A discrete element model (DEM) for predicting apple damage during handling

Scheffler, Otto Carl (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The Discrete Element Method (DEM) is used outside of its original purpose within the field of rock mechanics and applied to the agricultural sector. Apples are globally one of the most widely traded fruit and still suffer from significant mechanical injury in the form of bruise damage. Apple damage accounts for a significant proportion of financial losses in the postharvest handling of fresh produce. DEM allows for individual particle contacts and the dynamic mechanical behaviour of a group of particles to be studied through collisions. Successfully applying this method to fresh produce, such as apples, allows for future investigations into postharvest mechanical damage to be performed on a range of fruit and vegetables. A contact model that is capable of closely replicating the viscoelastic nature of apples was developed and applied. The mechanical material parameters required for the implementation of the viscoelastic model were successfully derived through an error minimisation algorithm and the use of a pendulum impactor. The accuracy of the contact force model was dependent on definite representation of the effective damping constant and was less sensitive to the elastic stiffness. Bruise damage models (in the form of bruise volume, bruise area and bruise depth) were coupled to the impact forces available through DEM. Bruise formations resulting from multiple impacts and variable time durations between impacts were studied. The resulting models were implemented in the numerical environment. A detailed particle shape representation along with a realistic contact point loading scheme was implemented in the DEM model. Overlapping bruises were studied and accounted for on a post-process level. The resulting DEM model was successfully validated with the use of numerous physical experiments. Experimental validation commenced at a single specimen contact level and expanded into realistic situations, with up to 90 specimens, representative of conditions that may be experienced in practice. The model was extended to include run-time bruise visualisation during the DEM simulations. Good correlation between the experimental and numerical results were achieved. Quantitatively, the model succeeded in accurately predicting the contact forces typically experienced by apples to within 11 %. The model predicted the mean bruise damage of a single apple for realistic situations within an accuracy of 50 % in terms of mean bruise volume, 35 % for bruise area and 30 % for bruise depth. Qualitatively good agreement of the dynamic mechanical behaviour predicted by the model and the experiments was achieved.

AFRIKAANSE OPSOMMING: Die Diskrete Element Metode (DEM) word aangewend buite die oorspronklike veld van rotsmeganika en toegepas in die landboumilieu. Appels is van die vrugte waarmee daar wêreldwyd die meeste handel gedryf word en doen steeds in die proses meganiese skade op. Appelskade as gevolg van die hantering tydens die na-oes, veroorsaak aansienlike finansiële verliese. Met behulp van DEM kan individuele partikel kontakte, asook die meganiese gedrag van ʼn groep partikels deeglik bestudeer word. Die suksesvolle toepassing van hierdie metode op vars produkte, soos appels, veroorloof (verdere) navorsing op meganiese skade tydens die na-oes van ʼn verskeidenheid vrugte en groente. ʼn Kontakmodel wat die visko-elastiese gedrag van appels noukeurig kan voorspel, is toegepas. Die meganiese parameters wat benodig word vir die toepassing van die model is suksesvol afgelei met behulp van ‘n foutminimeringsalgoritme, sowel as ʼn pendulumimpakapparaat. Dit blyk dat die akkuraatheid van die kontakmodel meer afhanklik van die effektiewe dempingskonstante as die elastiese styfheid is. Appelkneusingsmodelle (kneusvolume, kneusarea en kneusdiepte) is gekoppel aan impakkragte, soos beskikbaar gestel deur die gebruik van DEM. Die vorming. van kneusing as gevolg van herhaaldelike impak en die tydsverloop tussen die impakte wat lei tot kneusing is bestudeerder. ʼn Gedetailleerde partikel vormvoorstelling, tesame met ‘n realistiese kontakpunt laaiskema is geïmplementeer binne die DEM model. Modelle is gevolglik ontwikkel en daarna numeries toegepas. Die effek van oorvleuelende kneusings is ook bestudeer en in ag geneem op naprosseseringsvlak. Die DEM model wat tot stand kom, is suksesvol gevalideer met behulp van talle fisiese eksperimente. Eksperimentele validasie het begin op ‘n enkel monster kontakvlak en is uitgebrei tot realistiese situasies wat in die praktyk sal voorkom. Die model word dan nog verder verfyn om kneusingformasie visueel voor te stel soos dit tydens die DEM simulasie plaasvind. Uitstekende korrelasies is tussen die eksperimentele en numeriese resultate behaal. Die model het daarin geslaag om die kontakkragte wat tipies deur appels ondervind word met 'n 11% akkuraatheid te voorspel. Die model voorspel die gemiddelde skadeverlies, wat enkel appels ondervind in realistiese situasies, met ʼn akkuraatheid van 50 % met betrekking tot kneusvolume, 35 % vir kneusarea en 30 % vir kneusdiepte. 'n Uitstekende ooreenkoms is kwalitatief tussen die dinamiese gedrag van die model en die eksperimente behaal.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103771
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