Effects of mean stress and fibre volume content on the fatigue-induced damage mechanisms in CFRP

The effect of the load type (tension and compression) in quasi-static and of the applied mean stress in fatigue tests on the mechanical behaviour and on the damage mechanisms in unidirectional (UD) carbon/epoxy laminates has been studied in combination with the influence of fibre volume content. Results show that the fibre volume content increases the mechanical properties in tension–tension fatigue tests for all tested angles 0°, 45° and 90°. The tensile damage mechanisms of off-axis specimens depend on the fibre volume content and change from matrix cracking and matrix–fibre debonding to fibre-pull out with an increasing amount of fibres as investigated in detail in a previous work. In tension–compression tests, higher fibre volume contents are only beneficial in fatigue tests at angles of 0° and 45°. Fatigue strengths of UD 90° specimens in tension–compression tests are not significantly improved by the fibre volume content which can be ascribed to breakage of entire fibre bundles and crushed fibres on the fracture surfaces.     

Heterogeneous plastic deformation in bimodal bulk ultrafine-grained nickel

Heterogeneous plastic deformation behavior of two bimodal ultrafine-grained nickel materials with different ultrafine-grained (UFG) and coarse-grained (CG) components fractions was investigated experimentally at the grain level. The prismatic specimens were deformed quasistatically up to 10% axial plastic strain using compression test at room temperature. The local microstructure of the initial and deformed samples was measured by electron backscattered diffraction pattern analysis in a scanning electron microscope. It was found that the plastic deformation of bimodal materials is highly heterogeneous and the degree of heterogeneity depends strongly on the grain size distribution and the volume fraction of the CG component. The large localized plastic strain within the coarse grains was observed during compression. The strain localization resulted in occurrence of debonding and cracks in the UFG region or in the interface between CG and UFG components.     

EBSD study of micromechanisms involved in high deformation ability of DP steels

Notched and un-notched tensile specimens of fine grained commercial DP780 steel were deformed in uniaxial tension until fracture. Micro-texture analysis was performed by using an FE-SEM equipped with an EBSD detector and the data were analyzed to quantify orientation gradients within the microstructure of the deformed specimens in terms of Image Quality, Inverse Pole Figure and Taylor Factor map. High deformation ability of DP steels was found to be mostly due to such mechanisms as grain rotation, void creation and evolution, substructure formation within the ferrite grains and the highly plastic stretching of martensite during the deformation process. The true strain of martensite was measured up to 64% and 74% for the un-notched and notched specimens, respectively.     

Strain rate dependence of HPFRCC cylinders in monotonic tension

High-Performance Fiber-Reinforced Cementitious Composite (HPFRCC) materials exhibit strain hardening in uniaxial, monotonic tension accompanied by multiple cracking. The durability of HPFRCC materials under repeated loading makes them potentially suitable for seismic design applications. In this paper, the strain rate dependence of tensile properties of two HPFRCC materials in cylindrical specimens is reported from a larger study on strain rate effects in tension, compression and cyclic tension–compression loading. The cylindrical specimens were loaded in monotonic tension at strain rates ranging from quasi-static to 0.2 s⁻¹. To evaluate the impact of specimen geometry on tensile response, coupon specimens loaded in monotonic tension under a quasi-static strain rate were compared to corresponding cylindrical specimens made from the same batch of material. Tensile strength and ductility of the HPFRCC materials were significantly reduced with increasing strain rate. Multiple cracking, strain hardening, strain capacity, and the shape of the stress–strain response were found to be dependent on specimen geometry. SEM images taken of the fracture plane of several specimens indicated that pullout and fracture of the fibers occurred for both HPFRCC materials studied here.     

Orientation-dependent compression/tension asymmetry of short glass fiber reinforced polypropylene: Deformation,damage and failure

Short glass fiber reinforced polypropylene (sgf-PP) is increasingly employed in structural components which are subjected to a variety of loading conditions including tensile, compressive and bending loading modes. Since typical industrial components exhibit a wide range of fiber orientation distributions, their mechanical response to these loading conditions is also highly anisotropic. In this paper, the compression/tension asymmetry in the stress–strain behavior of sgf-PP is investigated from a macroscopic engineering and a micro-mechanisms of deformation and failure point of view for specimens with varying, precisely defined fiber orientations. Furthermore, we performed volume strain measurements and two-cyclic tests. We used the results to deduce the onset of damage due to cavitational mechanisms under tension and compared this to the onset of deviation of the tensile from the compressive stress–strain behavior. The results showed a good correlation for specimens with high fiber orientation, whereas for specimens with low fiber orientation results deviate due to the high deviatoric matrix volume strain contribution.     

The response of a multi-directional composite laminate to through-thickness loading

The through-thickness mechanical response of a carbon fibre/epoxy laminated composite of lay-up [0/45/−45]_ns is measured at low rates of strain. Uniaxial tension and compression experiments are carried out on dogbone specimens cut from a thick laminate along different directions, and failure mechanisms are observed via optical and electron microscopy. The effect of direct and shear stresses at the ply interfaces on the onset of failure is measured, and a failure envelope is constructed. The compressive response of specimens of different shape is investigated. Composite beams of different volume and aspect ratios are tested to failure in three-point bending and these tests reveal a strong dependence of the apparent out-of-plane tensile strength of the composite on the beam volume; this effect is modelled by Weibull theory.     

Investigating the static and dynamic tensile mechanical behaviour of polymer‐bonded explosives

The tensile properties of a polymer‐bonded explosive (PBX) were systematically studied by using quasi‐static and dynamic experiments. A non‐linear constitutive relation was developed to describe the tensile behaviour of the PBX. The tensile properties of the PBX under different strain rates and temperatures were measured in quasi‐static tests. The tensile behaviour of the PBX was found to exhibit high strain rate and strong temperature dependence, attributable to the large fraction of the polymer binder. To obtain the rational dynamic tensile results, a modified split Hopkinson tensile bar (SHTB) setup was designed such that the specimens were in dynamic stress equilibrium and deformed homogeneously at nearly constant strain rates. To characterise the viscoelastic behaviour, the master modulus curve was derived from the tensile stress relaxation tests at different temperatures. The non‐linear constitutive model was implemented in ABAQUS to predict the tensile behaviour of the PBX. The computational results were found to be in good agreement with the experimental results.     

Effects of gage length, loading rates, and damage on the strength of PPTA fibers

Axial tension and transverse compression experiments on single fibers were performed to investigate the mechanical behavior of three high-performance fibers (Kevlar^®, Kevlar^® 129, and Twaron^®) with diameters in the order of 9-12 μm. The single fibers were manufactured from 1998 through 2008. A miniaturized tensile Kolsky bar was used to determine the tensile response of PPTA single fibers at a high strain rate. Gage length and strain rate were found to have minimum effects on the tensile strength of PPTA single fibers. Manufacturing time over a decade was found to have negligible effects on the tensile strength of the fibers. Initial transverse compression on the fibers reduces their ultimate tensile strengths. A high resolution scanning electron microscope (SEM) was also used to examine the fracture modes of transversely deformed fibers. Different types of fracture morphology were observed.     

Effect of Strain Rate,Adiabatic Heating and Phase Transformation Phenomena on the Mechanical Behaviour of Stainless Steel

Abstract: The influence of strain rate on the stress–strain behaviour of an AISI 304 austenitic stainless steel sample was investigated. For this purpose, uniaxial tensile tests were performed at room temperature for different strain rates. Microstructural measurements of transformed martensitic phase as a function of plastic strain, and thermal analyses of the specimens were carried out as well. It was found that increasing the strain rate from 10^?4 to 10^?1 s^?1 leads to a 25% improvement in uniform elongation. Moreover, a ‘curve‐crossing’ phenomenon was observed for the hardening behaviour measured at different strain rates. These results were rationalized in terms of martensitic phase transformation suppressed by a temperature increase in the specimens deformed with high strain rates.     

Investigation of the Uncertainty of DIC Under Heterogeneous Strain States with Numerical Tests

Abstract: In this research, numerical 2D digital image correlation (DIC) tests are carried out to assess the uncertainty of DIC under heterogeneous strain states. DIC is implemented to measure the deformation of the numerically deformed images with respect to the undeformed counterparts, which are taken from the real tensile specimens. The tensile specimens are made of three materials, that is, steel DC06, steel DX54D+Z and aluminium alloy Al6016 and cut into three different geometries, namely one standard design and two complex designs. The specimens are all painted manually with random speckle patterns. The original images are deformed by imposed displacement fields, which are obtained by simulating uni‐axial tensile tests of the specimens with finite element analysis (FEA). In this way, the errors sourcing from the hardware of the image system are excluded. According to the geometries of the specimens, homogeneous and heterogeneous strain states are achieved by FEA. The optimum mesh sizes of the models are identified to minimise theirs influence on the imposed fields. The impacts of subset sizes, step sizes and strain window sizes are studied for an optimum correlation. Finally, the influence of the strain state is investigated. It is found that the DIC accuracy and precision decrease under highly heterogeneous strain states.     

Local and global instabilities in nanosize rectangular prismatic gold specimens

We use molecular mechanics simulations with the tight-binding potential to study local and global instabilities in initially defect-free nanosize rectangular prismatic specimens of gold deformed in tension/compression and simple tension/compression. Whereas in simple tension/compression atoms on end faces are constrained to move axially but are free to move laterally and the cross-sectional dimensions of end faces can change, in tension/compression all three components of displacements of atoms on end faces are prescribed and the cross-section of an end face does not change. The three criteria used to delineate local instabilities in a specimen are: (i) a component of second-order spatial partial derivatives of the displacement field has large value relative to its average value in the body, (ii) the minimum eigenvalue of the Hessian of the potential energy of an atom is negative, (iii) a relatively high value of the common neighborhood parameter. A specimen becomes globally unstable when its potential energy decreases noticeably with a small increase in its deformations. It is found that the three criteria for local instability are met essentially simultaneously at the same atomic position. Deformations of interior points of a specimen are different when it is deformed in simple tension/compression from those in tension/compression. It is found that the initial unloaded configuration (or the reference configuration) of the minimum potential energy has significant in-plane stresses on the bounding surfaces and non-zero normal stresses at interior points. This initial stress distribution satisfies Cauchy’s equilibrium equations for a continuum. In deformations of a nanobar studied here, the yield stress defined as the average axial stress when the average axial stress vs. the average axial strain curve exhibits a sharp discontinuity depends upon the specimen size. It is shown possibly for the first time that deformations of the specimen are reversible if it is unloaded prior to yielding but have a permanent strain if unloaded after it has yielded. Because of residual stresses in the reference configuration, the average axial stress at yield in compression is nearly one-half of that in tension. The slope of the average axial stress vs. the average axial strain curve during unloading after it has yielded is the same as that during initial loading up to the yield point.     

三向编织玻璃/环氧复合材料刚度性能

通过实验研究了三向编织玻璃/环氧复合材料的刚度性能 , 并考虑编织角和试件宽度参数的影响 , 探讨了拉伸和压缩刚度性能的差异。实验结果表明 : 在同一纤维体积分数条件下 , 随着编织角的增大 , 试件的纵向弹性模量有所减小 , 泊松比 (在编织角约大于 35° 时) 也有所减小 ; 宽度为两倍和三倍单胞宽度的试件的刚度性能基本相同; 试件的纵向弹性模量和泊松比远大于横向弹性模量和泊松比; 拉伸和压缩时试件的弹性模量和泊松比基本接近 ; 在横向拉伸和压缩时试件的应力2应变曲线具有明显的非线性特征。实验结果为编织复合材料结构设计提供了数据参考。      

Fracture of plasma-sprayed thick thermal barrier coatings under multiaxial stress states

Failure behaviour of free‐standing plasma‐sprayed coatings was investigated under combined axial and shear loading. Thin‐walled tubular specimens were loaded with various combinations of tension/compression and torsion. This allows the failure surface to be established for loading situations where the two principal stresses are of opposite signs. Specimens failed in one of the two modes, a tensile failure perpendicular to the maximum principal stress or a compression shear failure through the thickness. Failure data were adequately described by the maximum principal stress theory. Stress–strain curves fall within a single scatter band depending on the failure mode. In situ deformation tests showed that the mechanism was microcrack closing and sliding in compression and microcrack opening, coalescence and the development of new microcracks in tension.     

Failure of aluminium self-piercing rivets: An experimental and numerical study

The present paper investigates the fracture mechanisms of AA7278-T6 aluminium self-piercing rivets under compression during the riveting process. First, a microstructure investigation was conducted to disclose the grain structure and the particle distribution of the extruded aluminium alloy. Transmission electron micrographs revealed precipitate free zones along grain boundaries. Uniaxial tensile tests in three different directions with respect to the extrusion direction revealed anisotropy of the alloy in strength and ductility and a change in fracture mode with tensile direction. The behaviour of the alloy under compression was studied experimentally using upsetting tests and self-piercing riveting tests. Micrographs of the deformed specimens provided insight into the influence of the microstructure on the deformation and fracture of the alloy under compression. Second, numerical analyses were carried out using a 2-D axisymmetric model in LS–DYNA in an attempt to investigate the role of different physical variables on the final failure of the rivet. The numerical results revealed that constituent particles, precipitate free zones, and friction between the rivet and plates are important for strain localisation and fracture in the rivet.     

Determining true stress–strain curve for isotropic and anisotropic materials with rectangular tensile bars: method and verifications

Based on an extensive numerical study, the authors have recently developed a method for determining the true stress–strain curve for isotropic materials by using rectangular tensile specimens. In this method only load versus thickness reduction (at the diffuse necking zone) curve is needed. This method has been further developed for anisotropic materials. For anisotropic materials, both thickness and width reduction at the diffuse necking zone should be measured. In this paper, the effect of plastic anisotropy on the deformed cross-section profile has been further studied, and the proposed method has been experimentally verified against steel which was assumed to be isotropic (width to thickness strain ratio, r_y≈1) and aluminium alloys which were anisotropic with r_y varied in the range 0.28<r_y<0.66. It has been shown that the numerically predicted deformed cross-section shape is very close to the experimental one, measured from sectioned interrupted specimens. The true stress–strain curves determined from the rectangular tensile specimens were compared with the one determined from the conventional round tensile specimens, and very good agreement has been obtained. It has been found that the method using rectangular tensile specimens gives very consistent true stress–strain curves with small scatter.     

Partially coupled anisotropic fracture model for aluminum sheets

The objective of the present paper is to incorporate the effect of plastic anisotropy on the fracture modeling of aluminum alloy 6061-T 6 sheets. Six different types of tests were performed to fracture, including tensile tests on classical dog-bone specimens, flat specimens with cutouts, plane strain grooved specimens, and punch indentation tests on circular blanks. A limited number of shear/tension tests on butterfly specimens were performed on a dual-actuator loading frame. Plastic properties were determined from the dog-bone tensile tests, and were verified by the remaining tests. It was found that the sheets exhibited little planar anisotropy but substantial out-of-plane anisotropy, characterized by the Lankford parameter, r. A comprehensive numerical analysis of the experiments revealed that the Hill 1948 quadratic anisotropic yield model is able to describe, with good accuracy, the plastic response of all five types of tests.Fracture surface strains were measured using a digital image correlation system. Average fracture strains were determined by measuring post-fracture thickness reduction. Local fracture strains were determined by means of an inverse engineering method involving matching the displacement to fracture from numerical simulations to those measured. Possible discrepancies between the magnitudes of the fracture strain in the three above methods are discussed. Based on the previous experience of the investigating team, the experimental fracture data were analyzed within the realm of the three-parameter Modified Mohr-Coulomb fracture model. Using the plane stress anisotropic plasticity equations, the calibrated fracture model was then transformed to the space of the equivalent strain to fracture and stress triaxiality. An alternative representation of the fracture locus in the space of principal strains was also constructed. Other important factors influencing the form of the fracture locus, such as mesh-size effect and solid versus shell representation, were also investigated.     

Influence of stress state and strain rate on the behaviour of a rubber-particle reinforced polypropylene

This article presents an experimental investigation of a ductile rubber-modified polypropylene. The behaviour of the material is investigated by performing tension, shear and compression tests at quasi-static and dynamic strain rates. Subsequently, scanning electron microscopy is used to analyse the fracture surfaces of the tension test samples, and to relate the observed mechanical response to the evolution of the microstructure. The experimental study shows that the material is highly pressure and strain-rate sensitive. It also exhibits significant volume change, which is mainly ascribed to a cavitation process which appears during tensile deformation. Assuming matrix-particle debonding immediately after yielding, the rubber particles might play the role of initial cavities. It is further found that the flow stress level is highly dependent on the strain rate, and that the rate sensitivity seems to be slightly more pronounced in shear than in tension and compression. From the study of the fracture surfaces it appears that the fracture process is less ductile at high strain rates than under quasi-static conditions.     

Elastic moduli of a lean and a pavement quality concrete under uniaxial tension and compression

Results are given of strain measurements taken during a limited proramme of tests on a lean concrete and a pavement quality concrete to compare the elastic moduli under direct tension and compression. For each type of concrete differences of less than 10% were found between the tensile and compressive tangent moduli. In tension tests deviation from linearity, indicating commencement of microcracking, occurred at a lower percentage of the ultimate tensile strength for the concrete having the higher cement content. Modulus values were higher for the lean concrete than for the richer pavement quality concrete.     

Energy dissipation capacity of fibre reinforced concrete under biaxial tension–compression load. Part I: Test equipment and work of fracture

The objective of this research was to analyse the differences in the dissipated energy under uniaxial tension and biaxial tension–compression load of fibre reinforced concretes using the Wedge Splitting Test. Under biaxial load the specimens were subjected to compressive stress ratios from 10% to 50% of the concrete compressive strength perpendicular to the direction of the tensile load.Under biaxial tension–compression load the energy dissipation capacity of the specimens decreases compared to the uniaxial tension load case on average 20–30%. It is believed that the decrease is a result of the damage mechanism of the concrete matrix and deterioration of the fibre–matrix and/or aggregate–cement paste interfaces in case the section is additionally loaded with compression stresses. This indicates that dimensioning of concrete elements under biaxial stress states using material parameters obtained from tests conducted on specimens under uniaxial tensile load is unsafe and could potentially lead to a non-conservative design.In the second part of this paper the extent of the fracture process zone under uniaxial tension and biaxial tension–compression load will be examined with the Acoustic Emission technique and the reasons for decrease of the energy dissipation capacity under biaxial load will be further discussed.