Material Fatigue has always been a critical issue regarding the design of components. The number of failure cycles have exceeded 10E7cycles. This interest in the behaviour of materials at numbers of loading cycles beyond 10E7 results mainly from the demand of robust life assessment for the increasing number of components, such as engine parts, turbine disks and blades in aerospace and load-carrying structural parts of automobiles. The material under investigation is 42CrMo4 (AISI 4140), cyclic softening material. The foundation of the present research proposal is to better understand the effect of small external mean stresses on the fatigue life of the material. Previous research with the same material showed an increase in the fatigue life due to shot peening of the material. The objective of the present research is to further understand this behaviour. Two aspect were investigated to achieve the objective. The objective of performing the experimental tests is to understand how fatigue strength is affected only by applying external global mean stresses. Fatigue strength of the specimens can be compared to understand if the material is showing an increased fatigue strength when small mean stresses (R<-0.4) are applied. SEM analysis of fractured specimens is performed to look at the size of the inclusions, FGA and fish eye. There is local plasticity around the inclusion during cyclic loading of the material. During LCF investigations of the material, it was observed that the area of the stabilized hysteresis loop was increasing until mean strain is 0.3 % and then decreasing with increasing mean strain level from 0.3%. Numerical analysis is performed by taking the experimental LCF data and evaluating relevant parameters, building material model in Abaqus and finding the stress-strain relationships. Defining the material model which completely describes the effect of inclusion around the matrix. Two different models were developed to illustrate the interaction between inclusion and matrix. The effect of externally applied mean stresses is analysed around the inclusion. Kinematic hardening and isotropic softening model parameters were used to define the material behaviour at different mean stresses and strains. Combined hardening model was not enough to explain the complicated material behaviour and ratchetting experiments are needed to make a better material model by using a UMAT file. The number of failure cycles increase when stress intensity factor range at the edge of the inclusion decreases. So, small value of SIF range leads to higher fatigue life.
La fatica del materiale è sempre stata una questione critica per quanto riguarda la progettazione dei componenti. Il numero di cicli di guasti ha superato i 10E7 cicli. Risultati 10E7 per valutazione del ciclo di vita, caricamento e trasporto di parti per automobili. Il materiale in esame è 42CrMo4 (AISI 4140), materiale ammorbidente ciclico. La base della presente proposta di ricerca è di capire meglio l'effetto del materiale esterno. Ricerche precedenti con lo stesso materiale hanno mostrato un aumento della durata a fatica due alla pallinatura del materiale. L'obiettivo della presente ricerca è comprendere ulteriormente questo comportamento. Due aspetti sono stati studiati per raggiungere l'obiettivo. L'obiettivo dell'esecuzione di test sperimentali è capire in che modo la resistenza alla fatica viene influenzata solo applicando sollecitazioni medie globali esterne. La resistenza a fatica dei campioni può essere confrontata per capire se il materiale mostra una maggiore resistenza a fatica quando si applicano piccole sollecitazioni medie (R <-0,4). L'analisi al SEM di campioni fratturati viene eseguita per osservare le dimensioni delle inclusioni, l'FGA e l'occhio di pesce. C'è plasticità locale attorno all'inclusione durante il caricamento ciclico del materiale. Durante le indagini LCF del materiale, è stato che l'area del circuito di isteresi stabilito era in aumento fino a quando lo sforzo medio era dello 0,3% e quindi in diminuzione con l'aumento del livello medio dallo 0,3%. L'analisi numerica viene eseguita dai dati LCF sperimentali e dalla valutazione dei parametri rilevanti, dal modello dei materiali da costruzione in Abaqus e dalla ricerca delle relazioni sforzo-deformazione. Definire il modello materiale che descrive l'effetto dell'inclusione attorno alla matrice. Due diversi modelli sviluppati per illustrare l'interazione tra inclusione e matrice. L'effetto degli stress medi applicati esternamente viene analizzato attorno all'inclusione. I parametri del modello di indurimento cinematico e di rammollimento isotropico sono stati utilizzati per definire il comportamento del materiale a differenti tensioni e sforzi medi. Il modello combinato di tempra non era sufficiente per spiegare il complicato comportamento del materiale e gli esperimenti di cricchetto. Il numero di cicli di guasti aumenta quando lo stress aumenta. Quindi, il basso valore della gamma SIF porta ad una maggiore durata della fatica.
The influence of tension mean stress on fatigue strength of 42CrMo4 steel in VHCF regime
HASEEB, ABDUL
2018/2019
Abstract
Material Fatigue has always been a critical issue regarding the design of components. The number of failure cycles have exceeded 10E7cycles. This interest in the behaviour of materials at numbers of loading cycles beyond 10E7 results mainly from the demand of robust life assessment for the increasing number of components, such as engine parts, turbine disks and blades in aerospace and load-carrying structural parts of automobiles. The material under investigation is 42CrMo4 (AISI 4140), cyclic softening material. The foundation of the present research proposal is to better understand the effect of small external mean stresses on the fatigue life of the material. Previous research with the same material showed an increase in the fatigue life due to shot peening of the material. The objective of the present research is to further understand this behaviour. Two aspect were investigated to achieve the objective. The objective of performing the experimental tests is to understand how fatigue strength is affected only by applying external global mean stresses. Fatigue strength of the specimens can be compared to understand if the material is showing an increased fatigue strength when small mean stresses (R<-0.4) are applied. SEM analysis of fractured specimens is performed to look at the size of the inclusions, FGA and fish eye. There is local plasticity around the inclusion during cyclic loading of the material. During LCF investigations of the material, it was observed that the area of the stabilized hysteresis loop was increasing until mean strain is 0.3 % and then decreasing with increasing mean strain level from 0.3%. Numerical analysis is performed by taking the experimental LCF data and evaluating relevant parameters, building material model in Abaqus and finding the stress-strain relationships. Defining the material model which completely describes the effect of inclusion around the matrix. Two different models were developed to illustrate the interaction between inclusion and matrix. The effect of externally applied mean stresses is analysed around the inclusion. Kinematic hardening and isotropic softening model parameters were used to define the material behaviour at different mean stresses and strains. Combined hardening model was not enough to explain the complicated material behaviour and ratchetting experiments are needed to make a better material model by using a UMAT file. The number of failure cycles increase when stress intensity factor range at the edge of the inclusion decreases. So, small value of SIF range leads to higher fatigue life.| File | Dimensione | Formato | |
|---|---|---|---|
|
2019_04_Haseeb.pdf
non accessibile
Descrizione: Text of the Thesis
Dimensione
6.89 MB
Formato
Adobe PDF
|
6.89 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/146619