Damage Tolerance Characteristics Research Articles

A study on the damage tolerance of durable redundant composite sandwich joints

A patented Durable Redundant Joint (DRJ) concept featuring multiple adhesive load-paths, via a novel composite preform insert, is being considered for joining composite sandwich panel segments. Applications are wide ranging from spacecraft bus structure to launch vehicle primary structure (i.e. interstage cylinders and payload fairings). Numerical and experimental investigations were performed to assess the DRJ’s performance. A fracture-based approach was used to evaluate this design. Ply-level mechanical properties for the material systems were generated, and a double cantilever beam coupon was designed and tested to estimate the Mode I critical energy release rate for the interface between two different composite prepreg material systems. The DRJ was tested and compared to testing of the more conventional splice joint (CSJ) design. Polytetrafluoroethylene (PTFE) inserts were used at the free edges of the joints to simulate debonds between the doubler and facesheet laminates. The DRJ coupons reached 32% greater in-plane tensile failure load compared to the CSJ coupons. Furthermore, the predicted increased strength for the DRJ design compared to the CSJ design was in remarkably good agreement with test data. Using the insight gained from these studies, three design attributes were investigated relative to increasing damage tolerance characteristics of the DRJ: (1) Insert stiffness, (2) Relative length between the doubler and the insert, and (3) The use of tapers at the ends of doublers.

Effects of impactor mass on the low-velocity impact behaviour of thick GFRP pultruded laminates

This paper reports on the results of an experimental study on glass fibre reinforced polymer (GFRP) pultruded profiles subjected to low-velocity impact loading. The effects of impactor mass on the impact response of pultruded composites were investigated. Composite specimens were tested under two series of loading conditions using a drop-weight impact facility. The first set of tests was conducted with different impactor masses (i.e. 5.5, 10.5, 15.5 and 20.5 kg) under the same impact energy of 67 J. The specimens in the second set were subjected to varying impactor masses of 6, 12, 24 and 36 kg at the same initial velocity of 4.4 m/s, resulting in different impact energies. The internal and external damage of pultruded composites was analysed to compare the failure mechanisms, including matrix cracking, delamination, and fibre breakage, as functions of the initial impact energy. The results showed that the extent of impact-induced damage increased with ascending impact energies (ascending impactor masses) up to the point at which the specimen was perforated. The initial impact energy was the dominant parameter that controlled the damage tolerance characteristics of pultruded composites. Furthermore, it was found that a significant longitudinal crack was a precursor to ultimate failure.

Damage Tolerance Capability of Retrogression and Re-aged 7010 Aluminum Alloy Under FALSTAFF Loading

The present work deals with the damage tolerance characteristics of high strength aluminum alloy tempered in T6 and reversion condition. The fatigue experiments were carried out by applying a service simulating load spectrum, i.e., standard mini FALSTAFF loading. The crack propagation speed was found to be lower and the total crack propagation life was longer by 22% for reversion-treated alloy. The crack growth was also predicted to be using two parameter crack driving force approach. The fatigue data of these treated alloys under constant amplitude loading at various stress ratios were analyzed to obtain crack growth law. The predicted crack growth behavior was conservative and followed similar trend in both the alloys as observed in experiments. Predicted results of reversion-treated alloy also showed longer crack growth life. The modified microstructure after reversion treatment was attributed for the observed improvement in damage tolerance capability.

Comparative Evaluation of Creep-Rupture Behavior of P9 Steel Plate and Thick Section Tubeplate Forging

In the present investigation, creep-rupture behavior of P9 steel in two different product forms of 20-mm plate and 300-mm-thick tubeplate forging has been studied at 793 and 873 K. It has been found that steady-state creep rate and rupture life followed power law dependence on applied stress at low and high stress regimes for both forms of products. At high stress regime, P9 steel plate exhibited better creep strength with respect to tubeplate forging in terms of lower creep rate and higher rupture life at 793 and 873 K. On contrary, both product forms exhibited similar creep rate and rupture behavior in the low stress regime at 873 K. Difference in creep ductility has been found to be insignificant in both product forms. Irrespective of test conditions and product forms, fracture appearance remained transgranular ductile characterized by dimples. The comparative evaluation of creep-rupture properties of both product forms has also been described in terms of creep rate-rupture life relationships of Monkman–Grant type, creep damage tolerance and tertiary creep characteristics at 793 and 873 K.

Reinforcement of CFRP joints with fibre metal laminates and additional adhesive layers

The use of fibre metal laminates (FMLs) offers significant improvements over more traditional materials applied in aircraft structures, such as metallic alloys. The use of this type of materials offers weight reduction, improved damage tolerance characteristics and enhanced safety due to a synergetic combination of the advantageous properties of both composites and metallic alloys.The main objective of this work was to study the effect of reinforcing a basic carbon fibre reinforced polymer (CFRP) joint with titanium laminates and/or with additional adhesive layers in the interfaces between titanium and composite, following different lay-up configurations. The lay-up configuration that led to the best results in terms of failure mode and failure load, was found to be the configuration using titanium laminates on a basic CFRP substrate and additional layers of film adhesive in between the metal laminate and the CFRP. A numerical model using finite element analysis with cohesive zone elements was also developed, with the aim of studying the performance of the different proposed configurations, correlate the models with the experimental results, and to aid the identification of the optimal material to use for reinforcement of the CFRP adherend.

Precast Ultra-High-Performance Fiber-Reinforced Concrete (UHP-FRC) for Fast and Sustainable Pavement Repair

This paper presents a new methodology, which enables streets, roads, highways, bridges, and airfields to use an advanced fiber-reinforced concrete material, which can delay or prevent the deterioration of these transportation infrastructure when subjected to traffic and environmental loadings. The major problem of concrete is its considerable deterioration and limited service life due to its brittleness and limited durability. As a result, it requires frequent repair and eventual replacement, which consumes more natural resources. Ultra-high-performance fiber-reinforced concrete (UHP-FRC) introduces significant enhancement in the sustainability of concrete structures due to its dense microstructure and damage-tolerance characteristics. These characteristics can significantly reduce the amount of repair, rehabilitation, and maintenance work, thereby giving the transportation infrastructure a longer service life. This research addresses the strong need to develop fast and sustainable UHP-FRC materials for pavement repair that can be easily cast onsite without special treatments. This avoids any major changes to current concrete production practice and accelerates the use of UHP-FRC materials. This research investigated a new method for concrete repair by combining precast UHP-FRC panels with a small quantity of cast-in-place UHP-FRC for pavement repair without any dowel bars.

Tailoring of AP-PLY composite laminates for improved performance in the presence of delaminations

A fibre placement architecture, Advanced Placed Ply (AP-PLY), has been shown to give significantly improved damage tolerance characteristics of composite structures. The behavior of delaminations resulting from low speed impact damage is of particular concern. Emphasis is placed on using developed methodology to expand current knowledge on the delamination response and optimization of simple AP-PLY composite structure. A proposed energy method to analyze the strain energy release rate (SERR) of an existing crack in a composite laminates, was applied to an AP-PLY beam structure. A step by step approach was developed to obtain the SERR of a crack that grows in regions of variable stiffness. The analytical SERR determined when two materials are used in sequence, sets the stage for optimization of AP-PLY composite laminates to tailor stiffness and minimize SERR. The results show promise in providing designs where the stiffness is altered so that delamination growth is delayed.

Effect of Embedded Strain Gage on the Mechanical Behavior of Composite Structures

Fiber reinforced composites are increasingly used in several fields such as aeronautics and civil engineering due to their increased strength, durability, corrosion resistance, resistance to fatigue and damage tolerance characteristics. The embedding of sensor networks into such composite structures can be achieved. In the present study, glass fiber reinforced Epoxy composite with integrated strain gage was analysed. Firstly, the mechanical behaviour of this material with embedded strain gage is investigated. The as-prepared samples have been tested under tensile and flexural loading in order to study the effects of the strain gage embedding on the structural stiffness and strength of the composite. It was found that the tensile stiffness decreases by 5.8% and the tensile strength decrease by 1.5% when the strain gage embedded in the material. On the other hand, the flexural strength and stiffness is increased, respectively, by 1.5% and 5.5% with an embedded strain gage. The experiments showed that embedded strain gage is functional and demonstrated the successful integration of sensor networks into composite parts. The obtained results confirm that integrated strain gage can be used for the Structural Health Monitoring (SHM) of glass fiber reinforced Epoxy composite.

Prediction of delamination onset and growth for AP-PLY composite laminates using the finite element method

A recently developed fibre placement architecture, Advanced Placed Ply (AP-PLY), has been shown to have promising damage tolerance characteristics for composite structures. Understanding delamination creation and growth in such architecture is of particular concern. Approaches to predict delamination onset in AP-PLY architectures and to analyse delamination growth are presented. The recovered interlaminar stresses between layers combined with a maximum stress criterion were used to determine delamination onset of simple AP-PLY composite laminates under out-of-plane loads. 2D finite element models with cohesive elements inserted at the interfaces of woven layers were used to evaluate the delamination initiation and propagation in different woven patterns of simple AP-PLY composite beams. The predictions were compared to results from the stress recovery method and to test results. The parameters of the woven pattern, such as woven angle, number of woven plies, number of straight filling plies, and the location of the woven patterns through the thickness direction, were investigated and shown to have a significant effect on delamination creation and growth.

Effects of bolt torque tightening on the strength and fatigue life of airframe FRP laminate bolted joints

The experimental study presented herein, investigated the effects of bolt torque tightening on the strength and fatigue design of bolted AS7/8552 fibre reinforced polymer laminates. Damage initiation and final failure manifestation on the joints was investigated and presented using optical microscopy. Subsequent experimental result analysis explored the application domain of bolted joints within the airframe design sector, bound by the current airworthiness certification requirements and expected airframe design life. The reasons for the static strength of the joint laminates or the fatigue failure of the bolt being the main design drivers for the tested joints were highlighted. The study concluded with comments and suggestions on the application of bolt torque tightening in relation to the strength, fatigue life and damage tolerance characteristics of joints on similar fibre reinforced polymer laminate composite material systems.

Experimental investigation of reinforced bonded joints for composite laminates.

An experimental study has been carried out to investigate the behaviour of co-bonded carbon fibre reinforced plastics joints with a novel design incorporating a through the thickness local reinforcement. Different specimens were manufactured to investigate static and fatigue behaviour, as well as delamination size after impact and damage tolerance characteristics. The mechanical performances of the specimens with local reinforcement, consisting of the insertion of spiked thin metal sheets between co-bonded laminates, were compared with those ones obtained from specimens with purely co-bonded joints. This novel design demonstrated by tests that damage progression under cycling load results significantly delayed by the reinforcements. A significant number of experimental results were obtained that can be used to define preliminary design guidelines.

Analytical formulation of a criterion for adiabatic shear failure

Numerical modeling and prediction of adiabatic shear localization require a criterion that is both robust and simple enough so that it can be implemented into numerical codes. This criterion should have a minimum number of adjustable parameters, each of which being measurable, with sufficient generality to avoid adjustments for each new investigated case.This paper presents a detailed analytical study of such a criterion formulated in terms of plastic strain energy density. The criterion contains 3 physical parameters: an exponent b and two energy parameters corresponding to the initiation of the shear band and its final failure. It describes both the initiation and the propagation of the damage (shear band) in the dynamically loaded solid, while b represents the damage tolerance characteristics of the material. We show that the general stress–strain relationship decays exponentially (or close to) with ongoing damage, which allows attainment of very large strains in the shear band. A detailed procedure is reported for the experimental identification of each of the above-mentioned parameters, for any value of the exponent b.The criterion was previously used for dynamic shear localization simulations on empirical grounds [1], that are now firmly established for the first time through a rigorous analysis, including systematic parameters' identification.

Tensile Property Evaluation of Woven Glass Fiber Reinforced Plastic and Aluminium Stack

The desire of weight reduction and improved damage tolerance characteristics of the aircraft structures throws a light on the development on Fiber Metal Laminates (FML), one of the hybrid composites, with the combination of metallic and non-metallic layers. In this study, laminates of alternating layers of aluminium (metal) and glass fibers with Woven Roving mat is fabricated. Tensile test based on ASTM standard are then conducted on the laminates to study their yield properties. The interfacial bonding between the layers are analyzed using the Scanning Electron Microscopy of tested specimens.

An experimental investigation into the mechanical behaviour of 3D woven fabrics for structural composites

3D woven composites provide efficient delamination suppression, enhanced damage tolerance, superior impact, ballistic and blast performance characteristics over 2D fabric laminates. At the same time, this type of composites, having practically straight in-plane fibers, show significantly better in-plane stiffness and strength properties than respective properties of conventional composites. But, how the in-plane elastic and strength characteristics of this type of fabrics compare with respective in-plane properties of equivalent 2D woven fabrics. This paper presents a comprehensive experimental study of the comparison of in-plane tensile, bending, crimp interchange properties of UD, 2D, 3D orthogonal, 3D angle-interlock and 3D warp interlock multi-layer structures woven from E-glass tow. The results depict that the 3D woven fabrics have considerably superior mechanical properties with much lesser crimp compared to 2D fabrics.

Compression after impact strength of out-of-autoclave processed laminates

Out-of-autoclave processable fiber/resin systems have been gaining much attention due to the elimination of needing a costly autoclave large enough to hold the part to be cured. For large composite structures this can pose a challenge. However, for these fiber/resin systems to replace conventional autoclave fiber/resin systems for use on aerospace structures, the damage tolerance capabilities need to meet (or exceed) those of current autoclave fiber/resin systems. In this experimental study, compression-after-impact strengths of two commercially available out-of-autoclave fiber/resin systems are compared to compression-after-impact strengths of a conventional autoclave fiber/resin system used as a baseline in this study. compression-after-impact testing was chosen since this is the most common method to assess laminates damage tolerance capabilities. Three different levels of impact severity were chosen and information on damage size and morphology are assessed along with compression-after-impact strength values. The results show the two out-of-autoclave fiber/resin systems examined in this study have similar damage tolerance characteristics to the autoclave fiber/resin system used in this study.

Change in damage tolerance characteristics of sandwich structure with a Thermal Protection System (TPS)

Most composite damage tolerance assessments are made on bare laminates where the impactor comes into direct contact with the outermost ply. However, structures such as those used on launch vehicles are often covered with a thermal protection system (TPS) during the majority of the life of the part. This TPS covering may change the impact characteristics of the laminate rendering damage tolerance testing on bare laminates irrelevant to the part. This study examines the composite interstage structure of the ARES I launch vehicle which is scheduled to be covered with a sprayable foam TPS after manufacture. Damage tolerance testing is performed on bare sandwich structure and sandwich structure covered with the TPS selected for use on the ARES I composite interstage. Instrumented impact, infrared thermography, visual, cross-sectional and compression after impact CAI data are compared. Results show that the TPS covering does change most of the impact characteristics of the sandwich structure. It was found that the TPS created a larger damage zone as detected by IRT, however the TPS covered specimens possessed a higher residual compression strength for a given impact energy and damage size. These results are attempted to be explained by the different damage morphology that occurs between the bare and TPS covered specimens.

Notch Effect on Discontinuous Fiber Reinforced Thermoplastic Composites

Increasing utilization of thermoplastic composites in the structural application necessitates understanding of damage tolerance characteristics. In this work, unreinforced, 20 % short, 20 % long glass fiber reinforced polypropylene were injection molded and considered. Test specimens with different notch sizes were tested under static as well as fatigue loading conditions. Under static load condition, short fiber reinforced and unreinforced test material exhibited notch strengthening effect; whereas long fiber reinforced material exhibited notch weakening effect. Failure morphology under fatigue condition exhibited the influence of notch size and length of reinforced fibers over performance. Significant difference between notched and unnotched specimens is observed at low cycle fatigue and very less difference in performance is observed at high cycle fatigue condition.

Fracture toughness and fatigue crack growth characteristics of nanotwinned copper

Recent studies have shown that nanotwinned copper (NT Cu) exhibits a combination of high strength and moderate ductility. However, most engineering and structural applications would also require materials to have superior fracture toughness and prolonged subcritical fatigue crack growth life. The current study investigates the effect of twin density on the crack initiation toughness and stable fatigue crack propagation characteristics of NT Cu. Specifically, we examine the effects of tailored density of nanotwins, incorporated into a fixed grain size of ultrafine-grained (UFG) copper with an average grain size of 450 nm, on the onset and progression of subcritical fracture under quasi-static and cyclic loading at room temperature. We show here that processing-induced, initially coherent nanoscale twins in UFG copper lead to a noticeable improvement in damage tolerance under conditions of plane stress. This work strongly suggests that an increase in twin density, at a fixed grain size, is beneficial not only for desirable combinations of strength and ductility but also for enhancing damage tolerance characteristics such as fracture toughness, threshold stress intensity factor range for fatigue fracture and subcritical fatigue crack growth life. Possible mechanistic origins of these trends are discussed, along with issues and challenges in the study of damage tolerance in NT Cu.

Exploring the Use of Cork Based Composites for Aerospace Applications

Aerospace components are characterized by having high strength to weight ratios in order to obtain lightweight structures. Recently, different types of sandwich components using composite materials have been developed with the purpose of combining the effect of reinforced face-sheets with low weight core materials, such as honeycombs and foams. However, these materials must combine damage tolerance characteristics with high resistance under both static and dynamic loads. Cork composites can be considered as an alternative material for sandwich components since cork is a natural material with some remarkable properties, such as high damage tolerance to impact loads, good thermal and acoustic insulation capacities and excellent damping characteristics for the suppression of vibrations. The experiments carried out in this investigation were oriented in order to optimize the specific strength of cork based composites for sandwich components. Static bending tests were performed in order to characterize the mechanical strength of different types of cork agglomerates which were obtained considering distinct production variables. The ability to withstand dynamic loads was also evaluated from a set of impact tests using carbon-cork sandwich specimens. The results from experimental tests showed that cork agglomerates performance depends on the cork granulate size, the type of reinforcing elements and the bonding procedure used for the cohesion with the matrix material.

Evaluation of Low Cost Titanium Alloy Products

Titanium has extremely attractive properties for air vehicles ranging from excellent corrosion resistance to good compatibility with graphite reinforced composites and very good damage tolerance characteristics. At current Buy to Fly ratios, the F-35 Program will consume as much as seven million pounds of titanium a year at rate production. This figure is nearly double that of the F-22 Program, which has a much higher titanium content. Lockheed Martin has initiated “Project Black Ti” to reduce the cost of titanium parts by reducing the titanium consumption but not the quantity of titanium parts. Ultimately, we want to reduce the inherent waste in the current processing of titanium alloy products. The Kroll process, by which most titanium product is made today, is nearly 60 years old. Kroll himself predicted the process would be replaced within 15 years due to inherent inefficiencies – in 1950. Titanium is also mis-characterized as a precious metal, which it is not. It is the ninth most abundant element on the earth’s surface. Aluminum by comparison is the third most abundant but has a much more efficient method to convert it to a usable form. Until the turn of the 20th century, aluminum was considered to be as precious as platinum until the Bayer Process brought prices down from $1200/kg to $0.60/kg. Regarding titanium, one way to improve efficiency and buy less material to make the same parts is via Powder Metallurgy (PM). Until recently, titanium alloy powder was very expensive. However, new methods of producing titanium alloy have been developed which generate powder as an output versus massive ingots, which require multiple melts to achieve homogeneity. With powder, in theory, we should be able to get much closer to net shape and reduce the initial buy and reduce significant machining costs. These low cost titanium powders are becoming commercially available, which has the potential to initiate a paradigm shift in the applications of titanium. PM technologies and the consolidation of these new powders are now economically viable with the potential cost of the new powders running approximately an order of magnitude less than conventional PM grade powders. This paper will present the current status of “Project Black Ti” and its potential impact to the F-35 program.