Specimen Configuration Research Articles

In-situ strength effects in long fibre reinforced composites: A micro-mechanical analysis using the phase field approach of fracture

Transverse intralaminar cracks in layers with perpendicular orientation referred to the main loading direction have a significant effect on the apparent ultimate strength of the corresponding composite laminate. This effect stems from the fact that such transverse cracks generally promote the occurrence of other failure mechanisms leading to the specimen collapse in subsequent stages of the loading process. With the aim of conducting a careful investigation regarding the onset and progression of transverse intralaminar cracking events, a micro-mechanical analysis of cross-ply laminates is herein proposed. Particularly, the cross-ply laminates belonging to the family [0°2/90°n/0°2] are considered via the generation of high-fidelity micro-mechanical models, which reproduce the internal fiber arrangements related to the reference results in Saito et al. 2012, Experimental evaluation of the damage growth restraining in 90° layer of thin-ply CFRP cross ply laminates, Adv. Comp Mat, 21:1,57–66. Differing from alternative approaches, current predictions are equipped with the combined used of two fracture-based modeling methods: (i) the variational phase field (PF) approach for triggering crack events into the matrix, and (ii) the bilinear cohesive zone model (CZM) for the simulation of fibre-matrix decohesion failures. Relying on this computational methodology, the focus of this work is to analyze the influence of the transverse ply thickness (n) on the onset and propagation of damage under tensile conditions, in conjunction with the transition from micro-cracking to meso-scale damage states. For this purpose, several models are generated into the FE package ABAQUS using user-defined capabilities, replicating configurations of specimens included in previous experimental investigations, i.e. through the consideration of different transverse ply thickness (n = 1, 2, 4). Present results show the potential of the proposed methodology to predict the transverse matrix cracking phenomena and the ability and reliability for capturing the delay of the crack with thinner plies, a phenomenon which is usually denominated as in-situ effect in the related literature.

Investigation on the Durability of Ti-6Al-4V Alloy Designed in a Harmonic Structure via Powder Metallurgy: Fatigue Behavior and Specimen Size Parameter Issue

In the present work, the four-point bending loading fatigue properties of a heterogeneously distributed grain size microstructure consolidated from Ti-6Al-4V alloy powder are studied. The microstructure involved here, a so-called “harmonic structure”, possesses quasi-spherical large grain regions (“cores”) embedded in a continuous fine grain region (“shell”). Unlike the previous reports dealing with this issue, the effect of the specimen size on the fatigue characteristics is also probed, since two distinct specimen configurations are considered. Furthermore, the obtained experimental data are compared with the corresponding fatigue results derived from homogeneous coarse grain counterparts. Contrary to homogeneous structure material, discrepancies on both the fatigue strength and the fatigue crack initiation aspects are found for the harmonic structure material. Consequently, the present work aims to clarify the underlining phenomena involved in the specimen size effect detected for Ti-6Al-4V designed in the harmonic structure. A less active interface surface between the core and the shell combined with a wider critical volume in the large size specimen should be the main reasons of the fatigue strength discrepancy.

Experimental and numerical analysis of PMMA impact fracture

The work presents experimental and numerical results on dynamic fracture of PMMA plates subjected to impact loading. The experimental tests were conducted using steel cylinder-shaped projectile accelerated using a gas gun. In order to evaluate performance of the tested specimens, residual impactor velocity was assessed using high-speed photography setup. Square-shaped PMMA specimens with three thicknesses were investigated using various projectile velocities. For all the three specimen types the ballistic limits were experimentally obtained. The conducted experiments were numerically simulated using finite element method with explicit time integration scheme and incubation time fracture model for the material failure prediction. Experiments with all three specimen configurations were successfully simulated using one parameter – incubation time, which was evaluated from existing experimental data on the dynamic fracture of PMMA. In addition to the simulations of the real experiments estimates on performance of a sample with a virtual geometry were made using the developed numerical approach.

Examination of the block shear fracture test method to measure the Mode-II critical stress intensity factor

Block shear fracture tests were conducted using specimens of western hemlock to determine the properties of the fracture mechanics under the Mode-II condition. In the test, a fixture standardised as the block shear test of solid wood in the Japanese Industrial Association (JIS Z2101-2009) was used. The specimen configuration, location of the crack, and initial crack length were varied, and the effects of these conditions were examined. The Mode-II critical stress intensity factor was effectively obtained when the initial crack was introduced at the opposite location to that where the shear load was applied using a specimen with a cubic configuration. Additionally, the introduction of the additional crack length due to the fracture process zone ahead of the crack tip was also effective in the block shear fracture test and other Mode-II fracture tests.

Fundamental study of corrosion-preventive compounds: part III—soft films

ABSTRACT Soft film corrosion-preventive compounds (CPC) were studied using ASTM B117 accelerated corrosion testing method and a special test specimen. The tested CPCs included a commercial product and homemade CPCs with a generic formulation containing paraffin wax, petrolatum, corrosion inhibitors and surfactants. Carbon steel substrates coated with CPCs containing corrosion inhibitors and surfactants showed little corrosion after 21 days of exposure in an ASTM B117 chamber. However, the carbon steel substrates coated with CPCs containing no inhibitors or surfactants showed severe corrosion after 7 days of exposure. The effect of CPCs on galvanic corrosion prevention was studied using a special test specimen configuration consisting of a primer-coated AA7075-T6 panel coupled with SS316 fasteners. The galvanic currents between the panel and fastener were measured using a zero-resistance ammeter during exposure of the test specimen in the ASTM B117 chamber. The galvanic attack of the panels coated with different CPCs was evaluated.

Optimization of chevron-notched short bar specimen configuration for determination of the fracture toughness and crack growth rate of aluminum alloys

The goal of the study is optimization of the fracture toughness determination procedure on the samples with a chevron notch. Well-known principles of fracture mechanics are used to determine the geometry of a specimen with a chevron notch, which ensures the independence of the stress intensity factor from the crack length in the range sufficient for its practical application. A requirement to the shape of the dependence «compliance – crack length» is formulated for a chevron-notched specimen. The choice of the cross-sectional shape of the specimen with a chevron notch, the geometric dimensions of the specimen, and the drawing of the gripping device are justified. The formula for calculation of the fracture toughness from the maximum test load and thickness of the specimen is derived and experimentally validated. The results of the fracture toughness tests of the samples with a chevron notch and standard samples are compared for a number of aluminum alloys. The data on the scale factor in the range of a sample thicknesses of 12.5, 25, and 50 mm and variation coefficient obtained upon the fracture toughness determination revealed the absence of the significant effect of the scale factor, 6% variation under the normal law of the fracture toughness distribution in the tests of chevron-notched specimens. The results on the fracture toughness distribution in the height of the thick plates of alloys 1163T1 and V95pchT2 are presented. The temperature dependence of the fracture toughness of the plate (alloy 1201T1) are constructed for chevron-notched specimens within a temperature range of –196 – +200°C. The features of using chevron-notched specimens for determination of the crack growth rate attributed to independence of the stress intensity factor at the crack tip on crack length are considered in a rather wide range. The method of the crack growth rate determination is described and the results of assessing d a /d N – Δ K for an aluminum alloy plate 1163T are presented. The simplicity and reliability of the methods for determination of the fracture toughness and fracture growth rate (fatigue, creep, corrosion) on chevron-notched specimen of the proposed geometry allow us to recommend them for control tests and research in various test conditions.

An extended edge-notched disc bend (ENDB) specimen for mixed-mode I+II fracture assessments

A new test specimen for mixed-mode I+II fracture studies is proposed. An extended version of edge notch disc bend (ENDB) specimen, which was recently proposed to assess mixed-mode I+III fracture toughness, is designed and developed for introducing opening-in-plane-shear sliding mode. This new specimen that is called asymmetric ENDB specimen (A-ENDB), was numerically analysed to determine the stress intensity factors and T-stress along the crack front for a wide range of geometric and loading conditions. The finite element results demonstrated that (i) the contribution of mode III component is negligible under asymmetric loading of ENDB specimen and; (ii) the full modes I and II mixities including pure mode I, pure mode II and intermediate mode mixities can be achieved by changing loading support distances. The capability of the proposed asymmetric ENDB specimen was then experimentally assessed by conducting several mixed-mode I+II fracture toughness tests on a brittle model polymer (PMMA). The mode II fracture toughness value obtained from the A-ENDB specimen was noticeably greater than KIc value. Although the fracture trajectory of all A-ENDB specimens was nearly self-similar and along the plane of initial crack, different fracture surface morphologies were observed for mixed-mode I+II. The fracture surface consists of mirror area and ridge markings. While, towards to pure mode II, both density and length of the ridge marking were increased. It is shown that by using the same shape test specimen, loading type, and test configuration, both mixed-mode I+II and I+III can be studied.

A simple 3-point flexural method for measuring fracture toughness of the dental porcelain to zirconia bond and other brittle bimaterial interfaces

PurposePorcelain fused to zirconia prostheses are widely used, but porcelain chipping, fracture, spalling and delamination are common clinical problems. Conventional bond strength testing is inherently unsuited for studying interfacial failure by cracking in brittle materials. Instead, fracture toughness is a more meaningful parameter because it can assess the robustness of the interface when subjected to loading, but fracture mechanics approaches have only rarely been used. Our purpose was to develop a novel, simple, 3-point flexural methodology and mathematical analysis to measure the fracture toughness of the porcelain to zirconia interface. MethodsEquations were derived to estimate the fracture toughness of the bond by computing the interfacial energy release rate for a novel simple 3-point flexural test model. The test was validated using two different configurations of layered zirconia/porcelain beams (n = 10), approximating the dimensions of a fixed dental prosthesis, fabricated from a tetragonal polycrystalline zirconium dioxide partially stabilized with yttria and a feldspathic dental porcelain. ResultsCracking along the bimaterial interface was produced and measured as a discrete event. Fracture toughness means (standard deviations) computed from the measured energy release rate, for the porcelain to zirconia interface in two different specimen configurations were 7.9 (1.3) and 5.3 (1.6) J/m2. ConclusionsEquations were derived to measure interfacial fracture toughness of brittle materials using a novel simple 3-point flexural test method. The test was then validated; estimates for the fracture toughness for the porcelain to zirconia bond, overlapped with previously published data derived from more complex 4-point notched tests.

Elastic–plastic damage prediction in notched epoxy resin specimens under mixed mode I/II loading using two virtual linear elastic failure criteria

Elastic–plastic damage of a ductile epoxy resin is investigated for the first time in the configuration of semicircular bend specimen weakened by U-shaped notches under mixed mode I/II loading conditions. U-notched specimens are prepared from the characterized epoxy material with different notch rotation angles and notch tip radii. Load-carrying capacities of the U-notched specimens are experimentally obtained by performing fracture tests under various combinations of mode I and mode II loading. The reformulated Equivalent Material Concept is employed for the polymeric material in conjunction with the maximum tangential stress and mean stress criteria to provide the theoretical predictions without any necessity for elastic–plastic analyses of their damage. Scanning electron microscopy micrographs are also taken from the fracture surfaces and utilized for realizing the micromechanical processes of damage in the tested specimens. A very good consistency is found between the experimental results and the predictions of the combined Equivalent Material Concept-maximum tangential stress criterion, as well as those of the Equivalent Material Concept-mean stress criterion.

Experimental Identification of Statistical Correlation Between Mechanical Properties of FRP Composite.

Recent prediction on the heavy statistical correlation between the mechanical properties of fiber reinforced composite (FRP) raises new concerns on the accurate reliability evaluation of composite structures, but such statistical correlation still lacks experimental verification. In this work, an experimental methodology is proposed to determine the statistical correlation between mechanical properties of unidirectional FRP composite. A rectangular shaped carbon fiber reinforced plastic (CFRP) specimen with a circular hole is loaded by tension, and 3D digital image correlation (DIC) is employed to characterize the heterogeneous strain field around the hole. Virtual field method (VFM) is used to derive E11, E22, ν12, and G12 of specimens. Specimen configuration considering fiber angle and hole diameter is optimized to achieve accurate determination of correlation coefficients. Experimental results on the linear correlation coefficients between E11, E22, ν12, and G12 agree well with previous theoretical predictions.

Experimental study of wave loads on elevated pile cap of pile group foundation for sea-crossing bridges

Elevated pile cap, which transits the load from sea-crossing bridge pylon to pile group, is always close to the sea level and suffering from extreme wave loadings. This paper describes a wave flume experimental program to investigate wave loads on the elevated pile cap of pile foundation for sea-crossing bridges. Two specimen configurations including isolated cap and cap with pile group were set up with different clearance heights. Wave forces on the cap, including horizontal and vertical force and moment, were measured simultaneously under five nonlinear regular wave trains. The effects of pile group, clearance height and wave height on wave forces on the cap were investigated. The experimental results show that the clearance height and wave height affect wave forces significantly. The largest horizontal force occurs when the cap is fully submerged, while the largest vertical force occurs when the bottom of the cap locates at still water level (SWL). As the wave height increases, the horizontal forces increase except for the fully submerged case and the vertical forces increase firstly but decrease when the wave height exceeds 0.18 m. Pile group increases the vertical uplift forces on the cap when the bottom of the cap locates above SWL.

Effect of type of contact, counterface surface roughness, and contact area on the wear and friction of extensively cross-linked, vitamin E stabilized UHMWPE.

Novel extensively cross-linked, vitamin E stabilized polyethylene (VEXLPE) materials are expected to provide improved wear and oxidation resistance in orthopedic implants. Noncyclic, multidirectional pin-on-disk (POD) wear tests were performed for VEXLPE with flat-on-flat (FoF) and ball-on-flat (BoF) specimen configurations against CoCr counterfaces of varying surface roughness (Sa = 0.02-0.74 μm). In addition, wear tests (FoF) were performed for VEXLPE pins of varying nominal contact area (7.07-113 mm2 ) with consistent load regimen against polished CoCr disks. All specimen couples were also friction tested with a multidirectional, circularly translating POD device. In all tests, calf serum was used as the lubricant. In comparison with earlier, similar tests for conventional, gamma-sterilized ultra-high molecular weight polyethylene (UHMWPE) and for extensively cross-linked, heat treated UHMWPE (XLPE), the tribological findings for the present VEXLPE appeared promising with respect to its possible clinical use in prosthetic joints, particularly as an acetabular liner against large-diameter femoral heads, and in nonconforming contacts. Contrary to the well-known, paradoxical behavior of conventional UHMWPE, the VEXLPE wear factor decreased with increasing contact area.

Evaluation of temperature and loading rate effect on fracture toughness of fiber reinforced asphalt mixture using edge notched disc bend (ENDB) specimen

A number of specimen configuration and testing method have been developed till now for determining the fracture toughness of asphalt concrete. The Edge Notch Disc Bend (ENDB) specimen is among the suitable test specimens in this regard due to its some primary advantages such as simple geometry and testing procedure and also covering different combination of mode I and III fracture case. In this paper the influence of fiber, temperature and loading rate is investigated experimentally on mode I and III fracture toughness of asphalt mixtures tested with the ENDB specimen. The fracture toughness experiments were conducted on three different low temperature value, three loading rates and different fiber percentages.

A Novel Tensile Specimen Configuration for the Characterization of Bulk Mycelium Biopolymer

Mycelium is a biopolymer that has the potential to be a sustainable replacement for petroleum based foams in some engineering applications. During a recent study, with the objectives to first relate the effects of growing conditions on material properties and second to develop a model to predict the bulk behavior of mycelium, it was found that a variety of standard test methods are being used for the tensile characterization of mycelium in the literature. In the current study, these test methods were found to cause stress concentrations that resulted in maximum loads away from the gauge section, griping conditions that crushed the test samples through the thickness, and gauge sections that were smaller than the level of heterogeneity typically observed in samples of mycelium biopolymer. A new tensile specimen configuration has been developed that mitigates the issues associated with other specimen configurations. This new specimen configuration is used to determine the tensile strength, tensile modulus, and Poisson’s ratio of mycelium biopolymer specimens.

Spatially resolved thermoreflectance techniques for thermal conductivity measurements from the nanoscale to the mesoscale

Thermoreflectance techniques, namely, time- and frequency-domain thermoreflectance (TDTR and FDTR, respectively), are ubiquitously used for the thermophysical characterization of thin films and bulk materials. In this perspective, we discuss several recent advancements in thermoreflectance techniques to measure the thermal conductivity of solids, with emphasis on the governing length scales and future directions in expanding these advances to different length scales and material structures. Specifically, the lateral resolution of these techniques, typically on the order of several micrometers, allows for an understanding of the spatially varying properties for various materials. Similarly, limitations of TDTR and FDTR with respect to their volumetric probing regions are discussed. With a recently developed steady-state thermoreflectance technique, these limitations are overcome as probing volumes approach spot sizes. Finally, recent pushes toward the implementation of these techniques without the use of a thin metal transducer are presented, with guidelines for future avenues in the implementation under these specimen configurations.

Development of Modified Arcan Fixture for biaxial loading response of fiber-reinforced composites

A Modified Arcan Fixture (MAF) was developed to study the biaxial mechanical behavior of fiber reinforced composites. The MAF was calibrated compared with standard ASTM tests for an epoxy/twill woven-ply laminate. Material properties obtained with ASTM tests are implemented in ABAQUS to find the optimal notch radius that creates the most uniform stress state in gauge area. The optimized configuration of Arcan specimen was adopted to design the entire fixture numerically in preload and load steps to avoid plastic deformation. Stress concentration factor was considered in Arcan specimen under tensile loading. Biaxial experimental data for woven-ply composite in different loading angles was obtain with the MAF. Failure modes under biaxial loading show that fibers rotated towards the principle tensile direction and failure was caused by excessive stress which reached fiber limit. The ultimate shear strength corrected by considering the fiber rotation effect are in good agreement with ASTM standard results.

Study on uniform parameters characterizing the crack-tip constraint effect of fracture toughness

The plastic zone of crack-tip is connected with the constraint level inherently. Based on this, a unified crack-tip constraint parameter Cp was proposed. And through finite element analysis (FEA), the Cp can be recognized as a characteristic parameter in the description of equivalent stress fields. Moreover, the ductile material SA-508 steel was used to test the effectiveness of Cp as a uniform parameter, three different configurations (CT, SENB, Mini-SENB) of specimens with different crack lengths and thicknesses were employed to distinguish the effects of constraints on ductile fracture toughness. The result showed that clear linear relationship between the Cp and J0.2BL is shown for all specimens with different dimensions, which can be used to determine constraint-dependent fracture toughness for the specimens or flaw components of SA-508 steel, regardless of in-plane or out-of-plane constraint or both.

Experimental Study on the Effect of Two Equal-Sized Holes Lying at Different Angles in a CFRP Plate under Bending

A bending test method was designed to investigate the failure characteristics of drilling carbon-fiber reinforced composite (CFRP) plates. The CFRP plates were fabricated and drilled to join other parts for use in aircraft structures (riveting or bolting). Each sample had two equal-sized holes at 0, θ, or -θ degrees at a fixed distance of 36 mm in the longitudinal direction. The sample dimensions were identified following a study on tensile testing singleholed samples based on the ASTM D5961 standard. The tensile test focused on three cases of laminate: [0]14, [22.5]14, and [45]14. The maximum mean forces for [0]14, [22.5]14, and [45]14 laminate configurations were 6.32, 6.59, and 7.12 kN respectively. These results were implemented into a bending test which included two configurations of laminate: [0]14 and [45]14 and two equal-sized holes with a variable distance of 0 and 9 mm in the transverse direction of the plate. The bending test configuration is presented in this paper. The results revealed different load-bearing capabilities in each of the three types of specimen configuration. Samples with two holes lying at ±9 degree affected the overall strength in the [45]14 case, and as in [0]14 there was no difference between the eccentricity and the aligned holes. In [45]14, the eccentricity decreased the maximum allowable force by 1.57 percent in the normal eccentricity case and by 15.94 percent in the alternate eccentricity case compared to the aligned case.

Compression Capacity of Corrugated Core Hybrid Composite Sandwich Structure

Improvement of mechanical properties of light-weight corrugated core sandwich structures is a big demand in aerospace applications. Among these applications, space vehicles which encounter pressure loads and severe aerodynamic heating during ascent and reentry. The open-cell corrugated core is useful for active cooling of the sandwich structures. In this work, hybrid composite structural members with fiberglass corrugated core and carbon fiber skin facings were manufactured using vacuum bag technique. Different specimen configurations with rectangular cross-section area have been subjected to the load in the longitudinal direction of the corrugation and examined by edgewise compression test. The proposed testing has been applied to take advantage of the highest inertia of the specimen in such orientation. The test provides a basis for estimating the load carrying capacity when these structure members are used as individual webs in the aircraft interiors. Also as the core sheet is turned by 90° to the regular load direction, this structure member is similar to the so-called honeycomb when ordered in parallel rows and hence it is appropriate for floor sandwiching. In contrary to a honeycomb, this core consists of fiberglass laminate and therefore higher compressive resistance is associated. The results exhibit high values of both stiffness and ultimate compression force in the corrugation direction. For the rectangular area and the open corrugated contour, specific properties relative to the weight are extremely high. Also, the results and graphs indicate that there must be at least three corrugated ligaments with a trapezoidal cross section of 0.5” height and 63o per cell to grantee stability under load and high absorbed energy in the non-linear stage as well.

Influence of curved delamination front on R-curve of DCB specimen

This work investigated the evolution of the non-uniform distribution of Mode-I strain energy release rate (SERR) along a straight pre-crack front of double cantilever beams (DCB), caused by the anticlastic deformation and the longitudinal free-edge effects. A modified DCB specimen with a curved pre-crack front was introduced in order to alleviate this problem and produce an uniform SERR distribution at the initiation and propagation stages. The first order optimization method was used to determine the appropriate shape of the curved pre-crack front in order to guarantee an uniform SERR during the fracture process. The introduced DCB specimen configuration with the curved pre-crack front was shown to be more convenient for the estimation of a consistent SERR value throughout the fracture procedure without been affected by the visual deviation of the crack length and the non-uniformity ratio of Mode-I SEE distribution along the crack front.