Through-thickness Strength Research Articles

Tailoring precipitation/properties and related mechanisms for a high-strength aluminum alloy plate via low-temperature retrogression and re-aging processes

The retrogression and re-aging (RRA) processes, aimed mainly at tailoring intergranular precipitates, could significantly improve the corrosion resistance (i.e., stress corrosion cracking resistance) without considerably decreasing the strength, which signifies that an efficient control of the size, distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the (mid-)thick high-strength Al alloy plates. Compared to RRA process with retrogression at 200 °C (T77), this study investigated the impact of a modified RRA process (MT77) with lower retrogression temperatures (155-175 °C) and first-stage under-aging on the properties of a high-strength AA7050 Al alloy, in combination with detailed precipitate characterization. The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time, along with the increased electrical conductivity. The rapid responsiveness of microstructure/property typical of retrogression at 200 °C was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the (mid-)thick plates. On the other hand, higher retrogression temperature facilitated more intragranular η precipitates, coarse intergranular precipitates and wide precipitate free zones, which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys. With the preferred MT77 process, the high strength approaching T6 level as well as good corrosion resistance was achieved. However, though a relatively homogeneous through-thickness strength was obtained, some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed, possibly related to the quenching sensitivity. The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization.

Optimization of HNT nanoparticle distribution based on EPD process in epoxy–CFRP composites

There are numerous methods for reinforcing laminate composites using nanoadditives. However, the random dispersion of nanoparticles in polymer resins leads to critical defects owing to excessive dispersion. This study investigated a method of depositing halloysite nanotubes (HNTs) on a carbon fabric surface via electrophoresis deposition (EPD) for reinforcing the through-thickness strength of carbon fiber-reinforced plastic (CFRP) composites. The voltage during the EPD process was set as 6–12 V, which is the working range for nanoparticles during EPD. In addition, CFRPs were fabricated from modified carbon fabrics using vacuum-assisted resin transfer molding. The optimum voltage was determined as 6 V. At higher voltages, the mechanical properties deteriorated owing to the distribution of HNTs and damage to the fibers. The performance of the EPD-modified CFRPs was better than that of neat CFRPs. The highest strength was observed at 0.7 wt.% HNTs and 6 V, which indicated the feasibility of the EPD process and a well-dispersed morphology. The strong interaction of HNTs with carbon fabric influences the through-thickness mechanical properties of CFRP composites.

Experimental and Computational Analysis of Low-Velocity Impact on Carbon-, Glass- and Mixed-Fiber Composite Plates

One of the problems with composites is their weak impact damage resistance and post-impact mechanical properties. Composites are prone to delamination damage when impacted by low-speed projectiles because of the weak through-thickness strength. To combat the problem of delamination damage, composite parts are often over-designed with extra layers. However, this increases the cost, weight, and volume of the composite and, in some cases, may only provide moderate improvements to impact damage resistance. The selection of the optimal parameters for composite plates that give high impact resistance under low-velocity impact loads should consider several factors related to the properties of the materials as well as to how the composite product is manufactured. To obtain the desired impact resistance, it is essential to know the interrelationships between these parameters and the energy absorbed by the composite. Knowing which parameters affect the improvement of the composite impact resistance and which parameters give the most significant effect are the main issues in the composite industry. In this work, the impact response of composite laminates with various stacking sequences and resins was studied with the Instron 9250G drop-tower to determine the energy absorption. Three types of composites were used: carbon-fiber, glass-fiber, and mixed-fiber composite laminates. Also, these composites were characterized by different stacking sequences and resin types. The effect of several composite structural parameters on the absorbed energy of composite plates is studied. A finite element model was then used to find an optimized design with improved impact resistance based on the best attributes found from the experimental testing.

Numerical and Experimental Investigation of the Through-Thickness Strength Properties of Woven Glass Fiber Reinforced Polymer Composite Laminates under Combined Tensile and Shear Loading

The purpose of this paper is to investigate the through-thickness stresses of woven glass fiber reinforced polymer (GFRP) composite laminates under combined tensile and shear loading. Tensile tests were carried out with cross specimens at room temperature under various stacking angles, and the through-thickness strength properties of the woven GFRP laminates were evaluated. The failure characteristics of the woven GFRP laminates were also studied by optical microscopy observations. A three-dimensional finite element analysis (FEA) was carried out to calculate the stress distributions in the cross specimens, and the failure conditions of the specimens were examined. The numerically determined interlaminar tensile and shear stresses at failure location were consistent with Hoffman and Mohr-Coulomb failure criteria when the stacking angle was relatively small. This work is the first attempt to quantify the relation between interlaminar tensile and shear strengths of GFRP composite laminates under tensile and shear loading simultaneously using a combined numerical and experimental approach. A method based on finite element stress analysis was developed for estimating the through-thickness strength of the composite laminates using the experimentally determined fracture load and location. The results suggest that the through-thickness strength under combined tensile and shear loading can be determined effectively by this approach for relatively small stacking angles.

Electrophoresis deposited halloysite nanotubes as modifying additive for carbon fiber and CFRP

Carbon-fiber reinforced polymers (CFRPs) are widely applied to high-grade, precision and advanced industry fields due to its high strength-to-weight and stiffness-to-weight ratios. However, its poor through-thickness properties is becoming an issue. The objective of this study is to examine electrophoresis deposition (EPD) process of depositing halloysite nanotubes (HNTs) onto the carbon fabric surface for enhancing the through-thickness strength. The neutral HNTs were negatively charged and dispersed by adding sodium dodecyl sulfate (SDS) in aqueous solution and the CFRPs were fabricated using Vacuum-assisted Resin Transfer Molding (VaRTM). The bending strength and modulus, interlaminar shear strength, Mode I and Mode II fracture toughness were obtained according to ASTM standards. Electronic scanning microscope (SEM) were adopted to do the micromorphology observation.

Analysis and experiment on DCB specimen using I-fiber stitching process

Several studies related to the reinforcement of composite structures in the through-thickness direction have been conducted recently. In this study, a new reinforcement process is proposed based on previous experimental results; the proposed novel process involves the stitching of T-joints and one-stitched specimens. The need for additional analysis was established under standard DCB tests, for better understanding. Finite element modeling (FEM) results were compared after performing mode I interlaminar fracture toughness tests, using different stitching patterns, for analyzing the through-thickness strength, using reference laminates without stitching. The stitching patterns were defined as 2 × 2 and 3 × 3, where the upper and lower head of the non-continuous stitching process (I-fiber) was shown to strongly affect the through-thickness strength of the laminate. To design a numerical model, cohesive parameters were required for defining surface-to-surface bonding elements using the cohesive zone method (CZM) and for simulating the crack-opening behavior from the DCB test.

Experimental studies on through-thickness shear behavior of EPS based precast concrete sandwich panels with truss shear connectors

Effective use of building materials for achieving sustainable construction is of prime importance today due to fast depletion of the natural resources by the construction industries for obtaining raw materials and aggregates. Precast insulated concrete sandwich panels (CSPs) consisting of two reinforced concrete wythes, an Expanded Poly-Styrene (EPS) core and truss-type shear connectors can be considered as an innovative construction material for acting as load bearing walls and floors in building systems. The two wythes are connected by the shear connectors through the EPS core for transferring shear forces (henceforth called as through-thickness shear) between the wythes and achieving composite action in the panels. In this paper, details of experimental studies conducted on small-scale precast CSP specimens under through-thickness shear loading are presented. The major parameters considered in the experimental program were the thickness of EPS core, gap between the wythes (in CSPs without EPS core), presence/absence of EPS core and number of shear connector lines. The effect of the major parameters on the failure mode, through-thickness shear strength, load vs relative displacement (between top and bottom wythes) behavior and load vs strain behavior of the CSPs are presented and discussed. The test results indicate that the through-thickness shear strength and behavior of the CSPs are significantly influenced by the parameters considered. Reduction in thickness of the EPS core and presence of the EPS core improved the through-thickness shear strength of the CSPs.

Rate-dependent mode I delamination in ballistic composites – Experiment and simulation

The mode I fracture toughness of Dyneema® HB26 was experimentally measured for three different loading rates. The experiments showed an apparent increase of fracture toughness with increasing loading rate. Based on this information, and available information on the through-thickness strength, a cohesive zone model was developed incorporating both, rate-dependent mode I strength as well as rate-dependent mode I fracture toughness. The model was implemented into Abaqus/Explicit and used to reproduce the double cantilever beam experiments. A good correlation was found between simulation and experiment. A parameter study revealed new relationships between material parameters and the cohesive zone length. This is an important finding with respect to mesh-size effects, which supports the practicing engineer in selecting appropriate mesh sizes in impact applications.

Failure strength of composite T-joints prepared using a new 1-thread stitching process

The interlaminar or through-thickness strength of composite materials is lower than their 2-D plane strengths, which can cause failure of the composite structure. Z-pinning and stitching are two typical methods to increase through-thickness material strength. Z-pinning is a technique that inserts reinforcing pins or fibers along the thickness direction of composite materials, which improves their z-directional properties. Stitching, on the other hand, is typically carried out using aramid or carbon fibers. However, the strength of aramid fibers is relatively low, while carbon fibers fracture easily during the stitching process due to their high stiffness.In this study, a new 1-thread stitching process is proposed to minimize bending of carbon fibers and prevent their fracture. Composite T-joints were made, stitched using the new stitching process, and their strength compared to that of T-joints made using the Z-pinning method.

Mode II delamination resistance of composites reinforced with inclined Z-pins

Z-pin reinforcements in composite materials significantly enhance the through-thickness strength by providing strong bridging forces on the delaminated surfaces of a damaged laminate. The bridging forces have been shown to be more effective in mode I dominated delamination cracks than in mode II shear dominated cracks. One solution to help improve the Mode II delamination resistance of Z-pins is to incline the pins such that the angle between the longitudinal axis of the pin and the shear force load vector is reduced. In this study, the Mode II resistance of laminated composite reinforced through the thickness with inclined Z-pins is characterised. There is a notable increase in the apparent fracture toughness of composites when Z-pins are better aligned with the shear load vector (inclined) compared to the conventional, orthogonally inserted Z-pins. Brittle, catastrophic failure however occurs when the inclined Z-pins are angled against the shear load vector. For many structures the direction of the local direction of shear load cannot always be predicted, therefore a general approach for inclined Z-pins using a±θ configuration was also investigated. With this setup, a modest improvement in the apparent fracture toughness was obtained.

Cryogenic through-thickness tensile characterization of plain woven glass/epoxy composite laminates using cross specimens: Experimental test and finite element analysis

This paper investigates the through-thickness tensile behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. Tensile tests were carried out with cross specimens at room temperature and liquid nitrogen temperature (77 K), and the through-thickness elastic and strength properties of the woven GFRP laminates were evaluated. The failure characteristics of the woven GFRP laminates were also studied by optical and laser scanning microscopy observations. A three-dimensional finite element analysis was performed to calculate the stress distributions in the cross specimens, and the failure conditions of the specimens were examined. It is found that the cross specimen is suitable for the cryogenic through-thickness tensile characterization of laminated composite materials. In addition, the through-thickness Young's modulus of the woven GFRP composite laminates is dominated by the properties of the matrix polymer in the given temperature, while the tensile strength is characterized by both, the fiber to matrix interface energy and the cohesion energy of the matrix polymer.

English

Textile composites consist of interlaced tows, which are impregnated with a matrix material and then cured. The interlacing of the tows offers the potential for increased through-thickness strength compared to conventional laminated composites. However, one disadvantage of textile composites is the difficulty in predicting their per formance due to the complex geometry of their internal architectures. Finite element analysis (FEA) has become an effective means to predict the response of complex textile composite structures. When an attempt is made to perform a conventional FEA, one of the tough issues faced is how to deal with the topologically complex internal geometries. To overcome this difficult issue, a domain superposition technique (DST) has been proposed to implement free vibration analysis of woven composite structures. The significant advantage of the DST over traditional FEA is that it does not need to directly deal with the likely degenerated resin-rich region, thus the DST model is much easier to establish. Numerical results show that DST predictions correlate excellently with traditional FEAs.

Microstructure and Mechanical Properties of Al-Li Alloy 2397-T87 Rolled Plate

The microstructure, tensile property and fracture toughness of Al-Li alloy 2397-T87 rolled plate were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, tensile and plane-strain fracture toughness tests. The results show that a pronounced texture variation through the plate thickness was found. Near the surface, Goss texture dominated. While in the center of the plate, typical β fiber texture and a scattering of cube texture were observed. And the subsurface layer exhibited a very weak texture. From the center to the subsurface, the fraction of β fiber texture and cube texture decreased. In contrast, the fraction of shear type texture reaching the maximum in subsurface layer increased. The tensile properties in different layers along the thickness direction were inhomogeneous. The strengths near the surface were lower than those in the center. And the through-thickness strength properties variation in the rolling direction was more remarkable than that in the long transverse direction. In the same thickness layer, the fracture toughness and the strengths were anisotropic. The strengths in the rolling direction were higher than those in the long transverse direction and the short transverse direction, and the strengths in the short transverse direction were the lowest. The fracture toughness in L-T orientation was the highest, followed by that in T-L orientation, and the fracture toughness in S-L orientation was the lowest.

Stresses in Half-Elliptic Curved Beams Subjected to Transverse Tip Forces

In-plane bending of curved beams produces substantial through-thickness normal and shear stresses that can result in structural failures. A half-elliptic curved beam, having a known prescribed variable radius of curvature, is studied as an extension of the previously published circular arc beam. The equations for the normal, tangential, and shear stresses are developed for a curved beam outlined by two confocal half ellipses loaded by a pair of concentrated perpendicular forces on its ends. Closed-form analytical solutions for the stresses are found using an elasticity approach, where the solution is found by using selected terms of the biharmonic equation in elliptic coordinates. For the case of an elliptic beam with an aspect ratio of very close to unity, the solution closely agrees with published circular beam solutions. For other elliptic beam aspect ratios, the calculated stresses display good correlation to detailed finite element model solutions for thickness to semi-axis ratios < 0.1. A parametric study revealed that the maximum normal stress is located at the midplane for high-aspect ratio (a/b ≥ 1) half-elliptic beams, but shifts toward the load tip for low aspect ratio (a/b < 1) beams due to local curvature effects. Moreover, the peak shear stress location moves toward the midplane and the magnitude greatly increases as the aspect ratio is increased. Thus, there are large normal and shear stress interactions occurring near the midplane for high-aspect ratio half-elliptic beams, which is not observed for circular beams. These stress interactions can produce unique failures in materials having low shear strength and through-thickness strength. The current closed-form solution is an improvement on previously published approximate solutions.

Numerical Analysis of Three-Dimensional Braided Composite by Means of Geometrical Modeling Based on Machine Emulation

Three dimensional braided composite materials exhibit better performance in terms of structural integrity, through-thickness strength and impact resistance, than laminated composites. Other advantages regarding production are the ability to produce composite structures with intricate geometries close to the near-net-shape, and also with continuous changes in its cross-section. These properties are a consequence of the internal geometry of the 3D braided composites, in which yarns are interlaced following specific patterns programmed on a braiding machine. In this article, the operation of a typical rotary braiding machine is emulated in order to automatically obtain the geometry of a braiding unit cell. Once the geometrical description is obtained, a finite element model has been developed in order to carry out virtual testing of 3D composite materials. An AS4/epoxy braided material system has been virtually tested and a theory-experimental correlation has been performed for the longitudinal modulus and yield strength compression, using a failure criterion based on constituents and interface failures.

Through-thickness strength measurements using Arcan’s method

This paper presents results obtained from the experimental study on the behaviour of glass/epoxy butterfly-shaped specimens tested under pure shear and biaxial loadings using an Arcan device. Notch-to-notch longitudinally and perpendicularly oriented fibre specimens, respectively referred to as Mat31 and Mat32 ones have been tested with different loading angles ranging from 0° to 90°. The fracture mode was more or less repetitive for Mat31 specimens whereas it depended on the loading direction for Mat32 type, for which the number of cracks and the obliqueness relative to the notch-to-notch line, decreased as the test angle increased. The comparison of failure envelopes has demonstrated that Mat31 specimens are much more resistant whatever the loading angle. However, strengths at failure of Mat32 type are in good agreement with Hashin’s failure criterion, for pure shear tests up to 30° loading angles. Mat31 specimens’ failure stresses were overestimated by the criterion, although similarities in trends were noticed.

Through-Thickness Microstructure, Texture and Strength Gradients in AA 7055 Rolled Plate

Through-thickness microstructure, texture and mechanical property gradients in the longitudinal section of AA 7055 rolled plate have been investigated using electron back-scattered diffraction (EBSD) technique. Quantitative analysis of the microstructure and texture through the plate thickness was conducted. It was found that the microstructure and texture are obviously non-homogeneous through the plate thickness. From center to surface, more equiaxed grains are observed. As expected, the degree of recrystallization increases with increasing the S value. The grains in the center layer exhibited preferential orientations while the most grains near the surface presented a random one. The strength of each observed layer has also been tested.

Tensile and Toughness Properties of Pipeline Girth Welds

The primary objective of this study was to develop a better understanding of all-weld metal tensile testing using both round and strip tensile specimens in order to establish the variation of weld metal strength with respect to test specimen through-thickness position as well as the location around the circumference of a given girth weld. Results from a series of high strength pipeline girth welds have shown that there can be considerable differences in measured engineering 0.2 % offset and 0.5 % extension yield strengths using round and strip tensile specimens. To determine whether or not the specimen type influenced the observed stress-strain behaviour a series of tests were conducted on high strength X70, X80 and X100 line pipe steels and two double joint welds produced in X70 line pipe using a double-submerged-arc welding process. These results confirmed that the same form of stress-strain curve is obtained with both round and strip tensile specimens, although with the narrowest strip specimen slightly higher strengths were observed for the X70 and X100 line pipe steels. For the double joint welds the discontinuous stress-strain curves were observed for both the round and modified strip specimens. Tests conducted on the rolled X100 mechanized girth welds established that the round bar tensile specimens exhibited higher strength than the strip specimens. In addition, the trends for the split-strip specimens, which consistently exhibit lower strength for the specimen towards the OD and higher for the mid-thickness positioned specimen has also been confirmed. This further substantiates the through-thickness strength variation that has been observed in other X100 narrow gap welds. A second objective of this study was to provide an evaluation of the weld metal toughness and to characterize the weld metal microstructure for the series of mechanized girth welds examined.

Compression properties of z-pinned sandwich composites

Sandwich composites are ultra-light weight structural materials consisting of thin face skins covering a lowdensity core. The skins are made of metal alloy sheet or fibre-reinforced polymer laminate, and provide the sandwich composite with high in-plane mechanical properties. A variety of low-density materials are used in the core, with the most common being metallic honeycomb, polymer foam, syntactic foam, Nomex and balsa. Sandwich composites are used in a large number of structural applications because of their high in-plane properties and light-weight, including in aircraft, ships, rail carriages and buildings. A major limitation of sandwich composites is their low through-thickness compression properties, which are determined by the compliant and weak core material. The compression modulus and strength in the through-thickness direction is less than 10% of the in-plane properties for most types of sandwich composites, and this has limited their use in structures that must support high throughthickness compression loads. A method to improve the through-thickness compression properties is z-pinning, which involves inserting high modulus, high strength pins through the sandwich material. The mechanical performance of z-pinned sandwich composites have not been thoroughly characterised, and there are few micromechanical models to predict their structural properties. Previous research has revealed that z-pinned sandwich composites possess improved damage tolerance [1, 2], and Cartie and Fleck [3] recently showed that the throughthickness compression strength of foam core sandwich materials is increased with fibrous composite or metal pins. However, the effects of the volume content and diameter of the pins on the compression properties has not been determined. This paper presents a study to determine the effect of z-pinning on the through-thickness compression properties of sandwich composites. Improvements to the through-thickness compression modulus and strength with increasing volume content and size of z-pins is determined by experimental testing, and a model for predicting the compression properties of z-pinned sandwich composites is presented. A sandwich composite consisting of fibreglass skins and a poly(vinyl chloride) foam core was reinforced with fibrous composite pins. The skins were a woven glass/ epoxy laminate and the core was a closed cell PVC foam with a density of 90 kg/m. The sandwich material was reinforced with thin (0.28 mm diameter) T300 carbon/ bismaleimide pins to volume contents of 0.5%, 2% and 4%. The material was also pinned using thick (0.51 mm diameter) T650 carbon/bismaleimide pins to the volume content of 2%. Aztex Inc. (Waltham, MA) produced the pins using a pultrusion process that aligned the carbon fibres in the lengthwise direction. The mechanical properties of the pins are given in Table 1. The z-pins were inserted through the sandwich composite using a hand-held ultrasonically actuated horn in a process described by Partridge et al. [4]. The process used is the standard z-pinning technique for reinforcing composite laminates and sandwich materials. A problem with this process is the difficulty in accurately inserting the z-pins in the orthogonal orientation, and the pins are usually offset at a shallow angle. This is a problem often encountered with the z-pinning process, and is difficult to avoid when the pins are inserted manually. Figure 1 presents a histogram showing the percentage of pins offset at different angles from the orthogonal direction in the sandwich composite. The pins are A. P. Mouritz (&) School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Vic 3001, Australia e-mail: adrian.mouritz@rmit.edu.au J Mater Sci (2006) 41:5771–5774 DOI 10.1007/s10853-006-0114-8

Adhesively bonded lap joints from pultruded GFRP profiles. Part I: stress–strain analysis and failure modes

Quasi-static axial tension experiments were performed in a laboratory environment on epoxy bonded, balanced double-lap and single-lap joints from pultruded GFRP flat sections. Full-scale specimens were used to avoid size effects. Parameters investigated were the overlap length (from 50 to 200 mm), the adhesive layer thickness (from 1 to 3 mm) and the adherend thickness (from 3 to 12 mm). On nine of the double lap joints 40 strain gages were installed to measure the axial strain profiles along the overlaps. The measured axial strain profiles in the joints correlated well with numerical results obtained from a 2D finite element analysis. Failure initiation and progression were observed with a high-speed camera. Failure was initiated by the combination of local through-thickness tensile (peeling) and shear stresses at two different locations: in the adhesive fillet and in the outer fiber-mat layers of the adherends below the joint edges. Failure propagation always occurred in the outer fiber-mat layers of the adherends. The through-thickness strength of the adhesive–adherend interface was considerably higher than the through-thickness strength of the interfaces between the fiber-mats. The experimental investigation showed that the ultimate failure load converged to a constant value with increasing overlap length. The adhesive layer thickness had an insignificant influence on the ultimate failure loads and the stress-strain distributions along the overlaps.