Fracture Toughness Of Engineering Materials Research Articles

A new technique to measure the dynamic fracture toughness of solids

We propose and assess a new experimental technique to measure the fracture toughness of engineering materials and its sensitivity to strain rate. The proposed method is based on a ring expansion technique and it overcomes the limitations of current dynamic fracture tests, as it is not affected by transient stress wave propagation during loading and it results in spatially uniform remote stress and strain fields prior to fracture; the method is also suitable to achieve remote strain rates well in excess of 1000 s−1. We demonstrate the technique by measuring the plane-stress Mode I fracture toughness of PMMA specimens at remote strain rates ranging from 10−3 s−1 to 102 s−1. The experiments show an increase of the toughness of the material with increasing strain rate.

Calibration of a Test Method to Measure Mode I, Mode II and Mode III Fracture Toughness of Engineering Materials

A refined finite element model and a new post processing sub program to determine mixed mode stress intensity factors and their variation along a surface crack front in components and structures is presented. The proposed finite element model employs a fine mesh of singular isoparametric pentahedral solid element with user specified number and size from one crack face to another and a number of such segments along a surface crack front. A compatible mesh of regular elements namely hexahedral solid element and pentahedral solid element is used to discretize the rest of the domain under consideration. Consistent with the use of the singular element, formulae to compute the Mode I, Mode II and Mode III stress intensity factors using displacements only of flagged nodes on flagged singular elements are implemented in a special purpose post processing sub program named SIF 1-2-3. In the present work the finite element models developed using ANSYS, a commercial FEA program, and the stress intensity factors determined using SIF 1-2-3 are validated using benchmarks. This finite element model is then used to calibrate a proposed test method to measure Mode I, Mode II and Mode III fracture toughness of engineering materials.

A new spherical indentation approach to determine fracture toughness of high strength steels

Fracture toughness indicates the resistance to unstable crack growth that is very important in the assessment of structural integrity. As the Instrumented Indentation Test (IIT) technique is relatively simple to perform, it has been suggested as an alternative way to determine fracture toughness of engineering materials. This paper presents a new spherical IIT approach to predicting fracture toughness of engineering materials using the critical contact pressure. Firstly, according to Johnson’s maximum strain energy theory under indentation, the formula of critical contact pressure under IIT is derived by considering strain hardening and fundamental tensile properties. Secondly, by using an automatic iterative procedure of translating the input and output data between MATLAB and ANSYS, finite element analysis is carried out to build a dummy database of critical contact pressure versus strain hardening and tensile properties for a broad scale of engineering materials. Finally, for the sake of engineering applications, a general analytical formula is put forward according to the dummy database, by which the critical contact pressure and thus the critical indentation depth can be read directly. Fracture toughness of engineering materials is then determined using the concept of indentation energy to fracture at the critical indentation depth. Validation of predicted fracture toughness and tensile properties of four high strength steels is taken out using compact tension (CT) and standard tensile tests, respectively. Good agreements are found between the standard tests and IIT.

Investigation into dynamic response of a three-point bend specimen in a Hopkinson bar loaded fracture test using numerical methods

Dynamic fracture toughness of engineering materials at loading rates greater than [Formula: see text] is widely investigated using the modified Hopkinson pressure bar apparatus. For accurate measurement of dynamic fracture toughness, it is essential to thoroughly understand the dynamic effects excited by the stress wave, such as stress wave propagation characteristics in bars/cracked specimen, the contact situation between the specimen and loading point or supports, and the dynamic response of the fracture specimen. In this work, full transient dynamic analysis techniques are used to comprehend “loss of contact” situation of cracked fracture specimen with an incident bar (impactor) and a transmission bar (supports) in a Hopkinson bar loaded two-bar/three-point bend test. A modified Hopkinson bar loaded experimental setup, including striker, incident, and transmission bars and three-point bend fracture specimen, is modeled using the commercial software ANSYS. The dynamic responses of the specimens made of titanium alloy, high-strength steel, and aluminum alloy are analyzed, and the specimen contact states with the incident and transmission bars are investigated using stress state contours of the specimen along with nodal displacement of the specimen and the bars. The dynamic fracture toughness values for the three specimens are also calculated and compared with the experimental results. The simulation results from the current two-bar/three-point bend test indicated that no “loss of contact” occurs during the first load duration as is previously proved experimentally.

New Post-Yield Fracture Toughness Parameters for Engineering Materials

Abstract Micromechanics approach is employed to investigate the constraint effect on post-yield fracture toughness. Relationships between the conventional post-yield fracture toughness values, J1c and δc, and crack tip constraint characterized by the crack tip stress triaxiality are derived on the basis of an improved micromechanics criterion for ductile fracture. Then, new crack tip parameters Jmc and δmc (and associated new criteria for ductile fracture) are proposed, in which the effects of crack tip deformation and constraint are taken into account. Experiments show that both Jmc and δmc are material constant independent of stress state or specimen geometry. They can serve as new post-yield fracture toughness parameters to differentiate the fracture toughness of engineering materials, which provide new approaches for fracture assessments of engineering structures.

Mixed Mode Fracture Toughness of Engineering Materials

The effect of mixed mode loading on the fracture toughness of engineering materials is rationalized in terms of the operative fracture mechanism as well as the deformation field ahead of the crack tip. It was found that mixed mode fracture is of utmost importance in materials which fail by stable ductile fracture caused by second phase particles, since in such materials there is a significant reduction in fracture toughness on mixed-mode loading. An autocatalytic shear localization-void formation mechanistic model was found to qualitatively explain the mixed mode fracture behavior in such materials.

An engineering approach to remove the specimen geometry constraint dependence of elastic-plastic fracture toughness

Constraint effects on fracture have been the subject of a number of recent studies. In this paper, the effects of constraint changes on the elastic-plastic fracture toughness values J lc are investigated and an engineering approach for constraint correction is proposed. A relationship between the conventional elastic-plastic fracture toughness, J lc, and crack tip constraint characterized by crack tip stress triaxiality, T, is derived from a modified damage criterion for ductile fracture. Then, a new parameter J sc and associated J-like criterion, J s = J sc, for ductile fracture are proposed, in which both the crack tip deformation characterized by the J-integral and the crack tip constraint characterized by stress triaxiality are included. Several experiments show that the toughness variation with specimen geometry constraint changes can effectively be removed by use of the constraint correction procedure proposed in this paper and that the new parameter J sc is a material constant independent of specimen geometry (constraint). This parameter can serve as a new parameter to differentiate the elastic-plastic fracture toughness of engineering materials, which provides a new approach to the fracture assessments of structures. It is not necessary to determine which laboratory specimen matches the structural constraint; rather, any specimen geometry can be tested to measure the size independent fracture toughness J sc. The potential advantage is clear and the results are very encouraging.

DAMAGE MECHANICS APPROACH TO REMOVE THE CONSTRAINT DEPENDENCE OF ELASTIC–PLASTIC FRACTURE TOUGHNESS

AbstractIt is now generally agreed that the applicability of a one‐parameter J‐based ductile fracture approach is limited to so‐called high constraint crack geometries, and that the elastic‐plastic fracture toughness J1c, is not a material constant but strongly specimen geometry constraint‐dependent. In this paper, the constraint effect on elastic‐plastic fracture toughness is investigated by use of a continuum damage mechanics approach. Based on a new local damage theory for ductile fracture(proposed by the author) which has a clear physical meaning and can describe both deformation and constraint effects on ductile fracture, a relationship is described between the conventional elastic‐plastic fracture toughness, J1c, and crack tip constraint, characterized by crack tip stress triaxiality T. Then, a new parameter Jdc (and associated criterion, Jd=Jdc) for ductile fracture is proposed. Experiments show that toughness variation with specimen geometry constraint changes can effectively be removed by use of the constraint correction procedure proposed in this paper, and that the new parameter Jdc is a material constant independent of specimen geometry (constraint). This parameter can serve as a new parameter to differentiate the elastic‐plastic fracture toughness of engineering materials, which provides a new approach for fracture assessments of structures. It is not necessary to determine which laboratory specimen matches the structural constraint; rather, any specimen geometry can be tested to measure the size‐independent fracture toughness Jdc. The potential advantage is clear and the results are very encouraging.

The fracture process in a fine-grained superplastic 7475 Al alloy

In order to contribute towards alloy design and therefore an improvement in fracture toughness of engineering materials in general, the effect of temperature, strain rate and strain level on the superplastic deformation, cavity nucleation and growth, and fracture behaviour are studied in an important rate-sensitive structural engineering material, 7475 Al, in the light of current models and thinking. The efficacy of hydrostatic pressure in reducing cavitation during superplastic deformation is considered.

Stability Conditions in Quasi-Static Crack Propagation for Constant Strain Energy Release Rate

Measurement of valid fracture toughness of engineering materials and design assurance of quasi-static crack growth in structures before the onset of fracture instability are the important factors in the study of fracture mechanics. Generalized equations of the stability conditions are presented not only for linear but also for nonlinear load-displacement relationships at constant crack area. Examples are used to illustrate the applications of the proposed equations, and stability conditions of different crack geometries and loadings commonly observed in engineering structures are discussed. Experiments conducted to verify the stability conditions are described and found to be satisfactory.