Synonyms for Unyielding (same or very similar meaning)
WordNet sense 1 (stubbornly unyielding):
dogged, dour, pertinacious, tenacious, doggedWordNet sense 2 (tenaciously unwilling or marked by tenacious unwillingness to yield):
obstinate, stubborn, unregenerateWordNet sense 3 (resisting weight or pressure):
hardFrom the ODE community, based on WordNetadd/edit