Effect of the loading rate on mode I interlaminar fracture toughness of laminated composites

The objective of the present study is to determine the influence of the loading rate on the critical energy release rate G_Ic of fibre-reinforced epoxy laminates. In order to perform pure mode I loading at higher opening velocities, a new test device is developed. The approach is based on a symmetrical opening displacement applied to a DCB specimen. In the data reduction, the influence of the kinetic energy has to be taken into account. The results obtained on the unidirectional carbon-epoxy laminate T300/914 at crack opening rates up to 1.6 m/s show a slight effect of the loading rate on G_Ic.     

Analysis of stitched laminated ENF specimens for interlaminar mode II fracture toughness

Two simple micromechanics based models are proposed to predict the effect of through-thickness reinforcement (stitching) on the improvement of delamination crack growth resistance in end-notched flexure (ENF) specimens. In the first model, it is assumed that stitches stretch elastically and then rupture when the load carried approaches the failure load. In the second model, it is assumed that stitches are discontinuous and that the stitch thread-matrix interface is completely frictional. Approximate closed form solutions for energy release rates are obtained, and the effects of stitch density, matrix-stitch thread interfacial shear stress, stitch thread diameter, volume fraction of stitches, critical energy release rate and Young's modulus are then examined. A simple design study for sizing the ENF specimen to minimise geometric nonlinear response is presented. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.     

Investigation of wood fracture toughness using mode II fracture (shearing)

The test results of fracture toughness for three wood species, such as pine, alder and birch are presented. Examination of fracture toughness is carried out using mode II fracture (shearing). Values of the stress intensity factor,K _IIc, are determined for the three main anatomic directions of wood. Microstructural tests of particular wood species, performed on specimens along the three main anatomic directions of wood, are discussed. Qualitative relationships are found to exist between the microstructure of wood and the obtained values of the stress intensity factor,K _IIc.     

Developing a four point bend specimen to measure the mode I intralaminar fracture toughness of unidirectional laminated composites

The use of a four point bend test is investigated for the measurement of intralaminar toughness of unidirectional carbon fibre composites. A key element of the investigation is the process of producing a sharp pre-crack and the effect of this on the measured toughness. A method is presented in which the pre-crack is produced during the layup and cure process and this is shown to cause minimal distortion of the plies of the laminate. The test results show good consistency and low scatter.     

Mixed mode I/III fracture toughness in extruded magnesium alloys

Fracture toughness under mode I and mixed mode I/III loading were determined for magnesium (Mg) as well as binary Mg-Al and Mg-Zn alloys in as-extruded condition. It was found that in Mg and in Mg-1.25Zn alloy the fracture toughness under mixed mode I/III loading was higher than that under mode I loading whereas in binary Mg-1Al and Mg-3Al alloys it was lower than that under mode I loading. The results have been explained on the basis of the fracture mechanism and the nature of the stress fields ahead of the crack tip under mixed mode I/III loading.     

Combined mode I - mode III fracture toughness of a high carbon steel

• 1.1. Brittle materials exhibit minimum energy dissipation under pure mode I loading
• 2.2. In these materials, the mode IIIcomponent essentially provides redundant plastic work and increases J_totalc.
• 3.3. This behavior contrasts with more ductile materials where combined mode I - mode III loading produces The minimum energy dissipation.

     

Nesting effect on the mode I fracture toughness of woven laminates

The fracture toughness of two woven laminates is evaluated for different nesting/shifting values between advanced layers. The analysed woven composites are manufactured using the same resin-reinforcement and same architecture, but have a different tow size (3K/12K). Three different nesting/shifting configurations are applied to the plies at the fracture surface: zero shifting, middle shifting and maximum shifting. Before being tested, the internal geometry of the material is evaluated and any shifting error is measured. For all these configurations mode I fracture tests are carried out. The differences obtained between 3K and 12K cases can be explained by fibre bridging, but not the differences between the nesting configurations. Depending on the nesting/shifting value the delaminated surface waviness is different, and consequently the fracture toughness is also influenced.     

Measurement of the fracture toughness associated with the longitudinal fibre compressive failure mode of laminated composites

The fracture toughness associated with the fibre compressive failure was obtained from testing notched unidirectional carbon/epoxy four-point-bend specimens. Microscopy of failed specimens revealed that onset of damage was characterised by the formation of a single line of fibre breaks at approximately 45° to the plane of the initial notch. A micromechanical finite element model was used to investigate this failure scenario and it was concluded that the most probable cause of the damage morphology was compression-induced shear failure of the composite. An intrinsic material property in this case was deemed to be the mode II critical strain energy release rate associated with the initiation of the 45° crack. For IM7/8552, this was measured to be G_IIc = 4.5 ± 0.8 kJ/m².     

Effect of temperature on the mode I and mixed mode I/III fracture toughness of SA333 steel

The effect of temperature on tensile properties, mode I and mixed mode I/III fracture toughness of SA333 Grade 6 steel was investigated. The variation of ultimate tensile strength and strain hardening exponent with temperature as well as the appearance of serrations in the stress-strain plots indicated that dynamic strain aging regime in this steel is in the temperature range 175-300 °C at a nominal strain rate of 3 × 10⁻³ s⁻¹. Both mode I and mixed mode I/III fracture toughness values were found to exhibit a significant reduction in the DSA regime. The mixed mode I/III fracture toughness was found to be significantly lower than the mode I fracture toughness at all temperatures. However, the difference between the two toughness values was much higher prior to the onset of DSA. The results are explained on the basis of the nature of deformation fields under mode I and mixed mode I/III loading as well as the fracture mechanism prevalent in these steels at different temperatures.     

Prestandardization study on mode I interlaminar fracture toughness test for CFRP in Japan

This paper summarizes results from a series of interlaboratory round robin tests (RRTs) performed in order to establish a JIS standard for mode I interlaminar fracture toughness test using double cantilever beam (DCB) specimens. For the case of unidirectional laminates, brittle and toughened CF/epoxy, and CF/PEEK systems were used. Only a brittle CF/epoxy system was used for woven laminates. The round robin tests were conducted with two main aims: first, to examine the influence of starter films and the precracking condition on the initial mode I fracture toughness values; and second, to establish the definition of initial fracture toughness. Polyimide starter films stuck to the epoxy matrix, and caused unstable crack growth from starter films. Comparison of the tests with and without mode I precracks from starter films indicated that tests with precracks gave lower values of initial fracture toughness. The definition of initial fracture toughness values was discussed, based on the reproducibility. A 5% offset point was recommended as the initial fracture toughness from the RRT results. The influence of loading apparatus, data reduction methods, etc. was also discussed.     

Comparison of mode I and mode II elastic-plastic fracture toughness for two low alloyed high strength steels

In the present work, mode I and mode II tests were carried out on two low alloyed high strength steels. An asymmetrical four point bend specimen and J _II-integral vs. crack growth resistance curve technique were used for determining the mode II elastic-plastic fracture toughness, J _IIc · J _II-integral expression of the specimen was calibrated by finite element method. The results indicate that the present procedure for determining the J _IIc values is easy to use. Moreover, the mode I fracture toughness J _Ic is very sensitive to the rolling direction of the test steels, but the mode II fracture toughness J _IIc is completely insensitive to the rolling direction of the steels, and the J _IIc /J _Ic ratio is not a constant for the two steels, including the same steel with different orientations. Finally, the difference of the fracture toughness between the mode I and mode II is discussed with consideration of the different fracture mechanisms.     

Z—pins几何分布对其增强复合材料双悬臂梁I型层间韧性的影响

复合材料z—pinning增强技术通过在层合板内嵌人体分比小于5％的z—pins，能大幅度提高层合板的层间断裂韧性，减少因低能量冲击所产生的分层损伤。本文作者基于细观力学模型，构造了相应的z—pin单元；结合梁单元，建立了用于分析含z—pins的双悬臂梁（Double　cantilever　beam，DCB）的有限元模型；在分层裂纹面上引人接触单元以防止分析过程中2个分层子梁在分层前缘处的相互嵌人。通过数值算例分析了z—pins对层间韧性增强效果的影响及其原因。数值计算结果表明，z—pins的几何分布对其增强层间韧性的影响相对较小。     

Thickness and moisture content effect in the fracture toughness of Scots Pine

Fracture toughness tests were performed on specially prepared specimens taken from a single tree. These were kiln dried, machined to various thicknesses and then sets of them conditioned to various moisture contents. It was found that about half the specimens were rejected because of warping and splitting for thicknesses greater than 10 mm, but below this the rejection rate decreased. Fracture toughness values showed thickness variations in the kiln dried state, but the toughness decreased and became constant when the moisture content was changed. An explanation is proposed in terms of residual stresses induced by the drying which increases the toughness and constraint stresses which cause a decrease. A characteristic length of the annual ring spacing seems appropriate in defining the various transitions.     

Modeling the mode I fracture toughness of anisotropic low-density rigid PUR and PIR foams

Low-density foams have to possess a sufficient resistance to cracking in order to ensure the mechanical integrity of foam materials in service, even when not intended for load-bearing applications. In this study, mode I fracture toughness in the foam rise direction has been experimentally characterized for anisotropic rigid commercial polyurethane foams as well as for polyisocyanurate foams produced using polyols derived from rapeseed oil and filled with a montmorillonite nanoclay. Rectangular parallelepiped unit-cell based scaling relations expressing foam toughness via its relative density, cell dimensions, geometrical anisotropy, and the ultimate tensile stress of the base polymer have been employed for prediction of foam toughness. Assuming a brittle fracture of foam struts, a conservative estimate of toughness is obtained. It is demonstrated that considering the yielding of foam struts at the crack front as the criterion of crack extension provides a closer estimate of foam toughness.     

Mode I and mode II fracture toughness of E-glass non-crimp fabric/carbon nanotube (CNT) modified polymer based composites

In this study, mode I and mode II interlaminar fracture toughness, and interlaminar shear strength of E-glass non-crimp fabric/carbon nanotube modified polymer matrix composites were investigated. The matrix resin containing 0.1 wt.% of amino functionalized multi walled carbon nanotubes were prepared, utilizing the 3-roll milling technique. Composite laminates were manufactured via vacuum assisted resin transfer molding process. Carbon nanotube modified laminates were found to exhibit 8% and 11% higher mode II interlaminar fracture toughness and interlaminar shear strength values, respectively, as compared to the base laminates. However, no significant improvement was observed for mode I interlaminar fracture toughness values. Furthermore, Optical microscopy and scanning electron microscopy were utilized to monitor the distribution of carbon nanotubes within the composite microstructure and to examine the fracture surfaces of the failed specimens, respectively.     

Interfacial fracture energy and the toughness of composites

The premises upon which prevailing composite toughness theories are based are discussed in the light of observed strength variations in boron-epoxy composites with differing shear strengths of the interfacial bond. None of the extant toughness theories (pull-out, debonding, stress redistribution) successfully predicts the work of fracture of the boronepoxy system. However, incorporation of the work to create new surfaces into the total toughness analysis gives better agreement with experiment, and work of fracture predictions for other sytems, such as carbon-polyester, can also be modified. The approach is more generalized than the Outwater/Murphy debonding explanation for toughness, which in the way usually presented only applies when the filament fracture strain is greater than the matrix fracture strain. The present analysis suggests how to tailor the interfacial shear strength in order to obtain a reasonable toughness yet still maintain strengths of the order of the rule of mixtures.