Farhad Rezai-Aria

Image analysis of microscopic crack patterns applied to thermal fatigue heat-checking of high temperature tool steels

Surface cracking or heat-checking is investigated at a microscopic scale on a hot work tool steel (X38CrMoV5) tested under thermal fatigue. Thermal fatigue tests are periodically interrupted to observe the surface of the specimens by scanning electron microscopy (SEM). A non destructive and semi-automatic method is developed to assess and evaluate the two-dimensional crack pattern initiated on the oxide scale layer formed on the specimen surface. The crack pattern is characterized by image analysis in terms of density, morphological and topological features. This technique allows to determine the number of cycles to initiate the microscopic heat-checking and to follow its evolution.

Experimental conditions and environment effects on thermal fatigue damage accumulation and life of die-casting steel X38CrMoV5 (AISI H11)

Thermal Fatigue (TF) is a life-limiting factor in die-casting dies. The effect of the maximum temperature of thermal cycle, Tmax, and the heating (or heat flux density) of thermal fatigue (X38CrMoV5 AISI H11) at 47 HRC is investigated. Two microscopic and macroscopic heat-checking cells are identified. The microscopic heat checking is limited to the double-layer oxide scales formed on the external surface of TF specimens. An engineering Paris-type law using Kmax, can describe TF surface crack propagation of all tests examined. TF life based on σmax is used to rationalise TF micro- and macroscopic cracking.

A New Method For Advanced Virtual Design Of Stamping Tools For Automotive Industry: Application To Nodular Cast Iron EN‐GJS‐600‐3

This contribution presents an approach combining the stamping numerical processing simulations and structure analysis in order to improve the design for optimizing the tool fatigue life. The method consists in simulating the stamping process via AutoForm® (or any FEM Code) by considering the tool as a perfect rigid body. The estimated contact pressure is then used as boundary condition for FEM structure loading analysis. The result of this analysis is used for life prediction of the tool using S‐N fatigue curve. If the prescribed tool life requirements are not satisfied, then the critical region of the tool is redesigned and the whole simulation procedures are reactivated. This optimization method is applied for a cast iron EN‐GJS‐600‐3 as candidate stamping tool materiel. The room temperature fatigue S‐N curves of this alloy are established in laboratory under uniaxial push/pull cyclic experiments on cylindrical specimens under a load ratio of R (σmin/σmax) = −2.

Normalization of fatigue crack growth data in AISI H11 tool steel at room and elevated temperature

The fatigue crack growth rate of materials is shown to be dependent on the testing conditions like load ratio R and testing temperature. Great interest exists in normalizing this data onto a single curve. In this research, some methods commonly used to normalize the effect of R ratio are tested on Fatigue Crack Growth Rate (FCGR) curves of AISI H11 tool steel. These methods are based on either purely mathematical relations or on the effect of crack closure in variable R ratio tests. A model based on the crack tip opening displacement measurements to normalize the effect of R ratio as well as temperature is also used. This model takes into account the material-hardening coefficient, yield stress, Young’s modulus, and the crack tip opening displacement measurements. Crack tip opening displacement measurements have also been directly used to characterize the FCGR. A method is presented to find out crack closure as well as crack tip opening displacement using 2D digital image correlation measurements near the crack tip. At the end, a critical analysis of the four normalization techniques is presented.