“…As a result, it was calculated using the off-axis tensile strength of 10° off-axis specimen. The principal unnotched strengths obtained in this study differ by only about 5% from the values obtained in the previous study 34 for the unidirectional composite laminates fabricated using the same prepreg tape. Figure 2.Fiber orientation dependence of off-axis unnotched strength.…”
The multiaxial quasi-brittle failure criterion for notched orthotropic composites that was formulated in the first part of this study is evaluated on the basis of the off-axis notched strength of a unidirectional carbon/epoxy composite. Static tension tests are first carried out on center circular hole specimens of different hole diameters and fiber orientations to examine the notch size effect as well as the fiber orientation effect on the off-axis notched strength of the unidirectional composite. Experimental results show that the tensile strength in the fiber direction is more notch-sensitive than that in the transverse direction. The notch sensitivity in off-axis strength increases as the off-axis angle increases from the 0 fiber direction to about 10 off-axis direction, but it turns to decrease monotonically as the off-axis angle increases further in the range up to 90 . Then, the analytical formulas to predict the off-axis notched strength, off-axis notch sensitivity, off-axis intrinsic equivalent mode-I fracture toughness, and off-axis apparent equivalent mode-I fracture toughness of unidirectional composites are established on the basis of the multiaxial quasi-brittle failure criterion. These analytical formulas succeed in efficiently predicting both the effects of notch size and fiber orientation on the off-axis notched strength of the unidirectional composite. It is also shown that the intrinsic and apparent off-axis equivalent mode-I fracture toughness values predicted for brittle failure and quasi-brittle failure, respectively, correlate well with the experimental results.
“…As a result, it was calculated using the off-axis tensile strength of 10° off-axis specimen. The principal unnotched strengths obtained in this study differ by only about 5% from the values obtained in the previous study 34 for the unidirectional composite laminates fabricated using the same prepreg tape. Figure 2.Fiber orientation dependence of off-axis unnotched strength.…”
The multiaxial quasi-brittle failure criterion for notched orthotropic composites that was formulated in the first part of this study is evaluated on the basis of the off-axis notched strength of a unidirectional carbon/epoxy composite. Static tension tests are first carried out on center circular hole specimens of different hole diameters and fiber orientations to examine the notch size effect as well as the fiber orientation effect on the off-axis notched strength of the unidirectional composite. Experimental results show that the tensile strength in the fiber direction is more notch-sensitive than that in the transverse direction. The notch sensitivity in off-axis strength increases as the off-axis angle increases from the 0 fiber direction to about 10 off-axis direction, but it turns to decrease monotonically as the off-axis angle increases further in the range up to 90 . Then, the analytical formulas to predict the off-axis notched strength, off-axis notch sensitivity, off-axis intrinsic equivalent mode-I fracture toughness, and off-axis apparent equivalent mode-I fracture toughness of unidirectional composites are established on the basis of the multiaxial quasi-brittle failure criterion. These analytical formulas succeed in efficiently predicting both the effects of notch size and fiber orientation on the off-axis notched strength of the unidirectional composite. It is also shown that the intrinsic and apparent off-axis equivalent mode-I fracture toughness values predicted for brittle failure and quasi-brittle failure, respectively, correlate well with the experimental results.
“…A reason for this could be severe size effects at = 10°. The use of rectangular clamps prevents shear deformation caused by extension‐shear coupling, which is expected to be severe at low off‐axis angles such as = 10°, inducing stress inhomogenity throughout the specimen 36 and higher stress levels 26 . Longer specimens or the use of oblique tabs were reported to alleviate this effect 26,36 .…”
Section: Resultsmentioning
confidence: 99%
“…1 Linearity in double logarithmic scale is nevertheless associated with crack growth controlled failure, rather than plasticity, and as mentioned previously, the identification of the failure mechanisms is essential before choosing the right lifetime prediction strategy. 12 It can also be shown that the normalization mehthod used by Kawai et al 26 to account for the angle dependence does not agree with the plasticity-controlled failure kinetics either. 4,32 In the light of these observations, we aim to predict the off-axis time-dependent behavior of glass/iPP using a lifetime prediction method within the framework of plasticity-controlled failure, based on the previous experience on short fiber-reinforced composites 4 and oriented polymers.…”
Section: Introductionmentioning
confidence: 99%
“…Common lifetime prediction methods for UD composites are based on the rate theory, 18–20 time–temperature superposition, 21,22 and models involving viscoelasticity or viscoplasticity 23–26 . Some of these methods 18–22 predict time‐to‐failure without describing the evolution of stress and strain during creep loading, while others 23–25 propose constitutive models describing the evolution of stresses and strains and apply a failure criterion based on energy or continuum damage mechanics to predict time‐to‐failure.…”
In our previous study, we demonstrated that the time‐dependent failure of transversely loaded UD (unidirectional) glass/iPP is fully plasticity‐controlled and proposed a lifetime prediction method based on plasticity to predict transverse failure. In the present work, extending our previous study to other off‐axis angles, we aim to investigate the effect of the off‐axis angle on the time‐dependent failure of UD glass/iPP, and to propose a lifetime prediction method for the off‐axis failure. Glass/iPP specimens with different off‐axis angles are tested at various strain rates, creep, and fatigue loads to characterize the anisotropic, time‐dependent mechanical response. It is demonstrated that the influences of strain rate and fiber orientation angle on tensile strength are multiplicatively separable; also referred to as factorizable, enabling one to characterize the angle dependence at a single strain rate and the strain rate dependence at a single angle. Moreover, similar to transverse loading, off‐axis failure is also observed to be plasticity‐controlled. Based on these observations, the lifetime prediction method for the plasticity‐controlled transverse failure is extended to off‐axis loading using the aforementioned factorazibility, which resulted in lifetime predictions in agreement with the experimental creep and fatigue data.
“…According to Jumahat et al [31], the fibre waviness with a waviness angle of 2°reduces the compressive strength of unidirectional CFRP by 39%. On the other hand, Kawai et al [32] found that the fibre misorientation, which is macroscopic misalignment, reduces Young's modulus of unidirectional CFRP by 15% with a misorientation angle of 5°. As seen from these kinds of literature, detecting the fibre misalignment by NDT early in the life cycle is important for the quality assurance of CFRP products.…”
This paper describes the detectability of eddy current testing (ECT) using directional eddy current for detection of in-plane fibre waviness in unidirectional carbon fibre reinforced plastic (CFRP) laminate. Three different types of probes, such as circular driving, symmetrical driving and uniform driving probe, were proposed, and the waviness angle was extracted from the contour map of the ECT signal by applying a Canny filter and a Hough transform. By comparing both the waviness angle estimated by ECT and that obtained by an X-ray CT image, the standard deviation (precision) and root mean square error (accuracy) were evaluated to discuss the detectability of these probes. The directional uniform driving probe shows the best detectability and can detect fibre waviness with a waviness angle of more than 2° in unidirectional CFRP. The probe shows a root mean square error of 1.90° and a standard deviation of 4.49° between the actual waviness angle and the angle estimated by ECT.
This article is part of the theme issue ‘Advanced electromagnetic non-destructive evaluation and smart monitoring’.
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