Off-axis Tensile Tests Research Articles

Nylon 66/poly(vinyl pyrrolidone) reinforced composites: 2

The effect of interphase microstructure on the bulk mechanical properties and moisture absorption of Nylon 66 reinforced with E-glass or high-modulus (AS4) carbon fibers was examined. These particular composite systems were chosen because of their propensity to form distinct transcrystalline interphases. A comparison of mechanical properties was made by dynamic mechanical analysis (DMA) and Minimat off-axis tensile testing of samples with and without this unique interphase. Moreover, a newly developed vibrational testing technique was used and correlated with DMA to ascertain changes in the damping characteristics of the composites. The major contribution of this work is the systematic translation of single-filament (microscopic) results to bulk composite mechanical property results in the same well-characterized system. This was accomplished by studying both glass- and carbon-fiber/Nylon 66 composites. The E-glass- and carbon-fiber-reinforced Nylon 66 composites have better ultimate properties when the transcrystalline interphase is absent, but the composites have better damping properties when transcrystallinity is present. Finally, upon moisture exposure, suppression of the glass transition temperature was evident, as the water served as a plasticizer in all composites. The presence of transcrystallinity did not alter the adsorption kinetics of the composites.

Whole-field assessment of the effects of boundary conditions on the strain field in off-axis tensile testing of unidirectional composites

The objective of this paper is to present an experimental assessment of the effects of boundary conditions on the strain field in 10° off-axis tensile tests on unidirectional carbon/epoxy composites. It is shown experimentally by using a whole-field phase stepped grid technique that oblique tabbing of off-axis coupons results in a homogeneous strain field over the whole specimen, thus preventing premature failure near the tabs. A numerical sensitivity study reveals that the oblique angle is not very sensitive to the elastic moduli values. Therefore only estimates of the moduli are needed to derive the oblique angle. It is also shown by finite-element analysis that a 3° error on the oblique angle is acceptable in terms of stress concentrations near the tabs, but that a 7° error leads to significant stress concentrations for this material.

Interaction Term of Tsai-Wu Theory for Laminated Veneer

A stochastic-based method of evaluating the interaction parameter (\iF\d1\d2) of the Tsai-Wu strength theory has been presented in this paper. Treating all strength parameters of the strength theory as random variables, the mean and standard deviation of \iF\d1\d2, under plane stress conditions, have been estimated for Douglas-fir laminated veneer. This estimation has been managed through a nonlinear least-squares fit of the parameters to a cumulative probability distribution of off-axis tensile data. For the purpose of comparison, a sample evaluation of \iF\d1\d2 using deterministic methods has been presented. This evaluation showed strong dependence of \iF\d1\d2 on angle to grain. A subsequent sensitivity analysis of the off-axis tests on the value of \iF\d1\d2 indicated that data from 15° off-axis tensile tests were more stable in establishing \iF\d1\d2 than that of other angles tested: 30°, 45°, and 60°.

Insertion of an interphase synthesised from a functionalised silicone into glass-fibre/epoxy composites

The fibre/matrix interface in unidirectional composite materials is modified by introducing a controlled interphase having a low glass transition temperature. The synthesis of this interlayer is based on polyurethanes and sol-gel chemistries. The interphase is introduced by a solvent process on-line during filament-winding. The mechanical behaviour of composite materials studied by means of off-axis tensile tests and three-point bending measurements (by varying the span-to-depth ratio) is the same than that for composite materials based on commercially sized glass fibres. Thus, the elastic behaviour is not modified by the introduction of a low-modulus and low-thickness interphase. On the other hand, the impact toughness, studied by using the Charpy test, is enhanced by the soft interphase. These experimental studies confirm the theoretical models developed previously in the literature and the conclusions obtained on microcomposites.

A tensor function for the R-value of sheet metal

Tensile testing of sheet metals shows that both the orientation of the testpiece in the plane of the sheet and the degree of rolling reduction can influence the R-value. To explain this a quadratic and a cubic yield function are adapted to published results for a number of sheet metals. The cubic function can reproduce every dependence of R upon orientation as found from off-axis tensile tests. Present work shows that a simpler quadratic function lies within the material deviations. The theory of tensor functions enables this function to be expressed in terms of prior plastic strain, and so they provide a description of the dependence of the R-value on the processing history of a sheet metal. It is demonstrated how this junction can be applied to account for the in-plane R-variations observed when rolling a copper sheet under plane strain laboratory conditions. Of particular interest are the predictions that lead to improved drawability, i.e., the rolling reduction and sheet orientation that optimize R. A further examination is made of the stability of the R-variation pertaining to a given rolling reduction. It is shown that R remains stable under subsequent uniaxial plastic straining for any direction in the plane of the sheet. Moreover R is not altered appreciably by relatively small amounts of tensile plastic prestrain aligned with the rolling direction. That R is sensibly constant is the usual assumption made in plasticity theory.

New Ideas on the Measurement of the In-Plane Shear Strength of Unidirectional Composites

This paper addresses the issue of the measurement of the in-plane shear strength of unidirectional composites. It is shown from recently published material that for both Iosipescu and 100 off-axis tensile tests, failure under a homogeneous stress state is achieved. However, these stress states are not pure shear and it is shown that ignoring the presence of transverse tensile and compressive stresses, respectively in the 100 off-axis and the Iosipescu tests, would lead to important apparent differences in the in-plane shear strength measured from both tests. An analysis using a quadratic failure criterion underlines the role of the transverse stresses and values of 95 and 98 MPa for the shear strength of unidirectional T300/914 are derived respectively from the 100 off-axis and the Iosipescu tests.

Effect of strain rate on nonlinear deformation behavior in CFRP composites

This paper presents the effect of strain rates on nonlinear deformation behavior of CFRP composites. An elastic-plastic constitutive model is developed on the basis of both a complementary elastic energy function and a plastic potential, assuming that linear elastic deformation occurs in the fiber direction and that nonlinear strain in the shear direction is decomposed into nonlinear elastic strain and plastic strain. A one-parameter and a two-parameter plastic potential are employed to describe post-yielding behavior of composite laminae. Nonlinear elastic shear strain is measured by means of loading/unloading tensile tests of [±45°]s specimens. Off-axis tensile tests are performed to obtain the universal effective stress-effective plastic strain relation under monotonic loading. It is shown that rate-dependence of effective stress-effective plastic strain relation can be explained by determining anisotropy parameters of the plastic potential as functions of initial effective plastic strain rate.

The 10 ° off-axis tensile test: A critical approach

This paper addresses the issue of the improvement of the experimental procedure of the 10 ° off-axis tensile test by using oblique tabs. It is shown that oblique tabs significantly increase the maximum shear stress at failure and that unlike straight-tabbed specimens failing in the tabbed area, oblique-tabbed specimens failed neatly in the gauge section. It also appears that composite tabs are more appropriate than aluminium tabs. Finally, it is shown that although a state of homogeneous shear stress is now achieved in oblique-tabbed specimens, the presence of transverse tensile stress prevents the direct measurement of the shear strength and that only a lower bound is obtained.

Tensile flow and instability in galvanized, rolled steel sheet

This paper examines flow and instability of a zinc-clad sheet used for the manufacture of car panels. Three forming properties are reported from off-axis tensile testing: (a) anisotropy in the plane of the sheet, (b) tensile instability and (c) homogeneity of deformation. The analysis correlates the directional flow behaviour using the Hill yield function (1)† as a basis for equivalence. This function explains satisfactorily the linear plastic strain paths and the variation in r values observed but does contract flow behaviour to a single curve. A subtangent is derived for the off-axis testpiece in terms of its orientation and two strain ratios referred to its axis. This is used to derive the limiting uniform strain that is found to remain constant in regions exterior to local neck formation. A condition is established for which the principal axes of stress and plastic strain remain coincident. It is shown, from the measured rotations of the principal material axes, that deformations conform to standard strain transformation equations upon removal of rigid-body rotations.

Modelling of the mechanical behaviour of a 2-D SiC-SiC composite at a meso-scale

An original meso-model of the mechanical behaviour of a 2-D SiC-SiC composite is proposed. In order to take into account the main elementary constituents of the composite (which govern the material damage), the composite material and the damage mechanisms are described at an intermediate scale referred to as the meso-scale. The laminate meso-model is defined with an arrangement of one ‘inter-tow matrix and pores’ ply and four ‘unidirectional composite’ plies. The model is validated on the basis of comparisons between numerical simulations of the mechanical behaviour of the composite under both off-axis tensile (or biaxial stress state) testing conditions, and experimental data.

Fracture Modes of Oriented Polypropylene

This paper describes the fracture modes of both notched and unnotched polypropylene which has been oriented with a solid phase roll-drawing process. The shear strength of the unnotched material is estimated with a single off-axis tensile test. The notched specimens, when stressed in the roll-drawn direction, fractured normal to the fibrous structure and with a brittle mode. This brittleness enables a KIc to be reported. The inapplicability of linear elastic fracture mechanics at other angles of loading is also iden tified.

Evaluation of three in-plane shear test methods for advanced composite materials

This paper is concerned with the evaluation of three in-plane shear test methods for advanced carbon fibre composites for aerospace applications. To accomplish this goal, the losipescu, ± 45° tensile and 10° off-axis tensile shear test methods were evaluated for three advanced epoxy matrix materials (Narmco 5245C, Hexcel F584 and American Cyanamid Cycom 1806) reinforced with Hercules IM6 carbon fibres. The values of in-plane shear moduli obtained from the three test methods and three materials were then used with other previously determined elastic constants to predict the tensile moduli of (+45°/0°/−45°/90°) 6s laminates. Comparison of the predicted and experimental laminate tensile moduli showed that any one of the three shear test methods was appropriate for determining the in-plane shear modulus to predict tensile moduli of symmetric laminates which consist of equal numbers of 0°, +45°, −45° and 90° oriented laminae.

Fracture toughness testing of graphite/epoxy and graphite/PEEK composites

The combined Mode I (crack opening) and Mode II (forward shearing) fracture behaviour of unidirectional T300/1034C graphite/epoxy and graphite-reinforced APC-1 polyetheretherketone is characterized using the notched off-axis tensile test. The results and limitations of the test are discussed. The modified three rail shear test is used to obtain the in-plane Mode II fracture toughness, and results for the epoxy and thermoplastic system are compared. Scanning electron microscope images of the epoxy and thermoplastic at various mixed-mode ratios are presented and discussed.

Synthesis and characterization of bisimide amines and bisimide amine‐cured epoxy resins

AbstractA series of novel bisimide amines were synthesized and characterized and then utilized as curing agents with a standard epoxy resin, N,N′‐ tetraglycidyl‐methylenedianiline (TGMDA), known commercially as MY720. The bisimide amines (BIA's) were synthesized by reaction of 4,4′‐hexa‐ffuoroisopropylidine (biphthalic acid dianhydride) (6F anhydride) with aromatic and aliphatic diamines in dimethyl formamide at reflux temperatures in yields ranging 24 to 99 percent. The diamines used were 3,3′‐diaminodiphenylsulfone (3,3′‐DDS), 4,4′‐diaminodiphenylsulfone (4,4′‐DDS), 1,12‐dodecanediamine (1,12‐DDA), alone and as mixtures to produce the BIA's 6F‐3,3′‐ DDS, 6F‐4,4′‐DDS, 6F‐3,3′‐DDS‐4,4′‐DDS, 6F‐3,3′‐DDS‐1, 12‐DDA with various compositions, depending on the mode of addition and stoichiometry. The BIA's are isolated as mixtures containing monomer, oligomer, and polymeric species. They were characterized by elemental analysis, and high pressure liquid chromatography (HPLC). Epoxy resin specimens were fabricated by reaction of the standard epoxy (MY720) with the BIA and in some cases, mixtures of BIA and aromatic diamine, such as 3,3′‐DDS. The bisimide amine cured epoxies (IME's) were characterized for moisture absorption, thermal properties, physical and mechanical properties. The bisimide amine epoxy (IME) resins were also characterized as matrices in Celion 6000/bisimide amine cured epoxy (IME) composites. The relative toughness characteristics of each IME formulation was measured by the 10° offaxis tensile test by measuring the uniaxial tensile, shear strengths and shear‐strain‐to‐failure of the composite systems.

A comparison of simple shear characterization methods for composite laminates

Various methods for the shear stress/strain characterization of composite laminates are examined and their advantages and limitations are briefly discussed. Experimental results and the necessary accompanying analysis are then presented and compared for three simple shear characterization procedures. These are the off-axis tensile test method, the [±45°] s tensile test method and the [0°/90°] s symmetric rail shear test method. It is shown that the first technique indicates the shear properties of the graphite/epoxy laminates investigated are fundamentally brittle in nature while the latter two methods tend to indicate that these laminates are fundamentally ductile in nature. Finally, predictions of incrementally determined tensile stress/strain curves utilizing the various different shear behaviour methods as input information are presented and discussed.

Evaluation of lamina strength criteria by off-axis tensile coupon tests : Schneider, G. J. Fibre Science and Technology, Vol 5, No 1, p 29 (January 1972)

Evaluation of lamina strength criteria by off-axis tensile coupon tests : Schneider, G. J. Fibre Science and Technology, Vol 5, No 1, p 29 (January 1972)

Evaluation of lamina strength criteria by off-axis tensile coupon tests

The use of off-axis tensile coupon tests was evaluated as a means of determining the applicability of five strength criteria for predicting the ultimate strength of boron-epoxy lamina. The five criteria, which include one micromechanics theory and four macromechanics theories, were compared with the results of tests on unidirectional, off-axis, boron-epoxy tensile coupons. All five strength criteria gave similar results for the off-axis tensile coupons, and they were all in reasonably good agreement with the data. It is concluded that off-axis tensile tests do not provide sufficient discrimination among the strength criteria considered because they only reproduce a narrow segment of the failure surface. Biaxial test data is therefore required in order to differentiate among these strength criteria.

More on the Influence of End Constraints on Off-Axis Tensile Tests

An analytical investigation has been made of gripping effects on the off-axis tensile test results for flat specimens. It is an extension of the earlier work of Pagano and Halpin [1] and aims at establishing approxi mate bounds on the accuracy of the subject test. Considered are the cases of rigid clamping with and without end rotation for selected properties which are typical of glass, boron and graphite composites. Solutions of the corresponding mathematical models are effected numerically, utilizing a computerized finite-element approach. The results support the findings of [1] in demonstrating that length/width ratio and type of constraint can significantly modify the intended uniform stress/strain fields in the subject test. Despite this adverse effect, the analysis suggests that if strain measurements can be properly recorded, reasonably accurate test results should be obtain able for even small length/width specimens. Moreover, though the numerical results lack the generality of closed-form solutions, it would still appear that the trends estimated for the special glass, boron, and graphite systems are typical of other hard composite systems in these respective families.