Critical Notch Root Radius Research Articles

Fracture behaviour of p-Aramid and ultra high molecular weight polyethylene based hybrid composite

Damage in the form of delamination and debonding are the most common failure modes observed in polymeric composites used for high end structural applications. The principal aim of this work is to investigate the fracture behaviour of Ultra High Molecular Weight Polyethylene (UHMWPE) and p-aramid based hybrid composite. In this work, an optimized processing condition based on highest flexural strength has been evaluated using Box-Behken design of experiment method to fabricate the hybrid composite. Based on most optimized processing conditions process variable such as 14.2 MPa for pressure, 120 oC for temperature and 12.7 minutes for time, compression moulded hybrid composites were prepared and the same were studied in detail for their interlaminar fracture toughness. In the present study, a finite notch radius that varied from 110 – 750 µm has been employed. The fracture toughness/resistance (K1c) of the material has been evaluated and analyzed by investigating the influence of varying notch root radii. The results obtained from the analysis showed that the critical notch root radius of the hybrid composite is around 340 µm. The load displacement data of three point bend test with varied crack length are used to evaluate plain strain fracture toughness (K1c). The K1c has been found 1.5 MPa√m for the KQ values derived from specimens with a/W of 0.45-0.55.

Effect of Notch Root Radius on the Fracture Toughness of Epoxy and 0.5 wt% Amino MWCNTs-Reinforced Nanocomposite

Significantly improved fracture toughness can be imparted by introducing amino multi-walled carbon nanotubes to neat epoxy resin. The present work reports on the fracture toughness evaluation of neat epoxy resin and its nanocomposite. Both the materials with varied finite notch root radii ρ, (in the range of 110–750 µm) are subjected to mode-I (tensile) fracture. The study reveals that the critical notch root radius (below which the apparent fracture toughness KIQ does not vary with ρ) of neat epoxy is 340 µm and the same for nanocomposite is 300 µm.

Effect of Notch Root Radius on the Apparent Fracture Toughness in CNT and Carbon Fiber Reinforced, Epoxy-Matrix Hybrid Composite

Significantly improved fracture resistance (in terms of fracture toughness) can be imparted to monolithic materials by adopting composite design methodology based on fiber and nano particulate reinforcement technology. The present work describes the fracture behaviour of one such reinforced material; in this case, carbon fiber (Cf)- and carbon nanotube (CNT)- reinforced epoxy composite. The Cf and CNT reinforced, epoxy-matrix hybrid composite in longitudinal and transverse orientations with varied finite notch root radii (in the range of 120–750 μm) are subjected to mode-I (tensile) fracture. The fracture toughness/resistance (KQ) of the material is then evaluated and analyzed by investigating the influence of varying notch root radii in longitudinal and transverse orientations. Such an analysis has revealed that the present unidirectional epoxy hybrid composite exhibits a critical notch root radius of 270 μm in longitudinal and 390 μm in transverse orientation.

Fracture toughness prediction of low alloy steel as a function of specimen notch root radius and size constraints

The effect of specimen notch root radius ρ and specimen size (thickness B and ligament b) on the apparent fracture toughness KI,app is investigated. Series of fracture tests on nonstandard compact tension CT specimen of low alloy steel EN34NiCrMo6, which is equivalent to AISI 4340 low alloy steel with different values of ρ and crack length to width ratio a/W were conducted to measure the KI,app. The experimental results show that, KI,app depends on ρ and specimen size. There is a critical notch root radius ρc under which KI,app becomes independent of ρ. Two empirical models have been proposed to predict KI,app and can be used to transfer the data among specimens of different ρ. One additional empirical model is developed to transfer the data among specimens of different ligament, b. Finally, a model is developed by modifying the ASTM E1921 standard equation (which predicts the thickness effect only) to include the combined effect of B and b. The model predictions are in good agreement with the experimental results.

A notch root radius to attain minimum fracture loads in plates weakened by U-notches under Mode I loading

The paper deals with the minimum value of a fracture load, with respect to the notch root radius, in plates weakened by U-notches under Mode I loading. It has been found that the fracture load has a minimum value at a critical value of the notch root radius (ρ), using four criteria, namely, Mean Stress (MS), maximum tangent stress or Point Stress (PS), Critical Strain-Energy (CSE) and Averaged Strain-Energy Density (ASED). Using a characteristic length (lch), which is a function of material properties, the results showed that the dimensionless critical notch root radius (ρ/lch)cr depended on w/a ratio (the specimen width to the notch depth), Poisson ratio, and loading condition (tensile or bending loading) under Mode I loading. In other words, according to these criteria, a notch root radius different from zero exists, providing a minimum fracture load. Therefore, a crack is not more dangerous rather than a U-notch under Mode I loading. This critical notch root radius is important for a quasi-brittle material, but may not be significant for brittle ones in practical engineering situations. Good agreement was found between theoretical predictions and experimental results on Al356-T6.

Notch Radius and Specimen Size Effects on Fracture Toughness of Low Alloy Steel

The present work is amid to investigate the effect of notch root radius ρ and specimen size (thickness B and ligament b) on the apparent fracture toughness KI,app. Series of fracture tests on non standard compact tension CT specimen of AISI 4340 low alloy steel with different values of notch root radius (ρ=0.08 mm-3 mm) and ligament size/crack length to the specimen width ratio (a/W=0.1-0.9) have been conducted to measure the apparent fracture toughness KI,app. The experimental results show that KI,app depends on ρ and specimen size (B and b). There is a critical notch root radius ρc under which the apparent fracture toughness becomes independent of the notch radius ρ. Two empirical models have been proposed to transfer the data among specimens of different notch radii and predict the experimental values of KI,app considering the effect of ρ value. Also, Two models have been developed to transfer the data among specimens with different sizes and predict KI,app considering the effect of specimen thickness B, specimen ligament band the combined effect of B and b values. The combined effec –based model is developed by modifying the E 1921 standard equation (that is usually used to predict the thickness effect only). The models predictions are in good agreement with the present work experimental results and results reported in the literature.

Study of influence of notch root radius on fracture behaviour of extra deep drawn steel sheets

ABSTRACTFracture tests are carried out on extra deep drawn steel CT specimens containing notches with different values of notch root radius (ρ= 0.07–0.75 mm). Experimental findings clearly show a critical notch root radius (ρc) below which the fracture toughness remains independent of ρ and above which it varies linearly with ρ. The 3D finite element analysis shows that the location of maximum stress level causing crack initiation is in the vicinity of notch tip. The maximum stress level is independent of ρ; however, its location is shifted away from notch tip along unbroken ligament length with increase in ρ.

Effect of notch root radius on fracture toughness of Ti–18Al–8Nb alloy

The effect of notch root radius on the mode I fracture toughness of Ti–18Al–8Nb alloy in beta solution treated and water quenched condition was investigated. The apparent fracture toughness KIA was found to be independent of the notch root radius below a critical notch root radius ρ0 and subsequently increase linearly with the square root of notch root radius ρ1/2 beyond ρ0. The critical notch root radius in this alloy was found to be ∼50 μm. The results were explained on the basis of strain controlled fracture model.

Evaluation of static and dynamic fracture toughness using apparent fracture toughness of notched specimen

In this study, intrinsic static/dynamic fracture toughness of Al 7175-T74 and 17-4PH casting steel are evaluated from the apparent static/dynamic fracture toughness of notched specimen. Fracture toughness tests are conducted using an instrumented impact and three-point bending testers. The critical average stress fracture model is used to establish the relationship to predict the intrinsic fracture toughness from the apparent fracture toughness of a notched-cracked specimen. The critical average stress fracture model is suggested using the relationship between the notch root radius and the effective distance calculated by FE analysis. The results show that fracture toughness decreases as the notch root radius decreases until a limiting of critical radius. However, fracture toughness is independent of notch root radius below the limiting critical notch root radius. It is concluded that the true fracture toughness can be predicted from test results of apparent fracture toughness measured by using a notched specimen. Also, with the critical average stress fracture model, the equation predicting relation of notch root radius and apparent fracture toughness can be calculated.

Effect of notch root radius on ductile fracture toughness of armco iron

There are situations when fatigue precracking of fracture toughness test specimens is not feasible. A case in point is when mixed mode fracture toughness is to be evaluated [1]. It, then, becomes necessary to determine the influence of the notch root radius on the fracture toughness of a material. It is well established that notch root radius has a significant influence on the apparent fracture toughness of a material [2-5]." Usually a critical notch root radius, below which fracture toughness is independent of notch root radius, exists and is measured. We present in this communication the results of an investigation of the effect of notch-root radius on mode I fracture toughness (J~o) of Armco iron and a procedure to correct for the same to arrive at the t rue Jic value.

Effect of notch root radius on fracture toughness of an 8090 aluminium - lithium alloy

There was no effect of orientation (L-T or T-L) on the variation of apparent fracture toughness with a notch root radius in 8090 type AlLi alloys. The fracture toughness was independent of the notch root radius below a critical notch root radius of approximately 140 μm and increased linearly with the square root of notch root radius for ϱ greater than 140 μm, for both the test directions. The fracture process in such alloys was strain controlled.

Condition of ductile and brittle fracture in notched polycarbonate bars.

The fracture behavior of notched plates and grooved shafts of polycarbonate under tension has been investigated for a wide range of notch geometries and dimensions of specimens. Careful observations were made on the fracture process at the notch root during tests. Bluntly notched specimens failed in a fully ductile manner with large shear bands, and sharply notched specimens failed in a brittle manner after the formation of a small crack at the tip of the plastic zone. By varying the notch root radius and thickness of plates we have been able to distinguish clearly how both influence the brittle-ductile transitions. The critical notch root radius at the brittle-ductile transition increases according to increased thickness of the plate. Experimental results were discuss-ed in terms of a combination of the maximum elastic stress at the notch root and the notch root radius. It has been established that applying a fracture criterion derived here, we can make an accurate estimate of the strength of notched polycarbonate specimens of any shape and size.

遅れ破壊き裂発生挙動に及ぼす試験片形状の影響（見掛けの応力拡大係数による検討）

The crack initiation behavior in delayed failure was studied on various specimen configurations, using the apparent stress intensity factor, Ka, which is obtained by substituting notch length for crack length in the equation of stress intensity factor K.When Ka is constant, the time to crack initiation is independent of notch length a0 and loading methods, but is dependent on specimen thickness B and notch root radius ρ. When the crack initiation time is constant, the next experimental equation holds:Ka=KI(B0/B)δ(ρ/ρ0)λ, ρ≥ρ0where KI is the stress intensity factor under plane strain state, B0 is the critical specimen thickness over which crack initiation time is constant, ρ0 is the critical notch root radius under which Ka at crack initiation is constant, and δ and λ are constant. ρ0 varies with crack initiation time and seems to depend not only on mechanical factors but also on chemical factors, such as the formation of protective film on the notch root surface.

Fatigue Limit of Steel with Cracks

Fatigue tests were carried out on mild steel plate specimens with various lengths of cracks. The relation between the alternating fatigue limit σw and the crack length was well expressed by following equation. Rp(c)w = αe(sec πσw/2σy - 1 ) = const. where, Rp(c)w is the cyclic plastic zone size at fatigue limit, αe is the equivalent crack length of an infinite plate, and σy is the yield strength of the material. The size of the persistent slip band zone on the fatigue limit is about one tenth of Rp(c)w and is nearly equal to the measured mean grain size of the material. Rp(c)w has close relations with the crack length corresponding to the fatigue limit of a plain specimen, the material constant which was already obtained in the criteria of fatigue crack initiation at notch roots, and the critical notch root radius at the branch point.