Open-hole Tensile Test Research Articles

Strain sensing in GFRP via fully integrated carbon nanotube epoxy film sensors

Structural health monitoring of composite structures enables early damage detection to prevent critical component failure. Surface-mounted strain gauges are commonly applied to monitor the integrity of composite structures in highly loaded areas. However, strain gauges can only measure strain on the structure’s surface and are exposed to environmental influences. Within this paper, fully integrated carbon nanotube thin-film sensors for strain and damage sensing in glass fibre reinforced polymers (GFRP) via electrical resistance measurements are presented. Single wall carbon nanotube epoxy thin-films were manufactured using a manual film applicator, partially pre-cured, placed on dry glass fabrics and infused in a resin transfer moulding process. The mechanical properties of the composites and strain sensing capabilities of the integrated sensors were studied for various load cases and different laminate lay-ups. Results of quasi-static and step-wise three-point bending and tensile tests show that the integrated films allow for localised strain measurements in GFRP without significant loss of mechanical properties. Open hole tensile tests proof the ability to monitor local strain and damage in highly loaded areas enabling failure prediction via threshold resistance change values. The proposed thin-films enable a tailored strain and damage monitoring of GFRP offering the possibility for measurements at different material depths, over large sections or selectively in highly loaded areas. The manufacturing process is easily automatable and suitable for large scale manufacturing.

A pragmatic orthotropic elasticity-based damage model with spatially distributed properties applied to short glass-fibre reinforced polymers

This article presents a simple progressive damage model for quasi-brittle materials, combining orthotropic elasticity with a scalar damage model including spatial variation of the damage initiation strain and the crack band method for softening regularization. The model’s performance is first analyzed from a numerical point of view and then demonstrated for tensile tests (0°, 45° and 90°), open-hole tensile tests (0°) and three-point bending (0° and 90°) tests of short fibre-reinforced polypropylene with 15 wt.% and 30 wt.% glass fibres. Despite its simplicity, the model captures the anisotropic elastic and inelastic behaviour observed in experiments. The model is applicable for orthotropic brittle or quasi-brittle materials, where the variability in elastic properties is negligible and the orientation dependency of the fracture strain is small or not relevant for the application.

Influence of Shelf Life on Mechanical Properties of Glass Fibre Reinforced Composites

The present paper was focused on studying the influence of shelf life of an epoxy matrix on the mechanical properties of glass fibre reinforced composites. For the study, two types of the same epoxy system were used, one during its shelf life and one out of its shelf life. The reinforcement used consisted in E-glass fibre fabric. Mechanical investigations were realized in order to compare the materials in terms of loss of mechanical strength and elastic properties. Therefore, three mechanical tests were performed: tensile tests, in-plane shear and open-hole tensile tests. The results showed that the shelf life affects the mechanical properties of the polymeric composite. A decrease of 24% in tensile strength was recorded along with a 28% decrease of the in-plane shear strength and 55% of open-hole tensile strength for the composite manufactured with the out of shelf life epoxy system compared with the other composite. An overall reduction of mechanical strength and elastic properties of the composite material was observed, primarily due to polymeric matrix degradation, which after long periods it could be prone to brittleness and susceptible to delamination and fracture. The thermogravimetric analysis showed that thermal induced changes are happening at a higher speed in the out of shelf life composite, a lower mass loss being registered for new epoxy composite.

Analysis of process-induced damage in remote laser cut carbon fibre reinforced polymers

In this paper, an approach is presented that allows for a linkage between cutting process induced damage and the mechanical properties of the machined structures. For carbon fibre reinforced polymers, the milling and remote laser cutting processes are analysed. Open hole tensile test specimens, that are either milled or remote laser cut with three different cutting parameter configurations are tested. With a two-dimensional heat conduction simulation, the temperature field resulting from laser cutting is determined and thus the thermally induced damage is quantified. Those results are compared to micro-sections. The following structural analysis is based on an anisotropic damage model and is taking the thermal pre-damage into account. For this purpose two different thermal damage modelling approaches, based on damage variables and material parameter reduction, are compared. The influence of the cutting process on the structural properties is determined and compared with experimental results.

Unhole and open hole tensile properties of hybrid Kevlar/glass fiber polymer composites with different stacking sequence

The effects of stacking sequence play a vital role in determining the mechanical properties of fiber-reinforced polymer composite materials. This study was conducted to investigate the effects of a hole and unhole Kevlar/glass fiber hybrid composite specimens at four different stacking sequences. Other than stacking sequence, the damage tolerance properties were also analyzed in this study to find out the strength reduction between the hole specimen and the unhole specimens. The damage fractured on the surface of the specimens was also analyzed through macroscopic observation and scanning electron microscope. All the specimens in this study were fabricated using vacuum bagging method according to the ASTM D5766 for open-hole tensile test. In the open-hole tensile test, unhole and hole hybrid composite specimen (H4) with alternate layered fiber, projected the highest value of tensile properties. The alternate layered fiber can improve the interlaminar adhesion; thus, increase the tensile strength as compared to the sandwich stacking sequence (H2 and H3). Based on this study, GFRP and hybrid composite specimens displayed approximately 40%-50% strength reduction, while KFRP recorded only 30% of strength reduction. From the damage observation, the failure characteristics change from fiber pull out in glass fiber composite to fiber breakage in Kevlar fiber composite specimens.

A new interface element connecting 3D finite elements with non-coincident nodes to simulate delamination in composite laminates

Composite failure phenomena remain complex and lead to over-size structures in many industries, involving long and costly experimental campaigns. Numerical approaches are a good alternative to decrease sizing costs. The Discrete Ply Model developed in Institut Clément Ader over these last ten years shows good results in simulation of impact and compression after impact of composite laminates, ply drop-offs, open hole tensile tests… But the present approach leads to certain limitations for complex stacking sequences and induces unwanted characteristic lengths between consecutive plies. A user element for delamination calculation is developed that allows to define an interface corresponding to the overlapping zone between the two volume elements of upper and lower plies, whereas this overlapping zone is not directly defined by nodes of the mesh (volume elements have non-coincident nodes). The present paper details the new interface element implementation, its validation from DCB and ENF test simulations, and then its improvement is discussed.

Strength improvement of bolted joints in composite materials by use of patched metal inserts

Metal inserts are occasionally used to improve bearing load capacity of bolted joints in laminated composite materials. This paper investigates a new reinforcement concept where inserts are built by locally replacing composite plies with metal patches of various diameters, surrounding the holes. The inserts are built during composite manufacturing by alternately placing the metal patches through the thickness of the laminate at locations where holes are to be drilled after consolidation. An extensive experimental study including pin-loaded, open–hole tensile, and single-shear testing of bolted specimens is presented. Considerable improvement of the bearing strength – 50-60% – is attained for pin-loaded specimens with inserts, demonstrating the potential of the reinforcement concept. The open–hole tensile tests show that the by-pass strength can be maintained or even improved with up to 20% if the inserts are properly designed. Finally, the results from the single-shear tests of bolted joints show more than 25% improvement in strength for reinforced single- and double-bolt specimens. It is possible that the inserts would maintain clamping pressure over time, which could then almost double the imrovement (47%) for bolted joints.

Open hole tensile testing of 3D printed continuous carbon fiber reinforced composites

In this study, the effects of stress concentration on the tensile properties of a 3D printed carbon fiber-nylon composite were investigated. The samples were 3D printed with continuous carbon fiber and chopped fiber reinforced nylon. Samples were manufactured with four different open hole sizes as 3. 175 mm (⅛ in), 6.35 mm (¼ in), 9.25 mm (⅜ in), and 12.7 mm (½ in). Five samples were manufactured for each hole size group. Continuous carbon fibers were printed in the longitudinal direction. Additional reinforcements were placed around the periphery of the open hole. Samples were tested under uniaxial tension. The results were compared with the prediction of fracture mechanics theories namely Average and Point Stress Criteria. The results show that failure was initiated at the stress concentration region but the progression into the hole was prevented with the presence of continuous fiber. The experimental findings show that the samples with larger holes are more sensitive to discontinuity than the ones with smaller holes. The results confirm that 3D printing can be used to strengthen the parts at the discontinuity region to mitigate the effect of stress concentration.

Damage to carbon fibre reinforced polymers (CFRP) in hole-clinched joints with aluminium alloy and CFRP

This study aimed to investigate the damage to carbon fibre reinforced polymers (CFRP) in hole-clinched joints composed of CFRP and aluminium alloy 5754. A continuous damage model (CDM) based on the modified Hashin failure criterion was proposed to predict the mechanical behaviour and damage evolution of the CFRP, and the CDM was validated using open hole tensile tests. The effect of the CFRP hole radius on the joint quality was investigated to obtain the optimal joining conditions. Furthermore, a numerical model for the hole-clinching process was developed to understand the forming mechanism of the joint and the damage patterns in the CFRP. The CFRP damage patterns with different ply angles in the hole clinching process were evaluated using numerical simulations. The proposed CDM can predict the mechanical behaviour and complex damage modes of CFRP. An increased hole radius generally corresponds to an increased neck thickness and a decreased undercut of the joints. Hole clinching cannot effectively join CFRP with a 0° ply, and delamination is most severe in joints with a complex angle ply.

Open hole tensile testing of 3D printed parts using in-house fabricated PLA filament

PurposeFused deposition modeling (FDM) is being increasingly used in automotive and aerospace industries because of its ability to produce specimens having difficult geometrical shape. However, owing to lack of critical information regarding the reliability and mechanical properties of FDM-printed parts at various designs, the use of 3D printed parts in these industries is limited. Therefore, the purpose of this paper is to investigate the impact of process parameters of FDM on the tensile strength of open-hole specimen printed using in-house-fabricated polylactic acid (PLA).Design/methodology/approachIn the present study, three process parameters, namely, raster angle, layer thickness and raster width, are selected for investigation of tensile strength. To produce the tensile specimens in the FDM machine, the PLA filament is used which is fabricated from PLA granules using a single-screw extruder. Further, the experimental values are measured and critically analysed. Failure modes under tests are studied using scanning electron microscopy (SEM).FindingsResults indicate that the raster angle has a significant effect on the tensile strength of open-hole tensile specimen. Specimens built with 0° raster angle, 200-µm layer thickness and 500-µm raster width obtained maximum tensile strength.Originality/valueIn this work, a new concept of testing a plate that has a rectangular shape and a circular hole at the centre is tested. Open-hole tensile test standard ASTM D5766 has been implemented for the first time for the FDM process.

Peridynamic Open-Hole Tensile Strength Prediction of Fiber-Reinforced Composite Laminate Using Energy-Based Failure Criteria

In the present study, peridynamic (PD) open-hole tensile (OHT) strength prediction of fiber-reinforced composite laminate using energy-based failure criteria is conducted. Spherical-horizon peridynamic laminate theory (PDLT) model is used. Energy-based failure criteria are introduced into the model. Delamination fracture modes can be distinguished in the present energy-based failure criteria. Three OHT testing results of fiber-reinforced composite laminate are chosen from literatures and used as benchmarks to validate the present PD composite model with energy-based failure criteria. It is shown that the PD predicted OHT strength fits the experimental results quite well. From the predicted displacement field, the fracture surface can be clearly detected. Typical damage modes of composite, fiber breakage, matrix crack, and delamination, are also illustrated in detail for each specimen. Numerical results in the present study validate the accuracy and reliability of the present PD composite model with energy-based failure criteria.

Open hole tensile tests for the determination of the edge-crack sensitivity of sheared holes dependent on specimen geometry, cutting parameters, and the notch factor

Abstract Taking into account current trends in the automotive industry, one can see that components having a high functional integration in combination with a high material strength are integrated into the body shell in order to reduce the total weight. Regardless of the type of sheet metal component to be produced, a shear cutting process is involved in almost every case. Separation with unsuitable parameters leads to the risk that sheared component edges will fail during the forming process due to edge-cracks. Steel and automotive manufacturers as well as scientific institutes have developed a variety of non-standardized test methods to identify the edge-crack sensitivity of high-strength steels. The only standardized test set-up for identifying the edge-crack sensitivity of sheared edges, ISO16630, proved to be unsuitable due to the scatter of results and induced friction. An alternative method could be the Open Hole Tensile Test, which was developed for fiber composites testing. This method makes it possible to classify steels with regard to their edge-crack sensitivity depending on the machining method and the notch factor of the initial holes. The investigations described in this paper show that the specimen geometry and the process parameters used in the shearing process significantly influence the edge-crack sensitivity. For the identification of the respective influences the logarithmic major strain at the beginning of local necking is used. The characteristics are related to the cut-surface characteristics of the shear-cut starting holes.

Exploration of the design freedom of 3D printed continuous fibre-reinforced polymers in open-hole tensile strength tests

In the current work, the design freedom of 3D printed composite parts reinforced with continuous fibres is explored. The placement of continuous fibres to reduce stress concentrations around the hole is exploited in open-hole tests. Distinct counterintuitive performance in tensile strength is observed. The expected gain in ultimate tensile strength by adding contour reinforcement around the hole could only be observed for a rectangular reinforcement. Digital Image Correlation monitoring of the strain pattern demonstrated promising performance in strain reduction and the ability to locate the damage patterns outside the regions prone to stress concentrations (i.e. the hole).

Matrix-graded and fibre-steered composites to tackle stress concentrations

This paper studies the feasibility of improving structural performance of composites in the presence of stress concentrators. Matrix grading through local deposition of additive-enhanced matrices and fibre steering by varying fibrous architecture are examined independently and in combination on a glass-fibre triaxial braided composite subjected to open hole tensile test. Stiffened and toughened matrices were incorporated through precise point-wise injections of liquid reactive resin into dry preforms (Liquid Resin Printing). Fibre steering was implemented by varying the braiding angle along the length of the braided sleeve. It has been shown that these novel forms of architecture modification enable a significant improvement in composite strength through a variety of deformation mechanisms. This includes local stiffening of composite in the direct vicinity of the stress concentrator and damage accumulation away from the stress concentrators. The experimental observations are explained by using simple finite-element models.

Towards aerospace grade thin-ply composites: Effect of ply thickness, fibre, matrix and interlayer toughening on strength and damage tolerance

Thin-ply composites represent a promising approach to further improve the performance of carbon fibre composite structures thanks to their ability to delay the onset of matrix cracking and delamination up to the point of fibre dominated failure. However, this increased strength comes with a more brittle failure response which raises concerns on damage tolerance. Thus a careful material optimization is needed to address this trade-off. In this work, eight different formulations of thin-ply composites ranging from low modulus to high modulus carbon fibres are evaluated to understand the effects of the fibre and matrix constituents on the onset of damage and strength in unnotched tensile (UNT) tests of quasi isotropic laminates for ply thicknesses between 300 and 30 microns. The obtained experimental data are combined in master curve diagrams for simplified material selection process. It is observed that certain thin-ply composites with a ply thickness t < 134 μm can reach UNT strength corresponding to or approaching the ultimate strain of the fibres as well as UNT stress at onset of damage as high as 92% of the latter. Based on this knowledge, a novel aerospace grade toughened thin-ply composite system is developed which can reach a quasi-isotropic UNT strength above 1 GPa (>95% of the fibre strain). The newly developed composite is further optimized to improve damage tolerance by toughening the resin and selected interfaces. The effect of those modifications on damage tolerance are evaluated through compression strength after impact (CAI) tests and open hole tensile tests (OHT). It is found that an optimized interlayer toughened thin-ply composite based on 68 microns plies of intermediate modulus fibre can reach both outstanding strength properties with comparable or better CAI and OHT strength compared to current aerospace grade composites.

A study on the influence of drilling and CO2 laser cutting in carbon/epoxy laminates

Composite structure made of thermoset matrix often requires a circular hole to join with a steel or aluminium. Circular holes are introduced into the carbon/epoxy laminates by means of drilling and continuous wave (cw) CO2 laser. In this study, the quality of circular hole produced by drilling and laser processes was investigated by means of delamination factor and cone angle respectively. This paper also identifies the optimum drilling and laser machining parameters to make a hole in carbon/epoxy laminates. After identifying the optimum parameters, open hole tensile test was performed on the notched specimens. Moreover, finally a comparison with the help of finite element analysis for further understanding the stress concentration around the circular hole is presented.

Open Hole Tensile Behaviour of Nano Fillers (SiC & Banana) in CNSL/Epoxy Resin Reinforeced with Basalt fiber

In recent years Basalt Fiber Reinforced Polymers (BFRP) are used as a better alternative for Glass Fiber Reinforced Polymers (GFRP). This paper deals with study of standard tensile test and open hole tensile test to understand the maximum strength of composite. The aim of this paper is to fabricate a composites where the CNSL & Epoxy resin mixed with organic (SiCp) and inorganic (Banana fiber) nano fillers is used as matrix material reinforced with basalt fiber by hand layup technique. The open hole tensile test is conducted as per the ASTM D5766 standard to understand the effect of nano-fillers on tensile properties of the composites. The results indicate that the SiC Nano-filler reinforced composites have the better performance than Banana Nano-fillers reinforced composites.

Effect of Needle Punching Process on a Chopped Strand Mat Composite with an Open Hole

The easiest and most reliable joining method is the mechanical joint with a bolt and nut or rivet. However, in the case of composite laminates, mechanical joint properties decrease because of lower interlaminar properties compared to in-plane properties around hole. This study investigated needle punching process with the aim of improving the mechanical properties in the thickness direction of fiber-reinforced plastic composite laminates with an open hole. Needle punching process was applied to glass fiber chopped strand matused as the reinforcement for the composite laminates. Open-hole tensile tests and observations of end cross-sections after the tests were performed. The tensile properties and fracture mechanism of the specimens subjected to needle punching process were investigated. In addition, characteristic distance (a parameter for evaluating resistance to fracture in open-hole tensile test specimens) was also calculated to examine the effects of needle punching process conditions on fracture toughness. Tensile strength was improved by more than 15% by needle punching process. However, when a certain needle punching density was exceeded, the mechanical properties worsened. In addition, characteristic distance increased with increasing needle punching density. Thus, these results suggest that there is an optimal needle punching density with respect to strength and characteristic distance.

A blended continuum damage and fracture mechanics method for progressive damage analysis of composite structures using XFEM

Progressive damage analysis of composite structures remains problematic, holding back the full potential of these materials. Widely used continuum damage models feature a heuristical stiffness reduction to reflect damage, resulting in an unrealistic representation of damage patterns. To the end of a more realistic failure representation, this paper proposes a blended methodology for progressive damage analysis of such structures implemented in ABAQUS, combining continuum damage models with a more physically based approach from a fracture mechanics perspective. Matrix cracks are modelled through the eXtended Finite Element Method and delaminations through a cohesive zone model. Validation of the blend on an experimental campaign of open-hole tensile tests shows remarkable predictive capability, in good conformance to experimental failure loads, digital image correlation and acoustic emission measurements - particularly yielding more realistic damage patterns than state-of-the-art continuum damage model implementations.

Experimental and numerical analysis of Carbon Fiber Reinforced Polymer notched coupons under tensile loading

The behavior of composite laminates subject to notch based stress concentrations is difficult to apprehend, especially the mechanisms of damage progression leading to total failure. Numerical and experimental investigations were carried out on three different stacking sequences of notched, thin ply carbon/epoxy laminates. This paper presents a computational study of notched tensile tests (U-notch) using the Discrete Ply Modeling (DPM) method, which has already proved efficient on both in-plane and out-of-plane loading cases, such as pull through, low velocity impact and compression after impact. The specificities of this finite element model are its discrete nature (interface elements to model delamination and matrix cracks), the small number of parameters required, and its robustness. This work follows on from the study of open-hole tensile tests (same three layups) by the same authors [1] and analyzes the influence of layup and notch shape. Comparisons with experiments (using infrared technology) demonstrate that tensile strengths, and failure scenarios and patterns are predicted with acceptable accuracy.