailoring performs as a damage arrest mechanism.

Theoretical and experimental investigation of stress redistribution in open hole composite laminates due to damage accumulation

Two methods were examined for the prediction of stress redistribution due to subcritical damage accumulation near open holes in composite laminates. A finite element analysis in conjunction with the method of material property degradation was performed to assess the accuracy of the fiber direction stress redistribution prediction due to formation of longitudinal splitting. A simple case of a unidirectional composite with an open hole was considered. The method was unable to accurately predict the fiber stress relaxation due to longitudinal splitting, which in the case of laminates with holes is of paramount importance for the accurate prediction of ultimate strength. Three-dimensional ply level modeling of discrete damage near an open hole in a quasi-isotropic composite laminate was subsequently considered. Normally the mesh configuration is dictated by the boundaries of the specimen, such as the presence of a hole, creating formidable difficulties to modeling matrix cracking, which is aligned with the fiber direction in a given ply. A mesh independent displacement discontinuity modeling method based on higher order shape functions was constructed for this purpose. The surface of the displacement jump associated with matrix cracking was defined in terms of the domain Heaviside function approximated by using higher order polynomial B-splines. Moiré interferometry was used to determine the strain and displacement fields in the surface layers of a quasi-isotropic composite, previously prestressed beyond the damage initiation load. Good correlation between the experimental data and the stress redistribution predicted by the mesh independent damage modeling technique was observed.